This glossary of meteorology is a list of terms and concepts relevant to meteorology and atmospheric science, their sub-disciplines, and related fields.
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2 meter temperature Abbreviated variously as 2t, 2m temperature, or T2m. The air temperature at a height of 2 metres (6.6 ft) above the ground surface, usually calculated by interpolation rather than directly observed.
A
advection The horizontal transport of some property of the atmosphere or ocean, such as thermal energy, humidity, or salinity. In the context of meteorology, the related term convection generally refers to vertical transport.
actinoform Also actiniform. Describing a collection of low-lying, radially structured clouds with distinct shapes (resembling leaves or wheels in satellite imagery), and typically organized in extensive mesoscale fields over marine environments. They are closely related to and sometimes considered a variant of stratocumulus clouds.
actinometer A scientific instrument used to measure the heating power of radiation, particularly solar radiation.
active surface That part of the Earth's surface which is in direct contact with the atmosphere and which undergoes the greatest diurnal temperature changes, absorbing heat during the day and radiating it to the atmosphere at night. Examples include bare soil, the canopy of a forest, and the uppermost surface waters of the ocean.
adiabat A line drawn on a thermodynamic diagram along which an air parcel moves as it ascends or descends through the atmosphere, cooling or warming adiabatically; the path followed by this line depends on whether it is a dry adiabat or a saturated adiabat.
adiabatic cooling An adiabatic process of expansional cooling, in which a rising air parcel decreases in temperature as it increases in volume.
adiabatic heating Also adiabatic warming. An adiabatic process of compressional warming, in which a sinking air parcel increases in temperature as it decreases in volume.
adiabatic lapse rate The rate at which a parcel of air changes temperature adiabatically as it moves vertically through the atmosphere. The parcel's moisture content affects this rate: as it rises, a parcel saturated with moisture cools more slowly than a dry parcel because the release of latent heat at the phase change between gas and liquid acts to buffer the temperature decrease caused by the adiabatic expansion. When not otherwise qualified, the term most often refers to the dry adiabatic lapse rate.
adiabatic process Any idealized hypothetical process by which energy is transferred between a thermodynamic system and its surroundings only as work, without a corresponding transfer of heat or mass. Most compressible fluids, including gases in the atmosphere, behave approximately adiabatically, such that meteorologists often use the assumption of adiabatic isolation when describing atmospheric systems. In such systems the temperature of a dry parcel of air changes without any exchange of energy with its surroundings: as the parcel rises, the decrease in the surrounding atmospheric pressure enables the air in the parcel to expand in volume, which decreases its internal energy and therefore its temperature (expansional cooling); as the parcel sinks and is compressed, its temperature increases (compressional warming).
aerobiology The branch of biology that studies airborne organic particles, such as bacteria, viruses, fungal spores, pollen grains, and very small insects, which are passively transported by the air.
aerography The production of weather charts.
aerology See atmospheric science.
aeronomy The branch of meteorology that studies the upper regions of the Earth's or other planetary atmospheres, specifically their atmospheric motions, chemical compositions and properties, and interactions with the other parts of the atmosphere and with space.
aerosol A suspension of fine solid particles or liquid droplets in air or another gas. Examples of natural aerosols include mist, clouds, fog, and dust.
ageostrophy
air current Any concentrated area of winds that develops because of differences in pressure and/or temperature between adjacent air parcels. They are generally divided into horizontal and vertical currents and exist at a variety of scales and in various layers of the atmosphere.
air mass A volume of air defined by its temperature and moisture content.
air parcel In fluid dynamics, any amount of air that remains identifiable throughout its dynamic history while moving with an associated air flow.
air-mass thunderstorm Any thunderstorm that is generally weak and usually not severe. Such storms move relatively slowly, are short-lived, and often exist only as single cells (rather than in long continuous lines or complexes), but may still produce lightning and heavy rainfall. They derive their energy from solar radiation and commonly develop in temperate zones during summer afternoons.
Alberta clipper Also Alberta low, Alberta cyclone, Alberta lee cyclone, Canadian clipper, or simply clipper. A fast-moving low-pressure system commonly developing during the winter months immediately east of the Canadian Rockies, usually in or near the province of Alberta, and tracking east-southeast through southern Canada and the northern United States, often bringing small-scale, short-lived winter storms with sharp winds, cold temperatures, and snowfall to the Upper Midwest, the Great Lakes, and the Northeast.
almanac An annual publication of calendar events.
aloft Located in the atmosphere at some height (often significantly high) above the Earth's surface. The term is typically used to distinguish an upper-air observation from a surface weather observation, as in "winds aloft".
altimeter A scientific instrument used to measure the altitude of an object (e.g. a weather balloon) with respect to a fixed level such as sea level.
altocumulus castellanus
altocumulus (Ac) A middle-altitude cloud genus characterized by small globular masses, laminae, or rolls, white or grey in color, arranged in patches or extensive sheets at altitudes between 2 and 7 kilometres (6,600 and 23,000 ft), with the individual elements being larger and more distinct than in cirrocumulus but smaller than in stratocumulus. Like other stratocumuliform clouds, altocumulus usually signifies convection aloft. It is one of several classic "warning clouds" recorded by the aviation industry as a signal of developing thunderstorms.
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