Now put it together: station models, isobars, highs and lows, and the fronts of a mid-latitude storm. A forecaster reads a surface map like a story about to happen.
Key idea
On a surface map, isobars are drawn every 4 hPa around H and L centers. Winds circle lows counterclockwise and highs clockwise (Northern Hemisphere), crossing the isobars toward the low, strongest where the isobars crowd. Rain falls near the low and along its fronts. In the middle latitudes, the whole pattern usually travels from west to east, steered by the winds aloft, often 500 to 1,000 km a day.
Why it works
The map shows forces and motion together. Isobar spacing tells you the pressure gradient, and so the wind speed. Friction and Coriolis tell you the direction: counterclockwise and inward around a low. Converging air at the low and lifting along fronts tell you where the clouds and rain are. And because the jet stream blows from the west, systems drift east: the weather to your west is a preview of your weather.
Worked example: A forecast from the map
A town in Missouri has a low 600 km to its west, with a cold front trailing south of the low. The system moves east at 40 km/h. What happens over the next day?
- The low reaches the town's longitude in about $600 \div 40 = 15$ hours.
- Before then, southerly winds bring warm, humid air; pressure falls.
- As the cold front passes: a band of storms, a wind shift to the northwest, then colder, drier air and rising pressure.
Answer: Warm and humid, then storms, then colder
Worked example: Counting isobars
The innermost isobar around a low is 996 hPa, and a town lies three isobars farther out. Isobars are drawn every 4 hPa. About what is the pressure at the town?
- Each isobar outward from a low is 4 hPa higher.
- Three isobars: $996 + 3 \times 4 = 1008$ hPa.
Answer: About 1008 hPa
Worked example: The falling barometer
Near a town the pressure rises 2 hPa for every 100 km toward the east of an approaching low, and the pattern moves east at 50 km/h. About how fast does the town's barometer fall?
- Each hour the pattern moves 50 km, half of 100 km.
- So the town's pressure drops by half of 2 hPa: about 1 hPa per hour.
Answer: About 1 hPa per hour
Common mistake
Thinking weather moves at random. In the middle latitudes most systems travel from west to east, steered by the westerlies aloft, so the weather to the west is a good preview. Check: in a series of maps a day apart, the same low and fronts appear farther east each day.