This image shows atmospheric water vapor in Earth's upper
troposphere, about 10 kilometers (6 miles) above the surface, as
measured by the Microwave Limb Sounder (MLS) instrument flying aboard
the Upper Atmosphere Research Satellite. These data collected in early
October 1997 indicate the presence of El Nino by showing a shift of
humidity from west to east (blue and red areas) along the equatorial
Pacific Ocean. El Nino is the term used when the warmest equatorial
Pacific Ocean water is displaced toward the east. The areas of high
atmospheric moisture correspond to areas of very warm ocean water.
Warmer water evaporates at a higher rate and the resulting warm moist
air then rises, forming tall cloud towers. In the tropics, the warm
water and the resulting tall cloud towers typically produce large
amounts of rain. The MLS instrument, developed at NASA's Jet Propulsion
Laboratory, measures humidity at the top of these clouds, which are very
moist. This rain is now occurring in the eastern Pacific Ocean and has
left Indonesia (deep blue region) unusually dry, resulting in the
current drought in that region. This image also shows moisture moving
north into Mexico, an effect of several hurricanes spawned by the warm
waters of El Nino.