Weather System

British rainfall

Depressions pass over Britain frequently. They form in the Atlantic and move east across the country, bringing changeable weather.

Why does most rain fall in the west?

  • Highland areas receive more rain – many of these are in the west.

  • Prevailing winds come from the south west carrying moisture from the Atlantic Ocean.

Which types of rainfall commonly affect Britain?

Relief rainfall

  1. Prevailing winds bring warm, moist air to the western British Isles.

  2. Air is forced to rise over high areas.

  3. Air cools and condenses.

  4. Clouds form and it rains.

  5. Air descends on the other side of the mountains.

  6. It warms up and therefore becomes drier.

Frontal rainfall

The British Isles are affected by a number of different air masses. When warm and cold air meet, a depression forms:

  1. When a cold polar air mass meets a warm tropical air mass they do not mix – they form fronts.

  2. The colder air mass is heavier than the warmer air mass, therefore the lighter, warmer air rises over the top of the heavier, colder air.

  3. As the warm air is forced to rise it cools. Also, the warm air is in contact with the cold air along the fronts, and this also cools.

  4. Condensation occurs and clouds form.

  5. Rain occurs along the front.

In the UK, depressions often follow a similar pattern. First, a warm front passes over, bringing rain and then warmer air. Then a cold front follows, bringing more rain and cooler air.

On synoptic maps, warm fronts are shown by a red line with red semi-circles. Cold fronts are shown by a blue line with blue triangles.

Convectional rainfall

When the land warms up, it heats the air above it. This causes the air to expand and rise. As the air rises it cools and condenses. If this process continues then rain will fall. This type of rainfall is very common in tropical areas but also in areas such as South East England during warm sunny spells.

Weather hazards

A weather hazard is an extreme weather event that threatens people or property.

Weather hazards include:

  • tropical storms

  • tornadoes

  • droughts

  • storms and floods

  • fog

Tropical storms

Tropical storms are given different names in different parts of the world.

A tropical storm is a hazard that brings heavy rainfall, strong winds and other related hazards such as mudslides and floods.

Tropical storms usually form between approximately 5° and 30° latitude and move westward due to easterly winds. The Coriolis force sends them spinning towards the poles.

In most areas, tropical storms are given names. The names are alphabetical and alternate between male and female. This makes storms easier to identify, especially when they are close together.

It is hard to predict the path of a tropical storm, and therefore difficult to manage an adequate evacuation of an area if needed.

How do tropical storms form?

  • Hurricanes need a lot of heat to form, which is why they usually occur over tropical seas (at least 26°C).

  • The sun is close to the equator, providing energy to heat the ocean.

  • The warm ocean heats the air above it causing it to rise rapidly.

  • Water evaporates quickly from the hot surface of the ocean, so the rising air contains great amounts of water vapour.

  • The rising air starts to spin (clockwise in the northern hemisphere)

  • The centre of the storm – the eye – is calm.

  • As the air rises it cools, condenses and forms towering cumulonimbus clouds.

  • The rapidly rising air creates an area of intense low pressure. The low pressure sucks in air, causing very strong winds

  • Once the storm moves over land it starts to lose energy and fades.

Case Study: Hurricane Katrina

Hurricane Katrina was a devastating storm that hit the area around New Orleans, USA, on 25 August 2005. It had social, economic and environmental impacts:

Social

  • 1,800 people died

  • 300,000 homes were destroyed

  • 3 million people were left with no electricity

  • people had to move out of the area

Economic

  • $300 billion of damage

  • oil platforms were destroyed

  • shops were looted

  • fuel prices rose

  • tourism decreased

Environmental

  • the storm surge flooded large areas of the coast

  • 80% of New Orleans flooded as man-made levees, overwhelmed by extra water, broke

  • cotton and sugar cane crops were destroyed

  • delicate coastal habitats were destroyed

  • tornadoes were created

 
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