Introduction

The word ‘cyclone’ comes from the Greek word ‘KUKLOMA’, which means snake coil. There are two types of cyclones, tropical cyclones, and extratropical. Cyclones are a natural phenomenon. If a small space becomes too hot then a deep depression center is formed there. In order to maintain the balance of the air pressure, the air from the adjacent place rushes towards the center of low pressure and rises upwards thus called a cyclone.

A deep depression is created in the center of the cyclone. There is a strong air pressure slope around this center. So the air pressure from all sides rushes towards the center to maintain the pressure balance. Each cyclone has a circular center in the middle. This is called the eye of a cyclone. Cyclones can be observed in tropical and subtropical regions. Therefore, cyclones are divided into two groups based on location, namely tropical and extratropical cyclones (1) & (4).

What are tropical cyclones?

Definition

Tropical cyclones occur in the tropics of both hemispheres between 5° to 20° north and south of the equator. In this region, the air gets warmer and lighter due to extreme heat in summer. The result is a deep depression in the region. Then the cool air around them rushed towards the center of the deep depression. In a tropical region, such a strong, centrifugal upward-moving wind is called a tropical cyclone.

Characteristics

1. Strong depression prevails in the center of tropical cyclones.

2. Wind speed is very high in tropical cyclones.

3. Tropical cyclonic winds are centrifugal and wind flows from cool regions to warmer regions.

4. The oceans are the main source of this type of cyclone.

5. Tropical cyclones receive a lot of rainfall.

6. There is no anticyclone in this cyclone.

7. This cyclone is destructive and its progression is slow.

8. Tropical cyclones form only in warm seas.

9. This type of cyclone spreads over a small area.

10. Tropical cyclones are seen in summer and autumn.

11. It has no front.

12. This cyclone is created in the same type of air (2) & (4).

What are Extratropical cyclones or temperate cyclones?

Definition

Cyclones are formed by the conjunction of two different air masses in the mid-latitude or temperate zone between the latitudes of 30° to 65° north and south of the equator. This type of cyclone is called an extra-tropical cyclone or a temperate cyclone. It is called a mid-latitudinal cyclone because it is mainly formed in the middle latitudes (1) & (3).

Location

Extra-tropical cyclones occur in both hemispheres between 30° to 65° latitudes.

Formation of extra-tropical cyclone

The main cause for the formation of extratropical cyclones is the merging of air masses with two different physical properties. These air masses are of different types according to temperature, density, and flow direction.

For example, in the northern hemisphere, warm and humid westerly winds from the southwest and cool and dry northeastern polar winds from the north combine in the mid-latitudes to form extra-tropical cyclones.

In the subtropics, cold and dry air masses from the Polar Regions collide with warm and humid air masses coming from the tropics. Cold and heavy air takes the place of warm air at the junction of warm and cool air masses. The result is instability. Then the light water vapor warms up at high speed and causes rain. This is how extra-tropical cyclones form. It is also known as a wave cyclone (5).

Characteristics

1. Extra-tropical cyclones originate at sea or on land.

2. These cyclones are confined to temperate regions. Never enters the tropics.

3. This type of cyclone spreads over a large area.

4. It is strongly seen in winter.

5. This cyclone must have a front.

6. Extra-tropical cyclones form as a result of the conjunction of two different properties.

7. A strong depression present in the center of an extra-tropical cyclone.

8. The primary area of this type of cyclone is the sea region.

9. At the center of this cyclone, the air is turbulent and upward.

10. This cycle is not as strong and destructive as the tropical cyclone.

11. Here the temperature is not evenly distributed.

12. In this case, there is uneven light rain.

13. Nimbostratus and alto-stratus clouds form a few days of light rain in temperate cyclones.

14. Cold air circulates in temperate cyclones, only warm air rises.

15. This cyclone is moving much faster.

16. Usually, anticyclones occur with this cyclone.

17. The position of the eye of the cyclone cannot be observed in temperate cyclones like tropical cyclones (1) & (5).

Stage of formation of extra-tropical cyclone

1918. Berkness and J. Berkness introduced the polar frontier theory of the formation of extra-tropical cyclones in 1918. They explain the life cycle of temperate cyclones divided into six stages. These are-

First stage

In the first stage, two opposite air masses are located opposite each other. One of these two air masses is hot and humid and the other is cold and dry. A natural front is formed between these two air masses. This time there is calm weather.

Second stage

At this stage, the cool air swells in the form of waves and enters the warm air. As a result, when a wave-like fold is created on the natural front, two fronts are formed. The front at which the cool air mass is higher is called the cold front and the front where the warm air mass is stronger is known as the warm front. This stage is the beginning of extra-tropical cyclone formation, so it is known as the beginning phase of cyclones.

Third stage

In the third stage, the sharpness of the fold increases as the cold air mass pushes the warm air mass upwards. At this stage, the cyclone develops completely. So it is called the mature stage. As the warm air mass rises, the cool air mass descends and pushes the warm air upwards. The cold front and the warm front are closer and the waves rise sharply above the top.

Fourth stage

When the warm and humid air rises diagonally over the cold air in this way, it is blocked by the cold air around it and condenses and causes heavy rainfall. At this stage in the life cycle of a cyclone, the occluded front is formed. This condition is known as occlusion. It usually rains for several days.

In this case, the cold front moves further towards the warm front. So the warm front gradually shrinks towards the polar wind. This warm air is surrounded by cold air from three directions. At this stage the intensity of the cyclone is high.

Fifth stage

At this stage, the cyclone gradually gets weaker. As the warm air is surrounded by the cold air at the occluded front, the cyclonic energy supply decreases and it is located on a cold front instead of a warm front.

Sixth stage

In the sixth stage, the cyclone ends and the occluded front is removed. The curvature of the front is eliminated and the warm and cold air masses come in opposite directions and flow in parallel as before. At this stage, there is no longer a difference in temperature between the warm and cold front, resulting in the extinction of the cyclone (4).

Structure

• The average diameter of a temperate cyclone is 200 to 3000 km.
• Its vertical depth is less than that of tropical cyclones.
• The average wind speed here is 30 to 35 km.
• Extra-tropical cyclones are elliptical, oval, and ‘V-shaped.
• These cyclones form along warm and cold fronts (1).

Effects

• The presence of different weather conditions can be observed in different parts of extra-tropical cyclones.
• Before the extra-tropical cyclone arrives, the air pressure decreases, and the circular luminosity of the cirrus cloud is centered on the moon and the sun.
• The effect of this cyclone is a sudden increase in temperature and the direction of airflow gradually changes from east to southeast.
• This causes the sky to be covered with a dense black of nimbostratus clouds and the onset of rainfall.
• In this case, as the warm air gradually rises above the cool air, the amount of precipitation increases, and the visibility is less due to the presence of foggy air in the precipitation.
• Eventually, it rains with thunderstorms, and this is when the anticyclone arrives (1) & (2).

Q&A

1. In what direction do extra-tropical cyclones typically track across the US?

Extra-tropical cyclones typically track across the southwest to northeast in the US.

2. In what direction do extra-tropical cyclones typically track across the US?

Southwest part to the northeast.

3. In what direction do extra-tropical cyclones typically track across the United States?

These cyclones typically track across the southwest part to the northeast of the United States.

4. Why extra-tropical (i.e. middle latitude) cyclones generate fronts?

In extratropical cyclones, two different properties of air masses (warm and cold) are merged. The temperature and humidity of these air masses are different. When these two heterogeneous air masses merge with each other, they form a front. This is why extra-tropical (i.e. middle latitude) cyclones generate fronts.

5. Which of the following statements is not true about extra-tropical cyclones?

Extra-tropical cyclones are not disastrous hazards in temperate regions.