Extreme Weather Conditions
- Introduction of extreme weather conditions
- Droughts
- Flood

Extreme weather conditions, such as floods or droughts, sometimes interrupt the usual pattern of weather in some areas. In other places, extreme conditions are experienced each year as part of the seasonal pattern.

Drought

A drought occurs when there is less than 0.2mm of rain, or other type of precipitation, over a period of at least fifteen days. Droughts may occur because of a high blocking of the passage of rain- bearing lows across an area. They also occur when areas of land are cleared of vegetation in areas which are already very dry.

Flood

Floods can cause billions of pounds- worth of damage and many deaths. Many people still choose to live on flood plains because the land is good for farming and for building on. But if these areas are hit by unusually stormy weather, the overflowing rivers can bring disaster.


>> In Bangladesh's captial, Dhaka, people standing waist- high in floodwater queue to collect clean drinking water.

When more than 15mm of rain falls in 3 hours, meteorologists describe the conditions as a "flash flood". Here are some major reasons for flash floods to occur.

1. It sometimes occur when motionless or slow- moving thunderstorms produce heavy rain over a particular area. When this happens, over 50 mm of rain can fall in an hour. The ground very quickly becomes full of water and cannot absorb any more. Water then runs along the surface and into rivers, causing them to overflow.

2. Melting snow can also cause flash floods, sometimes many hundreds of kilometres from the areas of heavy snowfalls.

Flash floods are very destructive, not only because there is so little warning of them but also because the water flows so powerfully and contains a lot of mud, sediment and boulders.

Flood Protection

Since floodwater needs to drain away quickly, many cities have huge storm drains to carry away rushing water. In coastal towns, sea walls and dykes are built to keep out waves. To prevent rivers bursting their banks, walls or embankment are sometimes raised. Special flood barriers have been built in some large cities to protect them from the water that sometimes surges upriver during storms.

>> The Thames Barrier was opened in 1984. It protects the people of London from floods. If exceptionally high tides surge up the river, 20 metre- high steel gates close to protect the city.

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