Theories of Geography Part 13 – Winds, Rainfall and Cyclones


Global Winds

When air moves in a definite direction, it is called wind. If the winds move from west to east, they are called westerlies. If they move from east to west, they are called easterlies. There are winds because there are differences in pressures. The direction of wind is also affected by coriolis affect.

Due to Coriolis Force, the wind flowing from equator towards the North Pole and from North Pole towards the equator are deflected to their right while the winds flowing north- south and south-north in the southern hemisphere are deflected towards their left. The magnitude of the deflection, or “Coriolis effect,” varies significantly with latitude. The Coriolis Effect is zero at the equator and increases to a maximum at the poles. The effect is proportional to wind speed; that is, deflection increases as wind strengthens. The resultant balance between the pressure force and the Coriolis force is such that, in the absence of surface friction, air moves parallel to isobars (lines of equal pressure). This is called the geotropic wind. The Coriolis force explains why winds circulate around high and low pressure systems as opposed to blowing in the direction of the pressure gradient. Central idea behind the Coriolis force is that when the earth rotates from west to east, it produces the centrifugal force and due to this force, there is a change in the direction of the wind. There is Ferrel’s law derived from Coriolis Effect, which says that in northern hemispheres, wind deflects towards the right and in southern hemisphere wind deflects towards left. This means that in northern hemisphere, wind deflects clockwise, while in southern hemisphere, wind deflects anti- clockwise.

Trade winds

Trades wind blow out from the Subtropical High Pressure belts. In the northern hemisphere, they blow towards the equatorial low and called North East Trade Winds. In the Southern hemisphere they blow towards the equatorial low and become the South East Trade winds. This implies that Trade winds blow from North east towards equator in Northern hemisphere and South East Towards equator in southern hemisphere.  It has been shown in the following graphics.

The trade winds are most regular winds of all kinds on earth. They blow with great force and in constant direction that is why they are preferred by the sailors. The trade winds bring heavy rain falls and sometimes contain intense depressions.

Trade winds and Hadley cells

There are three primary circulation cells on earth known as the Hadley cell, Ferrel cell, and Polar cell.  The Hadley cell mechanism provides an explanation for the trade winds. Hadley cell is a closed circulation loop, which begins at the equator with warm, moist air lifted aloft in equatorial low pressure areas (the Intertropical Convergence Zone, ITCZ) to the tropopause and carried pole ward.

At about 30°N/S latitude, it descends in a high pressure area. Some of the descending air travels equatorially along the surface, closing the loop of the Hadley cell and creating the Trade Winds. Hadley Cells is described to be lying on equator but it follows sun’s zenith point, or what is termed the “thermal equator”.

Origin of Trade Winds

Trade winds are part of the Hadley cell circulation. At the equator, a low-pressure area of calm, light variable winds is known Intertropical Convergence Zone as we discussed above. The air lifts from here and at around 30° North and South, the air begins to descend toward the surface in subtropical high-pressure belts known as subtropical ridges. At the surface, the air flows from these subtropical high-pressure belts toward the Equator but is deflected toward the west in both hemispheres by the Coriolis Effect. Thus, these winds blow predominantly from the northeast in the Northern Hemisphere and from the southeast in the Southern Hemisphere. Because winds are named for the direction from which the wind is blowing, these winds are called the northeast trade winds in the Northern Hemisphere and the southeast trade winds in the Southern Hemisphere. The trade winds meet at the doldrums.

Implications of Trade winds

  • Trade winds are the surface winds in low latitudes, representing the low-level airflow. Back in history, two large belts of winds were discovered blowing toward the equator called North East and South East trade winds. The word trade in those days referred to advance steadily and was synonymous with efficient sailing. The trade winds allowed the sailing vessels to advance steadily—and, of course, to set up patterns of international trade. However, you must note that trade winds are not totally steady in force or direction, but they do trend in the general direction of southwest and northwest.
  • Hawaii is located south of Tropic of Cancer, yet, the temperatures are pleasant, temperatures and humidity tends to be a bit less extreme. This makes it one of the most famous tourist destinations of the world. What make such a climate are Trade Winds.


The directions of the Westerlies are opposite to trade winds and that is why they are also called antitrade winds. Westerlies blow in the middle latitudes between 30 and 60 degrees latitude, and originate from the high pressure area in the horse latitudes towards the poles. Under the effect of the Coriolis force, they become the south westerlies in the northern hemisphere and Northern westerlies in the southern hemisphere. Please note that in the southern hemisphere, there is more of ocean and less of land in comparison to the northern hemisphere. Due to this reason, the westerlies blow with much greater force in southern hemisphere in comparison to northern hemisphere.

This also has implications in the Ocean currents. The currents in the Northern Hemisphere are weaker than those in the Southern Hemisphere due to the differences in strength between the Westerlies of each hemisphere.

Generally, Westerlies are strongest in the winter hemisphere and at times when the pressure is lower over the poles, while they are weakest in the summer hemisphere and when pressures are higher over the poles. Please note the westerlies are also associated with the “extra tropical” cyclones which refer to the fact that this type of cyclone generally occurs outside of the tropics, in the middle latitudes of the planet, where the Westerlies steer the system generally from west to east. Whenever there is a convergence of the cold and denser polar winds and warm and light westerlies, there are much variation in the weather. The velocity of the westerlies increases southward and they become stormy. When we move towards poles, the velocity of the westerlies is given different terms as follows:

  • Roaring Forties between the 40-50°S
  • Furious Fifties at the 50°S and Shrieking Sixties at 60°S.

Polar Easterlies

Polar easterlies blow from the polar high pressure belts towards the temperate low pressure belts. These are extremely cold winds that come from the Tundra and Icecap regions of the poles. The Polar Easterlies are more regular in the southern hemisphere in comparison to the northern hemisphere. These polar cold winds converge with the warm easterlies near 60° latitudes and form the Polar front or Mid Latitude front. This mid-latitude front becomes the centre of the origin of the Temperate Cyclones.

Local Winds

The Local winds around the world are formed through the heating of land. In coastal regions, the sea breezes and land breezes are important factors in a location’s prevailing winds. The sea is warmed by the sun more slowly because of water’s greater specific heat compared to land. As the temperature of the surface of the land rises, the land heats the air above it by conduction. The warm air is less dense than the surrounding environment and so it rises. This causes a pressure gradient of about 2 millibar from the ocean to the land. The cooler air above the sea, now with higher sea level pressure, flows inland into the lower pressure, creating a cooler breeze near the coast.  At night, the land cools off more quickly than the ocean because of differences in their specific heat values. This temperature change causes the daytime sea breeze to dissipate. When the temperature onshore cools below the temperature offshore, the pressure over the water will be lower than that of the land, establishing a land breeze, as long as an onshore wind is not strong enough to oppose it.

Local winds near Mountains

There is a different explanation for local winds near mountains. Over elevated surfaces, heating of the ground exceeds the heating of the surrounding air at the same altitude above sea level, creating an associated thermal low over the terrain and enhancing any thermal lows that would have otherwise existed, and changing the wind circulation of the region. In areas where there is rugged topography that significantly interrupts the environmental wind flow, the wind circulation between mountains and valleys is the most important contributor to the prevailing winds.

Barrier Jet

The mountains and valleys are capable to distort the airflow by increasing friction between the atmosphere and landmass by acting as a physical block to the flow, deflecting the wind parallel to the range just upstream of the topography, which is known as a barrier jet. This barrier jet can increase the low level wind. Wind direction also changes because of the contour of the land. If there is a pass in the mountain range, winds will rush through the pass with considerable speed because of the Bernoulli principle that describes an inverse relationship between speed and pressure. The airflow can remain turbulent and erratic for some distance downwind into the flatter countryside. These conditions are dangerous to ascending and descending airplanes.

List of major Local Winds

  • Abroholos: squall frequent wind that occurs from May through August between Cabo de Sao Tome and Cabo Frio on the coast of Brazil
  • Amihan: northeasterly wind across the Philippines
  • Bayamo : violent wind on Cuba’s southern coast
  • Bora : northeasterly from eastern Europe to northeastern Italy
  • Calima : dust-laden south to southeasterly wind blowing in the Saharan Air Layer across the Canary Islands
  • Cape Doctor : dry south-easterly wind that blows on the South African coast in summer
  • Chinook : warm dry westerly off the Rocky Mountains
  • Elephanta : strong southerly or southeasterly wind on the Malabar coast of India
  • Föhn : warm dry southerly off the northern side of the Alps and the North Italy, the name gave rise to the fén-fēng or ‘burning wind’ of Taiwan
  • Fremantle Doctor : afternoon sea breeze from the Indian Ocean which cools Perth, Western Australia during summer
  • Gregale : northeasterly from Greece
  • Habagat : southwesterly wind across the Philippines
  • Harmattan : dry northerly wind across central Africa
  • Karaburan : “black storm”, a Spring and Summer Katabatic wind of central Asia
  • Khamsin : southeasterly from north Africa to the eastern Mediterranean
  • Khazri : cold north wind in the Absheron Peninsula of the Azerbaijan Republic
  • Kona : southeast wind in Hawaii, replacing trade winds, bringing high humidity and often rain
  • Košava : strong and cold southeasterly season wind in Serbia
  • Lodos : southwesterly towards Turkey. Strong “Lodos” events occur 6 – 7 times a year bringing 35 kt winds into Marmara Sea. The winds are funneled SE from the Mediterranean and through the Dardanelles Strait.
  • Loo : hot and dry wind which blows over plains of India and pakistan.
  • Mistral : cold northerly from central France and the Alps to Mediterranean
  • Monsoon : mainly south-westerly winds combined with heavy rain in various areas close to the equator
  • North wind : northern cold winds blowing from the Gulf of Mexico to the Isthmus of Tehuantepec
  • Nor’easter : strong storm with winds from the northeast in the eastern United States, especially New England
  • Nor’wester : wind that brings rain to the West Coast, and warm dry winds to the East Coast of New Zealand’s South Island, caused by the moist prevailing winds being uplifted over the Southern Alps, often accompanied by a distinctive arched cloud pattern
  • Pampero : Argentina, very strong wind which blows in the Pampa
  • Simoom : strong, dry, desert wind that blows in the Sahara, Israel, Jordan, Syria, and the desert of Arabia
  • Sirocco : southerly from north Africa to southern Europe
  • Sundowner : strong offshore wind off the California coast
  • Zonda wind: on the eastern slope of the Andes in Argentina

Climate Related Topics

Monsoon Mechanism

The word monsoon derived from the Arabic word mausim means seasonal winds. In this system, the direction of the winds reverses seasonally. The first thing we note is that Monsoon is typically considered a phenomenon of tropical south Asia, but it is also experienced over parts of North America and Africa.

Mechanism of Monsoon: Traditional View

Traditionally, monsoon has been considered a result of the differential heating of land and sea. In summer, southern Asia develops a low pressure while the pressure over the sea is relatively higher. As a result the air starts flowing towards land from the Indian oceans. The winds coming from ocean carry moisture and thus cause rainfall in summer reason. This is known as the southwest monsoon or summer monsoon. In winter, the pressure over land is higher than over the sea and consequently the air starts flowing from land to sea. The air coming from land being dry, these winds do not cause rainfall. The above explanation is known as the thermal theory of monsoon. This theory explains monsoon as a regional phenomenon but fails to explain the total amount of energy / processes involved in the global monsoon circulation.

Mechanism of Monsoon: Modern View

The modern meteorologists seek explanation for the phenomenon of monsoon on the basis of seasonal shift in the position of the global belts of pressure and winds. This is also known as Dynamic Theory. According to the dynamic theory, monsoons are a result of the shift of the inter-tropical convergence zone (ITCZ) under the influence of the vertical sun. Though the average position of the ITCZ is taken as the equator, it keeps shifting vertical sun towards with the migration of the vertical sun towards the tropics during the summer of the respective hemisphere.

  • During summer in the northern hemisphere in the months of May and June, the sun shines vertically over the tropic of cancer. Due to the northward shift of the zone of maximum heating and low pressure at this time the ITCZ also shifts northwards and approaches, the tropic of cancer. The ITCZ being the zone of the lowest pressure in the tropical region is the destination of the trade winds blowing from both the hemispheres.
  • With ITCZ situated close to the tropic of cancer the northeast trade winds are confined to an area extending to its north while the southeast trade winds blowing from the southern hemisphere have to cross the equator to reach this area of low pressure. However as the winds  blowing  from the southern  hemisphere cross the equator  their direction is altered due to Coriolis effect, i.e. they are direction is their right  and thus it give rise to the formation  of a belt of equatorial  westerlies  in the months of many of June northeast and they are called the southwest monsoon.
  • As the ITCZ again moves southwards at the end of the summer of the northern hemisphere the areas north of the equator which experienced the equatorial  westerlies during the summer season come under the influence of the northeast trade winds. These northeasterly winds are called the northeast monsoon. The onset of winter season the ITCZ shifts south of the equator and reaches as far south at this time.  in this season the northeast trades blowing towards the ITCZ have to cross the equator  towards  south and as a result they get deflected giving rise to the equatorial westerlies in the southern hemisphere. These westerlies blow form the northwest to the southwest, replacing the trade winds of the southern hemisphere between the ITCZ and the equator. They form the summer monsoon of the southern hemisphere.

We can say that due to the seasonal shift of the wind belts under the influence of the north-south migration of the vertical sun the areas situated in the tropical  zone in the both the hemisphere come under the influence  of the trade winds during the respective winter and the equatorial westerlies during the respective  summer season. The direction of the winds is thus reversed seasonally and it makes up the monsoon system of these regions. Please note that though, dynamic theory provides a much better explanation of the system of monsoon as a global phenomenon, it does not negate the influence of differential heating of land and sea. Differential heating still plays an important role in making monsoon much stronger in certain of the south-west monsoon factor that explains the extension of the southwest monsoon even to the north of the tropic of cancer in northern India.


The amount of moisture in air is commonly recorded as relative humidity; which is the percentage of the total water vapour air can hold at a particular air temperature. The presence of warm, moist and unstable air and sufficient amount of the hygroscopic nuclei is a prerequisite condition for rainfall. The warm and moist air after being lifted upwards becomes saturated and clouds are formed after condensation of water vapour around the hygroscopic nuclei such as dust particles. How much water vapour a parcel of air can contain before it becomes saturated (100% relative humidity) and forms into a cloud (a group of visible and tiny water and ice particles suspended above the Earth’s surface) depends on its temperature. Warmer air can contain more water vapour than cooler air before becoming saturated.


The process of condensation begins only when the relative humidity of the ascending air becomes 100% and air is cooled through four main mechanisms to its dew point: adiabatic cooling, conductive cooling, radiational cooling, and evaporative cooling. Adiabatic cooling occurs when air rises and expands. The air can rise due to convection, large-scale atmospheric motions, or a physical barrier such as a mountain (orographic lift). Conductive cooling occurs when the air comes into contact with a colder surface, usually by being blown from one surface to another, for example from a liquid water surface to colder land. Radiational cooling occurs due to the emission of infrared radiation, either by the air or by the surface underneath. Evaporative cooling occurs when moisture is added to the air through evaporation, which forces the air temperature to cool to its wet-bulb temperature, or until it reaches saturation. Further, we note that the very small rain drops are almost spherical in shape. As drops become larger, they become flattened on the bottom, like a hamburger bun. Very large rain drops are split into smaller ones by air resistance which makes them increasingly unstable. When water droplets fuse to create larger water droplets, it is called Coalescence. When water droplets freeze onto an ice crystal, which is known as the Bergeron process. Air resistance typically causes the water droplets in a cloud to remain stationary. When air turbulence occurs, water droplets collide, producing larger droplets. As these larger water droplets descend, coalescence continues, so that drops become heavy enough to overcome air resistance and fall as rain. Coalescence generally happens most often in clouds above freezing, and is also known as the warm rain process.

Convectional Rainfall

The convectional rainfall occurs due to the thermal convection currents caused due to the heating of ground due to insolation. The convectional rainfall is prevalent in equatorial regions. In these, the warm air rises up and expands then, reaches at a cooler layer and saturates, then condenses mainly in the form of cumulus or cumulonimbus clouds. In the equatorial regions, the precipitation due to convectional rainfall occurs in the afternoon. The rainfall is of very short duration but in the form of heavy showers.

Cyclonic / Frontal Rainfall

Frontal rainfall occurs due to the upward movement of the air caused by the convergence of different air masses with different temperatures. The warm air rises over the cold air and cyclonic rain occurs. The cold air pushes up the warm air and sky gets clear again.

Orographic Rainfall

The orographic rainfall occurs due to the ascent of air forced by the mountain barrier. The mountain barrier should be across the wind direction. So that the moist air is forced in obstruction to move upward and get cooled. In Rajasthan, the Aravalli is not an obstructing barrier to the highly moist air coming from Arabian Sea and that is why they don’t play very important role in rainfalls. Thus they produce a Rain shadow area. A rain shadow is a dry area on the lee side of a mountainous area. The mountains block the passage of rain-producing weather systems, casting a “shadow” of dryness behind them. In south India, the Mangalore is located on the western windward slope and gets 2000 mm of rainfall. But Bangalore is in rain shadow area and that is why receives less than 500 mm of rainfall.

Please note that the amount of the rainfall increases with increasing height of the barrier such as mountain, but this is up to a certain limit. After that there is a marked decrease due to lesser moisture content of the air and this phenomenon is called “Inversion of Rainfall”

Distribution of Rainfall

The regions having high temperature and abundance of water receive higher amount of rainfall, such as equatorial regions. In the subtropical regions, the western parts receive lesser rainfalls. This is due to anticyclone activities.

Mean annual rainfall for earth is 970mm. The equatorial regions receive rainfall throughout the year while the other regions receive rainfall seasonally. The Mediterranean region receives rainfall during the winter generally.

Air Mass & Fronts

Air mass is a volume of air defined by its temperature and water vapour content. An air mass may be of many hundreds or thousands of square miles, and adopt the characteristics of the surface below them. An air mass can be so extensive that it may cover the large portion of a continent below it and may be vertically so thick that may cover the troposphere. The vertical distribution of the temperature in an air mass and moisture content of the air are the two properties of air. Air mass which control the weather conditions of an area under that particular air mass. The air mass is considered to be cold air mass if its temperature is lower than the underlying surface, while an air mass is terms warm air mass when its temperature is higher than the underlying surface. The boundary between the two air masses is called the front.

Air masses are classified according to latitude and their continental or maritime source regions. Colder air masses are termed polar or arctic, while warmer air masses are deemed tropical. Continental and superior air masses are dry while maritime and monsoon air masses are moist. Weather fronts separate air masses with different density (temperature and/or moisture) characteristics. Once an air mass moves away from its source region, underlying vegetation and water bodies can quickly modify its character.

Frontogenesis and Frontolysis

The boundary between the two air masses is called the front.  A temperature difference is essential in the definition of a front because it implies a density difference. The air masses of different densities don’t mix readily and tend to retain their identity as far as we care for the moisture. The front represents a transition zone between two air masses of different density. Generally, an air mass from one region moves to the other region which is occupied by some other air mass. When a warmer and lighter air mass moved against a cold and denser air mass, the former rides over the other and it is called warm front. If the cold air mass forces its way under a warm air mass, it is called cold front. When new fronts are created or old fronts are regenerated, it is called Frontogenesis.  Please note that fronts don’t appear all of a sudden. They appear only after a process of Frontogenesis which is there in place for quite some time. When winds converge towards a point it would lead to Frontogenesis.

Frontogenesis takes place only when two conditions are met. First, two air masses of different densities must exist adjacent to one another; and second, a prevailing wind field must exist to bring them together.   There are three basic situations, which are conducive to Frontogenesis and satisfy the two basic requirements. The wind flow is cross isothermal and flowing from cold air to warmer air. The flow   must be cross isothermal, resulting in a concentration of isotherms (increased temperature gradient).  The flow does not have to be perpendicular; however, the more perpendicular the cross isothermal flow, the greater the intensity of Frontogenesis. On the other hand, the dying of a front is called Frontolysis.

Frontolysis also does not happen all of a sudden. The process of Frontolysis must happen for quite some time to destroy the existing front.

Types of Fronts

Cold Front

When a cold air invades the warm air, it remains at the ground and forcibly uplifts the warmer and lighter air mass. This is known as Cold front. This upward motion causes lowered pressure along the cold front and can cause the formation of a narrow line of showers and thunderstorms when enough moisture is present. Cold fronts can move up to twice as fast as warm fronts and can produce sharper changes in weather. Since cold air is denser than warm air, it rapidly replaces the warm air preceding the boundary. Cold fronts are usually associated with low-pressure areas. Cold front usually causes a shift of wind from southeast to northwest, and in the southern hemisphere a shift from northeast to southwest.

Warm front

When a warmer and lighter air mass moved against a cold and denser air mass, the former rides over the other and it is called warm front.  Being lighter, the warm air mass is unable to displace the cooler air mass and instead is forced upward along the upper boundary of the colder air in a process known as overrunning. The boundary between the two air masses has a gradual slope of 130 and lifting is slow but persistent. As the air mass rises into regions of lower pressure, it expands and cools. As it cools, water vapour condenses and forms extensive cloud coverage. The first clouds to form along the sloping surface of the cold air are high cirrus, which thicken to cirrostratus and altostratus.

Occluded front

An occluded front is a front that is formed when a cold front overtakes a warm front. The cold front moves rapidly than the warm front. Ultimately, the cold front overtakes the warm front and completely displaces the warm air at the ground.


Cyclone is a system of low atmospheric pressure in which the barometric gradient is steep. Cyclones represent circular fluid motion rotating in the same direction as the Earth. This means that the inward spiralling winds in a cyclone rotate anticlockwise in the Northern Hemisphere and clockwise in the Southern Hemisphere of the Earth. Most large-scale cyclonic circulations are centred on areas of low atmospheric pressure. The cyclones can be tropical cyclones or temperate cyclones (extra- tropical cyclones).

Basic difference between Tropical Cyclone and Extra-tropical Cyclone

The term “tropical cyclone” is used to refer to warm-core, low-pressure systems that develop over tropical or subtropical oceans. This definition differentiates tropical cyclones from extra tropical (midlatitude) cyclones that exhibit a cold-core in the upper troposphere and often form along fronts in higher latitudes. Subtropical cyclones are hybrid systems that exhibit some characteristics of tropical cyclones and some characteristics of extra-tropical cyclones. Tropical cyclones extract much of their energy from the upper layer of the ocean, while extra tropical cyclones derive much of their energy from the baroclinic temperature gradients in which they form.

Tropical Cyclones

The tropical cyclone is a system of low pressure occurring in tropical latitudes characterized by very strong winds. Here are the important notes which you must note about the Tropical Cyclones: Distribution

The tropical cyclones are found over the North Atlantic Ocean, Southern Atlantic Ocean, the eastern, central and western North Pacific Ocean, the central and western South Pacific Ocean and the northern and southern Indian Ocean.

Formation in Low Pressure areas

All tropical cyclones are formed in areas of low atmospheric pressure in the Earth’s atmosphere.

Minimum Pressure is at centre

The pressures recorded at the centers of tropical cyclones are among the lowest that occur on Earth’s surface at sea level.

Driver is the Large Heat of Condensation

Tropical cyclones are driven by the release of large amounts of latent heat of condensation, which occurs when moist air is carried upwards and its water vapour condenses. This heat is distributed vertically around the center of the storm. Thus, at any given altitude, environment inside the cyclone is warmer than its outer surroundings.

Eye is the sinking air

There is an area of sinking air at the center of circulation, which is known as Eye. Weather in the eye is normally calm and free of clouds, although the sea below it may be extremely violent. Eye is normally circular in shape, and is typically 30–65 km in diameter.

Stadium Effect

The mature tropical cyclones sometimes exhibit an outward curving of the eye wall’s top, making it resemble an arena football stadium. It is called Stadium Effect.

Greatest Wind speeds are at eye walls

Greatest wind speeds in a tropical cyclone is found at the eye wall, which is a circle of strong thunderstorms that surrounds the eye. Here, the clouds reach the highest, and precipitation is the heaviest. The heaviest wind damage occurs where a tropical cyclones eye wall passes over land.

Source of the huge Energy

Primary energy source is the release of the heat of condensation from water vapour condensing, with solar heating being the initial source for evaporation. So a tropical cyclone can be visualized as a giant vertical heat engine supported by mechanics driven by physical forces such as the rotation (Coriolis force) and gravity of the Earth. Inflow of warmth and moisture from the underlying ocean surface is critical for tropical cyclone strengthening.

Impact of Earth’s Rotation

The rotation of the Earth causes the system to spin (Coriolis Effect) giving it a cyclonic characteristic and affecting the trajectory of the storm. In Northern Hemisphere, where the cyclone’s wind flow is counterclockwise, the fastest winds relative to the surface of the Earth occur on the eastern side of a northward-moving storm and on the northern side of a westward-moving one; the opposite occurs in the Southern Hemisphere, where the wind flow is clockwise.

Movement of Clouds

In Lower troposphere, motion of clouds is toward the center. At upper-level, there is outward flow of clouds.

Formation in Northern Atlantic Ocean

Northern Atlantic cyclone season occurs from June 1 to November 30, sharply peaking from late August through September. The statistical peak of the Atlantic hurricane season is 10 September.

Formation in North East Pacific

The Northeast Pacific Ocean has a broader period of activity, but in a similar time frame to the Atlantic.

Formation in North West Pacific

The Northwest Pacific sees tropical cyclones year-round, with a minimum in February and March and a peak in early September.

Formation in North Indian basin

Storms are most common from April to December, with peaks in May and November.

Formation in Southern Hemisphere

Tropical cyclone year begins on July 1 and runs all year-round and encompasses the tropical cyclone seasons, which run from November 1 until the end of April, with peaks in mid-February to early March.

Requirements for formation

  • Water temperatures of at least 26.5 °C down to a depth of at least 50 m, so that it may cause the overlying atmosphere to be unstable enough to sustain convection and thunderstorms.
  • Rapid cooling with height, so that it may cause release of the heat of condensation that powers a tropical cyclone.
  • High humidity
  • Low amounts of wind shear as high shear is disruptive to the storm’s circulation.
  • A distance from the Equator is necessary, which should be at least 555 km or 5° of latitude, so that it allows the Coriolis effect to deflect winds blowing towardsthe low pressure center and creating a circulation. Because the Coriolis effect initiates and maintains tropical cyclone rotation, tropical cyclones rarely form or move within about 5° of the equator,where the Coriolis effect is weakest.
  • A pre-existing system of disturbed weather.


Coriolis Effect causes cyclonic systems to turn towards the poles in the absence of strong steering currents. The pole ward portion of a tropical cyclone contains easterly winds, and the Coriolis effect pulls them slightly more pole ward. The westerly winds on the Equatorward portion of the cyclone pull slightly towards the equator, but, because the Coriolis effect weakens toward the equator, the net drag on the cyclone is pole ward. Thus, tropical cyclones in the Northern Hemisphere usually turn north (before being blown east), and tropical cyclones in the Southern Hemisphere usually turn south (before being blown east) when no other effects counteract the Coriolis Effect.

High speed of rotation

It is caused by Coriolis effect as well as energy released by heat of condensation.

Fujiwhara effect

When two cyclones approach one another, their centers will begin orbiting cyclonically about a point between the two systems. The two vortices will be attracted to each other, and eventually spiral into the center point and merge. When the two vortices are of unequal size, the larger vortex will tend to dominate the interaction, and the smaller vortex will orbit around it. This phenomenon is called the Fujiwhara effect.

Impact on passing over land

We should note that the deep convection is a driving force for tropical cyclones. The convection is strongest in a tropical climate; it defines the initial domain of the tropical cyclone. This is a major difference between the Tropical cyclones with other mid-latitude cyclones as the later derive their energy mostly from pre-existing horizontal temperature gradients in the atmosphere. To continue to drive its heat engine, a tropical cyclone must remain over warm water, which provides the needed atmospheric moisture to keep the positive feedback loop running. When a tropical cyclone passes over land, it is cut off from its heat source and its strength diminishes rapidly. The moving over land deprives it of the warm water it needs to power itself, quickly losing strength. Thus, most strong storms lose their strength when the pass on to land, but if it manages to move back to ocean, it will regenerate.

Impact of passing over cold water

When a tropical storm moves over waters significantly below 26.5 °C, it will lose its strength. This is because of losing its tropical characteristic of the warm core.

Project Stormfury

The United States Government attempted in 1960s and 1970s to artificially weaken the Cyclones. During this project, Cyclones were seeded with silver iodide. It was thought that the seeding would cause supercooled water in the outer rainbands to freeze, causing the inner eye wall to collapse and thus reducing the winds. The Hurricane Debbie lost as much as 31% of its strength, when seeded with Silver Iodide in this project but Debbie regained its strength after each of two seeding forays. So, it was not a good idea. There were some more ideas applied which were as follows:

  • Cooling the water under a tropical cyclone by towing icebergs into the tropical oceans and covering the ocean in a substance that inhibits evaporation
  • Dropping large quantities of ice into the eye at very early stages of development (so that the latent heat is absorbed by the ice, instead of being converted to kinetic energy that would feed the positive feedback loop)
  • Blasting the cyclone apart with nuclear weapons.
  • A Project called Project Cirrus involved throwing dry ice on a cyclone.
  • None of the idea was very much practical because the tropical storms are too large and too momentary.

Naming of Cyclones

Tropical cyclones are classified into three main groups, based on intensity: tropical depressions, tropical storms, and a third group of more intense storms, whose name depends on the region. If a tropical storm in the North-western Pacific reaches hurricane-strength winds on the Beaufort scale, it is referred to as a typhoon. If a tropical storm passes the same benchmark in the Northeast Pacific Basin, or in the Atlantic, it is called a hurricane. Neither “hurricane” nor “typhoon” is used in either the Southern Hemisphere or the Indian Ocean. In these basins, storms of tropical nature are referred to simply as “cyclones”.

Types of the Tropical Cyclones

There are three kinds of Tropical cyclones:

  • Tropical Depression: A tropical depression is a system with low pressure enclosed within few isobars and with the wind speed of 60 kmph. It lacks marked circulation
  • Tropical Storm: It is a system with several closed isobars and a wind circulation of 115 kmph.
  • Tropical Cyclone: It is a warm core vortex circulation of tropical origin with small diameter, circular shape and occurs in oceanic areas.


An ‘anticyclone’ is opposite to a cyclone, in which winds move into a low-pressure area. In an anticyclone, winds move out from a high-pressure area with wind direction clockwise in the northern hemisphere, anti-clockwise in the southern hemisphere. Such a high pressure area is usually spread over a large area, created by descending warm air devoid of moisture. The absence of moisture makes the dry air denser than an equal quantity of air with moisture. When it displaces the heavier nitrogen and oxygen, it causes an anti-cyclone.

Temperate Cyclones

Temperate cyclones are generally called depressions. They have low pressure at the centre and increasing pressure outwardly. They are of varying shapes such as circular, elliptical. The formation of tropical storms as we read above are confined to oceans, the temperate cyclones are formed over land and sea alike. Temperate Cyclones are formed in 35-65° North as well as South Latitudes. While the tropical cyclones are largely formed in summer and autumn, the temperate cyclones are formed in generally winter. Rainfall in these cyclones is low and continuous not as furious as in case of tropical cyclones.


Basically, hurricanes and typhoons form over water and are huge, while tornados form over land and are much smaller in size. A tornado is a violent windstorm characterized by a twisting, funnel- shaped cloud. In the United States, twister is used as a colloquial term for tornado.

What is it?

Technically, a tornado is a rotating column of air that is in contact with both the surface of the earth and a cloud, which is generally cumulonimbus and occasionally cumulus. Most tornadoes have wind speeds less than 110 miles per hour and travel several kilometers before dissipating.

How it is formed?

First the rotating cloud base lowers. This lowering becomes a funnel, which continues descending while winds build near the surface, kicking   up dust and other debris. Finally, the visible funnel extends to the ground, and the tornado begins causing major damage.

Where they are seen?

Tornadoes have been observed on every continent except Antarctica.

How they are detected?

Tornadoes can be detected before or as they occur through the use of Pulse-Doppler radar by recognizing patterns in velocity and reflectivity data.

What is Fujitsa Scale?

Fujita scale rates tornadoes by damage caused, and has been replaced in some countries by the updated Enhanced Fujita Scale. An F0 or EF0 is the weakest tornado, while F5 or EF5 is the strongest tornado.

What is Torro Scale?

TORRO scale ranges from a T0 for extremely weak tornadoes to T11 for the most powerful known tornadoes.

Funnel Cloud as predecessor

Tornadoes often begin as funnel clouds with no associated strong winds at the surface, although not all evolve into a tornado. However, many tornadoes are preceded by a funnel cloud. Most tornadoes produce strong winds at the surface while the visible funnel is still above the ground, so it is difficult to discern the difference between a funnel cloud and a tornado from a distance.

Infrasonic signature

Tornadoes produce identifiable inaudible infrasonic signatures. Due to the long distance propagation of low-frequency sound, efforts are ongoing to develop tornado prediction and detection devices with additional value in understanding tornado morphology, dynamics, and creation. Electromagnetic Spectrum Tornadoes emit on the electromagnetic spectrum. There are observed correlations between tornadoes and patterns of lightning.

When they occur?

Tornadoes are most common in spring and least common in winter. Spring and fall experience peaks of activity as those are the seasons when stronger winds, wind shear, and atmospheric instability are present. Tornado occurrence is highly dependent on the time of day, because of solar heating. Worldwide, most tornadoes occur in the late afternoon, between 3 pm and 7 pm local time, with a peak near 5 pm.

January 14, 2018

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