What are cyclones? What are the differences between cyclones and anticyclones? What are their types? How do they originate? What are the conditions for their formation? Read to know more here.
Cyclones are also termed atmospheric disturbances.
When the velocity of winds increases to such an extent that they attain strong force, a cyclonic storm is formed.
What is a Cyclone?
Cyclones are the centers of low pressure. They are surrounded by closed isobars having increasing pressure outward.
The wind circulation is from outside towards the central low pressure. They rotate anti-clockwise in the Northern hemisphere and clockwise in the southern hemisphere.
The inward flow of air is due to pressure gradient force and the lowest pressure at the center.
Due to Coriolis force, blowing winds are deflected from their paths- rightwards in the Northern hemisphere and leftward in the Southern hemisphere.
Types of cyclones
From the location point of view cyclones are classified into two principal types:
- Extratropical cyclones Temperate cyclones or wave cyclones
- Tropical cyclones
(1) Temperate Cyclone
Temperate cyclones also called extratropical cyclones or waves cyclones, are low-pressure centers produced in the mid-latitudes during winters in both hemispheres.
They are formed in regions extending between 35-65 degree latitudes in both hemispheres due to the convergence of two contrasting air masses.
They are most intensified and frequent during winters as thermal contrast is high during that time.
The polar front theory discusses and explains the origin of temperate cyclones, according to this theory frontogenesis is a precondition for the formation of temperate cyclones.
Hence the conditions necessary for the development of a front are also required for the development of a temperate cyclone:
- The presence of two opposing air masses has contrasting properties of air temperature and pressure density and humidity.
- The two air masses must move in the face direction.
Temperate cyclones cover a large area as these are formed due to the convergence of large and contrasting air masses. Sometimes temperate cyclones are so extensive that they cover an area of about 10 lakh square kilometers.
Temperate cyclones extend up to 10 km from sea level in the upper troposphere.
Moisture present in hot humid and lighter air mass is the source of energy in temperate cyclones. This moisture later becomes a source of cooling condensation cloud formation and rainfall. Since the source of energy in temperate cyclones is moisture present in warm air mass they can occur and develop over both oceanic and continental surfaces.
Temperate cyclones are cyclones of mid-latitudes and hence are primarily under the influence of permanent winds of mid-latitude that is the westerlies.
The movement is these cyclones are eastwards of the origin with an average velocity of 32 km per hour in summers and 48 km per hour in winters.
The formation of temperate cyclones is a quick process but passes through a series of successive stages:
The period of a cyclone from its inception (cyclogenesis) to its termination (frontolysis or occlusion) is called the life cycle of a cyclone. It is completed through 6 successive stages and is described by the polar front theory.
Stage 1: It involves the convergence of two masses of contrasting physical properties and directions.
Stage 2: It is also called an incipient stage during which the warm and cold air masses penetrate the territories of each other and a wave-like front is formed under the influence of Coriolis force.
Stage 3: It is the mature stage when the cyclone is fully developed and isobars become almost circular. The energy and intensity of the cyclone are greatest in this stage. Rainfall is generated by nimbostratus clouds along the warm front and cumulonimbus clouds generate high-intensity rainfall at cold fronts.
Stage 4: The warm sector of the cyclone is narrow due to the more advancement of the cold front full stop as a warm front declines energy reservoir of the cyclonic system starts declining.
Stage 5: It starts with the occlusion of the cyclone when the advancing cold front finally overtakes the warm front and an occluded front is formed. The intensity of rainfall and winds declines further.
Stage 6: In this warm sector completely disappears occluded front is eliminated cold air mass covers the surface and ultimately cyclone dies out.
Temperate cyclones are relatively regular and predictable compared to tropical cyclones and a primarily observed in the winter seasons.
Wave cyclones originating over the Mediterranean sea and Caspian sea flow eastward under the influence of subtropical westerly jet stream. The cyclones enter into North-Western parts of India during the winter season to generate snowfall on the Himalayan mountain ranges of Jammu and Kashmir Himachal Pradesh and Uttarakhand. They also cause rainfall in the plains of Punjab Haryana Western Uttar Pradesh Delhi and northern Rajasthan and support Rabi crops.
(2) Tropical cyclone
Tropical cyclones are formed between 5 degrees to 30 degrees in both the Northern and Southern hemispheres. These are not formed over or near the equator due to a lack of Coriolis force.
They are low-pressure atmospheric systems of low latitudes.
Cyclones developed in the regions lying between the tropics of Capricorn and cancer are called tropical cyclones which are not regular and uniform like extratropical or temperate cyclones.
Numerous forms of cyclones vary considerably in shape size velocity and weather conditions.
Different names of tropical cyclones in different regions of the world:
- Southeast Asia- typhoons
- Caribbean sea- hurricane
- Indian Ocean- tropical cyclones
- Northeast coast of Australia- Willy Willy
- A tornado is the smallest and most deadly form of cyclone in the Mississippi valley of the USA. They are funnel-shaped storms that are small but the most violent and disastrous of all storms. The center of a tornado is characterized by extremely low pressure.
Because of such steep pressure gradient winds rush hour with great force towards the center having a high velocity of 600 to 800 km per hour. The approach of tornadoes is heralded by dark and thick clouds in the sky resulting in complete darkness and minimum visibility and low air pressure.
Conditions required for the origin of tropical cyclones:
- Presence of a warm Ocean surface with a temperature of 27 degrees Celsius or more. The warm surface is the source of thermal convection and strong hot and humid currents. It is because of this factor that tropical cyclones are characteristically developed over the east coast of continents under the influence of warm ocean currents.
- The presence of ITCZ (Inter Tropical Convergence Zone) supports the intensification of low pressure and augments the supply of moisture due to the inflow of trade winds.
- Coriolis force helps in deflection of strong influence wind to generate circulatory system. In the absence of Coriolis force tropical cyclones do not originate near the equator.
- Presence of high moisture in the atmosphere as it becomes the source of latent heat of vaporization which is the basic source of energy for a tropical cyclone.
- Minimum vertical wind shear to support the unrestricted flow of air currents upwards.
- Divergence of air in the upper troposphere.
Tropical cyclones cover relatively smaller areas but are more destructive. On average their diameters range between 80-300 km but sometimes they become so small that the diameter is restricted to 50 km or less. But they are relatively taller and extend up to 15 km from sea level in the upper troposphere.
The basic source of energy is the latent heat of vaporization present in hot and humid thermal convective currents. This latent heat of vaporization later becomes a source of cooling condensation cloud formation and rainfall.
Normally they move from east to west under the influence of trade winds hence the general direction is there for the West word from their origin.
A fully developed tropical cyclone consists of:
- Eye: the pressure is lowest in the eye. It is a zone of high temperature and high humidity. It is also a cloudless, windless, and rain-less zone.
- Inner eyewall: It is the most destructive part of a tropical cyclone. It is characterized by strong winds and the greatest intensity of rainfall. Rainfall is generated through cumulonimbus clouds.
- Outer eyewall: It is the outermost part of a tropical cyclone. In this zone, wind velocity is relatively low and so is the intensity of rainfall.
Tropical cyclones become very vigorous with high velocity over the oceans but become weak while moving over land areas and ultimately die out after reaching the interior portion of the continents. That is why the cyclones affect only the coastal areas of the continents.
Anticyclones are centers of high pressure. They are surrounded by closed isobars having decreasing pressure outward.
The circulation is from central high pressure towards the periphery in such a way that air blows outwards in a clockwise direction in the Northern hemisphere and anticlockwise direction in the southern hemisphere.
Due to Coriolis force, blowing winds are deflected from their paths to the right in the Northern hemisphere and left in the Southern hemisphere, that’s how it gets circular with a flowing system.
The difference in pressure between the center and periphery of anticyclone ranges between 10 to 20 mb and sometimes higher.
There are much larger in size and area than temperate cyclones as the diameter is 75% larger than that of temperate cyclones. Temperate anticyclones are very extensive that a single anticyclone can cover nearly half of the USA.
The track is highly variable and unpredictable. They move very sluggishly and sometimes they become stationary over a particular place for 4 days. The average velocity of an anticyclone is 30 to 50 km per hour.
Anticyclones originated due to the descent of either polar cold air mass or warm tropical air mass.
These anticyclones are high-pressure systems and are more common in subtropical high-pressure belts and polar high-pressure belts where the air is sinking from the upper troposphere to the lower troposphere but are practically absent in equatorial regions.
The formation of anticyclonic conditions at polar high-pressure belts is a thermal phenomenon as these bills a thermally direct whereas the formation of anticyclonic conditions at subtropical high-pressure belts is a dynamic phenomenon as these bills are thermally indirect.
Hence, anticyclones of polar high-pressure belts are termed thermal anticyclones, and anticyclones of subtropical high-pressure belts are termed dynamic anticyclones.
Types of anticyclones:
- Cold anticyclones or thermal anticyclones: They are formed above the Polar Regions due to the sinking of air. Post subsidence of air outflows from the polar region in an easterly and southeasterly direction.
- Warm anticyclones or dynamic anticyclones: They are formed above warm subtropical regions due to the sinking of air from the upper troposphere to the lower troposphere and consequent divergence of air.
- Blocking anticyclones: These developed due to obstruction in the air circulation in the upper troposphere that develops over mid-latitudes and are called blocking because they obstruct the flow of temperate cyclones in mid-latitudes.
Anticyclones tend to produce fairly uniform weather. Whence descends from above at the center and the weather becomes clear and rainless because the descending wind brings atmospheric stability. The weather of Canada USA and northern Eurasia is mostly affected by anticyclones.
Colour code for Cyclones by IMD
Green: The color green denotes that everything is smooth and in order, or that “everything is well” with no bad weather.
Yellow: The yellow color code requests that the security personnel “be updated” to deal with adverse weather that could linger for days and could have an impact on everyday activities.
Orange: The color-coded alert meaning “be prepared” is orange. It may serve as a warning of severe damage to communication breakdowns that could result in power outages, traffic and rail jams, and other problems. The orange notice also serves as a warning to evacuate and to prepare the essentials for families.
Red: The color red represents the strongest level of warning and urges authorities to “take action.” In this situation, the worst weather conditions pose a hazard to human life. With the assistance of disaster management response teams, all necessary steps are taken in this circumstance to manage the situation.
UPSC mains question 2022
Discuss the meaning of color-coded weather warnings for cyclone-prone areas given by the India Meteorological Department. (Answer in 150 words) 10 marks (GS I)