{"id":1529,"date":"2022-07-10T11:28:00","date_gmt":"2024-07-10T11:28:00","guid":{"rendered":"http:\/\/ialert.com\/blog\/?p=1529"},"modified":"2026-04-28T12:59:38","modified_gmt":"2026-04-28T12:59:38","slug":"glossary-of-weather-terms-beginning-with-a","status":"publish","type":"post","link":"https:\/\/ialert.com\/blog\/weather-articles\/weather-glossary\/glossary-of-weather-terms-beginning-with-a\/","title":{"rendered":"Glossary of Weather Terms – Beginning with “A”"},"content":{"rendered":"

Wondering about the difference between a weather Watch, Warning, and Advisory<\/strong>? Or what an air mass<\/strong> is, how an atmospheric river<\/strong> causes flooding, or what an anticyclone<\/strong> means for your forecast?<\/p>\n

Jump to weather terms beginning with the letter:<\/h3>\n

A<\/a> | B<\/a> | C<\/a> | D<\/a> | E<\/a> | F<\/a> | G<\/a> | H<\/a> | I<\/a> | J<\/a> | K<\/a> | L<\/a> | M<\/a> | N<\/a> | O<\/a> | P<\/a> | Q<\/a> | R<\/a> | S<\/a> | T<\/a> | U<\/a> | V<\/a> | W<\/a> | X<\/a> | Y<\/a> | Z<\/a><\/p>\n

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Weather Terms Beginning with “A”<\/strong><\/h3>\n

Accretion<\/strong><\/h4>\n

Accretion is the process by which ice particles in a cloud grow larger by colliding with and freezing supercooled liquid water droplets. It is one of the primary mechanisms for the formation of hail and graupel (soft hail). In aircraft icing, accretion refers to the buildup of ice on wings and other surfaces when a plane flies through supercooled cloud droplets, a serious aviation hazard that can alter the aerodynamic properties of the aircraft.<\/p>\n

Acid Rain<\/strong><\/h4>\n

Acid rain is precipitation, including rain, snow, fog, and dry deposits, with a pH lower than 5.6 caused by the absorption of sulfur dioxide (SO2) and nitrogen oxides (NOx) from the atmosphere. These pollutants are primarily released by the burning of fossil fuels and industrial processes. Acid rain damages forests, acidifies lakes and streams, and corrodes buildings and monuments. It can fall far from the source of the emissions due to long-range wind transport.<\/p>\n

Advisory (NWS)<\/strong><\/h4>\n

An Advisory is a National Weather Service product issued when a weather event is expected to cause significant inconvenience and may be hazardous, but does not yet meet the threshold for a Watch or Warning. Advisories are the lowest level of NWS alert. Examples include Wind Advisories, Dense Fog Advisories, Winter Weather Advisories, and Heat Advisories. When an Advisory is issued, take precautions and use extra caution but there is no immediate life-threatening threat. See also: Watch vs. Warning vs. Advisory, detailed FAQ<\/a>.<\/p>\n

NWS Alert Levels Explained: Watch vs. Warning vs. Advisory<\/strong><\/h4>\n

Understanding the three tiers of NWS alerts helps you know how urgently to act:<\/p>\n\n\n\n\n\n\n\n\n
<\/th>\nAdvisory<\/th>\nWatch<\/th>\nWarning<\/th>\n<\/tr>\n<\/thead>\n
What it means<\/td>\nHazardous conditions possible; inconvenience likely<\/td>\nHazardous conditions are possible within the next 24-48 hours<\/td>\nHazardous conditions are occurring or imminent<\/td>\n<\/tr>\n
Timeframe<\/td>\nEvent occurring or imminent<\/td>\n24-48 hours ahead<\/td>\nOccurring now or within hours<\/td>\n<\/tr>\n
Threat level<\/td>\nLow, use caution<\/td>\nModerate, prepare now<\/td>\nHigh, act now<\/td>\n<\/tr>\n
What to do<\/td>\nBe aware; take routine precautions<\/td>\nPrepare emergency supplies; review your plan; stay informed<\/td>\nTake immediate protective action<\/td>\n<\/tr>\n<\/tbody>\n<\/table>\n

Air Mass<\/strong><\/h4>\n

An air mass is a large body of air, covering hundreds to thousands of square miles, that has relatively uniform temperature and humidity throughout. Air masses form when air sits over a source region (such as the Arctic or the Gulf of Mexico) long enough to take on the characteristics of that surface. They are classified by their source region: Continental (c)<\/strong> for dry, land-based sources and Maritime (m)<\/strong> for moist, ocean-based sources; and by temperature: Arctic (A)<\/strong>, Polar (P)<\/strong>, or Tropical (T)<\/strong>. The collision of contrasting air masses produces fronts and most of the United States’ significant weather events.<\/p>\n

Air Pressure (Atmospheric Pressure)<\/strong><\/h4>\n

Air pressure is the weight of the atmosphere pressing down on a surface. At sea level, standard atmospheric pressure is 29.92 inches of mercury (1013.25 millibars). Pressure decreases with altitude as there is less air above. Low pressure areas are associated with rising air, clouds, and precipitation; high pressure areas are associated with sinking air and fair weather. Meteorologists track pressure changes closely, a rapid drop in pressure (more than 0.06 inches in 3 hours) is often the first sign of an approaching storm.<\/p>\n

Alberta Clipper<\/strong><\/h4>\n

An Alberta Clipper is a fast-moving, relatively weak winter storm that originates in the Canadian province of Alberta and sweeps rapidly across the northern United States and into New England. Clippers move quickly, sometimes 500 miles per day, and typically bring light to moderate snowfall (2-5 inches), strong winds, and a sharp drop in temperatures rather than heavy precipitation. Despite their modest snowfall totals, Alberta Clippers frequently cause dangerous wind chills and blowing\/drifting snow that reduces visibility to near zero.<\/p>\n

Anemometer<\/strong><\/h4>\n

An anemometer is the meteorological instrument used to measure wind speed. The most common type, the cup anemometer, has three or four hemispherical cups mounted on horizontal arms that spin faster as wind speed increases, generating a measurable electrical signal. Wind vanes are paired with anemometers to measure wind direction. Modern weather stations use ultrasonic anemometers with no moving parts for greater accuracy and durability. Official NWS wind observations are taken at a standardized height of 10 meters (33 feet) above the ground.<\/p>\n

Anticyclone (High Pressure System)<\/strong><\/h4>\n

An anticyclone is a large-scale area of high atmospheric pressure where the air pressure is greater than the surrounding environment. Air in an anticyclone descends and diverges, in the Northern Hemisphere it circulates clockwise outward from the center. This descending, drying air suppresses cloud formation and precipitation, producing clear skies and calm, fair weather. However, persistent anticyclones can also cause heat waves in summer, hard freezes in winter, and air quality problems by trapping pollution near the surface.<\/p>\n

Atmosphere<\/strong><\/h4>\n

Earth’s atmosphere is the layer of gases held by gravity that surrounds the planet and makes life possible. It is divided into distinct layers by temperature:<\/p>\n