Planetary Winds

Imagine Earth as a vast stage where the winds perform an endless, synchronized dance, governed by the forces of pressure, temperature, and rotation. These winds, known as planetary winds or permanent winds, are like dedicated marathon runners—they blow in almost the same direction throughout the year, covering vast regions of the planet.

Now, let’s understand these winds, feeling their presence as we move across different latitudes. But before that look at the following diagram once again and try to have a feel for the atmospheric circulation systems of earth:

The Trade Winds

Let’s start near the equator, where the sun blazes down with all its might, heating the land and air intensely. As the air warms, it rises, creating a low-pressure zone. Nature, never allowing a vacuum to persist, pulls in air from the subtropical high-pressure belts (25°–30° latitudes). This incoming air is what we call the trade winds.

Characteristics of Trade Winds:

• These winds are the most reliable and consistent of all planetary winds. If Earth had a clock, these winds would be its most punctual tick.
• They blow from the subtropical high-pressure belt to the equatorial low-pressure belt, curving slightly westward due to the Coriolis effect (caused by Earth’s rotation).
• As these winds travel from the cooler subtropics to the warmer equator, they gain high moisture-holding capacity. This is why, when they hit the eastern coasts of continents, they unleash heavy rainfall—creating lush, tropical rainforests like the Amazon and Congo.
• However, the story is different on the western coasts of continents. Here, trade winds blow from land towards the ocean, carrying no moisture. The result? Hot deserts like the Sahara, Atacama, and Kalahari, where rain is almost a myth.

Some Important Points:

• Onshore Winds: Blow from the sea to the land.
• Offshore Winds: Blow from the land to the sea, leading to desert formation.
• Trade Winds:
  • Act as onshore winds for the eastern part.
  • Act as offshore winds for the western part.
•  Wind Movement (HP → LP):
  • Winds with high moisture-carrying capacity result in high rainfall.
  • High moisture leads to significant rainfall, forming equatorial rainforests.
• What is Intertropical Convergence?
  • The equatorial front formed due to convergence of two trade winds is called as intertropical fronts (ITF) or intertropical convergence (ITC). We will talk about fronts later in upcoming chapters.

The Westerlies

Now, let’s move further away from the equator, between 40° and 60° latitudes, where a different set of winds takes over—the westerlies. Unlike the disciplined trade winds, these winds are more like wandering travellers, constantly shifting and changing.

Characteristics of Westerlies:

• They originate from the subtropical high-pressure belt and move towards the temperate low-pressure belt, blowing from west to east.
• In the Northern Hemisphere, they are less consistent due to the complex mix of land and water, which disrupts their flow.
• But in the Southern Hemisphere, where oceans dominate, these winds are ferocious, earning dramatic names like:
  • Roaring 40s (at 40° S)
  • Furious 50s
  • Shrieking 60s
  • Screaming 70s
    These winds sweep across the Southern Ocean with incredible force, creating some of the roughest seas in the world.
• The westerlies bring heavy rainfall to the western coasts of continents between 40° and 60° S, giving rise to temperate rainforests in regions like Washington, Oregon, and British Columbia.
• However, if a mountain range stands in their way, it acts as a natural rain barrier. The windward side gets drenched, while the leeward side remains dry. This effect is visible in:
  • The Patagonia Desert, lying in the rain shadow of the Andes.
  • The Great Plains of the U.S., where the Rockies block moisture, resulting in semi-arid prairies.
• Refer to the following figure: The westerlies bring moisture-laden air from the Atlantic Ocean, influencing the climate of Western and Central Europe, making it relatively mild and wet.

The Polar Easterlies

Finally, let’s head to the polar regions, where freezing air dominates. Here, we encounter the polar easterlies, the weakest and coldest of all planetary winds.

Characteristics of Polar Easterlies:

• These winds originate from the polar high-pressure belts and move towards the temperate low-pressure belt.
• Since polar areas receive very little sunlight, the air remains cold and heavy, sinking to create high pressure. The air then drifts outward, forming the polar easterlies.
• However, these winds lack the force and consistency of the trade winds and westerlies because:
  1. The polar regions receive less solar energy, keeping air circulation weak.
  2. There’s less air mass available compared to lower latitudes.

Conclusion

Picture the Earth’s atmosphere as a grand orchestra, where the trade winds, westerlies, and polar easterlies each play their part in maintaining balance.

• Trade winds bring moisture and sustain tropical rainforests but also create deserts.
• Westerlies carry warmth and rain to temperate lands, shaping landscapes like the Pacific Northwest and Patagonia.
• Polar easterlies, though weak, maintain the cold air dominance near the poles.

Together, these winds form the invisible yet powerful forces that shape weather patterns, ocean currents, and entire ecosystems across the planet. 🌍💨