Guide To..... Charts - SLP & 500hPa Height and Temp

[PB22]

Understanding what the charts show - Mean Sea Level Pressure and the 500hPa height and temperature:

The white lines are used to show points of equal atmospheric pressure. On one side of the isobar pressure will be smaller than the isobar value, whilst on the other side its greater. Wind follows roughly the direction of the isobars so that if you walk along an isobar (not literally!) and the wind is blowing onto the back of your neck, the lower pressure is at your left hand side (in the Northern Hemisphere).

The dark grey dashed lines gives the forecast temperature at 500hPa height.

The colour shading is used to determine the 500hPa height (there is a key on the right hand side of the chart). The level of 500 hPa is roughly dividing the mass of the atmosphere in two. It lies near 5km, and its height is normally analysed at intervals of 40 or 80m, corresponding to MSLP isobars at intervals of 5 or 10 hPa.

Paul Barber - December 2003