On Tuesday, 24 September 2013 10:25:52 UTC+1, John Hall wrote:

Should that read "not have spread out"?

Indeed it should. What was I thinking of? Don't ask.

Tudor Hughes, Warlingham, Surrey

On Tuesday, 24 September 2013 10:25:52 UTC+1, John Hall wrote:
In article ,
Tudor Hughes writes:
On Monday, 23 September 2013 17:07:22 UTC+1, Steve Wood wrote:

I do not understand why cloud
formed as the sun came up and soon blocked out the sun.

How does cloud (not ground level mist) form in sinkingair?



The air in which the cloud is formed is not sinking. The base of
the subsidence inversion rarely descends to below about 3000 ft.
If it had been in midsummer the cooler air beneath the inversion
could have been warmed out enough for the inversion to disappear.

There would still have been convective cloud, maybe quite large,
but it would [not] have spread out in the way it has done in
the last few days.

Another factor about the current situation is that the lower layers
are rather moist, more so than usual.

I think this must be a tricky time of year for forecasters with this
set-up.

Hence the recent earthquake. Trick situations = bad situations. Easy situations = easy money. Ergo, someones are getting paids for not many lots aren't they, my precious? Super-duper computers be damned.

Now for my two half pence:

To start with it is cold air. This is coming down from above. It comes down from above because it can. It can because:
1. It is heavier than warm moist air and
2. The situation up top is "unstable" enough for the stability to enact some really beautiful physics.

Falling cold air can only fall as far as the pressure it exerts allows it to fall. If it meets resistance it gets warm or stops falling. If it can still fall it has to fall through lower layers and it can only do that as a vortex, having gained a certain degree of streamlining.

The situation is now exactly the same as a cyclone and not a cyclone in reverse too neither. It is a cyclone. But it's base is at the tropopause. From several miles up it is rotating in the same direction as a cyclone rotates at sea level.

It then takes on all the mechanics of a cyclone with reference to water content and it's dispersal. The pressure at ground level then changes according to what is happening on it's shoulders.

All this without the benefit of Coriolis Force. I love it. How unstable does something have to be before that cuts in? If you want to see how a tropical low is influenced by the disposal of precipitate, take a look at the run on here for today:

http://www.bom.gov.au/australia/char...Refresh+ View

I'll put an animation of it on my blog later so it won't get lost to eternity. The depiction is so clear it would be a shame to lose it. Follow the stuff south of Africa. It grows into a large complex and the length and "degree of parallelism" (there isn't a term for that, is there) of the outer isobars is the key.