09th Sept 2021

 

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Cyclones

• Cyclones are rapid inward air circulation around a low-pressure area. The air circulates in an anticlockwise direction in the Northern hemisphere and clockwise in the Southern hemisphere.
• Cyclones are usually accompanied by violent storms and bad weather.
• The word Cyclone is derived from the Greek word Cyclos meaning the coils of a snake. It was coined by Henry Peddington because the tropical storms in the Bay of Bengal and the Arabian Sea appear like coiled serpents of the sea.

Types of Cyclone

There are various types of cyclones depending on the type of prevailing low-pressure system.

1. Tropical cyclone
2. Extra-tropical cyclone
3. Tornadoes

How are Cyclones addressed in Different Locations?

Cyclones are addressed by different names in different locations.

1. Hurricanes – In the Atlantic and Eastern Pacific.
2. Typhoons – In Southeast Asia
3. Cyclone – In the Indian Ocean and Western Pacific around Australia.

What are tropical Cyclones?

• A Tropical cyclone is an intense circular storm that originates over warm tropical oceans and is characterized by low atmospheric pressure, high winds, and heavy rain.
• Cyclones are formed over slightly warm ocean waters. The temperature of the top layer of the sea, up to a depth of about 60 meters, need to be at least 28°C to support the formation of a cyclone.
• This explains why the April-May and October-December periods are conducive for cyclones.
• Then, the low level of air above the waters needs to have an ‘anticlockwise’ rotation (in the northern hemisphere; clockwise in the southern hemisphere).
• During these periods, there is an ITCZ in the Bay of Bengal whose southern boundary experiences winds from west to east, while the northern boundary has winds flowing east to west.
• Once formed, cyclones in this area usually move northwest. As it travels over the sea, the cyclone gathers more moist air from the warm sea which adds to its heft.

Formation of a Cyclone

(The above figure shows how cyclones form. The green arrows show where warm air is rising. The red arrows indicate where cool air is sinking)

Requirements for a Cyclone to form

There are six main requirements for tropical cyclogenesis:

• Sufficiently warm sea surface temperatures.
• Atmospheric instability.
• High humidity in the lower to middle levels of the troposphere.
• Enough Coriolis force to develop a low-pressure center.
• A preexisting low-level focus or disturbance.
• Low vertical wind shear.

Origin

• The formation and initial development of a cyclonic storm depends upon the transfer of water vapour and heat from the warm ocean to the overlying air, primarily by evaporation from the sea surface.
• It encourages formation of massive vertical cumulus clouds due to convection with condensation of rising air above the ocean surface.
• Under favorable conditions, multiple thunderstorms originate over the oceans. These thunderstorms merge and create an intense low pressure system (wind is warm and lighter).

Early Stage 

• In the thunderstorm, air is uplifted as it is warm and light. At certain height, due to lapse rate and adiabatic lapse rate, the temperature of air falls and moisture in the air undergoes condensation.
• Condensation releases latent heat of condensation making the air more warmer. It becomes much lighter and is further uplifted.
• The space is filled by fresh moisture laden air. Condensation occurs in this air and the cycle is repeated as long as the moisture is supplied.
• Due to excess moisture over oceans, the thunderstorm intensifies and sucks in air at much faster rate. The air from surroundings rushes in and undergoes deflection due to Coriolis force creating a cyclonic vortex (spiraling air column. Similar to tornado).
• Due to centripetal acceleration (centripetal force pulling towards the center is countered by an opposing force called centrifugal force), the air in the vortex is forced to form a region of calmness called an eye at the center of the cyclone. The inner surface of the vortex forms the eye wall, the most violent region of the cyclone.
• All the wind that is carried upwards loses its moisture and becomes cold and dense. It descends to the surface through the cylindrical eye region and at the edges of the cyclone.
• Continuous supply of moisture from the sea is the major driving force behind every cyclone. On reaching the land the moisture supply is cut off and the storm dissipates.
• If ocean can supply more moisture, the storm will reach a mature stage.

Mature Stage

• At this stage, the spiraling winds create multiple convective cells with successive calm and violent regions.
• The regions with cumulonimbus cloud (rising limbs of convective cell) formation are called rain bands below which intense rainfall occurs.
• The ascending air will lose moisture at some point and descends (subsides) back to surface through the calm regions (descending limbs of convection cell – subsiding air) that exist between two rain bands.
• Cloud formation is dense at the center. The cloud size decreases from center to periphery.
• Rain bands are mostly made up of cumulonimbus clouds. The ones at the periphery are made up of nimbostratus and cumulus clouds.
• The dense overcast at the upper levels of troposphere is due to cirrus clouds which are mostly made up of hexagonal ice crystals.
• The dry air flowing along the central dense overcast descends at the periphery and the eye region.

Structure of a tropical cyclone

Eye

• The “eye” is a roughly circular area of comparatively light winds and fair weather found at the center of a severe tropical cyclone.
• There is little or no precipitation and sometimes blue sky or stars can be seen.
• The eye is the region of lowest surface pressure and warmest temperatures aloft (in the upper levels) – the eye temperature may be 10°C warmer or more at an altitude of 12 km than the surrounding environment, but only 0-2°C warmer at the surface in the tropical cyclone.
• Eyes range in size from 8 km to over 200 km across, but most are approximately 30-60 km in diameter.

Eye wall

• The eye is surrounded by the “eye wall”, the roughly circular ring of deep convection, which is the area of highest surface winds in the tropical cyclone. Eye Wall region also sees the maximum sustained winds i.e. fastest winds in a cyclone occur along the eye wall region.
• The eye is composed of air that is slowly sinking and the eye wall has a net upward flow as a result of many moderate – occasionally strong – updrafts and downdrafts [Explained in ‘Thunderstorms’].
• The eye’s warm temperatures are due to compressional warming (adiabatic) of the subsiding air.
• Most soundings taken within the eye show a low-level layer, which is relatively moist, with an inversion above – suggesting that the sinking in the eye typically does not reach the ocean surface, but instead only gets to around 1-3 km of the surface.

Spiral bands

• Another feature of tropical cyclones that probably plays a role in forming and maintaining the eye is the eye wall convection.
• Convection in tropical cyclones is organized into long, narrow rain bands which are oriented in the same direction as the horizontal wind.
• Because these bands seem to spiral into the center of a tropical cyclone, they are called “spiral bands”.
• Along these bands, low-level convergence is a maximum, and therefore, upper-level divergence is most pronounced above.
• A direct circulation develops in which warm, moist air converges at the surface, ascends through these bands, diverges aloft, and descends on both sides of the bands.
• Subsidence is distributed over a wide area on the outside of the rain band but is concentrated in the small inside area.
• As the air subsides, adiabatic warming takes place, and the air dries.
• Because subsidence is concentrated on the inside of the band, the adiabatic warming is stronger inward from the band causing a sharp contrast in pressure falls across the band since warm air is lighter than cold air.
• Because of the pressure falls on the inside, the tangential winds around the tropical cyclone increase due to increased pressure gradient. Eventually, the band moves toward the center and encircles it and the eye and eye wall form.
• Thus, the cloud-free eye may be due to a combination of dynamically forced centrifuging of mass out of the eye into the eye wall and to a forced descent caused by the moist convection of the eye wall.

Vertical Structure of a Tropical Cyclone

There are three divisions in the vertical structure of tropical cyclones.

• The lowest layer, extending up to 3 km and known as the inflow layer, is responsible for driving the storm.
• The middle layer, extending from 3 km to 7 km, is where the main cyclonic storm takes place.
• The outflow layer lies above 7 km. The maximum outflow is found at 12 km and above. The movement of air is anticyclonic in nature.

Categories of Tropical Cyclones

Characteristics of Tropical Cyclones

The main features of tropical cyclones are as follows.

Size and Shape

• Tropical cyclones have symmetrical elliptical shapes (2:3 ratio of length and breadth) with steep pressure gradients. They have a compact size—80 km near center, which may develop up to 300 km to 1500 km.

Wind Velocity and Strength

• Wind velocity, in a tropical cyclone, is more in poleward margins than at center and is more over oceans than over landmasses, which are scattered with physical barriers. The wind velocity may range from nil to 1200 km per hour.

Path of Tropical Cyclones

• These cyclones start with a westward movement, but turn northwards around 20° latitude. They turn further north-eastwards around 25° latitude, and then eastwards around 30° latitude. They then lose energy and subside.
• Tropical cyclones follow a parabolic path, their axis being parallel to the isobars.
• Coriolis force or earth’s rotation, easterly and westerly winds influence the path of a tropical cyclone.
• Tropical cyclones die at 30° latitude because of cool ocean waters and increasing wind shear due to westerlies.

Tides

• The periodical rise and fall of the sea level, once or twice a day, mainly due to the attraction of the sun and the moon, is called a tide.
• Movement of water caused by meteorological effects (winds and atmospheric pressure changes) are called surges (storm surge during cyclones).
• The study of tides is very complex, spatially and temporally, as it has great variations in frequency, magnitude and height.
• The moon’s gravitational pull to a great extent and to a lesser extent the sun’s gravitational pull, are the major causes for the occurrence of tides.
• Another factor is centrifugal force which acts opposite to gravitational pull of earth.
• Tides occur due to a balance between all these forces.

Factors Controlling the Nature and Magnitude of Tides

• The movement of the moon in relation to the earth.
• Changes in position of the sun and moon in relation to the earth.
• Uneven distribution of water over the globe.
• Irregularities in the configuration of the oceans.
• On the surface of the earth, the horizontal tide generating forces are more important than the vertical forces in generating the tidal bulges.
• The tidal bulges on wide continental shelves, have greater height. When tidal bulges hit the mid-oceanic islands they become low.
• The shape of bays and estuaries along a coastline can also magnify the intensity of tides.
• Funnel-shaped bays greatly change tidal magnitudes. When the tide is channeled between islands or into bays and estuaries they are called tidal currents (tidal bore is one such tidal current).

Types of Tides

• Tides vary in their frequency, direction and movement from place to place and also from time to time.
• Tides may be grouped into various types based on their frequency of occurrence in one day or 24 hours or based on their height.

Tides based on Frequency

Semi-diurnal tide

• The most common tidal pattern, featuring two high tides and two low tides each day [Actually it varies between 3 tides to 4 tides –– 3 tides in rare cases but 4 is normal]. The successive high or low tides are approximately of the same height.

Although tides occur twice a day, their interval is not exactly 12 hours. Instead, they occur at regular intervals of 12 hours and 25 minutes.

• This is because the moon revolves around the earth from west to east, and each day it moves a bit to the east if observed from the same place on earth at the same time on two consecutive days.
• This time lag explains the tide interval of 12 hours and- 25 minutes, as tides occur twice a day.
• A place in England—Southampton—experiences tides 6-8 times a day [2 high tides from North Sea + 2 high tides from English Channel + 2 neap tides from North Sea + 2 neap tides from English Channel]. This happens because the North Sea and the English Channel push the water at different intervals.

Diurnal tide

• There is only one high tide and one low tide during each day. The successive high and low tides are approximately of the same height.

Mixed tide

• Tides having variations in height are known as mixed tides. These tides generally occur along the west coast of North America and on many islands of the Pacific Ocean.

Tides based on the Sun, Moon and the Earth Positions

• The height of rising water (high tide) varies appreciably depending upon the position of sun and moon with respect to the earth. Spring tides and neap tides come under this category.

Spring tides

• The position of both the sun and the moon in relation to the earth has direct bearing on tide height.
• When the sun, the moon and the earth are in a straight line, the height of the tide will be higher.
• These are called spring tides and they occur twice a month, one on full moon period and another during new moon period.

Neap tides

• Normally, there is a seven day interval between the spring tides and neap tides.
• At this time the sun and moon are at right angles to each other and the forces of the sun and moon tend to counteract one another.
• The Moon’s attraction, though more than twice as strong as the sun’s, is diminished by the counteracting force of the sun’s gravitational pull.
• Like spring tides, these tides also occur twice a month.

The magnitude of tides based on Perigee and apogee of moon

• Once in a month, when the moon’s orbit is closest to the earth (perigee), unusually high and low tides occur. During this time the tidal range is greater than normal.
• Two weeks later, when the moon is farthest from earth (apogee), the moon’s gravitational force is limited and the tidal ranges are less than their average heights.

The magnitude of tides based on Perigee and Apogee of earth

• When the earth is closest to the sun (perihelion), around 3rd January each year, tidal ranges are also much greater, with unusually high and unusually low tides.
• When the earth is farthest from the sun (aphelion), around 4th July each year, tidal ranges are much less than average.

Ebb and Flood

• The time between the high tide and low tide, when the water level is falling, is called the ebb.
• The time between the low tide and high tide, when the tide is rising, is called the flow or flood.

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