What Makes a Heatwave?

Step 1 of 5

Normally the weather carries heat off.

Wind moves warm air on and brings cooler air in, and clouds shade the ground. A heatwave starts when both of those stop happening.

Step 2 of 5

Sinking air cannot make clouds.

In a high-pressure dome, air descends slowly. Sinking air warms and dries, so vapour never condenses — which means no cloud and no shade.

Step 3 of 5

Each day starts warmer than the last.

Without cloud the ground bakes all day, and clear nights still leave it warm by morning. The starting point climbs steadily.

Step 4 of 5

Dry ground makes it worse.

Damp soil spends the Sun's energy evaporating water instead of heating air. Once the soil is dry that cooling stops and every ray goes into temperature.

Step 5 of 5

Cities heat up hardest.

Brick, tarmac and concrete store heat all day and release it at night, so a city can stay several degrees warmer than the fields around it.

The hot nights do the real damage — a body needs a cool spell to recover, and a heatwave never gives it one.