Guide To.....Charts - SLP, Cloud (%) & 500-1000hPa thick

[PB22]

Understanding what the charts show - Mean Sea Level Pressure; Cloud (%) and 500-1000hPa thickness:

The white lines are points of equal atmospheric pressure.

The coloured lines are referring to thickness. This is a useful in finding warm and cold air in the lower 'half' of the troposphere (the surface to about 5.5km), where a large part of the 'weather' is to be found. Bulk movement of warm air (high values - 564DAM) and cold air (low values - 510DAM) can be followed by looping the fields. Look for areas where the thermal contrast is increasing as these are regions of potential major baroclinic development (i.e. low pressure development).

It is advisable to avoid working out things like maximum temperatures, snow-level etc. from these charts.

The monochrome shading is used to give cover forecasts. These give an output relating to the percentage of sky forecast to be covered with cloud. Hence 100% would be overcast, and 50% (equivalent to 4 oktas) predicts a half cloudy sky. This is useful for picking out where the frontal systems are and determining where they are most likely to be the most intense from the higher percentages given (scale on right hand side). *NOTE - This is for medium/high level cloud only. Low level cloud is not included.*

Paul Barber - December 2003

Edited May 26, 2007 by Paul B