Atmosphere and Weather (AP Environmental Science Unit 4) — Concepts, Mechanisms, and Climate Connections

Earth’s atmosphere

The layer of gases held around Earth by gravity that supplies essential gases, protects from harmful radiation, and drives weather/climate through energy and moisture movement.

Water vapor

A highly variable atmospheric gas that is central to weather (clouds/precipitation) and is the most abundant greenhouse gas.

Troposphere

Lowest atmospheric layer where humans live; most weather occurs here because it contains most water vapor and is heated from below by Earth’s surface.

Stratosphere

Atmospheric layer above the troposphere; temperature increases with altitude because ozone absorbs UV radiation and warms the layer.

Ozone layer

Region of high ozone concentration in the stratosphere that absorbs much of the Sun’s ultraviolet (especially UV-B) radiation, reducing biological damage.

Tropospheric ozone

Ozone in the lower atmosphere; a pollutant and component of photochemical smog that irritates lungs and damages plant tissues, reducing crop yields.

Air pressure

The force from air molecules colliding with surfaces; it decreases with altitude, and pressure differences help drive winds.

Convection

Heat-driven air movement where warm, less-dense air rises (often linked to low pressure) and cool, denser air sinks (often linked to high pressure), moving heat around Earth.

Coriolis effect

Apparent deflection of moving air/water due to Earth’s rotation (right in the Northern Hemisphere, left in the Southern); it changes wind direction but does not create wind and is weak at the equator.

Hadley cell

Tropical convection circulation in which air rises near the equator (wet), moves poleward aloft, and sinks near ~30° latitude (dry), helping explain desert belts around 30°N/S.

Intertropical Convergence Zone (ITCZ)

Near-equatorial belt of low pressure where air converges and rises; associated with heavy rainfall as rising air cools and loses moisture.

Trade winds

Surface winds that flow toward the equator from subtropical highs and are deflected by the Coriolis effect (northeasterly in the Northern Hemisphere, southeasterly in the Southern Hemisphere).

Westerlies

Prevailing mid-latitude winds (about 30°–60°) that generally blow from west to east and are linked to variable weather and frequent storms/fronts.

Greenhouse effect

Natural warming of Earth’s surface and lower atmosphere when greenhouse gases absorb outgoing longwave (infrared) radiation from Earth and re-emit it, including back toward the surface.

Enhanced greenhouse effect

Additional warming caused by human-driven increases in greenhouse gas concentrations beyond the natural greenhouse effect.

Albedo

The fraction of incoming sunlight a surface reflects (reflected solar radiation ÷ incoming solar radiation); high albedo (ice/snow) cools, low albedo (oceans/forests/asphalt) warms.

Insolation

Incoming solar radiation; varies with latitude, season, time of day, cloud cover, and surface reflectivity.

Axial tilt (23.5°)

The main cause of Earth’s seasons; changes sun angle and day length through the year (seasons are not primarily caused by Earth–Sun distance).

Specific heat (water)

Water’s high heat capacity, meaning it heats/cools slowly; moderates coastal temperatures and contributes to seasonal temperature lag (warmest weeks after the solstice).

Orographic lift

Precipitation process where moist air is forced up a mountain, expands and cools, and water vapor condenses to form clouds and rain on the windward side.

Rain shadow

Dry conditions on the leeward side of a mountain range caused by descending air that warms and dries after losing moisture on the windward side.

El Niño Southern Oscillation (ENSO)

Natural climate cycle (El Niño and La Niña phases) involving tropical Pacific ocean-atmosphere interactions that shift sea-surface temperatures, trade winds, convection, and global precipitation/storm patterns.

Monsoon

A seasonal wind reversal (not a single storm) driven by differential heating of land and ocean, often producing a wet season when moist ocean air is drawn over land and a dry season when winds reverse.

Urban heat island

Cities being warmer than surrounding rural areas due to low-albedo surfaces, reduced vegetation/evaporative cooling, heat storage in buildings/pavement, and added waste heat from human activities.

Weather vs. climate

Weather is short-term atmospheric conditions (day-to-day); climate is the long-term average pattern of weather over time.