How Do You Read a Weather Map?

Step 1 of 5

The curved lines are pressure contours.

Every point on one line has the same air pressure, exactly like the height contours on a hill map. They never cross.

Step 2 of 5

Close lines mean strong wind.

Air flows from high pressure toward low. Where the contours crowd together the change is steep over a short distance, and the wind is fierce.

Step 3 of 5

Highs are dry, lows are wet.

In a high, air sinks — warming, drying, clearing the sky. In a low, air rises, cools and makes cloud. That is the whole forecast in one word.

Step 4 of 5

Fronts are where two air masses meet.

A front is the boundary between warm and cold air. Triangles mean cold air advancing; half-circles mean warm air advancing.

Step 5 of 5

Fronts move, so the map is a film not a photograph.

Systems usually travel at thirty to fifty kilometres an hour. Knowing which way the fronts are heading turns today's chart into tomorrow's forecast.

The units are hectopascals; around 1013 is average, below 980 is a serious storm, above 1030 is a settled spell.