Naked Science Forum

Non Life Sciences => Physics, Astronomy & Cosmology => Topic started by: thedoc on 07/10/2013 13:30:01

Title: Why does smoke sink at night and rise during the day?
Post by: thedoc on 07/10/2013 13:30:01

tonya soderquist asked the Naked Scientists:
   My family lives in Utah (USA) and we are currently having some severe wildfires.

When we get up in the morning, the smoke from the nearby mountain fires is low and mostly against the mountain. By evening however, the smoke is substantially thicker, is high in the sky and covering the valley (much like heavy cloud cover).

We have observed this consistently for several days over which there has not been a change in weather patterns.

Why is it that the smoke would be heavier in the evening than in the morning?
 
Does it have anything to do with our "high desert" temperatures?

Love the podcast by the way. I'm originally from Oxford and you all keep me from getting too homesick!

Regards,
Tonya, John & Alex Soderquist

What do you think?

Title: Re: Why does smoke sink at night and rise during the day?
Post by: webplodder on 07/10/2013 15:33:47

I imagine this is due to the heating effect of the atmosphere during the day. Heat rises so lifting the smoke.

Title: Re: Why does smoke sink at night and rise during the day?
Post by: Phractality on 07/10/2013 15:40:05

In daytime, solar energy heats the ground, and the ground heats the atmosphere by convection. This generates updrafts which carry the smoke higher.
At night, the ground radiates heat, making it cooler than the air. There is less turbulence in the atmosphere, and there may also be a temperature inversion holding smoke close to the ground.
If you camp near a stream on a mountainside when there is no strong wind, you may observe that the breeze is usually upstream in the day and downstream at night.