What is a thunderstorm?

Back <<

A thunderstorm is nothing more than millions of droplets of water vapor suspended in the atmosphere that produces lightning, rainfall, winds, hail or tornados. Thunderstorms form when instability in the atmosphere causes large clouds called cumulonimbus to form that reach great heights in the atmosphere. It is the great heights at which the storms grow to that cause a phenomena called lightning. Towards the tops of the thunderstorms the water droplets become super cooled to temperatures as low as -60 degrees celsius. Supercooled means that the drops of water have not turned into a solid (ice) however it is still at the very cold temperatures. At the very top of a thunderstorm there is a region known as the anvil. This is where the thunderstorm cannot reach anymore height in the atmosphere due to the stratosphere being stable so the cloud only has one way to go now instead of straight up. The cloud starts to spread out along the bottom of the stratosphere, much like steam spreads out along a roof in the kitchen. This is often the high whispy looking cloud you see at the top of a thunderstorm. Dust and other matter gets sucked into the thunderstorm when it is growing in its size and it is at this point that if a supercooled water droplet comes in contact with a piece of dust, it will immediately turn to a solid (ice).

Towering cumulus clouds signal the buildup of an afternoon thunderstorm

If enough of these ice particles clump together, a small hailstone forms and under the gravity of the earth, it will fall back towards the earth. If the storms updrafts are strong enough, the hail stone will be caught in the updraft of the storm and the hail stone will be sucked back up into the tops of the storm where more ice particles and water droplets will freeze together to make the stone larger. This process continues until the hail stone is too large for the updraft of the storm to support and the hail stone falls to earth.

Vertical wind shear or uplift is the winds rising vertically from the earths surface into the atmosphere. The strength of these winds are generally governed by temperature differences between the different atmospheric levels. Basically, the larger the difference between the surface and say 500mb (5.5km / 17,000ft) the faster the "parcel" will rise. The speed at which this parcel of air will rise from the surface into the atmosphere is quite often called the "lapse rates" amongst storm chasers. A strong lapse rate will generally have very warm temperatures at the surface and rather cold air at the 500mb level. During the Sydney hailstorm back in 2005, the Sydney area recorded hailstones up to 12cm in diameter. The vertical wind speed needed to sustain such large hail would of had to have been in excess of 250km/hr.

Lightning occurs when the ice in the clouds rub by each other causing friction that in turn charges the water droplets and clouds that they are in. Once enough charge builds up from the ice particles rubbing against each other, a spark occurs between another cloud or the ground which is an opposite charge. This is the lightning that you see. Thunder is caused when the air super heats to around 30,000 degrees celcius and then suddenly cools back down again. This produces a massive pressure wave which we hear as thunder.

Picture yourself trying to blow air underneath a cricketball to keep it from falling and hitting you in the face, this is impossible. The only way for the cricket ball to be suspended in the air is if you use a very strong fan to force the ball up. This is the same principal as how hail in a storm works. So, what causes these massive updrafts to suspend the hail?

As the earth warms up during the day, small thermals of warm air begin to rise into the atmosphere. As this happens, it carries moisture up with it. The higher you go into the atmosphere, the colder it gets. Because of the temperature of the air that is rising into this colder air, it accelerates upwards faster due to the larger temperature difference surrounding this "parcel" of air. Think about if you have tipped a cup of hot coffee out on the lawn on a very cold winter morning, the steam from the coffee will rise quicker into the air due to the higher temperature difference. The same effect happens with clouds except on a much larger scale.

First signs: early morning cumulus building up

Of course as the air parcel rises into the atmosphere, it will begin to cool down as it ascends into the atmosphere until it is the same air temperature as the surrounding air. This is said to be "stable" and the updraft will no longer rise. The hotter the earth is, the more updrafts can ascend into the atmosphere. The same goes with the colder the atmosphere is, the greater the temperature is between the parcel of air and the surroundings. More moisture in the updraft also helps to get clouds going faster.

Towering cumulus: as more heat becomes available during the day, the clouds grow vertically in height.

We don't want too much moisture in the upper atmosphere though because as the updraft cools, it releases heat into the atmosphere. If there is too much moisture in the atmosphere, it will not cool the atmosphere down as much and will help to stabilize the atmosphere to prevent any updrafts from going too high.

So, the things that help to get a thunderstorm going is heat on the earths surface, moisture, cold atmosphere temperatures and dry air in the atmosphere about 10,000 feet high. If these conditions are met then we can get a thunderstorm in motion! Small cumulus clouds start to form in the morning as the earth heats up. As the day goes on, these cumulus grow larger and become more organised. By the late afternoon, there is only a few large cumulus left which keep towering upwards and become a thunderstorm eventually.

Afternoon thunderstorm starting to build

Our thunderstorm cant do very much at the moment because it aint moving anywhere! this is where wind shear comes in. Wind shear is the horizontal wind that blows across the earths surface and in the atmosphere. This helps to push the storms along and keep them going. Without wind shear, a thunderstorm would simply sit in one location, build up, start to precipitate, cool down the earth that it is sitting on and eventually die because it will run out of heating to keep it going.

Fully mature afternoon thunderstorm

So there you have it, that's all the basic ingredients to a thunderstorm. Of course in real life, things aren't this simple and other factors such as inversions, caps and triggers play an important part in a thunderstorms development.