This
tutorial is probably the most difficult so far. Take a deep breath.
This was a midnight sounding
on the right.
It looks dreadful, doesn't
it?
Lets blow up the bottom
bit
With some difficulty,
it can be seen that the dry and the dewpoint lines are very close or even
coincide near the surface.
The surface dewpoint Tdew
and dry temperature Tdry are both about 14°C.
Tdew probably won't change
much during the day. But Tdry will rise to an expected 20°C (obtained
from various forecast sources).
From the expected surface
temperature of 20° follow the dry adiabatic line upward diagonally
to the left until it meets the environmental line.
This occurs at about 880
mbs.
However, now we must consider
the moisture content of the air.
From the surface dewpoint
of about 14°, follow (or parallel) a purple/brown line upwards
to the right.
This will intersect the dry
adiabatic from 20° surface at about 920 mbs. This is the expected
cloudbase.
A cross
check using the Bradbury 400 ft / degree rule would come up with the same
result - ie 6° difference means cloudbase of 2,400 feet. So we
have confirmed expected cloud base.
Remember that the 400
ft / degree rule is only appropriate if cumulus develops and cannot be
used to determine thermal depth on blue days.
The relevant
part of the sounding is shown
The maximum temperature
is assumed to be 20°C
Earlier, we saw how to work
out the cloud base.
But the convection does
not stop at cloud base. It continues at the SALR until reaching the
ELR at around 400 mbs (~23,000 feet).
Blue
Haze caps or Cumulus? On a later page, we will
see how it can be determined (with, it must be said, a certain degree of
uncertainty) whether or not it will be a blue day.
How
tall will the clouds grow? It is important to have
an idea how tall the cloud might grow as this gives an indication of potential
shower activity (or otherwise).
Showers are unlikely if
the cloud tops do not penetrate the height at which -10°C occurs.
In this example, clouds
do indeed go to heights where the temperature is -25°C
The later midday sounding
shows that we were correct with the earlier interpretations. Indeed,
the airmass looks even more likely to result in heavy showers - there is
a bigger area between the black and the red line.
(to be technical, larger
CAPE).
And it all went to plan:
