Pressure Systems, Synoptic Charts and Weather Forecasting
What the exam tests
This domain tests pressure systems, synoptic chart interpretation and the use of forecasts.
Core rules
- Isobars join equal mean sea level pressure; closer spacing means a stronger pressure gradient and stronger wind. Highs are associated with subsidence and generally fine weather; lows with convergence, cloud and precipitation.
- In the Southern Hemisphere, stand with your back to the surface wind and lower pressure is on your right (Buys Ballot). In the Northern Hemisphere it is on your left.
- Ridges and troughs distort the isobar pattern; a trough favours lifting and showers, a ridge favours sinking air and clearing.
- Always check the chart valid time, the pressure reduction (QNH) and whether the forecast is area, aerodrome or trend.
Worked example
A chart shows a low with tightly packed isobars and a ridge extending east. Near the low, expect strong winds, showers and low cloud; under the ridge, expect lighter winds and improving weather. The reasoning is gradient strength plus vertical motion: convergence near the low, subsidence under the ridge.
Traps
Reading isobar spacing backwards; applying Northern Hemisphere circulation rules in the Southern Hemisphere; treating a forecast as valid outside its stated period; ignoring troughs and ridges.