Drakensberg Weather
30 Apr 2018 11:01 - 30 Apr 2018 15:13 #73328
by tiska
Drakensberg Weather was created by tiska
There is a major, annual thread on Snow under the General category on the forum, but no thread that I can find on Weather.
From time to time there are points of discussion on the weather - which might belong in a thread like this - posted under Drakensberg Environment
Ghaz posted the following:
Dew will form when water vapour condenses into liquid water.
The relationship that governs whether condensation happens is reasonably simple.
The temperature of the air has to cool to be the same as dew point temperature for condensation to occur.
Dew point temperature is the temperature at which molecules of water vapour will begin to change phase from gas to liquid. It is also roughly proportional to the number of water vapour molecules in the air. Air with a dew point of 10 deg C has more water vapour in it that air with a dew point of 9 deg C, no matter what the temperature of the air is.
If the temperature is 20 deg C and the dew point is 16 deg C, the air needs to cool by 4 degrees from 20 deg C to 16 deg C for condensation to start and for the first dew to form. If the air cools from 20 deg C to 16.5 deg C, no dew will form.
If the temperature is 10 deg C and the dew point is 9 deg C, the air needs to cool by 1 deg C from 10 to 9 for dew to form.The closer the dew point and the temperature measurements are to each other, the greater the likelihood of dew formation.
If the temperature is 4 deg C and the dew point is -1 deg C, and the air cools by 5 deg C, frost will form.
How does the cooling occur?
Typically the cooling occurs at night once solar heating stops. The earth cools by longwave (infrared) radiation all the time but this usually only has a cooling effect once the heating from the sun stops. The earth's solid surface cools much, much more efficiently than air, especially dry air. As a result, the cooling happens fastest at the surface. The air a few tens of meters above the surface does not cool much by longwave radiation. It cools becomes the surface below it cools.
On windy nights the surface can cool through radiation but the wind mixes down the warmer air from above which does not cool much. As a result, it is difficult for the dew point temperature to be reached and unusual for dew to form. There is an additional influence: the wind mixes down dryer air from above to the surface, lowering the dew point. As a result, the temperature might have to drop even more to reach the lower dew point.
How did the outside of Ghaz sleeping bag get dew on it on a windy night?
The explanation so far is about how cooling happens through radiation. It is very unlikely that radiation caused the cooling that led to dew given the strong wind.
But there are other ways to cool air. One is to blow a wind from a colder source region. That is how the midlatitude atmosphere exchanges most of its heat across latitudes. If a cold wind blows, the drop in temperature can be achieved by the cold wind. Then dew point can be reached and dew will form.
Another possibility is that colder air blew in AND the dew point temperature was raised along with it because air containing more moisture that was also cooler pitched up.
The numerical weather prediction simulations from the US GFS on Ventusky show a change from air at escarpment top height (700 hPa) originating from the Bie plateau in Angola at noon on Thursday 26 April 2018 backing to air from the subtropical SE Atlantic by dawn on Friday 27 April 2018. That airstream would likely have been characterised by lower temperatures and higher dew point temperatures. At lower than escarpment levels, a cold front passed through KZN bringing moister, cooler conditions and low cloud up against the escarpment around 10pm Thursday night.
From time to time there are points of discussion on the weather - which might belong in a thread like this - posted under Drakensberg Environment
Ghaz posted the following:
ghaznavid wrote: Despite quite a strong wind, somehow we had dew (can someone explain this to me - I remember learning in geography that dew doesn't fall if there is wind).
My pack looked like this the next morning:
Dew will form when water vapour condenses into liquid water.
The relationship that governs whether condensation happens is reasonably simple.
The temperature of the air has to cool to be the same as dew point temperature for condensation to occur.
Dew point temperature is the temperature at which molecules of water vapour will begin to change phase from gas to liquid. It is also roughly proportional to the number of water vapour molecules in the air. Air with a dew point of 10 deg C has more water vapour in it that air with a dew point of 9 deg C, no matter what the temperature of the air is.
If the temperature is 20 deg C and the dew point is 16 deg C, the air needs to cool by 4 degrees from 20 deg C to 16 deg C for condensation to start and for the first dew to form. If the air cools from 20 deg C to 16.5 deg C, no dew will form.
If the temperature is 10 deg C and the dew point is 9 deg C, the air needs to cool by 1 deg C from 10 to 9 for dew to form.The closer the dew point and the temperature measurements are to each other, the greater the likelihood of dew formation.
If the temperature is 4 deg C and the dew point is -1 deg C, and the air cools by 5 deg C, frost will form.
How does the cooling occur?
Typically the cooling occurs at night once solar heating stops. The earth cools by longwave (infrared) radiation all the time but this usually only has a cooling effect once the heating from the sun stops. The earth's solid surface cools much, much more efficiently than air, especially dry air. As a result, the cooling happens fastest at the surface. The air a few tens of meters above the surface does not cool much by longwave radiation. It cools becomes the surface below it cools.
On windy nights the surface can cool through radiation but the wind mixes down the warmer air from above which does not cool much. As a result, it is difficult for the dew point temperature to be reached and unusual for dew to form. There is an additional influence: the wind mixes down dryer air from above to the surface, lowering the dew point. As a result, the temperature might have to drop even more to reach the lower dew point.
How did the outside of Ghaz sleeping bag get dew on it on a windy night?
The explanation so far is about how cooling happens through radiation. It is very unlikely that radiation caused the cooling that led to dew given the strong wind.
But there are other ways to cool air. One is to blow a wind from a colder source region. That is how the midlatitude atmosphere exchanges most of its heat across latitudes. If a cold wind blows, the drop in temperature can be achieved by the cold wind. Then dew point can be reached and dew will form.
Another possibility is that colder air blew in AND the dew point temperature was raised along with it because air containing more moisture that was also cooler pitched up.
The numerical weather prediction simulations from the US GFS on Ventusky show a change from air at escarpment top height (700 hPa) originating from the Bie plateau in Angola at noon on Thursday 26 April 2018 backing to air from the subtropical SE Atlantic by dawn on Friday 27 April 2018. That airstream would likely have been characterised by lower temperatures and higher dew point temperatures. At lower than escarpment levels, a cold front passed through KZN bringing moister, cooler conditions and low cloud up against the escarpment around 10pm Thursday night.
Last edit: 30 Apr 2018 15:13 by tiska. Reason: typo
The following user(s) said Thank You: Serious tribe, diverian, ghaznavid, tonymarshall, Macc, AndrewP, saros, biomech, TheRealDave, Gudstff
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