Reading Weather Maps

About this topic

Weather maps (synoptic charts) use symbols and lines to show conditions. Isobars join points of equal pressure: closely spaced lines mean strong wind. H and L mark high- and low-pressure centres; fronts are lines with triangles or semicircles; station symbols give temperature, wind direction and speed, and cloud cover.

High pressure generally means settled, clear weather; low pressure means cloud, wind and rain. Forecasters combine maps, satellite and radar images and computer models to predict changes.

A simple way to read a chart: find the closed circles of isobars and mark the H and L, then look at spacing. If isobars are tightly packed between a high and a low, expect strong winds; if they are far apart, expect light winds. In the Northern Hemisphere, wind blows roughly parallel to isobars with low pressure on your left. A station model uses a small circle shaded to show cloud cover in eighths, a line with feathers for wind speed and direction (each full barb being 10 knots), and numbers for temperature and dew point.

Weather maps are used daily in aviation, shipping, farming, and TV broadcasts, and can be paired with satellite images that show clouds and radar that shows rain intensity. Numerical weather prediction divides the atmosphere into a grid and solves physics equations on supercomputers, with ensembles run many times to show uncertainty. A common misconception is that isobars show temperature; they show pressure, and temperature is shown by isotherms. Another is that a forecast percent chance of rain means rain over that percent of time; it refers to the probability that rain occurs at a given place.