N.K. Chauhan 
Rising air creates low pressure systems, and sinking air creates high pressure - these high and low pressure systems influence climatic phenomena.
Warm air contains more water vapor than cold air, which is why we experience humidity during the summer and not during the winter.
The water content of air roughly doubles with every 10° C increase in temperature.
As air is warmed at the equator it becomes less dense and rises, the rising air is warm and full of water vapor; as it rises into the upper atmosphere it cools, and the cool air can no longer hold as much water vapor, so the water condenses and forms rain.
Therefore, low pressure systems are associated with precipitation, and we see wet habitats like tropical rainforests near the equator.
After rising and producing rain near the equator, the air masses move towards 30° latitude and sink back towards Earth as part of the Hadley convection cells.
This air has lost most of its moisture after producing the equatorial rains, so the sinking air is dry, resulting in arid climates near 30° latitude in both hemispheres.
Many of the major desert regions on Earth are located near 30o latitude, including much of Australia, the Middle East, and the Sahara Desert of Africa.
The air also becomes compressed and heats up as it sinks, absorbing any moisture from the clouds and creating clear skies. Thus high pressure systems are associated with dry weather and clear skies.
The Ferrel convection cells (Low Pressure Zone) leads to rain and the boreal forests at 60° latitude in the Northern Hemisphere (there are no corresponding large land masses at these latitudes in the Southern Hemisphere).
At the poles, descending, dry air produces little precipitation, leading to the polar desert climate.
Role of mountains
As moist air moves over land and encounters mountains it rises, expands, and cools because of the declining pressure and temperature. The cool air holds less water vapor, so condensation occurs and rain falls on the windward side of the mountains.
As the air passes over the mountains to the leeward side, it is now dry air, and as it sinks the pressure increases, it heats back up, any moisture revaporizes, and it creates dry, deserts regions (rain shadow) behind the mountains..
Tibetan Plateau and Gobi Desert behind the Himalayas, Death Valley behind the Sierra Nevada mountains, and the dry San Joaquin Valley in California.
Indian Monsoons
The climate of India is described as the "monsoon" type.Monsoons are seasonal winds which reverse their direction with the change of season. It blows from the northeast during cooler months and reverses direction to blow from the southwest during the warmest months of the year.
Generally, across the world, the monsoons are experienced in the tropical area roughly between 20? N and 20? S.
Out of a total of 4 seasonal divisions of India, monsoon occupy 2 divisions, namely.
The southwest monsoon season - It occurs between June and September, due to intense low pressure system formed over the Tibetan plateau.
The retreating monsoon season (North-east) - The months of October and November are known for retreating monsoons.
India, Indonesia, Bangladesh, Myanmar receive most of the annual rainfall during south-west monsoon season where as SE China, Japan receive most of the annual rainfall during north-east monsoon season.
The Thermal Concept
According to this view monsoon is a result of differential rates of heating and cooling of land and sea.The sun is vertical over the Tropic of Cancer in summer season of northern hemisphere and the Indian landmass at this time gets heated to a greater extent than the neighbouring sea.
This leads to formation of low pressure conditions over the Indian subcontinent in comparison to over Indian Ocean.
Therefore, thermally induced pressure gradient is produced from ocean towards Indian sub-continent leading to the onset of southwesterly winds blowing from Indian ocean towards India.
These winds, called southwest monsoon, blowing from sea towards land carry a large amount of moisture and cause copious rainfall over the landmass.
The land not only gets heated faster, it also cools faster and to a greater extent in summer than the ocean. Hence the Indian Ocean is warmer than the Indian subcontinent in winter.
This causes the pressure gradient to be reversed towards sea. This altered pressure gradient leads to the onset of winds blowing from northeast to southwest, i.e. winds blowing from Indian subcontinent towards Indian Ocean.
This wind system is called the northeast monsoon.
Since the winds are at this time blowing from land towards sea, they carry little moisture. The winter season over the Indian landmass thus remains largely dry.
It fails to explain the uncertain and irregular character of dynamic monsoon.
Role of Inter Tropical Convergence Zone (ITCZ)
The planetary winds of tropics are trade winds.In the months of March and September, when sun is overhead in equatorial area low pressure belt is created near equator and north-east trade winds of northern hemisphere and south-east trade winds of southern hemisphere converge in this belt of low pressure.
The ITCZ is associated with the zone of highest temperature and the lowest pressure.
During summer sun's rays are vertical over the Tropic of Cancer. At this point of time ITCZ shifts northwards and becomes NITCZ (Northern Inter Tropical Convergence Zone).
It extends up to 30? N Latitude in South and South-East Asia. The excessive heating of Indian sub-continent further intensifies this process.
At this point of time the south-east trade winds of southern hemisphere shift northward and get extended as southwest monsoon winds.
The south-east trade winds of southern hemisphere become south-westerly under the influence of coriolis force as they cross equator.
During winter season due to southward shifting of ITCZ north-east trade winds gets re-established over this region.
These north-east trade winds are called north-east winter monsoons.
When the ITCZ shifts towards Tropic of Capricorn in winter, the Trade Winds of northern hemisphere will cross the equator, will be deflected to left hand side and the southern hemisphere tropical zone will experience northwesterly winds.
Therefore are generally dry and devoid of rains. But on Tamil Nadu coast they are on-shore and bring precipitation in winter months
This theory does not negate the effect of the differential heating of land and sea. This dynamic concept also fails to explain the complex monsoon mechanism.
The recent concepts mainly focus on the role of jet streams and Tibetan plateau and ENSO (El Nino and Southern Oscillation).
The upper atmospheric conditions over Tibetan Plateau and positions and intensity of sub tropical westerly jet stream and tropical easterly jet stream play a significant role in the onset, withdrawal and intensity of Indian monsoon.
Role of jet streams
The sub-tropical westerly jet occupies a position over north India in winter season and the Himalayas and the Tibet plateau lead to bifurcation of this jet stream into two branches.The northern branch occupies a position to the north of the Tibetan Plateau and the southern branch is located over north India to the south of the Himalayas.
In the upper troposphere a high pressure system (anticyclonic conditions with clockwise air circulation) develops towards south of southern branch of sub-tropical jet stream over Afghanistan and north-west Pakistan.
Consequently, the winds tend to descend over the north-western parts of India, resulting into atmospheric stability and dry conditions.
During the summer season (April, May and June) due to shift in the overhead position of sun, low pressure areas develop at the surface near Pakistan and north-west India.
The winds descending from the upper air high pressure restrict ascend of winds from the surface low pressure.
This results into warm and dry weather conditions.
This is why the months of April and May (hottest month) are dry in spite of high temperature and evaporation.
Contrary to it, upper air low pressure is formed in the eastern Himalayan region due to upper air seasonal easterly jet streams.
Due to these conditions the winds coming from southern Myanmar are forced to ascend and produce rainfall in Myanmar, Bangladesh and North East India.
After the first week of June the southern branch of sub-tropical westerly jet steam disappears and only northern branch operates to the north of the Tibetan plateau.
This results into the development of a dynamic depression over north western part of Indo-Pakistan. As this dynamic depression gets established over the thermal depression present in this area, burst of monsoon takes place.
Role of Tibetan plateau
In summer season the Tibetan Plateau gets heated and acts as a high altitude heat source which produces a thermal anticyclone over this region (in the upper atmosphere).This anti-cyclone weakens the westerly sub-tropical jet stream south of the Himalayas, and gives rise to the tropical easterly jet stream and intensifies the high pressure cell over the Indian Ocean.
Thus a surface pressure gradient is produced from the Indian Ocean towards India and it activates south-west monsoon.
When the summer temperature over Tibetan Plateau remains high for a sufficiently long duration, it provides strength to the easterly jet stream and results in heavy rainfall over India.
Role of El Nino and Southern Oscillation (ENSO)
El Nino is a temporary warm ocean current which appears off the coast of Peru in December in some years.When the pressure is high over equatorial south Indian Ocean, it is low over the equatorial south Pacific and vice-versa.
This pressure variation gives rise to circulation along the equator known as Walker Circulation.
The pressure variation is stated as Southern Oscillation Index (SOI) and it is measured as difference in pressure between Tahiti in French Polynesia, representing the southern Pacific Ocean and Port Darwin, in northern Australia, representing the Indian Ocean.
Sustained positive SOI values are indicative of La Nina conditions while negative values suggest El Nino conditions.
There is tendency of monsoons being poor in El Nino years but the relationship is not deterministic.
El Nino is a phenomenon in the equatorial Pacific, in which sea-surface temperatures rise over a threshold of +0.5 degree Celsius (and cools by the same margin during alter ego La Nina).
Retreating Monsoon
During the months of October-December, the south-west monsoon winds become weaker and start to retreat from the skies of North India. This phase of the monsoon is known as the retreating monsoon.Unlike the sudden burst of the advancing monsoons, the withdrawal is rather gradual and takes about three months.
The retreating monsoons absorb moisture while passing over the Bay of Bengal and cause this rainfall.
From October to December the coast of Tamil Nadu state receives at least half of its roughly 40 inches (1,000 mm) of annual precipitation.
With retreat of the monsoons, the clouds disappear and the sky becomes clear.
As the monsoons retreat, the monsoon trough weakens and gradually shifts southward.
Most severe and devastating tropical cyclones originate in the Indian seas especially in the Bay of Bengal.
Western Disturbance
A western disturbance is an extratropical storm originating in the Mediterranean region that brings sudden winter rain to the northern parts of the Indian subcontinent.It is a non-monsoonal precipitation pattern driven by the westerlies.
The moisture in these storms usually originates over the Mediterranean Sea, the Caspian Sea and the Black Sea.
The western disturbances are embedded in the mid-latitude subtropical westerly jet stream.
The southwest monsoon current generally progresses from east to west in the northern Himalayan region, unlike western disturbances which follow a west to east trend in north India with consequent rise in pressure carrying cold pool of air.
