MadSci Network: Environment & Ecology

Re: Does smoke from fires (forest, etc.) cause rain?

Date: Mon Nov 17 13:40:28 2003
Posted By: Denni Windrim, Interpolator
Area of science: Environment & Ecology
ID: 1069038771.En

An interesting question, Savonnah. Your hunch is probably correct. The easterly Santa Anna winds which were partly responsible for the ferocity of the fires carried smoke well out into the Pacific to the extent that it was easily visible on GOES weather satellite images. Combined with the moist westerly airflow typical of mid-latitude fall/winter, heavy rain is an entirely plausible consequence.

Rain forms in a cloud through the consolidation of very small droplets into larger ones. Typically, rain droplets are formed when moisture in a cloud nucleates onto a particle of airborne dust, salt, smoke. or other natural or artificial pollutant. These droplets then merge until their weight is enough to overcome whatever updraft may be supporting them and they fall as rain.

(Contrary to popular belief, it is not necessary for these rather larger nuclei to be present. Droplets can nucleate on something as small as a molecule of gas, though the process is much slower. The vast majority of droplets nucleate on solid particles. But thatís an aside.)

It is this process that we consciously manipulate in cloud-seeding. Silver iodide makes an excellent condensation nucleus, and many tests have been made to determine its usefulness not only in bringing precipitation to regions where rainclouds typically pass overhead without dropping rain, but in forcing potentially dangerous rainstorms into dropping some of their precipitation early so they are not as intensive later.

I appreciate your question for another reason: I learned something new when I researched it. It turns out (as is true in many situations) that more is not always better. If a cloud, or a warm, moist airmass, is seeded with too many condensation nuclei, rainfall may actually be impeded. With so many nuclei available on which water vapor may condense, there may not be enough vapor to produce droplets that are both sufficiently large and sufficiently dense to combine to make rain where one would expect it to fall.

You can find out more about weather in general at the BBC Weather Centre, and there is information about cloud seeding, whether natural or artificial, at the University of Colorado. The second is a .pdf document, so you will need Adobe Reader to see it. If you don't have the reader, you can download the latest version at no cost from Adobe.

Thanks for a fun question!

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