Where Storm Clouds Gather
Rain clouds form when moisture-laden air is driven skyward by warm updrafts emitted from a sun-warmed land or ocean surface; or when mountain slopes push moist air aloft; or when a wedge of colder, denser air plows warmer, moist air upward to higher elevations. Because cold air cannot hold as much water vapor as warm air, and because the atmosphere cools at higher elevations, water vapor condenses readily into liquid droplet or ice crystal form, in the presence of seed aerosol particles. Were there no aerosol particles in the Earth’s atmosphere, there would be no fog, no clouds, no mist, and probably no rain. When water evaporates at the surface, it absorbs energy from its surroundings and stores it as “latent heat.” When water vapor condenses back into liquid or ice form it releases its latent heat into its surroundings. Only about 25 percent of the energy contained within the atmosphere comes directly from the sun’s rays; the remaining 75 percent comes from the release of latent heat contained in water vapor. Most atmospheric water vapor originates from the tropical oceans, where it rises high into the atmosphere to form towering thunderheads. Encircling the globe along the equator is an almost continuous band of thunderheads known as the Intertropical Convergence Zone (ITCZ), producing roughly three quarters of the energy in our atmosphere that helps to drive its circulation patterns.

The above image shows the orbit of the Tropical Rainfall Measuring Mission (TRMM). See the corresponding animation for more information. (Image courtesy of the Scientific Visualization Studio, NASA GSFC)

We cannot measure the latent heat contained within clouds. We can, however, measure tropical rainfall. Currently, there is a 50 percent uncertainty in estimates of annual global rainfall. If we are to more accurately determine how much energy our atmosphere receives from latent heat, then we must more accurately measure rainfall. In 1997, NASDA and NASA jointly developed and launched the Tropical Rainfall Measuring Mission (TRMM) into a mid-inclination (35°) precessing orbit. Scientists estimate about 60 percent of our world’s precipitation falls within the band spanning ± 30° north and south of the equator. TRMM carries three instruments designed to measure rainfall—the Precipitation Radar (PR), the TRMM Microwave Imager (TMI), and the Visible and Infrared Scanner (VIRS). Designed and built by NASDA, the Precipitation Radar is the first satellite sensor to provide three-dimensional images of the internal structures of storm clouds. Its measurements show scientists the intensity and distribution of rain within a storm, the total height of a storm, and the elevation at which ice crystals melt into raindrops. Most importantly, the Precipitation Radar can measure rain rates as accurately as 0.7 mm per hour. While scientists expected to use ground-based Doppler Radar stations to validate TRMM’s Precipitation Radar measurements, much to their pleasant surprise they found that the latter exceeds most ground-based measurements in accuracy and spatial resolution.

Hurricane Bonnie as observed by the TRMM/Precipitation Radar on August 22, 1998. Red shows intense precipitation, green and yellow hues are intermediate values, and blues are low values. The eye of the storm reached to 16 km.

The TMI is a “passive” sensor designed to measure minute amounts of microwave energy emitted by the Earth’s surface and from within its atmosphere. (Whereas “active” sensors send pulses of energy and then measure how much gets absorbed and reflected by the target, “passive” sensors measure only energy originating from, or reflected by external sources.) These measurements allow TMI to quantify the amount of water vapor, cloud water, and rainfall intensity within the atmosphere. Based upon the design heritage of the Defense Meteorological Satellite Program’s Special Sensor Microwave/Imager (SSM/I), the TMI has a wider viewing swath (780 km) and finer spectral resolution than its predecessors. The TRMM VIRS detects radiant energy in five spectral bands, ranging from visible to infrared wavelengths (from 0.63 to 12 microns). Ideally designed to measure temperature, VIRS can precisely determine cloud top temperatures that scientists can then indirectly correlate with rainfall amounts.

next: Conclusion
back: The Chemistry of Earth’s Atmosphere

Remote Sensing
Introduction
Balancing Earth’s Radiant Energy Budget
Dust in the Wind
Abstract Art or Arbiters of Energy?
Serendipity and Stratospheric Ozone
The Chemistry of Earth’s Atmosphere
Where Storm Clouds Gather
Conclusion

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Aerosols and Climate Change
Clouds and Radiation
Why isn’t Earth Hot as an Oven?

Related Datasets
TOMS Ozone
Precipitation
Cloud Fraction