500 charts

[moses]

Could someone please tell me the difference between 500 and 850 charts - sometimes the 500 charts have lots of blue - but there are mild temperatures over the same region

What 850 temps do we need for snow?

What about the figures 530 dam on the 500 charts - what use are they?

Also - the phrase "height rises over greenland" - what is this in relation to the charts?

thanks

Guy

[Paul]

The 500 charts, such as this one shows the heights at 500mb along with sea level pressure.

The colours correspond to the height in metres of the 500hpa level (which is a measure of air pressure).

The 850's such as the one below show the temperature at the 850hpa level, with -5 and below a benchmark for a reasonable chance of snow.

When people talk of height rises over greenland they're talking about the 500hpa charts, height rises mean that real high pressure may be building..

All the charts are available here:

http://www.netweather.tv/index.cgi?action=nwdc;sess=

[BrickFielder]

At the surface of the earth where we live presure is typically around 1013hPa, but depending on the weather systems then it can vary between 920hPa and 1060hPa. High pressure systems where the pressure is above 1000hPa are anticyclonic (winds go clockwise in the northern hemisphere) and are associated with calmer weather. Low pressure systems where the pressure is below 1000hPa are cyclonic (winds go anti clockwise in the northern hemisphere) and are associated with active windy and wet weather.

As you go up through the atmosphere the pressure decreases so at about 4500 feet the pressure is about 850hPa and at 18000 feet the pressure is about 500hPa.

Round about 300hPa (30000 feet) is the tropopause which is the boundary between the troposphere and stratosphere.In terms of temperatures required for snow at 850hPa then you should be looking for temperatures below -10C, although snow can fall with 850hPa temperatures at -5C it is a much more marginal and depends on a number of complex factors.

Geopotential heights (dam lines) show the height in the atmosphere of a particular pressure. So for a air pressure that you would typically find at 500hPa a dam line of 530 shows that the 500hPa typical pressure is to be found some 1000 feet lower. This shows that the air below this level is cold and you would expect a trough below it. Typically the 528 dam is a good indicator that air is cold enough below 500hPa for snow.The 500hPa eights tend to be usefull for determnning where ridges and troughs will move to. Height rises over greenland is often taken to mean high pressure is building there.

Probably the best indicator for snow is actually the wet bulb temperature being below 0C with precipitation falling.