What do we need for snow?

Slippin Jimmy

This is going to sound really silly for you's weather folks. But what do we need for snow to fall?

MarkR

Hmm... Winds from the north I suppose, to get it nice and cold. That and clouds. (Me obviously no scientist - I don't even play one on tv!)

Makes me think though, what's all this about "It's too cold for snow". Does that make any sense?

Snowbie

Nothing wrong with that Donny.
Simple but there is many a factor.
1.Below 2c anything thing above is wet snow/sleet.
2.Dewpoint at or below 0c
3.A consistent Northerly blast with low 850 temps.
4.Rely on Polar low formation off our North coast in the very N Atlantic or trough lines moving southwards in the airstream.
5.A very strong easterly wind or an easterly airflow pooling from a cold siberian origin moving up against moist air coming in from atlantic for frontal snow to occur.(this is a mid to late Winter event)
6.Also showers building over the Irish sea on the easterly airflow.

There are more factors but these are the main ones put in layman terms.

Snowbie

MarkR wrote:

Hmm... Winds from the north I suppose, to get it nice and cold. That and clouds. (Me obviously no scientist - I don't even play one on tv!)

Makes me think though, what's all this about "It's too cold for snow". Does that make any sense?

Too cold for Snow,some agree and some disagree.
It can be too cold for snow at the poles as the air is so chilled that moisture cannot be held in the air.I think after -30c this happens and residual snow which is blown around in the air makes it look like its snowing when it warms up a tad during the summer.
So what you see from the poles is total ice,permafrost covering.

This point is being argued all the time by scientists.

Actually the lower the dewpoint when its sub zero and related Relative humidity the higher chance there is of snow at higher temperatures.

For instance if theres precipitation at +4c with a dewpoint of -4c you might well get snow and dry snow at that.It would just want to be heavy to stick and it would go about melting very quickly when it stopped falling.

Snowbie

Well in a way Tristrame thats true,but in any form of precipitation the RH rises regardless of temp and in this instance the dewpoint and RH would rise on the onset of snow when clouds gather.

For snow to occur,lets say we had a good northerly blast over a number of days in the heart of winter,the obvious temp would be low and the dewpoint will be subzero where we are in this country.IE:we would be too far away from the North facing windward coast and in the crisp clear air giving those low DP.

On the other hand the the North facing coasts would have also low temps but higher DP levels(close to or just above zero) as clouds and snow showers would be more widespread up there.Same too for the east coast if we had an easterly blast.

Meaning the Dewpoint temp is air when cooled and saturated enough when vapour condenses into clouds,the lower the temp and dewpoint the better it is for snow.So DP would be lower in clear conditions and a bit higher in cloudy conditions and higher RH in precip.

To get the proper white stuff,from 2c or lower and the depoint below freezing is a better chance of big snowflakes and powder stuff to occur.
I also saw snow falling at 5c before with a DP of 0c in the rouge showers passing down from the north.
Its all about the right ingreidents at the end of the day.

Strasser

There is a good explanation of what is needed here

http://www.netweather.tv/forum/index.php?showtopic=20474

Takeshi_Kovacs

Snowbie you mention low 850 temps... now i've been following the weather forum here the last 2 years, and while i don't know enough about making informed predictions and the like, i do like the science and stuff behind it all, and i am just wondering what are these '850 temps', as i see they come up a lot in different threads.
I could never figure them out, are they to do with the temp of the atmosphere at some height above sea level or something?

Snowbie

Takeshi_Kovacs wrote:

Snowbie you mention low 850 temps... now i've been following the weather forum here the last 2 years, and while i don't know enough about making informed predictions and the like, i do like the science and stuff behind it all, and i am just wondering what are these '850 temps', as i see they come up a lot in different threads.
I could never figure them out, are they to do with the temp of the atmosphere at some height above sea level or something?

Exactly,the 850mb temps are located in the lower troposphere which is about mile up above sea level.When the temps are at or below 0c,well to put it simple it would be ideal for snow to occur.The lower the temp in or within the boundaries of the isotherms(lines of adjoing temperature)the greater the chance of snow falling.
So within these 850mb temps is where you can forecast rain or snow etc at the surface obviously dependant on the temperature roughly in the troposphere.

Takeshi_Kovacs

Snowbie wrote:

Exactly,the 850mb temps are located in the lower troposphere which is about mile up above sea level.When the temps are at or below 0c,well to put it simple it would be ideal for snow to occur.The lower the temp in or within the boundaries of the isotherms(lines of adjoining temperature)the greater the chance of snow falling.
So within these 850mb temps is where you can forecast rain or snow etc at the surface obviously dependent on the temperature roughly in the troposphere.

Thanks for explanation.. makes a bit more sense now... the 850 mb temps being the part of atmosphere where the snow flakes would start to occur in the cloud..