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Jet Streams - Climatology

What are Jet Streams?

The Jet Streams on earth are powerful and fast-moving circumpolar westerly (flowing west to east) air circulation that occurs above the troposphere's upper boundary in a narrow band that is a few hundred kilometres wide.

They are found on the margins of the 3 circulation cells at a height of around 12000 meters. They occur at the margins of the circulation cells as its a boundary between hot and cold air masses. These margins keep on shifting in a north-south direction due to the tilt of the earth and variable incident solar radiation that the earth's surface receives.

The most common application of jet streams is in air travel, where flying with or against the flow can substantially cut flight time. In order to save money and time, airlines frequently strive to fly 'with' the jet stream.

Factors affecting Jet Streams on Earth

Jet streams on earth are the result of two factors: 

1. Differential Solar Heating: The heating of the atmosphere by solar radiation, which forms the large-scale Polar, Ferrel, and Hadley circulation cells

2. Coriolis Force: The effect of the Coriolis force on those moving masses. The spinning of the planet on its axis causes the Coriolis force to determine the direction of flow.

Other planets' jet streams are powered by internal heat rather than solar heating.

Discovery of Jet Streams

The weather trackers in 19ᵗʰ century would track volcanic smoke and data from pilot balloons to understand upper atmosphere wind patterns. However, the jet streams were critically understood during WWII when American jet bomber combat planes flew from east to west towards Japan and faced impediments to an airflow flowing in different directions (from West to East).

Properties of Jet Stream

  • Tropospheric Westerly Flow: They flow at a height of 7.5-14 kilometres from west to east in a narrow strip - a few hundred kilometres wide.

  • Thousands of Kilometeres Long: Jet streams can be thousands of kilometres long, hundreds of kilometres wide, and only a few kilometres deep (2-4 kilometres).

  • Found around both poles: Because they round the poles in both hemispheres, jet streams are sometimes known as circumpolar whirls.

  • Wavy and Meandering Path: The path of their circulation is wavy and meandering due to the shifting of ITCZ over time.

  • Variation in Velocity: The wind velocity of the jet streams fluctuates seasonally. During the winter, these become especially forceful because the pressure difference is stark.

Why do jet streams move faster in the winter than they do in the summer?

Because jet streams occur at the boundary of hot and cold air masses, and the temperature difference between these masses is greater in winter than in summer, the speed and strength of the jetstreams are affected.

Their top speed ranges from approximately 110 km/h in the summer to approximately 184 km/h in the winter.

Types of Jet Streams

Jet streams are classified as follows based on their location:

Polar Front Jet Streams: These jet streams arise when winds from sub-tropical and polar regions combine and form over sub-polar low pressure belts. These aren't your average jet streams. They're heading east i.e they are easterlies.

Sub-Tropical Westerly Jet Stream is found above 30-35 degrees latitude, poleward of the sub-tropical high pressure band. They flow from west to east in a regular pattern

Sub-Tropical Easterly Jet Stream develop during summer in upper troposphere over Indian subcontinent. They play an important role in bringing monsoon winds to India.

Sub-polar stratospheric jet streams are another name for polar night jet streams. They arise primarily in the winter due to a high temperature gradient in the stratosphere near the poles at a height of 30 kilometres.

Indian Monsoon Mechanism and Role of Sub-Tropical Jet Stream (STJ)

The Sub-Tropical Jet Stream (STJ) is important for both hindering monsoon winds and hastening monsoon arrival. The Indian monsoon season is however unaffected by Polar Jet which influences weather close to poles than the tropics.

The subtropical westerly jet streams that blow at 27° and 30° latitudes widely near the northern edge of the Indian subcontinent dominate the upper air circulation over India.

During the summer, these subtropical westerly jetstreams migrate north of the Himalayas because the northern hemisphere receives more solar energy. Simultaneously, another subtropical easterly jet stream flows over peninsular India, about north of 14°N.

During the rest of the year, particularly in winter, the subtropical westerly jet stream shifts south of the Himalayas, bringing with it western cyclonic disturbances from the Mediterranean. These storms deliver winter rain to the Indian subcontinent's north and northwest.

Tropical cyclones, on the other hand, form during the monsoon season and in October-November as part of the easterly jet stream flow. These disturbances impact the country's eastern coastal regions, including West Bengal, Odisha, Andhra Pradesh, and Tamil Nadu.

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