Thunderstorms

This section concerns the subject of thunderstorms. These are a worldwide phenomenon (with some areas experiencing a lot more than others!). At any one time there are no less than 7,000 different thunderstorms in progress right across the planet with up to 8 million lightning discharges every day. This equates to some 100 discharges per second! This is, it is believed to help balance out the electrical energy right across the World. There is one location in South America which has almost continuous thunderstorms – with this being the place with the most thunderstorms in the World! Lightning travels at 60 million M.P.H. and individual discharges can be around 60,000 Amps (sometimes up to 200,000 Amps) with up to 1 billion volts of electricity. They can be 6 times hotter than the surface of the sun and the air inside a lightning bolt can reach a temp. of 50,000F. Thunderstorms generally travel in the same direction as the prevailing jet stream at the time. Some thunderstorms can be very intense, slow moving & localised. This sort can produce flash flooding with a lot of rain falling in a very short time. Hail and gusty winds or squalls can also occur (any time that there is hail, then thunder & lightning is also possible). Lightning strikes can result in the production of small ionized balls of gas plasma (usually blue in colour) to be given off. These usually drift and then gradually disperse & disappear over a matter of about a minute or so. Another (fairly rare) phenomenon is ball lightning. This usually occurs in the immediate vicinity of a very close, or overhead thunderstorm and happens often after an especially vicious lightning discharge. Electrified air or plasma (ionized gas) merges into a ball like structure and again this can drift and soon disperses. In rare cases, they can touch an ‘earthable’ object and either connect (by a ‘lightning’ discharge such as into a metal railway line) or even explode. These have been known to severely burn skin in some people, but in others, it can harmlessly pass by them. The distance of a thunderstorm can easily be calculated by using the ‘5 second rule’ (e.g. 5 seconds for 1 mile, 10 seconds for 2 miles, etc, etc.) Count the seconds from seeing the lightning to hearing the thunder and you can then work out how far it is away! Thunder rarely travels for more than 12 miles! There are two main sorts of thunderstorms – airmass & convergence. Airmass storms are the result of a clash of different airmasses. One of the most common is the undercutting of warm or very warm & humid air by colder air. This often happens as a cold front moves eastwards, forcing the warm air in front of it upwards. The air is often unstable to begin with (often shown up as lines or clusters of medium level Altocumulus castellanus). These clouds can sometimes produce their own lightning and/or rain showers if the air is unstable enough. Some thunderstorms can be very heavy or severe. The ‘lapse rate’ of temperature is also important. The faster that the temperature decreases with height, so the more likely are thunderstorms to occur. Deep, cold polar air often has large singular cumulonimbus cells (thunderclouds) and these can produce storms too. These often only produce 1 or 2 discharges of lightning though. A lesser known form are ‘convergence’ thunderstorms which are usually the result of air rapidly rising in the centre of a deepening depression or a hurricane. Shower troughs (shown as a solid black line across the isobars on a synoptic chart) can also produce thunderstorms. These can also be associated with a ‘line squall’. These are shown up on rainfall radar as a sharp line of very heavy rain and/or hail as a part of a larger area of rain. Occasionally there can be ‘microbursts’. These are intense bursts of rain underneath the cloud, often associated with cloud downdrafts. These can cause localised flooding and also be an extreme danger to aircraft, more especially when they are taking off or landing. Cases have been known of aircraft being pushed back down by these and subsequently crashing. Air traffic usually actively avoid going any where near full cumulonimbus (Cb) due to the dangers involved, not least, lightning, hail & moderate to severe air turbulence. A commercial airliner is usually struck by lightning at least once per year, but with special discharge bars fitted, any major structural damage is usually very limited and often the aircraft are entirely unaffected by a lightning strike. (Strong winds in any form can be a hazard to air traffic and (in the U.S.) any crosswind that is 46 M.P.H. or more is inadvisable for any flight passage in that particular Country). In the U.K. an airliner can generally land safely with crosswinds of around 35 M.P.H. (some ‘crabbing’ may have to be carried out – this involves kicking the rudder (or adjusting electronically) to twist the landing aircraft into the prevailing crosswind and then, at the last second, kicking, or electronically adjusting the rudder back to land safely). When a cumulonimbus (CL9) is forming, there are very strong updrafts, with these sometimes reaching 60 M.P.H. (and occasionally a combination of both updrafts & downdrafts). In it’s dying stages the whole cloud produces steady downdrafts and it is these which often show up as ‘mammatus’ (udder or dome like projections on the underside of the cloud). Even these can be strong and gusty in nature. The cloud to look out for thunderstorms is the classic ‘cumulonimbus’. In it’s mature stages this cloud is shaped like an ‘anvil’ and when it is ‘fibrous’ at the top it means that it is ‘iced’ or has formed ice crystals and at this level, it often stops growing any more. This is when it is most likely to produce thunderstorms, rain, hail & strong winds or squalls. A new distance record for a single lightning discharge occurred during April 2020 & was measured at travelling in excess of 477 miles across 3 separate U.S. States, namely Texas, Louisiana & Mississippi. The tops of mature iced cumulonimbus cells are usually limited to around 15Km above the surface. Having said that, a number of years ago, an astronomical observer in the plains of the U.S.A. was out one night with his telescope when he saw (at the top of a huge ‘thunderhead’ towards the horizon) a series of discharges in an upward direction. This eventually turned out to be the first fully described occurrence of ‘sprites’. These are still being scientifically investigated as they may well be a form of a lightning discharge actually into the edge of space. This phenomena has also been observed from the International Space Station (ISS).

Here are two pages of a very old March 1942 document. The science of thunderstorms and lightning has moved on immeasurably since those days, but the basic principles are still very much the same! (both pages courtesy of Newnes Practical Mechanics.)

stithiansweather.co.uk/thunder-lightning-the-phenomena-of-a-thunderstorm-and-its-effects-page-1/ : Page 1

stithiansweather.co.uk/thunder-lightning-the-phenomena-of-a-thunderstorm-and-its-effects-page-2/ : Page 2