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By definition a cyclone is a large mass of air which revolves around a strong centre of low atmospheric pressure zone. The direction of this rotation is anticlockwise in the Northern hemisphere while clockwise in the Southern hemisphere. In simple words a cyclone is an inward spiralling wind which rotates around a zone of low pressure. The largest low pressure system is called polar vortices. Warm core cyclones such as tropical cyclones and subtropical cyclones are under the gambit of synoptic scale ( these are the cyclones of largest scale e.g. extratropical cyclones). Also, cyclones are not just limited to earth but have been seen on extra terrestrial planets as well for example Mars, Jupiter and Neptune.
What are Cyclones?
Cyclones are just low-pressure centres, surrounded by closed isobars of increasing pressure. In simplest words, cyclones are defined as the system of winds that are rotating inwards to an area of low barometric pressure. Cyclones are formed with an enormous amount of energy from the ocean to the atmosphere. The surface winds get spiralled towards the low-pressure system due to the Coriolis force. The cyclonic systems are not formed in the equatorial regions because the Coriolis force is negligible between the latitudes 5 degrees north and 5 degrees south.
Cyclones are generally classified on the basis of their geographical location, namely Temperate cyclones and tropical cyclones.
What are Temperate Cyclones?
Temperate Cyclones are basically mid-latitude depression developed in the region extending from 35° to 65° in the both Northern as well as Southern Hemisphere. Temperate Cyclones, often termed as extra-tropical cyclones, are formed because of the intersection of two air masses of different contrasts, i.e. light tropical air masses and dense polar air masses. The major cause for the formation and origin of these cyclones are polar fronts.
Characteristics of Temperate Cyclones
Location: Temperate Cyclones are usually formed between 35-65 in both the hemispheres where the cold polar mass from the poles and warm tropical air mass collide with each other. Generally, these cyclones are pronounced more in the northern hemisphere because of higher temperature contrast.
Velocity: As the pressure gradient of these cyclones are comparatively less, the velocity of such cyclones is moderate and it ranges from 30 to 40 kmph.
Frequency: As we know, these cyclones are formed due to the convergence of air masses, and polar fronts are responsible for the same, there is no such confined season for these cyclones. These cyclones occur throughout the year, but the frequency is much more in winters as compared to what it is during summer.
Air Pressure: As the isobars of air are widely spread, the pressure isn’t that harsh and is very gentle.
Area covered by Temperate Cyclones: Generally, these cyclones take over a large area and occupy over 1500 kilometres in diameters. And if we consider the vertical area, it can reach up to Tropopause.
Formation: The major phenomena responsible for the formation of these cyclones are the formation of fronts due to variations in contrast of the air. Basically, it’s the thermal contrast of the air which causes a low-pressure centre, leading to these cyclones.
Direction of Movement: As we know, these cyclones are developed in medium latitude, wherein westerlies are prominent. Thus, the direction of the cyclone is from west to east. Considering the northern hemisphere, temperate cyclones move from southwest to northeast, whereas in the southern hemisphere, these cyclones move from northwest to southeast.
Life Cycle of Temperate Cyclones
The life cycle of temperate cycles comprises of four stages, namely
- The Initial Stage
- The Incipient Stage
- The Mature Stage
- The occlusion Stage
The Initial Stage
In the first stage, the chilly polar air and the hot tropical air masses blow parallel to the front. The polar air mass to the north of the front usually blows from east to west, whereas the warm air mass blows from west to east. At this stage, no wind shift occurs and thus the weather is completely fine. The cyclone is harmless at this stage.
The Incipient Stage
In this stage, a wave has formed on the front. Cold air turns in the south direction and warm air in the north direction. Each air mass encroaches into the domain of the other. A cyclonic circulation is initiated around a low centre at the apex of the wave. The new depression developing at the crest of the wave is called the “nascent cyclone”. This process is called cyclogenesis.
The Mature Stage
In this stage, the intensity of the cyclone increases. The curvature and amplitude of the wave undergo a marked increase. Now the cyclone is fully developed. Since the position of the cold front advances faster than the warm front, the warm sector becomes progressively narrower.
The Occlusion Stage
This is the final stage where the advancing cold front ultimately overtakes the warm front. And this results in the formation of an occluded front. Occlusion starts first near the apex of the wave where the warm front is closest to the cold front. Ultimately, the two cold air masses mix across the front and die out. The occlusion is of two types i.e. cold front occlusion and warm front occlusion. The lifespan of a single frontal cyclone is 5 to 7 days.
Origin and Development of Temperate Cyclones
Even though the exact mechanism behind the formation and origin of a temperate cyclone is still unknown, two Norwegian Meteorologists V. Bjerknes and J.Bjerknes proposed the Polar Front theory of temperate cyclones.
According to their theory, cyclonic unsettling influences structure along a front where polar and tropical air masses with differentiating actual properties are moving corresponding to it in inverse ways. Subsequently, cyclone arrangement happens where the front facing surface is misshaped into a wave-moulded intermittence.
The arrangement of a wave is demonstrative of thermal contrast.
There are a few elements which might create an unsteady wave. These variables are mountains, temperature differentiation or some close by aggravations.
As the cool polar air and warm air is avoided toward the equator and post individually, a tornado framing wave is shaped along the front. The wave subsequently shaped is isolated into two sections.
The eastern piece of the wave is known as the warm front. The western piece of the brokenness where cold polar air is supplanting the warm air is known as a cold front.
The checking movement added by the Coriolis power builds the amplitude of the wave.
Since the cold front advances at a quicker rate than the warm front, it surpasses the warm front. Presently the course of impediment begins and the cold air behind the cold front meets cold air in front of the warm front. The beginning of the course of impediment brings the tempest near its demise.
Size of Temperate Cyclones
There is a colossal variety in the shape and size of mild twisters. No calm tornado is by and large like some other. The isobars shaped in these tornadoes are practically round or circular. These isobars are described by low strain in their middle. The width of these tornadoes may shift from 160 km to 3200 km. The majority of these twisters have widths estimating 300 to 1500 km. The assessed region covered by a normal twister is around 1.6 million sq. km. The upward degree of a normal measured twister is assessed to be 10 to 12 km.
Direction of Temperate Cyclones
Temperate Cyclones move easterly, affected by westerly breezes. In the middle for uniting and rising air, these moving twisters produce mists and precipitation. These typhoons control the climate conditions in the centre scopes.
The temperate cyclone develops in the extra-tropical areas. There are important zones for the development of temperate cyclones.
- Atlantic- Arctic front and cyclone.
- North-America- Polar front and cyclone.
- Mediterranean front and cyclone.
- Extra-tropical cyclones in the Southern Hemisphere.
Conclusion
Cyclones are generally classified on the basis of their geographical location, namely Temperate cyclones and tropical cyclones. Temperate Cyclones are basically mid-latitude depression developed in the region extending from 35° to 65° in the both Northern as well as Southern Hemisphere. The major cause for the formation and origin of these cyclones are polar fronts. Characteristics of Temperate Cyclones. Location: Temperate Cyclones are usually formed between 35-65 in both the hemispheres where the cold polar mass from the poles and warm tropical air mass collide with each other. The cyclones occur throughout the year, but the frequency is much more in winters as compared to what it is during summer. As the isobars of air are widely spread, the pressure isn’t that harsh and is very gentle. Generally, the cyclones take over a large area and occupy over 1500 kilometres in diameters. The major phenomena responsible for the formation of these cyclones are the formation of fronts due to variations in contrast of the air. Direction of Movement: As we know, these cyclones are developed in medium latitude, wherein westerlies are prominent. Thus, the direction of the cyclone is from west to east. In the first stage, the chilly polar air and the hot tropical air masses blow parallel to the front. The cyclone is harmless at this stage. Each air mass encroaches into the domain of the other. The new depression developing at the crest of the wave is called the “nascent cyclone”. In this stage, the intensity of the cyclone increases. The curvature and amplitude of the wave undergo a marked increase. Occlusion starts first near the apex of the wave where the warm front is closest to the cold front. The occlusion is of two types i.e. Bjerknes and J.Bjerknes proposed the Polar Front theory of temperate cyclones. According to their theory, cyclonic unsettling influences structure along a front where polar and tropical air masses with differentiating actual properties are moving corresponding to it in inverse ways. The arrangement of a wave is demonstrative of thermal contrast. As the cool polar air and warm air is avoided toward the equator and post individually, a tornado framing wave is shaped along the front.
