🌍 Class 11 Geography – Chapter 10
Atmospheric Circulation and Weather Systems
☁️ Atmospheric Pressure (वायुमंडलीय दाब)
- Definition → Atmospheric pressure is the weight of the air column on 1 unit area from sea level to the top of the atmosphere.
- Measurement Units → Millibars (mb) & Hectopascals (hPa) 📏
- Average Air Pressure on Ocean Surface → 1013.25 mb
- Representation on Map → By Isobars (समान दाब रेखाएँ)
⬇️ Decrease of Air Pressure with Altitude
- Air pressure is higher near the surface 🌍
- Decreases with altitude at a rate of 1 mb per 10 meters ⛰️
➰ Isobars (समान दाब रेखाएँ)
- Definition → Lines joining places of equal air pressure above sea level.
- Interval → Drawn at equal pressure differences.
- Importance →
- If isobars are close → High Pressure Gradient
- If isobars are far → Low Pressure Gradient
📉 Pressure Gradient (दाब प्रवणता)
- Definition → Difference in pressure from one place to another.
- Stronger gradient → Faster wind 🌬️
🌬️ Local Winds (स्थानीय पवनें)
- Definition → Movement of wind in a specific region due to temperature differences and other weather factors.
- Examples → Loo (लू), Föhn, Chinook, Land & Sea Breeze
🌪️ Tornado (बवंडर)
- Found in mid-latitudes 🌍
- Formed with thunderstorms ⛈️
- Very low pressure at the center
- Air moves violently downward like the trunk of an elephant 🐘
🌫️ Air Mass (वायु राशि)
- When air stays long enough over a region → it acquires temperature & humidity properties of that region.
- Features →
- Hundreds of km wide 🌐
- Multiple vertical layers
🌀 Coriolis Force (कोरिओलिस बल)
- Definition → Deflection of winds due to Earth’s rotation 🌍
- Effect:
- In Northern Hemisphere → Winds deflect to the right ➡️
- In Southern Hemisphere → Winds deflect to the left ⬅️
- Zero effect → At Equator
- Maximum effect → At Poles ❄️
- Ferrel’s Law → Proved by scientist Ferrel 📚
✅ Exam Ready Quick Notes:
- Air pressure unit → mb / hPa
- Average sea level pressure → 1013.25 mb
- Rate of decrease with altitude → 1 mb / 10 m
- Isobars → Equal pressure lines
- Coriolis Force → Right deflection (North), Left deflection (South)
🌬️ Types of Winds (Class 11 Geography – Chapter 10)
✅ There are 3 main types of winds:
- 🌎 Planetary Winds (वैश्विक पवन)
- 🍃 Seasonal Winds (ऋतुजन्य पवन)
- 🏞️ Local Winds (स्थानीय पवन)
🌎 1. Planetary Winds (वैश्विक पवन)
- These winds blow permanently in the same direction throughout the year.
- They move regularly from high-pressure zones → low-pressure zones.
- Main 3 Types:
- 🌐 Trade Winds (व्यापारिक पवन)
- 💨 Westerlies (पश्चिमी पवन)
- ❄️ Polar Winds (ध्रुवीय पवन)
🌐 Trade Winds (व्यापारिक पवन)
- Blow from sub-tropical high pressure → equatorial low pressure zone.
- Deflected by Coriolis Force (कोरिओलिस बल):
- 👉 Northern Hemisphere → North-East direction.
- 👉 Southern Hemisphere → South-East direction.
- At the equator → winds get moist & cause heavy rainfall.
- “Trade” word (German: Trade) means a fixed path.
💨 Westerlies (पश्चिमी पवन)
- Blow from sub-tropical high pressure → sub-polar low pressure zone.
- Found between 30°–60° latitude in both hemispheres.
- Direction:
- 👉 Northern Hemisphere → South-West
- 👉 Southern Hemisphere → North-West
- Less regular, disturbed by cyclones & anticyclones.
❄️ Polar Winds (ध्रुवीय पवन)
- Blow from polar high pressure → sub-polar low pressure zone.
- Found between 60° latitude → poles.
- Very cold & dry, as they come from snow-covered regions.
🍃 2. Seasonal Winds (ऋतुजन्य पवन)
- Change direction according to season.
- Example → Monsoon Winds (मॉनसून पवन).
🌧️ Monsoon Winds (मॉनसून पवन)
- Word “Monsoon” comes from Arabic ‘Mausim’ = Season (ऋतु).
- Direction completely reverses with season:
- ☀️ Summer → From Sea to Land (rainy season).
- ❄️ Winter → From Land to Sea (dry season).
- Two types: Summer Monsoon & Winter Monsoon.
- Found mainly in Indian Subcontinent.
🏞️ 3. Local Winds (स्थानीय पवन)
- Blow in small areas due to differential heating & cooling of land and water.
- Examples:
- 🌊 Land Breeze & Sea Breeze (स्थल समीर व समुद्र समीर)
- 🔥 Loo (लू – hot & dry wind in North India)
- 🌄 Föhn & Chinook (warm winds in mountains)
- ❄️ Mistral (cold wind in France)
✨ Exam Tip:
Planetary winds = Permanent winds,
Seasonal winds = Monsoon,
Local winds = Small scale effect (like Loo, Föhn, Sea breeze).
🌬️ Local Winds, Cyclones, Anticyclones & Air Pressure Distribution
🌊 Land Breeze & Sea Breeze
🏞️ Land Breeze (Sthal Sameer)
- These winds blow from land ➝ towards the sea at night 🌙.
- Land cools faster than the sea, creating low pressure over the sea.
- Hence, air moves from land (high pressure) ➝ sea (low pressure).
🌊 Sea Breeze (Samudra Sameer)
- Blows from sea ➝ towards the land during the day ☀️.
- Land heats faster than water, creating low pressure over land.
- These winds are cool & humid, bringing relief in coastal areas.
🏔️ Valley Breeze & Mountain Breeze
🌄 Valley Breeze
- During the daytime, slopes facing the sun get heated quickly.
- Warm air rises, and cooler air from the valley moves upward.
- Speed is fastest around 2 PM.
- Sometimes causes cloud formation & rainfall on slopes. ⛈️
🏔️ Mountain Breeze
- At night, mountain slopes cool quickly due to terrestrial radiation.
- Cold, heavy air flows down into the valley.
- Valley temperature drops before sunrise 🌅 → leads to temperature inversion.
- These winds are dry & strongest before sunrise.
🌪️ Cyclones & Anticyclones
🌪️ Cyclone (Low Pressure System)
- Low pressure at the center, high pressure around it.
- Winds move towards the center but get deflected due to Earth’s rotation 🌍.
- Northern Hemisphere → Anti-clockwise
- Southern Hemisphere → Clockwise
🌞 Anticyclone (High Pressure System)
- High pressure at the center, low pressure around it.
- Winds move outward from the center.
- Northern Hemisphere → Clockwise
- Southern Hemisphere → Anti-clockwise
🌬️ Fronts (Vatagra)
Boundary where two different air masses meet. Four types:
- ❄️ Cold Front
- ☀️ Warm Front
- ↔️ Stationary Front
- ⚡ Occluded Front
🌍 Horizontal Distribution of Air Pressure
🌴 Equatorial Low Pressure Zone
- Lies between 5° N – 5° S.
- Sun’s rays fall directly year-round → air rises upward.
- No horizontal winds → only vertical currents.
- Known as Doldrums (Calm Zone).
🏜️ Subtropical High Pressure Zone
- Between 30° – 35° N & S latitudes.
- Air is calm, dry & cloudless 🌞.
- Most world deserts 🏜️ lie here (offshore winds are dry).
🌧️ Subpolar Low Pressure Zone
- Between 60° – 65° latitudes.
- Frequent cyclonic depressions, especially in winters ❄️.
🧊 Polar High Pressure Zone
- Found near North & South Poles (90° N & S).
- Extremely cold, snow-covered ❄️ → permanent high pressure.
✨ These winds & pressure zones together create the global circulation system of the atmosphere 🌍💨.
