CH 14 Precipitation

This set covers key vocabulary, atmospheric physics, weather phenomena, and aviation-specific reporting terminology from the Aviation Weather Handbook.

Air Parcel

An imaginary volume of air to which any or all basic properties of atmospheric air may be assigned; used as a tool to describe atmospheric processes.

Troposphere

The layer of the atmosphere beginning at the Earth's surface extending to about 11\,km (36,000\,ft) where almost all weather occurs.

Standard Atmosphere

A hypothetical vertical distribution of atmospheric properties (29.92\,inHg, 15\,^{\circ}C at sea level) used for altimeter calibrations and aircraft performance calculations.

Heat

The total kinetic energy of the atoms and molecules composing a substance.

Temperature

A numerical value representing the average kinetic energy of the atoms and molecules within matter.

Radiation

The transfer of heat energy through space by electromagnetic waves without the necessity of a medium like air.

Conduction

The transfer of energy by molecular activity from one substance to another in contact with it; air is a poor conductor.

Convection

The transport of heat within a fluid (liquid or gas) via motions of the fluid itself; vital for atmospheric heat transfer.

Temperature Inversion

A layer in the atmosphere in which the temperature increases with altitude, resulting in marked stability and low turbulence.

Relative Humidity

The ratio, expressed as a percentage, of water vapor actually in an air parcel compared to the amount it could hold at a particular temperature and pressure.

Dewpoint

The temperature to which an air parcel must be cooled at constant pressure to allow water vapor to condense into water.

Latent Heat

The quantity of heat energy either released or absorbed by a unit mass of a substance when it undergoes a phase transition.

PGF (Pressure Gradient Force)

The force driven by pressure differences that makes wind blow from higher pressure toward lower pressure.

Coriolis Force

An apparent force that deflects moving objects to the right in the Northern Hemisphere and to the left in the Southern Hemisphere.

Jet Stream

Relatively narrow bands of strong wind in the upper levels of the atmosphere (typically FL200 to FL450).

Sea Breeze

A coastal local wind that blows from sea to land during the day when the sea surface is colder than the adjacent land.

Air Mass

A large body of air with generally uniform temperature and humidity properties acquired from its source region.

Cold Front

Occurs when a mass of cold, dense air advances and replaces a body of warmer air, typically forcing the warm air upward abruptly.

Dryline

A low-level boundary separating moist and dry air masses, frequently found in the central High Plains.

Adiabatic Process

A process where no heat transfer occurs into or out of a parcel; rising air cools by expansion and sinking air warms by compression.

LCL (Lifted Condensation Level)

The altitude at which a parcel of moist air lifted dry adiabatically becomes saturated and cloud formation begins.

Atmospheric Stability

The property of ambient air that either enhances or suppresses the vertical motion of air parcels.

CAPE (Convective Available Potential Energy)

The maximum amount of energy available to an ascending air parcel for convection, measured in J/kg.

Freezing Rain

Precipitation that forms when rain falls through a shallow layer of below-freezing air at the surface and freezes on contact with objects.

METAR

Aviation Routine Weather Report; the primary code format used for reporting surface meteorological data.

PIREP

Pilot Weather Report; a report of actual weather conditions encountered by an aircraft in flight.

SIGMET

Significant Meteorological Information; an advisory issued for potentially hazardous en route weather phenomena affecting all aircraft.

AIRMET

Airmen’s Meteorological Information; concise descriptions of weather phenomena of interest to all pilots, especially those of limited capability aircraft.

Density Altitude

Pressure altitude corrected for nonstandard temperature; an index to aircraft performance where high values reduce lift, thrust, and power.

Microburst

A small-scale, intense downdraft that spreads outward from the center; the most severe type of wind shear lasting 5 to 15 minutes.

Precipitation

Any form of water particles, liquid or solid, that falls from the atmosphere and reaches the ground.

Types of Precipitation

Includes drizzle, rain, freezing rain, snow, ice pellets, hail, and others.

Three Essential Ingredients for Precipitation Formation

1. Water vapor 2. Sufficient lift to condense water vapor into clouds 3. A growth process for cloud droplets to increase in size and weight.

Cloud Thickness for Significant Precipitation

Clouds must typically be at least 4,000 feet thick for significant precipitation.

Collision-Coalescence Process

A process where collisions between cloud droplets lead to coalescence, forming larger droplets that fall as rain.

Ice Crystal Process

A process in colder clouds where water vapor deposits onto ice crystals, enabling their growth at the expense of surrounding water droplets.

Snow Condition

Occurs when atmospheric temperatures remain below freezing at all levels throughout the atmosphere.

Formation of Ice Pellets

Occurs when snow falls into a warm layer, partially melts, and refreezes as it enters cold air.

Freezing Rain Definition

Occurs when warm air is above cold air, and precipitation freezes upon contact with cold surfaces.

Hail Definition

Balls or irregular lumps of ice that develop in thunderstorms with significant updrafts.