This image represents the total precipitable water vapor for May, 2009 as
observed by AIRS, the Atmospheric Infrared Sounder (AIRS) on NASA's
Aqua satellite. It shows the total amount of water vapor present in the
atmospheric column above each point of the Earth's surface. If all the
water vapor in the column were forced to fall as rain, the depth of the
resulting puddle on the surface at that point is equal to the value shown
on the map. Fifty millimeters is about 2 inches. The water vapor measured
above the Antarctic is not shown (black color), since the elevation there
is generally very high and the water vapor content extremely low—so
low that the color scale would have to be “stretched” to show any details, which
would reduce the details for the rest of the globe.
The large area of maximum water vapor in the neighborhood of the equator
is the Intertropical Convergence Zone or ITCZ, a region of strong
convection and powerful thunderstorms. It is particularly intense in a
region around the Philippines and Indonesia called the warm pool. The
air’s ability to contain moisture Is related to its temperature—the
higher the air temperature, the more moisture can be contained before it
must condense and fall out as rain. This image confirms that, as one would
expect, the greatest amounts of moisture occur in the tropics. At
extremely cold temperatures the atmosphere can hold very little water
vapor, which is reflected by the strong correspondence between light brown
dry areas of this map and cold areas in temperature maps. Very low
precipitable water vapor occurs because the air is dry, such as over
deserts. But because of the temperature relationship, most of the “column
water vapor” is contained in the lowest portion of the atmosphere, and
when a high mountain makes it impossible to measure down to sea level, the
total is much smaller. Mountain regions therefore appear to be abnormally
dry. That is sometimes true but not always - sometimes it just reflects
the fact that the “measurement column” has been cut off by the mountain.
The AIRS Public Web site can be found at http://airs.jpl.nasa.gov.
AIRS, in conjunction with the Advanced Microwave Sounding Unit (AMSU) also
on Aqua, sense emitted infrared and microwave heat radiation from Earth to
provide a three-dimensional look at Earth's weather and climate. Working
in tandem, the two instruments can make simultaneous observations all the
way down to Earth's surface, even in the presence of heavy clouds. With
more than 2,000 channels sensing different regions of the atmosphere, the
system creates a global, 3-D map of atmospheric temperature and humidity,
cloud amounts and heights, greenhouse gas concentrations and many other
atmospheric phenomena. AIRS and AMSU are managed by NASA's Jet Propulsion
Laboratory, Pasadena, Calif., under contract to NASA's Science Mission
Directorate, Washington. JPL is a division of the California Institute of
Technology in Pasadena.