Hi Will, I don't know if you remember many years ago sending me a little
simple rule of thumb for estimating the different probabilities of
snow risk, based on adding the DP and temp together. Well I've used it
ever since and it has proved to be very accurate on many occasions,
today being one such.
I realise there are other more sophisticated factors, evaporative
cooling etc. but it has nevertheless proved extremely useful
Thanks,
Dave

Dave Cornwell wrote:

Hi Will, I don't know if you remember many years ago sending me a
little
simple rule of thumb for estimating the different probabilities of
snow risk, based on adding the DP and temp together. Well I've used it
ever since and it has proved to be very accurate on many occasions,
today being one such.
I realise there are other more sophisticated factors, evaporative
cooling etc. but it has nevertheless proved extremely useful
Thanks,
Dave

I'm intrigued now Dave .. share it with us all please.

Hi Will, I don't know if you remember many years ago sending me a
little
simple rule of thumb for estimating the different probabilities of
snow risk, based on adding the DP and temp together. Well I've used it
ever since and it has proved to be very accurate on many occasions,
today being one such.
I realise there are other more sophisticated factors, evaporative
cooling etc. but it has nevertheless proved extremely useful
Thanks,
Dave

I'm intrigued now Dave .. share it with us all please.

==== Will's rule of thumb ====

Hope he doesn't mind me re-posting his post....

"I have a rule of thumb which has worked for over 30 years!
Just add the temperature and dewpoint together in the rain and call the sum
S.
If S is =7 prob turning to snow in the next half-hour is 10%
S =6 prob is 20%
S =5 prob is 30%
S =4 prob is 40%
S =3 prob is 50%
S =2 prob is 60%
S =1 prob is 70%
S =0 prob is 80%

(S 7 prob is NIL)

E.g. T 5.0 D -2.0 gives S=3 and a snow prob of 50%
T 3.0 D 3.0 gives S=6 and a prob. of 20%
T 7.0 D -6.0 gives S=1 and a prob. of 70%

HTH
Will"

Joe

JCW wrote:
Hi Will, I don't know if you remember many years ago sending me a
little
simple rule of thumb for estimating the different probabilities of
snow risk, based on adding the DP and temp together. Well I've used it
ever since and it has proved to be very accurate on many occasions,
today being one such.
I realise there are other more sophisticated factors, evaporative
cooling etc. but it has nevertheless proved extremely useful
Thanks,
Dave

I'm intrigued now Dave .. share it with us all please.

==== Will's rule of thumb ====

Hope he doesn't mind me re-posting his post....

"I have a rule of thumb which has worked for over 30 years!
Just add the temperature and dewpoint together in the rain and call the sum
S.
If S is =7 prob turning to snow in the next half-hour is 10%
S =6 prob is 20%
S =5 prob is 30%
S =4 prob is 40%
S =3 prob is 50%
S =2 prob is 60%
S =1 prob is 70%
S =0 prob is 80%

(S 7 prob is NIL)

E.g. T 5.0 D -2.0 gives S=3 and a snow prob of 50%
T 3.0 D 3.0 gives S=6 and a prob. of 20%
T 7.0 D -6.0 gives S=1 and a prob. of 70%

HTH
Will"

Joe


---------------------------------
Beat me to it! Thanks Joe, yes that's the one. Or for the youngsters who
like online stuff similar here based on RH:-
http://www.sciencebits.com/SnowProbCalc

(plus a link to the theoretical equations)
Dave

In article ,
Dave Cornwell writes:
Or for the youngsters who like online stuff similar here based on
RH:-
http://www.sciencebits.com/SnowProbCalc

(plus a link to the theoretical equations)

Presumably using dry-bulb and dewpoint, or dry-bulb and RH, or dry-bulb
and wet-bulb are all alternative ways of doing what's effectively the
same calculation. (Wet-bulb temperatures seem to have rather gone out of
fashion for some reason. It's surprising, since if the wet-bulb is below
0C you know at once that anything frozen isn't going to melt.)
--
John Hall

"Whenever people agree with me I always feel I must be wrong."
Oscar Wilde

in 372979 20130211 103728 John Hall wrote:
In article ,
Dave Cornwell writes:
Or for the youngsters who like online stuff similar here based on
RH:-
http://www.sciencebits.com/SnowProbCalc

(plus a link to the theoretical equations)

Presumably using dry-bulb and dewpoint, or dry-bulb and RH, or dry-bulb
and wet-bulb are all alternative ways of doing what's effectively the
same calculation. (Wet-bulb temperatures seem to have rather gone out of
fashion for some reason. It's surprising, since if the wet-bulb is below
0C you know at once that anything frozen isn't going to melt.)

Snowing steadily here*, temp = 2.2, DP 0.9 and rh 91%

Those valies in http://www.sciencebits.com/SnowProbCalc
say "Very slim change of snow, but don't count on it!"

* : Waterlooville, 50m asl)

"JCW" wrote in message ...
Hi Will, I don't know if you remember many years ago sending me a
little
simple rule of thumb for estimating the different probabilities of
snow risk, based on adding the DP and temp together. Well I've used it
ever since and it has proved to be very accurate on many occasions,
today being one such.
I realise there are other more sophisticated factors, evaporative
cooling etc. but it has nevertheless proved extremely useful
Thanks,
Dave

I'm intrigued now Dave .. share it with us all please.

==== Will's rule of thumb ====

Hope he doesn't mind me re-posting his post....

"I have a rule of thumb which has worked for over 30 years!
Just add the temperature and dewpoint together in the rain and call the
sum
S.
If S is =7 prob turning to snow in the next half-hour is 10%
S =6 prob is 20%
S =5 prob is 30%
S =4 prob is 40%
S =3 prob is 50%
S =2 prob is 60%
S =1 prob is 70%
S =0 prob is 80%

(S 7 prob is NIL)

E.g. T 5.0 D -2.0 gives S=3 and a snow prob of 50%
T 3.0 D 3.0 gives S=6 and a prob. of 20%
T 7.0 D -6.0 gives S=1 and a prob. of 70%

HTH
Will"

Joe

Thanks Dave and Joe. Glad it has worked for you. Too warm at my location
with S=8 for most of yesterday.

Will
--
http://www.lyneside.demon.co.uk/Hayt...antage_Pro.htm
Will Hand (Haytor, Devon, 1017 feet asl)
---------------------------------------------

On Feb 10, 6:39*pm, Dave Cornwell wrote:
Hi Will, I don't know if you remember many years ago sending me a little
* simple rule of thumb for estimating the different probabilities of
snow risk, based on adding the DP and temp together. Well I've used it
ever since and it has proved to be very accurate on many occasions,
today being one such.
I realise there are other more sophisticated factors, evaporative
cooling etc. but it has nevertheless proved extremely useful

imple rule # 1.

When it is cold enough to snow it won't unless it looks like misty
weather and then gets warm.
Hence the saying:
"Warm enopugh to snow."

"Bob Martin" wrote in message
...
in 372979 20130211 103728 John Hall wrote:
In article ,
Dave Cornwell writes:
Or for the youngsters who like online stuff similar here based on
RH:-
http://www.sciencebits.com/SnowProbCalc

(plus a link to the theoretical equations)

Presumably using dry-bulb and dewpoint, or dry-bulb and RH, or dry-bulb
and wet-bulb are all alternative ways of doing what's effectively the
same calculation. (Wet-bulb temperatures seem to have rather gone out of
fashion for some reason. It's surprising, since if the wet-bulb is below
0C you know at once that anything frozen isn't going to melt.)

Snowing steadily here*, temp = 2.2, DP 0.9 and rh 91%

Those valies in http://www.sciencebits.com/SnowProbCalc
say "Very slim change of snow, but don't count on it!"

Interesting.
As a purely theoretical exercise, a 0% RH gives a melting
temperature of 10.5°C.
--
Col

Bolton, Lancashire
160m asl

On 11/02/2013 10:37, John Hall wrote:
In article ,
Dave Cornwell writes:
Or for the youngsters who like online stuff similar here based on
RH:-
http://www.sciencebits.com/SnowProbCalc

(plus a link to the theoretical equations)
Presumably using dry-bulb and dewpoint, or dry-bulb and RH, or dry-bulb
and wet-bulb are all alternative ways of doing what's effectively the
same calculation. (Wet-bulb temperatures seem to have rather gone out of
fashion for some reason. It's surprising, since if the wet-bulb is below
0C you know at once that anything frozen isn't going to melt.)

I have one of those two-a-penny electronic weather stations. It gives
me the air temperature, relative humidity and dewpoint.

To apply Will's formula is a simple matter of noting two readings,
adding them together, and seeing from Will's list what the snow
probability is. And the consensus of opinion here seems to be that this
works well enough for most practical purposes.

Unless you actually have a psychrometer or a properly exposed wet-bulb
thermometer, how easy is it to obtain the wet-bulb temperature without
solving what I believe is a fairly complex equation or resorting to
tables or even one of those special slide rules?

Need I say more?

[P.S. Will's formula is also a lot more accurate than the "Precipitype"
predictions you see, some of which seem to rely mostly on the 850hPa
temperature (= -3C rain; -4C or -5C sleet; = -5C snow) regardless of
the temperature and humidity structure below.]

--
- Yokel -

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