Cloud

[AMiller]

Hi all,

I am sorry if this is posted in the wrong section of pprune...

Can anyone briefly explain to me why in the Summer in the mornings there is no cloud then during the day Cu builds up...but then goes away again in the early evening....

Andy

[Farmer 1]

Yes, Andy.

When the sun rises, it warms the earth's surface. Different areas warm at different rates, depending on the make-up of the surface. The surface then warms the air in contact with it, causing it to expand. As the air expands, it becomes less dense than the cooler air around it, and so rises.

As it rises, it expands more, because the atmospheric pressure decreases as you ascend. Because of the drop in pressure, the rising air cools adiabatically (i.e. without loss of heat). Its rate of cooling will probably be different from that of the surrounding air: if it is a lower rate it becomes relatively warmer than the surrounding air, and will continue to ascend at an increasing rate; if it cools at a higher rate it will soon reach the same temperature as the surrounding air, and stop ascending.

If the rising air has a high enough relative humidity, it will cool to its dew point. It is then saturated, and any further cooling will cause water vapour within it to condense into water droplets, and cloud is the result.

Referring to my first paragraph, small areas of surface heated cause small pockets of air to warm and rise, which is why you see small, puffy clouds on a summer's morning. Fair weather cumulus is one name for it.

When the surface, and the air in contact with it, cools in the evening the air is no longer heated, so is not caused to rise.

[Trentino]

If you ever have noticed, thunderstorms are generally more powerful in the equator than more polar locations. One of the reasons for this is that thunderstorm buildup is limited to the Tropopause. Because the Tropopause is higher at the Equator than at the poles, the thunderstorms lift higher and have more potential.

So, if you have the same exact weather conditions in Quito and in Helsinki lets say, the potential storm in Quito will be stronger because the 'weather roof' is higher

[Farmer 1]

Trentino,

That's either something I've long since forgotten, or something you've just taught me. Thank you.

Farmer.

[Trentino]

Anytime Farmer! I used to hate weather theory and then came the day that I had to teach it. I have no choice now but to love it.

weather is so comprehensive, PPrune should make a forum just dedicated to weather.

[faultygoods]

wow you guys i'm impressed

[Clandestino]

OUCH!!!!

Explanation that thunderstorm growth is limited by the height of the tropopause is mixing up cause and consequences. In the equatorial areas there's much stronger insolation than in polar and that's what causes stronger thunderstorms and heightened tropopause. Stonger insolation is due to sun being higher over horizon, therefore same amount of solar radiation is spread over lesser surface in lower geographical lattitudes and path of solar rays through the atmosphere is shorter, reducing losses due to reflection and absorbtion.

[selfin]

http://www.net-weather.co.uk/forum/ is a very useful location for this.