"Andrew Sarangan" wrote in message
1...
Instability produces cumulus clouds and stability produces stratus clouds.
We know that. However, since the saturated and unsaturated lapse rates are
significantly different (1C/1000' compared to 3C/1000'), it seems quite
possible to get cumulus clouds even when the atmosphere below is stable.
For instance, if the environmental lapse rate is 2C/1000', the unsaturated
air is stable. Once clouds form (how they form without vertical currents
is
a different matter), the air inside the clouds will become unstable. Does
this seem reasonable?

I think there's an aspect to this that hasn't been discussed. It *does*
require instability to produce cumulus cloud, but that instability can be
very local. So you may see an average environmental lapse rate of 2
degC/1000' through the lowest 3000' of the atmosphere, but actually you've
got at least patches of surface being heated by the sun, producing higher
temperatures and local instability. In that simple example, if you heat a
thin layer at the surface by just 3 degC, you've now got instability and the
makings of vertical convection.

That's not to say that stratiform clouds can't become unstable by the
mechanism you propose, but cu can form, particularly close to the surface,
in atmospheres that start off looking stable.

Julian Scarfe