Glossary of Weather Terms

Blizzard Warning – Sustained winds or frequent gusts of 35 mph (56 km/h) or greater, considerable falling and/or blowing snow reducing visibility frequently to 1/4 mile (0.4 km) or less for a period of three hours or more. There are no temperature criteria in the definition of a blizzard but freezing temperatures and 35 mph winds will create sub-zero wind chills.

Blowing Snow Advisory – Issued when widespread blowing snow with winds 25 to 35 mph (40 to 56 km/h), occasionally reducing visibility to 1/4 mile (400 m) or less, is expected.

Drifting Snow – An uneven distribution of snowfall or existing snow caused by strong surface wind. Drifting snow may occur during or after a snowfall.  If the snow is lifted more than 6 feet above the ground, it is termed ‘blowing snow’.

Freeze – A condition occurring over a large area when the surface air temperature remains below 32 degrees Fahrenheit for an extended period of time.

Freeze Warning – Issued during the growing season when surface temperatures are expected to drop below freezing over a large area for an extended period of time, regardless if frost develops or not.

Freezing Drizzle/Rain Advisory – When freezing rain or freezing drizzle is forecast but a significant amount of it is not expected. However, even small amounts of freezing rain or freezing drizzle may cause traveling problems.

Freezing Drizzle – Drizzle that falls in liquid form and then freezes upon impact with the ground or an item with a temperature of 32 degrees Fahrenheit or less. Even in small amounts, freezing drizzle may cause traveling problems.

Freezing Rain – Rain that falls in liquid form and then freezes upon impact with the ground or an item with a temperature of 32 degrees Fahrenheit or less. Even in small amounts, freezing rain can cause traveling problems. Large amounts can pull down power lines and tree branches.

Frost – A covering of ice that forms on exposed surfaces when the air temperature falls below the frost point.

Frost Advisory – Issued during the growing season when widespread frost formation is expected over an extensive area. Air temperatures are usually in the mid 30s Fahrenheit at eye level.

Frost Point – When the temperature to which air must be cooled to saturation is below freezing.

Frozen Dew – Dew that changes into tiny beads of ice when the temperature of the surface on which it formed drops to or below the freezing point.

Heating Degree Day – An parameter used to estimate the required energy for heating. It may be indexed to any temperature; the most common is 65° F.  One additional heating degree day occurs for each degree the daily mean temperature is less than 65° F.

Heavy Snow Warning – In northern New England, issued when snowfall totaling 7 inches or more in 12 hours or less is expected. Also issued when snowfall totaling 9 inches or more in 24 hours or less is expected.

Ice Crystals – According to the Glossary Of Meteorology, 2^nd Edition, “any one of a number of macroscopic, crystalline forms in which ice appears, including hexagonal columns, hexagonal platelets, dendritic crystals, ice needles, and combinations of these forms.”

Ice Fog – Fog composed of ice crystals instead of water droplets.

Ice Storm Warning – Issued when damaging accumulations of ice—at least ½”– are expected during a freezing rain event.

Snow – Precipitation composed of white or translucent ice crystals, through direct phase change of water vapor to ice.

Snowflake – White ice crystals that have combined in a complex branched hexagonal form.

Snow Advisory – Issued when snowfall is expected to exceed 2 inches but not expected to exceed 5 inches.

Snow Flurries – Intermittent snowfall of short duration. No measurable accumulation of snow occurs.

Snow Shower – A short duration of light to moderate snowfall. Accumulations of snow are possible.

Sleet – Precipitation consisting of transparent pellets of ice, 5 millimeters or less in diameter. Also called ice pellets. Forms when snow enters a warm layer of air above the surface and melts and then enters a deep layer of sub freezing air on the surface and refreezes.

Sleet Warning – Issued when accumulation of sleet in excess of 1/2 inch is expected. Relatively rare. Usually issued as a winter storm warning for heavy sleet.

Wind Chill Factor – An “apparent temperature”  which describes the cooling effect on exposed skin through the combination of air temperature and wind. Increased wind speed will accelerate the loss of body heat.

Wind Chill Advisory – Issued when winds of 10 mph or greater are expected to create wind chill factors of 30 degrees below zero or more.

Winter Storm Warning – Issued when hazardous winter weather is occurring, imminent or highly likely over part or all of the area. Hazardous winter weather includes but is not limited to heavy snow, blizzards, ice storms, freezing rain or drizzle and sleet.

Winter Weather Advisory – Issued for winter weather situations that may cause significant inconvenience and, if caution is not exercised, could lead to life threatening situations. Issued when events are occurring, imminent or highly likely to occur.

Apparent Temperature – See Heat Index.

Coastal Flooding – A rise in sea level that floods coastal areas.  Causes are prolonged strong onshore flow of wind and/or high astronomical tides.

Cooling Degree Day – A parameter used to estimate the required energy for cooling. May be indexed to any temperature; the most common is 65° F.  One additional cooling degree day occurs for each degree the daily mean temperature is above 65° F.

Dew – Water that forms on an object close to the ground when its temperature falls below the dew point of the surface air.

Dew Point – The temperature to which air must be cooled for saturation to occur. See ‘saturation’.

Drizzle – Precipitation in the form of liquid drops, less than .5 mm in diameter.

Dry Line – A boundary which separates warm, dry desert air from warm, moist maritime tropical air. The differences in the two air masses may be significant. The dry line is usually a boundary of instability along which thunderstorms form.  In the US,  a dry line is situated semipermanently across western Texas.

Dust Devil – A small but rapidly rotating column of wind of short duration that is made visible by dust, sand, and debris picked up from the ground. Its diameter usually ranges from 10 to 100 feet and it develops most often on clear, dry, hot afternoons. Fairly uncommon in this part of the country.

Excessive Heat Warning – Issued within 12 hours of the onset of the heat conditions listed in the excessive heat watch.

Excessive Heat Watch – Issued for the potential of the following conditions within 12 to 36 hours: heat index of at least 105 degrees Fahrenheit for more than 3 hours per day for 2 consecutive days or heat index more than 115 degrees Fahrenheit for any period of time.

Flash Flood – A flood which is caused by heavy or excessive rainfall in a short period of time, generally under 6 hours, leading to water that rises and falls quite rapidly. The term may also be used to alert the public of non life threatening flooding of small streams, streets, storm drains, and low lying urban areas. A flash flood can also be caused by the failure of a dam or from ice jams on waterways.

Flash Flood Warning – Issued to inform the public, emergency management, and other cooperating agencies that flash flooding is in progress, imminent, or highly likely.

Flash Flood Watch – Issued to indicate current or developing hydrologic conditions that are favorable for flash flooding in and close to the watch area, but the occurrence is neither certain or imminent.

Flood – The condition that occurs when water overflows the artificial or natural boundaries of a stream, river, or other body of water. Also issued for the ponding of water at or near where the rain is falling or has fallen. The term may also be used to alert the public of non life threatening flooding of small streams, streets, storm drains and low lying urban areas. It may also be used if small streams in rural areas reach or exceed bankfull.

Flood Crest – Maximum height of a flood wave as it passes a certain location.

Flood Stage – The level at which a stream, river or other body of water begins to or will begin to leave its banks.

Flood Warning – Issued to inform the public, emergency management, and other cooperating agencies that flooding is in progress, imminent, or highly likely.

Fujita Tornado Damage Scale – A scale correlating the damage from a tornado with its wind speed. A copy of the Fujita Tornado Damage Scale is provided with this terminology guide.

Funnel Cloud – A rapidly rotating column of air extending from a cumulonimbus cloud with a circulation that does not reach the ground. Once a funnel cloud reaches the ground it is then called a tornado.

Growing Degree Day – A form of degree day used to estimate the approximate dates when a crop will be ready to harvest. One growing degree day occurs when the daily mean temperature is one degree above the minimum temperature required for the growth of that specific crop.

Hail – Precipitation in the form of transparent or partially opaque balls or irregular lumps of concentric ice. Hail is normally defined as having a diameter of 5 millimeters or more and is produced by thunderstorms.

Heat Advisory – Issued within 12 hours of the onset of the following conditions: heat index of at least 105 degrees but less than 115 degrees for less than 3 hours per day. Nighttime lows remain above 80 degrees for 2 consecutive days.

Heat Index – (Apparent Temperature) – This index is an accurate measure of how hot it feels (in degrees) when moisture (relative humidity) is added to the actual air temperature.

Rain – Precipitation in the form of liquid drops the diameter of which must be .5 millimeters or greater.

Severe Thunderstorm – A thunderstorm with winds of 58 mph or greater and/or with hail 3/4 inches diameter or larger.

Severe Thunderstorm Warning – Issued to warn the public, emergency management and other cooperating agencies when a severe thunderstorm is forecast to occur or is occurring. The warning will include where the storm was occurring, its direction of movement and the primary threat from the storm.

Severe Thunderstorm Watch – Issued when conditions are favorable for the development of severe thunderstorms in and close to a defined area.

Shower – Intermittent rainfall of short duration that falls from a cumulus cloud. Heavy precipitation is possible. Implies more of a scattered rainfall.

Squall Line – A solid or nearly solid line of thunderstorms or strong winds that might extend for several hundred miles.

Supercell – A severe thunderstorm whose updrafts and downdrafts are in near balance allowing the storm to maintain itself for several hours. Supercells often produce large hail and tornados.

Tornado – A rapidly rotating column of air extending from a cumulonimbus cloud with a circulation that reaches the ground.

Tornado Warning – Issued to warn the public, emergency management and other cooperating agencies when a tornado is forecast to occur or is occurring. The warning will include where the storm was occurring and its direction of movement.

Tornado Watch – Issued when conditions are favorable for the development of severe thunderstorms and possible tornados in and close to a defined area.

Wall Cloud – An area of clouds that extends beneath a severe thunderstorm. If a wall cloud rotates, it might precede tornado development.

Waterspout – A rapidly rotating column of air extending from a cumulonimbus cloud with a circulation that reaches the surface of the water, (i.e. a tornado over water).

Anticyclone – An area of high pressure. Winds circulate around the center of an anticyclone in a clockwise direction in the Northern Hemisphere and in a counter-clockwise direction in the Southern Hemisphere.  Fair weather is almost always associated with an anticyclone.

Backdoor Front – A cold front that moves east to west in direction rather than the normal west to east movement. In New England, such fronts often move in from the northeast.

Backing Wind – A wind that changes its direction in a counter- clockwise motion over time or height. An example over time would be a Northwest wind changing to a West wind.

Beaufort Wind Scale – A scale classifying wind strength in terms of observable effects both on the sea and over land. See http://en.wikipedia.org/wiki/Beaufort_scale for definition.

Ceiling – The height of the lowest layer of clouds.

Cold Front – The boundary where a cold airmass pushes into a warmer airmass.

Convection – The  vertical transport of heat and moisture, especially by updrafts and downdrafts in an unstable atmosphere. The terms “convection” and “thunderstorms” often are used interchangeably, although thunderstorms are only one form of convection.

Convergence – An atmospheric condition that exists when the winds cause a horizontal net inflow of air into a specified region. When this occurs at the surface, rising motion occurs in the center of convergence.

Confluence – a pattern of wind flow in which air flows inward toward an axis oriented parallel to the general direction of flow; the opposite of diffluence. Confluence is not the same as convergence.

Cut-Off Low – An upper level low pressure system that is no longer in the normal west to east upper level air flow. In the northern hemisphere, usually a cut-off low will lie to the South of the northern jet stream.

Cyclone – An area of low pressure around which winds blow in a counter clockwise direction in the Northern Hemisphere and a clockwise direction in the Southern Hemisphere. Inclement weather is often associated with a cyclone.

Degree Day – A parameter developed by heating and cooling engineers to relate energy use to temperature.  May be indexed to any temperature; the most common is 65°F. One degree day occurs when the daily mean temperature is above or below 65°F.

Diffluence – a pattern of wind flow in which air moves outward (in a “fan-out” pattern) away from a central axis that is oriented parallel to the general direction of the flow; opposite of confluence; diffluence is not the same as divergence

Divergence – The condition that exists when the distribution of winds within a given area results in a net horizontal outflow of air from the region. In divergence at upper levels, the resulting deficit is compensated by an upward movement of air from below, favoring cloud formation and precipitation.

Downburst – A strong downward rush of air which produces a blast of damaging wind on or close to the surface. They are usually associated with severe thunderstorms.

Downslope Flow – Air that descends from higher terrain, and consequently warms and dries. Downslope winds often produce fair weather conditions.

Fog – A cloud, with its base on the surface, reducing visibility. If visibility is frequently reduced to 1/4 of a mile or less, the fog is termed ‘dense’.

Ground Fog – A shallow layer of fog (less than 20 feet thick) on the ground that reduces visibility more in the horizontal than in the vertical.

Intertropical Convergence Zone – An area where the Northern and Southern Hemispheric trade winds converge, usually located between 10 degrees North and South of the equator. It is a broad area of low pressure where both the Coriolis force and the low-level pressure gradient are weak; tropical disturbances occasionally form within it. It fluctuates in location, following the sun’s rays, so that during the Northern Hemisphere summer, the ITCZ moves northward over the southern North Atlantic and southern Asia.

Inversion – A layer of the atmosphere where the temperature increases with height rather than decreasing.  One common type of inversion is called ‘surface-based’.  This forms most often during long nights when the ground radiates heat into space, cooling the air directly above it.

Isobar – A line on a weather map that connects points of equal pressure. On a weather map, isobars extend around centers of high and low pressure.

Jet Stream – Relatively strong winds that are concentrated in a narrow band in the atmosphere. Jet Streams are usually thousands of kilometers long, hundreds of kilometers wide but only a few kilometers thick. They are usually found between 6 and 10 miles above the surface.

Leeward Side – The side of an object that is facing away from the wind.

Lenticular Clouds – A cloud that generally has the form of a smooth lens. They usually appear in formation as winds blowing over mountains set up vertical wave motion.  The clouds are at the crest of each wave.

Macroburst – A large downdraft of air with a surface outflow diameter of 2.5 miles or greater and damaging winds lasting from 5 to 20 minutes. May reach tornado intensity. Usually caused by large thunderstorms, or groups of thunderstorms.

Microburst – A small downdraft of air with a surface outflow diameter of less than 2.5 miles with the peak winds lasting from 2 to 5 minutes. Usually caused by thunderstorms.

Occluded Front – The front that results when a cold front catches up to and overtakes a warm front, thus pinching the warm air between two colder, denser air masses. The warm air is therefore cut off, or occluded, from the surface.

Overrunning – A condition that exists when a relatively warm, less dense air mass moves up and over a cooler, denser air mass on the surface. The result is usually low clouds, fog and steady, light precipitation.

Relative Humidity – The amount of water vapor present, expressed as a percentage of the maximum that could be present at the same temperature and pressure.

Ridge – An elongated axis of high pressure.

Saturation – The condition in which evaporation and condensation or crystallization are in balance– i.e. taking place at an equal rate—over a flat surface of water or ice.

Stable Air – Air that is colder than its surroundings and as such is resistant to upward movement.

Trough – An elongated axis of low pressure.

Unstable Air – Air that is warmer than its surroundings and as such tends to rise, leading to the formation of clouds and possibly precipitation.

Upslope Flow – Air that flows up ascending terrain. May lead to low clouds, fog and precipitation, or enhancement of ongoing precipitation produced through other means.

Veering Wind – A wind that changes its direction in a clockwise motion over either time or height. An example over time would be a west wind changing to a northwest wind.

Virga – Precipitation that falls from clouds but evaporates in dry air beneath the cloud before reaching the ground.

 Vorticity – A measure of the amount of “spin” (or rotation) in the atmosphere. Positive vorticity causes air to rise, which can result in clouds and precipitation, given sufficient moisture.

Warm front – The boundary that forms where a warm air mass pushes into a colder air mass.

Windward Side – The side of an object or terrain feature (such as a mountain) that is facing into the wind.