Factors affecting wind: Pressure and Coriolis Force

Wind

• Air moves from high to low pressure areas
• Wind refers to the horizontal movement of air
• Air current refers to vertical or nearly vertical movement of air
• Winds and air currents maintain atmospheric circulation

Factors Affecting Wind

• Wind direction and speed are influenced by:
  • Pressure gradient force
  • Coriolis force
  • Altitude
  • Latitude
  • Earth's rotation
  • Centripetal acceleration
  • Friction

Pressure Gradient

• A greater pressure difference between two points results in a steeper pressure gradient and higher wind speed
• Wind blows parallel to the gradient and at right angles to isobars, moving from high to low pressure

Coriolis Force

• The earth’s rotation on its axis creates the Coriolis force, deflecting air
• Deflection is minimal at the Equator and maximal at the Poles
• Winds deflect to the right in the Northern Hemisphere
• Winds deflect to the left in the Southern Hemisphere
• Northern hemisphere winds that blow from north become northeasterly
• Southern hemisphere winds that blow from south become southeasterly
• The maximum speed of rotation at the Equator results in less deflection

Rotation of the Earth

• Earth's rotation causes air at the Poles to move toward the Equator
• Air piles up along the Equator, creating a high-pressure belt
• Low pressure develops at the Poles

Altitude

• Pressure decreases with increasing altitude
• The rate of pressure decrease with height is not constant
• Factors like land slope, winds, and temperature affect vertical air pressure
• Ground-level pressure varies from 940 mb to 1040 mb
• At 3000 meters, pressure is approximately 700 mb
• The rate of decrease is 100 millibars per 1000 meters
• High altitudes have thin air with low pressure and heat capacity
• Mountaineers require oxygen at high altitudes

Latitude

• Rising temperatures cause air to expand, reducing density and pressure
• Falling temperatures cause air to become dense, increasing pressure
• Temperature and pressure are inversely related.
• A low-pressure belt exists around the Equator due to high temperatures
• High pressure exists at the Poles due to extremely low temperatures
• In-blowing winds and uplifting air currents occur in hot, low-pressure regions
• Out-blowing winds and sinking air currents occur in cold, high-pressure regions
• Uplifting air currents are called convection currents
• Higher pressure increases wind speed
• Wind direction and speed are affected by the migration of the sun and shifting heat zones

Friction

• Earth's surface irregularities impede horizontal wind movement
• Friction determines the angle at which air flows across isobars and can alter wind direction
• Low friction over smooth ocean surfaces allows air to move at a lower angle to isobars and at a greater speed
• High friction over rugged terrain increases the angle of airflow and reduces speed

Types of Winds

• Permanent, prevailing, or planetary winds blow throughout the year due to latitudinal pressure differences
• Periodic winds reverse direction seasonally
• Local winds occur in specific areas and have unique characteristics

Permanent Winds: The Trade Winds

• Trade Winds lie between 5° and 30° North and South of the Equator
• Trade Winds result from pressure gradients between the Sub-Tropical High Pressure Belt and the Equatorial Low Pressure Belt
• Trade winds are steady winds blowing from the Sub-Tropical High Pressure area (30° N and S) towards the Equatorial Low Pressure belt
• The term "trade" originates from the German word meaning "track," signifying steady blowing in a constant direction
• In the Northern Hemisphere, winds are deflected southwestward and named 'North-East Trade'
• In the Southern Hemisphere, winds are deflected leftward, creating the 'South-East Trades'
• Trade winds bring heavy rainfall to eastern coasts of continents within the Tropics and no rainfall to the western coasts
• Western areas within the tropics experience aridity
• Deserts like the Sahara, Kalahari, Atacama, and Great Australian Deserts are located on the western margins of continents within tropical latitudes

Characteristics of Trade Winds

• Blow from Sub-tropical High Pressure to Equatorial Low Pressure
• Warm winds pick up moisture
• Responsible for heavy rainfall on eastern sides of tropical lands
• Called North-East Trades in the Northern Hemisphere and South-East Trades in the Southern Hemisphere
• Winds and pressure belts shift north and south with the overhead sun
• Have fixed velocity and are regular
• Are permanent or prevailing winds

The Westerlies

• Westerlies blow between 35° and 60° North and South latitudes from the Sub-Tropical High Pressure Belts towards the Sub-Polar Low-Pressure Belts
• In the Northern Hemisphere, westerlies generally blow from the southwest to the northeast
• In the Southern Hemisphere, westerlies generally blow from the northwest to the southeast
• Westerlies are onshore winds on west coasts and offshore winds on east coasts
• Onshore winds bring rainfall, while offshore winds do not

Characteristics of Westerlies

• Blow from Sub-tropical High Pressure to Sub-polar low
• Very strong winds
• Blow from the western side of landmass
• Interspersed by cyclones, causing light drizzle
• Stronger in the southern hemisphere due to absence of landmasses

The Polar Easterlies

• Polar Winds originate in the North and South Polar Regions and blow towards the Circum-polar Low Pressure Zone.
• Start from the Polar High Pressure Zone, and come from ice-capped landmasses in Arctic and Antarctic latitudes
• In the Northern Hemisphere, they are called the Northeast Polar Winds
• In the Southern Hemisphere, they are called the Southeast Polar Winds
• Deflected to the west in both hemispheres, hence known as Polar Easterlies

Characteristics of Polar Winds

• Very cold winds
• Referred to as Polar Easterlies from their direction
• Become warm blowing over oceans

Periodic Winds

• Periodic winds blow at regular intervals or cycles
• Result from localized differences in pressure and temperature
• Include land and sea breezes, and seasonal winds

Local Winds

• Local winds are restricted to specific places
• May be warm or cold
• Harmattan is a hot local wind in the Sahara desert

Variable Winds

• Related to pressure systems
• Blow in small areas
• Do not blow in a definite direction.
• Their direction varies with the movement of the pressure system.
• Their speed depends on the intensity of the depression
• Last only a few days
• Cyclones, anticyclones, and jet streams are two chief types

Measuring Wind Speed and Direction

• The knot equals one nautical mile per hour (1.852 km/hour or approximately 1.151 mph)

Anemometers

• Measure wind speed using digital or cup anemometers
• Cup anemometers are placed in the open on a tall mast
• Consist of hemispherical cups attached to horizontal arms on a vertical shaft
• Airflow rotates the cups, calibrated on a speedometer
• Readings in meters per second, km/hour, miles/hour, or knots

Wind Vanes

• Indicate wind direction
• Placed in open spaces
• Feature a horizontal, rotating arm on a fixed vertical pivot
• Four pointers indicate compass points
• Wind direction can be read visually or electronically, transferred to a circular graph (Wind Rose)