"gb" wrote in message
...
| I'm not sure, but I got this rule of thumb from Will some time ago,
which is | one of many rough guide criteria for snow (or not).
| Add together the Dew point and the temperature = A then if:-
| If A 7 prob. snow is very small
| If A=7 prob. snow = 10%
| A=6? 20%
| A=5 30%
| A=4 40%
| A=3 50%
| A=2 60%
| A=1 70%
| A=0 80% or more

The dew point is the much more important of these two figures and the
formula should be weighted in some way to reflect this. On most occasions
where I live, a temperature of 2C and dewpoint of 1C will result in rain,
perhaps a trace of snow in it if you look very hard. Temperature 3C,
dewpoint 0C will be snow most times. Combining these may produce the 50%
shown above for A=3, but does not really tell the story.
The temperature structure of the atmosphere above is also important. Last
week we had rain with A= -9!
- Yokel - Yokel @ Ashurst New Forest SU 336 107 17m a.s.l.

|Only a layman's inexpert curiosity but:
|At this late date, Yokel, do you remember what were the temp and
|dewpt that gave A= - 9 and rain, not snow ?
|The formula has worked brilliantly so far here ( E.Anglian coast) .
|
|From the forum's FAQ: Dew Point (strictly dew-point temperature) The
|temperature (of an air sample that contains water vapour), to which
|that sample must be cooled (Pressure and humidity content being held
|constant) to achieve saturation with respect to a water
|surface...Frost point (strictly frost-point temperature) The
|temperature (of an air sample that contains water vapour), to which
|that sample must be cooled (Pressure and humidity content being held
|constant) to achieve saturation with respect to an ice surface. (see
|also dew point).
|
|Why would the inventor imagine that adding these two temperatures
|would give an indication of snow; it seems to be an attempt to balance
|two conflicting parameters ?
|And why would dewpoint of a surface be more important than air temp
|through which the precipitation is falling, as you observed
|empirically?

Looking up the records of the "Ashurst Observatory"...

23 December 0700. Recorded minimum -6.3C Current temperature -5C, Dew
Point -6C (actually, that is A= -11) Weather "Rain shower in past hour".
Intermediate notes to the evening obervation record the moderate to heavy
freezing rain or showers during the day.

Admittedly the site is non-standard, the temperature record being a max/min
thermometer in the most sheltered place I can find and the dewpoint being an
electronic temperature / humidity sensor set to record this directly. But
under cloudy conditions (as this was) these readings are reasonably
comaparable with local synoptic observations.

Whether snow survives to the ground depends not just on the air temperature
but also on the fact that evaporation and melting both require heat input.
The drier the air, the more evaporation there is. This evaporation tends to
cool the air, as does melting of the falling snow. The maximum it can be
cooled down to by this means is the dewpoint - the temperature at which
water (or ice if below 0C) will condense onto a surface from the air. So if
the dewpoint is above 0C, water vapour will tend to condense on any ice
precipitation and the latent heat release will accelerate the melting. If
the dewpoint is below 0C, there is a chance, increasing with the amount the
dewpoint is below, that cooling by evaporation of any water that forms will
prevent much of the snow from melting even though the air temperature may be
above freezing. This effect can - if the air is dry enough - allow snow to
reach the ground with air temperatures of 4 or 5 degrees C.

In practice, the heat input from the air will prevent the temperature of a
wet surface from falling as far as the dewpoint. The level to which it will
fall can be measured by a special thermometer which has a normal "dry bulb"
and an additional "wet bulb" with a muslin coat kept moist by a wick into a
water container below. This "Wet Bulb" temperature can also be derived by
thermodynamics and so can be calculated if you know the dry bulb and the dew
point. The "A" formula appears to me to be a simplified attempt to estimate
the wet bulb temperature which, as others on this group will state, is a
more accurate measure of whether snow will melt to rain than the dew point
temperature. But the dew point temeperature is still a better indication
than the standard ("dry bulb") air temperature is.

Freezing rain is a different kettle of fish and requires a certain
temperature structure in the atmosphere. The precipitation will start as
snow, so somewhere aloft there has to be a layer of air warm enough to melt
the snow. The resulting rain then falls into another sub-zero air layer.
Water drops are surprisingly reluctant to freeze if left to themselves -
small cloud droplets can remain liquid down to -40C. Raindrops cannot be
"supercooled" this much but can be cooled several degrees below freezing
before turning to ice.

All this changes once the supercooled drops are disturbed, normally by
hitting the ground, trees, power lines, etc. They will then freeze into
ice, providing the surface they have landed on is cold enough to take away
the latent heat released by freezing. As the water tends to flow a bit
before freezing, this produces a uniquely smooth and slippery form of ice
which causes major transport disruption. Its weight can bring down tree
branches and power lines if the freezing rain or "ice storm" is severe
enough. This intensity of "ice storm" is very rare in the UK, but much more
common on the other side of the Atlantic where serious "ice storms" cause
major damage somewhere in the eastern states in most winters.

What happened in our event is that mild air over the Channel (some coastal
stations were reporting +5C or more while we were reporting -3C) overran a
shallow layer (1500 - 2000 feet) of stagnant very cold air. Showers which
had formed over the Channel were carried inland on this flow, but it could
not disturb the surface cold pool. This led to an ideal temperature
structure for the freezing rain event.
--
- Yokel -
Yokel @ Ashurst New Forest
SU 336 107 17m a.s.l.

"Yokel" posts via a spam-trap account which is not read.