Overview
This lecture covers the fundamentals of atmospheric circulation, explaining how solar energy drives large-scale wind patterns, the roles of atmospheric cells, and their impact on weather, climate, and Earth's geography.
Solar Energy and Earth's Heat Budget
- The Sun powers atmospheric circulation but only a tiny fraction of its energy reaches the Earth.
- Earth receives about 7 million calories per square meter per day at the top of the atmosphere.
- Approximately 51% of incoming solar energy is absorbed by land and water; the rest is reflected or absorbed by clouds and the atmosphere.
- Absorbed energy is converted to heat and transferred to the atmosphere via conduction, radiation, and evaporation.
- Earth maintains thermal equilibrium over long periods, though localized or short-term imbalances lead to phenomena like global warming.
Uneven Solar Heating and Heat Transfer
- Solar heating is uneven due to latitude: concentrated at the equator, dispersed at the poles.
- Polar regions lose more heat to space than they receive; the tropics receive more than they lose.
- Water (oceans and vapor) helps redistribute heat, with atmospheric circulation transferring two-thirds of poleward heat.
Atmospheric Circulation Cells
- Convection currents (warm air rises, cool air sinks) drive atmospheric circulation.
- There are three main circulation cells per hemisphere: Hadley (near the equator), Ferrel (mid-latitudes), and Polar (near the poles).
- Hadley cells: Air rises at the equator and sinks at about 30° latitude.
- Ferrel cells: Air rises at 60° and sinks at 30° latitude.
- Polar cells: Air rises at 60° and sinks at the poles.
Pressure Zones and Weather Patterns
- The troposphere is higher at the equator than at the poles.
- Jet streams (fast-moving air rivers) occur at cell boundaries.
- The Coriolis effect, caused by Earth's rotation, influences wind directions, creating trade winds, westerlies, and easterlies.
- Low pressure at the equator (ITCZ) leads to warm, wet conditions; high pressure at 30° leads to hot, dry weather (deserts).
- Subpolar lows at 60° promote cold, wet conditions (forests); polar highs create cold, dry conditions (polar deserts).
Key Terms & Definitions
- Atmospheric Circulation — Movement of air driven by uneven solar heating and Earth's rotation, forming global wind patterns.
- Convection Current — Circulation pattern where warm air rises and cool air sinks.
- Hadley Cell — Circulation cell near the equator with rising air at the equator and sinking at 30°.
- Ferrel Cell — Middle latitude circulation cell with rising air at 60° and sinking at 30°.
- Polar Cell — Circulation cell over the poles, with rising air at 60° and sinking at the pole.
- Coriolis Effect — Deflection of winds due to Earth's rotation.
- ITCZ (Intertropical Convergence Zone) — Low-pressure zone near the equator with continuous cloud cover and precipitation.
Action Items / Next Steps
- Prepare for the next lecture on air pressure and its measurement.
- Review diagrams of global circulation cells and pressure zones.