As that precipitation falls towards the ground, it exerts a drag on the… — Page 260, Lesson 222

I want to walk you through the behaviour of a thunderstorm once it has formed, and what determines whether it will be a short-lived single cell or a long-lasting, self-sustaining system. Let's start with the key condition. If there is no vertical windshear — meaning the wind doesn't change speed or direction with height — then as the cloud grows and the updraught strengthens, precipitation forms in the upper parts of the cloud. As that precipitation falls towards the ground, it exerts a drag on the updraught. That drag weakens the updraught, and the cloud simply decays. So without windshear, the storm kills itself. But here is where it gets interesting for a professional pilot. For a storm that has the downdraught offset from the updraught — particularly where the updraught is not cut off at the surface by the spreading out of the cold downdraught — it can develop into a self-generating system. That means it can last for hours, independently of any surface heating. So the storm no longer needs the sun to keep it going; it sustains itself through its own internal dynamics. Now let's look at the intensity of these draughts. The updraughts and downdraughts are of comparable intensity, often in close proximity to each other. They frequently reach speeds in excess of 3000 feet per minute. That is already severe. But sharp gusts with vertical speeds of 10 000 feet per minute have been measured. Let that sink in — that is over 100 knots vertically. The horizontal extent of these vertical draughts may, occasionally, be more than a mile. So a single updraught or downdraught can be over a mile wide. The top of a developing cell has been observed to rise at more than 5000 feet per minute. When thunderstorms are associated with frontal conditions, areas of 'line squall' activity can extend for more than 100 miles. So a line of storms along a front can stretch over a hundred miles. The vertical extent of storms varies considerably, but it is not uncommon for them to penetrate the tropopause. Cloud tops can exceed 40 000 feet in temperate latitudes and 60 000 feet in sub-tropical and tropical regions. So in the tropics, these storms can punch well into the stratosphere. Now, the lifetime of an individual cell is usually less than an hour. But a storm system, with new cells developing and old ones decaying, may persist for several hours. So the system as a whole can last much longer than any single cell within it. Let's move to forecasting. Areas where conditions will be favourable for thunderstorm development can usually be forecast successfully several hours in advance. However, it is not possible at present to determine the precise location and distribution of individual storms. So we know a region is likely, but not exactly where each storm will pop up. That said, where up-to-date ground weather radar information is available, useful information on the expected movement of an individual storm can be forecast for periods of up to an hour or so ahead. So radar gives us a short-term nowcast for individual storm movement. Finally, a useful rule of thumb for the UK. As a general rule, the movement of a cumulonimbus cloud is in the direction of the 10 000 foot wind, which is the 700 hectopascal level. But keep in mind that large storms tend to distort wind fields, and the development of new cells will cause variations in this general movement. So the 700 hPa wind gives you a starting point, but you must expect deviations. Let me show you the three stages visually. shows the building stage of a thunderstorm. shows the mature stage. And shows the dissipating stage, where the cloud extends to the tropopause and is spread out by the upper wind.