October 27, 1999

Forest Fires, Smoke, and Rain in the Tropics

Composite image of a section across Kalimantan (Borneo), Indonesia, and Sarawak, Malaysia, showing areas of water clouds (white and gray pattern), smoke from forest fires (diffuse gray pattern to the right), forest fires (orange), cloud droplet size (green), and rainfall (blue). This composite image and other data, obtained by the Tropical Rainfall Measuring Mission (TRMM), demonstrate that in smoke-infected clouds rainfall is inhibited, while in adjacent areas with cleaner air the moisture content of the clouds precipitates as rain.

For the individual images of this composite, click here.

Image Credit: Daniel Rosenfeld, Institute of Earth Sciences, Hebrew University of Jerusalem; Tropical Rainfall Measuring Mission (TRMM); and the Scientific Visualization Studio, NASA Goddard Space Flight Center.

The individual images of the composite shown above and other satellite data demonstrate that heavy smoke from forest fires inhibits rainfall. Although known before, the full extent of this inhibiting effect was not determined until the TRMM spacecraft returned contemporaneous data of water clouds, cloud droplets suspended in the atmosphere, rainfall, and smoke from biomass burning over selected areas in the tropics.

What is happening is this. Burning vegetation emits large numbers of very tiny smoke particles on which the water content of the clouds condenses. Because the number of these smoke particles (called "cloud condensation nuclei") is very large, the water content of the clouds becomes distributed over a great many very small water droplets (with radii smaller than 14 micrometers) that remain suspended in the air; and their coalescence into raindrops large enough to fall to the ground becomes greatly reduced. Thus, precipitation is essentially shut off.

For precipitation in smoke-infected clouds to occur, the cloud tops have to rise to above about 16,000 ft. where they cool to temperatures of less than 0° C (32° Fahrenheit). Only at such low, subfreezing temperatures do ice particles form, which then collect efficiently the small cloud droplets that remain liquid at these low temperatures. As the ice particles grow in size and become heavier, they fall to lower and warmer heights and turn into raindrops. In contrast, in clouds containing clean air most of the water content precipitates at much lower heights and above freezing temperatures.

Studies such as these, based mainly on satellite data of the tropics and subtropics, are not just of interest to meteorologists making local weather forecasts. The reason is as follows. Only about one-fourth of the sunlight striking the Earth goes into local heating. The remainder, namely three-fourths, goes into evaporating water from ocean and land surfaces and gets stored as energy in the water content (i.e., separate water molecules) of the Earth's atmosphere. This so-called "latent heat" is released when the water molecules coalesce to form rain or snow and precipitate to the Earth's surface. The warming that results from this energy release is the major driver of the Earth's global weather systems. Thus, learning about the causes as well as the suppression of precipitation is important to understanding global weather patterns, carrying out long-term climate studies, and predicting the extent to which global warming will affect the future of our planet.

More Cool Stuff

This Observation of the Week is based on NASA press release 99-110 of October 5, 1999, and a scientific paper by Daniel Rosenfeld (Institute of Earth Sciences, Hebrew University of Jerusalem) published in the Geophysical Research Letters of October 15, 1999 (Vol. 26, No. 20):

ftp://ftp.hq.nasa.gov/pub/pao/pressrel/1999/99-110.txt
http://www.agu.org/pubs/toc/gl/gl_26_20.html

The Tropical Rainfall Measuring Mission (TRMM) is a joint effort between NASA and the National Space Development Agency (NASDA) of Japan. The mission's goal is to monitor and study tropical rainfall and the associated release of energy that helps to power the global atmospheric circulation, weather, and climate around the globe. For more information about this mission, science results, and images, go to the TRMM homepage:

http://trmm.gsfc.nasa.gov/

We have featured results from the Tropical Rainfall Measuring Mission on our site before:

Measuring Global Rainfall (July 1, 1998):

http://observe.ivv.nasa.gov/nasa/ootw/1998/ootw_980701/ob980701.html

The Fury of Hurricanes (May 26, 1999):

http://observe.ivv.nasa.gov/nasa/ootw/1999/ootw_990526/ob990526.html

As part of its Learning Technologies Project (LTP), NASA supports a number of educational Web sites that have excellent material on the Earth's atmosphere and oceans:

http://observe.ivv.nasa.gov/nasa/education/edu/edudocs/topic_atmos.html
http://observe.ivv.nasa.gov/nasa/education/edu/edudocs/topic_water.html