How thunderstorms form, live and die
1The lesson in plain words
Thunderstorms need moisture, unstable air and lift. An ordinary cell lasts about half an hour; multicell storms and supercells can last hours.
Three ingredients
A thunderstorm needs moisture, unstable air that keeps rising once lifted, and a trigger that lifts it. Triggers at sea include cold fronts and troughs, sea-breeze fronts near a hot coast, and air flowing up mountains. Cold air crossing a much warmer sea is heated from below and often makes storms too. Clouds and humidity explained stability; here you see what it produces.
The life cycle
An ordinary single cell has three stages. In the towering cumulus stage, warm air rises and the cloud grows fast, with little rain. In the mature stage, rain and hail fall and drag cold air down in a downdraught beside the rising updraught: lightning, heavy rain and the strongest gusts. In the dissipating stage, the downdraught takes over, rain eases and the cloud spreads out.
How long they last
An ordinary single cell lasts about 30 minutes. Most storms are multicell clusters or lines, where new cells keep forming on one side as old ones die, so the storm lasts hours. A supercell has a rotating updraught, can last hours, and may bring large hail, violent gusts and tornadoes. Do not judge the danger from one cell's life: watch the whole group.
The anvil
At the top of a mature storm the rising air spreads out under the stable layer above, making a flat, spreading anvil. Strong upper winds blow the anvil far ahead of the storm. Lightning can strike from it many kilometres from the rain. A sharp, hard-edged top or a dome bulging above the anvil shows a strong updraught.
When and where at sea
Over land, storms peak in the afternoon after the sun has heated the ground. Over a warm sea they are often commonest at night and in the early morning. In the Mediterranean, a warm sea in late summer and autumn feeds heavy storms. Read the forecast for words such as thundery showers or isolated thunderstorms before you leave.
2Key data
- Ingredients
- Moisture, unstable air and a lifting trigger
- Life cycle
- Towering cumulus → mature → dissipating
- Ordinary single cell
- Lives about 30 minutes
- Most dangerous stage
- Mature: updraught and downdraught side by side
- Over a warm sea
- Often commonest at night and early morning
3Key points
- 1
Moisture, unstable air and a trigger make a thunderstorm.
- 2
The mature stage brings lightning, heavy rain, hail and the strongest gusts.
- 3
An ordinary single cell lives about 30 minutes; multicell storms and supercells can last hours.
- 4
Lightning can strike from the anvil far ahead of the rain.
- 5
Over a warm sea, storms are often commonest at night.
4Frequent mistakes
Wrong: Thunderstorms only happen on hot summer afternoons.
Right: Over the sea they are often commonest at night. Cold fronts and cold air over a warm sea bring them in any season.
Wrong: An ordinary single cell lasts half an hour, so the storm will be gone soon.
Right: New cells can keep a multicell cluster or line going for hours. A supercell can also last hours.
5Practice case & exam keys
Off the Balearics in September, isolated thunderstorms are forecast overnight. Your crew say the clear evening rules them out and any storm will be gone in 30 minutes. What is wrong with that assessment?
Show the answer
A warm sea can support night-time storms despite a clear evening. Mediterranean late summer and autumn favour this pattern. Thirty minutes describes an ordinary single cell; multicell storms and supercells can last hours. Lightning can also reach ahead of the rain.
Exam keys
- Ingredients: moisture, instability, trigger.
- Life cycle: towering cumulus, mature, dissipating; ordinary single cell about 30 minutes.
- Mature stage: lightning, hail, heavy rain, strongest downdraught gusts.
- Over warm seas, storms often peak at night and early morning.
Finish this unit
When you've read it, match the terms, then answer three questions. Get them all right to finish the unit; opening the page alone doesn't count.
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