Storms and weather.

Step 1 of 6

Thermals

The Sun heats the ground far more than it heats the air, so the air is warmed from below. A bubble of warm air over a sunlit field, a thermal, is lighter than the cooler air around it and floats up. As it rises the pressure on it falls, so it expands and cools, always by 9.8 °C for every kilometre. It keeps rising as long as it is warmer than the air around it.

The Scene is a block of farmland 24 km by 16 km and the air above it to 16 km. The air aloft is fixed; only the Ground temperature, the Dew point and the Wind shear change. It is a cool day now, 20 °C.

Step 2 of 6

The surrounding air’s gradient

On a hot day the thermals stir the lowest kilometres until the air there cools with height almost as fast as a thermal does. That rate, the lapse rate, is the first ingredient of a storm: the steeper it is, the more readily air rises. In the chart beside the Scene the black line is the surrounding air, and the amber line the rising air.

Step 3 of 6

The cloud base

Every thermal carries the water vapour of the air it left. Its dew point is the temperature at which that vapour will start to condense. As the thermal rises and cools, it closes on its dew point by about 8 °C every kilometre. At the height where it reaches it, the vapour condenses into droplets and a cloud begins. Every thermal reaches that height together, which is why cumulus clouds have flat bases, all at one level.

Step 4 of 6

Without moisture

It is a hot day now, 33 °C, with a dew point of 18 °C: a cloud towers up to the tropopause. Predict what will happen if the air is dry, at exactly the same heat.

Step 5 of 6

The fuel

Cloudy air cools more slowly than dry air as it rises, about 4–7 °C per kilometre instead of 9.8, so it can stay warmer than its surroundings all the way up. In the cut-open cloud, the updraft’s amber beads brighten from the cloud base upwards: the brighter they are, the warmer the air is than its surroundings. Forecasters measure that energy as CAPE, the amber area in the chart.

Step 6 of 6

Wind shear

Wind shear is how much the wind changes with height: in the Scene, the difference between the wind at the ground and the wind 6 km up, shown by the arrows at the left. Rain and hail falling from the cloud drag cold air down with them, and it spreads along the ground as a gust front. Build the storm is at An hour on, and the shear is 0 m/s.