- The weight of an air column exerted per unit area
- Pressure created by a mercury column 76 cm or 760 mm high
- Unit: Millibar (mb) or Pascal (Pa)
- Instrument: Barometer – weather indicator
- Maximum air pressure: 1013.2 millibar (at sea level)
- Isobar: A line connecting areas of equal atmospheric pressure
Stormy
Rapidly falls in reading
Rain
Initially falls, then gradually rises
Anticyclone / Clear Weather
Continuous rise in reading
⚡ Quick Recall
A rapidly falling barometer reading warns of an incoming storm.
| Belt | Location & Key Feature |
|---|
| Polar High Pressure | 90°N and 90°S – Cold, descending air |
| Subpolar Low Pressure | 60°–65°N and S – Rising air, cyclones |
| Subtropical High Pressure (Horse Latitudes) | 30°–35°N and S – Descending air, dry regions, calm winds |
| Equatorial Low Pressure (Doldrums) | 0°–5°N and S – Rising air, calm winds, heavy rainfall |
⚠ Exam Trap
Both the
Horse Latitudes (30–35°) and the
Doldrums (0–5°) have calm winds, but for opposite reasons — Horse Latitudes = descending dry air; Doldrums = rising moist air. Don't mix them up.
- Horizontal air flow: High Pressure → Low Pressure (Pressure Gradient)
- Warm air (low pressure) → Rises upward
- Cold air (high pressure) → Sinks downward
- Wind deflects – Right in Northern Hemisphere, Left in Southern Hemisphere (Ferrel's Law)
- Magnitude: Zero at the Equator, Maximum at the Poles
- Proportional to angle of latitude
📌 Geostrophic Winds
Flow parallel to Isobars
⚡ Quick Recall
NHern → deflect Right · SHern → deflect Left (Ferrel's Law) — Coriolis Force is zero at the Equator.
- Zone where NE Trade Winds and SE Trade Winds converge
- Shifts to 20°N–25°N latitude in July (Northern Hemisphere summer)
- Forms a low pressure trough in equatorial latitudes
- Region of calm winds (Doldrums), flanked by Northeastern and Southeastern Trade Winds
📌 Naming Winds
Based on the direction they blow FROM (direction of origin)
⚡ Quick Recall
ITCZ's seasonal shift towards 20–25°N in July is a key driver of the Indian Monsoon.
Trade Winds (Easterlies)
- Converging winds
- Flow from subtropical high pressure zones towards the equatorial low pressure belt
Westerlies
- Flow from subtropical high pressure zones towards subpolar low pressure areas
- Southern Hemisphere: 40° – Roaring Forties, 50° – Furious Fifties, 60° – Shrieking Sixties
Polar Winds
- Flow from Polar High Pressure to Subpolar Low Pressure
- Dry, cold, prevailing winds (Polar Easterlies)
📌 Horse Latitudes (30–35°N/S)
Boundary separating Trade Winds and Westerlies
⚠ Exam Trap
The Southern Hemisphere Westerlies naming sequence —
Roaring Forties (40°) → Furious Fifties (50°) → Shrieking Sixties (60°) — increases in intensity as latitude increases; a commonly tested sequence.
- Change in wind direction according to season and time
- Monsoon Winds – seasonal change according to weather
Sea Breeze (Day)
- Flows from Sea → Land
- Sea – High Pressure
- Land – Low Pressure
Land Breeze (Night)
- Flows from Land → Sea
- Land – High Pressure
- Sea – Low Pressure
📌 Land vs Water heating
Land heats & cools quickly • Water heats & cools slowly
⚡ Quick Recall
Sea Breeze blows during the
day (land is hotter, low pressure); Land Breeze blows at
night (sea is warmer, low pressure).
| Hot Wind | Region of Flow |
|---|
| Chinook | Rocky Mountains, USA (Snow Eater) |
| Foehn (Föhn) | Alps – France, Italy |
| Simoom | Sahara, Jordan, Iraq, Syria |
| Khamsin | Egypt |
| Harmattan | Sahara region – West Africa ("Doctor Wind" on Guinea coast) |
| Sirocco | Northern Africa |
| Zonda | Argentina |
| Norwester | New Zealand, India |
| Brick Fielder | Australia (Desert Region) |
| Terral | Chile |
| Cold Wind | Region of Flow |
|---|
| Mistral | France and Rhine River Valley |
| Bora | Adriatic Sea |
| Blizzard | Polar Regions |
| Levanter | Spain |
| Papagayo | Coast of Mexico |
| Pampero | Argentina, Uruguay, Chile |
| Purga | Russian Tundra Region |
| Buran | Russia and Central Asia |
| Joran | Switzerland (Jura mountains) |
| Nevadas | Ecuador |
⚠ Exam Trap
Chinook (Hot, Rocky Mountains) and Bora (Cold, Adriatic Sea) are frequently confused with each other in region-matching MCQs — remember Chinook = "Snow Eater," a warming wind.
- Found in Upper Troposphere / Tropopause – 9–13 km
- In winter, the subtropical westerly Jet Stream flows over the Western Indian region
- This causes Western Disturbances – affecting Punjab, Haryana, Western Uttar Pradesh
| Fact | Details |
|---|
| Standard air pressure at sea level | 1013.2 millibar |
| Instrument to measure air pressure | Barometer |
| Calmest wind zone | Doldrums (Equatorial Low Pressure belt – 0°–5°) |
| Horse Latitudes | 30°–35° N and S – Subtropical High Pressure belt |
| Roaring Forties / Furious Fifties / Shrieking Sixties | Westerlies in Southern Hemisphere (40°, 50°, 60°) |
| Chinook | Hot wind – Rocky Mountains USA (Snow Eater) |
| Foehn | Hot wind – Alps (France, Italy) |
| Mistral | Cold wind – France and Rhine River Valley |
| Blizzard | Cold wind – Polar Regions |
| Harmattan | Hot dry wind – Sahara region (West Africa) – "Doctor Wind" on Guinea coast |
| Jet Stream location | 9–13 km (Upper Troposphere / Tropopause) |
⚡ Quick Recall
Winter Western Disturbances over North-west India are caused by the subtropical
Jet Stream — a frequently tested India-specific weather fact.