The equatorial and tropical regions get hotter than the Polar Regions. The warm air at the equator rises, and the cold air moves in from the Polar Regions. The air moves due to uneven heating of the earth’s surface between the equator and poles, which results in circulation of wind across the globe. Lets talk about some main patterns of winds.
Trade winds
The air in the equator region gets heat up as this region gets direct rays of sun. When the hot air from this region rises up, the cooler air from the regions of 0-30 latitudes on either sides of the equators move in. These winds are called trade winds. The trade winds blow predominantly from the northeast in the Northern Hemisphere and from the southeast in the Southern Hemisphere. These winds strengthen during the winter and when the Arctic oscillation is in its warm phase. Trade winds have been used by captains of sailing ships to cross the world's oceans for centuries, and enabled trade routes to become established across the Atlantic and Pacific oceans, hence the name "trade wind".
Westerlies
Above the subtropical highs in the Northern Hemisphere, and below the subtropical highs in the Southern Hemisphere, winds blow from the west towards the east. These winds are called westerly winds, after the direction from where the winds come. The westerlies generally blow between 30 ° and 60 ° latitude in both the Northern and Southern hemispheres. The higher one travels into the atmosphere, the more noticeable these westerly winds are.
At the core of the westerly winds lies what scientists call a jet stream. Jet streams are super high-speed winds. Each hemisphere’s westerly has two main jet streams. Closer to the poles we find the polar jet stream. At slightly lower latitude we find the subtropical jet stream.
Polar winds
Polar winds begin near the North and South Poles. Frigid air in the winter sinks toward the ground creating a high pressure area at the poles. These winds occur in both hemispheres. Approximately 60 degrees north and south latitude warm air is rising creating low pressure areas. Winds form as they more from high pressure areas near the poles toward the low pressure areas 30 degrees below each of the poles. As these winds blow from the east to the west, so these winds are also called polar easterlies.
Monsoon winds
The uneven heating of land and water also causes temperature differences, which in turn cause air currents (wind) to develop. The term Monsoon is generally used for the big seasonal winds blowing from the Bay of Bengal and Arabian Sea in the southwest bringing heavy rainfall in the India, Bangladesh and Pakistan.
The southwestern summer monsoons occur from June through September. The Thar Desert and adjoining areas of the northern and central Indian subcontinent heats up considerably during the hot summers, which causes a low pressure area over the northern and central Indian subcontinent. Thus, a high pressure area lies over the Indian Ocean while a low exists over the Asian continent. The air masses move from the high pressure over the ocean to the low over the continent, bringing moisture-laden air to south Asia. These winds, rich in moisture, are drawn towards the Himalayas, creating winds blowing storm clouds towards the subcontinent. The Himalayas act like a high wall, blocking the winds from passing into Central Asia, thus forcing them to rise. With the gain in altitude of the clouds, the temperature drops and precipitation occurs.
During winter, the process is reversed. The cooler air above the land and a low sits over the Indian Ocean while a high lies over the Tibetan plateau so air flows down the Himalaya and south to the ocean. The migration of trade winds and westerlies also contributes to the monsoons.
