This false-color image over Australia,
produced using NOAA Advanced Very High Resolution Radiometer (AVHRR) data, shows where pollution from human
industry reduced clouds' particle sizes. Polluted clouds may rain less frequently then unpolluted clouds
because the pollutants prevent water droplets from growing large enough to precipitate.
Blue areas are cloudless, while purplish-red areas
are covered by thick clouds comprised of large droplets. The
yellowish-green and orange streaks are clouds comprised of small
droplets. These latter clouds are more polluted than the purplish-red
clouds and literally pointing to their sources of pollution.
Daniel Rosenfeld, a scientist collaborating with NASA, colored the visible, midwave-infrared, and thermal infrared
AVHRR data as red, green, and blue, respectively, in this image to
differentiate clouds with different properties in a three-dimensional
way. Red was used to indicate the reflectiveness of the cloud in
visible wavelengths. Green corresponds to droplet size. The more green
there is in an area (as determined by AVHRR measurements of energy
reflected at the 3.7 micrometer wavelength) the smaller the droplets. Blue was used to represent
the clouds' temperature. The deeper the blue, the warmer the
temperature (determined by AVHRR measurements of brightness temperature
at 10.8 micrometer wavelength).