Mid-latitude weather systems
Depressions
The zone of change between different air masses is called a FRONT. The front is named after the air mass BEHIND the front. DEPRESSIONS, linked to the upper air flows (jet streams) , are low-pressure systems that develop a wave along boundaries between colder polar air and warmer tropical air, THE POLAR FRONT. As the warmer air meets the colder air the less dense air rises leading to low pressure at the surface.
The CONVEYOR BELT MODEL is a model for depression formation. It suggests that cold, dry air near the Tropopause divides as it sinks and meets warmer air rising from beneath. this helps to transfer heat from tropical to polar areas and from lower to higher altitudes.
ANA FRONTS- Ana Fronts occur where the air rises in relation to the front, creating more unstable conditions and heavy rainfall.
KATA FRONTS- Kata Fronts occur where the air is sinking in relation to the fronts, producing more stable conditions and only light rain or drizzle.
Once depressions are formed they are carried eastwards along the Rossby wave system. Distinctive warm and cold fronts develop, when the cold front moves faster than the warm front and when it catches up with it is called an OCCLUDED FRONT.
The zone of change between different air masses is called a FRONT. The front is named after the air mass BEHIND the front. DEPRESSIONS, linked to the upper air flows (jet streams) , are low-pressure systems that develop a wave along boundaries between colder polar air and warmer tropical air, THE POLAR FRONT. As the warmer air meets the colder air the less dense air rises leading to low pressure at the surface.
The CONVEYOR BELT MODEL is a model for depression formation. It suggests that cold, dry air near the Tropopause divides as it sinks and meets warmer air rising from beneath. this helps to transfer heat from tropical to polar areas and from lower to higher altitudes.
ANA FRONTS- Ana Fronts occur where the air rises in relation to the front, creating more unstable conditions and heavy rainfall.
KATA FRONTS- Kata Fronts occur where the air is sinking in relation to the fronts, producing more stable conditions and only light rain or drizzle.
Once depressions are formed they are carried eastwards along the Rossby wave system. Distinctive warm and cold fronts develop, when the cold front moves faster than the warm front and when it catches up with it is called an OCCLUDED FRONT.
Cyclones and Anticyclones
Areas of low pressure are called CYCLONES which flow ANTI-CLOCKWISE , while areas with high pressure are called ANTICYCLONES which flow CLOCKWISE.
Low pressure area = Cyclone: Cloudy or rainy weather
High pressure area = Anticyclone: Fair weather
Isobars on surface maps show areas of high and low pressure. On Upper-level charts, height contours look like waves. The highest section of the wave is called a RIDGE, and the lower section of the wave is called a TROUGH. Both are shown opposite.
Anticyclones
Anticyclones are enclosed areas of diverging subsiding air. As the air descends it warms adiabatically, this creates high pressures at the surface and decrease in relative humidity; moving clockwise in the northern hemisphere. In the summer anticyclones shift pole ward due to the changing position of the sun. Conditions within the anticyclone are usually stable with calm winds, but can vary with times of the year.
RIDGES are the small areas of high pressures between depressions and are small extensions of the larger anticyclone
body. Ridges are linked to upper air flow patterns; they can develop to produce enclosed anticyclones forming BLOCKING HIGHS.
BLOCKING HIGHS block the paths of depressions and direct north/ south of their normal path. This produces long periods of settled weather in the affected area and more depressions than normal in the areas north and south of it.
Areas of low pressure are called CYCLONES which flow ANTI-CLOCKWISE , while areas with high pressure are called ANTICYCLONES which flow CLOCKWISE.
Low pressure area = Cyclone: Cloudy or rainy weather
High pressure area = Anticyclone: Fair weather
Isobars on surface maps show areas of high and low pressure. On Upper-level charts, height contours look like waves. The highest section of the wave is called a RIDGE, and the lower section of the wave is called a TROUGH. Both are shown opposite.
Anticyclones
Anticyclones are enclosed areas of diverging subsiding air. As the air descends it warms adiabatically, this creates high pressures at the surface and decrease in relative humidity; moving clockwise in the northern hemisphere. In the summer anticyclones shift pole ward due to the changing position of the sun. Conditions within the anticyclone are usually stable with calm winds, but can vary with times of the year.
RIDGES are the small areas of high pressures between depressions and are small extensions of the larger anticyclone
body. Ridges are linked to upper air flow patterns; they can develop to produce enclosed anticyclones forming BLOCKING HIGHS.
BLOCKING HIGHS block the paths of depressions and direct north/ south of their normal path. This produces long periods of settled weather in the affected area and more depressions than normal in the areas north and south of it.
extreme weather: low pressure systems: hurricanes
Hurricanes
Follow this link to the Weather Channel's stories on the 2013 Hurricane season and weather forecasts: WEATHER CHANNEL
- Hurricanes are tropical low-pressure systems they are also called typhoons near Japan and cyclones off India and Australia.
- Hurricanes mainly occur near the Equator, in regions with prevailing easterly winds.
- To form they require the ocean to be
a minimum of 27 degrees and 60 m deep to provide heat and moisture. - Southeast and Northeast trade winds converge at the surface and, due to the CORIOLIS FORCE, air spirals as it converges. The spiraling develops around a calm central eye.
- The whole system can be up to 300 km in diameter with wind speeds of at least 118 km per hour.
- Hurricanes lose their source of heat and
moisture as they cross land and so decline and dissipate. - The Atlantic hurricane season is
officially from the 1st of June until the 30th of November. The season peaks in the Atlantic basin from August to October. Maximum activity is in early to mid September.
Follow this link to the Weather Channel's stories on the 2013 Hurricane season and weather forecasts: WEATHER CHANNEL
extreme weather: hurricane katrina
Hurricane Katrina was part of the 2005 Atlantic hurricane season and is one of the five deadliest hurricanes, in the
history of the United States. Approximately 1,836 people lost their lives in the hurricane. The total property damage was estimated at $81 billion, triple the damage caused by Hurricane Andrew (1992).
Hurricane Katrina formed over the Bahamas on August 23, 2005 and crossed southern Florida as a moderate Category 1 hurricane before strengthening rapidly in the Gulf of Mexico. The storm made its second landfall as a Category 3 storm on Monday the 29 th of August in southeast Louisiana.
Katrina caused destruction from central Florida to Texas all along the Gulf Coast, much was due to the storm surge. The most serve loss of life occurred in New Orleans, Louisiana, which flooded as the levee system failed. The worst property damage was in the coastal areas, such as Mississippi, which was over 90% flooded with waters spreading 6 -12 miles (10-19 km) from the beach.
history of the United States. Approximately 1,836 people lost their lives in the hurricane. The total property damage was estimated at $81 billion, triple the damage caused by Hurricane Andrew (1992).
Hurricane Katrina formed over the Bahamas on August 23, 2005 and crossed southern Florida as a moderate Category 1 hurricane before strengthening rapidly in the Gulf of Mexico. The storm made its second landfall as a Category 3 storm on Monday the 29 th of August in southeast Louisiana.
Katrina caused destruction from central Florida to Texas all along the Gulf Coast, much was due to the storm surge. The most serve loss of life occurred in New Orleans, Louisiana, which flooded as the levee system failed. The worst property damage was in the coastal areas, such as Mississippi, which was over 90% flooded with waters spreading 6 -12 miles (10-19 km) from the beach.