How to read a weather map

There's method in the madness of your local TV weathercaster when he/she is rambling on about curved, studded lines hanging from a little red “L”. The fronts on a basic weather map were devised by Bergen School meteorologists early in the twentieth century. It took years for them to be accepted by scientists in much of Europe and the US, and they didn't become part of pop culture until TV and newspapers made them a fixture in the 1950s. Here's a meteorological phrasebook to help you interpret surface weather maps.

? “L”denotes low pressure,”H” high.

? The thin lines sometimes shown as wrapping around Hs and Ls are isobars – lines that connect points of equal surface pressure. By and large, the surface wind blows along the isobars, but it tilts slightly inward toward low pressure. Because winds are driven by pressure contrast, they're almost always strongest where the isobars are most tightly packed. Quickly intensifying lows or strengthening highs may induce the wind to cross isobars at a sharper than usual angle from high toward low.

? Lows, highs and isobars are drawn on a map only after the pressure at each station is adjusted to a sea-level equivalent.The idea is to keepthe lower pressure that's always present at high-altitude stations from skewing the entire map (and producing a misleading “L” across the mountains day after day).

? Cold fronts are usually drawn in blue with triangular barbs; warm fronts, in red with semicircles. (The barbs and semicircles allow fronts to be recognized in black and white.) Occluded fronts, less common than the first two, occur where a cold front overtakes a warm front; they're traditionally drawn in purple with rectangles. Troughs of low pressure that have little temperature change across them, such as dry lines in the southern US plains, are sometimes depicted as dashed lines or as fronts spiked with brown rectangles.