Pressure Systems and Synoptic Meteorology
This domain covers anticyclones, depressions, troughs, ridges and cols, and how the surface chart translates into weather. It is the domain where you must read a pattern of isobars and state wind, cloud, visibility and pressure tendency.
Isobars and pressure gradient
- Isobars are drawn at fixed intervals, typically 4 hPa on surface analyses.
- Closely spaced isobars mean a strong pressure gradient and strong wind; widely spaced isobars mean light or variable wind.
- Pressure tendency over three hours indicates development: a fall of 3 hPa or more suggests a deepening low and increasing wind; a steady rise suggests a building anticyclone.
Anticyclones and ridges
- High pressure with subsidence, surface divergence and descending air aloft.
- Weather: light or variable wind, clear skies or a sheet of stratocumulus beneath the subsidence inversion, haze, mist and radiation fog in winter, and smooth air above the inversion.
- A ridge is an elongated extension of high pressure; it brings settled weather but often the same low-level haze and fog.
- Blocking highs can persist for days and stall frontal systems on their flanks.
Depressions and troughs
- A mid-latitude depression is a low with converging surface flow, ascent, and a warm sector between cold and warm fronts; pressure falls as it approaches.
- Families of depressions travel eastward with the polar front; secondary lows can deepen rapidly and produce severe winds.
- A trough is an elongated area of low pressure. It usually carries cloud and precipitation, and an upper trough with a cold pool aloft produces instability showers and thunderstorms.
- A col is the saddle between two highs and two lows: very light variable wind, and a strong tendency to fog or low stratus.
Thermal systems
- Heat lows form over strongly heated land in summer and are shallow; they give dust, haze and convective cloud rather than frontal weather.
- Cold highs (for example over the Siberian plateau in winter) give extreme cold, strong inversions and dense fog.
Worked example
A chart shows a 980 hPa low with isobars 4 hPa apart and 200 km between them. Another chart shows the same 4 hPa spacing but 400 km apart. The first has twice the pressure gradient and therefore roughly double the geostrophic wind. Combine this with a 3 hPa fall in three hours and the forecast is for a rapidly deepening system with gale-force winds and heavy frontal rain.
Traps that cost marks
- Assuming a high always means clear skies; stratocumulus sheets and fog are common beneath the inversion.
- Confusing a col with a ridge, or a trough with a col.
- Forgetting that a deepening low gives a falling pressure tendency and strengthening wind, not just cloud.