Thunderstorms — Page 238, Lesson 194

I want to walk you through the development of a single-cell thunderstorm. This is the classic thunderstorm that forms from one main updraft, and we break it into three distinct stages: the initial stage, the mature stage, and the dissipating stage. Let's start with the initial stage. In the initial stage, several small cumulus clouds combine to form one large cumulus cell about 5 nautical miles across. That's roughly the width of the cloud at this point. Inside this cell, we have strong upcurrents — that's air rising rapidly — at speeds of 1000 to 2000 feet per minute. Exceptionally, those upcurrents can reach up to 6000 feet per minute. As this air rises, air from the sides and from below is drawn in to replace the lifted air, and that inflow causes turbulence. The initial stage lasts about 15 to 20 minutes. Now, the mature stage begins the moment precipitation occurs. That's the key trigger: when rain or hail starts falling from the cloud, the storm has reached the mature stage. The rain or hail causes strong down currents — descending air — of up to 2400 feet per minute. This descending air also brings cold air down to lower levels. Here's an important mechanism: those down drafts warm initially at the saturated adiabatic lapse rate, or SALR. Because they warm very slowly at that rate, the descending air stays colder than the surrounding air, which makes it sink even faster. There's another factor that aids these down drafts: some of the rain evaporates, and that evaporation absorbs latent heat from the air, making the air even colder and more dense. The intensity of this cooling can lead to the formation of what we call the gust front. Meanwhile, the up currents in the mature stage remain strong and can be up to 10,000 feet per minute. The cloud tops may rise at 5000 feet per minute or more. There can be moderate to severe turbulence in, under, over, and all around the cloud. At the bottom leading edge of the storm, there can be a roll of stratocumulus cloud, and a strong gust front can be experienced up to 13 to 17 nautical miles — that's 24 to 32 kilometres — ahead of the storm, and that gust front can be up to 6000 feet in depth. Below the cloud, you can expect a squall and associated windshear. Also at this stage, downbursts may occur — these are either microbursts or macrobursts, which we discuss later in the chapter. Another key feature of the mature stage: rising and falling water droplets produce a considerable build-up of static electricity. Typically, a positive charge builds up at the top of the cloud and a negative charge at the bottom. These charge build-ups eventually lead to lightning discharge and thunder. The gust front I mentioned is a characteristic of the mature stage — it's produced by the force of the descending air, and it may extend 13 to 17 nautical miles (24 to 32 km) ahead of the storm centre. The mature stage lasts for a further 15 to 20 minutes. Now let's move to the dissipating stage. At this stage, there is heavy precipitation and extreme turbulence. Thunder and lightning may possibly still occur at this stage. The cloud extends up to the tropopause, where it is spread out by the upper wind to form an anvil shape. At these high levels, the cloud thins to form cirrus cloud. Large variations in static charge in and around the cloud cause discharge in the form of lightning, which can appear in the cloud, from the cloud to the ground, or from the cloud to the air alongside. The dissipating stage lasts for a further 1 and a half to 2 and a half hours. Finally, regarding the movement of thunderstorms: single-cell thunderstorms usually move in the direction of the 10,000-foot wind, which corresponds to the 700 hPa pressure level. However, large storms and newly developed ones may differ from this general rule.