Why Is It Drizzling At 3 Celcius?

[snowflake]

Hi,

Can anyone explain to me why it continues to rain when the temperatures are so cold?

Last night was even colder, just hovering above freezing, about one degree, and it rained then too.

I would have expected snow at these kind of temperatures.

I have definitely known snow on days at four celcius. How can this be?

Thanks for any replies!

Claire

[londonsnow]

it's about upper level air temperatures. your surface level temps may be low but aloft it isnt as cold. so you end up with rain, sometimes freezing rain.

[snowflake]

Hi Londonsnow,

Thanks for replying. Sorry to sound thick here, but why are the upper air levels warmer than surface air? Especially after the recent days of sub-zero temps?

I thought it was colder higher up?

Claire

[shuggee]

Not to mention the variance in the wet bulb temp, the dew point etc.

[Ross B]

Hi Londonsnow,

Thanks for replying. Sorry to sound thick here, but why are the upper air levels warmer than surface air? Especially after the recent days of sub-zero temps?

I thought it was colder higher up?

Claire

Cold air sinks and heat rises so it can be cold at the surface and warm above that. Therefore warm upper air temps can spread over the top of cold, a little like the start of the cold spell where upper air temperatures were between 0 to +5c but it was very cold and frosty on the ground. The cold temps at low levels can be hard to shift, that is why people often say this in the model topic as most people will just look at the upper air temperatures

(I hope that makes some sense)

[londonsnow]

Cold air sinks and heat rises so it can be cold at the surface and warm above that. Therefore warm upper air temps can spread over the top of cold, a little like the start of the cold spell where upper air temperatures were between 0 to +5c but it was very cold and frosty on the ground. The cold temps at low levels can be hard to shift, that is why people often say this in the model topic as most people will just look at the upper air temperatures

(I hope that makes some sense)

couldn't have put it better myself. thanks ross

[Yeti]

It's not all about upper air temps - unless your dewpoint is at 0 or below, it's almost guaranteed rain. Normally, snow can fall with upper air temps of -5 or below (depending on altitude) but sometimes it can snow LOTS with -3 or -2, even at low levels. However, if you're temp is 1c and dewpoint 1c, it rains. If your temp is 3c, and your dewpoint -1, you have a very good chance of snow

[londonsnow]

it's a good point as made by shuggs and yeti, the dew point is most definitely a factor

[Somerset Squall]

Temp here is 0C and dewpoint is -2C, and it's raining.

[Thundery wintry showers]

It will probably be freezing rain then- rain that freezes as it hits the ground, leading to dangerous walking and driving conditions. It is the result of relatively warm upper air that is moving down from the north. Our weather has been anticyclonic, and often with relatively mild upper air recently, hence rain falling at surprisingly low temperatures.

In most cases 4C is the upper limit for snow although during spring northerlies I've known snow showers fall with a steady temperature of 5-6C, and with a starting temperature as high as 8C. This happens because of a combination of cold upper air, low dewpoints and strong solar heating of the surface.

[Dave J]

A Tephigram for yesterday will probably illustrate it best.

Below is Albemarle's midnight ascent for the night just gone. Ok it's not exactly Lancashire but it's the nearest one we have..

As you can see, at the surface the air temp is 0.2C and dew point -1.2C which you would expect to produce precipitation in the form of snow- however in the profile between approx. 1000ft and 3000ft AMSL there is a wedge of relatively warmer and moist air which acts to melt the snowflakes as they fall through that relatively warmer layer on their way to the surface. The melting snowflakes will fall to the surface as either a mix of raindrops and big snowflakes (that survive the warm layer) or if the cold surface layer is deep and cold enough, result in ice pellets or even the very hazardous and relatively rare freezing rain!

There are other complications such as evaporative cooling which can, over time and with little mixing, cause the profile in the lowest few thousand feet to became isothermal along the 0C line and bring the freezing level to the surface - and with it rain changing over to snow!

It all adds up to make snow forecasting a total nightmare...

Edited January 7, 2009 by Dave J

[Summer of 95]

Following on from this, what exactly determines/sets the dewpoint at a particular place and time? I understand dewpoint being "the temperature to which air must cool to reach 100% relative humidity"- but why do some airmasses have higher dewpoints than others, and why looking at maps can dewpoint vary by 3-5C over a few dozen miles, when the same air mass is present everywhere? (I have seen this phenomenon this week).

I can understand why it rains with a dewpoint 1C and temp 1C as it can't cool below freezing before precip occurs, but why do higher dewpoints stop snow settling?

[Dave J]

Following on from this, what exactly determines/sets the dewpoint at a particular place and time? I understand dewpoint being "the temperature to which air must cool to reach 100% relative humidity"- but why do some airmasses have higher dewpoints than others, and why looking at maps can dewpoint vary by 3-5C over a few dozen miles, when the same air mass is present everywhere? (I have seen this phenomenon this week).

The dew point is effectively a measure of how moist an airmass is, this will vary considerably within a particular airmass particularly over the UK because of the variations between drier inland observation sites and those that are near lakes or by the coast where there are significant moistures sources. Airmasses that originate over the warm Azores (Tm/Tropical maritime) pick up the characteristics of where they are formed and therefore have high moisture content (typically in the summer as high as 15C) whereas those that form over the dry continent of eastern Europe/Russia such as our current Pc/Polar continental airmass, have low moisture content which can be as low as MS10C. Dew points will also fall at night at varying rates as moisture is deposited on surfaces as hoar frost. The dew point can also vary with pressure though this effect is only really noticeable with altitude rather than over distances at the surface, where the variations are an order of magnitude higher.

I can understand why it rains with a dewpoint 1C and temp 1C as it can't cool below freezing before precip occurs, but why do higher dewpoints stop snow settling?

The dew point is the temperature at which evaporation and condensation of water vapour is in equilibrium - it has a very high positive correlation with snowmelt such that the higher the dew point the faster the melting/evaporation of snowcover. This is because the vapour pressure of the snow (frozen water) becomes increasing less than that of air (water vapor) and therefore condensation occurs, which releases latent heat melting the snow (a process which in itself takes latent heat).

This is often observed in the form of low stratus and/or fog developing (depending on surface flow) as positive dew points advect over snow fields.

[Summer of 95]

The dew point is effectively a measure of how moist an airmass is, this will vary considerably within a particular airmass particularly over the UK because of the variations between drier inland observation sites and those that are near lakes or by the coast where there are significant moistures sources. Airmasses that originate over the warm Azores (Tm/Tropical maritime) pick up the characteristics of where they are formed and therefore have high moisture content (typically in the summer as high as 15C) whereas those that form over the dry continent of eastern Europe/Russia such as our current Pc/Polar continental airmass, have low moisture content which can be as low as MS10C. Dew points will also fall at night at varying rates as moisture is deposited on surfaces as hoar frost. The dew point can also vary with pressure though this effect is only really noticeable with altitude rather than over distances at the surface, where the variations are an order of magnitude higher.

The dew point is the temperature at which evaporation and condensation of water vapour is in equilibrium - it has a very high positive correlation with snowmelt such that the higher the dew point the faster the melting/evaporation of snowcover. This is because the vapour pressure of the snow (frozen water) becomes increasing less than that of air (water vapor) and therefore condensation occurs, which releases latent heat melting the snow (a process which in itself takes latent heat).

This is often observed in the form of low stratus and/or fog developing (depending on surface flow) as positive dew points advect over snow fields.

Is it therefore possible that snow (when it falls) finds it so difficult to settle in Shrewsbury because of the River Severn forming a loop around the town, increasing the dewpoint, with hills on 3 sides preventing the moistened air dispersing? This week wasn't the first time I've seen the area around Meole Brace (the furthest part of the "urban" area from the river, lower than many other areas) being the place that snow sticks most readily. If this is the case, Shawbury is a different planet!

Edited January 7, 2009 by Summer of 95

[Snowyowl9]

Had some drizzle with a touch of sleet in yesterday morning at 0.7c

And heres a recent year after the heavy snow, when there was freezing light drizzle after with a SE-ly it didn`t last that long that day a hour or so, in March 2006.

http://www.wetterzentrale.de/archive/ra/20...00120060312.gif

http://www.wetterzentrale.de/archive/ra/20...00220060312.gif

The thing was though temps reached 1.3c but it was still freezing on surfaces in a very cold SE-ly must of been some very low dewpoints or something like that.

Quite an incredible cold spell that was in recent times.