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Weather and climate

Climate

Weather describes the day-to-day conditions of the atmosphere. Weather can change quickly – one day it can be dry and sunny and the next day it may rain.

Climate describes average weather conditions over longer periods (30 years) and over large areas.

Measuring weather
Meteorologists measure weather conditions and they use this information to report and make forecasts about future weather conditions.

What do we measure?
Temperature
Precipitation
Wind speed and direction
Cloud cover
Air pressure
Humidity
Sunshine

Temperature: is measured in degrees Celsius with a thermometer. Mercury in the glass bulb expands as it gets hotter and rises up the scale to show the temperature of the air.

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Precipitation: Rainfall is measured in millimetres using a Rain Gauge. Rain is collected through the funnel. The tube has a scale which shows the amount of rainfall.

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Wind: Weather Vane measures wind direction using the points of the compass. The weather vane is designed so that the arrow will point to the direction the wind is blowing from.

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Anemometer measures wind speed. Four hemispherical cups are blown by the wind and spin faster the stronger the wind. The speed of the wind is shown on a dial at the bottom.

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Cloud cover: is measured in oktas. Meteorologists look at the sky and decide how many eighths of the sky are covered in cloud. 0 oktas means the sky is clear, 8 oktas means the sky is covered with cloud, 4 oktas means half the sky is covered with cloud.

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Air Pressure: is measured in millibars using a barometer. Changes in atmospheric pressure cause a chamber inside the device to expand or contract. A series of levers cause the dial to move showing whether pressure is rising or falling.

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Humidity: (the amount of moisture in the air) is measured using a hygrometer. The device contains a strand of human or animal hair which lengthens when there is more moisture in the air and shortens when it is dryer. The hair is attached to a pointer which shows the humidity.

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Sunshine: A Campbell-Stokes sunshine recorder is used to record the sunshine duration. Sunshine recorder essentially consists of a glass sphere mounted in a spherical bowl and a metallic groove which holds a record card.

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Synoptic charts and the symbols

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A synoptic chart is any map that summarises atmospheric conditions (temperature, precipitation, wind speed and direction, atmospheric pressure and cloud coverage) over a wide area at a given time. They display an overview of the weather conditions observed from many different weather stations, aeroplanes, balloons and satellites.

What do all the symbols mean?

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What do the symbols look like on a synoptic chart?

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Clouds

Clouds are categorised according to height and shape. The different categories of clouds are then given names based on Latin words, eg nimbus clouds bring rain, stratus clouds appear as layers.

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Factors affecting climate

Five factors affect climate. They are: latitude, altitude, winds, distance from the sea and aspect

Latitude
Temperature range increases with distance from the equator. Also, temperatures decrease as you move away from the equator. This is because the suns rays are dispersed over a larger area of land as you move away from the equator. This is due to the curved surface of the earth.

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Altitude
Temperatures decrease with height. The air is less dense and cannot hold heat as easily.

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Winds
If winds are warm – they have been blown from a hot area – they will raise temperatures. If winds have been blown from cold areas they will lower temperatures.

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Distance from the sea (continentality)
Land heats and cools faster than the sea. Therefore coastal areas have a lower temperature range than those areas inland. On the coast winters are mild and summers are cool. In inland areas temperatures are high in the summer and cold in the winter.

Aspect
Slopes facing the sun are warmer than those that are not. Thus south facing slopes in the northern hemisphere are usually warm. However, slopes facing north in the southern hemisphere are warmest.

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Types of rainfall

Precipitation is caused when moist air rises; water vapour in the air-cools & condenses & forms clouds. Air cools upwards through the atmosphere because temperature falls with altitude because the Earth’s atmosphere is heated from the Earth’s surface. When the water droplets in clouds grow to a certain size, gravity causes them to fall because of their own weight. There are 3 different types of Precipitation formation; conventional, frontal and relief.

Convection rainfall

Very common in areas where the ground is heated by the hot sun, such as the Tropics. This is why those areas experience heavy rainfalls most afternoons. The United Kingdom does experience some convectional rainfall during the summer, particularly in the South East of the country.

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Convectional rainfall occurs when:
The surface of the earth is heated by the sun.
The warm surface heats the air above it. Hot air always rises so this newly heated air does so.
As it rises the air-cools and begins to condensate.
Further rising and cooling causes a large amount of condensation to occur and rain is formed.
Convection tends to produce towering cumulo-nimbus clouds, which produce heavy rain and possible thunder and lightning

Frontal rainfall

The United Kingdom experiences a lot of frontal rainfall, as it is associated with the movement of depressions over the country, which are described in more detail elsewhere in this topic.

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Frontal rainfall occurs when:
Two air masses meet, one a warm air mass and one a cold air mass.
The lighter, less dense, warm air is forced to rise over the denser, cold air.
This causes the warm air to cool and begin to condense.
As the warm air is forced to rise further condensation occurs and rain is formed.
Frontal rain produces a variety of clouds, which bring moderate to heavy rainfall.

Relief rainfall

This is also called orographic rainfall, which is very common in the United Kingdom, especially on the West coast since the prevailing weather comes from that direction.

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Relief Rainfall occurs when:
The prevailing winds pick up moisture from the sea as they travel across it, making the air moist.
The moist air reaches the coast and is forced to rise over mountains and hills.
This forces the air to cool and condense, forming clouds.
The air continues to be forced over the mountains and so it drops its moisture as relief rain.
Once over the top of the mountain the air will usually drop down the other side, warming as it does so. This means it has a greater ability to carry water moisture and so there is little rain on the far side of the mountain. This area is called the rain shadow.

However, too much rainfall, as we recently found out, is not fun at all.

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2 Comments
  1. xavier permalink

    good website and very helpful

  2. ollie wright permalink

    Very informative and easy to understand

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