Dew Point And Its Function In Snowy Weather

[greybing]

hello,i have read a few people mention dew point in relation top the chance of snow.I was just wondering if people would explain to me how the dew point affects the chance of snow and its general function in relation to snow.

Edited October 19, 2010 by greybing

[Backtrack]

hello,i have read a feww people mention dew point in relation top the cahnce of snow.I was just wondering if people would explain to me how the dew point affesctsthe chance of snow and its general function in relation to snow

Hello Greybing.

I am not the best person to answer this. That role would be taken by John Holmes. But I can tell you that as a general rule, you need the dewpoint to be at 0C for it to snow!

Here's a Google reference to Dew Point:

The dew point is the temperature to which a given parcel of air must be cooled, at constant barometric pressure, for water vapor to condense into water. The condensed water is called dew. The dew point is a saturation oint.

Which I think means the temperature in which precipitation can fall as snow. I'm sure John Holmes will have a post for you which will help much more. Sorry I couldn't be of more help!

Backtrack.

Edited October 19, 2010 by Backtrack

[johnholmes]

Uses of dewpoints in weather analysis-especially its use with regard to will it snow?

A fuller explanation is given on this web site and is as good as anyone could give you.

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. It can be measured indirectly using a wet & dry hygrometer (ordinary dry bulb thermometer, and another/adjacent thermometer with its bulb covered in a damp muslin - hygrometric tables or calculator then being used to calculate the dew point, relative humidity, vapour pressure); also by a 'dew-cell' type of instrument that measures relative humidity, from which the dew point can be calculated, or it can be measured directly by a dew-point hygrometer. The screen/surface dew-point temperature is used in air mass analysis, and also in the calculation of night-minimum and fog-point temperatures, as well as being used in the estimation of convective condensation levels, human-comfort indices, probability of snow at the surface etc. Dew point values above the surface (from radio-sonde ascents) are used to define cloudy or potentially cloudy layers etc., in the upper air (see also Frost point).

For help in forecasting snow then the lower it is, put very simply, then the more likely precipitation is to be snow.

There are subtle differences between snowfall from convective clouds, Cumulus (Cu) Cumulonimbus (Cb) and layer type Nimbostratus/Altostratus( Ns and As) but for most purposes it needs to be at zero C or below, better below 0C.

Most rainfall starts life as snow or ice particles at high levels in clouds and changes to rain as the particles fall through the cloud when its above zero C. A better indicator of how likely snow is than the surface temperature is the dewpoint. This is because it only changes slowly and by relatively small amounts both in a 24 hour cycle and on perhaps long track from its source.

Forecasters use dewpoints not just for trying to decide will it snow or rain but in many more ways. Will it be foggy, will there be showers or thunderstorms, how much cloud will there be, where on a weather map are the weather fronts and others.

Forecasting snow is even more difficult than forecasting rain as there are so many factors to take into account.

Try this link into the Net Weather Guides.

http://forum.netweat...-forecast-snow/

this one will give ideas on what synoptic set ups can give snowfall

http://forum.netweat...-winter-setups/

and in general do read the Net Weather Guides as they will help both at a basic and more advanced level for most things discussed on the forum

enjoy

jh

Do ask questions as that is the way many of us increase our knowledge.

Edited October 19, 2010 by johnholmes

[Guest]

Dew points are fascinating....I'm still amazed every June when we get a high air temp with a winter style dew point. I've seen air temps of +26C with a dew point of -2C.

Please explain that one John! :)

[Isolated Frost]

Dew Point

I posted this on 'Cold Spell Discussion'. Quite interesting. Type in a (or your) temp and humidity, go on Celsius, and press calculate. Important for this winter possibly?

[johnholmes]

Dew points are fascinating....I'm still amazed every June when we get a high air temp with a winter style dew point. I've seen air temps of +26C with a dew point of -2C.

Please explain that one John! :)

very dry air for one reason or another, could be a high with a large area of descending air and more likely air flowing over a mountain, the fohn effect to some extent. There are other reasons but at this time of the morning my brain cells are not really operating!

[greybing]

thankyou

[Isolated Frost]

http://upload.wikimedia.org/wikipedia/commons/f/f6/Ice_Storm_Chart_words_small.png

This helps aswell.

[summer blizzard]

Effectively, a large range between the dew point and air temperature means dry air, and a low range means high humidity, if the dew point is within 1C of the air temperature, fog can form. It is also important to note that the dew point can never be warmer than the air temperature, the dew point is the reason why you get snowfall with an air temperature of 2C.

[Thundery wintry showers]

The dew point also accounts for why we sometimes get snow at surprisingly high temperatures in spring. The airmass over the UK may support dewpoints of -3C, with overnight minima just below freezing, and then as the sun emerges during the day and heats up the surface, we see a significant rise in air temperature but no significant change in the dew point, giving respective values of, say, a dry bulb temperature of +7C and a dew point of -3C, and the relative humidity falls considerably as a result.

So, when a shower comes over, the low relative humidity and the sharp temperature gradient just above the surface enables falling snowflakes to reach the ground without melting significantly. In heavy showers, though, it doesn't snow at 7C for very long because the downdraughts associated with the showers and absorption of latent heat cause the temperature to fall very quickly as the shower gets underway.

As some places saw on the 11th May this year, you can even get sleet & snow in the heavier showers on days with a maximum temperature of 11-12C, because as the shower gets underway the surface temperature plummets to around 4-6C and if the dewpoints are low enough this drags the "snow level" down to the surface.

Edited October 27, 2010 by Thundery wintry showers

[greybing]

The dew point also accounts for why we sometimes get snow at surprisingly high temperatures in spring. The airmass over the UK may support dewpoints of -3C, with overnight minima just below freezing, and then as the sun emerges during the day and heats up the surface, we see a significant rise in air temperature but no significant change in the dew point, giving respective values of, say, a dry bulb temperature of +7C and a dew point of -3C, and the relative humidity falls considerably as a result.

So, when a shower comes over, the low relative humidity and the sharp temperature gradient just above the surface enables falling snowflakes to reach the ground without melting significantly. In heavy showers, though, it doesn't snow at 7C for very long because the downdraughts associated with the showers and absorption of latent heat cause the temperature to fall very quickly as the shower gets underway.

As some places saw on the 11th May this year, you can even get sleet & snow in the heavier showers on days with a maximum temperature of 11-12C, because as the shower gets underway the surface temperature plummets to around 4-6C and if the dewpoints are low enough this drags the "snow level" down to the surface.

,often have i experienced events such as you describe above and wondered about the mechanisms that enable them,thankyou for this information.

Edited October 27, 2010 by greybing

[Guest]

very dry air for one reason or another, could be a high with a large area of descending air and more likely air flowing over a mountain, the fohn effect to some extent. There are other reasons but at this time of the morning my brain cells are not really operating!

Thanks John, I'd always assumed a fohn effect was localised to the immediate vincinity of mountains, Glasgow Airport is 15-20miles from the nearest proper hill...but the high temp/low dewpoint effect does happen in a northerly airflow most Junes. I just find it very interesting that we can get sub-zero dew points in very high air temps.