Fronts and Cyclones

  • A jet stream develops where air masses of differing temperatures meet. Therefore, the surface temperatures determine where the jet stream will form.
    • The greater the difference in temperature, the faster the wind velocity inside the jet stream.
    • Jet streams can flow up to 200 mph (322 km/h), are 1000’s of miles long, 100’s of miles wide, and a few miles thick.

Where the jet stream begins?
• Air warmed in the tropics around the equator fuels the jet stream as it rises.
• Hitting the tropopause at about 58,000 feet (the layer of the atmosphere separating the troposphere from the stratosphere), it is drawn toward the colder air at the north and south poles.


How does it form a convection cell?
At higher latitudes, the warm air cools and sinks, drawing more warm air in behind it. The cooled air flows back towards the equator, creating a loop or convection cell.


Why the jet stream flows on an easterly course?
As the earth rotates on its axis, so does the air around it. Due to this easterly rotation, rising warm air builds up momentum going the same direction. Thus, the jet stream cannot flow due north or due south, but makes an angular approach from the west, toward both poles.


Subtropical Jet Streams


• These jets, like the polar-front jets, are best developed in winter and early spring.
• During summer, in the Northern Hemisphere, the subtropical jet weakens considerably, and it is only identifiable in sporadic velocity streaks around the globe.
• During winter, subtropical jets intensify and can be found between 20° and 50° latitude.
• Their maximum speed approaches 300 knots, although these higher wind speeds are associated with their merger with polar-front jets.
• The core is most frequently found between 35,000 and 40,000 feet.
• A subsidence motion accompanies subtropical jets and gives rise to predominantly fair weather in areas they pass over.
• These jets are also remarkably persistent from time to time, but they do fluctuate daily.
• Sometimes they drift northward and merge with a polar-front jet. Over Asia in summer, the subtropical jet is replaced by the tropical easterly jet stream.

Tropical Easterly Jet Stream
• This jet occurs near the Tropopause over Southeast Asia, India, and Africa during summer.
• The strongest winds are over southern India, but they are not as intense as the winds encountered in polar-front or subtropical jet streams.
• This jet is closely connected to the Indian and African summer monsoons.
• The existence of this jet implies that there is a deep layer of warm air to the north of the jet and colder air to the south over the Indian Ocean.
• This warm air is of course associated with the maximum heating taking place over India in summer, while the colder air is over the ocean.
• The difference in heating and cooling and the ensuing pressure gradient is what drives this jet stream

Polar-Night Jet Stream
• This jet meanders through the upper stratosphere over the poles.
• It occurs only during the long winter night. Night is 6 months long over the pole in which winter is occurring.
• The polar stratosphere undergoes appreciable cooling due to the lack of solar radiation. The horizontal temperature gradient is strongly established between the equator and the pole, and the pressure gradient creates this westerly jet.
• The temperature gradient breaks down intermittently during middle and late winter in the Northern Hemisphere, therefore, the jet is intermittent at these times.
• In the Southern Hemisphere the temperature gradient and jet disappear rather abruptly near the time of the spring equinox.

• In terms of commercial usage, the jet stream is important for the airline industry. By flying well within the jet stream at 25,000 feet (7,600 meters), the flight time gets reduced significantly. The reduced flight time and aid of the strong winds also allows for a reduction in fuel consumption.

• One of the most important impacts of the jet stream though is the weather it brings. Because it is a strong current of rapidly moving air, it has the ability to push weather patterns around the world.
• As a result, most weather systems do not just sit over an area, but they are instead moved forward with the jet stream. The position and strength of the jet stream then helps meteorologists forecast future weather events.
• In addition, various climatic factors can cause the jet stream to shift and dramatically change an area’s weather patterns.
• The world’s jet streams are also impacted by El Nino and La Nina. During El Nino for example, precipitation usually increases in California because the polar jet stream moves farther south and brings more storms with it.
• Conversely, during La Nina events, California dries out and precipitation moves into the Pacific Northwest because the polar jet stream moves more north. In addition, precipitation often increases in Europe because the jet stream is stronger in the Northern Atlantic and is capable of pushing them farther east.
• Today, movement of the jet stream north has been detected indicating possible changes in climate.
• Whatever the position of the jet stream, though, it has a significant impact on the world’s weather patterns and severe weather events like floods and droughts.

How do the Jet Streams affect the Monsoons and the Indian Sub Continent?
Over the Indian subcontinent, there are a number of separate jet streams whose speed varies from 110 km/h in summer to about 184 km/h in winter.

• In winter the sub-tropical westerly jet streams bring rain to the western part of India, especially Himachal Pradesh, Haryana and Punjab.

• In summer the sub-tropical easterly jet blows over Peninsular India approximately at 14N and bring some rain and storm.


• With respect to the monsoons of India it is the Subtropical Jet Stream (STJ) and the countering Easterly Jet that are most important. As the summertime approaches there is increased solar heating of the Indian subcontinent, this has a tendency to form a cyclonic monsoon cell situated between the Indian Ocean and southern Asia.
• This cell is blocked by the STJ which tends to blow to the south of the Himalayas; as long as the STJ is in this position the development of summer monsoons is inhibited.
• During the summer months the STJ deflects northwards and crosses over the Himalayan Range. The altitude of the mountains initially disrupts the jet but once it has cleared the summits it is able to reform over central Asia.
• With the STJ out of the way the sub continental monsoon cell develops very quickly indeed, often in a matter of a few days. Warmth and moisture are fed into the cell by a lower level tropical jet stream which brings with it air masses laden with moisture from the Indian Ocean.
• As these air masses are forced upward by north India’s mountainous terrain, the air is cooled and compressed, it easily reaches its saturation vapor point and the excess moisture is dissipated out in the form of monsoon rains.
• The end of the monsoon season is brought about when the atmosphere over the Tibetan Plateau begins to cool; this enables the STJ to transition back across the Himalayas. This leads to the formation of a cyclonic winter monsoon cell typified by sinking air masses over India and relatively moisture free winds that blow seaward.
• This gives rise to relatively settled and dry weather over India during the winter months.
• Atmospheric changes over the southern Pacific Ocean led to warmer than usual waters flowing into the Indian Ocean. This provided additional moisture to feed the monsoon systems.
• Further to the north the polar jet stream stalled due to being countered by Rossby Waves, there was a large kink in the stream and this was centred over Russia. The stalled system prevented weather systems being drawn across Russia and the kink acted as a barrier trapping hot air to the south and cold air to the north.
• The consequence of this static mass of hot air was the heat wave that devastated Russia. With the jet stream stalled the STJ was unable to transit across the Himalayas as it would do ordinarily, the monsoon cell to the south, fed by warmer waters in the Indian Ocean, had nowhere to go and as a consequence it deposited vast amounts of rain over Pakistan, Himachal Pradesh and Jammu and Kashmir and this led to extensive flooding.

  1. The Somali Jet Stream
    • The monsoon wind that is deflected to the north as it crosses the equator is further deflected to the east by the mountains of Africa.
    • Further, the progress of the southwest monsoon towards India is greatly aided by the onset of certain jet streams including the crucial Somali jet that transits Kenya, Somalia and Sahel and exits the African coast at 9 degrees north at low level and very fast.
    • This low level jet stream was found to be most pronounced between 1.0 and 1.5 km above the ground.
    • It was observed to flow from Mauritius and the northern part of the island of Madagascar before reaching the coast of Kenya at about 3º S. Subsequently it ran over the plains of Kenya, Ethiopia and Somalia before reaching the coast again around 9º N.
    • The jet stream appears to be fed by a stream of air, which moves northwards from the Mozambique Channel.
    • The major part of this low level jet penetrates into East Africa during May and, subsequently, traverses the northern parts of the Arabian Sea before reaching India in June.
    • Observations suggest that the strongest cross equatorial flow from the southern to the northern hemisphere during the Asian Summer Monsoon is in the region of the low level jet.
    • This has intrigued meteorologists, because it is not clear why the major flow of air from the southern to northern hemisphere should take place along a narrow preferred zone off the East African coast.
    • The importance of the low level jet arises from the fact that its path around 9º N coincides with a zone of coastal upwelling. As the strong winds drive away the surface coastal waters towards the east, extremely cold water from the depths of the sea rise upwards to preserve the continuity of mass. This upwelling is brought about by strong low level winds.
    • After the low level jet moves towards the Indian coastline around 9º N, it separates into two branches. One appears to move to the northern parts of the Indian Peninsula while the other recurves towards the southern half of the Indian coastline and Sri Lanka.
    • Conclusively, an increase in the cross-equatorial flow was followed by an increase in rainfall over the west coast.js4
  2. The Somali Ocean Current
    • This ocean current named the Somali Current, flows northward from the equator to 9º N, where it separates from the coast. It is a fairly strong current.
    • The Somali Current may be considered to be a western boundary current of the Indian Ocean. But, its peculiar feature is a reversal in direction with the onset of the summer monsoon.
    • In winter, this current is from north to the south running southwards from the coast of Arabia to the east African coastline; but with the advent of the summer monsoon it reverses its direction and flows from the south to the north.
    • This suggests a relationship with the reversal of monsoon winds, but usually the oceans respond very slowly to changes in atmospheric circulation.


  1. Sub-tropical Westerly and Tropical Jet Streams
    • Certain interesting changes take place in the upper atmosphere with the advent of the summer monsoon.
    • Towards the end of May, a narrow stream of air, which moves from the west to the east over northern India, suddenly weakens and moves to a new location far to the north of the Himalayas. This is known as sub-tropical westerly jet stream.
    • Its movement towards the north is one of the main features associated with the onset of the monsoon over India.
    • As the westerly jet moves north, yet another jet stream sets in over the southern half of the Indian peninsula. This flows in the reverse direction from the east to west. It is called tropical easterly jet, and it exhibits periodic movements to the north and south of its mean location during the hundred-day monsoon season beginning with the first of June and ending around mid-September.
    • The altitude at which the winds attain their maximum strength in the tropical easterly jet is around 150 hPa, but the maximum winds associated with the sub-tropical westerly jet occur at a lower altitude of 300 hPa. (HPa refers to ‘hecta Pascal’ and is a unit of measure of atmospheric air pressure)
    • A remarkable feature of the tropical easterly jet is that it can be traced in the upper troposphere right up to the west coast of Africa.


  • When the air remains over a homogenous area for a sufficiently longer time, it acquires the characteristics of the area. The homogenous regions can be the vast ocean surface or vast plains. The air with distinctive characteristics in terms of temperature and humidity is called an Airmass.
    Warm Air Mass is that whose temperature is greater than the surface temperature of the areas over which it moves. Thus this indicates that the surface underlying the air mass is cold. Due to the presence of cool surface the warm air mass gets cooled from below. The lower layer becomes stable and stops the vertical movement of air. Due to this No adiabatic cooling of air formation of cloud halts precipitation stops creates anti-cyclonic stable conditions Warm air mass can further be divided as continental warm air mass and maritime warm air mass (based on the source of origin whether continent or ocean respectively).
    Cold Air Mass originates in the polar and arctic regions. Temperature and specific humidity is very low. Its temperature is lower than the surface temperature of the areas over which it moves. Thus the air mass is warmed from below and becomes unstable. Due to heating up of air from below, the air rises vertically and because of adiabatic cooling condensation process starts. This will lead to formation of clouds and finally will result in precipitation. But the precipitation will occur only when the air mass is above the warm ocean as it will get the unobstructed supply of moisture whereas if lies above warm continent then leads to clear weather.
    • Tropical air masses are warm and polar air masses are cold.

Fronts and Cyclones

When two different air masses meet, the boundary zone between them is called a front. The process of formation of the fronts is known as frontogenesis. Front is the leading edge of an advancing air-mass It’s a line of contrasting weather conditions. The leading edge of a cold air – mass is a cold front, whereas, the leading edge of a warm air mass is a warm front.

Types of Fronts

1. Cold Front: On a weather map shown by a line marked with triangular spikes, pointing in the direction of frontal

2. Warm Front: A warm front is denoted on weather maps by a line marked with semi-circles facing the direction of frontal

3. Occluded Fronts: Occluded front is formed when cold front overtakes warm front and warm air is completely displaced from the ground surface. The temperature drops as the warm air mass is occluded, or “cut off,” from the ground and pushed upward.
4. Stationary Front
· A stationary front forms when a cold front or warm front stops moving. This happens when two masses of air are pushing against each other but neither is powerful enough to move the other. Winds blowing parallel to the front instead of perpendicular can help it stay in place.

1. Temperate Cyclone
Isobars are generally elongated or oval shaped.
Air pressure 940-930 mb.
– The size (diameter) may by 150-3000 km (100-200 miles), but mostly vary between 300-1500 m.
Speed: It may be practically stationary or moving at a speed of 900-1000 Km per day. Aleutain and Iceland places of origin track.
– They originate where warm tropical air mass meets cold polar air.
– Characterized with unsettled and variable weather. They change their path with season.
– General direction of movement from west to East (in the belt of westerlies).
– The averages speed is about 30 to 50 km per hour.
– Rainfall is light to moderate light shower.
– Fogginess and poor visibility.
– A few hours after the front has passed, clear weather (anticyclone) prevails.
– Thunder and lightning occurs in the rear part of a cyclone.

Weather Associated with Temperate Cyclones
– Air temperature changes as we move from behind the cold front to a position ahead of the warm front.
– Behind the surface position of the cold front, forward moving cold dense air causes the uplift of the warm lighter air in advance of the front.
– Because this uplift is relatively rapid along a steep frontal gradient, the condensed water vapor quickly organizes itself into cumulus and then cumulonimbus clouds.
– Cumulonimbus clouds produce heavy precipitation and can develop into severe thunderstorms if conditions are right.
– Along the gently sloping warm front, the lifting of moist air produces first nimbostratus clouds followed by altostratus and cirrostratus.
– Precipitation is less intense along this front, varying from moderate to light showers some distance ahead of the surface location of the warm front.

Stage of Development of a Temperate Cyclone
– The stage in the development of temperate cyclone (depression) as per Bjerknes, are as under (shown in the figure as A, B,C, D, E and F):
Stage I: A Warm and cold air are present side by side separated by a stationary quasi-stationary front.
Stage II: A wave has formed on the fornt and a centre of low pressure is developing at the apex of wave.
Stage III: The young and developing cyclone.
Stage IV: The cold front overtakes the warm front and the system is set to occlude.
Stage V: The occlusion process continues, the warm air is lifted to higher levels.
Stage VI: The cyclone comes into existence.

  1. Tropical Cyclone
    – A powerful manifestation of Earth’s energy and moisture system is the tropical cyclone.
    – These cyclones originate entirely within tropical air masses, i.e. 23 ½°N and 23 ½°S
    – The air of the tropics is essentially homogenous, with no front or conflicting air masses.
    – In addition, the warm air and warm seas ensure an abundant supply of water vapor and thus the necessary latent heat to fuel (generate) these cyclones (stroms).
    – The tropical cyclones generally originate on the western margins of oceans around 8° to 12° N and South.
    – With the help of latent heat, the tropical cyclones start whirling like a chimney, pulling more moisture-laden air into the developing system.
    – Tropical cyclones tend to occur when the equatorial low pressure trough is the farthest from the equator that is during the month that follow the summer solstice in each hemisphere.
    – Tropical cyclones are most destructive.
    – Vertically, these storms dominate the full height of the troposphere.
    – A system of low pressure occurring in tropical latitude is known as tropical cyclone. It is a general term for any type of cyclonic storm at low latitudes for which many other terms (local names) like hurricanes, typhoons etc. are used.

Origin of Tropical Cyclones
The exact mechanism that leads to the origin and development of tropical cyclones is not fully known due to lack of climatic data. There are however certain basic requirements which results in the origin of a tropical cyclone. The required conditions are as under:
Large and continuous supply of warm and moist air: Tropical cyclones develop over the warm tropical oceans where the surface temperature is around 27°C. The high temperature near the surface of the oceans makes the air full of water vapor. The latent heat is transported into the storms and released in the process of cloud and rain formation. Tropical cyclones consequently originate in the western parts of the ocean where temperatures are relatively higher than their eastern parts. In the eastern parts of the ocean the existence of cold water currents reduce the surface temperature of the oceans.
Large value of Coriolis force: The maximum value of Coriolis force is along the equator. Consequently, the tropical cyclones do not originate in the belt of doldrums. Most of them have their origin around 15° latitude on the western margins of the oceans.
Upper level outflow: At a height of 9000 to 1500 meters above surface of the ocean, there must be an anti cyclonic circulation, so that the ascending air currents within the cyclone may continue to be pumped on in order to maintain the low pressure at the centre of the cyclone.
Weak vertical wind shear in the basic current: Because of weak vertical wind shear, hurricane formation processes are limited to latitudes equator wards of the subtropical jet stream.
Presence of anti cyclonic circulation: There should be an anticyclonic circulation at the height of 9 km to 15 km above the surface disturbance.

Main Characteristics of Tropical Cyclones
• They have circular and closed isobars.
• Their diameter varies between 150 to 300 km. And in exceptional cases to 10 km. vertically, these cyclones dominate the full height of the troposphere. The inwards spiraling clouds from dense rain bands, with a central area designated the eye, around which a thunderstorm cloud called eye-wall swirls, producing the area of most intense rainfall. The eye has quite, warm air with even a glimpse of blue sky or stars possible.
• They do not have fronts and developing the Inter Tropical Convergence Zone (ITCZ) over the oceans. They tend to occur when the equatorial low-pressure through is the farthest from the equator.
• They drive their energy from the latent heat.
• They are irregular. They occur in the autumn season in the Northern Hemisphere and in March and April in the Southern Hemisphere.

  • In the initial stage their speed varies between 15 to 30 km per hour which accelerates up to 200 and even more km per hour.
    • There is an eye of the cyclone about 30 km in diameter in which the atmosphere is calm, clear and the air pressure as low as 892 mb. The winds are light and variable. The clouds are either absent or scattered. The eye of the cyclone is the warmest part.
    • There are towering cumulonimbus clouds, torrential rainfall and violent winds accompanying a tropical cyclone.
    • The majority of cyclones decay when they come over the land or when they resurvey northward and reach over oceans.
    • Heavy rain may continue even after winds have become weak.

Nomenclature of Cyclones


The tropical cyclones occur mainly in the following regions:
1. Tropical North Atlantic – Caribbean Sea, West Indies, Gulf of Mexico, Western coastal areas of Mexico and Gulf of Panama (Hurricanes).
2. Tropical part of the North pacific and the Philippines (Baguio) the China Sea and areas around Japan (Taifu)
3. The Bay of Bengal and the north east Arabian Sea Gulf of Cambay of Kathiawar coast (Cyclones).
4. The south Indian Ocean, Coral Sea, Fiji, and the north and north east coast of Australia (Willy Willies).
5. The south western parts of the Indian Ocean, Madagascar, Mozambique, and Tanzania.


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