• The winds blow from the sub tropical high pressure belt towards the equatorial low pressure belt.
• Due to Coriolis Effect, these winds are deflected to the right in the northern hemisphere and to the left in the southern hemisphere.
• As winds are named after the direction from which they originate they are called as the North East and South east trade winds.
• As the winds favoured trading ships they are called as ‘Trade winds’.

Side Topic : Tropic Deserts & Trade Winds (aka Trade Wind Deserts or Trade Deserts)

• Tropical easterlies/ Trade Winds flow from east to west  . Hence, windx becomes dry when they reaches the western coast of continent as all the moisture  is already shed in form of rainfall on eastern coast .As a result, in tropical region, deserts are found on western coasts of continents .They are also known as Trade Deserts.
• Apart from that, Cold Currents near the western coasts of continents also provides desiccating imapct on the surrounding lands leading to more dryness .

• Westerlies flow towards the Sub Polar High from Sub-Tropic Low
• They turn towards right and left in northern and southern hemisphere respectively due to Coriolis force.
• As they flow from West to East, they are called Westerlies
• Ocean is dominant in the southern hemisphere between the latitudes 40º and 60ºS. Hence the westerlies are so powerful and persistent that the sailors used such expressions as “Roaring Forties”, “Furious Fifties” and “Screeching Sixties” for these high velocity winds in the latitudes of 40º, 50º and 60º respectively.

• Sea Breeze :  During daytime, land heats up much faster than water. The air over the land warms and expands leading to formation of low pressure. At the same time, the air over the ocean remains cool because of water’s slower rate of heating and results in formation of high pressure. Air begins to blow from high pressure over ocean to the low pressure over the land. This is called as ‘Sea breeze’.

• Land Breeze : During night time, the wind blows from land to sea and it is called as ‘Land breeze’

Note :  Sea breeze and land breeze influence the movement of boats near the coastal region and fisher men use these winds for their daily fish catching. Fishermen go for fishing at early morning along the land breeze and return to the shore in the evening with the sea breeze.

Cold Local Winds

• High Pressure  conditions are created in areas situated in high latitudes due to cold weather. As a result,   air starts to come down and diverge into different directions (forming anti-cyclones) blowing as ‘Cold and Dry Local Winds’ and reducing temperature of surrounding regions as well.
• In Siberia such anti cyclonic winds are created and diverge in different directions  . These winds are called Buran

Warm Local Winds

• Low  pressure develops  over Deserts and low latitudes in summers due to excessive heating of land . The air starts to move upward and diverges in different directions blowing as upper tropospheric wind. They carry sand and dust with them and raises temperature of regions over which they flow. These winds are known with different names in different regions like

• Other  type of warm and local winds like Chinook winds develop when warm, moist air blows from the adjoining ocean ( Pacific Ocean in this case) towards the Mountain range situated near the coast (Rockies in this case). In such situation, dry and warm air over the mountain will descend in the adjoining valley on the leeward side of wind .  Other such type of winds are Fohn and Zonda

• Cause of Jetstreams is the pressure difference (due to temperature difference) in the upper atmosphere. Hence , higher the pressure gradient (or temperature gradient) higher will be speed of Jetstream.
• Temperature variations are more in winters (lowest ~ -70 C & highest ~ 15 C) compared to summers (lowest ~ 20 C & highest ~ 55 C) . Hence, Jetstreams are faster in winters of the respective hemisphere.

1 . Sub Tropical Westerly Jet Stream (STWJ) & Indian Weather

• It is centred around 25° N & S at altitude of 12 km & is strong in winter season with velocity of 40 mph.
• It greatly determines the weather of Indian Sub continent .
  1. During summer until it is present over Indian subcontinent , High Pressure is maintained over there & monsoon can’t start. Only when STWJ moves above Himalayas & low pressure is created over Indian subcontinent  , monsoon hits India.
  2. Western Disturbances : STWJ comes to India after passing over Mediterranean Sea where rainfall occurs during  winter. STWJ bring those cyclonic disturbances to north India along with it. This results in winter rain & hailstorms in North India  & occasional high snowfall in hilly areas  .

2. Jetstreams and Frontal / Temperate Cyclones

• Jetstreams play important role in formation of Temperate Cyclones which are important feature of the climate of temperate regions like Britain etc .

3. Tropical Easterly Jet and Somali Jetstream

• These Jetstreams play important role in the Indian Monsoon.
• More about this can be read in (chapter) Indian Climate .

4. Role in Aviation Industry

• If aeroplanes moves in the direction of Jetstream, it can lead to large fuel savings and vice-versa

Due to Climate change and Global warming, Earth’s Polar regions are warming more rapidly than other parts . This has resulted in weakening of Polar Jet Streams because temperature contrast that drives Jetstreams has decreased.