GR3AFET Chapter 8 part 2 Fronts.pdf
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This document discusses different types of weather fronts: - Cold fronts occur when cold air advances on warm air, pushing it upwards and forming cumulonimbus clouds along the boundary. Rain, gusty winds and thunderstorms often accompany cold front passage. - Warm fronts mark the transition where warm air replaces cold air. Heavier precipitation falls ahead of the front, with lighter widespread rain following the front. - Occluded fronts form when a cold front overtakes a warm front, causing the three air masses to converge. Precipitation and changing winds are common along occluded fronts. - Stationary fronts occur when opposing air masses balance each other, preventing frontal movement. Unsettled weather with rain or
GR3AFET Chapter 8 part 2 Fronts.pdf
- 1. Chapter 8 Lecture © 2015 Pearson Education, Inc. Weather Part 2 Fronts Geosystems 9th Edition
- 2. © 2015 Pearson Education, Inc. Part 2 Fronts • In this section: • Cold fronts – Squall line – Berg winds – Coastal lows • Warm fronts • Occluded fronts • Stationary fronts
- 3. © 2015 Pearson Education, Inc. 1. Cold Front • A cold front occurs where a large mass of cold air meets a mass of warmer air, and the cold air advances on the warmer air. • The cold air undercuts the warm air pushing it upwards. • Cumulonimbus clouds form a well-defined line along the boundary between the air masses.
- 4. © 2015 Pearson Education, Inc. 1. Cold Front • As the cold front passes, the air temperature may become noticeably cooler, with temperatures dropping by 5 °C or more within the first hour. • Rain, gusty winds, and, sometimes, thunderstorms occur with the passage of the cold front. • On a synoptic chart, a cold front is represented by a solid line with blue triangles along the front pointing towards the warmer air and in the direction of movement.
- 5. © 2015 Pearson Education, Inc. 1. Cold Front • Cold air is more dense than warm air • Cold air forces warm air up. • Days before arrival of a cold front – cirrus clouds, shifting winds, T°drops, pressure drops • Up to 400 km wide • Precipitation is behind the cold front and often heavy precipitation and thunderstorms
- 6. © 2015 Pearson Education, Inc. Fronts • Video: Cold warm occluded stationary – types of weather fronts • Duration: 3:07 • https://www.youtube.com/watch?v=-ez4QoQLnZ8 • Video: What exactly is a cold front? • Duration: 3:53 • https://www.youtube.com/watch?v=YJCW4zuoMM0 • Video: Weather fronts explained • Duration: 1:52 • https://www.youtube.com/watch?v=naarbGHoAGU
- 7. © 2015 Pearson Education, Inc. Squall Line • Fast advancing cold front can cause violent lifting creating a squall line slightly ahead of the front creating high winds and intense storms. • Squall lines generally form along or ahead of cold fronts and can produce severe weather in the form of heavy rainfall, strong winds, large hail, and frequent lightning. • Video: What is a squall? • Duration: 1:51 • https://www.youtube.com/watch?v=JKR910Dyc2c
- 8. © 2015 Pearson Education, Inc. Squall Line
- 9. © 2015 Pearson Education, Inc. Cold Front
- 10. © 2015 Pearson Education, Inc. Berg Winds • Berg winds are warm, dry, gusty winds that blow from the SA plateau towards the coast in winter. • In winter, when there is a strong HP cell over the interior and a LP cell at the coast (coastal LP), wind spirals downwards and outwards around the HP. • As the wind descends from the plateau to the coast, it is heated by compression, arriving at the coast as a hot, dry wind (sometimes over 35 ºC) and can last for two or three days. • Berg winds precede Coastal Low Pressures • Video: South African Berg Winds • Duration: 2:58 • https://www.youtube.com/watch?v=Tt4mqc8XVAM
- 11. © 2015 Pearson Education, Inc. Coastal Lows – South Africa • When a depression and its attendant cold front passes over South Africa, there is a predictable sequence of weather events throughout the country. • Map1 shows warm Berg Winds over the KZN coast. • Ahead of the coastal low (L1), the weather along the KZN coast is fine, but behind it onshore, foggy weather is likely to be experienced. • Because a cold front has invaded from the Atlantic, the weather over the Western Cape will be cold and wet.
- 12. © 2015 Pearson Education, Inc. Coastal Lows – South Africa • The subsequent maps trace the advance of the cold front across SA over the next few days. • Map 2 shows the weather about one or two days later. • By this stage, the Berg winds over KZN have been replaced by cold, southerly air flows which may bring snow to the Drakensberg and Cape mountains. • If the cold air moves inland, as it often happens in winter, the Free State, and the northern provinces will experience a cold snap for a few days. • The same airflow will cause south- easterly winds to blow over Cape Town.
- 13. © 2015 Pearson Education, Inc. Coastal Lows – South Africa • A day or two later (Map 3), the front has moved well to the east and the Berg wind cycle has started again on the west coast. • While warm northeasters blow offshore over the west coast, those on the east coast may experience cold weather due to the passing front.
- 14. © 2015 Pearson Education, Inc. Coastal Lows – South Africa • The cycle is completed when the Berg winds and a coastal low have reached the southeast coast (Map 4) and a new cold front approaches the Cape. • Some fronts are very cold, bringing wet weather and even snow to the Cape, where others are poorly developed and only bring a slight drop in temperature.
- 15. © 2015 Pearson Education, Inc. Coastal Lows – South Africa • Develops with Berg Winds. • Imagine cells of low pressure as slow moving shallow spirals of air, about 200 – 500 km wide, and with the spiral centred on the coast. • Starting on the west coast, the spiral moves over a period of six to eight days around SA’s coast as far as northern KZN, and then disappears. • Often, the coastal migration is squeezed in a few hundred kilometres ahead of an advancing cold front. • When a coastal low advances from west to east, the weather along the coast is predictable – typified first by a period of clear dry weather as the air ahead of the low is moving offshore. • This is followed 6 to 18 hours later by cooler, foggy, and rainy weather as the LP pushes sea air onshore.
- 16. © 2015 Pearson Education, Inc. Coastal Lows – South Africa
- 17. © 2015 Pearson Education, Inc. 2. Warm Front • A warm front is defined as the transition zone where a warm air mass is replacing a cold air mass. • The air behind a warm front is warmer and more moist than the air ahead of it. • When a warm front passes through, the air becomes noticeably warmer and more humid than it was before.
- 18. © 2015 Pearson Education, Inc. 2. Warm Front • The leading edge of an advancing warm air mass is unable to displace cooler, passive air which is dense along the surface. • Instead the warm air moves up and over cold air creating a temperature inversion • This can be 1000 km wide
- 19. © 2015 Pearson Education, Inc. 2. Warm Front • First before the warm front arrives the pressure in area starts to steadily decrease and temperatures remain cool. • The winds tend to blow north to northeast in the southern hemisphere. • The precipitation is normally rain, sleet, or snow. Common cloud types that appear - stratus, cumulus, and nimbus clouds. • The dew point also rises steadily • While the front is passing through a region temperatures start to warm rapidly. • The atmospheric pressure in the area that was dropping starts to level off.
- 20. © 2015 Pearson Education, Inc. 2. Warm Front • The winds become variable and precipitation turns into a light drizzle. • Clouds are mostly stratus type clouds formations. • The dew point then starts to level off. • Heavy precipitation occurs ahead of the warm front • Lighter, wide-spread precipitation for a longer period of time follows the front, thereafter clear skies Symbolically, a warm front is represented by a solid line with semicircles pointing towards the colder air and in the direction of movement. • If warmer air is replacing colder air, then the front should be analysed as a warm front. • If colder air is replacing warmer air, then the front should be analysed as a cold front.
- 21. © 2015 Pearson Education, Inc. 2. Warm Front
- 22. © 2015 Pearson Education, Inc.
- 23. © 2015 Pearson Education, Inc. 3. Occluded Front • When a cold front overtakes a warm front • The cold air mass from the cold front meets the cool air that was ahead of the warm front. • The warm air rises as these air masses come together. • Occluded fronts usually form around areas of low atmospheric pressure.
- 24. © 2015 Pearson Education, Inc. Occlusion (Southern hemisphere) • When 2 distinctly different air masses meet, they mix very slowly and begin to form a barrier between them. • When the cold air meets the warm subtropical air, a front forms between them. • The front does not remain a simple boundary at points along it, some of the warmer air pushes against it, ‘denting’ it and rising over the colder, heavier air. • Where a ‘dent’ occurs, depressions (LP cells) develop and the air starts to move in a circular pattern. • The warm air rises and spirals toward the centre, advancing poleward (south), while the cold air moves equatorward (north) by pushing under the warm air. • As the rising air is lifted and cooled, it gives rise to clouds along both the warm and cold fronts. • Unsettled, rainy and even stormy weather will accompany the depression as it moves eastwards with the frontal system.
- 25. © 2015 Pearson Education, Inc. Occlusion
- 26. © 2015 Pearson Education, Inc. 3. Occluded Front • There is often precipitation along an occluded front from cumulonimbus or nimbostratus clouds. • Wind changes direction as the front passes and the temperature changes too. • The temperature may warm or cool. • After the front passes, the sky is usually clearer and the air is drier. • Video: What are Weather Fronts? Warm Front, Cold front? | Weather Wise • Duration: 2:18 • https://www.youtube.com/watch?v=5Cnx5Bzctas
- 27. © 2015 Pearson Education, Inc. 3. Occluded Front • An occluded front is a composite of two frontal systems that merge as a result of occlusion. • Cold fronts generally move faster than warm fronts (sometimes double the speed) • As a result, a cold front will overtake an existing warm front. • an occluded front forms as the three air masses meet.
- 28. © 2015 Pearson Education, Inc. 3. Occluded Front • When the cold air pushes underneath the warm air, it lifts the warm air up from the ground, which makes it hidden, or "occluded.“ • These fronts are symbolized on a weather map by a purple line with both semicircles and triangles on it.
- 29. © 2015 Pearson Education, Inc. 3. Occluded Front
- 30. © 2015 Pearson Education, Inc. 4. Stationary Front • A stationary front forms when a cold front or warm front stops moving. • This happens when two masses of air are pushing against each other but neither is powerful enough to move the other. • Winds blowing parallel to the front instead of perpendicular can help it stay in place. • A stationary front may last for days. • If the wind direction changes the front will start moving again, becoming either a cold or warm front. Or the front may break apart. • .
- 31. © 2015 Pearson Education, Inc. 4. Stationary Front • Because a stationary front marks the boundary between two air masses, there are often differences in air temperature and wind on opposite sides of it. • The weather is often cloudy along a stationary front and rain or snow often falls, especially if the front is in an area of low atmospheric pressure. • On a weather map, a stationary front is shown as alternating red semicircles and blue triangles • The blue triangles point in one direction and the red semicircles point in the opposite direction.
- 32. © 2015 Pearson Education, Inc. 4. Stationary Front
- 33. © 2015 Pearson Education, Inc. End of Part 2 • Online class test this week Thursday 20 April covering Parts 1 and 2