OK, convective cloud formation question.

Say you get a parcel of air that gets warmed by surface heating. It rises, cooling at the dry adiabatic lapse rate until it has reached the dew point, at which point moisture condenses and a cloud starts to form. OK so far - and it provides a resonable match to METAR data.

However, how does the process stop?? Above the cloud base the air cools at the moist adiabatic lapse rate (MALR). This is given as 1.5 deg C / 1000 ft, but the average environmental lapse rate (ELR) is also given as 2 deg C / 1000 ft - i.e. the air is always conditionally unstable (but as it is saturated it will continue to rise). Ultimately the ELR starts to increase at the tropopause, but only serious thunderstorms get that high.

Now I guess either the MALR or ELR (or both) are oversimplified in the simple stuff I'm reading - at least one must change so that the air becomes stable and stops rising. Can anyone explain, or point me in the direction of something that does?

<img src="confused.gif" border="0">

Evo 7

It is a little confusing that MALR and ELR are presented in such a similar way in many texts.

The moist (or saturated) adiabatic lapse rate is, in effect, a law of physics. If you change the pressure of a parcel of air without allowing heat to flow in or out, the temperature will change by a predictable amount. It varies with the temperature you start from, but not from day to day.

The environmental lapse rate of 2 degC/1000 ft is a day-to-day 'average', which is also used to define the international standard atmosphere.

If you look at an . .<a href="http://weather.uwyo.edu/upperair/sounding.html" target="_blank">Atmospheric Sounding</a> you'll see that the real temperature profile of the atmosphere can be nothing like the smooth 2 degC/1000 ft average. It's a jaggy line that goes all over the place.

The shape of the real environmental lapse rate, not the day-to-day average, determines the stability of the atmosphere and the convective cloud bases and tops.

Hope that helps.

Thanks bookworm (and Irv) - helps a lot to see what it really looks like. Still plenty of things I'm confused about, but they are a bit OTT for the PPL. I'll get the exam out of the way at the weekend, but I'm not done with Met yet. I've become rather interested in it now... <img src="redface.gif" border="0"> <img src="smile.gif" border="0">

Passed - just one question wrong. Thanks for the help chaps! <img src="smile.gif" border="0">

Congrats!! You must have been that worried looking chap moping around at Goodwood yesterday when I paid my landing fee!!. .. .DOC