in feet = 400 times (temperature / dewpoint split)|
|Many pilots will be familiar
with this rule of thumb, named after glider pilot / professional meteorologist
The rule links temperature,
dewpoint and cloudbase with a very simple formula. (the rule can
easily be verified from a soundings diagram)
difference between surface temperature and surface dewpoint multiplied
by 400 (Tdry-Tdew) *400
gives an excellent estimation
of cloud base in feet above ground level
This method is better for
cumulus cloud base than Tafs
Tafs usually give the lowest
base during the forecast period
this works out depth of convection ONLY if cumulus actually
blue days, the height of thermals has to be determined by other methods
measurements or from metars during morning
during morning = 12°C
change in basic airmass is anticipated, so when Tdry reaches around 19°C,
(ie Tdry / Tdew split = 7°), a cloud base of around 2,800 feet can
afternoon Tdry maximum is 23°C, so eventual split = 11° giving
best cloud base of 4,400 feet
profile (lapse rate) below cloud or in blue thermals
temperature (below cloud), either rising in thermal or having been previously
affected by convection, decreases by 3.0°C for every 1,000 feet
increase in height.
is called the "Dry Adiabatic Lapse Rate".
practice, there is usually an excess of around 1° near the ground.
is called a super-adiabatic
Tdry = 23°C
4,000 feet it is therefore 4 * 3°C lower, ie = 11°C
adding that super-adiabatic in the lowest 1,000 ft the temperature at 1,000
agl is more like 23 - 4 = 19°C and thus at 4,000 feet, probably around
|These figures have been
verified by actual in-flight temperature measurements.
Above cloud base or tops of blue thermals, different parameters apply.
in practice - an example
Assume dewpoint remains unchanged
at 11 °C during day (a not unreasonable assumption unless change
When Tdry rises to
18°C during the morning with that Tdew = 11°C , cloudbase = 2,800
(remember, Tdry -
Tdew = 7 equating to a cloudbase of 7 x 400 feet = 2,800 feet)
But it is not always necessary
to know Tdew. Many modern cars have thermometers. Note the
temperature, get a cloudbase from eg a glider already soaring during the
morning, and you can easily infer the dewpoint at present and thus infer
expected best cloudbase at maximum Tdry.
|However, that was during
the morning. The surface temperatures is forecast to reach
Assuming Tdew remains unchanged
at 11°C, then cloud base should eventually rise to 4,000 feet
It can be seen that each
degree rise in temperature means a 400 foot rise in cloudbase
A very good guide to
Tdew can be found from various internet reports such as Metars and can
form the basis for predictions.
There are also specialized
instruments for calculating dewpoint.
- The forecast Max Tdry minus present Tdry times
400 = expected cloudbase at best time of day.
rule of thumb of course goes wrong if cloud base rises so much as to overtake
cloud tops, ie the day turns blue.
are lots of ifs and buts, but while bearing these limitations in mind,
this method gives a very useful guide.
is a term to describe what happens sometimes after cumulus has been forming
for some while. The developing clouds run up against a lid (maybe
an inversion) and can rise no further, They spread sideways and cut
off the sun thus affecting the amount of heat reaching the ground.
An extensive sheet of strato cumulus develops at say 4 to 6,000 feet.
Surface temperatures cease to rise and can even fall by a degree or two.
Convection is now very poor. This of course not only is of major
significance to glider pilots, but the general public get irritated when
a beautifully sunny start turns to extensive cloud during the late morning.
is notoriously difficult to predict and may indeed affect only some areas
(but why?). A clue, but no more than that, is provided by the time
when the first puffs of cumulus appear. If this happens early, then
spreadout is POSSIBLE. On the other hand, if the first cumulus do
not occur until late (and by implication, quite at high base and at a surface
temperature only just short of the expected maximum) then spreadout is
very useful for convective situations. But do bear in mind its limitations.
Temperatures are often known only to the nearest degree.
Thus if Tdry is in fact
0.5°C lower than reported (metars) and Tdew 0.5°C higher, then
cloudbase using the quoted temperatures would be 400 feet in error.
But of course, with 'rounding',
things tend to average out