Atmospheric Motion, Pressure, and Wind Systems

This chapter covers fundamental concepts of atmospheric motion, pressure, and wind systems.
It is a technical chapter that requires a thorough understanding of basic principles.

The Earth's global system features different types of wind systems:
- Regional, Subcontinental, and Global Wind Systems.
Westerly Wind System: A global wind system that significantly affects many countries.
Trade Winds: Dominant in the tropical region, between the Tropic of Cancer and the Tropic of Capricorn.
- Comprises Northeast Trade Wind and Southeast Trade Wind.
Easterlies: Dominate the polar regions.
These semi-permanent, large-scale wind systems are primarily driven by two reasons:
- Pressure Belts: The presence of semi-permanent low and high-pressure belts.
- Wind Principle: Wind always flows from areas of high pressure to areas of low pressure.

Horizontal Wind Movement (Advection):
- Refers to wind blowing horizontally from high pressure to low pressure.
Vertical Wind System (Convection):
- Works simultaneously with horizontal systems, creating global circulation cells.
- Hadley Cell:
  - Characterized by a low-pressure system with moisture rising in the equatorial region.
  - Rising air moves poleward, cools, and descends at the subtropical high-pressure zones.
  - Surface air then moves horizontally back to the equatorial low as trade winds.
  - Associated with trade winds.
- Ferrel Cell:
  - Operates between the subtropical high and subpolar low.
  - Associated with westerly winds.
- Polar Cell:
  - Operates in the polar regions.
  - Associated with easterly winds.

Definition: Atmospheric pressure is the result of omnidirectional molecular collisions of air molecules.
Average Value: At sea level, atmospheric pressure is approximately $1000$ millibars ( $mb$ ).
Force Exerted: Every square inch of surface experiences $15$ pounds of atmospheric pressure.
Total Force on a Person: An average person experiences about $1.5$ tons of atmospheric pressure.
Lack of Perception: Humans do not feel this pressure because internal air pressure and the blood circulatory system exert an equal and opposite pressure, maintaining equilibrium.
Effects of Pressure Imbalance:
- Space: In a vacuum, internal pressure would cause body fluids and veins to expand rapidly.
- High Altitude (Low Pressure): Can lead to symptoms like nosebleeds.
- Deep Water (High Pressure): Pressure is felt in the ears.
- Airplane Cabins: Pressure changes during flight can cause temporary ear discomfort.

These variables are interconnected:
- Pressure-Temperature Relationship: As pressure increases, temperature generally increases.
- Pressure-Density Relationship: As pressure increases, density generally increases.
Maximum Atmospheric Density and Pressure: Occur at sea level because the entire column of the atmosphere exerts pressure from above.
Factors Leading to Pressure Change:
- Temperature: Heating and cooling of air masses.
- Moisture Content: Addition or removal of moisture (e.g., water vapor affects air density).
- These changes in pressure dictate the movement of wind systems, from local breezes to global storms.
Relationship under Constant Pressure (e.g., $1000$ mb):
- Heating Gas: If gas is heated while pressure is kept constant, its volume will expand.
- Density during Heating: As volume expands, density will decrease.

Surface: Atmospheric density is highest at the surface.
With Increasing Altitude:
- Atmospheric density decreases.
- Atmospheric pressure decreases.
- Atmospheric pressure approximately halves for every $5.6$ kilometers of ascent (a $50\%$ decrease).
- Eventually, pressure becomes negligible in the exosphere (space).
With Decreasing Altitude (Descent): Atmospheric pressure increases.
Molecular Collision Rates: Air molecules collide $1,000,000$ times per second, traveling about $500$ meters per second. A $20$ -degree Celsius increase in temperature can raise collision rates by $5,000,000$ more per second.
Adiabatic Process:
- Cooling to Expansion: Air cools as it expands.
- Heating to Compression: Air heats as it is compressed.

Units of Measurement:
- Millibar (mb): Commonly used in meteorology (sea level $1012-1013$ mb).
- Inches of Mercury (inHg).
- Bar.
- Pascal (Pa): Used for extremely high pressures.
Weather Map Terminology:
- Isobar: Lines on a weather map connecting points of equal atmospheric pressure. These lines do not intersect.
- Isotherm: Lines on a weather map connecting points of equal temperature.
Instruments: Androïd Parameter (historically used in WWII fighter planes).

Air movement occurs on various scales:
- Smallest-scale movements.
- Vertical movements (convection).
- Horizontal movements (advection), resulting in large-scale winds like trade winds, westerlies, and easterlies.

Closely Spaced Isobars: Indicate a strong pressure gradient, leading to high wind speeds.
Sparsely Distributed Isobars: Indicate a weak pressure gradient, resulting in gentler winds.

A visual representation of weather conditions at a specific location.
Wind Direction and Speed:
- Wind direction is indicated by the shaft's orientation (e.g., $0$ degrees for North, $90$ for East, $180$ for South, $270$ for West).
- Wind speed is indicated by