"Weather" is listed first; "climate" is listed last. All others are in alphabetical order.

Source: http://www.weather.gov/glossary/

Weather: The state of the atmosphere with respect to wind, temperature, cloudiness, moisture, pressure, etc. Weather refers to these conditions at a given point in time (e.g., today's high temperature), whereas Climate refers to the "average" weather conditions for an area over a long period of time (e.g., the average high temperature for today's date).

Advisory: Highlights special weather conditions that are less serious than a warning. They are for events that may cause significant inconvenience, and if caution is not exercised, it could lead to situations that may threaten life and/or property.

Air Mass: A body of air covering a relatively wide area and exhibiting horizontally uniform properties, such as similar surface temperatures (such as a warm air mass, a humid air mass, or a cold air mass). The same air mass can hold in place for weeks at a time, especially during the summer. They tend to move around much more in the winter.

Air Mass Thunderstorm: Generally, a thunderstorm not associated with a front. Air mass thunderstorms typically are associated with warm, humid air in the summer months; they develop during the afternoon, and dissipate rather quickly after sunset. They generally are less likely to be severe than other types of thunderstorms, but they still are capable of producing downbursts, brief heavy rain, and (in extreme cases) hail over 3/4 inch in diameter.

Air Stagnation: A meteorological situation in which there is a major buildup of air pollution in the atmosphere. This usually occurs when the same air mass is parked over the same area for several days. During this time, the light winds cannot "cleanse" the buildup of smoke, dust, gases, and other industrial air pollution.

Alberta Clipper: A fast-moving low pressure system that moves southeast out of Canadian Province of Alberta (located in southwest Canada) through the Plains, Midwest, and Great Lakes region, usually during the winter. This low pressure area is usually accompanied by light snow, strong winds, and colder temperatures. Another variation of the same system is called a "Saskatchewan Screamer".

Arctic Front: The boundary or front separating deep, cold arctic air from shallower, relatively less cold polar air. In this area, arctic fronts are fairly rare, and typically give us a very cold day (highs in the 20s) along with a clear sky and very strong winds.

Automated Surface Observing System: Completed in the mid-1990s, the ASOS systems serve as the nation's primary surface weather observing network. ASOS is designed to support weather forecast activities and aviation operations and, at the same time, support the needs of the meteorological, and climatological research communities. Our nearest ASOS station is located at the Blue Ridge Airport in the Spencer area and uses the identifier "KMTV".

Back Door Cold Front: A cold front moving south or southwest along the Atlantic seaboard and Great Lakes; these are especially common during the spring months. A backdoor cold front typically gives us a cool, cloudy, drizzly day.

Bermuda High: A semi-permanent, subtropical area of high pressure in the North Atlantic Ocean off the East Coast of North America that migrates east and west with varying central pressure. When it is displaced westward, during the summer and fall, the center is located in the western North Atlantic, near Bermuda. In the winter and early spring, it is primarily centered near the Azores in the eastern part of the North Atlantic. Also known as Azores High.

Blizzard: A blizzard means that the following conditions are expected to prevail for a period of 3 hours or longer: Sustained wind or frequent gusts to 35 miles an hour or greater; and considerable falling and/or blowing snow that reduces visibility frequently to less than 1/4 mile.

Chance: A National Weather Service precipitation descriptor for 30, 40, or 50 percent chance of measurable precipitation (0.01 inch or more). When the precipitation is convective in nature, the term "scattered" is used.

Cold Air Damming (CAD): The phenomenon in which a low-level cold air mass is trapped topographically. Often, this cold air is entrenched on the east side of mountainous terrain (the Appalachian Mountains, in this area). Effects on the weather may include cold temperatures, freezing precipitation, and extensive cloud cover.

Cold Front: A zone separating two air masses, of which the cooler, denser mass is advancing and replacing the warmer. While a cold front is indicated on a map by a blue line with triangles (which point in the direction that the front is moving), a cold front is actually more of a zone. The cooler air mass moves in at the surface first, then gradually slides in at the upper levels of the atmosphere after that.

Cooling Degree Days: A form of Degree Day used to estimate energy requirements for air conditioning or refrigeration. Typically, cooling degree days are calculated as how much warmer the mean temperature at a location is than 65°F on a given day. For example, if a location experiences a mean temperature of 75°F on a certain day, there were 10 CDD (Cooling Degree Days) that day because 75 - 65 = 10.

Dew Point: A measure of atmospheric moisture. It is the temperature to which air must be cooled in order to reach saturation (which is a relative humidity of 100%). A higher dew point indicates that there is more moisture present in the air, while a lower dew point indicates that a drier air mass is in place. Temperatures often fall to the dew point around sunrise, which can lead to the formation of dew or fog. It is sometimes referred to as Dew Point Temperature.

Diurnal Temperature Range: The temperature difference between the minimum at night (low) and the maximum during the day (high). A typical temperature range in this area is around 25 degrees. It is typically much lower on cloudy days, and can only be a few degrees during a steady, all-day rain or snow event. In contrast, with sunny and very dry conditions, the temperature range can be above 40 and, on rare occasions, around 50 degrees.

Dry Line: A boundary separating moist and dry air masses. A typical dry line passage results in a sharp drop in humidity (hence the name), a clearing sky, and a wind shift from south or southeasterly to west or southwesterly. Severe and sometimes tornadic thunderstorms often develop along a dry line or in the moist air just to the east of it. Dry lines are rare in the East, but occur frequently from the Midwest down into Texas, especially in the springtime.

ECMWF: Acronym for the European Centre for Medium-Range Weather Forecasts. More often, it is used to refer to the ECMWF's medium-range numerical forecast model (computer model), which runs out to 10 days. Access to the model's precipitation forecast maps is tightly restricted, but this model typically is the most accurate in handling the overall weather pattern, as well as such things as the timing of an approaching cold front.

Eta Model: Now referred to as North Amercian Mesoscale (NAM), this is one of the operational numerical forecast models (computer models) run at NCEP. The Eta is run four times daily (every 6 hours), with forecast output out to 84 hours (3 1/2 days). It is generally considered to be the most accurate short-range model, but it has struggled of late with the timing of low-pressure systems.

Eye: The relatively calm center in a hurricane that is more than one half surrounded by the wall cloud (eye wall). The winds are light, the sky is partly cloudy or even clear (it is usually free of rain), and radar depicts it as an echo-free area within the eye wall.

Eye Wall: An organized band of clouds that immediately surrounds the center (eye) of a hurricane. This is generally the most dangerous part of a hurricane. The fiercest winds and most intense rainfall typically occur in or near the eye wall. Eye wall and wall cloud are used synonymously, but it should not be confused with a wall cloud of a thunderstorm.

Flurries: Snow flurries are an intermittent light snowfall of short duration (generally light snow showers) with no measurable accumulation (trace category).

Fog: Formed with water droplets that are suspended in the air at the Earth's surface. Fog often forms the morning after a moderate or heavy rainfall the previous evening, because there is still a lot of moisture in the air, but it typically lifts fairly quickly after sunrise. Fog is often hazardous when the visibility is reduced to 1/4 mile or less.

Freezing Fog: A suspension of numerous tiny ice crystals in the air, or water droplets at temperatures below 32°F, near the Earth's surface, which reduces horizontal visibility; also called ice fog. Freezing fog is not too common in this area, because our temperatures rarely fall below the mid or upper 20s unless we are in a cold, dry air mass, and don't have the moist conditions necessary to form fog.

Freezing Rain: Rain that falls as a liquid but freezes into glaze upon contact with the ground. Freezing rain actually falls from the cloud to the ground in the form of raindrops, just as in a typical rain shower, because temperatures in the atmosphere are just enough above freezing to support liquid precipitation. However, surfaces at or near the ground are below freezing. That allows the liquid water to freeze on contact, creating a sheet of ice.

Global Forecast System (GFS): One of the operational forecast models run at NCEP. The GFS is run four times daily (every 6 hours), with forecast output out to 384 hours (16 days). While the GFS can be useful in predicting longer-term pattern changes at times, the European model (ECMWF) typically does a much better job. The GFS forecast tends to flunctuate (or "flip-flop") a lot from one run to the next.

Ground Clutter: A pattern of radar echoes from fixed ground targets (buildings, hills, etc.) near the radar site. Ground clutter may hide or confuse precipitation echoes near the radar antenna. Our radar is located on a mountain in Floyd County, and ground clutter can sometimes be seen in Floyd and surrounding counties, especially during times of dry weather.

Hail: Precipitation in the form of irregular pellets or balls of ice, falling from a cumulonimbus (thunderstorm) cloud. A hailstone starts as a small ball of ice, falling toward the ground. However, the strong updraft (upward wind motion) in the storm pulls it back up into the cold part of the cloud. More ice accumulates, making the ball bigger. That up-and-down motion continues until it becomes heavy enough to break through the updraft and fall to the ground.

Heat Index: The Heat Index or the "Apparent Temperature" is an accurate measure of how hot it really feels when the Relative Humidity is added to the actual air temperature. The more humid it is, the hotter the air feels to the human body.

Heating Degree Days: A form of degree day used to estimate energy requirements for heating. Typically, heating degree days are calculated as how much colder the mean temperature at a location is than 65°F on a given day. For example, if a location experiences a mean temperature of 55°F on a certain day, there were 10 HDD (Heating Degree Days) that day because 65 - 55 = 10.

High Pressure System: An area of a relative pressure maximum that has diverging winds and rotates in the opposite direction as the earth (clockwise in the Northern Hemisphere, and counter-clockwise in the Southern Hemisphere). Also known as an anticyclone, high pressure systems typically bring sunny and dry conditions. In winter, strong systems with very high pressure can contain very cold, arctic air.

Hurricane: A tropical cyclone in the Atlantic Ocean, Caribbean Sea, Gulf of Mexico, or Eastern Pacific Ocean, in which the maximum 1-minute sustained surface wind is 64 knots (74 mph) or greater.

Ice Pellets: Same as Sleet; defined as pellets of ice composed of frozen or mostly frozen raindrops or refrozen, partially melted snowflakes. These pellets of ice usually bounce after hitting the ground or other hard surfaces. Heavy sleet is a relatively rare event defined as an accumulation of ice pellets covering the ground to a depth of 1/2" or more.

Infrared Satellite Imagery: This type of satellite imagery senses surface and cloud top temperatures by measuring the wavelength of electromagnetic radiation emitted from these objects. High clouds are very cold, so they appear white. Mid-level clouds are somewhat warmer, so they will be a light gray shade. Low cloud are warmer still, so they appear as a dark shade of gray or black. They are especially useful at night, because visible satellites rely on sunlight.

Jet Stream: Relatively strong winds concentrated in a narrow stream in the atmosphere, normally referring to horizontal, high-altitude winds (usually about 30,000 feet above the surface). The position and orientation of jet streams vary from day to day. General weather patterns (hot/cold, wet/dry) are related closely to the position, strength and orientation of the jet stream(s). Low pressure systems often travel along the jet stream, especially in winter.

Likely: A National Weather Service precipitation descriptor. In probability of precipitation statements, the equivalent of a 60 or 70 percent chance.

Low Pressure System: An area of a relative pressure minimum that has converging winds and rotates in the same direction as the earth (counter-clockwise in the Northern Hemisphere and clockwise in the Southern Hemisphere). Also known as a cyclone, low pressure systems typically contain clouds and precipitation.

Mostly Cloudy: When the predominant/average sky condition is covered by more than half, but not completely covered by opaque (not transparent) clouds. In other words, 5/8 to 7/8 of the sky is covered by opaque clouds. Same as Considerable Cloudiness.

Mostly Clear: When the predominant/average sky condition is covered 1/8 to 2/8 with opaque (not transparent) clouds. Sometimes referred to as Mostly Sunny if this condition is present during daylight hours.

Nowcast: A short-term weather forecast, generally out to six hours or less. This is also called a Short Term Forecast. A nowcast typically discusses the current radar picture, and what is expected to happen over the next few hours.

NWS: National Weather Service. An agency of the Federal Government within the Department of Commerce, National Oceanic and Atmospheric Administration, which is responsible for providing observations, forecasts and warnings of meteorological and hydrological events in the interest of national safety and economy. The NWS has around 130 local offices across the country; the office that serves our area is located in Blacksburg.

Occluded Front: A composite of two fronts, formed as a cold front overtakes a warm or stationary front. Two types of occlusions can form depending on the relative coldness of the air behind the cold front to the air ahead of the warm or stationary front. A cold occlusion results when the coldest air is behind the cold front and a warm occlusion results when the coldest air is ahead of the warm front.

Partly Cloudy: When the predominant/average sky condition is covered 3/8 to 4/8 with opaque (not transparent) clouds. In other words, roughly half of the sky is covered by clouds. This is the same as Partly Sunny.

Popcorn Convection: Slang for showers and thunderstorms that form on a scattered basis with little or no apparent organization, usually during the afternoon in response to diurnal heating. Individual thunderstorms typically are of the type sometimes referred to as air-mass thunderstorms: they are small, short-lived, very rarely severe, and they almost always dissipate near or just after sunset.

Precipitation: The process where water vapor condenses in the atmosphere to form water droplets that fall to the Earth as rain, sleet, snow, hail, etc. Annual precipitation amounts vary widely across the country and the world, from 4 inches in Las Vegas (and much less than that in some desert locations), to several hundred inches on some mountain peaks.

Probability of Precipitation: The probability that precipitation will be reported at a certain location during a specified period of time. This term is frequently abbreviated as "POP".

RADAR: Acronym for RAdio Detection And Ranging; a radio device or system for locating an object by means of ultrahigh-frequency radio waves reflected from the object and received, observed, and analyzed by the receiving part of the device in such a way that characteristics (as distance and direction) of the object may be determined.

Relative Humidity (RH): A dimensionless ratio, expressed in percent, of the amount of atmospheric moisture present relative to the amount that would be present if the air were saturated. RH by itself does not directly indicate the actual amount of atmospheric moisture present. In other words, the RH doesn't tell you anything about the actual moisture content of the air, and is generally used to make it easier for the public to understand how moist the air is.

Scattered: A National Weather Service precipitation descriptor predicting areal coverage of convective weather affecting 30 percent to 50 percent of a forecast zone.

Severe Thunderstorm: A thunderstorm that produces a tornado, winds of at least 58 mph (50 knots), and/or hail at least 3/4" in diameter. Structural wind damage may imply the occurrence of a severe thunderstorm. A thunderstorm wind equal to or greater than 40 mph (35 knots) and/or hail of at least 1/2" is defined as approaching severe.

Severe Thunderstorm Warning: Issued when either a severe thunderstorm is indicated by radar or a spotter reports a thunderstorm that meets severe criteria. People in the affected area should seek safe shelter immediately. Severe thunderstorms can produce tornadoes with little or no advance warning. Lightning frequency is NOT a criteria for issuing a Severe Thunderstorm Warning. They can be issued without a Severe Thunderstorm Watch being already in effect.

Severe Thunderstorm Watch: Issued by the NWS when conditions are favorable for the development of severe thunderstorms in and close to the watch area. The size of the watch can vary depending on the weather situation. They are usually issued for a duration of 4 to 8 hours. They are normally issued well in advance of the actual occurrence of severe weather. During the watch, people should be prepared to move a place of safety if threatening weather approaches.

Showers (SH): A descriptor used to qualify precipitation characterized by the suddenness with which they start and stop, by the rapid changes of intensity, and usually by rapid changes in the appearance of the sky. In simple terms, you can expect on-and-off rain (or snow) throughout the forecast period.

Slight Chance: In probability of precipitation statements, usually equivalent to a 20 percent chance.

Stationary Front: A front between warm and cold air masses that is moving very slowly or not at all. If a front stalls and becomes stationary nearby (which most often happens in the springtime), it typically causes increased cloud cover and periods of rain for up to several days.

Straight-line Winds: Generally, any wind that is not associated with rotation, used mainly to differentiate them from tornadic winds. Straight-line winds have been known to cause significant structural damage, but move in a straight line (as the name implies) as opposed to tornadic winds, which move in a circular rotation.

Subtropical Cyclone: A low pressure system that has characteristics of both tropical and extratropical (non-tropical) cyclones. These typically have maximum sustained winds occurring at a radius of about 100 miles or more from the center. In comparison to tropical cyclones, such systems have a relatively broad zone of maximum winds that is located farther from the center, and typically have a less symmetric wind field and distribution of convection.

Sunny: The sky condition when there are no opaque (not transparent) clouds. Same as Clear, which is typically used in nighttime forecasts, but can also be used during the day.

Tornado: A violently rotating column of air, usually in a cumulonimbus cloud, with circulation reaching the ground. It nearly always starts as a funnel cloud (which is similar to a tornado but doesn't actually touch the ground) and may be accompanied by a loud roaring noise. On a local scale, it is the most destructive of all atmospheric phenomena.

Tornado Warning: This is issued when a tornado is indicated by radar or sighted by spotters; therefore, people in the affected area should seek safe shelter immediately. They can be issued without a Tornado Watch being already in effect.

Tornado Watch: This is issued by the NWS when conditions are favorable for the development of tornadoes in and close to the watch area. Their size can vary depending on the weather situation. They are usually issued for a duration of 4 to 8 hours. They normally are issued well in advance of the actual occurrence of severe weather. During the watch, people should review tornado safety rules and be prepared to move a place of safety if threatening weather approaches.

Tropical Depression: A tropical cyclone in which the maximum 1-minute sustained surface wind is 33 knots (38 mph) or less. When a system first becomes a depression, it is given a number.

Tropical Storm: A tropical cyclone in which the maximum 1-minute sustained surface wind ranges from 34 to 63 knots (39 to 73 mph). When a depression turns into a storm, it is given a name.

Trough: An elongated area of relatively low atmospheric pressure, usually not associated with a closed circulation, and thus used to distinguish from a closed low. The opposite of ridge. Troughs tend to bring cold air to a large area, while ridges tend to bring warmer air to a large area.

Typhoon: A tropical cyclone in the Western Pacific Ocean in which the maximum 1-minute sustained surface wind is 64 knots (74 mph) or greater. Essentially, a typhoon is equivalent to a hurricane; it's just in a different part of the world.

Universal Time (UT): By international agreement, the local time at the prime meridian, which passes through Greenwich, England. Prior to 1972, this time was called Greenwich Mean Time (GMT) but is now referred to as Coordinated Universal Time or Universal Time Coordinated (UTC). It is a coordinated time scale, maintained by the Bureau International des Poids et Mesures (BIPM). It is also known a "Z time" or "Zulu Time".

Upper Level: In weather observing, the term applies to the portion of the atmosphere that is above the lower troposphere, generally 850 mb (roughly 5000 feet above the surface) and above. Temperatures in the upper levels of the atmosphere are especially critical in determining the type of precipitation that falls in an area during a winter storm.

Variable Wind (Direction): A condition when the wind direction fluctuates by 60° (1/6 of the way around a compass) or more during the 2-minute evaluation period and the wind speed is greater than 6 knots; or the direction is variable and the wind speed is less than 6 knots.

Virga: Streaks or wisps of precipitation falling from a cloud but evaporating before reaching the ground. In certain cases, shafts of virga may precede a microburst.

Visibility: The distance at which a given standard object can be seen and identified with the unaided eye. For many locations (including Martinsville), the maximum visibility that can be reported is 10 miles. This is 20 miles in some places, and can be as high as 100 miles on a clear day at the top of Mount Washington, NH.

Visible Satellite Imagery: This type of satellite imagery uses reflected sunlight to see things in the atmosphere and on the Earth's surface. Clouds and fresh snow are excellent reflectors, so they appear white on the imagery. Clouds can be distinguished from snow, because clouds move and snow does not move. Meanwhile, the ground reflects less sunlight, so it appears black on the imagery. Since this imagery relies on reflected imagery, it cannot be used during night.

Warm Front: A transition zone between a mass of warm air and the colder air it is replacing. It is indicated on a map by a red line with half circles (which point in the direction that the front is moving). Opposite of a cold front, warmer air moves in at the upper levels of the atmosphere first, then at the surface after that. A "wedge" has a similar effect as it erodes from the top of the atmosphere down to the surface.

Warning: A warning is issued when a hazardous weather or hydrologic event is occurring, is imminent, or has a very high probability of occurring. A warning is used for conditions posing a threat to life or property. Warnings can mean different things, though. A tornado warning means that a tornado is likely on the ground, while a blizzard or hurricane warning means that such conditions are expected to occur within 12 to 24 hours.

Watch: A watch is used when the risk of a hazardous weather or hydrologic event has increased significantly, but its occurrence, location, and/or timing is still uncertain. It is intended to provide enough lead time so that those who need to set their plans in motion can do so.

Wind Chill Factor: A formula for determining the effect that a combination of cold temperatures and high winds have on the human body. Increased wind speeds accelerate heat loss from exposed skin. No specific rules exist for determining when wind chill becomes dangerous. As a general rule, the threshold for potentially dangerous wind chill conditions is about -20°F.

Windy: 20 to 30 mph winds; breezy: 15 to 25 mph winds; calm: a weather condition when no air motion (wind) is detected.

Zonal Flow: Large-scale atmospheric flow in which the east-west component is dominant. The accompanying meridional (north-south) component often is weaker than normal. In simple terms, the national temperature map during a zonal flow typically features nearly straight horizontal lines across the country. For example, all the northern states may be in the 40s, with 80s across all the southern states.

Climate: The composite or generally prevailing weather conditions of a region, throughout the year, averaged over a series of years.