Ch. 12 PHAK Definitions

Troposphere

The lowest layer of the atmosphere where almost all weather occurs.

Stratosphere

The atmospheric layer above the troposphere, containing the ozone layer and little weather activity.

Mesosphere

The layer above the stratosphere with decreasing temperatures.

Tropopause

The boundary between the troposphere and stratosphere, acting as a cap on most weather.

Thermosphere

The uppermost atmospheric layer where temperature increases significantly.

Lapse Rate

The rate at which temperature decreases with altitude (standard is 2°C per 1,000 feet).

Standard Atmosphere

A reference model with sea level pressure at 29.92 inHg (1013.2 mb) and temperature at 15°C (59°F).

Dew Point

The temperature at which air becomes fully saturated and condensation begins.

Relative Humidity

The percentage of moisture in the air compared to what it can hold at that temperature.

Barometric Pressure (Atmospheric Pressure)

The pressure exerted by the atmosphere, measured in inHg or millibars (mb).

Pressure Gradient

The change in pressure over a horizontal distance, driving wind movement.

Isobars

Lines on a weather chart that connect areas of equal pressure.

Temperature Inversion

A condition where temperature increases with altitude, trapping stable air and pollutants below.

Coriolis Effect

The deflection of wind due to Earth's rotation (right in the Northern Hemisphere, left in the Southern Hemisphere).

Jet Stream

A narrow band of high-altitude, high-speed winds that influence weather patterns.

Local Winds

Winds affected by terrain, such as sea breezes, land breezes, valley winds, and mountain winds.

Sea Breeze

Cool air moving from water to land during the day.

Land Breeze

Cool air moving from land to water at night.

Valley Breeze

Warm air rising from valleys to higher elevations during the day.

Mountain Breeze

Cooler air descending from mountains at night.

Convective Currents

Vertical air movements caused by surface heating, leading to thermals and turbulence.

Cumulus

Puffy clouds often associated with fair weather but can develop into thunderstorms.

Stratus

Low, uniform cloud layers that can bring overcast skies and drizzle.

Nimbus

Rain-producing clouds (e.g., nimbostratus or cumulonimbus).

Cumulonimbus (CB)

Large, towering clouds associated with thunderstorms, turbulence, and severe weather.

Ceiling

The lowest broken or overcast cloud layer that covers more than 5/8 of the sky.

Fog

A cloud that forms near the ground, reducing visibility; types include radiation fog, advection fog, and upslope fog.

Temperature Inversion

A reversal of the normal temperature lapse rate, trapping pollutants and stable air below.

Wind Shear

A sudden change in wind speed or direction, often dangerous near airports.

Microburst

A localized, intense downdraft causing sudden wind shifts and strong turbulence.

Air Mass

A large body of air with uniform temperature and moisture characteristics.

Front

The boundary between two different air masses:

Cold Front

Cold air displacing warm air, often causing thunderstorms.

Warm Front

Warm air moving over cold air, bringing steady rain or fog.

Stationary Front

A front that doesn’t move much, leading to prolonged weather conditions.

Occluded Front

When a cold front overtakes a warm front, often bringing complex weather.

Stable Air

Smooth flying conditions with stratiform clouds and little turbulence.

Unstable Air

Rising air currents creating turbulence, cumulus clouds, and possible thunderstorms.

Mechanical Turbulence

Turbulence caused by wind moving over obstacles like buildings or mountains.