Chapter – 9

Atmospheric Circulation and Weather Systems

In this post we have given the detailed notes of class 11 Geography Book 1 Chapter 9 (Atmospheric Circulation and Weather Systems) in English. These notes are useful for the students who are going to appear in class 11 board exams.

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Chapter 9: Atmospheric Circulation and Weather Systems

Atmospheric Pressure

• Definition: The weight of a column of air contained in a unit area from the mean sea level to the top of the atmosphere.  
• Units: Millibar (mb)  
• Average at sea level: 1,013.2 mb  
• Factors influencing atmospheric pressure:
  • Altitude: Air pressure decreases with increasing altitude because there is less air above pressing down.  
  • Temperature: Warm air is less dense than cold air, so it rises and exerts less pressure. Cold air is denser and exerts higher pressure.  
  • Moisture: Moist air is lighter than dry air. This is because water vapor (H2O) has a lower molecular weight than the nitrogen (N2) and oxygen (O2) that make up most of the atmosphere.  
• Measurement:
  • Mercury Barometer: Measures the height of a column of mercury that is balanced by atmospheric pressure.  
  • Aneroid Barometer: Uses an airtight metal box that expands or contracts with changes in air pressure.  
• Vertical Variation: The pressure decreases rapidly with height in the lower atmosphere, at a rate of about 1 mb for each 10m increase in elevation.  
• Horizontal Distribution: * Isobars: Lines connecting places having equal pressure.  
  • Pressure Systems:
    • Low-Pressure System (Cyclone): Enclosed by one or more isobars with the lowest pressure in the center. Associated with rising air, cloud formation, and precipitation.  
    • High-Pressure System (Anticyclone): Enclosed by one or more isobars with the highest pressure in the center. Associated with sinking air and clear skies.  

World Distribution of Sea Level Pressure

• Pressure Belts:
  • Equatorial Low: Low-pressure area near the equator due to intense heating and rising air.  
  • Subtropical Highs: High-pressure areas found along 30° N and 30° S due to the sinking of air from the Hadley cells.  
  • Subpolar Lows: Low-pressure belts along 60° N and 60° S, where warm air from the subtropics meets cold air from the poles.  
  • Polar Highs: High-pressure areas near the poles due to the sinking of cold, dense air.  
• Migration of Pressure Belts: These belts are not stationary; they shift north and south with the changing seasons and the apparent movement of the sun.  

Wind

• Definition: Air in horizontal motion.  
• Causes: Differences in atmospheric pressure create pressure gradients, causing air to move from high pressure to low pressure areas.  

Forces Affecting Wind Velocity and Direction

• Pressure Gradient Force: The force that drives wind from high to low pressure. The steeper the pressure gradient (closer isobars), the stronger the wind.  
• Frictional Force: The force that opposes wind motion due to the contact between air and the Earth’s surface. Friction reduces wind speed and is greatest at the surface.  
• Coriolis Force: The force caused by the Earth’s rotation that deflects winds to the right in the Northern Hemisphere and to the left in the Southern Hemisphere.  
  • The Coriolis force is strongest at the poles and zero at the equator.  
  • It is directly proportional to wind speed; faster winds are deflected more.  
• Gravitational Force: Pulls the air downwards.  

Types of Wind

• Geostrophic Wind: A theoretical wind that results from a balance between the pressure gradient force and the Coriolis force. It blows parallel to straight isobars in the absence of friction.  
• Surface Winds: Winds near the Earth’s surface are influenced by all three forces: pressure gradient force, Coriolis force, and friction.  
• Gradient Winds: Winds that blow parallel to curved isobars, experiencing a balance between pressure gradient force, Coriolis force, and centrifugal force.

General Circulation of the Atmosphere

• Definition: The global pattern of wind and pressure belts.  
• Factors Affecting:
  • Latitudinal Heat Imbalance: The tropics receive more solar radiation than the poles, creating a temperature gradient that drives atmospheric circulation.  
  • Pressure Belts: The global distribution of pressure belts (Equatorial Low, Subtropical Highs, Subpolar Lows, Polar Highs) influences wind patterns.  
  • Earth’s Rotation: The Coriolis effect modifies wind direction, leading to the formation of easterlies, westerlies, and other wind patterns.  
  • Distribution of Land and Water: Continents and oceans heat and cool differently, affecting air pressure and wind patterns.  
• Atmospheric Cells:
  • Hadley Cells: Large-scale circulation patterns in the tropics, with air rising near the equator, flowing poleward at high altitudes, sinking around 30° latitude, and returning to the equator as trade winds.  
  • Ferrel Cells: Mid-latitude circulation cells, with air sinking near 30° latitude, flowing poleward at the surface as westerlies, rising around 60° latitude, and returning equatorward at high altitudes.  
  • Polar Cells: Circulation cells at high latitudes, with air sinking near the poles, flowing equatorward at the surface as polar easterlies, rising around 60° latitude, and returning poleward at high altitudes.  

General Atmospheric Circulation and its Effects on Oceans

• Ocean Currents: The large-scale wind patterns drive major ocean currents, redistributing heat and influencing climate.  
• El Niño-Southern Oscillation (ENSO): A periodic fluctuation in sea surface temperature and atmospheric pressure in the tropical Pacific Ocean, affecting weather patterns worldwide.
  • El Niño: Warmer-than-average sea surface temperatures in the central and eastern Pacific Ocean.  
  • Southern Oscillation: Seesaw pattern of atmospheric pressure between the eastern and western Pacific.  
  • Impacts: ENSO can cause droughts, floods, and other weather anomalies in various parts of the world.  

Seasonal and Local Winds

• Monsoons: Seasonal reversal of wind direction, often associated with heavy rainfall.  
• Local Winds:
  • Land and Sea Breezes: Daily wind patterns caused by the differential heating of land and sea. During the day, land heats faster, creating a sea breeze; at night, land cools faster, creating a land breeze.  
  • Mountain and Valley Breezes: Daytime upslope winds (valley breezes) as valley air heats up and rises, and nighttime downslope winds (mountain breezes) as mountain slopes cool and air sinks.  
  • Katabatic Winds: Cold, dense air draining down from high plateaus and ice fields.  
  • Foehn/Chinook Winds: Warm, dry winds that descend on the leeward side of mountains, having lost their moisture on the windward side.  

Air Masses

• Definition: Large bodies of air with relatively uniform temperature and humidity.  
• Source Regions: Areas where air masses originate, such as oceans, continents, or polar regions.  
• Classification: Based on source region and temperature:
  • Maritime (m): Originating over oceans, moist.  
  • Continental (c): Originating over continents, dry.  
  • Tropical (T): Warm.  
  • Polar (P): Cold.  
  • Arctic (A): Very cold.  

Fronts

• Definition: Boundaries between different air masses.  
• Types:
  • Cold Front: Cold air mass advances and forces warm air to rise abruptly, leading to cumulonimbus clouds, heavy precipitation, and thunderstorms.  
  • Warm Front: Warm air mass rises over a cold air mass, producing a gradual slope and widespread stratiform clouds with light to moderate precipitation.  
  • Stationary Front: Boundary between two air masses that are not moving, resulting in little or no change in weather.  
  • Occluded Front: Occurs when a cold front overtakes a warm front, lifting the warm air mass completely off the ground.  

Extra Tropical Cyclones

• Definition: Large low-pressure systems that form in the middle and high latitudes, characterized by fronts and a wide range of weather conditions.  
• Formation: Develop along the polar front, where warm and cold air masses meet.  
• Characteristics:
  • Frontal System: Well-defined warm and cold fronts.  
  • Size: Larger than tropical cyclones.  
  • Origin: Can form over land or sea.  
  • Movement: Generally move from west to east.  
  • Weather: Associated with a variety of weather, including precipitation, wind, and temperature changes.  

Tropical Cyclones

• Definition: Intense low-pressure systems that form over warm tropical oceans, characterized by strong winds, heavy rainfall, and storm surges.  
• Other Names: Hurricanes (Atlantic), Typhoons (Western Pacific), Willy-Willies (Australia)  
• Conditions for Formation:
  • Warm Sea Surface Temperatures: At least 27°C (80°F).  
  • Coriolis Force: Needed for rotation.  
  • Low Wind Shear: Minimal variation in wind speed with height.  
  • Pre-existing Low-Pressure Area: Provides the initial disturbance.  
  • Upper Divergence: Outflow of air aloft helps to maintain low pressure at the surface.  
• Structure:
  • Eye: Calm center with low pressure and light winds.  
  • Eye Wall: Ring of intense thunderstorms surrounding the eye, with the strongest winds and heaviest rainfall.  
  • Rain Bands: Spiraling bands of clouds and precipitation extending outward from the eye wall.  
• Movement: Generally move from east to west, steered by prevailing winds.  
• Landfall: When a tropical cyclone reaches land, it loses its energy source (warm ocean water) and weakens.  
• Storm Surge: A rise in sea level caused by the strong winds and low pressure of a cyclone, leading to coastal flooding.  

Thunderstorms and Tornadoes

• Thunderstorms: Localized storms produced by cumulonimbus clouds, characterized by thunder, lightning, heavy rain, and sometimes hail.  
  • Formation: Intense convection on hot, humid days.  
  • Updrafts and Downdrafts: Rising warm air (updrafts) fuels the storm, while sinking cool air (downdrafts) brings precipitation.  
• Tornadoes: Violently rotating columns of air extending from a thunderstorm to the ground.  
  • Formation: Associated with severe thunderstorms.  
  • Characteristics: Extremely low pressure at the center, very strong winds, capable of causing significant damage.  
  • Water Spouts: Tornadoes that form over water.  

Key Concepts

• Atmospheric Pressure: The weight of the air above us. It changes with height and from place to place.
• Wind: Air moving horizontally. It is caused by differences in air pressure.
• Global Circulation: The large-scale pattern of winds around the Earth. It is driven by differences in heating between the equator and the poles.
• Air Masses: Large bodies of air with similar temperature and humidity.
• Fronts: Boundaries between different air masses. They often bring changes in weather.
• Cyclones: Low-pressure systems with rotating winds. They can be extra-tropical (mid-latitudes) or tropical.
• Thunderstorms: Storms with thunder and lightning.
• Tornadoes: Violently rotating columns of air.

Remember these points from the chapter:

• Air pressure is measured in millibars (mb).  
• Wind blows from high pressure to low pressure.  
• The Coriolis effect deflects winds to the right in the Northern Hemisphere and to the left in the Southern Hemisphere.  
• There are three major types of fronts: cold fronts, warm fronts, and occluded fronts.  
• Tropical cyclones form over warm ocean waters.    

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