2. Parameters
Table of Contents
 Introduction
 Moisture/Humidity
 Temperature/Levels
 Virtual Temperature (T_{v})
 Potential Temperature (Theta)
 Lifting Condensation Level (LCL)
 Equivalent Temperature (T_{e})
 Equivalent Potential Temperature (Thetae)
 WetBulb Temperature (T_{w})
 WetBulb Potential Temperature (Thetaw)
 Convective Condensation Level (CCL)
 Convective Temperature (T_{c})
 Thickness
 Other Levels
 Stability Assessment
 Shear Assessment
Introduction
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 (w_{s})
Moisture/Humidity » Saturation Mixing Ratio (w_{s}) Definition
Definition: Saturation Mixing Ratio
The saturation mixing ratio (w_{s}) is the ratio of the mass of water vapor (M_{v}) to the mass of dry air (M_{d}) in a parcel of air at saturation. In other words w_{s} is the maximum amount of water vapor that a parcel can hold without condensation.
w_{s} = M_{v} / M_{d}
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 (w_{s}) Tephigram Procedure
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 mixingratio line that crosses the T curve at that pressure.
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 mixingratio 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 (w_{s}) Question
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 (M_{v}) to the mass of dry air (M_{d}):
w = M_{v} / M_{d}
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 = M_{v} / (M_{v} + M_{d})
However, for synoptic forecasting purposes, the mixing ratio is sufficiently representative and is easier to evaluate.
Moisture/Humidity » Mixing Ratio (w) Tephigram Procedure
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 mixingratio line that crosses the T_{d} curve at that pressure.
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 mixingratio line that crosses the T_{d} 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
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
The relative humidity can be computed from the mixing ratio (w) and the saturation mixing ratio (w_{s}) by the following equation:
RH = 100 * (w/w_{s})
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
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
Moisture/Humidity Saturation Vapor Pressure (e_{s})
Moisture/Humidity » Saturation Vapor Pressure (e_{s}) Definition
Definition: Saturation Vapor Pressure
The saturation vapor pressure (e_{s}) 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 (e_{s}) Tephigram Procedure
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.
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 (e_{s}) Question
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
From the dewpoint (T_{d}) 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.
From the dewpoint (T_{d}) 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
Temperature/Levels
Temperature/Levels Virtual Temperature (T_{v})
Temperature/Levels » Virtual Temperature (T_{v}) Definition
Definition: Virtual Temperature
The virtual temperature (T_{v}) 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.
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 (T_{v}) Tephigram Procedure
At a given pressure level, do the following:
 Determine the mixing ratio (w, in g/kg), which is the value of the saturation mixing ratio line passing through the dewpoint (T_{d}), at a given pressure.
 Determine the mixing ratio (w, in g/kg), which is the value of the saturation mixing ratio line passing through the dewpoint (T_{d}), at a given pressure.
 The virtual temperature is then computed as follows:
T_{v} ~ T + (w / 6)
In this example:
T = 5°C and T_{d} = 6°C.
Thus, w = 2.9 g/kg.
T_{v} = T + w/6
= 5 + 2.9/6
= 5.5°C
Temperature/Levels » Virtual Temperature (T_{v}) Question
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 dryadiabatically 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
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
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 dryadiabatically.
Temperature/Levels » Lifting Condensation Level (LCL) Tephigram Procedure
The LCL is located on a sounding at the intersection of the saturation mixingratio 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 T_{d}=0°C will become saturated if lifted dryadiabatically to 870 hPa, which is the lifting condensation level.
The LCL is located on a sounding at the intersection of the saturation mixingratio 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 T_{d}=0°C will become saturated if lifted dryadiabatically to 870 hPa, which is the lifting condensation level.
Note: When the moisture content in the nearsurface layers varies significantly, an average moisture value of the lower layer may be used in place of the surfaceparcel moisture value to compute the LCL.
Temperature/Levels » Lifting Condensation Level (LCL) Question
Temperature/Levels Equivalent Temperature (T_{e})
Temperature/Levels » Equivalent Temperature (T_{e}) Definition
Definition: Equivalent Temperature
The equivalent temperature (T_{e}) is the temperature a sample of air at a pressure level would have if all its moisture were condensed out by a pseudoadiabatic 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 (T_{e}) Tephigram Procedure
 From the dewpoint at the given pressure, draw a line upward parallel to the saturation mixingratio 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.
 From the dewpoint at the given pressure, draw a line upward parallel to the saturation mixingratio 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.
 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.
 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.
 From this pressure, follow a dry adiabat back to the original pressure. The isotherm value at this point is equal to the equivalent temperature (T_{e}).
 From this pressure, follow a dry adiabat back to the original pressure. The isotherm value at this point is equal to the equivalent temperature (T_{e}).
In this example, air at 850 hPa with T = 10°C and T_{d} = 8°C has an equivalent temperature of 17°C.
Temperature/Levels » Equivalent Temperature (T_{e}) Question
Temperature/Levels Equivalent Potential Temperature (Thetae)
Temperature/Levels » Equivalent Potential Temperature (Thetae) Definition
Definition: Equivalent Potential Temperature
The equivalent potential temperature (thetae) is the temperature a sample of air would have if all its moisture were condensed out by a pseudoadiabatic process (i.e., with the latent heat of condensation being used to heat the air sample), and the sample then brought dryadiabatically back to 1000 hPa.
The equivalent potential temperature is identical to the equivalent temperature, except the sample is brought dryadiabatically 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 (Thetae) Tephigram Procedure
Tephigram Procedure:
 From the dewpoint at the given pressure, draw a line upward along a saturation mixingratio 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.
 From the dewpoint at the given pressure, draw a line upward along a saturation mixingratio 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.
 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.
 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.
 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 (thetae).
In this example, air at 850 hPa with T = 10°C and T_{d} = 8°C has an equivalent potential temperature just above 30°C.
Temperature/Levels » Equivalent Potential Temperature (Thetae) Question
Temperature/Levels WetBulb Temperature (T_{w})
Temperature/Levels » WetBulb Temperature (T_{w}) Definition
Definition: WetBulb Temperature
The wetbulb temperature (T_{w}) 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 wetbulb temperature is reached when the air parcel achieves saturation.
The wetbulb temperature will always fall between the dewpoint and the temperature, unless the air is saturated. At saturation, the temperature, dewpoint, and wetbulb temperature are equal.
In the real atmosphere T_{w} often provides a good estimate of what the surface temperature will become after the onset of precipitation and once conditions become saturated.
Temperature/Levels » WetBulb Temperature (T_{w}) Tephigram Procedure
At a given pressure level, do the following:
 From the temperature, proceed up along a dry adiabat.
 From the temperature, proceed up along a dry adiabat.
 From the dewpoint proceed up along a mixing ratio line.
 From the dewpoint proceed up along a mixing ratio line.
 From where the two lines intersect, proceed down the saturation adiabat to the original level.
 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 T_{d} = 0°C has a wetbulb 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 » WetBulb Temperature (T_{w}) Question
Temperature/Levels WetBulb Potential Temperature (Thetaw)
Temperature/Levels » WetBulb Potential Temperature (Thetaw) Definition
Definition: WetBulb Potential Temperature
The wetbulb potential temperature (thetaw) is the wetbulb temperature a sample of air would have if it were brought along a saturation adiabat to a pressure of 1000 hPa.
The wetbulb potential temperature (thetaw) is the wetbulb temperature a sample of air would have if it were brought along a saturation adiabat to a pressure of 1000 hPa.
The wetbulb potential temperature is identical to the wetbulb temperature, except the sample is brought moistadiabatically from the wetbulb temperature at the initial level to the wetbulb potential temperature at the 1000 hPa level.
Wetbulb 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 » WetBulb Potential Temperature (Thetaw) Tephigram Procedure
At a given pressure level, do the following:
 Find the wetbulb temperature
 Find the wetbulb temperature
 From the wetbulb temperature, follow the saturation adiabat to the 1000 hPa isobar.
 From the wetbulb temperature, follow the saturation adiabat to the 1000 hPa isobar.
 The isotherm value at this intersection equals the wetbulb potential temperature at the given pressure.
 The isotherm value at this intersection equals the wetbulb potential temperature at the given pressure.
In this example, air at 700 hPa with T = 5°C and T_{d} = 13°C has a wetbulb temperature of 7°C and a wetbulb potential temperature of 10°C.
Temperature/Levels » WetBulb Potential Temperature (Thetaw) Question
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
To determine the CCL on a sounding, start at the surface dewpoint, proceed upward along the saturation mixingratio line until this line intersects the temperature profile on the sounding. The level of the intersection is the CCL.
To determine the CCL on a sounding, start at the surface dewpoint, proceed upward along the saturation mixingratio 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 nearsurface layers varies significantly, an average moisture value of the lower layer may be used in place of the surfaceparcel moisture value in computing the CCL.
Temperature/Levels » Convective Condensation Level (CCL) Question
Temperature/Levels Convective Temperature (T_{c})
Temperature/Levels » Convective Temperature (T_{c}) Definition
Definition: Convective Temperature
The convective temperature (T_{c}) is the surface temperature that must be reached to start the formation of convective clouds caused by solar heating of the nearsurface layer.
Temperature/Levels » Convective Temperature (T_{c}) Tephigram Procedure
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 (T_{c}).
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 (T_{c}).
In this example, the CCL lies at 750 hPa and the convective temperature is 20°C.
Temperature/Levels » Convective Temperature (T_{c}) Question
Temperature/Levels Thickness
Temperature/Levels » Thickness Definition
Definition: Thickness
Thickness refers to the vertical distance between two constant pressure surfaces (isobars).
Thickness refers to the vertical distance between two constant pressure surfaces (isobars).
The thickness of a layer is related to the mean virtual temperature (T_{v}) according to the formula:
Where:
 ΔZ = thickness (m)
 R_{d} = gas constant for dry air
 = 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 (T_{v}) in the layer. The 1000500 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 60meter contours. In this case, each contour reflects a mean virtual temperature change of about 3°C.
Note for a given pair of pressures, the thickness is proportional to the mean virtual temperature (T_{v}) in the layer. The 1000500 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 60meter 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 1000500 hPa layer is calculated and shown on the righthand 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 1000500 thickness of 5400 m (540 dm) or a 1000850 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
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
Other Levels WetBulb Zero Level
Other Levels » WetBulb Zero Level Definition & Tephigram Procedure
Definition: WetBulb Zero Level
The wetbulb zero level is the the lowest level in a sounding at which the wetbulb temperature is 0°C.
The wetbulb zero level is the the lowest level in a sounding at which the wetbulb temperature is 0°C.
During the onset of a coolseason precipitation event, the higher the initial wetbulb zero level, the less chance of the precipitation changing to freezing/frozen precipitation at the surface. Also, during convective season, lower wetbulb 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 wetbulb temperature profile. That level is the wetbulb zero level.
Other Levels » WetBulb Zero Level Question
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 dryadiabatically until saturated (at the LCL) and then moistadiabatically thereafter.
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 dryadiabatically until saturated (at the LCL) and then moistadiabatically thereafter.
Other Levels » Level of Free Convection (LFC) Tephigram Procedure
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.
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 T_{d}=0°C, resulting in an LCL of 870 hPa and an LFC of 675 hPa.
Other Levels » Level of Free Convection (LFC) Question
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
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.
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 equalarea method as shown on the tephigram.
The mean mixing ratio is determined from the sounding T_{d} curve by the equalarea method also as shown on the tephigram.
The MCL lies at the pressure level specified by the intersection of the mean saturation mixingratio 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
Other Levels » Mixing Condensation Level (MCL) Question 2
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
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
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).
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
From the CCL, proceed upward along a saturation adiabat until intersecting the temperature profile. The pressure at this intersection is the equilibrium level (EL).
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
Other Levels Maximum Parcel Level (MPL)
Other Levels » Maximum Parcel Level (MPL) Definition
Definition: 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.
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
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 computergenerated tephigrams, the MPL is usually computed automatically.
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 computergenerated tephigrams, the MPL is usually computed automatically.
Other Levels » Maximum Parcel Level (MPL) Question
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).
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).
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 
01000  Marginally Unstable 
10002500  Moderately Unstable 
25003500  Very Unstable 
3500 or greater  Extremely Unstable 
CAPE may also be related to updraft velocity via the relation:
W_{max} (in m/s) = sqrt(2 * CAPE)
Hence, for a CAPE of 2500 J/kg, the maximum updraft velocity, W_{max}, would be about 71 m/s! In reality, water loading, entrainment, and other factors can reduce W_{max} 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.
 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 lowlevel 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
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).
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
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
 Find the mean temperature (T) and dewpoint (T_{d}) in the lowest 100 hPa.
 From those mean T and T_{d}, located at the midpoint of the layer, find the LCL.
 From those mean T and T_{d}, located at the midpoint of the layer, find the LCL.
 From the LCL lift the parcel moistadiabatically to 500 hPa and find the parcel temperature (T′).
 From the LCL lift the parcel moistadiabatically to 500 hPa and find the parcel temperature (T′).
 Given the 500 hPa sounding temperature (T500), LI is computed as follows:
LI = T500  T′
Stability Assessment » Lifted Index (LI) Question
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 100hPa 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.
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 100hPa 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 underrepresent 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
 Find the temperature (T) and dewpoint (T_{d}) at 850 hPa
 From that T and T_{d}, find the LCL
 From that T and T_{d}, find the LCL
 From the LCL lift the parcel moistadiabatically to 500 hPa and find the parcel temperature (T′).
 From the LCL lift the parcel moistadiabatically to 500 hPa and find the parcel temperature (T′).
 Given the 500 hPa sounding temperature (T500), SSI is computed as follows:
SSI = T500  T′
Stability Assessment » Showalter Stability Index (SSI) Question
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 heavyrainproducing 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 850500 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) + T_{d}850  (T700  T_{d}700)
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
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 widelyused 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 (T_{d}850) and the 500 hPa temperature (T500), the TT is computed as follows:
TT = (T850 + T_{d}850)  (2 * T500)
Stability Assessment » Total Totals Index (TT) Question
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 
150300  Slight severe 
300400  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 lowlevel 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:
 Total Totals index (TT)
 Total Totals index (TT)
 Dewpoint (°C) at 850 hPa (T_{d}850)
 Wind speed (kt) at 850 hPa (V850)
 Wind direction (°) at 850 hPa (dd850)
 Wind speed (kt) at 500 hPa (V500)
 Wind direction (°) at 500 hPa (dd500)
The SWEAT index is computed as follows:
SWEAT= 12(850T_{d}) + 20(TT  49) + 2(V850) + (V500) + 125(sin(dd500  dd850) + 0.2)
Note the following rules:
 If TT is less than 49, then that term of the equation is set to zero.
 If any term is negative, then that term is set to zero.
 Winds must be veering with height or that term is set to zero.
Stability Assessment » SWEAT Index Question
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., multicell, supercell, etc.) and movement are related to the vertical shear.
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., multicell, supercell, etc.) and movement are related to the vertical shear.
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, highshear, lowCAPE 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 multicell 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 * (u_{6km}  u_{500m})^{2})
Where
u_{6km} is the mean wind speed in the lowest 6000 m and
u_{500m} is the mean wind speed in the lowest 500 m.
The BRN is difficult to calculate manually, but is typically calculated and displayed on computergenerated tephigrams. On the Interactive Tephigram, it is displayed on the righthand 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 m^{2}/sec^{2}) are usually associated with longlived 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. 

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. 
Shear Assessment » Helicity StormRelative Wind
When we compute helicity, it is most appropriate to use stormrelative winds. To find the stormrelative 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 clockwisecurving hodograph will move to the right of the 06 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
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 rightmoving storms, characterized by clockwisecurving hodographs (as shown here) and cyclonic rotation, while SREH values are negative for leftmoving, anticyclonicrotating storms with counterclockwisecurving hodographs.
Shear Assessment » Helicity Helicity, Vorticity, and Supercells
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 tornadoproducing 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.