Temperate Cyclones

Temperate Cyclones or Extra-Tropical Cyclones or Mid-latitude Cyclones or Frontal Cyclones or Wave Cyclones

Figure 1

Cyclones that develop beyond the tropics are known as temperate cyclones. They develop between 35° and 65° latitudes in the northern and southern hemispheres (Fig: 1) and for this reason they are called mid-latitude, temperate or extra-tropical cyclones. These three names tell us about the latitudinal location of these cyclones.

While frontal and wave suggest the mechanisms of these cyclones. We will read more on this below.

Polar front theory

Figure 2
Figure 3: Source regions of global air masses

This theory tries to explain the functioning of temperate cyclones. What it says is that warm and humid air from lower latitudes meets cold and dry air from higher latitudes or poles and forms a front as these two air masses don’t mix readily. Such a front formation is usually found between subtropical high and sub-polar low-pressure belts (Fig: 2) or between 35° and 65° latitudes.

Read more about ‘Air Masses’ & ‘Air Fronts’

Mechanism of a temperate cyclone

Figure 4: World heat map

Warm air from near the equator moves towards the north while cold air from the poles moves towards the equator or south (Fig. 4) and forms a front when it pushes against each other (Fig. 5 & 6).

Figure 6: Different air masses which affect North America as well as other continents, tend to be separated by frontal boundaries
Figure 5

Figure 7

Gradually pressure on the front drops and the warm air starts moving north and the cold air moves south. This moving air under the influence of Coriolis force is deflected towards the right in the northern hemisphere resulting in an anti-clockwise cyclonic circulation (Fig. 7). This cyclone circulation leads to a well developed temperate or extratropical cyclone and advances in the influence of jet streams.

Figure 8: Different air fronts

Depending on the direction of movement of air masses different fronts are formed. Figure 8 shows different weather fronts. These fronts are represented by different shapes and colours.

The air behind the warm front is warm and pushes the stationary cold air mass [Figure 9(a)].

The air behind the cold front is cold and pushes the stationary warm air mass [Figure 9(b)].

Figure 9(c) represents a condition when both the air masses become stationary. Such a front is known as a stationary front.

Figure: 10

Occluded Front: It is the last front or we can say the last stage of a temperate cyclone. When the lighter warm air is finally lifted up against the heavier cold air, the cyclone dissipates, thus forming an occluded front.

Different weather conditions prevail when different air masses interact with each other. To understand the weather conditions associated with the respective movement of air masses its important to have a look at the cross-section of these fronts. Read more about Air Fronts…