📖 PART 1: Atmospheric Pressure
Atmospheric Pressure is the weight of the air column acting perpendicularly on a unit area
of Earth's surface. It is measured in millibars (mb) using a barometer. Standard sea-level pressure is
1013.2 mb.
Factors Affecting Atmospheric Pressure
- Altitude: Pressure decreases as altitude increases (air becomes thinner and less
dense).
- Temperature: Inverse relationship. High temperature → air expands, becomes lighter, and
rises → Low Pressure. Low temperature → air contracts, becomes heavy, and sinks →
High Pressure.
- Water Vapour: Moist air is lighter than dry air. Therefore, high humidity → lower
pressure.
- Earth's Rotation: Centrifugal force pushes air away from poles towards the equator,
creating dynamic pressure belts.
🌍 PART 2: Major Pressure Belts of the World
The unequal heating of Earth and its rotation create 7 distinct pressure belts:
| Pressure Belt |
Location |
Characteristics |
| Equatorial Low Pressure Belt (Doldrums) |
5°N to 5°S |
Intense heating → air rises (convection). Very calm winds (hence "doldrums"). Heavy afternoon
rainfall daily. |
| Sub-Tropical High Pressure Belts (Horse Latitudes) |
30°N and 30°S |
Air from equator cools and sinks here → High pressure. Dry, calm, clear skies. Major world
deserts (Sahara, Arabian) are located here. |
| Sub-Polar Low Pressure Belts |
60°N and 60°S |
Created by Earth's rotation (dynamic). Warm Westerlies and cold Polar Easterlies meet here,
forcing air to rise → Cyclonic storms common. |
| Polar High Pressure Belts |
90°N and 90°S |
Extreme cold → heavy, dense air sinks. Permanent High Pressure. |
🔄 Shifting of Pressure Belts
The pressure belts are not static. Because of Earth's axial tilt (23.5°), the belts shift northwards by
about 5° in summer (June Solstice) and southwards by about 5° in winter (December Solstice), following the
apparent movement of the Sun. This shift causes seasonal monsoons and Mediterranean climates.
🌬️ PART 3: Winds
Wind is the horizontal movement of air from High Pressure to Low Pressure
areas. An anemometer measures wind speed, and a wind vane shows wind direction. Winds are always named after
the direction from which they blow (e.g., Westerly = blows from West to East).
Factors Affecting Wind Direction and Velocity
- Pressure Gradient: The rate of change in pressure over a distance. Steep gradient
(isobars close together) = strong winds. Gentle gradient (isobars far apart) = light winds.
- Coriolis Effect (Ferrel's Law): Due to Earth's west-to-east rotation, moving air is
deflected from its straight path. It deflects to the RIGHT in the Northern Hemisphere
(NH) and to the LEFT in the Southern Hemisphere (SH).
- Friction: Mountains, trees, buildings slow wind speed near the surface.
🌐 PART 4: Types of Winds
1. Permanent (Planetary / Prevailing) Winds
Blow constantly in the same direction throughout the year globally.
| Wind |
Flows From → To |
Characteristics |
| Trade Winds |
Sub-Tropical High (30°) → Equatorial Low (0°) |
Blow from NE in NH (North-East Trades) and SE in SH. Extremely steady. Sailors used them for
trade routes. Pick up moisture over oceans and give rain to eastern coasts. |
| Westerlies |
Sub-Tropical High (30°) → Sub-Polar Low (60°) |
Blow from SW in NH and NW in SH. Much stronger and stormier in the SH ("Roaring Forties",
"Furious Fifties") because of vast open oceans. |
| Polar Easterlies |
Polar High (90°) → Sub-Polar Low (60°) |
Blow from NE in NH and SE in SH. Extremely cold and dry winds coming from ice caps. |
2. Periodic Winds
Winds that change their direction periodically with the season or time of day.
- Monsoons: Seasonal reversal of winds. In summer (June-Sept), land gets very hot (Low
Pressure), ocean is cooler (High Pressure) → moist winds blow Sea to Land (SW Monsoon)
causing heavy rain in India. In winter (Oct-Nov), land cools rapidly (High Pressure), ocean is warmer
(Low Pressure) → dry winds blow Land to Sea (NE Monsoon).
- Sea Breeze (Day): Land heats faster than sea. Low Pressure over land, High over sea.
Wind blows from Sea to Land. Cools coastal areas.
- Land Breeze (Night): Land cools faster than sea. High Pressure over land, Low over sea.
Wind blows from Land to Sea.
3. Local Winds
Affect small areas; influenced by local topography and temperature differences.
| Wind Name |
Nature |
Region / Impact |
| Loo |
Hot & Dry |
Summer wind in Northern India & Pakistan plains. Causes heat strokes. |
| Chinook ("Snow-eater") |
Warm & Dry |
Flows down eastern slopes of Rocky Mountains (USA). Melts snow rapidly in winter, aiding
grazing. |
| Foehn |
Warm & Dry |
Similar to Chinook, but flows down the northern slopes of the Alps in Europe. Helps ripen
grapes. |
| Mistral |
Cold & Dry |
Blows rapidly down the Rhone Valley (France) towards the Mediterranean. Destructive to crops.
|
| Sirocco |
Hot, Dry, Dusty |
Blows from Sahara to Mediterranean (Italy/Spain). Often called "blood rain" due to red desert
dust mixed with rain. |
4. Variable Winds (Cyclones and Anticyclones)
- Cyclones: Swirling low-pressure systems. Winds blow inward (anticlockwise in
NH, clockwise in SH). Bring heavy rain, dark clouds, and stormy weather. (Tropical cyclones =
Hurricanes, Typhoons, Cyclones).
- Anticyclones: High-pressure systems. Winds blow outward (clockwise in NH,
anticlockwise in SH). Bring calm, clear, sunny, and dry weather.
✈️ PART 5: Jet Streams
Jet Streams are high-speed, narrow, meandering bands of very strong Westerly winds blowing
in the upper troposphere (9-12 km altitude) at speeds of 150-300 km/h.
- They significantly influence surface weather (e.g., they determine the arrival and intensity of the
Indian Monsoon).
- Pilots use them to save fuel when flying eastward (tail-wind) or avoid them when flying westward
(head-wind).
📌 Chapter Summary
- Pressure limits: Increases with colder temps and lower altitude. Belt Shifts:
Follow the Sun north/south.
- Belts: Equatorial Low (Doldrums), Sub-Tropical High (Horse Latitudes), Sub-Polar
Low, Polar High.
- Ferrel's Law / Coriolis: Winds deflect right in NH, left in SH.
- Planetary Winds: Trades (30°→0°), Westerlies (30°→60°), Polar Easterlies (90°→60°).
- Periodic: Monsoons (seasonal reversal), Sea/Land breezes (daily).
- Local: Loo (hot/India), Chinook/Foehn (warm/Rockies & Alps), Mistral
(cold/France).
- Cyclones: Low pressure inward swirl, storm. Anticyclones: High
pressure outward swirl, clear weather.