Reading weather charts

The essentials in two minutes

1. Isobars are the contour lines of pressure: tight, strong wind; the wind follows them, low pressure on its left, and converges towards the lows at the surface.
2. A front’s symbols (semicircles: warm; triangles: cold; alternating: occlusion) are always on the side towards which it is moving.
3. Six shapes to recognise: low, anticyclone, V-shaped trough, U-shaped ridge, slack pressure gradient, col.
4. The 500 hPa chart plots the height of the 500 hPa surface (≈ 5,500 m; “552” = 5,520 m): low, cold air (trough); high, warm air (ridge).
5. The front of a trough deepens the lows and makes the weather deteriorate; the back clears it. A cut-off low is almost invisible at the surface and explosive aloft.
6. Always cross-check the surface and the upper air: an anticyclone under the front of a trough is fine weather that is doomed, even with the barometer set fair.
7. T850 gives the freezing level (≈ 1,500 m + 150 to 160 m per degree); the T850 − T500 difference gives an idea of the instability (under 22: stable; over 28: stormy if moist).
8. Four typical situations — westerly flow, bise, southerly flow with foehn, slack pressure — and a method in five steps: centres, fronts, upper air, cross-check, one written sentence, on a series of charts.