Mid latitude cyclones 1

• Isobars are lines drawn on weather maps which join places with the same pressure
• Places that experience rain in winter brought by mid-latitude cyclones have a Mediterranean climate
• Mid-latitude cyclones occur in the middle, or temperate, latitudes or outside the tropics. They are also referred to as temperate cyclones or extra-tropical cyclones
• General characteristics of mid-latitude cyclones

  • Speed of movement from west to east: 30-50 km/h
  • Life span: 4-14 days
  • Pressure: extensive low-pressure centre
  • Diameter of whole system: up to 3,000 km
  • Pattern of isobars: oval shaped
  • Pressure gradient: not normally steep
  • Contains a warm front and a cold front
  • Occur in families: the cold front of an older depression is joined to the warm front of a younger depression
• Mid-latitude cyclone

  A cell of low pressure containing a warm front and a cold front, bringing rain in winter to the Western Cape
• Front


  The dividing line between two air masses with different properties
• A low-pressure cell may be referred to as a depression, so mid-latitude cyclones are also called temperate depressions. They are also known as frontal depressions as they contain fronts
• The zone of greatest heat, or thermal equator, is north of the equator when the sun is overhead at the Tropic of Cancer, and south of the equator when the sun is overhead at the Tropic of Capricorn. The pressure belts and planetary winds move north in the northern hemisphere summer and south in the southern hemisphere summer. As a result, the mid-latitude cyclones reach South Africa in winter and not summer
• A mid-latitude cyclone forms when warm moist air rises over cold dry air, causing condensation and precipitation.
• The movement of the system also influences wind speeds and directions, impacting weather conditions across wide regions.
• Coriolis force causes winds to move from high pressure areas towards low pressure areas, creating a circular motion around an area of low pressure called a cyclone.
• Mid-latitude cyclones can cause strong winds, heavy rain, snow, hail, lightning, tornadoes, and other severe weather events.
• The Coriolis effect is the deflection to the right (in the northern hemisphere) or left (in the southern hemisphere) of moving air due to the rotation of the Earth.
• Occluded Front - A boundary formed when a fast-moving cold front catches up with a slower moving warm front. This results in the formation of clouds and precipitation along the occlusion point.
• They play a significant role in shaping global climate patterns by redistributing energy and moisture through atmospheric circulation.
• In the Northern Hemisphere, mid-latitude cyclones rotate counterclockwise due to Coriolis effect while in Southern Hemisphere it rotates clockwise.
• Trough - An elongated region of relatively low atmospheric pressure that extends horizontally between two adjacent high-pressure systems.
• Where do mud latitude cyclones form?

  Polar front
• Deflection by the South Atlantic and South Indian high pressure systems

  These high pressure cells can prevent the depression from reaching South Africa and cause it to move in a southeasterly direction away from the land
• Heavy rain from the cold front crossing the Western Cape leads to poor visibility and traffic accidents, bringing hardship to people living in informal settlements
• Very cold conditions, which are experienced when a cold front crosses the plateau and moves onto the Highveld, can result in power cuts and a disruption to human activities
• The berg winds and hot dry conditions that are experienced ahead of a cold front can result in veld fires, damaging farming in the area and the ecology
• Berg winds


  Hot, dry winds occurring due to a steep pressure gradient between the Kalahari high pressure and the depression, heating adiabatically as it moves down the escarpment to converge on the low pressure
• Winter rainfall in the Western Cape determines the type of crops cultivated there, leading to the development of a profitable wine industry
• Mountain passes may be closed if there is snow
• Cold snap on the plateau

  The South Atlantic high pressure ridges in behind the cold front, reinforcing the southwesterly air behind the cold front and pushing it across the land, introducing very cold air from the south
• Snow on the Cape fold mountains


  Occurs if the air behind the cold front is very cold and cools further on rising up the mountains
• The southwesterly winds behind the cold front can create very stormy conditions and gale-force winds over the ocean, posing a danger to ships
• Mid-latitude cyclones move across South Africa in winter, affecting the Western and southern Cape with rain from passing cold fronts
• Rain brought by frontal depressions replenishes water in dams
• The cultivation of grapes has led to the development of a profitable wine industry in the Western Cape
• Wind direction changes in the southern hemisphere

  As the air circulates clockwise, the wind changes from northeasterly to northwesterly to southwesterly
• Cold air behind the front is warmer and lighter than the air ahead of the warm front in the northern hemisphere


  The warm front remains in contact with the ground and the cold front rises
• Cloud cover increases as a mid-latitude cyclone passes
• Temperatures drop during a cold front

  Wind direction changes to southwest
• Clouds become cumulonimbus during a mid-latitude cyclone
• Heavy rainfall occurs over a small area during a cold front
• The snow that falls on the Cape fold mountains and mountains of the escarpment is a danger to livestock
• Cold air behind the front is colder and denser than the air ahead of the warm front in the northern hemisphere


  The cold front remains in contact with the ground and lifts the warm front
• Winter rainfall in the Western Cape has determined the type of crops that are cultivated there