Introduction to Monsoon and Atmospheric Pressure
- Monsoon refers to seasonal wind patterns causing significant rainfall, especially in South and Southeast Asia.
- Atmospheric pressure is the weight of air above a unit area, measured in millibars (mb).
- Unequal heating of the earth and its rotation create pressure differences, driving wind systems.
- Pressure distribution is shown on maps using isobars (lines of equal pressure).
Diurnal and Seasonal Variation in Atmospheric Pressure
Diurnal Variation:
- Pressure rises and falls in a daily rhythm due to radiational heating and cooling.
- More pronounced near the equator and at sea level.
Seasonal Variation:
- Caused by annual changes in solar radiation (insolation).
- Greater in tropical regions than in mid or polar latitudes.
- High pressure over continents in winter, over oceans in summer.
Global Pressure Belts and Their Characteristics
- Seven alternating low and high pressure belts exist from equator to poles:
- Equatorial trough (5°N–5°S, low pressure)
- Subtropical highs (25°–35°N/S)
- Subpolar lows (60°–70°N/S)
- Polar highs (at the poles)
- Pressure belts result from uneven solar heating and earth’s shape.
Factors Affecting Atmospheric Pressure
- Temperature: Hot air expands (low pressure); cold air contracts (high pressure).
- Altitude: Pressure decreases with height; drops ~1 hPa per 10 m ascent.
- Water vapour: Moist, warm air exerts less pressure than dry, cold air.
- Earth’s rotation: Influences pressure at subpolar belts, contributing to global wind patterns.
Coriolis Force and Its Effect
- Described by G.D. Coriolis (1844).
- Apparent deflection of moving air due to earth’s rotation.
- Deflects right in Northern Hemisphere, left in Southern Hemisphere.
- Not a true force, but an effect of rotation.
Pressure Systems: Cyclones and Anticyclones
Low/Depression (Cyclone):
- Lowest pressure at center; isobars circular/elliptical.
- Anti-clockwise movement (Northern Hemisphere), clockwise (Southern Hemisphere).
- Wind speed up to 40 km/h.
Anticyclone:
- Highest pressure at center; isobars circular/elliptical.
- Clockwise movement (Northern Hemisphere), anti-clockwise (Southern Hemisphere).
Storms, Hurricanes, Thunderstorms, Tornadoes, and Waterspouts
Storm:
- Low pressure center, wind speed 40–120 km/h.
- Common in Bay of Bengal and Arabian Sea; causes heavy rain and damage.
Hurricane:
- Severe tropical cyclone, wind speed >120 km/h.
- Called typhoon (W. Pacific), willy-willy (Australia), cyclone (Indian Ocean).
Thunderstorm:
- Produced by cumulonimbus clouds, with lightning, thunder, strong winds, rain, sometimes hail.
Tornado:
- Violently rotating column of air, funnel-shaped cloud, short duration, causes severe crop loss.
Waterspout:
- Rotating air column over water, similar to tornado but weaker.
Global Wind Systems
- Major wind belts: Doldrums, Trade Winds, Prevailing Westerlies, Polar Easterlies.
Doldrums:
- Near equator (5°N–5°S), low pressure, calm, variable winds, vertical air movement.
Trade Winds (Tropical Easterlies):
- Flow from subtropical highs (25°–35°N/S) to equator.
- Deflected by Coriolis force: NE in Northern, SE in Southern Hemisphere.
- Most constant winds, important for global climate.
Anti-trade Winds:
- Upper-level winds, opposite to surface trades, generally dry.
Prevailing Westerlies:
- From subtropical highs to subpolar lows (60°–70°N/S).
- SW in Northern, NW in Southern Hemisphere; irregular, high precipitation.
Polar Easterlies:
- From polar highs to subpolar lows; cold, dry winds.
Local Winds: Mountain, Valley, Sea, and Land Breezes
Mountain Winds (Katabatic):
- Blow downslope at night due to cooling of air near slopes.
Valley Winds (Anabatic):
- Blow upslope during day due to heating of air near slopes.
Sea Breeze:
- Daytime wind from sea to land; brings moisture, cools coastal areas.
Land Breeze:
- Nighttime wind from land to sea; drier, weaker than sea breeze.
| Sea Breeze | Land Breeze |
|---|
| Daytime | Nighttime |
| From sea | From land |
| Moist | Drier |
| Stronger | Weaker |
Effect of Wind on Crop Plants
- Transports heat, affecting temperature and humidity around crops.
- Increases transpiration and CO2 intake; can cause mechanical damage.
- Facilitates pollination and seed dispersal.
- Gentle winds help clean produce; strong, dry winds cause water loss and injury.
- Prevents frost by disrupting atmospheric inversion; can cause soil erosion.
Wind Speed and Direction: Measurement and Seasonal Variation
- Wind speed measured at 3 m above ground over open terrain.
- Mean daily wind speed: average over 24 hours; annual mean from daily values.
- Winds named for direction they originate (e.g., south wind from south).
- Wind vane measures direction; windward = from, leeward = to.
- Prevailing wind: most frequent wind direction in a region.
Monsoon Winds in India: South West and North East Monsoon
South West Monsoon (June–September):
- Westerly winds prevail over Kerala; south winds over eastern India.
- High temperatures in north India create low pressure, drawing monsoon winds.
North East Monsoon (October–December):
- High pressure in northern India shifts winds southeastward.
- North-easterly winds bring rainfall to southern and southeastern India.
Seasonal Rainfall Patterns in India
Winter Rainfall:
- Occurs mainly in northern India; snow in hills, rain in plains.
- Western disturbances are key for winter precipitation.
Summer Rainfall:
- Received from March to May as local storms.
- Mainly in southeast peninsular India and Bengal; rare in western India.