High-Resolution Image

In addition to being intimately related to the daily weather systems we experience, clouds are recognized as critical components of the EarthÕs climate system. Two of the more basic questions related to the global distribution of clouds are (1) how often do they occur (that is, what is the frequency of occurrence for any region on Earth), and (2) how much condensed water do they contain?

The Moderate Resolution Imaging Spectroradiometer (MODIS) instrument, flying onboard the NASA Terra and Aqua spacecraft, retrieves a number of cloud parameters on an operational basis. An image of the retrieved cloud fraction and water path for liquid clouds (i.e., excluding ice cloud observations) from the MODIS Aqua instrument during July 2004 is shown. The "cloud fraction" is the fraction of MODIS pixels that are determined to contain clouds, determined using several wavelengths that are sensitive to cloudiness. The quantity Òwater pathÓ represents the mass of cloud water in a square meter column of the atmosphere. A global average for the month shown was determined to be about 100 gm-2; this is equivalent to a layer of liquid water that is only 0.1 mm thick. A relatively small amount of condensed water can have profound impacts on the EarthÕs weather and climate when distributed among small cloud droplets (on the order of 10 µm in size).

The global images represent an average of the two quantities over equal-angle 1¡ grids, and were generated from publicly available data sets available at the NASA Goddard Space flight Center Distributed Active Archive Data (DAAC) center.

(submitted by Steven Platnick)