This presentation provides an academic overview of the monsoon, focusing on its relationship with atmospheric pressure, global wind systems, and their effects on agriculture. Key concepts include the formation of pressure belts, the role of the Coriolis force, and the impact of various wind systems on weather and crop production. Understanding these processes is essential for managing agricultural practices and predicting weather patterns in monsoon-dependent regions. Important terms such as monsoon, atmospheric pressure, Coriolis force, wind systems, and crop impact are emphasized throughout.
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.
Objective Questions
Q1. What is the pressure exerted by the weight of air on the earth's surface?
A. 1.034 gm/cm2
B. 0.934 gm/cm2
C. 1.134 gm/cm2
D. 0.834 gm/cm2
Answer: A
Q2. Which region records the most prominent diurnal variation in atmospheric pressure?
A. Polar region
B. Equatorial region
C. Subtropical region
D. Mid-latitude region
Answer: B
Q3. What is the direction of wind movement in a cyclone in the Northern Hemisphere?
A. Clockwise
B. Anti-clockwise
C. North to South
D. South to North
Answer: B
Q4. Which pressure belt is located between 25° and 35° latitude in both hemispheres?
A. Equatorial trough
B. Subtropical high pressure belt
C. Subpolar low pressure belt
D. Polar high
Answer: B
Q5. What is the effect of the Coriolis force on winds in the Southern Hemisphere?
A. Deflects to the right
B. Deflects to the left
C. No deflection
D. Deflects upward
Answer: B
Q6. Which wind system is also known as 'Anabatic winds'?
A. Sea breeze
B. Valley winds
C. Mountain winds
D. Land breeze
Answer: B
Q7. What is the wind direction of prevailing westerlies in the Northern Hemisphere?
A. NE
B. SE
C. SW
D. NW
Answer: C
Q8. Which instrument is used to determine wind direction?
A. Barometer
B. Anemometer
C. Wind vane
D. Hygrometer
Answer: C
Q9. What is the name given to severe tropical cyclones in the North Atlantic?
A. Typhoon
B. Cyclone
C. Hurricane
D. Willy-willy
Answer: C
Q10. During which months does the North East Monsoon bring heavy rainfall to Southern India?
A. June to September
B. March to May
C. October to December
D. January to March
Answer: C