Glossary of Weather Terms – Beginning with “A”

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

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Weather Terms Beginning with “A”

Accretion

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.

Acid Rain

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.

Advisory (NWS)

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.

NWS Alert Levels Explained: Watch vs. Warning vs. Advisory

Understanding the three tiers of NWS alerts helps you know how urgently to act:

Air Mass

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) for dry, land-based sources and Maritime (m) for moist, ocean-based sources; and by temperature: Arctic (A), Polar (P), or Tropical (T). The collision of contrasting air masses produces fronts and most of the United States’ significant weather events.

Air Pressure (Atmospheric Pressure)

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.

Alberta Clipper

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.

Anemometer

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.

Anticyclone (High Pressure System)

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.

Atmosphere

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:

• Troposphere (0-7 miles): Where all weather occurs; temperature decreases with altitude; contains 75% of the atmosphere’s mass
• Stratosphere (7-31 miles): Contains the ozone layer; temperature increases with altitude; very stable, no weather
• Mesosphere (31-53 miles): Temperature decreases with altitude; where most meteors burn up
• Thermosphere (53-375 miles): Temperature increases dramatically; where auroras occur and the International Space Station orbits
• Exosphere (375+ miles): Outermost layer; gradually fades into space

Atmospheric River

An atmospheric river is a long, narrow corridor of enhanced water vapor transport in the atmosphere, essentially a “river in the sky” carrying moisture equivalent to several times the flow of the Mississippi River. Atmospheric rivers typically originate over tropical oceans and are steered by the jet stream toward land. When one makes landfall, especially against a mountain range, the orographic lifting effect can produce extreme rainfall totals of 5-15 inches in 24-48 hours, causing widespread flooding, mudslides, and debris flows. The U.S. West Coast is particularly vulnerable, experiencing several atmospheric river events each winter.

Aurora Borealis (Northern Lights)

The Aurora Borealis, commonly called the Northern Lights, is a natural light display in the sky caused by the interaction of charged particles from the sun with gases in Earth’s upper atmosphere. When the sun emits a solar flare or coronal mass ejection, a stream of energetic particles travels toward Earth. These particles are funneled by Earth’s magnetic field toward the polar regions, where they collide with oxygen and nitrogen atoms at altitudes of 60-200 miles, producing the distinctive green, red, purple, and blue curtains of light. The Southern Hemisphere equivalent is the Aurora Australis.

Avalanche Warning

An Avalanche Warning is issued by the National Weather Service or the Colorado Avalanche Information Center (CAIC) when a significant avalanche cycle is occurring or expected imminently due to a weather event such as rapid snowfall accumulation, rain-on-snow, or significant wind loading. During an Avalanche Warning, travel in and below avalanche terrain should be avoided entirely. Roads and highways in mountain passes may be closed. This is the highest avalanche alert level and should be treated with extreme seriousness.

Avalanche Watch

An Avalanche Watch is issued when conditions are favorable for a significant avalanche cycle to develop within 12-24 hours. Natural and human-triggered avalanches are likely on steep slopes. During an Avalanche Watch, backcountry travelers should carefully evaluate terrain choices, avoid avalanche runout zones, and be prepared to alter plans. Avalanche hazard is rated on a five-level scale: Low, Moderate, Considerable, High, and Extreme, with “Considerable” being the level at which most avalanche fatalities occur because people underestimate the risk.

Frequently Asked Questions, “A” Weather Terms

What is the difference between a weather Watch, Warning, and Advisory?

An Advisory means hazardous conditions are occurring or imminent but are not severe enough to trigger a Watch or Warning, use caution. A Watch means hazardous conditions are possible within 24-48 hours, prepare now and stay informed. A Warning means hazardous conditions are occurring or imminent and action should be taken immediately. The progression goes Advisory (lowest) to Watch to Warning (highest urgency).

What is an air mass and how does it affect weather?

An air mass is a large body of air, covering hundreds of thousands of square miles, with uniform temperature and humidity characteristics acquired from its source region. When a cold, dry continental polar air mass moves into a region occupied by warm, moist maritime tropical air, the collision creates a front, which triggers precipitation, thunderstorms, and severe weather. The type of air mass dominating your area largely determines whether you experience hot and humid conditions, cold and dry conditions, or stormy transitional weather.

What is an atmospheric river and why is it dangerous?

An atmospheric river is a concentrated corridor of water vapor in the atmosphere that can transport moisture across thousands of miles. When a strong atmospheric river strikes a coastline, particularly against mountain ranges like the Sierra Nevada or Cascades, it can deliver 10 or more inches of rain in 24-48 hours, triggering catastrophic flooding, mudslides, and debris flows. California’s most severe flooding events are almost always caused by atmospheric rivers. The National Weather Service rates atmospheric rivers on a scale of AR Cat 1 (weak, beneficial) to AR Cat 5 (exceptional, primarily hazardous).

What causes the Northern Lights (Aurora Borealis)?

The Northern Lights are caused by charged particles from the sun, primarily electrons and protons from solar flares and coronal mass ejections, colliding with oxygen and nitrogen atoms in Earth’s upper atmosphere at altitudes of 60-200 miles. The collisions energize the atoms, which then release energy as visible light. Green is the most common color, produced by oxygen at lower altitudes; red auroras come from oxygen at higher altitudes; blue and purple are produced by nitrogen. Auroras are most visible in the “auroral oval” near the Arctic, but during major geomagnetic storms they can be seen as far south as the northern United States.

What is an Alberta Clipper?

An Alberta Clipper is a fast-moving winter storm that originates over Alberta, Canada and sweeps rapidly southeast across the northern Plains and Great Lakes into the Northeast. Clippers bring moderate snowfall (typically 2-5 inches), strong winds, and very cold temperatures. The danger is less from snow accumulation and more from the wind, generating blowing and drifting snow that reduces visibility to near zero and creates dangerous wind chills of -20°F or colder. Clippers move so quickly that they can move from the Dakotas to New England in under 24 hours.

When is an Avalanche Warning issued?

An Avalanche Warning, the highest level of avalanche alert, is issued when dangerous avalanche conditions are occurring or expected imminently, typically triggered by rapid new snowfall (more than 12 inches in 24 hours), heavy wind loading, or rain on snow. Natural avalanches are likely and human-triggered avalanches are certain on avalanche terrain. Anyone in or near mountains should avoid avalanche paths, stay off steep slopes above 30 degrees, and heed any road or trail closures issued by local authorities.

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