Planetary Winds

Planetary winds are winds that flow across the planet and stay relatively stable throughout the year. There is a horizontal and vertical distribution of pressure belts over the surface of the earth. The difference in temperature causes pressure to change. Varying pressure causes air to move and we call this moving air wind. These winds can have far-reaching effects on the surface and atmosphere of a planet. These winds are responsible for distributing the insolation our atmosphere receives and regulating global weather patterns. Primarily all the planetary winds are either easterlies (coming from the east) or westerlies (coming from the west).

Types of winds

Winds can be classified based on their expanse and period, into two categories.

1. Planetary or Primary Winds
2. Secondary or Periodic Winds

Planetary Winds:

Primary winds or planetary winds are large-scale atmospheric movements. They are found flowing between the global pressure belts. They maintain a consistent pattern and remain relatively permanent throughout the year. In this article, we will delve into the intricacies of planetary winds. We will explore their causes, patterns, and impacts on the planet’s climate and weather systems. All the easterlies and westerlies are examples of planetary winds.

Secondary Winds:

Secondary winds are characterized by their seasonal or periodic nature. They are specific to particular regions, with each region having its own unique flow of seasonal wind. These winds are driven by regional factors such as differences in air pressure and temperature, topography, and land-sea distribution.

Examples of seasonal or secondary winds include monsoons, harmattan, bora, chinook, and Santa Ana, to name a few. Each of these winds has its own distinct characteristics and effects. These winds modify the climate and weather patterns of the regions they flow in.

Mechanisms of Planetary Winds

Now, we will try to understand the functioning of planetary winds. To understand these winds, it is important to know the reasons behind their flow. Various thermal and dynamic factors influence the flow of planetary winds. Insolation and the earth’s rotation play important roles in initiating and modifying the flow of these winds.

The pressure belts shown in Figure 1 initiate the flow of winds.

There are two extreme thermal points on Earth. The Equator and the Poles i.e. the 0° and the 90°. When the air gets heated up on the equator and rises, it tries reaching the poles. Theoretically, air would move from extremely high (poles) to extremely low (equator), as shown in Figure 2(a). Professor Brain Farrell in the 1990s theorised this condition. He said that air must have moved this way during the past climates. Hadley cells must have extended up to the poles.

Rising air at the equator

Figure 1 shows that 0° is a low-pressure area. Here, the sun’s rays fall directly. High temperature creates low pressure because the heated air on the surface rises up. This rising air moves towards the poles because its density is low. Centrifugal force aids in uplifting air (Centrifugal force is highest at the equator).

Air moving poleward cools adiabatically and is subjected to strong Coriolis force.

Descending air at 30° latitude

The risen air from the 0° loses inertia as it cools down. This air falls at 30° latitude. This creates an abundance of air at this latitude. Comparatively, the 30° latitude is cooler than the equator. The descending air from the equator further intensifies the higher pressure here.

As a result air from this region attempts to normalise the vacuum created by rising air at 0°.

The 90° high

Just like the 0° (where the air is subjected to intense outward centrifugal acceleration), at the poles this acceleration is minimal and the air isn’t thrown outward, making the already cold dense air even denser. Some air from the equator also manages to fall here making it an extremely high-pressure region.

Nomenclature of Planetary Winds

Trade Winds Or the North-Easterlies Or the Prevailing Winds

The North East Trade Winds are the prevailing winds that blow from the northeast towards the equator in the Northern Hemisphere (30° N to 0°). And from the southeast towards the equator in the southern hemisphere (30° S to 0°). These are two out of the six major wind belts that exist on Earth. The North-easterlies are important for global weather patterns and came to be known as Trade winds for maritime trade routes. As they allow ships to sail from Europe to the Americas and back again. These winds also play a role in the climate of the Caribbean and the Gulf of Mexico. They can bring moisture and rain to these regions.

The ITCZ (Inter Tropical Convergence Zone)

ITCZ or the Inter Tropical Convergence Zone is the region where the air from both tropics converges. This is the zone where the northeast and southeast trade winds merge. The ITCZ shown in the above figure is at the equinox or when the earth is not tilted towards or away from the sun. But when this position changes the then ITCZ shifts North and South in the summer and winter solstice respectively of the northern hemisphere. When this happens ITCZ is known as the NITCZ or North ITCZ when it’s in the northern hemisphere and SITCZ of the South ITCZ in the southern hemisphere.

Doldrums

The Equatorial Low or the ITCZ is also called Doldrums. This name highlights another characteristic of this region which is typically located around 5 degrees north and south of the Equator. It refers to a low-pressure area near the Equator where the prevailing winds are calm or very weak. This region between the Northern and Southern Hemisphere trade winds and is characterized by the absence of any significant winds.

In the past, ships would often become becalmed in the doldrums, and crews would suffer from a lack of fresh water, food, and ventilation. Today, modern ships and aircraft can navigate the doldrums more quickly, but it remains a challenging area for any travellers who rely on wind power.

The Mid-Latitude Westerlies

The westerlies are prevailing winds that blow from subtropical high-pressure belts (around 30° to 35°) towards sub-polar low-pressure belts (around 60° to 65°) in both hemispheres. They blow southwest to northeast in the Northern Hemisphere and northwest to southeast in the Southern Hemisphere.

In the Southern Hemisphere, the westerlies are generally stronger and more consistent due to the vast expanse of ocean, while those in the Northern Hemisphere can be more variable due to the uneven terrain of large landmasses. During summer in the Northern Hemisphere, the westerlies become more complex and less effective but become more vigorous during winter.

The westerlies carry significant amounts of moisture, especially over vast stretches of ocean, and can bring substantial precipitation to western regions of continents such as the northwest European coasts.

The westerlies are most pronounced between latitudes of 40° and 65°S. Here they are nicknamed the Roaring Forties, Furious Fifties, and Shrieking Sixties, which sailors fear.

The poleward boundary of the westerlies is highly variable and experiences many seasonal and short-term fluctuations that can cause wet spells and weather variability. Moreover, in the Southern Hemisphere, the westerlies can become stormy and associated with boisterous gales as they gain velocity towards the pole due to the ocean’s dominance and lack of land.

Polar Easterlies

Polar easterlies are prevailing winds that blow from the polar high-pressure areas (near the poles) towards the subpolar low-pressure belts at around 60-65 degrees latitude in both hemispheres. In the Northern Hemisphere, they blow from northeast to southwest, and in the Southern Hemisphere, they blow from southeast to northwest. These winds are characterized by their cold temperatures and weak intensity, as the low solar radiation levels at high latitudes influence them.

The Tri-cellular Model

The planetary wind system is complex. To understand it we try to oversimplify concepts. The tri-cellular model does the same. Even though this cell helps us understand planetary wind circulation it is not perfect.

The figure below shows all three planetary winds trade winds, westerlies and polar easterlies. these three winds form a band of air throughout the globe. A cross-section reveals three cells: the Polar, Ferrel and Hadley.

The Hadley cell is the largest and most important of the three cells. It is located near the equator and is responsible for the trade winds in the tropics. The cell is driven by warm, moist air rising at the equator and moving towards the poles, where it cools and sinks back to the surface. This creates a band of low pressure near the equator and a band of high pressure at around 30 degrees north and south latitudes.

The Ferrel cell is located in the mid-latitudes, between the Hadley and Polar cells. It is driven by the circulation of air between the Hadley and Polar cells. The Ferrel cell is not as well-defined as the other two cells, but it is responsible for the prevailing westerly winds in the mid-latitudes.

The Polar cell is located near the poles and is driven by the sinking of cold, dry air. This creates a high-pressure zone at the poles, which in turn drives the circulation of air towards the mid-latitudes.

We often consider Palmen’s model when we want a better understanding of planetary circulation. Palmen’s model defines planetary circulation considering air masses and upper air circulation. Palmen’s model has been influential in the study of mid-latitude cyclones and is still widely used by meteorologists today to understand and forecast weather patterns.