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GR3AFET Chapter 8 part 2 Fronts.pdf

Mphow Parvo
Mphow Parvo

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

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Chapter 8 Lecture 息 2015 Pearson Education, Inc. Weather Part 2 Fronts Geosystems 9th Edition
息 2015 Pearson Education, Inc. Part 2 Fronts In this section: Cold fronts Squall line Berg winds Coastal lows Warm fronts Occluded fronts Stationary fronts
息 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.
息 2015 Pearson Education, Inc. 1. Cold Front As the cold front passes, the air temperature may become noticeably cooler, with temperatures dropping by 5 属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.
息 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属drops, pressure drops Up to 400 km wide Precipitation is behind the cold front and often heavy precipitation and thunderstorms
息 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
息 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
息 2015 Pearson Education, Inc. Squall Line
息 2015 Pearson Education, Inc. Cold Front
息 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 尊C) 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
息 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.
息 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.
息 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.
息 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.
息 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 SAs 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.
息 2015 Pearson Education, Inc. Coastal Lows South Africa
息 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.
息 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
息 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.
息 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.
息 2015 Pearson Education, Inc. 2. Warm Front
息 2015 Pearson Education, Inc.
息 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.
息 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.
息 2015 Pearson Education, Inc. Occlusion
息 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
息 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.
息 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.
息 2015 Pearson Education, Inc. 3. Occluded Front
息 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. .
息 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.
息 2015 Pearson Education, Inc. 4. Stationary Front
息 2015 Pearson Education, Inc. End of Part 2 Online class test this week Thursday 20 April covering Parts 1 and 2

More Related Content

GR3AFET Chapter 8 part 2 Fronts.pdf

  • 1. Chapter 8 Lecture 息 2015 Pearson Education, Inc. Weather Part 2 Fronts Geosystems 9th Edition
  • 2. 息 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. 息 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. 息 2015 Pearson Education, Inc. 1. Cold Front As the cold front passes, the air temperature may become noticeably cooler, with temperatures dropping by 5 属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. 息 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属drops, pressure drops Up to 400 km wide Precipitation is behind the cold front and often heavy precipitation and thunderstorms
  • 6. 息 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. 息 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. 息 2015 Pearson Education, Inc. Squall Line
  • 9. 息 2015 Pearson Education, Inc. Cold Front
  • 10. 息 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 尊C) 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. 息 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. 息 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. 息 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. 息 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. 息 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 SAs 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. 息 2015 Pearson Education, Inc. Coastal Lows South Africa
  • 17. 息 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. 息 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. 息 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. 息 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. 息 2015 Pearson Education, Inc. 2. Warm Front
  • 22. 息 2015 Pearson Education, Inc.
  • 23. 息 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. 息 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. 息 2015 Pearson Education, Inc. Occlusion
  • 26. 息 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. 息 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. 息 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. 息 2015 Pearson Education, Inc. 3. Occluded Front
  • 30. 息 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. 息 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. 息 2015 Pearson Education, Inc. 4. Stationary Front
  • 33. 息 2015 Pearson Education, Inc. End of Part 2 Online class test this week Thursday 20 April covering Parts 1 and 2