Using RGBs in Different Situations

So far, we have seen three ways of viewing a scene that contains dust:

  • Single satellite images, such as longwave infrared images, where detecting dust depends on the thermal contrast between dust and the surface background
  • Channel differences (or BTDs, Brightness Channel Differences), which enhance the depiction of dust plumes
  • An RGB that combines the inputs from the first two options into something that is easy to interpret

The real test of an RGB is whether it can perform in varied conditions. In an operational setting, RGBs are often ‘tuned’ to account for seasonal and geographical differences, as well as different satellite viewing geometries, such as high versus low latitude views from a geostationary satellite.

Click each tab and see how well the dust product does.

Two RGBs, one dust, the other natural, depicting the same scene

These natural color and dust RGBs are from the next day. Which product does a better job of depicting dust over water and land? (Use the selection box to choose the answer that best completes the statement.)

a) Over water, the RGB better depicts dust.
The correct answer is natural color.
b) Over land, the RGB better depicts dust.
The correct answer is dust.

Although it is difficult to see dust over land in the natural color product, it shows up vividly against the dark water background. In contrast, the dust RGB is better at depicting dust over land. Notice the dust streaks that are not apparent in the natural color product. The thing to remember is that no one RGB accomplishes its purpose all the time. You need to know when one is suitable and when to seek out an alternative product.

This dust RGB animation shows how the dust storm evolves over four days and nights. You can tell when the sun is up since the heated land appears in warmer, bluish colors. Pinks and yellows predominate during the night and then fade to a bluish color typical of land during the day. Thick, high clouds are dark red while thin, high clouds are dark blue to black in appearance. Low cloud features, commonly water clouds, are shades of orange. Dust appears as magenta.

Dust RGB animation showing the evolution of a dust storm over several days over Saudi Arabia

Notice the strong system pushing through the Persian Gulf midday through the period followed by a major dust outbreak. At what time does the dust outbreak reach the southern shore of the Saudi Peninsula? (Choose the best answer.)

The correct answer is B.

Notice how the dust pushes offshore after it reaches the coast.

Dust RGB animation showing the evolution of a dust storm over several days over Saudi Arabia

This dust RGB animation occurs over the Atlantic Ocean for nearly one week. Which of the following are evident? (Choose all that apply.)

All three choices are correct.

Several dust plumes have arisen from specific source regions over Africa. This dust moves over the ocean and eventually reaches the Americas.

Dust is usually found at middle and low levels of the atmosphere and is often obscured by higher clouds on satellite products.

Note that this dust RGB can be useful for tropical cyclone forecasting over the Atlantic Ocean. This is because dust and the dry air that contains it tend to dampen storm strength.

Dust RGB over northern Africa overlaid with 1000-hpa winds and contours of divergence

RGBs do not by themselves provide quantitative information. But we can get this kind of information by overlaying derived satellite products or model data for example.

In this example, the RGB provides the location of the dust front while the model overlays provide information about the winds associated with that front and the airmass behind it.