Atmospheric Pressure and Wind

• Air expands when heated and compresses when cooled, which leads to atmospheric pressure variations.
• Air movement occurs from high to low pressure areas
• Horizontal motion of air defines Wind

Atmospheric Pressure

• It determines if air will rise or sink.
• Wind redistributes heat and moisture.
• Rising moist air cools, forms clouds, and causes precipitation.
• Atmospheric pressure results from the weight of an air column from sea level to the top of the atmosphere.
• It is measured in millibars (mb).
• Average atmospheric pressure at sea level is 1,013.2 mb.
• Air at the surface is denser due to gravity, leading to higher pressure.
• Pressure decreases with height.
• Pressure is measured using mercury or aneroid barometers.
• Pressure variations from place to place cause air motion or wind.

Vertical Variation of Pressure

• In the lower atmosphere, pressure decreases rapidly with height.
• The decrease is approximately 1 mb per 10 m increase in elevation.
• Vertical pressure gradient force is generally balanced by gravitational force.

Horizontal Distribution of Pressure

• Horizontal pressure distribution is analyzed using isobars that connect places with equal pressure at constant levels.
• Pressure is reduced to sea level to eliminate altitude effects.
• Low-pressure systems are enclosed by isobars with the lowest pressure in the center.
• High-pressure systems are enclosed by isobars with the highest pressure in the center.

World Distribution of Sea Level Pressure

• Near the equator, there is an equatorial low-pressure area.
• Subtropical highs are found along 30° N and 30° S.
• Subpolar lows are located around 60° N and 60° S.
• Polar highs are near the poles.

Factors Affecting Wind

• Wind is air in motion, moving from high to low pressure areas.
• Friction at the surface, Earth's rotation, and gravitational force affect wind.
• Coriolis force is exerted by the earth's rotation
• Horizontal winds respond to pressure gradient, friction, and Coriolis forces.

Pressure Gradient Force

• Differences in atmospheric pressure create this force.
• The pressure gradient is strong when isobars are close and weak when they are far apart.

Frictional Force

• Frictional force affects wind speed
• The influence of Frictional force is greatest at the surface and extends up to 1-3 km.
• Friction is minimal over the sea surface.

Coriolis Force

• The rotation of the Earth affects wind direction with Coriolis Force
• It deflects wind to the right in the Northern Hemisphere and to the left in the Southern Hemisphere.
• Deflection is proportional to wind velocity and latitude.
• Coriolis Force is maximum at the poles and absent at the equator.
• It acts perpendicular to the pressure gradient force.

Pressure and Wind

• Wind velocity and direction result from wind-generating forces.
• Upper atmosphere winds are controlled by pressure gradient and Coriolis force.
• Geostrophic winds blow parallel to isobars when pressure gradient force is balanced by the Coriolis force.
• Wind circulation around a low is cyclonic and around a high is anticyclonic.

General Circulation of the Atmosphere

• Planetary winds depend on latitudinal heating, pressure belts, migration of belts, distribution of continents and oceans, and Earth's rotation.
• General circulation influences ocean water circulation.

General Atmospheric Circulation and its Effects on Oceans

• Warming and cooling in the Pacific Ocean can trigger phenomena such as El Nino.
• El Nino involves warm water moving towards the South American coast.
• Southern Oscillation involves pressure changes in the Pacific.
• ENSO (El Nino-Southern Oscillation) can cause variations in weather patterns worldwide.

Seasonal Wind

• Wind circulation changes with seasons due to shifting heating, pressure, and wind belts.

Local Winds

• Common local and regional winds occur because of heating and cooling differences.

Land and Sea Breezes

• During the day, land heats faster than the sea, causing air to rise and creating a sea breeze (wind from sea to land).
• At night, the land cools faster, reversing the process and creating a land breeze (wind from land to sea).

Mountain and Valley Winds

• During the day, slopes heat up and air moves upslope, creating a valley breeze.
• At night, slopes cool and dense air descends into the valley as a mountain wind.
• Katabatic winds are cool air from high plateaus draining into valleys.

Air Masses

• Air that remains over a homogeneous area for a long time acquires the area's characteristics.
• An air mass is a large body of air with little horizontal variation in temperature and moisture.
• Source regions are the homogeneous surfaces where air masses form.
• Air masses are classified based on source regions: Maritime tropical (mT), Continental tropical (cT), Maritime polar (mP), Continental polar (cP), and Continental arctic (cA).
• Tropical air masses are warm and polar air masses are cold.

Fronts

• A front is a boundary zone between two different air masses.
• Frontogenesis is the process of front formation.
• Types of fronts: Cold, Warm, Stationary, and Occluded.
• Stationary Front: The front remains stationary
• Cold Front: Cold air moves towards a warm air mass.
• Warm Front: Warm air moves towards a cold air mass.
• Occluded Front: An air mass is fully lifted above the land surface.

Extra Tropical Cyclones

• Extra tropical cyclones form along the polar front.
• Initially, the front is stationary
• Warm air moves northwards, and cold air moves southwards, creating cyclonic circulation.
• The cyclone has a warm front and a cold front.
• Warm air glides over cold air causing cloud formation and precipitation.
• The cold front approaches and pushes warm air up, forming cumulus clouds.
• Occluded Front: Cold front overtakes the warm front and lifts the warm air, and cyclone dissipates.

Tropical Cyclones

• Tropical cyclones originate over oceans in tropical areas, leading to large-scale destruction through strong winds, heavy rainfall, and storm surges.
• Hurricanes (Atlantic), Typhoons (Western Pacific, South China Sea), Willy-willies (Western Australia), and Cyclones (Indian Ocean).
• Conditions favoring development: warm sea surface (over 27°C), Coriolis force, small variations in vertical wind speed, a pre-existing low-pressure area, etc.
• Intensification comes from condensation in cumulonimbus clouds.
• The storm cuts off moisture supply and dissipates, after reaching land.
• The landfall is the point where the cyclone crosses the coast.
• The place where a tropical cyclone crosses the coast is called the landfall of the cyclone.
• Eye: calm with subsiding air
• Eyewall: Strong spiralling ascent of air, with maximum wind velocity and torrential rain.
• Rain Bands: Trains of cumulus and cumulonimbus clouds that may drift into the outer region.

Thunderstorms

• Thunderstorms are caused by intense convection on moist hot days.
• A thunderstorm has thunder and lightning
• Hailstorms can occur, if clouds have sub-zero temperature
• The updraft of warm air causes clouds to grow.
• Downdraft brings rain and cool air.

Tornadoes

• In severe thunderstorms, spiralling wind descends with low pressure, causing destruction.
• Tornadoes occur in middle latitudes
• Water spouts are tornadoes over the sea.