- Page 2
|Look at this new graph
It shows yet another type
of presentation that is available on the internet
In fact, this is a Skew-T
and is very like the demonstration charts seen earlier but with minor variations.
The sounding is the midnight
one on what was arguably "the day of the year" when many big flights were
Points to note:
Very strong inversion at
850 mbs thus convection limited to about 5,000 feet
An inversion is when
the temperature increases with height - more usually it decreases
At 850 mbs the air is very
dry (the left dewpoint line is well separated from the right environmental
Winds are light at all levels,
but near coasts, sea breezes need to be considered
|Not shown on
this Skew-T, but using information from other sources, cooler air between
850 and 800 mbs was expected to move in from the west during the day raising
the depth of convection. That was taken into account when making
the forecast for 15th August
Breeze rule of thumb
Deep penetration inland
only likely if total depth of convection, including cumulus tops, is between
3,500 and 10,000 feet. Less or greater depth will probably mean any
sea breezes confined to very near the coast.
So with light winds and
appropriate depth of convection on 15th August, sea breezes were forecast
to move well inland. See the separate tutorial
on sea breezes
Two final lines on this
lines (arrowed in black) run diagonally up at 45° from the bottom
right to the top left.
These are the Dry Adiabatic
Lapse Rate (DALR) lines.
Adiabatic means no external
heat added nor taken away
from the air mass.
A rising parcel of air cools
(because it expands) at the DALR until such time as it becomes saturated.
DALR can be taken as
being about 3°C per 1,000 feet.
So a thermal which leaves
the surface with a temperature of 20°C will have cooled to 14°C
by the time it has reached 2,000 feet.
the thermal rises far enough and cools sufficiently for condensation to
occur, cloud forms. Condensation takes place if the air continues
to rise, and latent heat is given out by the condensation process.
Thus the temperature in the cloudy thermal falls off rather more slowly
than it does in a dry thermal.
At low levels, this can
be taken to be roughly 1½°C per 1,000 feet.
These Saturated Adiabatic
Lapse Rates lines (SALR) are curved and indicated by the blue
NOTE : Air behaves either
as being dry or saturated. It is not a gradual process of change
between one state and the other .
we should know what all the various lines mean.
So how do we use them?
ELR line starts from about the 1000 mb level.
Now using those red
temperature lines that go diagonally
upwards to the right, it can be seen that the ELR reaches the surface at
Just to the right of this
point, there is a black DALR lines (they go diagonally up to the
left) which intercepts the 1000 mb surface at about 16°C
So if the surface temperature
reaches 16°C, then a bubble of air (being warmer than the environment)
will rise as a thermal and follow that DALR line (temperature decreasing
by 3°C every 1,000 feet as it does so).
Eventually, that thermal
reaches the red ELR line at about 850
mbs (5,000 feet) and if it were somehow able to rise above that point,
would in fact be cooler than the environment. This cannot happen,
(ignoring orographic effects) so where this particular DALR line from a
surface temperature of 17°C reaches the environmental line, the air
stops rising, and this marks the
top of the thermal.
limitations of this simplistic interpretation are given below.
are rather "coarse" forecast soundings and don't always model the profiles
in great detail.
Above left is an enlargement
of part of the larger diagram. It might simply show that at 15°C,
there is a dry adiabatic to around 850 mbs
But it might hide an early
morning inversion as shown in the middle picture.
The right hand diagram shows
that 15°C is needed as trigger to "break the inversion".
Some of the more detailed
soundings such as those from Dr.Jack and Meteoblue deal with these lower
levels rather better, but still have their limitations.
In this illustration,
formation of cumulus has deliberately been ignored as a simplification.
That will be considered later.