Predicting a storm in the UK can be a hit and miss affair. Often the thunderstorms that occur in this country last a lot less longer than those in countries such as the USA. However, using the naked eye you can start to learn how to predict thunderstorms.

Of course, predicting takes time and practice, and we are only going to cover a few of the issues that may help you predict a storm. Often using resources such as those on this website will help you determine whether a storm is brewing or heading your way. The information on this page is courtesy of Stormchasers UK.

One of the most noticeable factors of any thunderstorm is humidity. In this country, thunderstorms are normally associated with areas of low pressure and cold fronts. They get their energy from moisture in "lower levels" of the atmosphere and need a good temperature or height gradient. This means that the air above the surface needs to be much cooler than any air at the surface. The temperature and height gradient allows warm, moist air at the surface to rise.

Provided the air surrounding the rising parcel of warm moist air is cooler it will continue to rise until the air parcel eventually cools to the temperature of the surrounding air. As the air parcel rises into the lower pressure air (air pressure decreases with height) some of the moisture condenses forming clouds. Depending on the Height/Temperature differential clouds can grow to extreme heights, often as much as 8 miles and its these towering "Cumulus" clouds that turn into Thunderstorms and "cumulonimbus" clouds.

The area of warm rising air within the Thunderstorm is called the "Updraft" and for the storm to survive it needs a constant flow of moist warm air from below (in a severe storm the updraft can be in excess of 100mph). As the cloud grows the water vapour in it will condense into small droplets, once the droplets grow to a size where they can no longer be held aloft by the flow of the updraft the water will fall as rain. In a typical British storm this rain will fall down through the updraft, some of it evaporating as it falls.

The evaporation cools the updraft and cuts off the storms supply of warm air and the storm commits suicide and dies. If you are near a Thunderstorm you can often feel the cold air falling out of the storm as it approaches, this is called a gust front.

Funnel Clouds, which often get reported in this country as Tornadoes occur when a storm starts to form with a strong updraft. This acts like water going down a plug-hole and starts to rotate, the rotation acts like a small centrifuge and a small area of low pressure will form within the rotating "vortex" under the base of the cloud. The drop in pressure causes water vapour to condense under the cloud within the vortex forming a visible "funnel" of cloud. This is called a "Funnel Cloud" it is NOT a Tornado. Funnel clouds don't make it to the ground and as a result do no damage.

This is where things start to get a bit more interesting. If the winds at the surface of the earth are blowing in a different direction or different speed to the winds higher in the atmosphere then the atmosphere is "sheared".

If a storm grows in a sheared atmosphere then it will get tilted over slightly by the wind (which normally increases in speed with height). The tilt means that when it starts to rain (the "Forward Flank Downdraft") the rain and cool downdraft falls away from the storm and not back down through the warm updraft.

As the updraft is not being cooled by the rain the storm can continue to grow. When the updraft meets the Jet Stream winds (at over 8 miles height) the top of the clouds get carried a down wind by the strong 100mph winds. This creates the characteristic "Anvil Cloud" (looks like the top of the storm is squashed ) of a severe Thunderstorm. The Anvil can be carried hundreds of miles ahead of the storm and will often be the first indication of a storm heading your way.

A storm that achieves this sort of height will have a strong updraft, towards the top of the storm the rising air will be below freezing and ice crystal form. These small ice pellets (graupnel) grow while they are held aloft in the updraft eventually falling as hail. It is believed that the hail within a storm rising and falling causes an electrical charge to build and it is this charge that we see as lightning.

Now that the warm updraft and cool downdraft (with the falling rain/hail) have become separated all sorts of interesting things start to happen under the storm.

The area under the storms starts to behave like a weather system in miniature. Where the cold downdraft meets the warm air feeding the storm a small "cold front" forms. Depending on the strength of the downdraft compared to the updraft many things can happen. The cold air may cut off the warm air and the storm may die or the cold front may increase the intensity of the storm by pushing the warm air upwards over the top of the cold air. Another thing that can happen is that the warm and cold air may brush alongside each other. Imagine holding a pen between the palms of your hands, one hand is the warm humid (moist) air the other is the cold "downdraft" air, as the one moves relative to the other the pen rotates. Similar things can happen under a severe storm, instead of a pen rotating its the air that starts to spin just above the surfaceof the land. Some Tornadoes actually form near the surface and then grow upwards towards the base of the storm. If the rotation near the surface is strong enough and it is under or near the bottom of the storms updraft (the "Updraft Base") the spinning air can be drawn up into the storm by the updraft creating a spinning column of air called a Tornado.

This type of Tornado (which is one of the more common UK Tornadoes) is called a "Landspout". A Landspout is a true Tornado and can do significant damage with winds in excess of 170mph. However the most violent types of Tornadoes form under an altogether different type of Thunderstorm.. a "Supercell".

Supercells are very rare (perhaps only one every couple of years in the UK). In the USA most of the big Tornadoes form under Supercells. The first storm to be identified as a Supercell was in fact in England.

So what do I look for? Well most strong/severe storms are driven by the winds in the upper levels of the atmosphere and will be tilted over to some degree. The first sign of an approaching storm may be an anvil cloud, this may look like a broad thin cloud very high in the sky. Next might be the rain (as this is being blown downwind of the main updraft) followed by any hail that the storm might be producing. At the same time as the rain and hail you may see the first lightning. The area where the rain and hail is falling is the "core". Finally if you are in the storms path the updraft will pass over. The updraft will often have a flat base and may be dark depending on the size of the storm. It is here normally that any funnel clouds or Tornadoes will form.

Quick Tips
» Is it a warm sticky day?
» Are there cumulus clouds in the sky?
» Are towering cumulus clouds forming?
» Is there cumulonimbus in the sky?
» Is the area under the cumulonimbus becoming gray?
» Does that area look like rain?
» If so, it could be a storm brewing...