What is Weather? - Introduction

High Pressure

Low Pressure

Snow

Thunderstorms & Tornados

Tropical Cyclones

Clouds

Precipitation

Reading Charts

Severe Weather

Learn About Seasons

Thunderstorms & Tornados
Thunderstorms can occur any time of the year but occur most frequently in hot and humid conditions - typically in the Summer. Thunderstorms occur due to the atmospheric conditions allowing the hot and humid air to rise and condense into large clouds. This sounds similar to the way normal shower clouds develop. However, thunderstorm clouds, known as Cumulonimbus, just take it that bit further and grow much taller than the standard Cumulus (read more about clouds later on). Lightning in a storm is caused by electricity discharging to the grown or to other clouds. Thunderstorm clouds grow very tall which aids the build up of electrostatic forces. When electrostatic charges build up in the cloud, electrons jump from the cloud to the ground or to other clouds. The ground or another cloud is positivly charged so the electons jump to where there is opposite charge (the law of static electricity).

Tornadoes occur an estimated 50 times per year in the UK - not all are spotted and reported. In the UK, the tornadoes occur less violetly than in the USA because we have less extreme air sources near us. Meteorologists still do not know exacly how or why tornadoes form but they estimate several conditions which help the tornado to develop. Tornadoes form from supercell clouds. These are a form of thunderstorm clouds, known as Cumulonimbus but are much taller and contain an area organised rotation within them - known as mesocyclones. The tornado begins when a downdraught of air is forced to the ground due to a period of very heavy rain or hail. This downdraught forces the mesocyclone closer to the ground and after just a few minutes, a funnel is formed. Once the funnel touches down, it is renamed a tornado and strong winds spiral around the column of air sucking debris into it and causes destruction wherever the tornado strikes. Tornadoes can last from several minutes to well over an hour depending if there is a continuous supply of moist air. After time, the downdraughts will supply the tornado with cool surface winds which will weaken and dissipate the tornado.

Tropical Cyclones
There are several names that all mean a Tropical Cyclone - these names vary from where they are located. A hurricane is the name of a storm system that has developped over the Atlantic or Eastern Pacific Oceans. In the South Pacific and Indian Oceans they are called "cyclonic storms" and in the Western North Pacific and Philippines they are called "typhoons".

These cyclones develop because there are very warm seas and oceans in parts of the world that allow moist air to rise and condense. Water takes longer to heat up and cool down than air so it takes utill late Summer before the seas and oceans are warm enough to fuel the developpment of these cyclones. However, the temperatures vary in different parts of the World. The exact conditions needed for these cyclones are still not yet known but temperatures of 26-27c in seas and oceans and a rapid cooling as the air rising helps to release latent heat.

In the centre of a cyclone is a gap called the "eye". If the eye passes over a specific location, very calm and still conditions will arrive and it would almost seem like the storm is over - but you would be mistaken to think that. The eye can last from minutes to hours depending on the speed of the storm. Once the eye has passed over, the storm will instantly return to full power once again. The main different is that the wind will be coming from the opposite direction. This is often the most destructive part of the storm because if there have been trees or buildings battered in one direction, if the winds come from the opposite direction, then the tree or buildings will become weak and collapse.

It is very hard to forecast exact locations of cyclones because they often shear in different directions during movement and there have been many known cases where the storms change direction at the last minute. The strength of cyclonic winds can reach in excessive of 190mph in extreme cases and are divided into seperate catagories to dertermain how damaging and strong the cyclone is. Below is the outline - Note that every storm is unique:

Category 1: 74-95 mph - Minor Damage
Category 2: 96-110 mph - Damage to Rooves, Windows, Trees
Category 3: 111-130 mph - Danger to Life, Damage to Houses/Mobile Homes, Storm Surges
Category 4: 131-155 mph - Severe Damage to Buildings, Severe Flooding, Danger to Life
Category 5: 155 mph+ - Major Storm, Buildings Destroyed, Severe Flooding, Evacuations/Deaths

Clouds
A cloud is formed from water vapour condensing in the lower atmosphere due to the cooler temperatures present. There are many variations in cloud shapes and structures and meteorologists study the clouds in order to know what types precipitation will fall from them. The specific name for studying clouds is called nephology and is just one aspect of meteorology. I will briefly explain the main types of clouds and some of the differences:

Cumulus: These are clouds you may see nearly everyday. They are small, fluffy clouds which often don't even produce any rain. They are also known as fair weather clouds and are a sign of very limited unstability in the atmosphere when very small. These clouds can often develop into much larger and taller clouds in warm/hot and humid conditions. There are several stages a cumulus cloud takes untill it reaches a thunderstorm - for example one of them is called Cumulus Congestus which is a taller and larger form of a cumulus cloud but not anywhere near the right size for a thunderstorm cloud. The cumulus cloud forms usually produce showers or thunderstorms and less organised rain.
Stratus: These clouds usually produce longer spells or organised light rain. They are very layered in shape. They can sometimes produce heavier spells of rain but usually Nimbostratus brings the heavy rain.

Cirrus: These clouds are formed very high up in the atmosphere, typically 23,000 feet or above. At these heights, water cannot be in liquid form so usually contain ice crystals. These look very thin and swirly and on their own don't indicate any real unstability in the atmosphere. However, if there is lot of thick cirrus in the troposphere and streak in a single direction across the sky, then it very often indicates an organised frontal system or developping storm.

I have only briefly touched on clouds as there is so much to nephology. You can learn in depth studies of clouds from places like WikiPedia.

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