Atmospheric Pressure, Circulation & Winds

1. Atmospheric Pressure

Atmospheric pressure is the weight of a column of air contained in a unit area from the mean sea level to the top of the atmosphere. It decreases with altitude because the air becomes less dense. Standard sea-level pressure is 1013.2 millibars (mb). Isobars are lines connecting places of equal pressure.

Global Pressure Belts

There are seven distinct pressure belts on Earth due to differential heating and Earth's rotation:

1. Equatorial Low-Pressure Belt (Doldrums) [10°N to 10°S]: Intense heating causes air to expand and rise. Extremely calm winds.
2. Sub-Tropical High-Pressure Belts (Horse Latitudes) [30° to 35° N & S]: The air that rose at the equator cools and descends here, creating intense, dry high pressure.
3. Sub-Polar Low-Pressure Belts [60° to 65° N & S]: Created dynamically where warm westerlies meet cold polar easterlies, forcing the warm air up.
4. Polar High-Pressure Belts [90° N & S]: Intensely cold, dense air subsides at the poles.

These belts shift slightly northwards during the northern summer and southwards during the southern summer due to the apparent movement of the sun.

2. Atmospheric Circulation and Winds

Wind is the horizontal movement of air from High Pressure to Low Pressure. The direction and velocity of winds are determined by three forces:

1. Pressure Gradient Force: The primary driving force. Steep gradients lead to strong winds.
2. Coriolis Force: An apparent force caused by the Earth's rotation. It deflects winds to the right in the Northern Hemisphere and to the left in the Southern Hemisphere (Ferrel's Law). It is zero at the equator and maximum at the poles.
3. Frictional Force: Slows wind down, greatest near the surface.

Geostrophic Wind: When isobars are straight and friction is absent, the pressure gradient force is perfectly balanced by the Coriolis force, resulting in wind blowing parallel to the isobars.

3. Types of Winds

A. Planetary (Primary / Prevailing) Winds

These blow persistently in the same direction year-round.

• Trade Winds: Blow from Sub-Tropical High to Equatorial Low. Deflected to become North-East trades in NH and South-East trades in SH. Very steady.
• Westerlies: Blow from Sub-Tropical High to Sub-Polar Low (SW to NE in NH, NW to SE in SH). Stronger in the continuous southern oceans (Roaring Forties, Furious Fifties, Screaming Sixties).
• Polar Easterlies: Cold dry winds blowing from Polar Highs to Sub-Polar Lows.

B. Secondary (Seasonal / Periodic) Winds

Change direction with seasons.

• Monsoons: Seasonal reversal of wind direction. Driven by the differential heating of land (Asia) and water (Indian Ocean). In summer, low pressure over Tibet draws wet SW Monsoons. In winter, high pressure over Siberia drives dry NE Monsoons.
• Land and Sea Breezes: Daily phenomenon. Day: Sea (High P) to Land (Low P). Night: Land (High P) to Sea (Low P).

C. Local Winds

Blowing only over small areas for a short period.

• Hot Local Winds:
  • Chinook (Snow-eater): Descends eastern slopes of Rockies (USA/Canada), melting snow rapidly.
  • Foehn: Similar to Chinook, in the Alps (Europe).
  • Sirocco (Blood rain): Hot, dusty wind from Sahara across the Mediterranean to Italy.
  • Loo: Hot, dry wind over Northern India/Pakistan in summer.
• Cold Local Winds:
  • Mistral: Cold wind funneled down the Rhone valley (France).
  • Bora: Cold Adriatic sea wind.

4. Jet Streams

Jet streams are narrow bands of strong, high-speed, meandering winds in the upper troposphere (9-12 km height), blowing from west to east.

• Types: Subtropical Jet Stream (influences Indian monsoon dynamics) and Polar Front Jet Stream.
• They heavily influence surface weather, steering extratropical cyclones and regulating the onset of monsoons.