Extratropical Cyclones

Extratropical cyclones are also called wave cyclone or mid-latitude cyclone or temperate cyclones. The systems developing in the mid and high latitude, beyond the tropics are called the middle latitude or extra tropical cyclones.

To understand and formation of cyclone formation, we have to understand several terms related to it.

Cyclogenesis

It is an umbrella term to identify several different processes that result in a cyclone.

Air Masses

When the air remains over a homogenous area for a sufficiently longer time, it acquires the characteristics of the area. The homogenous regions can be the vast ocean surface or vast plains. The air with distinctive characteristics in terms of temperature and humidity is called an air mass.

It is defined as a large body of air having little horizontal variation in temperature and moisture. The homogenous surfaces, over which air masses form, are called the source regions.

The air masses are classified according to the source regions. There are five major source regions. These are:

  • Warm tropical and subtropical oceans;
  • The subtropical hot deserts;
  • The relatively cold high latitude oceans;
  • The very cold snow covered continents in high latitudes;
  • Permanently ice covered continents in the Arctic and Antarctica.

Fronts

When two different air masses meet, the boundary zone between them is called a front. The process of formation of the fronts is known as frontogenesis. There are four types of fronts:

  • Stationary: When the front remains stationary, it is called a stationary front.
  • Warm: When the warm air mass moves towards the cold air mass, the contact zone is a warm front.
  • Cold: When the cold air moves towards the warm air mass, its contact zone is called the cold front
  • Occluded: When an air mass is fully lifted above the land surface, it is called the occluded front.

Cyclones and Anticyclones

The fronts occur in middle latitudes and are characterized by steep gradient in temperature and pressure. They bring abrupt changes in temperature and cause the air to rise to form clouds and cause precipitation.

Formation of Extratropical Cyclones

  • Extratropical cyclones form along the polar front.
  • Initially, the front is stationary.
  • In the northern hemisphere, warm air blows from the south and cold air from the north of the front.
  • When the pressure drops along the front, the warm air moves northwards and the cold air move towards south, causing an anticlockwise cyclonic circulation.
  • The cyclonic circulation leads to a well-developed extra tropical cyclone, with a warm front and a cold front.
  • The plan and cross section of a well-developed cyclone is given in Figure 2.
  • The warm air glides over the cold air and a sequence of clouds appear over the sky ahead of the warm front and cause precipitation.
  • The cold front approaches the warm air from behind and pushes the warm air up.
  • As a result, cumulus clouds develop along the cold front.
  • The cold front moves faster than the warm front ultimately overtaking the warm front.
  • The warm air is completely lifted up and the front is occluded and the cyclone dissipates.

Cyclones and Anticyclones