In summer the clear settled conditions associated with anticyclones can bring long sunny days and warm temperatures. The weather is normally dry, although occasionally, very hot temperatures can trigger thunderstorms. An anticyclone situated over the UK or near continent usually brings warm, fine weather. A low pressure system, also known as a depression occurs when the weather is dominated by unstable conditions.
Under a depression air is rising, forming an area of low pressure at the surface. This rising air cools and condenses and helps encourage cloud formation, so the weather is often cloudy and wet. In the Northern Hemisphere winds blow in anticlockwise direction around a depression. Isobars are normally closely spaced around a depressions leading to strong winds.
Depressions can be identified on weather charts as an area of closely spaced isobars, often in a roughly circular shape, where pressure is lower than surrounding areas. They are often accompanied by fronts. Using this information on pressure systems you should now be able to complete worksheet 1.
Then you can complete Extension 1 or worksheet 2. A front is a boundary between two different types of air masses, these are normally warm moist air masses from the tropics and cooler drier air masses from polar regions. Fronts move with the wind so over the UK they normally move from west to east. The notes below provide information about the most common types of fronts. The descriptions given apply to active well developed fronts, weaker fronts may not display all the characteristics or they may be less well defined.
A warm front indicates that warm air is advancing and rising up over the colder air. Therefore warm fronts occur where warmer air is replacing cooler air at the surface. As the warm front approaches there is a gradual deterioration in the weather. Clouds gradually lower from higher cirrus, through altostratus, to stratus and nimbostratus at the front. There is often a prolonged spell of rainfall which is often heavy. Behind the warm front the rain becomes lighter, turns to drizzle or ceases, but it remains cloudy.
Pressure falls steadily ahead of and during the passage of the warm front, but then rises slowly after its passage. The diagram below shows a cross section through a warm front, with associated cloud, temperature and weather changes. A cold front indicates that cold air is advancing and pushing underneath warmer air at the surface. Therefore cold fronts occur where cooler air is replacing warmer air at the surface.
The passage of weather associated with a cold front is much shorter lived than that with a warm front. In a low pressure system the warm front is the first to pass over. This occurs when warm air meets cold air and the warm air rises above it. Warm fronts bring steady continuous rain. The next front to pass over is the cold front. This occurs when cold air meets warm air. The cold air pushes the warm air upwards. Cold fronts bring heavy rain showers. Winds are normally stronger.
They usually form over the Atlantic Ocean and are carried across Britain by westerly winds. They produce cloudy, rainy and windy weather. These low-pressure systems often begin in the Atlantic, moving eastwards towards the UK. They are responsible for the UK's changeable weather. In the Atlantic Ocean, the cold polar maritime and warm tropical maritime air masses will meet. The lighter warm air will start to rise up over the denser, colder air and this creates an unstable area that will develop a front.
There is a favourite meeting place in the mid-Atlantic for cold polar air and warm sub-tropical air. Depressions usually have well defined warm and cold fronts, as the warm air is forced to rise above the cold air. Fronts and depressions have a birth, lifetime and death; and according to the stage at which they are encountered, so does the weather intensity vary. A depression appears on a synoptic weather chart as a set of closed curved isobars with winds circulating anticlockwise in the Northern Hemisphere and clockwise in the Southern Hemisphere due to the rotation of the Earth.
0コメント