Guide To.....Charts - 850hPa and temps

[PB22]

Understanding what the charts show - 850hPa and Temperature:

The white lines are contours of 850hPa isobaric surface.

The black lines are isotherms of 850hPa temperatures (deg C).

The colour shading is used to determine the 850hPa temperatures. 850hPa level is around 1.5km high, usually above the atmospheric boundary layer. Therefore there is no diurnal temperature variation, and the underlying surface such as a cool sea won't affect it's temperature. This is why the 850hpa temperature is used to distinguish air masses and thus to locate cold and warm fronts.

Generally, the 850hPa temperature can be related to the 2m temperature by adding around 10 degrees, and use it instead of surface maximum temperature. There is a scale on the right hand side of the chart.

However, this only works at low altitudes, and only when the sun is heating the ground; not at the sea or near to the coast. It doesn't apply at night-time either, and often not in the winter.

Paul Barber - December 2003

Edited May 26, 2007 by Paul B