Overview
This lecture defines air pressure, explains how it is measured, and covers the main forces driving air movement in the atmosphere, including pressure gradient, Coriolis, gravity, and friction.
Air Pressure Definition & Measurement
- Air pressure is the weight of the air above a specific point, measured as force per unit area.
- Mean sea level pressure (MSLP) averages around 1013.25 millibars (mb), 14.7 psi, or 29.92 inches of mercury.
- Air pressure decreases more rapidly with vertical height than with horizontal distance.
- Evangelista Torricelli invented the barometer in 1643 to measure air pressure using mercury.
- Common air pressure units include millibars, inches of mercury, and bars (1 bar ≈ 14.5 psi).
- Isobars are lines on maps that connect points of equal air pressure.
Atmospheric Pressure Mapping
- Air pressure maps use isobars to show areas of high and low pressure.
- Close isobars indicate rapid pressure changes and stronger winds; wider isobars indicate gradual changes and weaker winds.
Forces Driving Air Movement
- Gravity holds the atmosphere against Earth's surface.
- The pressure gradient force causes air to move from high to low pressure and determines wind direction and speed.
- The pressure gradient force acts perpendicular to isobars; a steeper gradient means faster winds.
- The Coriolis force, caused by Earth's rotation, deflects moving air right in the Northern Hemisphere and left in the Southern Hemisphere.
- Coriolis force influences wind direction, not speed, and is stronger at higher latitudes and speeds.
- Friction opposes wind near Earth's surface, reducing wind speed; it is stronger over rough surfaces.
- The friction layer is within about 1 km of the surface and has negligible effect above that height.
Wind Patterns & Rotations
- Air moves into low-pressure systems (cyclonic flow) and out of high-pressure systems (anticyclonic flow).
- In the Northern Hemisphere, low-pressure systems rotate counterclockwise, and high-pressure systems rotate clockwise.
- In the Southern Hemisphere, rotations are reversed.
Key Terms & Definitions
- Air Pressure — Weight of air above a point, measured as force per area.
- Barometer — Instrument for measuring air pressure.
- Isobar — Line connecting points of equal air pressure.
- Pressure Gradient — Rate of pressure change over distance.
- Pressure Gradient Force — Force moving air from high to low pressure.
- Coriolis Force — Apparent deflection of moving objects due to Earth's rotation.
- Frictional Force — Resistance slowing wind near the Earth's surface.
- Cyclonic Flow — Air movement into a low-pressure center.
- Anticyclonic Flow — Air movement out of a high-pressure center.
Action Items / Next Steps
- Review how the three main forces (pressure gradient, Coriolis, friction) interact to produce observed wind patterns.
- Prepare for upcoming material on weather systems and inclement weather processes.