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Post by chriso on Nov 25, 2012 20:37:07 GMT
If you live in certain areas, fog is fairly common. And it is also common that when the sun comes out, the fog will rapidly dissipate. I have heard this described as the sun "Burning" it away. Although I know the sun doesn't actually burn it, as far as I can see the effect is real*, leaving me wondering what the mechanics behind it is. So my question is, what about sunlight can remove or destroy fog? Just curious.
*Cloudy days, along with fog, means a miserable day, while on a sunny day it dissipates in a few hours. You can tell when on a hill which it is.
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Post by srmarti on Nov 25, 2012 21:17:12 GMT
Sunrise results in temperature increase and water evaporation.
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Post by chriso on Nov 25, 2012 21:27:10 GMT
Sunrise results in temperature increase and water evaporation. So in other words, it forces it back into a vapor? Rather then a very fine mist? |
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Post by srmarti on Nov 26, 2012 1:16:47 GMT
Yes, the droplets evaporate.
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Post by silverdragon on Nov 26, 2012 7:38:03 GMT
Relative Humidity, RH....
RH is dependant on heat, the colder the air, the less humidity it can hold, this is how de-humidifiers work, they pass air over cold plates that condensate out water, they then use that air to cool the machine as well, which is why you get warm air out a de-humidifier.
Therefore, if warm air cools down, the RH must drop into vapour, this is essentially how Fog is caused.
The sun warms up the air, and the potential for that air to increase its RH raises, so the water vapour is absorbed back into the surrounding air.
Thus the sun "Burning off" fog is almost exactly right?....
(Am I being any help here?..)
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Post by srmarti on Nov 26, 2012 15:04:01 GMT
Sure, or maybe more simply you can have more water evaporate into warmer air than cooler air. As the air warms up, the fog goes away.
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Post by the light works on Nov 27, 2012 15:36:15 GMT
to clarify Cyber's explanation: RH is the percentage of the maximum amount of water the air can hold that it is trying to hold. as the air warms up, it can hold more water - so the fog is absorbed by the warming air. when it cools off at night, it squeezes the water back out - as fog, mist, or dew.
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Post by chriso on Nov 27, 2012 22:01:23 GMT
Okay, now I understand. Another of life's little mysteries solved.
And you meant silver, not cyber, right?
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Post by the light works on Nov 28, 2012 2:19:04 GMT
yes, I got the icons confused this morning.
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Post by silverdragon on Nov 28, 2012 8:37:30 GMT
Take more coffee with it?......
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Post by c64 on Nov 28, 2012 8:57:33 GMT
Well, there has to be a very special condition to allow fog to happen. You need the correct temperature and moisture. It works best at around +4°C. Also you need different temperatures merging just above the ground or a slowly but steady rising or decreasing environmental temperature.
The trick is that while the water condensates, it just doesn't drop down as rain since the tiny water droplets are reabsorbed by the air before they can fall out of the air. The constant condensing and vaporising keeps the air full of visible water droplets which is what we call fog.
And direct sunlight heats up those water droplets, making them vaporize a lot faster than new droplets can condense. That's the "burn through". At the correct time, you can see the sun-rays shining through a forest burn holes into the fog until the air becomes to warm to make fog appear and everything is gone.
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Post by freegan on Nov 28, 2012 9:06:20 GMT
Just out of curiosity, is there any functional difference between fog and ground level cloud (as one might encounter on high ground) or is it just a case of semantics?
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Post by c64 on Nov 28, 2012 9:33:14 GMT
Just out of curiosity, is there any functional difference between fog and ground level cloud (as one might encounter on high ground) or is it just a case of semantics? Clouds can exist beneath a certain air pressure. So in case of a really, really tall mountain, you can have a cloud at ground level. Fog can exist at any pressure. On a slope where the wind is guided upwards by the ground, fog can appear much more easily so you have fog on hills much more often than on even ground. |
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Post by the light works on Nov 28, 2012 15:25:15 GMT
Just out of curiosity, is there any functional difference between fog and ground level cloud (as one might encounter on high ground) or is it just a case of semantics? semantics. |
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Post by freegan on Nov 28, 2012 16:36:42 GMT
So in case of a really, really tall mountain, you can have a cloud at ground level. I have ridden my motorcycle through low cloud, from clear air to minimum visibility to clear air, all below 150 meters altitude in the Yorkshire Dales. No "really, really tall mountain" there. |
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Post by c64 on Nov 28, 2012 17:30:13 GMT
So in case of a really, really tall mountain, you can have a cloud at ground level. I have ridden my motorcycle through low cloud, from clear air to minimum visibility to clear air, all below 150 meters altitude in the Yorkshire Dales. No "really, really tall mountain" there. That was plain old fog - or smog  |
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Post by freegan on Nov 28, 2012 17:33:49 GMT
I have ridden my motorcycle through low cloud, from clear air to minimum visibility to clear air, all below 150 meters altitude in the Yorkshire Dales. No "really, really tall mountain" there. That was plain old fog - or smog Looked a lot like cloud from both below and above. |
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Post by watcher56 on Nov 28, 2012 17:53:47 GMT
Any time the ambient temperature tries to fall below the dew point, fog may form. A cloud is a fogbank in the sky.
One of the things I find most interesting about fog is that it limits the temperature drop in a given mass of air. Condensing moisture releases a large amount of heat - if an air mass temperature tries to drop below it's dew point temperature, water vapor will condense out releasing heat.
You will never see an air temperature below the dew point, and dew point is characteristic of the air mass.
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Post by c64 on Nov 28, 2012 18:31:31 GMT
You will never see an air temperature below the dew point, and dew point is characteristic of the air mass. You do, it just takes a bit longer. ALso the dew point is pressure and temperature dependant… If the temperature is below the dew point over night, you don't have to scratch ice of your car in the morning any more, just dust of snow at best. Must be very cold to have this advantage. |
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Post by watcher56 on Nov 29, 2012 0:14:07 GMT
You do, it just takes a bit longer. ALso the dew point is pressure and temperature dependant… No and no. As soon as the temperature reaches the dew point, water vapor begins to actively condense out of the atmosphere, adding heat. At that point, the temperature will not decrease further unless enough water vapor condenses to also reduce the dew point temperature. Temperature (and perhaps to a lesser degree pressure) determines how much water vapor the atmosphere can hold. But by definition, dew point is that temperature at which the air becomes saturated - any further attempt to cool it will first start condensing out the water vapor. |
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