Temperate cyclones develop over mid-latitudes between 35 degrees and 65 degrees North and South in both the hemisphere. A temperate cyclone is called mid-latitude depression, extra-tropical cyclones, frontal depressions and wave cyclones. The wind direction movement blows from west to east and is more pronounced in the winter season.
Theory of Origin of Temperate Cyclones:
At the end of World War I, Norwegian meteorologists associated with ‘Bergen School of Meteorologist’ VilhelmBjerknes, his son J. Bjerknes and associates brought about significant advances in understanding the temperate cyclones. The first model identified and developed for the life cycle of mid-latitude cyclones. Being related to the development of an occluded front, wherein the mid-latitude cyclone’s cold front surpasses its warm front. Their examination explores outcomes about clarification of the Polar Front Theory of cyclones or simply as Wave Theory.
1. Polar Front Theory By Bjerknes:
As per this theory, the warm-humid air masses from the tropics meet the dry-cold air masses from the poles and thus, a polar front is formed. It develops a surface of discontinuity between two air masses. Such condition takes place over sub-tropical highs, sub-polar lows and along the tropopause. The cold air is denser and heavier, which occupies the lower ground space. Due to this reason, cold air pushes warm air upwards. The interaction between cold and warm air masses develops instability, and low pressure develops at the centre of interaction. The centre zone where warm and cold fronts are differentiated. Later on, this point creates the eye of the cyclone.
Stages of formation of temperate cyclone:
1. Initial Stage: The cold air and warm air mass tend to converge along an axis. In this condition, both the air masses are stable and in contact with each other. It is also called the stationary stage of front formation. Though the difference between temperature, moisture and pressure remains between two air masses. Which not create instability at a bigger stage. As time passes, warm air pushes to the eastern side, and cold air pushes to the western side. At the latter stage, the straight and flat formed front turned by the advancing air masses.
2. Incipient Stage: At the end of the first stage, the frontiers of the two air masses are different. Mainly impact on the warm air is observed. It is a warm front, and it blows towards the eastern part of the northern hemisphere. In the north and north-west part, the area occupied by cold air masses. Reverse to it, i.e. warm air masses influence the south and south-east area. It is due to the principles of the winds associated with the Coriolis effect and Ferrel’s law.
Warm air has found more moisture. Due to this, it is unstable. Opposite to this, cold air has less moisture. We also know that the addition or availability of more and more moisture causes the air to be more unstable. When at the centre of the two fronts, the instability is more. Which cold air mass pushes the warm air mass, and in this process, the centre becomes sharper. The warmer area starts shrinking and becomes bend sharp. The warm air mass is lighter in density and hence is pushed up on the cold air mass. The warm air is rising with moisture which cools down under cold air and increases the height. The air condenses, and after precipitation, latent heat released and then low pressure is developed at the centre of two fronts.
3. Mature: The low-pressure area at the centre and precipitation in the second stage. Because of the intensification, the isobars become closer. The pressure gradient becomes very sharp. Then air moves whirling way. The cold air mass is more capture area, and the warm air becomes small and shrinks. Now the winds are westerlies, and temperate cyclones are move towards the eastern side. Which result, weather condition changes.
4. Occlusion: Due to circulation, two different fronts are getting compressed. At the end of the third stage, two fronts were becoming closer and then merging, called Occluded front. In this stage, the cyclone starts, and low pressure gets weakened. The intensity becomes low, and the wind velocity also declines. Cold air at the ground and warm air move above the ground.
5. Dissipation: At the end of the occlusion stage, it is significantly weakened. In this stage, cold air occupies more areas. This condition happens due to dense and heavy in comparison to warm air. The warm air then cools down under the influence of the cold air, which eliminates low pressure and the usual condition reach.
Characteristics of Temperate Cyclone:
1. The size of the temperate cyclone is asymmetrical.
2. Shape of the temperate cyclone like inverted ‘V’.
3. This cyclone vary with season, location and from the cyclone to cyclone.
4. The wind force is more on the eastern and southern portions, mainly in Northern America.
5. The movement of the cyclone is westerlies and direct east-west.
6. Advances of the temperate cyclone is indicated by a fall in temperature, wind shift and a halo around the sun and the moon.
Distribution of the temperate cyclone:
1. Over USA and Canada, Mexican Gulf
2. The belt extending from Iceland to the Barents Sea and carry on with Russia and Siberia.
3. Winter storm over the Baltic Sea.
4. In the Winter season, it extends up to India due to western disturbances.
5. The Antarctic frontal zone.