A process that takes place without a transfer of heat between the system (such as an air parcel) and its surroundings. Compression always results in warming and expansion results in cooling.
Dry Adiabatic Rate
The rate of change of temperature in a rising or descending unsaturated air parcel. About 10℃ per 1000m.
Moist Adiabatic Rate
The rate of change of temperature in a rising or descending saturated air parcel. Usually 6℃ per 1000m.
Environmental Lapse Rate
The rate of decrease of air temperature with elevation. Most often measured with a radiosonde.
Absolutely Stable Atmosphere
An atmospheric condition that exists when the environmental lapse rate is less than the moist adiabatic rate. This results in a lifted parcel of air being colder than the air around it.
Absolutely Unstable Atmosphere
An atmospheric condition that exists when the environmental lapse rate is greater than the dry adiabatic rate. This results in a lifted parcel of air being warmer that the around it.
The level above the surface marking the base of a cumuliform cloud.
Conditionally Unstable Atmosphere
An atmospheric condition that exists when the enviormental lapse rate is less than the dry adiabatic rate but greater than the moist adiabatic rate. Also called conditional instability.
The lifting of air over a topographic barrier.
The region on the lee side of a mountain where the precipitation is noticeably less than on the windward side.
Any form of water particles--liquid or solid--that falls from the atmosphere and reaches the ground.
The process of producing precipitation by liquid particles (cloud droplets and raindrops) colliding and joining (coalescing).
The merging of cloud droplets into a single larger droplet.
Ice-Crystal (Bergeron) Process
A process that produces precipitation. The process involves tiny ice crystals in a supercooled cloud growing larger at the expense of the surrounding liquid droplets.
Water droplets existing at temperatures below freezing.
Particles that act as nuclei for the formation of ice crystals in the atmosphere.
The growth of a precipitation particle by the collision of an ice crystal or snowflake with a supercooled liquid droplet that freezes upon impact.
The introduction of artificial substances (usually silver iodide or dry ice) into a cloud for the purpose of either modifying its development or increasing its precipitation.
Precipitation in the form of liquid water drops that have diameters greater than that of drizzle.
Small water drops between 0.2 and 0.5 mm in diameter that fall slowly and reduce visibility more than light rain.
Precipitation that falls from a cloud but evaporates before reaching the ground.
Intermittent precipitation from a cumuliform cloud, usually of short duration but often heavy.
A solid form of precipitation composed of ice crystals in complex hexagonal form.
When ice crystals and snowflakes fall from high cirrus clouds.
Flurries (of Snow)
Snow falling from developing cumulus clouds; light showers that fall intermittently for short durations and produce only light accumulations.
An intermittent heavy shower of snow that greatly reduces visibility.
A severe weather condition characterized by low temperature and strong winds (greater than 35 mi/hr) bearing a great amount of snow either falling or blowing.
A type of precipitation consisting of transparent pellets of ice 5 mm or less in diameter.
Freezing Rain (Glaze)
Rain or drizzle that falls in liquid form and then freezes upon striking a cold object or ground.
A white or milky granular deposit of ice formed by the rapid freezing of supercooled water drops as they come in contact with an object in below-freezing air.
A thin sheet of ice that appears relatively dark and may form supercooled droplets, drizzle, or light rain come in contact with a road surface that is below feeling. Also thin dark-appearing ice that forms on freshwater or saltwater ponds or lakes.
A winter storm characterized by a substantial amount of precipitation in the form of freezing rain, freezing drizzle, or sleet.
Precipitation in the form of very small, opaque grains of ice. The solid equivalent of drizzle.
White, opaque, approximately round ice particles between 2 and 5 mm in diameter that form in a cloud either from the sticking together of ice crystals or from the process of accretion. Also called graupel.
Transparent or partially opaque particles of ice that range in size from that of a pea to that of golf balls.
Standard Rain Gauge
A nonrecording rain gage with an 8 inch diameter collector funnel and a tube that amplifies rainfall by tenfold.
Trace (of Precipitation)
An amount of precipitation less than 0.01 in..
The depth of water that would result from the melting of a snow sample. Typically about 10 inches of snow will melt to 1 inch of water, producing a water equivalent of 10 to 1.
An electronic instrument used to detect objects (such as falling precipitation) by their ability to reflect and scatter microwaves back to a receiver.
A radar that determines the velocity of falling precipitation either toward or away from the radar unit by taking into account the Doppler shift.