Trade Winds

What are the Trade Winds?

Trade winds are a persistent wind that can be defined as the wind that flows towards the equator from the north-east in the northern hemisphere or from the south-east in the southern hemisphere. These are also known as tropical easterlies and are known for their consistency in force and direction. It is stronger and more consistent over the oceans than over land and often produces partly cloudy sky conditions, characterized by shallow cumulus clouds, or clear skies that make trade-wind islands popular tourist resorts. To understand trade winds it is important to understand what wind is. The wind is defined as the flow of gases or air on a large scale from the high-pressure area to low pressure area.

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Classification of Wind

  • Planetary Winds: Winds that are caused due to air pressure difference from one latitude to another latitude, these winds are also called prevailing winds.

  • Trade Winds: Winds that blow as south-eastern trades in the Southern hemisphere and as north-eastern trades in the Northern hemisphere. And are primarily caused due to the Coriolis effect and Ferrel’s law.

  • The Westerlies: The winds that blow from the west towards the east in the middle latitudes between 30° and 60° latitude in both northern and southern hemispheres. These winds get their name from their direction of origin. These are blown from the horse latitudes towards the poles. In the northern hemisphere, these are predominantly from the southwest and in the southern hemisphere, they are from the northwest. 

  • Easterlies: The easterlies or polar easterlies are located between 60° and 90° latitude in both northern and southern hemispheres and are named after the direction they originate from. These are formed when the cool air from the poles sinks and moves towards the equator. These winds are important for sailors as with the help of trade winds only Christopher Columbus discovered America.

  • Periodic Winds: Winds that change their direction accordingly and are very much dependent on different seasons,  like monsoons.

  • Local Winds: Winds that are caused due to differences in temperature and pressure locally and can be classified into conventional,  hot, cold, and slope.


What is the Coriolis Effect?

The Coriolis effect is defined as the inertial or fictitious force responsible for the deflection of winds towards the right in the northern hemisphere and towards the left in the southern hemisphere. Coriolis effect is used for deriving Ferrel’s law. The Coriolis effect is defined as how a moving object seems to veer toward the right in the Northern hemisphere and left in the Southern hemisphere. The turning of Hurricane winds towards the left in the Northern hemisphere is an example of the Coriolis effect.

There is no Coriolis effect underneath a horizontally and freely moving object at the equator as there is no rotation of the surface of the Earth (sense of turning), and there is no curving of the path for the object as we measure relative to Earth's surface and the object's path is straight. Thus causing no Coriolis effect.

  • The equation of motion for an object in an inertial reference is as follows:

F = ma

Where,

F =  the vector sum of physical forces acting on the object

m  = the mass of the object

a = the acceleration of the object which is relative to the inertial reference frame.


  • The above equation can be turned into a non-inertial reference frame is:

F − m dΩ dt . r − 2mΩv′ − mΩ . (Ω . r)=ma′

Where,

Ω = the rotational vector

v′ = the velocity which is relative to the rotating reference frame

r = the position vector of the object

a′= the acceleration which is relative to the rotating reference frame


Did You Know?

In northern India, the average maximum temperature is above 33°C in  May. It is because of such a high temperature the air of that region heats up and the hot air rises causing a low-pressure area under it, also known as the monsoonal trough. On the contrary, the temperature over the Indian Ocean is relatively low therefore a relatively high-pressure region is created over the sea. The air from the high-pressure region moves towards the low-pressure region because of the pressure difference between the Indian Ocean and North Central Indian Plains. This signifies that the movement of air is from the equatorial region of the Indian Ocean to the Indian subcontinent in the South-West to North-East direction in June which is exactly opposite to that of the trade winds (North-East to South-West) in India prevailing during winter. The reversal in direction of the wind from North-East to South West and vice-versa is called monsoons. These winds are heavily moisture-laden and cause widespread rain throughout India and from June to September when they move over the Indian subcontinent.

FAQ (Frequently Asked Questions)

1. Why Do The Trade Winds Blow From East to West?

The trade winds blow toward the west partly because of Earth’s rotation on its axis. The trade winds begin as warm, moist air from the equator which rises in the atmosphere and as cool air closer to the poles sinks. Though the air cycles from the equator the trade wind does not blow from north to south it is because Earth rotates as the air is moving, the winds in the Northern Hemisphere curve to the right and air in the Southern Hemisphere curves to the left. This phenomenon is called the Coriolis Effect and it is why the trade winds blow toward the west in both the Northern Hemisphere and Southern Hemisphere. 

2. How Are Trade Winds Caused And What Weakens Trade Winds?

Trade winds are caused by strong warming and evaporation within the atmosphere around the equator where the warm air rises rapidly, carrying a lot of moisture. The Coriolis Effect, in combination with an area of high pressure, is responsible for causing the prevailing winds called the trade winds that move from east to west on both sides of the equator.

The surface pressure pattern of higher pressure in the eastern Pacific and the lower pressure in the west drives the easterly trade winds. The trade winds start to weaken when the pressure gradient weakens which drives the easterly trade winds.