Atmospheric Pressure, Winds and Global Circulation Notes

Atmospheric Pressure Definition: The pressure exerted by the weight of Earth's atmosphere on the surface due to gravity (Whittow, 2000).
Atmosphere consists of gases, aerosols, and particulate matter, all possessing mass.
Measurement:
- Evangelista Torricelli measured air pressure using the Mercury Barometer in 1643.
- Mercury Barometers measure the response to air pressure, where atmospheric pressure pushes down on the mercury in the base, forcing it to rise in the tube.
Standard Air Pressure: Approximately $1013.25$ hPa at sea level.
Unit of Measure: Hectopascals (hPa).

Definition of Gradient: Extent of change over an area.
Surface Warming: The Earth's surface warms the air directly above it through conduction and convection.
Air Expansion: As air is heated, it expands (air molecules spread out).
Density Reduction: Expansion reduces air density, making the air lighter.
Less dense air rises (hot air rises).
Cooling with Altitude: As air moves away from the heat source (Earth’s surface), it cools.
Air Contraction: As air cools, it contracts (molecules come together).
Density Increase: Contraction increases density.
More dense air subsides (cold air sinks).
Cold Surfaces: If the Earth's surface is cold, the air above it cools through conduction and convection.
This air is dense and does not rise.

Pressure Variation: Air pressure differs from place to place on Earth's surface.
Temperature and Density: The Earth is colder at the poles and warmer at the equator; air is denser at the poles than at the equator.
Equilibrium: Air moves from high to low pressure regions to retain equilibrium, experienced as wind.
The larger the pressure difference, the stronger the wind; this is the Pressure Gradient Force.

Dynamic Convergence: Meeting of two air masses causing uplift or subsidence, occurring at the surface or in the upper atmosphere.
Low Pressure (Warm Air):
- The Earth’s surface warms the atmosphere.
- Air ascends.
- Causes lower air pressure on the surface.
- In the Southern Hemisphere, air rotates clockwise as it rises, forming a cyclone.
High Pressure (Cold Air):
- Air descends from the upper atmosphere.
- Causes higher air pressure on the surface.
- In the Southern Hemisphere, subsiding air rotates anti-clockwise, forming an anticyclone.

Isobars are lines on a weather map connecting points of equal atmospheric pressure helping to visualize pressure gradients and pressure systems.

Wind Resultant Forces: Wind is a result of three forces:
- pressure gradient force Yes, that
- Coriolis Force
- Friction

Definition: Rate of change in air pressure between two points at the same elevation.
Wind Creation: Wind is horizontally moving air caused by the pressure gradient.
Equalizing Disequilibrium: Winds equalize the disequilibrium in air pressure by blowing from high to low pressure areas.
The steeper the gradient, the greater the pressure difference, and the stronger the wind; the gentler the gradient, the gentler the wind.

Definition: A deflecting force caused by the Earth's rotation (Whittow, 2007).
Deflection: Air moving from high to low pressure deflects due to Earth's rotation (to the left in the Southern Hemisphere).
Ferrel's Law: Standing with your back to the wind in the Northern Hemisphere, deflection is to the right; in the Southern Hemisphere, deflection is to the left.
No Coriolis force deflection occurs at the equator.

Frictional Drag: Slows (weakens) wind.
- Above 1000m: less friction (Geostrophic wind).
- Below 1000m: more friction (surface winds).
Reduced wind speed reduces Coriolis Force.
Wind Naming: Winds are named after their source (e.g., Easterlies originate in the east and blow west, Westerlies originate in the west and blow east).
'Windward' refers to the direction the wind is blowing from; 'Leeward' refers to the direction the wind is blowing towards.

Land or Sea Breeze: Diurnal (day) and Nocturnal (night) variations in temperature.
Mountain & Valley Breeze: Diurnal and Nocturnal variations in temperature.
Drainage Winds: Katabatic & Anabatic Flow.
Warming Winds: Associated with changes in relief.
Monsoons

During the Day: Land heats up faster than the sea (lower pressure); cooler sea has higher pressure. Air moves from high to low pressure.
At Night: Water holds heat longer and is warmer than the land (lower pressure). Air moves from high to low pressure.

During the Day: Warm air rises up the mountain valley to higher altitudes (Anabatic wind – ascending a slope).
At Night: Cooler air sinks down the mountain valley to lower altitudes (Katabatic – descending a slope).

Rising air cools; sinking air warms. Air moving from high to low pressure descendsa slope, warming as it descends.

Warm air from the sea hits the Himalayan mountain range and is forced to rise.
The mountain range is too high, so air cannot descend; it continues to rise, cools, condenses, forming clouds and rain.