Snow/uppers and low pressure

[smithyweather]

Lately it has cooled down significantly with yesterday being my 4th day of snow falling this season.

 

However, uppers are not really that cold with freezing levels at 700m yesterday. We had snow lying to 150m and max temps of 2/3c.

 

Looking at next week bar today it is looking sub 5c. Yet uppers are only -5c at lowest. Why is this? Someone on the scottish thread mentioned low pressure, but I don't understand what effect that would have?

 

And surely if the freezing level is 700m there would be no snow lying below that? Explain then why we had lying snow to 150m yesterday? 

Edited December 20, 2013 by smithyweather

[Nights King]

Evaporative cooling brings the freezing level down if only temporarily with heavy ppn.

 

West country saw snow also although it melted quickly after on high ground

[Summer of 95]

850 hpa temps are what "uppers" show; that height will be lower when pressure is. When it's 950hpa at sea level, 850 won't be very high up (about 800-900m?).

[smithyweather]

850 hpa temps are what "uppers" show; that height will be lower when pressure is. When it's 950hpa at sea level, 850 won't be very high up (about 800-900m?).

What do you mean? Are you saying that the 850 temps are not always at 5000feet? Explain to me how to work out what height it is please.

 

Evaporative cooling brings the freezing level down if only temporarily with heavy ppn.

 

West country saw snow also although it melted quickly after on high ground

No it wasn't evaporative cooling. It maxed out at 2c. 

Edited December 20, 2013 by smithyweather

[Nights King]

What do you mean? Are you saying that the 850 temps are not always at 5000feet? Explain to me how to work out what height it is please.

 

No it wasn't evaporative cooling. It maxed out at 2c. 

 

 

Oh okay it certainly played a part down south because it was around 5c as trough rolled in.

[Nick L]

What do you mean? Are you saying that the 850 temps are not always at 5000feet? Explain to me how to work out what height it is please.

 

 

The 850hPa temperature is the temperature at the level where the air pressure is 850hPa. On average, the 850hPa level is approximately 1500m. However, when it's lower pressure at the surface, this level is lower. So, say you've got -5c 850hPa temperature, this -5c level will be lower during lower pressure than higher pressure.

 

A hypothetical extreme case is where the surface pressure is at 850hPa, then the 850hPa temperature will be at the surface!

[johnholmes]

 

 

No it wasn't evaporative cooling. It maxed out at 2c. 

 

why was it not evaporative colling-can you give your reason(s) please?

[smithyweather]

The 850hPa temperature is the temperature at the level where the air pressure is 850hPa. On average, the 850hPa level is approximately 1500m. However, when it's lower pressure at the surface, this level is lower. So, say you've got -5c 850hPa temperature, this -5c level will be lower during lower pressure than higher pressure.

 

A hypothetical extreme case is where the surface pressure is at 850hPa, then the 850hPa temperature will be at the surface!

I get you now.

What you have to do is convert the pressure into altitude at sea level and then the pressure at 850 into an altitude and then calculate it from there. Or something like that. 

[gary1365]

the 850 charts we use in uni have the altitude printed on them aswell.

[Su Campu]

Pressure drops by 1 hPa for every 8 m increase in altitude (roughly, in the lower troposphere - it's less higher up). The average ISA 850 hPa height is 1,450 m, with a surface pressure of 1,013.25 hPa. In very cold airmasses the pressure fall with height is greater because the air is denser and the pressure levela are spaced closer together, and vice versa for warm airmasses.

In the case of low pressure (or depression, i.e. a depression in the pressure level surfaces through the lower atmosphere) all of the pressure levels are brought down towards the ground, so the 950 hPa level, which would normally be at around 500 m, could now be at the surface, and the 850 hPa level will be lowered by roughly the same amount (so now at around 950 m!).

Given this 500 m drop, the 850 hPa temperature can now afford to be warmer than we'd normally look for for snow, and we should really be looking at a new level 500 m higher up for our "850hPa uppers" (i.e. 500/8 m = ~63 hPa higher, or the 787 hPa level - roughly!).

People blindly look at the 850 hPa level for their "uppers" when in fact there is a lot more to it than that. In any case, the 850 hPa chart will also give the height of that level too, but a rough calculation of it is to take the difference between the pressure at the surface and 850 hPa and multiply by 8 m.

e.g. 960 at the surface so 850 hPa level is now at roughly

(960-850)x8 m = 880 m.

Edited December 22, 2013 by Su Campu