"Graham P Davis" wrote in message
...
On Sat, 02 Jul 2011 15:37:15 +0100, Togless wrote:

"Tudor Hughes" wrote:

On Jul 2, 11:59 am, "Togless" wrote:
Looking north from my house here in Portsmouth I can see large grey
clouds looming up rather quickly, and on the satellite photos it has
all developed just in the last 2 hours or so - what is that called,
when (presumably)
the heat of the sun makes the sky go from clear blue to masses of
cloud in just a couple of hours? Anyway, I'm hoping that an onshore
breeze will keep it away from us here on the coast. Sometimes being
on the coast works in our favour, and sometimes not (sea mist etc).

John.

It's convection over the heated land. The air will cool as it
rises and will form cloud when it reaches saturation. Today it won't
rise very far because there is warmer air aloft so the cloud spreads
out. We have similar cloud here.

Thanks Tudor - I knew about the first part, but hadn't appreciated the
second. What do you use to determine that there is 'warmer air aloft',
if you don't mind me asking? Presumably in different circumstances you
would get large high cumulus clouds and the risk of intense
precipitation - is that right?

Have a look at this ascent from Herstmonceux: http://tinyurl.com/6kbgvfe
It shows that there can be convection from the surface to the lid at
750hPa.

For other stations, dates, times, got to here -
http://weather.uwyo.edu/upperair/sounding.html - and select the region
(Europe) and type of plot (GIF skew-T). Date and time-frame will be
defaulted to the latest but you can select earlier periods. Then click on
the station you want. The graph will pop up in a separate window.

Thanks Graham, fascinating stuff. I can see what you mean about there being
a 'lid' at 750hPa. I guessed that the right-hand line must be temperature
and the left-hand must be dewpoint. Nice to learn something new :-)