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  • 1. Aims: • to know the stages of a depression • to know the weather conditions associated with each stage of a depression
  • 2. What is a depression? Depressions are areas of low pressure which usually bring rain, cloud and wind to the British Isles. Most depressions develop over the Atlantic Ocean. This is where a mass of warm, moist tropical air from the south meets a mass of colder, drier polar air from the north. The two air masses, because they have different temperatures, have different densities (weight). This prevents them from easily mixing. Instead, the warmer air, which is less dense (lighter) is forced to rise over the more dense (heavier) colder air. As the warm air rises it creates an area of low pressure at ground level. The boundary between two air masses is called a front. There are two fronts in a typical depression. 1. The warm front, which passes first, is where the advancing warm air is forces to rise over the cold air. 2. The cold front, which follows, is where the advancing cold air undercuts the warm air in front of it. Ingredients Lis t What do you ne ed to make a depression? • • • • • •
  • 3. What are the stages of a depression? As the warm front approaches The first sign of an approaching warm front of a depression is the formation of high, thin clouds (cirrus). In time, the clouds get lower and thicker (stratus). Winds slowly begin to increase in strength and blow in an anti-clockwise direction from the south east. As warm air rises there is a rapid fall in atmospheric pressure. After the cold front passes As the cold air replaces the warm air, temperatures fall and atmospheric pressure rises. In time the heavy rain gives way to frequent and heavy showers and winds slowly begin to decrease in strength. Temperatures remain low due to the winds which continue to come from the north west. Eventually the showers die out and the clouds disperse to give increasingly longer sunny intervals. An average depression can take between one and three days to pass over the British Isles. As the warm front passes As the warm front passes, temperatures rise and winds become stronger, blowing from a south westerly direction. Steady rain falls for a lengthly period from the low thick clouds (nimbo stratus). During the warm sector The weather within the warm sector is a little less predictable. Light rain or drizzle may continue, or the clouds may break to give weak sunshine. Winds usually decrease in strength. As the cold front passes The most extreme conditions occur as the cold front passes. Winds often reach gale force and swing round to the north west. Rainfall is very heavy, and can at times be accompanied by hail and even thunder (cumulonimbus clouds. The rain, however, is of shorter duration than that at the warm front.
  • 4. Use the text on the previous page to annotate the cross section of a depression below.
  • 5. Task: From the diagrams and the text, complete the following table summarising the weather found at different stages of a depression. 5 After the cold front passes 4 As cold front passes 3 During the warm sector 2 As warm front passes 1 As the warm front approaches Weather Conditions Temperature Pressure Cloud amount and type Precipitation Wind direction Wind speed
  • 6. Forecasting the weather conditions associated with a depression The weather in Plymouth will be… The weather in Plymouth will be… The weather in Plymouth will be…
  • 7. Forecasting the weather conditions associated with a depression The weather in Plymouth will be… The weather in Plymouth will be… The weather in Plymouth will be…