The other day I asked a question about the role volcanic activity played in the part of one of the theories behind the extinction of the dinosaurs. In Scott Rowland's reply about the ash particles affecting the temperature of the atmosphere. I do not understand why ash particles less than 2 microns would block out the sunlight and particles greater than 2 microns, sunlight could get through, but the heat radiation from the surface would not be able to get out. I do not understand the part about the particle size and why sunlight would be able to get through if larger than 2 microns. Please explain this to me. I do thank you for your response to my earlier question. Thanks again.

rocky Karen Gurley

Hi Karen,

Your questions are getting harder and harder, and further and further from what I know. In fact this is into the realm of physics. It has to do with the fact that for a wave to be affected by something, that something has to be the size of the wavelength or smaller. For example, ocean waves entering a wide-mouthed harbor come straight in whereas the same waves entering a harbor with a small opening get diffracted into circular waves that appear to radiate from that small opening. Well, for the case you are asking about, if the particles are large then incoming solar radiation (short wavelengths) don't get scattered by these big particles. Certainly some of them get blocked by the particles but they don't interact to scatter back to space. If, however, the particles are really small then they DO scatter the short-wavelength incoming solar radiation. That means less radiation gets in so the Earth cools off.

The radiation from the Earth's surface is mostly heat radiation and it has longer wavelengths. It CAN be scattered (back to the surface) by the larger particles and pretty soon the Earth starts to warm up. I know you're going to ask me why the small particles don't also scatter the heat radiation back, and I assume they do. But their effect on the incoming radiation is probably greater so that is the result that is seen.

Physics from a geologist is probably not the best you can get but hopefully this will help a little.

Sincerely,

Scott Rowland, University of Hawaii 

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