- 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
- Standard sea-level pressure: 1013.25 hPa/mb = 101325 Pa; not a maximum.
- 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
Rapidly falling pressure
can indicate an approaching/deepening low and rising storm potential; it does not guarantee a 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
- For fixed wind speed, proportional to sin(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
- Over South Asia, the summer ITCZ/monsoon trough can shift to ~20°–25°N or farther locally; not a fixed global July latitude.
- 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
Over
South Asia, the seasonal northward ITCZ/monsoon-trough migration supports the
Indian monsoon; 20–25°N is regional, not a fixed global line.
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
Roaring Forties, Furious Fifties, Shrieking/Screaming Sixties refer to broad Southern Ocean latitude bands, not exact lines or a universal stepwise wind-speed rule.
- 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 | Eastern Rockies, Canada and USA; warm, dry downslope wind (‘snow eater’) |
| Foehn (Föhn) | European Alps; warm, dry downslope wind |
| Simoom | Sahara, Jordan, Iraq, Syria |
| Khamsin | Egypt |
| Harmattan | Dry, dusty NE/E wind: Sahara → West Africa; often relatively cool; ‘Doctor’ in parts of the region |
| Sirocco | Northern Africa |
| Zonda | Argentina |
| Norwester | New Zealand: warm, dry foehn northwesterly; India’s Kalbaisakhi/Nor’westers: pre-monsoon thunderstorm squalls |
| Brick Fielder | Australia (Desert Region) |
| Terral | Málaga / Mediterranean Andalusia, southern Spain |
| Cold Wind | Region of Flow |
|---|
| Mistral | Southern France, Rhône Valley toward Mediterranean |
| Bora | Adriatic Sea |
| Blizzard | Severe snowstorm with strong winds and low visibility; not a single named local wind or exclusive to poles |
| Levanter | Gibraltar / southern Spain: humid easterly, not inherently cold |
| Papagayo | Gulf of Papagayo, near Nicaragua and Costa Rica; Central American gap wind |
| Pampero | Pampas of Argentina and Uruguay; cold S/SW wind |
| Purga | Russian Tundra Region |
| Buran | Russia and Central Asia |
| Joran | Switzerland (Jura mountains) |
| ‘Nevadas’ — not a standard local-wind name | Spanish term for snowfalls; do not memorize as an Ecuadorian local wind |
⚠ 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
- Western Disturbances are extratropical systems embedded in westerlies; the jet steers/supports their track and development over NW India and the western Himalaya.
| Fact | Details |
|---|
| Standard air pressure at sea level | 1013.25 hPa/mb (=101325 Pa) |
| 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 | Warm, dry wind east of Rockies, Canada/USA (‘snow eater’) |
| Foehn | Warm, dry downslope wind of European Alps |
| Mistral | Cold, dry wind: Rhône Valley, southern France |
| Blizzard | Severe snowstorm with strong winds/low visibility; not a single local wind |
| Harmattan | Dry, dusty NE/E Sahara-to-West-Africa wind; may be relatively cool (‘Doctor’) |
| Jet Stream location | 9–13 km (Upper Troposphere / Tropopause) |
⚡ Quick Recall
Western Disturbances are extratropical systems within westerly flow; the
subtropical westerly jet influences their track/development.