Wind is the flow of gases on a large scale. On the surface of the Earth, wind consists of the bulk movement of air. In outer space, solar wind is the movement of gases or charged particles from the Sun through space, while planetary wind is the out gassing of light chemical elements from a planet’s atmosphere into space.
In meteorology, winds are often referred to according to their strength, and the direction from which the wind is blowing. Short bursts of high-speed wind are termed gusts. Strong winds of intermediate duration around one minute, are termed squalls. Long-duration winds have various names associated with their average strength, such as breeze, gale, storm, and hurricane.
1. Planetary Winds:Planetary winds comprise winds distributed throughout the lower atmosphere. The winds blow regularly throughout the year confined within latitudinal belts, mainly in north-east and south-east directions or from high-pressure polar-regions to low-pressure regions.
2. Trade Winds: These are extremely steady winds blowing from sub-tropical high pressure areas (30°N and S) towards the equatorial low pressure belt. These winds should have blown from the north to south in Northern Hemisphere and south to north in Southern Hemisphere, but, they get deflected to the right in Northern Hemisphere and to the left in Southern Hemisphere due to Coriolis effect and Ferrel’s law. Thus, they blow as north eastern trades in Northern Hemisphere and south eastern trades in Southern Hemisphere.
They are also known as tropical easterlies, and they blow steadily in the same direction. They are noted for consistency in both force and direction.
3. The Westerlies: The Westerlies are the winds in the middle latitudes in the ranges of 35 to 65 degrees. These winds blow from the west to the east and determine the travelling directions of extra tropical cyclones in a similar direction. The winds are mainly from the northwest in the Southern Hemisphere and southwest in the Northern Hemisphere.
4. Periodic Winds: Periodic winds change their direction periodically with the change in season, e.g., Monsoons, Land and Sea Breezes, Mountain and Valley Breezes.
- Monsoon Winds: These winds are seasonal winds and refer to wind systems that have a pronounced, seasonal reversal of direction. According to ‘Flohn’, monsoon is a seasonal modification of general Planetary Wind System. Summer monsoon is called South Westerly Wind and is characterized by highly variable weather with frequent spells of drought and heavy rains. The winter monsoon is a gentle drift of air in which winds blow from the north-east and is known as North Easterly Wind.
- Land Breeze: At night, land masses cool quicker than sea due to rapid radiation which results in high pressure over land and low pressure over sea. And in calm, cloudless weather, air blows from land to sea. This breeze carries no moisture and is little warm and dry.
- Sea Breeze: In day time, the land being hotter than the sea develops low air pressure and the sea being cool develops high pressure. The air over land rises and is replaced by a cool breeze known as Sea Breeze from the sea, carrying some moisture.
- Mountain and Valley Breezes: A diurnal wind occurs in mountainous regions which are similar to Land and Sea Breezes. During the day the slopes of mountains are hot and air from valley flows up the slopes. This is known as “Valley Breeze”. After sunset the pattern is reversed and cold air slides from mountain to valley and is called “mountain breeze”.
5. Local Winds:The local difference in temperature and pressure causes local winds. It is of four types: hot, cold, convectional and slope.