Re: airplane watching questions

Date:         18 Mar 2001 12:06:14 
From:         "James Weber" <mattheww50@home.com>
References:   1
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>2.  It was a cool afternoon (temp had dropped to about 12 C) and very humid
>(raining on and off and very low cloud cover).  Most of the planes landing
>had a streamer of water vapor trailing from the wingtips like smoke
>streamers and some had a cloud of vapor on the upper surface of the wings.
>Someone once explained this to me and I forget what it is.  Something to do
>with low pressure around the wing?

The vortex behind the wings  tips are in fact low pressure areas. The sudden
reduction in pressue without doing 'work' is called adiabic expansion, which
cools the air.  If the relative humidity is very high, the lower temperature
can cause the relative humidity to exceed 100%, at which point the water
condeses out, and you get instant cloud formation.....

It the humidity is high enough, you can also get it toward the back of the
upper wing surface.   Shock wave from a supersonic aircraft does the
same thing. Low pressure behind the wave causes cloud formation, so you
can see an aircraft that seems to be draped in cloud.......