The velocity of air also depends on air pressure. The gradient of air pressure affects the speed of air. When the gradient of air pressure decreases, the wind blows at a high speed. Isobaric lines indicate the gradient. If the isobars are close to each other, then the air pressure is high and the winds blow at high speed. But when the isobars are far apart, the air pressure is low and the speed of the winds is slow.
Winds turn to the right in the northern hemisphere and to the left in the southern hemisphere due to the deflecting force generated by the motion of the earth. This is called Ferrel’s law. Mr. Bize Ballett stated that “winds flow parallel to the isobars at a height above the surface of the earth. In this, the low air pressure area in the northern hemisphere remains on the left side of the winds and in the southern hemisphere on the right side.”
Major Planetary Permanent Winds
Six major permanent winds blow on the earth. Permanent winds are also called Sanatani or Sanmargi winds. Two winds blow from the sub-tropical high-pressure belts towards the equatorial low-pressure area. These are called trade winds. Similarly, two winds blow from the sub-tropical high-pressure belts towards the sub-polar lowland region. These are called westerly winds. In addition, two winds move from the polar high-pressure belt to the subtropical low-pressure belt, where they meet the westerly winds. These are called polar winds.
Trade winds – These winds move between 100 to 300 latitudes, but their position shifts 5° north and south with the sun. These winds blow at a consistent speed over ocean areas, usually between 16 to 24 km/h. There is a significant difference in their direction and speed over the continents. The north-easterly winds blow in the northern hemisphere and the south-easterly trade winds blow in the southern hemisphere. These winds move in a consistent direction and speed day and night without any hindrance. Due to these winds, there is sufficient rainfall on the eastern edges of the continents, but the western edge is often dry. They cover 30.6 percent of the earth’s water body.
Westerly winds – These winds blow between 30° to 60° latitudes. Their belts also shift. The reason for their origin is the polar low-pressure belt at the high pressure of horse latitude and the daily motion of the earth.
The westerly winds are stormy and cause heavy rainfall throughout the year. Due to their origin from hot latitudes, they increase the temperature. Westerly cyclonic winds are found in the northern hemisphere, but in the southern hemisphere, the winds move freely on the vast seas at tremendous speeds. That’s why they are called thundering Chalisa or Veer Pachhua Pawan.
Polar Winds – Winds blow between 60° latitudes and the poles. Their belts shift according to the position of the Sun. When the Sun shines in the Northern Hemisphere, the polar winds shift to the south, and when the Sun shines in the Southern Hemisphere, they shift to the north. These are extremely cold winds.
The main reason for the mobility of the atmosphere is the Sun. Winds blow as a result of the uneven heating of the surface. Due to uneven temperature, there is a difference in the air pressure of different places. Winds start blowing from the place of high pressure to the place of low pressure. Thus, air pressure controls the wind direction.
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Land and Sea Breezes
- Terrestrial Breezes – The winds blowing from the land towards the ocean are called Terrestrial Breezes. These winds often become weaker as they travel further away from the shore. They are typically dry, and their effect on coastal areas is immediate.
- Sea Breezes – The winds moving from the sea towards the land are called Sea Breezes. Sea breezes do not usually blow up to high altitudes in the atmosphere, but their height can vary due to different atmospheric processes in different climatic regions. It has been found that sea breezes can be found near large lakes up to a height of 200-500 meters. In tropical and subtropical regions, their height can be estimated at up to 1000-2000 meters near the oceans. This is why it often rains daily around 3 pm in equatorial and tropical areas, which are heavily influenced by sea breezes. Sea winds also change their speed and direction depending on the latitude. In middle latitudes, their speed ranges from 12 to 50 km per hour, while in low latitudes, their speed can become stronger and take the form of storms, etc. The temperature of these winds can also vary greatly.
Valley and Mountain Breezes
Mountain and valley breezes are also local winds, like aquatic and terrestrial winds, that occur due to the variation in daily air masses. During the day, after receiving heat from the sun’s rays, the winds in mountain valleys become lighter and start rising above the valley. These winds then rise with the help of the mountains and reach the peaks of the mountains, creating a valley wind system known as Sameer.
Local Winds and Chinook Wind
Local winds are generated due to special reasons that disturb the order of planetary or permanent winds, causing special types of winds to blow in particular seasons or places. Due to temperature differences on the surface, many types of local winds are generated and have different names, such as the Loo in northern India, Sirakko blowing from northern Sahara to Europe, Khamsin blowing in Egypt, Spoon blowing in Arav, Crick Fielder blowing in New South Wales (Australia), and Santana blowing in California (USA).
Mountain and valley breeze is also a local wind. In mountainous regions, mountain peaks become low-pressure areas when heated, causing the air in the valley to rise. Fohn winds occur when warm air rises from the Southern Alps, expands, and drops its moisture in the form of frost. When it crosses the hill and descends to the north, the temperature rises, creating Chinook winds that warm up as they descend from the Rocky Mountains to the northern plains. The areas affected by Fohn and Chinook winds are wider, and they are economically significant for agricultural work.