The important temperature that needs to be considered in relation to the
phase of precipitation is the wet-bulb temperature. This is because each
particle will experience this temperature as it falls through the air, and
not the air temperature itself. The wet bulb temperature is a conservative
property of the atmosphere, namely, it remains constant during adiabatic
changes. It can, however, change when moisture is added to an air sample,
or
the temperature of the sample is changed by a non-adiabatic process, such
as
radiation.

This table shows the relationship between the wet bulb temperature and dew
point and relative humidity. Note, the melting process starts when the
particle experiences a wet bulb of 0.0 C, and for most particles such as
snow flakes, is completed for the majority by the time that the wet bulb
experienced is +2.0 C. For grauple and hail, particles can reach the ground
with higher wet bulb temperatures as they will fall further during the
melting process. Two other factors to consider are the energy taken from
the
air during the melting process, which lowers its temperature, and the
moisture added to the air due to evaporation of the particles, both
processes tending to lower the wet bulb temperature.

Wet bulb 0.0 C
Dry bulb Dew point RH
0.0 0.0 100
1.0 -1.6 83
2.0 -3.4 67
3.0 -5.4 54
4.0 -7.8 42
5.0 -10.6 31
6.0 -14.1 22
7.0 -18.8 14

Wet bulb +2.0 C
Dry bulb Dew Point RH
2.0 2.0 100
3.0 0.6 84
4.0 -0.9 70
5.0 -2.6 58
6.0 -4.6 47
7.0 -6.8 37
8.0 -9.4 28
9.0 -12.5 20
10.0 -16.7 13


Addendum
Just to clarify, when I say that the 'wet bulb temperature is conservative',
I should have made clear that I am referring to the wet bulb potential
temperature (also known as theta-w), as it is this that is conserved. The
wet bulb potential temperature is the wet bulb temperature that the air
would have if it is lifted from its cuurent level until saturation is
reached, and its temperature at that point is transferred to 1000 mbar along
the saturated adiabat. The temperature of falling ice/water particles in the
atmosphere will closely follow this temperature curve.

Also, in para two (above), the last sentence should read 'Two other factors
to consider are the energy taken from the
air during the melting process, which lowers its temperature, and the
moisture added to the air due to evaporation of the particles, the first
which tends to lower the wet bulb potential temperature and the second which
tends to raise it.'

The tables above refer to conditions at standard atmospheric pressure (ca
1013 mbar).


--
Bernard Burton

Wokingham Berkshire.

Weather data and satellite images at:
www.woksat.info/wwp.html

"mattmaz" wrote in message
...
Hello All,

I am a weather novice, there's no doubt of that having read some of
the posts on this group. I have a question regarding Dewpoint
temperature.

What is the effect of the dewpoint temperature on the likelyhood of
any precipitation falling as snow? i.e. my weather station is
currently reporting an external temp of +1.2 deg C but with a dewpoint
of -1.1 deg C, if there was precipitation right now would it fall as
rain or snow based on the figures ? I understand there are other
factors involved too, but I thought I'd ask as I've often wondered.

Thanks in advance,

Regards,
Matt

P.S. Is Eskimo Will snowed in yet? lol