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).
For more about interaction between pollution, clouds, and precipitation, visit Changing our Weather One Smokestack at a Time.
Image by Daniel Rosenfeld, Hebrew University of Israel