Learning Objectives: 1) To understand the formation and development of a depression 2) To know the sequence of weather associated with a depression 3) To know the characteristics of an anticyclone and the contrasting weather conditions between winter and summer. Depressions are areas of low pressure. There is a relatively low air pressure at the surface because the air is rising. Air, carried by surface winds, is drawn into the centre of he depression to replace the rising air. In the northern hemisphere, winds blow in an anti-clockwise direction around a depression, spiralling towards it centre. In general, depressions bring spells of unsettled weather with plenty of wind, cloud and rain. In the UK most of our weather is associated with the passage of a depressions which, having formed over the Atlantic, pass across the UK from west to east driven by the prevailing winds. How are frontal depressions formed? Frontal depressions are formed when warm air from the tropics meets- and is forced to rise above- cold air from the poles. The rising creates the centre of low pressure. The line that separates the two air masses is called a front. As the depression develops and moves east, driven by the westerly circulation, two fronts can be recognised. The leading front is the warm front, so called because once the warm front passes over a place, the warm air of the tropical air mass brings warm weather. The front at the rear of the depression is the cold front, so called because after it passes of the cold air of the polar air mass brings colder weather. In well developed depressions, the cold front catches up with the warm front and forms an occluded front. Frontal depressions and satellite photographs and synoptic charts In the satellite photo the centre of the low pressure can be detected by the swirl of cloud. This is where the wind is being sucked into the centre from the surrounding areas of high pressure. The swirl is caused by the rotation of the Earth. The fronts show up as trailing lines of cloud on the southern sides of the depression. It is along the fronts that most activity is occurring as air is being pushed upwards, leading to condensation of its moisture into cloud. The white speckles of cloud in the area behind the cold front indicate shower clouds. Cold air is being warmed up by its journey over the sea, encouraging air to rise to form cumulo-nimbus clouds shown. Depressions are isolated storms that form at the boundary of cold polar air moving south and warm tropical air moving north. This boundary is called the polar front. Once formed, the depression intensifies and starts to rotate, with air spiralling upwards to form cloud and rain. The warm front marks the front of warmer air and the cold front represents the front of colder air. At these fronts air is forced to rise, often forming distinct bands of cloud and rain. Over time the cold front catches up with the warm front to form a single boundary called an occluded front. Eventually the depression fizzles out and dies. Depressions often develop to the west of Britain, where tropical and polar air masses meet, and travel eastwards across the country, blown by the prevailing westerly winds. A depression is recognisable on a weather map because of the two fronts, a warm and a cold front, which meets at an apex. The cold front moves more quickly than the warm front so the warm sector gradually becomes smaller. As the fronts merge an occluded front forms, often bringing particularly bad weather conditions. As a depression crosses over the country it brings changeable weather. Anticyclones- high pressure systems. These large areas of descending air bring settled weather that can last for several days or weeks. Pressure in these systems is usually over 1,000 millibars. If they persist they are called ‘blocking highs’ because they block out depressions. Main features of the depression This depression is centred off Northern Ireland to the west coast of Scotland. The warm front stretches through the North Sea and the cold front runs through the centre of England. Ahead of the warm front, conditions slowly become cloudier before a prolonged period of steady rain sets. Behind the warm front the temperature increases in the warm sector. It remains cloudy with patchy rain and drizzle. The cold front brings a short period of heavy rain, often accompanied by strong and gusty winds. Behind the cold front the temperature drops sharply. The rain stops but heavy showers may occur. The one above takes 18h to cross the UK. Centre of low pressure Cold front Warm front Warm sector Anti-clockwise winds 3 (a) (ii) • Isobars are circular in shape. • Pressure is low/lowest in the middle (below 984mb) • Pressure increases from the centre • Closely spaced isobars Explain why the weather changes with the passage of a depression (6 marks) Temperature- cool initially and then getting warmer as the warm front approaches and takes over from the cold. Cloud- increases and cloud thickens, layer cloud- stratus and cirrus as warm air rises over cold, then thins and possible breaks as warm sector passes to be followed by thick, vertical, cumulus clouds as the cold front approaches and the cold air forces the warm air to rise quickly. Precipitation- prolonged, relatively light rain and quite mild as the wider warm front passes, then drier, possibly drizzle and breaks in rain and as the cold front passes heavy rain, temperatures fall and possibly snow. Rain is heavier due to the cold air undercutting the warm air and causes rapid uplift. Wind increases as warm front approaches and pressure falls as warm front approaches, steadies as warm sector passes and increases to be at its strongest as cold front passes. The wind direction will change from SW to NW around the centre of the low. Pressure starts to fall ahead of the warm front, continuing to do so as the warm front passes. It then steadies before rising as cold front passes. 1) Describe the climate of York (3 marks) 2) Explain why the climate of Aberystwyth is different to that of Edinburgh (4 marks) Temperature - Maximum temperature is 21°C - Minimum is 6°C - Temperature range is 15°C - Peak occurs in June and July - Falls faster than it rises Precipitation - Precipitation fluctuates throughout the year - No clear change with the seasons - Wettest in summer (July and August) - Driest during late winter/ early spring- February to April Edinburgh is cooler than Aberystwyth whilst Edinburgh is wetter than Edinburgh from the climate graphs Aberystwyth is wetter due to the prevailing winds and nearness to the sea, it is on the coast, in the path of the prevailing winds as they hit the shore. Areas on the east receive less rainfall, due to rainshadow. Depressions come from the west so more rainfall is released in the west. Edinburgh is cooler as it is further north/ at a higher latitude. This means that the sun’s rays are more concentrated in York as the sun is higher in the sky as insolation from the sun is higher and there is a smaller area for them to be spread over on the surface. Aberystwyth is on the west coast and is influenced by the sea and North Atlantic Drift, so is warmer than Edinburgh in winter on the eastern side of Scotland. 1) Study the climate graph for Tenby. Describe annual temperature range in Tenby (2 marks) 2) Study the map, which shows information about the climate at for weather stations in the UK. Explain the variation in rainfall shown. You should refer to reasons such as altitude, winds and distance from the sea (8 marks). Distance from the sea/ the direction of the prevailing winds- places nearer the sea, such as Tenby and St Mawgan tend to be wetter than those further away, such as Cambridge. This is because the south westerly winds will have crossed a large expanse of water know as the Atlantic Ocean and when they blow off the sea onto the land the winds are heavily laden with moisture nearer the coast than inland. The air will rise or meet air from other areas and deposit much of their rainfall on places in the west and thus the rain has already fallen by the time eastern areas like Cambridge is reached. Altitude exaggerates the effects as the winds blowing from the sea to the land have to rise more at Princetown, which is higher up and in the west, than at St Mawgan, so the winds are cooled more and so there is more condensation, more clouds more and greater precipitation occurs.