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Empirical cloud forecasts

EMPIRICAL 'RULES' FOR LOW CLOUD

General note:

When computing cloud heights from surface (i.e. screen) data, don't try and be too clever: the nearest 200ft or so is reasonable - the higher the cloud base, the more likely the error, because the methods below take no account of the humidity structure above the surface, and subsequent mixing (i.e. entrainment / detrainment) with the cloud environment as convective over-turning will alter the character / base / thickness of the cloud.

CLOUD HEIGHT: [ Cumuliform only ]

If T = screen (air) temperature and Td = air-mass dewpoint (degC),
then
base of Cu (in feet) = [ T - Td ] * 410
... sufficiently accurate to use 400: i.e. (difference T-Td) * (4) * (100) [ which is easier to do in your head!]
base of Cu (in metres) = [ T - Td ] * 125
... sufficiently accurate to use 120: i.e. (difference T-Td) * (12) * (10) [again, easier to do in the head.]
base of Cu (in kilometres)  =  [ T - Td ] / 8
... this gives approximately the same result as that for the 'metres' version above.

(remember, if using the screen dew point at the start of the day, this will change: often it will be lower during the afternoon as the low-level air is thoroughly mixed with air that has been dragged down from aloft, but during the post-dawn period, and for a little while afterwards, the dew-point could increase; this is particularly the case when there has been mist, fog or a heavy dew formation at the end of the night. Also, precipitating cloud bases tend to be lower, as the rain / snow will moisten up the below-cloud environment.)

CLOUD HEIGHT: [ Stratus / low-base Stratocumulus only ]

If T = screen (air) temperature and Td = air-mass dewpoint (degC),
then
base of St {or low Sc} (in feet) = [ T - Td ] * 400
(as above, use [difference] * [4] * [100])
base of St {or low Sc} (in metres} = [ T - Td ] * 122
(as above, use [difference] * [12] * [10])
(Precipitation such as drizzle or fine rain will moisten up the below-cloud environment and therefore lower the cloud base - perhaps significantly.)

For Stratus forming over a cooling land mass (i.e. inland over Eastern England on an east or northeasterly airflow), the approximate (very crude in my opinion) cloud base as the cloud forms is given by [ 75 * 10m wind speed ] (base in feet, wind in knots).

CLOUD AMOUNT: [ Cumuliform cloud ]

As a very rough guide, and given no tendency to thick Sc formation, then if the air-mass RH IN THE CLOUD ENVIRONMENT is around 50%, amounts will average out at half-cover (i.e. FEW/SCT, TEMPO BKN)

If the RH is around 75% , the amounts will average out at more than half-cover (i.e. BKN).

[ Stratocumulus formed by the spreading out of Cumulus ]

A reasonably accurate estimate can be made by dividing the relative humidity (U) of the air WHEN THE CLOUD FIRST FORMS by 6, and subtracting 6 from the answer. The resulting value will signify the number of tenths of sky likely to be obscured when convection is at a maximum. (This method dates from pre-WWII days, hence the reference to 'tenths')

So % cover = 10 * ( U / 6 ) - 6

(this implies that for U < 36%, any Sc Cugen should clear; U >~ 80%, and near-full cover can be expected to be maintained, perhaps only clearing late in the afternoon / early evening - depending upon the thickness of the cloud.)

As with methods above, this only uses the screen values and the cloud environment will dictate the exact evolution.