A perfect summer day is when the sun is shining, the breeze is blowing, the birds are singing, and the lawn mower is broken. - James Dent
Question #653 posted on 10/04/2003 2:17 a.m.

Dear 100 Hour Board,

I'm sure you noticed that there has been a lot of smoke here in Utah ever since that out-of-control control-fire. My question is: Why is it that each morning there is tons of smoke, but by the evening it is mostly gone, only to return the next morning?

- Anonymous

A: Air is mostly stagnant during the night due to lack of heating from the sun. That's one reason why fog forms (and stays) early in the morning -- well, okay, Utah's much too dry for that to happen on a regular basis, but in areas that have a respectable amount of water around it happens -- and clears up within an hour of sunrise. Pollutants of any sort still build up during the night, and just sit near the ground because there aren't any air currents to move them around. Once the sun comes up and the air starts moving, the pollutants get blown off the surface and carried into the upper atmosphere.

-- Misaneroth