Print Version

2. Parameters

Table of Contents

Collapse Menu | Expand Menu

Introduction

Photo of 3 men launching 2 weather balloons

Data plotted on the tephigram comprise temperatures, dewpoints, and wind speed and direction. Once plotted on the tephigram, other unreported meteorological quantities describing atmospheric moisture and thermodynamic properties, such as humidity, stability, and other temperature characteristics, can be determined using graphical techniques.

This section describes techniques for determining various calculated quantities or parameters from sounding data plotted on the tephigram.

Moisture/Humidity

Moisture/Humidity Saturation Mixing Ratio (ws)

Moisture/Humidity » Saturation Mixing Ratio (ws) Definition

Definition: Saturation Mixing Ratio

The saturation mixing ratio (ws) is the ratio of the mass of water vapor (Mv) to the mass of dry air (Md) in a parcel of air at saturation. In other words ws is the maximum amount of water vapor that a parcel can hold without condensation.

ws = Mv / Md

The saturation mixing ratio is expressed in parts per thousand, usually grams of water vapor per kilogram of dry air.

Moisture/Humidity » Saturation Mixing Ratio (ws) Tephigram Procedure

Tephigram Procedure to Determine Saturation Mixing Ratio

To find the saturation mixing ratio for a given temperature and pressure on a plotted sounding, read the value, either directly or by interpolation, of the saturation mixing-ratio line that crosses the T curve at that pressure.

In this example, a parcel of air at 850 hPa with a temperature of 5°C has a saturation mixing ratio of 6.5 g/kg.

Moisture/Humidity » Saturation Mixing Ratio (ws) Question

Sounding taken at Bahrain, 1200 UTC, 03 Feb 2006, showing drying and inversion at 750 hPa

This sounding comes from Bahrain. On this tephigram mixing ratio lines are dashed light brown.

What is the saturation mixing ratio (ws) for the following pressure levels on this sounding?
Enter your answers in the text boxes below, then click Done.

ws (surface) = g/kg   


ws (850 hPa) = g/kg   


ws (700 hPa) = g/kg   

Moisture/Humidity Mixing Ratio (w)

Moisture/Humidity » Mixing Ratio (w) Definition

Definition: Mixing Ratio

In a sample of moist air, the mixing ratio (w) is the ratio of the mass of water vapor (Mv) to the mass of dry air (Md):

w = Mv / Md

The mixing ratio is expressed in parts per thousand, usually grams of water vapor per kilogram of dry air.

The mixing ratio differs from the saturation mixing ratio in that it measures the actual amount of water vapor present, while the saturation mixing ratio measures the amount of water vapor that would be present at saturation.


Note for very precise physical computations, the "specific humidity" (q) is often a preferable quantity to use. Specific humidity is the mass of water vapor per mass of moist air:

q = Mv / (Mv + Md)

However, for synoptic forecasting purposes, the mixing ratio is sufficiently representative and is easier to evaluate.

Moisture/Humidity » Mixing Ratio (w) Tephigram Procedure

Tephigram procedure to determine mixing ratio

To find the mixing ratio (w) for a given pressure on the plotted sounding, read the value, either directly or by interpolation, of the saturation mixing-ratio line that crosses the Td curve at that pressure.

In this example, a parcel of air at 850 hPa with a dewpoint of -6°C has a mixing ratio of 2.9 g/kg.

Moisture/Humidity » Mixing Ratio (w) Question

Sounding taken at Honolulu, HI, 1200 UTC, 03 Feb 2006, showing drying at 700 hPa

This sounding from Pearl Harbor, HI, shows rapid drying at about 700 hPa. On this tephigram saturation mixing ratio lines are dashed, light brown.

What is the mixing ratio (w) for the following pressure levels on this sounding?
Enter your answers in the text boxes below, then click Done.

w (surface) = g/kg   


w (750 hPa) = g/kg   


w (650 hPa) = g/kg   

Moisture/Humidity Relative Humidity (RH)

Moisture/Humidity » Relative Humidity (RH) Definition & Tephigram Procedure

Definition: Relative Humidity (RH)

Relative humidity (RH) is the ratio (expressed as a percent) of the amount of water vapor in a given volume of air to the amount that volume would hold if the air were saturated.

Tephigram procedure to determine relative humidity

Tephigram Procedure

The relative humidity can be computed from the mixing ratio (w) and the saturation mixing ratio (ws) by the following equation:

RH = 100 * (w/ws)

In the example sounding above, a parcel of air at 850 hPa has a mixing ratio of 2.9 g/kg, a saturation mixing ratio of 6.5 g/kg, and a relative humidity of 45%.

Note the definition assumes a saturation mixing ratio (and relative humidity) for liquid water, not ice.

Moisture/Humidity » Relative Humidity (RH) Question

Sounding taken at Honolulu, HI, 1200 UTC, 03 Feb 2006, showing drying at 700 hPa

This sounding from Pearl Harbor, HI, shows rapid drying at about 700 hPa. On this tephigram, mixing ratio lines are dashed, light brown.

Determine the relative humidity at the surface and again at 600 hPa.
Enter your answer in the text boxes below, then click Done.

RH (surface) = %  


RH (600 hPa) = %  

Moisture/Humidity Dewpoint Depression

Moisture/Humidity » Dewpoint Depression Definition

Definition: Dewpoint Depression

The dewpoint depression is the difference between the temperature and the dewpoint temperature at a particular pressure level. The moisture output from rawinsonde observations is typically reported in terms of dewpoint depression. Saturated conditions have a dewpoint depression of zero, while dry conditions have a large dewpoint depression (30°C or more).

Moisture/Humidity » Dewpoint Depression Question

Sounding taken at Oakland, CA, 1200 UTC, 02 Aug 2005, associated with fog

This sounding is fairly typical of a summer morning along the coast in Northern California.

What is the dewpoint depression for the following pressure levels on this sounding?
Enter your answers in the text boxes below, then click Done.

Dewpoint depression (surface) = °C  


Dewpoint depression (850 hPa) = °C  

Moisture/Humidity Saturation Vapor Pressure (es)

Moisture/Humidity » Saturation Vapor Pressure (es) Definition

Definition: Saturation Vapor Pressure

The saturation vapor pressure (es) is that part of the total atmospheric pressure attributable to water vapor if the air were saturated. Because the air is saturated, this represents a maximum vapor pressure possible for a given pressure and temperature.

Moisture/Humidity » Saturation Vapor Pressure (es) Tephigram Procedure

Tephigram procedure to determine saturation vapor pressure

From the temperature (T) curve at the given pressure on the sounding, always follow the isotherm to the 622 hPa isobar. The value of the saturation mixing ratio line, read by interpolation if necessary, through this point at 622 hPa gives the saturation vapor pressure in hectopascals (hPa) at the given pressure. In this example, air at a pressure of 850 hPa with a temperature of 3°C has a saturation vapor pressure of 7.5 hPa.

Moisture/Humidity » Saturation Vapor Pressure (es) Question

Sounding taken at Fairbanks, AK, 1200 UTC, 06 Feb 2006, showing inversions at surface and 850 hPa

This sounding comes from Fairbanks, AK. On this tephigram, mixing ratio lines are dashed, light brown.

What is the saturation vapor pressure (es) at 850 hPa for the sounding?
Enter your answer in the text box below, then click Done.

es (850 hPa) = hPa  

Moisture/Humidity Vapor Pressure (e)

Moisture/Humidity » Vapor Pressure (e) Definition

Definition: Vapor Pressure

The vapor pressure (e) is that part of the total atmospheric pressure attributable to water vapor.

Moisture/Humidity » Vapor Pressure (e) Tephigram Procedure

Tephigram procedure to determine vapor pressure

From the dewpoint (Td) curve at the given pressure on the sounding, always follow the isotherm to the 622 hPa isobar. The value of the saturation mixing ratio line, read by interpolation if necessary, through this point at 622 hPa gives the vapor pressure in hectopascals (hPa) at the given pressure. In this example, air at a pressure of 850 hPa with a dewpoint of -9°C has a vapor pressure of 3.0 hPa.

Note the procedure to find vapor pressure is quite similar to the one already described to find saturation vapor pressure. The only difference is one starts with the dewpoint to find the vapor pressure, while one starts with the temperature to find the saturation vapor pressure.

Moisture/Humidity » Vapor Pressure (e) Question

Sounding taken at Bahrain, 1200 UTC, 03 Feb 2006, showing drying and inversion at 750 hPa

This sounding comes from Bahrain. On this tephigram, mixing ratio lines are dashed, light brown.

What is the vapor pressure (e) at 850 hPa for the sounding?
Enter your answer in the text box below, then click Done.

e (850 hPa) = hPa  

Temperature/Levels

Temperature/Levels Virtual Temperature (Tv)

Temperature/Levels » Virtual Temperature (Tv) Definition

Definition: Virtual Temperature

The virtual temperature (Tv) is the temperature at which dry air would have the same density as the moist air, at a given pressure. In other words, two air samples with the same virtual temperature have the same density, regardless of their actual temperature or relative humidity.

Scale with 2 air parcels on it. On left, t=27, td=24, and tv=30. On right, t=29, td=5, and tv=30. The 2 parcels balance.

Because water vapor is less dense than dry air and warm air is less dense than cool air, the virtual temperature is always greater than or equal to the actual temperature.

Note because the saturation mixing ratio increases exponentially with temperature (roughly doubling with every increase of about 10°C), the virtual temperature correction becomes increasingly important for higher dewpoints.

Temperature/Levels » Virtual Temperature (Tv) Tephigram Procedure

Tephigram procedure to determine virtual temperature

At a given pressure level, do the following:

  1. Determine the mixing ratio (w, in g/kg), which is the value of the saturation mixing ratio line passing through the dewpoint (Td), at a given pressure.
  2. The virtual temperature is then computed as follows:
    Tv ~ T + (w / 6)

In this example:
T = 5°C and Td = -6°C.
Thus, w = 2.9 g/kg.
Tv = T + w/6
     = 5 + 2.9/6
     = 5.5°C

Temperature/Levels » Virtual Temperature (Tv) Question

Tephigram with Td = 5, T = 10 plotted at 700 hPa

At 700 hPa, the temperature is 10°C and the dewpoint is 5°C.
What is the virtual temperature?

Step 1: Determine the mixing ratio in g/kg.
Enter your answer in the text box below, then click Done.

Mixing ratio = g/kg  

Temperature/Levels Potential Temperature (Theta)

Temperature/Levels » Potential Temperature (Theta) Definition

Definition: Potential Temperature

The potential temperature (theta) is the temperature that a sample of air would have if it were brought dry-adiabatically to a pressure of 1000 hPa.

Potential temperature is commonly expressed in kelvins. However, in this module we will use °C for the sake of convenience. To convert from K to °C, simply subtract 273.15.

Temperature/Levels » Potential Temperature (Theta)Tephigram Procedure

Tephigram procedure to determine potential temperature

From the temperature curve at the given pressure, follow the dry adiabat to the 1000 hPa isobar. The isotherm value at this point is equal to the potential temperature of the air parcel. The dry adiabat is an isotherm of constant potential temperature. Thus, air with a temperature of -30°C at 500 hPa (shown) has the same potential temperature as air with a temperature of 0°C at 750 hPa or 23°C at 1000 hPa.

Temperature/Levels » Potential Temperature (Theta) Question

Tephigram with sounding for potential temperature (theta) question

What is the potential temperature (theta) for the air parcel at 700 hPa in this example?

Enter your answer in the text box below, then click Done.

Theta (700 hPa) = °C  

Temperature/Levels Lifting Condensation Level (LCL)

Temperature/Levels » Lifting Condensation Level (LCL) Definition

Definition: Lifting Condensation Level (LCL)

The lifting condensation level (LCL) is the height at which a parcel of air becomes saturated when it is lifted dry-adiabatically.

Temperature/Levels » Lifting Condensation Level (LCL) Tephigram Procedure

Tephigram procedure to determine height of LCL

The LCL is located on a sounding at the intersection of the saturation mixing-ratio line that passes through the surface dewpoint temperature with the dry adiabat that passes through the surface temperature. In this example, air at the surface with T=9°C and Td=0°C will become saturated if lifted dry-adiabatically to 870 hPa, which is the lifting condensation level.

Note: When the moisture content in the near-surface layers varies significantly, an average moisture value of the lower layer may be used in place of the surface-parcel moisture value to compute the LCL.

Temperature/Levels » Lifting Condensation Level (LCL) Question

Tephigram with sounding for LCL, CCL, LFC questions

What is the lifting condensation level (LCL) for the surface layer in this sounding?
Enter your answer in the text box below, then click Done.

LCL = hPa  

Temperature/Levels Equivalent Temperature (Te)

Temperature/Levels » Equivalent Temperature (Te) Definition

Definition: Equivalent Temperature

The equivalent temperature (Te) is the temperature a sample of air at a pressure level would have if all its moisture were condensed out by a pseudo-adiabatic process (whereby all the condensed moisture is immediately removed from the air sample). The latent heat of condensation then heats the air sample. Sometimes, the equivalent temperature is termed the "adiabatic equivalent temperature."

Temperature/Levels » Equivalent Temperature (Te) Tephigram Procedure

Tephigram procedure to determine equivalent temperature
  1. From the dewpoint at the given pressure, draw a line upward parallel to the saturation mixing-ratio lines. Also, from the temperature curve at the given pressure, draw a line upward along a dry adiabat until it intersects the line drawn from the dewpoint. Recall that this level is the LCL.
  2. From the LCL, follow a saturation adiabat upward to a pressure where the saturation adiabat parallels the dry adiabat. This is the pressure level where all the moisture has been condensed out of the sample.
  3. From this pressure, follow a dry adiabat back to the original pressure. The isotherm value at this point is equal to the equivalent temperature (Te).

In this example, air at 850 hPa with T = 10°C and Td = -8°C has an equivalent temperature of 17°C.

Temperature/Levels » Equivalent Temperature (Te) Question

Tephigram with sounding for equivalent temperature (Te) question

What is the 900 hPa equivalent temperature (Te) in this sounding?

Enter your answer in the text box below, then click Done. Te = °C  

Temperature/Levels Equivalent Potential Temperature (Theta-e)

Temperature/Levels » Equivalent Potential Temperature (Theta-e) Definition

Definition: Equivalent Potential Temperature

The equivalent potential temperature (theta-e) is the temperature a sample of air would have if all its moisture were condensed out by a pseudo-adiabatic process (i.e., with the latent heat of condensation being used to heat the air sample), and the sample then brought dry-adiabatically back to 1000 hPa.

The equivalent potential temperature is identical to the equivalent temperature, except the sample is brought dry-adiabatically from the equivalent temperature at the initial level to the equivalent potential temperature at the 1000 hPa level.

Equivalent potential temperature is commonly expressed in kelvins. However, in this module we will use °C for the sake of convenience. To convert from K to °C, simply subtract 273.15.

Temperature/Levels » Equivalent Potential Temperature (Theta-e) Tephigram Procedure

Tephigram procedure to determine equivalent potential temperature

Tephigram Procedure:

  1. From the dewpoint at the given pressure, draw a line upward along a saturation mixing-ratio line. Also, from the T curve at the given pressure, draw a line upward along a dry adiabat until it intersects the line drawn from the dewpoint at the LCL.
  2. From this intersection, follow a saturation adiabat upward to a pressure where the saturation adiabat parallels the dry adiabat. This is the pressure level where all the moisture has been condensed out of the sample.
  3. From this pressure, follow the dry adiabat down to the 1000 hPa isobar. The temperature where the dry adiabat crosses the 1000 hPa isobar is the equivalent potential temperature (theta-e).

In this example, air at 850 hPa with T = 10°C and Td = -8°C has an equivalent potential temperature just above 30°C.

Temperature/Levels » Equivalent Potential Temperature (Theta-e) Question

Tephigram with sounding for equivalent potential temperature (Theta-e) question

Compute the equivalent potential temperature (theta-e) for the given temperature and dewpoint at 900 hPa shown in this sounding.

Enter your answer in the text box below, then click Done.

Theta-e = °C  

Temperature/Levels Wet-Bulb Temperature (Tw)

Temperature/Levels » Wet-Bulb Temperature (Tw) Definition

Definition: Wet-Bulb Temperature

The wet-bulb temperature (Tw) is the temperature to which a parcel of air at a constant pressure cools through the evaporation of water into it. At this temperature, the parcel becomes saturated.

In other words, take a parcel of air, not at saturation. Then, at constant pressure (with no vertical motion), evaporate water into the parcel. Evaporative cooling will occur until the parcel reaches saturation. The wet-bulb temperature is reached when the air parcel achieves saturation.

The wet-bulb temperature will always fall between the dewpoint and the temperature, unless the air is saturated. At saturation, the temperature, dewpoint, and wet-bulb temperature are equal.

In the real atmosphere Tw often provides a good estimate of what the surface temperature will become after the onset of precipitation and once conditions become saturated.

Temperature/Levels » Wet-Bulb Temperature (Tw) Tephigram Procedure

Tephigram procedure to determine wet-bulb temperature

At a given pressure level, do the following:

  1. From the temperature, proceed up along a dry adiabat.
  2. From the dewpoint proceed up along a mixing ratio line.
  3. From where the two lines intersect, proceed down the saturation adiabat to the original level.

In this example, air at 850 hPa with T = 20°C and Td = 0°C has a wet-bulb temperature of 10°C.

Recall that the definition on the previous page called for a process at constant pressure, which implies no vertical motion. Yet the tephigram procedure illustrated here requires the apparent lifting of an air parcel. This just illustrates that the tephigram is fundamentally a thermodynamic diagram that allows us to determine various thermodynamic properties graphically, as well as display sounding data and other vertical atmospheric profiles.

Temperature/Levels » Wet-Bulb Temperature (Tw) Question

Tephigram with Td and T plotted for wet-bulb temperature (Tw) question

Suppose you are stationed in Denver, Colorado, which lies at approximately 5000 feet.

Pressure = 850 hPa
Temperature = 4°C
Dewpoint = -10°C

What is the surface wet-bulb temperature?
Enter your answer in the text box below, then click Done.

Wet-bulb temperature = °C  

Temperature/Levels Wet-Bulb Potential Temperature (Theta-w)

Temperature/Levels » Wet-Bulb Potential Temperature (Theta-w) Definition

Definition: Wet-Bulb Potential Temperature

The wet-bulb potential temperature (theta-w) is the wet-bulb temperature a sample of air would have if it were brought along a saturation adiabat to a pressure of 1000 hPa.

The wet-bulb potential temperature is identical to the wet-bulb temperature, except the sample is brought moist-adiabatically from the wet-bulb temperature at the initial level to the wet-bulb potential temperature at the 1000 hPa level.

Wet-bulb potential temperature is commonly expressed in kelvins. However, in this module we will use °C for the sake of convenience. To convert from K to °C, simply subtract 273.15.

Temperature/Levels » Wet-Bulb Potential Temperature (Theta-w) Tephigram Procedure

Tephigram procedure to determine wet-bulb potential temperature

At a given pressure level, do the following:

  1. Find the wet-bulb temperature
  2. From the wet-bulb temperature, follow the saturation adiabat to the 1000 hPa isobar.
  3. The isotherm value at this intersection equals the wet-bulb potential temperature at the given pressure.

In this example, air at 700 hPa with T = -5°C and Td = -13°C has a wet-bulb temperature of -7°C and a wet-bulb potential temperature of 10°C.

Temperature/Levels » Wet-Bulb Potential Temperature (Theta-w) Question

Sounding taken at North Platte, NE, 1200 UTC, 24 Jul 2000, associated with a severe supercell thunderstorm

This sounding was taken at North Platte, Nebraska, at 1200 UTC 24 July 2000. It was associated with a severe supercell thunderstorm.

What is the wet-bulb potential temperature (theta-w) for the temperature and dewpoint at 850 hPa shown in this sounding? Enter your answer in the text box below, then click Done.

Theta-w (850 hPa) = °C  

Temperature/Levels Convective Condensation Level (CCL)

Temperature/Levels » Convective Condensation Level (CCL) Definition

Definition: Convective Condensation Level (CCL)

The convective condensation level (CCL) is the height to which a parcel of air, if heated sufficiently from below, will rise adiabatically until it is just saturated. Usually, it is the height of the base of cumuliform clouds produced by thermal convection caused solely by surface heating.

Temperature/Levels » Convective Condensation Level (CCL) Tephigram Procedure

Tephigram procedure to determine height of CCL

To determine the CCL on a sounding, start at the surface dewpoint, proceed upward along the saturation mixing-ratio line until this line intersects the temperature profile on the sounding. The level of the intersection is the CCL.

In this example, air at the surface with a dewpoint of 0°C would have a CCL of 750 hPa.

Note: When the moisture content in the near-surface layers varies significantly, an average moisture value of the lower layer may be used in place of the surface-parcel moisture value in computing the CCL.

Temperature/Levels » Convective Condensation Level (CCL) Question

Tephigram with sounding for LCL, CCL, LFC questions

What is the convective condensation level (CCL) for the surface layer in this sounding?
Enter your answer in the text box below, then click Done.

CCL = hPa  

Temperature/Levels Convective Temperature (Tc)

Temperature/Levels » Convective Temperature (Tc) Definition

Definition: Convective Temperature

The convective temperature (Tc) is the surface temperature that must be reached to start the formation of convective clouds caused by solar heating of the near-surface layer.

Temperature/Levels » Convective Temperature (Tc) Tephigram Procedure

Tephigram procedure to determine convective temperature

From the convective condensation level (CCL) on the temperature profile, proceed downward along a dry adiabat to the pressure level at the surface. The temperature read at this intersection is the convective temperature (Tc).

In this example, the CCL lies at 750 hPa and the convective temperature is 20°C.

Temperature/Levels » Convective Temperature (Tc) Question

Sounding taken at North Platte, NE, 1200 UTC, 24 Jul 2000, associated with a severe supercell thunderstorm

This sounding was taken at North Platte, Nebraska, at 1200 UTC 24 July 2000. It was associated with a severe supercell thunderstorm. North Platte lies at an elevation of approximately 2800 feet (850 m) or near 920 hPa.

What is the convective temperature (Tc) for this sounding?
Enter your answer in the text box below, then click Done.

Tc = °C  

Temperature/Levels Thickness

Temperature/Levels » Thickness Definition

Definition: Thickness

Thickness refers to the vertical distance between two constant pressure surfaces (isobars).

The thickness of a layer is related to the mean virtual temperature (Tv) according to the formula:

Equation used to define thickness

Where:

  • ΔZ = thickness (m)
  • Rd = gas constant for dry air
  • Symbol for mean virtual temperature = mean virtual temperature for the layer
  • p1 = pressure at the bottom of the layer
  • p2 = pressure at the top of the layer
  • g = gravitational acceleration

Note for a given pair of pressures, the thickness is proportional to the mean virtual temperature (Tv) in the layer. The 1000-500 hPa thickness is perhaps the most commonly used because it corresponds to the mean temperature in the lower half of the troposphere. It is typically plotted on weather charts with 60-meter contours. In this case, each contour reflects a mean virtual temperature change of about 3°C.

Temperature/Levels » Thickness Tephigram Procedure

In the past, the thickness of layers could be determined manually by reference to the plotted virtual temperature curve and a thickness scale on the printed tephigram. Nowadays, thickness is typically computed and displayed on electronic versions of the tephigram. On the Interactive Tephigram accompanying this module, the thickness of the 1000-500 hPa layer is calculated and shown on the right-hand side of the diagram once you click on the "CAPE" menu item at the top.

Temperature/Levels » Thickness Strengths and Limitations

Thickness values are widely used to forecast surface high and low temperatures, since these values are proportional to mean temperatures in layers of the atmosphere, and since operational numerical models often have difficulty with surface temperature forecasts. At a given location, depending on the season and cloud cover, typical values of thicknesses derived from empirical studies are utilized to get an estimate of daily maximum or minimum temperatures.

Thickness values are also used to forecast precipitation type. For example, the critical rain/snow threshold corresponds to a 1000-500 thickness of 5400 m (540 dm) or a 1000-850 thickness of 1300 m (130 dm)—with adjustments made for higher elevation locations and maritime regions.

Caution: Threshold thickness values are just one diagnostic tool and need to be considered with other important factors such as latent cooling, precipitation rate, cold advection, cloud depth, etc.

For more information on using thickness values to forecast precipitation type, see:
Topics in Precipitation Type Forecasting / Partial Thickness Analysis

Other Levels

Other Levels Freezing Level

Other Levels » Freezing Level Definition & Tephigram Procedure

Definition: Freezing Level

The freezing level is the lowest level in a sounding at which a temperature of 0°C is reported. (If the surface temperature is below freezing, then the surface level is the freezing level.)

Tephigram procedure to determine freezing level

Tephigram Procedure

From the surface, follow a 0°C isotherm upward until it crosses the temperature profile. That level is the freezing level.

The sounding above comes from Bahrain. A bold, blue line clearly denotes the 0°C isotherm, which crosses the temperature profile just above the 600 hPa isobar.

Other Levels » Freezing Level Question

Tephigram with sounding for freezing level question

In this sounding, what is the freezing level?
Enter your answer in the text box below, then click Done.

Freezing level = hPa  

Other Levels Wet-Bulb Zero Level

Other Levels » Wet-Bulb Zero Level Definition & Tephigram Procedure

Definition: Wet-Bulb Zero Level

The wet-bulb zero level is the the lowest level in a sounding at which the wet-bulb temperature is 0°C.

During the onset of a cool-season precipitation event, the higher the initial wet-bulb zero level, the less chance of the precipitation changing to freezing/frozen precipitation at the surface. Also, during convective season, lower wet-bulb zero levels can indicate a higher probability of hail occurrence.

Tephigram Procedure

From the surface, follow a 0°C isotherm upward until it crosses the wet-bulb temperature profile. That level is the wet-bulb zero level.

Other Levels » Wet-Bulb Zero Level Question

Tephigram with sounding for wet-bulb freezing level (wet-bulb zero) question

In this sounding, the wet-bulb temperature profile is represented by the solid blue curve. What is the wet-bulb zero level?
Enter your answer in the text box below, then click Done.

Wet-bulb zero level = hPa  

Other Levels Level of Free Convection (LFC)

Other Levels » Level of Free Convection (LFC) Definition

Definition: Level of Free Convection (LFC)

The level of free convection (LFC) is the height at which a parcel of air, when lifted, becomes warmer than its surroundings and thus convectively buoyant. The parcel is lifted dry-adiabatically until saturated (at the LCL) and then moist-adiabatically thereafter.

Other Levels » Level of Free Convection (LFC) Tephigram Procedure

Tephigram procedure to determine height of LFC

From the lifting condensation level (LCL) proceed upward along a saturation adiabat until you intersect the sounding temperature curve. The level of this intersection is the LFC.

In this example, the surface T = 9°C and Td=0°C, resulting in an LCL of 870 hPa and an LFC of 675 hPa.

Other Levels » Level of Free Convection (LFC) Question

Tephigram with sounding for LCL, CCL, LFC questions

What is the level of free convection (LFC) for the surface layer in this sounding?
Enter your answer in the text box below, then click Done.

LFC = hPa  

Other Levels Mixing Condensation Level (MCL)

Other Levels » Mixing Condensation Level (MCL) Definition

Definition: Mixing Condensation Level (MCL)

The mixing condensation level (MCL) is the height at which saturation occurs after the complete mixing of a layer.

Other Levels » Mixing Condensation Level (MCL) Tephigram Procedure

Tephigram procedure to determine height of MCL

The determination of the MCL first requires estimation of the height of the top of the mixed layer. This is done subjectively using local forecasting methods.

Once the top of the mixed layer is estimated, one must determine the mean dry adiabat and the mean mixing ratio of the mixed layer.

The mean dry adiabat is determined from the sounding T curve by the equal-area method as shown on the tephigram.

The mean mixing ratio is determined from the sounding Td curve by the equal-area method also as shown on the tephigram.

The MCL lies at the pressure level specified by the intersection of the mean saturation mixing-ratio line and the mean dry adiabat within the mixed layer. If these two lines intersect above the mixed layer, then the mixed air is too dry to reach saturation by the mixing process and no MCL exists.

In this example, there is an MCL at 830 hPa, since it lies below the top of the mixed layer at 780 hPa.

Other Levels » Mixing Condensation Level (MCL) Question 1

Tephigram with sounding for MCL question 1

It is a summer day at a location in the Northern Plains of the United States. The time is 1300 UTC (0700 local time), and you have the recent 1200 UTC sounding plot as well as the sounding plot from 0000 UTC the evening before.

Your task is use the sounding plots to determine the cloud base for later in the day, assuming a mixed layer depth similar to yesterday.

From the 0000 UTC sounding shown, what was the top of the mixed layer?
Enter your answer in the text box below, then click Done.

Top of the mixed layer = hPa  

Other Levels » Mixing Condensation Level (MCL) Question 2

Tephigram with sounding for MCL question 2

From the 1200 UTC sounding shown, what do you expect the mean mixing ratio to be in the mixed layer?
Enter your answer in the text box below, then click Done.

Mean mixing ratio = g/kg  

Other Levels Tropopause

Other Levels » Tropopause Definition

Definition: Tropopause

The tropopause is defined as the boundary between the troposphere and the stratosphere. It is usually marked by a significant change in lapse rate from less stable below in the troposphere to very stable above in the stratosphere.

Its height varies from 10 km or lower in polar regions to as high as 20 km in the tropics.

Since the temperature gradient reverses from cooling with increased height in the troposphere to warming with increased height in the stratosphere, the maximum wind speed is typically observed at or near the tropopause level.

The tropopause is a mandatory reported level for most rawinsonde soundings.

Other Levels » Tropopause Question

Tephigram with sounding for tropopause question

In this sounding, identify the level of the tropopause.
Enter your answer in the text box below, then click Done.

Tropopause = hPa  

Other Levels Equilibrium Level (EL)

Other Levels » Equilibrium Level (EL) Definition

Definition: Equilibrium Level (EL)

The equilibrium level (EL) is the height where the temperature of a buoyantly rising parcel again equals the temperature of the environment. The EL may be determined for surface parcels that are lifted or heated.

Other Levels » Equilibrium Level (EL) Tephigram Procedure for a Lifted Surface Parcel

Tephigram procedure to determine equilibrium level (EL) for a lifted parcel

From the LFC, proceed upward along a saturation adiabat until it intersects the temperature profile. The pressure at this intersection is the equilibrium level (EL).

In this example, an air parcel lifted mechanically from the surface has an equilibrium level of 198 hPa.

Other Levels » Equilibrium Level (EL) Tephigram Procedure for a Heated Surface Parcel

Tephigram procedure to determine equilibrium level (EL) for a heated parcel

From the CCL, proceed upward along a saturation adiabat until intersecting the temperature profile. The pressure at this intersection is the equilibrium level (EL).

In this example, an air parcel lifted convectively by heating has an equilibrium level of 140 hPa.

Other Levels » Equilibrium Level (EL) Question

Sounding taken at Tucson, AZ, 1200 UTC, 14 Jul 2002, associated with a severe thunderstorm outbreak in southeast Arizona

This sounding comes from the July 14, 2002 severe thunderstorm outbreak in southeast Arizona.

Find the equilibrium level (EL) for both lifted and heated surface parcels. Enter your answers in the text boxes below, then click Done.


EL (lifted) = hPa   


EL (heated) = hPa   

Other Levels Maximum Parcel Level (MPL)

Other Levels » Maximum Parcel Level (MPL) Definition

Definition: Maximum Parcel Level (MPL)

Animation showing depiction of maximum parcel level (MPL)

The maximum parcel level (MPL) is the level to which a parcel will travel before exhausting all of its upward momentum. When a parcel travels through the equilibrium level, its upward acceleration ceases as it becomes colder than its surroundings, but its upward momentum continues to propel the parcel to a higher level. Therefore, the MPL is always at a higher level than the equilibrium level. Practically speaking, the MPL is the maximum predicted height of a thunderstorm for a given sounding.

Other Levels » Maximum Parcel Level (MPL) Tephigram Procedure

Tephigram procedure to determine maximum parcel level (MPL)

First, determine the equilibrium level (EL) for either a lifted or heated parcel, whichever is most appropriate for the situation. Then continue upward along a saturation adiabat until the negative area above the EL is equal to the positive area (CAPE) below the EL. For computer-generated tephigrams, the MPL is usually computed automatically.

Other Levels » Maximum Parcel Level (MPL) Question

Sounding taken at Tucson, AZ, 1200 UTC, 14 Jul 2002, associated with a severe thunderstorm outbreak in southeast Arizona

This sounding comes from the July 14, 2002 severe thunderstorm outbreak in southeast Arizona.

Estimate the maximum parcel level (MPL) for a surface parcel that became convectively buoyant after it was lifted to its LFC LFC. Enter your answer in the text box below, then click Done.

MPL (lifted) = hPa   

Stability Assessment

Stability Assessment Convective Available Potential Energy (CAPE)

Stability Assessment » Convective Available Potential Energy (CAPE) Definition

Definition: Convective Available Potential Energy (CAPE)

The convective available potential energy (CAPE) is represented by the area on a tephigram enclosed by the environmental temperature profile and the saturation adiabat running from the LFC to the EL. This area, depicted in the diagram below, indicates the amount of buoyant energy available as the parcel is accelerated upward. CAPE is measured in units of joules per kilogram (J/kg).

Depiction of CAPE on a tephigram

The larger the positive area, the higher the CAPE value and instability, and the greater the potential for strong and perhaps severe convection. This table offers a general correlation between CAPE and atmospheric stability, however CAPE climatologies vary widely.

CAPE Value Stability
0 Stable
0-1000 Marginally Unstable
1000-2500 Moderately Unstable
2500-3500 Very Unstable
3500 or greater Extremely Unstable

CAPE may also be related to updraft velocity via the relation:

    Wmax (in m/s) = sqrt(2 * CAPE)

Hence, for a CAPE of 2500 J/kg, the maximum updraft velocity, Wmax, would be about 71 m/s! In reality, water loading, entrainment, and other factors can reduce Wmax by as much as a factor of 2.

Stability Assessment » Convective Available Potential Energy (CAPE) Strengths and Limitations

Strengths

  • CAPE is a robust indicator of the potential for deep convection and convective intensity.
  • CAPE provides a measure of stability integrated over the depth of the sounding, as opposed to other stability indices, such as Lifted Index, that use data from only a few mandatory levels.

Limitations

  • The computation of CAPE is extremely sensitive to the mean mixing ratio in the lowest 500 m. For instance, a 1 g/kg increase can increase CAPE by 20%.
  • Since the computation of CAPE is based on parcel theory, it does not take into account processes such as mixing, water loading, and freezing.
  • Surface layer based CAPE computations may underestimate the convective potential in situations with elevated convection.
  • Since CAPE, by itself, does not account for wind shear, it may underestimate the potential for severe convection where strong wind shear is present.

Stability Assessment » Convective Available Potential Energy (CAPE) Tephigram Procedure

Usually CAPE is computed automatically and displayed as output in electronic versions of the tephigram.

When calculating CAPE, we normally lift a parcel that reflects the mean values of the temperature and moisture in the lowest 50 to 100 hPa. This layer represents the average heat and moisture conditions fueling convective storms.

Remember, you should never rely solely on CAPE to evaluate the convective potential. Also, consider the strength of low-level inversions, the height of the LFC, and other factors related to the vertical distribution of CAPE that could also modulate convection.

For a complete discussion of CAPE, see the module: Buoyancy and CAPE.

Stability Assessment » Convective Available Potential Energy (CAPE) Question

2 tephigrams with soundings for CAPE, CIN questions

Which of these two soundings has greater CAPE?

Stability Assessment Convective Inhibition (CIN)

Stability Assessment » Convective Inhibition (CIN) Definition

Definition: Convective Inhibition

The convective inhibition (CIN) is represented by the area on a tephigram enclosed by the environmental temperature profile and the temperature of a parcel lifted from some originating level to the LFC. This area indicates the amount of energy required to lift the parcel to the LFC. CIN is measured in units of joules per kilogram (J/kg).

Hypothetical sounding showing CIN and CAPE

The larger the negative area, the higher the CIN value, and the lower the likelihood of convective storms. One caveat is that if the CIN is large but storms manage to form, usually due to increased moisture and/or heating overcoming the CIN, then the storms are more likely to be severe. CIN is usually the result of a capping stable layer or inversion, with values of over 200 J/kg significantly inhibiting convective potential.

Stability Assessment » Convective Inhibition (CIN) Tephigram Procedure

Usually CIN is computed automatically and displayed as output in electronic versions of the tephigram.

When calculating CIN, we normally lift a parcel that reflects the mean values of the temperature and moisture in the lowest 50 to 100 hPa. This layer represents the average heat and moisture conditions fueling convective storms.

For a complete discussion of CIN, see the module: Buoyancy and CAPE.

Stability Assessment » Convective Inhibition (CIN) Question

2 tephigrams with soundings for CAPE, CIN questions

Which of these two soundings has greater CIN?

Stability Assessment Lifted Index (LI)

Stability Assessment » Lifted Index (LI) Definition

Definition: Lifted Index

The lifted index (LI) is calculated as the difference between the observed temperature at 500 hPa and the temperature of an air parcel lifted to 500 hPa from near the surface. The more unstable the environment, the more negative the LI.

LI values have been empirically linked to convective events as follows:

LI Value Severe Weather Potential
-2 Weak
-3 to -5 Moderate
-6 or less Strong

These threshold values are valid for the eastern 2/3 of the United States. The values must be modified upward (i.e., less negative) for higher elevations such as in western Canada and the U.S. As with CAPE, you should never rely solely on LI to evaluate the convective potential.

Stability Assessment » Lifted Index (LI) Strengths and Limitations

LI is relatively easy to determine with the aid of a tephigram. It is limited because it relies on only 3 sounding inputs: temperature and dewpoint of the boundary layer and the temperature at 500 hPa. Thus, important sounding features may be obscured, such as dry layers and/or inversions. LI also does not take into account vertical wind shear, which is often an important element in the severe convective environment.

Stability Assessment » Lifted Index (LI) Tephigram Procedure

Tephigram procedure to determine lifted index (LI)
  1. Find the mean temperature (T) and dewpoint (Td) in the lowest 100 hPa.
  2. From those mean T and Td, located at the midpoint of the layer, find the LCL.
  3. From the LCL lift the parcel moist-adiabatically to 500 hPa and find the parcel temperature (T′).
  4. Given the 500 hPa sounding temperature (T500), LI is computed as follows:

LI = T500 - T′

Stability Assessment » Lifted Index (LI) Question

Sounding taken at Tucson, AZ, 1200 UTC, 14 Jul 2002, associated with a severe thunderstorm outbreak in southeast Arizona

This sounding comes from the July 14, 2002 severe thunderstorm outbreak in southeast Arizona.

Using the mean dewpoint and temperature in the lowest 100 hPa, determine the lifted index (LI). Enter your answer in the text box below, then click Done.

LI =   

Stability Assessment Showalter Stability Index (SSI)

Stability Assessment » Showalter Stability Index (SSI) Definition

Definition: Showalter Stability Index

The Showalter stability index (SSI) is a popular severe weather index. It is similar to the lifted index (LI), but while the LI starts with the mean of the lowest 100-hPa AGL (above ground level) layer, the SSI uses a parcel lifted from 850 hPa to 500 hPa. At 500 hPa the parcel temperature is subtracted from the sounding temperature. More negative SSI values indicate greater instability.

SSI values have been empirically linked to convective events as follows:

SSI Value Event
+3 to +1 Rain showers,
some thundershowers
+1 to -2 Thundershowers
-3 to -6 Severe thunderstorms
less than -6 Severe thunderstorms,
possible tornadoes

These threshold values are valid for lower elevation localities in the eastern 2/3 of the United States. As with LI or CAPE, you should never rely solely on SSI to evaluate the convective potential.

Stability Assessment » Showalter Stability Index (SSI) Strengths and Limitations

SSI is relatively easy to compute and is often useful to diagnose environmental instability. However, it has several limitations:

  • It may under-represent the instability if the top of the moist layer falls below 850 hPa.
  • It is intended for use at locations with a station elevation up to about 1000 feet.
  • It does not take into account vertical wind shear, which also affects storm potential.

Stability Assessment » Showalter Stability Index (SSI) Tephigram Procedure

Tephigram procedure to determine Showalter Stability Index (SSI)
  1. Find the temperature (T) and dewpoint (Td) at 850 hPa
  2. From that T and Td, find the LCL
  3. From the LCL lift the parcel moist-adiabatically to 500 hPa and find the parcel temperature (T′).
  4. Given the 500 hPa sounding temperature (T500), SSI is computed as follows:

SSI = T500 - T′

Stability Assessment » Showalter Stability Index (SSI) Question

Sounding taken at Fort Worth, TX, 1200 UTC, 06 May 2003, associated with a back-building MCS

This sounding comes from the vicinity of a mesoscale convective system in north Texas on 6 May 2003.

Determine the Showalter stability index (SSI). Enter your answer in the text box below, then click Done.

SSI =   


Stability Assessment K Index (KI)

Stability Assessment » K Index (KI) Definition, Strengths and Limitations

Definition: K Index

The K index (KI) is particularly useful for identifying convective and heavy-rain-producing environments. Its computation takes into account the vertical distribution of both moisture and temperature. It does not require a tephigram; it is simply computed from temperatures at 850, 700, and 500 hPa, and dewpoints at 850 and 700 hPa. The higher the moisture and the greater the 850-500 temperature difference, the higher the KI and potential for convection.

Thunderstorm probability ranges from very low when KI < 20 (KI < 15 west of the Rocky Mountains) to a likelihood of widespread activity when KI > 35 (KI > 30 west of the Rocky Mountains).

Strengths and Limitations

The K index is a useful tool for diagnosing the potential for convection. However, it can't be used to infer the severity of convection. Because it uses 850 hPa data, it is not applicable in the Rocky Mountain region, where the surface pressure is typically less than 850 hPa.

Stability Assessment » K Index (KI) Tephigram Procedure

From the sounding, read the temperature and dewpoint values at 850 and 700 hPa, and the temperature at 500 hPa, and use them to compute the K index as follows:

K index = (T850 - T500) + Td850 - (T700 - Td700)

If the surface pressure < 900 hPa, then use the temperature and dewpoint at 800 hPa instead of 850 hPa.

Stability Assessment » K Index (KI) Question

Tephigram with sounding for K index and Total Totals Index questions

What is the K index for this sounding?
Enter your answer in the text box below, then click Done.

KI =   

Stability Assessment Total Totals Index (TT)

Stability Assessment » Total Totals Index (TT) Definition, Strengths and Limitations

Definition: Total Totals Index

The Total Totals index (TT) is yet another severe weather index. It is computed using the temperature and dewpoint at 850 hPa and the temperature at 500 hPa. The higher the 850 hPa dewpoint and temperature and the lower the 500 hPa temperature, the greater the instability and the resulting TT value.

TT values are empirically related to severe weather likelihood as follows:

TT Event
44 Thunderstorms
50 Severe thunderstorms possible
55 or greater Severe thunderstorms likely;
possible tornadoes

Strengths and Limitations

TT is a widely-used severe weather index that is very easy to compute. However, it is limited in that it uses data from only two mandatory levels (850 and 500 hPa) and thus does not account for intervening inversions or moist or dry layers that may occur below or between these levels. In addition, it does not work for areas in the western Great Plains or the Rocky Mountains, where 850 hPa is near the surface or below ground. Last, like several other severe weather indexes, it does not take into account wind shear, which is a critical factor in many severe convective environments.

Stability Assessment » Total Totals Index (TT) Tephigram Procedure

From a sounding, using the 850 hPa temperature (T850) and dewpoint (Td850) and the 500 hPa temperature (T500), the TT is computed as follows:

TT = (T850 + Td850) - (2 * T500)

Stability Assessment » Total Totals Index (TT) Question

Tephigram with sounding for K index and Total Totals Index questions

What is the Total Totals index for this sounding?
Enter your answer in the text box below, then click Done.

TT =   

Stability Assessment SWEAT Index

Stability Assessment » SWEAT Index Definition

Definition: SWEAT Index

The Severe Weather Threat (SWEAT) index differs from many of the other severe weather indices in that it takes into account the wind profile in assessing severe weather potential. Inputs include:

  • Total Totals index (TT)
  • 850 hPa dewpoint
  • 850 hPa wind speed and direction
  • 500 hPa wind speed and direction

In general, the following conditions lead to a higher SWEAT index and greater probability of severe weather:

  • Higher temperature and moisture at low levels
  • Cooler temperatures aloft
  • Large vertical wind shear
  • Wind direction veering with height

SWEAT index values have been empirically linked to convective events as follows:

SWEAT Severe Weather Potential
150-300 Slight severe
300-400 Severe possible
400 or greater Tornadic possible

Stability Assessment » SWEAT Index Strengths and Limitations

The SWEAT index is advantageous for diagnosing severe convective potential since it takes into account many important parameters including low-level moisture, instability, and vertical wind shear (both speed and direction). However, a limitation is that inputs are only from 850 and 500 hPa levels, obscuring any inversions, dry layers, etc. that may be present in intervening layers. Also, it can be cumbersome to compute without an automated sounding routine such as the Interactive Tephigram.

Stability Assessment » SWEAT Index Tephigram Procedure

From the tephigram, determine the following values:

The SWEAT index is computed as follows:

           SWEAT= 12(850Td) + 20(TT - 49) + 2(V850) + (V500) + 125(sin(dd500 - dd850) + 0.2)

Note the following rules:

  1. If TT is less than 49, then that term of the equation is set to zero.
  2. If any term is negative, then that term is set to zero.
  3. Winds must be veering with height or that term is set to zero.

Stability Assessment » SWEAT Index Question

Tephigram with sounding for SWEAT index question

What is the SWEAT index for this sounding?
Enter your answer in the text box below, then click Done.

SWEAT =   

Shear Assessment

Shear Assessment Bulk Richardson Number (BRN)

Shear Assessment » Bulk Richardson Number (BRN) Definition

Definition: Bulk Richardson Number (BRN)

The Bulk Richardson Number (BRN) is the ratio of the buoyancy (as measured by the CAPE) to the vertical wind shear of the environment. As we have noted previously, updraft strength is directly related to CAPE, while the storm structure (e.g., multi-cell, supercell, etc.) and movement are related to the vertical shear.

Bulk Richardson Number values of observed storms

This graphic shows BRN values related to storm type. Generally, if the BRN is less than 10, there is much more shear than buoyancy, and the storms tend to be torn apart by the shear. The exception is in strongly forced, high-shear, low-CAPE environments where supercells are observed with BRN values less than 10. With BRN between 10 and 35, the balance between shear and buoyancy tends to favor supercells. With BRN greater than 50, buoyancy dominates over shear and single- or multi-cell storms are more likely to be observed.

Shear Assessment » Bulk Richardson Number (BRN) Tephigram Procedure

The Bulk Richardson Number is calculated as follows:

BRN = CAPE / (0.5 * (u6km - u500m)2)

Where
u6km is the mean wind speed in the lowest 6000 m and
u500m is the mean wind speed in the lowest 500 m.

The BRN is difficult to calculate manually, but is typically calculated and displayed on computer-generated tephigrams. On the Interactive Tephigram, it is displayed on the right-hand side of the diagram once you click on the "CAPE" menu item at the top.

Shear Assessment Helicity

Shear Assessment » Helicity Definition

Definition: Storm Relative Environmental Helicity (SREH)

Storm relative environmental helicity (SREH) provides an indication of an environment that favors the development of thunderstorms with rotating updrafts. High values of SREH (usually >150 m2/sec2) are usually associated with long-lived supercells with rotating updrafts, capable of producing tornadoes.

More technically, SREH is a measure of the streamwise vorticity within the inflow environment of a convective storm. What do we mean by that? Let's look at each piece of the definition.

Shear Assessment » Helicity Vorticity

Horizontal vorticity often results from vertical wind shear. A wind profile that maintains a single direction and increases its speed with height generates a shear vector parallel to the wind direction. This shear results in a horizontal vorticity whose axis (the vorticity vector) is perpendicular to the wind direction. We refer to this as crosswise vorticity.

3-D conceptual picture depicting unidirectional wind shear and resulting crosswise vorticity

On the other hand, a wind profile whose speed remains constant, but whose direction changes with height generates a shear vector perpendicular to the mean wind. The resulting vorticity vector is parallel to the mean wind. We refer to this as streamwise vorticity.

In the real world, vorticity is rarely perfectly crosswise or streamwise. Thus, when we say streamwise vorticity we refer to the vector component of the vorticity that is oriented parallel to the mean flow.

3-D conceptual picture depicting directional wind shear and resulting streamwise vorticity

Shear Assessment » Helicity Storm-Relative Wind

Hodograph with arrows for observed wind, storm motion, and storm-relative wind at 3 km AGL

When we compute helicity, it is most appropriate to use storm-relative winds. To find the storm-relative wind, we subtract the anticipated or observed storm speed and direction from the wind at every level of the sounding. This process requires a hodograph analysis of the wind profile to predict the storm motion.

Several methods have been proposed to determine supercell storm motion. Currently the most accepted approach is the ID (Internal Dynamics) method. Generally, storms in an environment with a clockwise-curving hodograph will move to the right of the 0-6 km mean wind, while storms in an environment with a counterclockwise curving hodograph will move to the left.

For more information on storm motion, the ID method, and supercell motion see this COMET module:

Shear Assessment » Helicity SREH Computation

Hodograph with arrows and shading to show the area storm-relative environmental helicity (SREH)

Automated sounding routines such as the Interactive Tephigram compute SREH from the hodograph. On the hodograph, SREH is proportional to the area swept out by the storm relative wind vector over the depth of the inflow, typically 3 km AGL, as depicted in this figure. SREH values are positive for right-moving storms, characterized by clockwise-curving hodographs (as shown here) and cyclonic rotation, while SREH values are negative for left-moving, anticyclonic-rotating storms with counterclockwise-curving hodographs.

Shear Assessment » Helicity Helicity, Vorticity, and Supercells

Animated image depicting how flow with high helicity encounters a convective updraft; the vorticity tilts up toward vertical vorticity

How do streamwise horizontal vorticity and helicity relate to thunderstorm rotation and possible tornadoes? Well, a significant streamwise component of vorticity aligned with the inflow leads to higher helicity value. When flow with high helicity encounters a convective updraft, the vorticity tilts up toward vertical, as this animation illustrates. Thus, high helicity leads to updraft rotation within a supercell. Rotating supercells are sometimes associated with the occurrence of tornadoes.

Shear Assessment » Helicity Strengths and Limitations

SREH is perhaps the parameter most widely used to provide a good diagnosis for the potential for tornado-producing supercells. Like CAPE values, there is no magic value of (positive) helicity over which rotating thunderstorms will develop. Furthermore, the calculation of SREH is quite sensitive to assumptions about storm motion and the environmental wind shear. SREH, like other parameters, must be used with caution, especially with rapidly changing environmental conditions.