A thunderstorm is unstable air let loose. Every thunderstorm needs three things, and you have met all of them: moisture, instability, and something to lift the air to its cloud base.
Key idea
A thunderstorm grows from moisture (a high dew point), instability (surrounding air that cools fast with height) and a lift (heating, a front, a mountain or converging winds). A single storm passes through three stages: the cumulus stage (all updraft, the cloud towers upward), the mature stage (rain and a cold downdraft begin beside the updraft; lightning and the heaviest rain) and the dissipating stage (the downdraft takes over and cuts off the warm, moist air). A single storm often lives less than an hour.
Why it works
Once lifted to its cloud base, a parcel in unstable air is warmer than its surroundings and keeps rising; condensation releases latent heat that keeps it warmer still. The updraft builds a tower up to the top of the troposphere, often 10 to 15 km, where the rising air spreads out into the flat anvil. Meanwhile droplets and ice grow big enough to fall. Falling rain drags air down with it, and some of the rain evaporates into drier air, chilling it; cold air is dense, so it plunges: the downdraft. When the downdraft spreads under the whole storm, it chokes off the updraft's supply of warm, humid air, and the storm dies.
Worked example: Riding the updraft
A storm's updraft rises at 10 m/s from a cloud base at 1.5 km to a top at 12 km. How long does the air take to rise from base to top?
- Distance: $12 - 1.5 = 10.5$ km = 10,500 m.
- Time: $10{,}500 \div 10 = 1050$ s, about 17.5 minutes.
Answer: About 17.5 minutes
Worked example: Why is the top flat?
Why does a mature thunderstorm spread into a flat anvil near 12 km?
- Above the troposphere lies the stratosphere, where temperature stops falling with height and starts rising.
- That layer is extremely stable: a rising parcel there is soon colder than its surroundings.
- So the updraft stops rising and spreads sideways: the anvil.
Answer: The stable stratosphere acts as a lid
Common mistake
Thinking a storm's cold downdraft is wind blowing down from high above. It starts inside the storm: falling rain and hail drag air down, and rain evaporating into the drier air beneath the cloud chills it, making it dense enough to plunge. That is why the gust of cool air and the sharp drop in temperature arrive just before the heavy rain. Check: the air in a downdraft is rain-cooled, so the temperature falls as the outflow arrives.
Storms that live much longer than an hour, like squall lines and supercells, keep their updrafts and downdrafts apart, often because the wind changes with height. Supercells are the subject of the tornado lesson.