More Wx Definitions & Terms

More Weather Terms & Definitions (Updated 20210808) COMBINED SOURCE WX GLOSSARY https://w1.weather.gov/glossary/ http://www.selstonweather.info/wxglossary.php https://en.wikipedia.org/wiki/Classifications_of_snow

Air Quality Index (AQI) - The United States Environmental Protection Agency (EPA) has developed an Air Quality Index that is used to report air quality. This AQI is divided into six categories indicating increasing levels of health concern. An AQI value over 300 represents hazardous air quality and below 50 the air quality is good. The AQI is based on the five "criteria" pollutants regulated under the Clean Air Act: ground-level ozone, particulate matter, carbon monoxide, sulfur dioxide, and nitrogen dioxide. The EPA has established National Ambient Air Quality Standards (NAAQS) for each of these pollutants in order to protect public health. An AQI value of 100 generally corresponds to the level of the NAAQS for the pollutant. The Clean Air Act (USA) (1990) requires the EPA to review its National Ambient Air Quality Standards every five years to reflect evolving health effects information. The Air Quality Index is adjusted periodically to reflect these changes. (Wikipedia) Clear - In terms of octants, with the sky having 1/8th or less of opaque cloud coverage. “Clear” is the term mostly used to describe night time sky conditions. Daytime term mostly used to describe a clear sky is “Sunny.” Aviation terminology uses the term “FEW (trace)” for cloud coverage of 1/8th to 2/8th sky cover. Cloud - A visible cluster of tiny water and/or ice particles in the atmosphere. Cloud Base - For a given cloud or cloud layer, it is the lowest level in the atmosphere where cloud particles are visible Cloudy - The state of the sky when When 7/8ths to 8/8ths of the sky is covered by clouds. Aviation forecasts utilize the term OVC (“Overcast”) with 8/8ths of opaque sky coverage. Cold Front - A narrow transition zone separating advancing colder air from retreating warmer air. The air behind a cold front is cooler and typically drier than the air it is replacing. Dense Fog - a fog in which the visibility is less than one-quarter mile. Drizzle - Small, slowly falling water droplets, with diameters between 0.2 and 0.5 millimeters. Precipitation consisting of numerous minute droplets of water less than 0.5 mm (500 micrometers) in diameter. Fair - describes weather in which there is less than 4/10ths of opaque cloud cover, no precipitation, and there is no extreme visibility, wind or temperature conditions. Flurries - Snow flurries are an intermittent light snowfall of short duration (generally light snow showers) with no measurable accumulation (trace category). Freezing Drizzle - A drizzle that falls as a liquid but freezes into glaze or rime upon contact with the cold ground or surface structures. Freezing Rain - Rain that falls as a liquid but freezes into glaze upon contact with the ground. Rain that freezes on objects such as trees, cars and roads, forming a coating or glaze of ice. Temperatures at higher levels are warm enough for rain to form, but surface temperatures are below 32 degrees Fahrenheit, causing the rain to freeze on impact. Front - The boundary or transition zone between two different air masses. The basic frontal types are cold fronts, warm fronts and occluded fronts. Occluded fronts usually form around mature low pressure areas. There are two types of occlusion, warm and cold: In a cold occlusion, the cold air mass overtaking the warm front is colder than the cool air ahead of the warm front, and plows under both air masses. In a warm occlusion, the cold air mass overtaking the warm front is warmer than the cool air ahead of the warm front, and rides over the colder air mass while lifting the warm air. Graupel - A type of winter precipitation that’s a mix of snow and hail. Also known as “snow pellets, soft hail, small hail, tapioca snow, rimed snow, and ice balls.” The World Meteorological Organization defines small hail as snow pellets encapsulated by ice, a precipitation halfway between graupel and hail. To tell the difference between graupel and hail, you simply have to touch a graupel ball. Graupel pellets typically fall apart when touched or when they hit the ground. Hail is formed when layers of ice accumulate and are very hard as a result. Graupel commonly forms in high-altitude climates and is both denser and more granular than ordinary snow, due to its rimed exterior. Macroscopically, graupel resembles small beads of polystyrene. Gust - A brief sudden increase in wind speed. Generally the duration is less than 20 seconds and the fluctuation greater than 10 mph. Heating Degree Day - A form of degree day used to estimate the required energy for heating. One heating degree day occurs for each degree the daily mean temperature is below 65 degrees Fahrenheit. Hail - Showery precipitation in the form of irregular pellets or balls of ice more than 5 mm in diameter, falling from a cumulonimbus cloud. Haze - Haze is traditionally an atmospheric phenomenon in which dust, smoke, and other dry particulates obscure the clarity of the sky. The World Meteorological Organization manual of codes includes a classification of horizontal obscuration into categories of fog, ice fog, steam fog, mist, haze, smoke, volcanic ash, dust, sand, and snow. Sources for haze particles include farming (ploughing in dry weather), traffic, industry, and wildfires. Seen from afar (e.g. an approaching airplane) and depending on the direction of view with respect to the Sun, haze may appear brownish or bluish, while mist tends to be bluish grey. Whereas haze often is thought of as a phenomenon of dry air, mist formation is a phenomenon of humid air. However, haze particles may act as condensation nuclei for the subsequent formation of mist droplets; such forms of haze are known as "wet haze." Heavy snow - Depending on the region of the USA, this generally means that four or more inches of snow has accumulated in 12 hours, or six or more inches of snow in 24 hours. This generally means... • snowfall accumulating to 4" or more in depth in 12 hours or less; or • snowfall accumulating to 8" or more in depth in 24 hours or less • also see “Snow Intensity In forecasts, snowfall amounts are expressed as a range of values, e g.. "8 to 12 inches." However, in heavy snow situations where there is considerable uncertainty concerning the range of values, more appropriate phrases are used, such as "...up to 12 inches..." or alternatively "...8 inches or more...". Isobar - A line of equal barometric pressure on a weather map. Jet Stream - Strong winds concentrated within a narrow band in the upper atmosphere. It normally refers to horizontal, high -altitude winds. The jet stream often "steers" surface features such as front and low pressure systems. Knot - A measure of speed. It is one nautical mile per hour (1.15 mph). A nautical mile is one minute of one degree of latitude. Low - An area of low pressure, usually accompanied by cyclonic and inward wind flow. Also known as a cyclone. METAR - A weather observation near ground level. It may include date and time, wind, visibility, weather and obstructions to vision, sky condition, temperature and dew point, sea level pressure, precipitation amount and other data used for aircraft operations. Millibar - A metric unit of atmospheric pressure. 1 mb = 100 Pa (Pascal). Normal surface pressure is approximately 1013 millibars. Mist - is a phenomenon caused by small droplets of water suspended in air. Physically, it is an example of a dispersion. It is most commonly seen where warm, moist air meets sudden cooling, such as in exhaled air in the winter, or when throwing water onto the hot stove of a sauna. It can be created artificially with aerosol canisters if the humidity and temperature conditions are right. It can also occur as part of natural weather, when humid air cools rapidly, notably when the air comes into contact with surfaces that are much cooler than the air. Mostly Clear - When 1/8th to 3/8ths of the night sky is covered by with opaque (not transparent) clouds. The sky is referred to as “Mostly Sunny” if this condition is present during daylight hours. Aviation terminology for this sky condition is FEW (“trace”). Mostly Cloudy - When 5/8ths to 7/8ths of the night time sky is covered by with opaque (not transparent) clouds. Daylight hours would us the term “Considerable Cloudiness.” Aviation terminology for this degree of sky coverage is BKN (“broken”). Overcast - Aviation term for a sky condition having 9/10 or more of the sky covered with opaque clouds. Abbreviated as OVC. An official sky cover classification for aviation weather observations, when the sky is completely covered by an obscuring phenomenon. This is applied only when obscuring phenomenon aloft are present—that is not when obscuring phenomenon are surface-based, such as fog. Corresponds with NWS definition of “Cloudy,” with 8/8ths coverage. Partly Cloudy - When the night time sky is covered with from 3/8ths to 5/8ths of opaque (not transparent) clouds. The daytime term used to describe this sky condition is “Partly Sunny.” Aviation sky conditions are described as SCT (“scattered”) with sky coverage from 3/8ths to 4/8ths. Partly Sunny - Similar to partly cloudy. Used to emphasize daytime sunshine. Precipitation - Liquid or solid water that falls from the atmosphere and reaches the ground. Pressure - The force exerted by the interaction of the atmosphere and gravity. Also known as atmospheric pressure. Relative Humidity - The amount of water vapor in the air, compared to the amount the air could hold if it was totally saturated. (Expressed as a percentage). Sky Condition - The state of the sky in terms of such parameters as sky cover, layers and associated heights, ceiling, and cloud types. Sky Cover - The amount of the sky which is covered by clouds or obscurations in contact with the surface. Sky Condition - Terminology used by NWS in forecasts to describe the predominant average sky condition based upon octants (eighths) of the sky covered by opaque (not transparent) clouds. Sky Condition Cloud Coverage, Octants Cloud Covarage, Percentage Clear/Sunny 0/8 to 1/8 00.0% - 12.5% Mostly Clear/Mostly Sunny 1/8 to 3/8 12.5% - 37.5% Partly Cloudy/Partly Sunny 3/8 to 5/8 37.5% - 62.5% Mostly Cloudy/Considerable Cloudiness 5/8 to 7/8 62.5% - 87.5% Cloudy 7/8 to 8/8 87.5% - 100.0% Fair (mainly for night) Less than 4/10 opaque clouds. No precipitation, no extremes of visibility/temperature/wind Sleet - (PL) - Sleet is defined as pellets of ice composed of frozen or mostly frozen raindrops or refrozen partially melted snowflakes. Transparent or translucent precipitation in the form of pellets of ice that are smaller than 5 mm (.2 in). These pellets of ice usually bounce after hitting the ground or other hard surfaces. “Heavy” sleet is a relatively rare event and can be defined as an accumulation of ice pellets covering the ground at a rate of 0.30”/hr or more. Snow - Frozen precipitation composed of ice particles in complex hexagonal patterns. Snow forms in cold clouds by the direct transfer of water vapor to ice. Snow Depth - The vertical height of frozen precipitation on the ground. For this purpose, frozen precipitation includes ice pellets, glaze, hail, any combination of these, and sheet ice formed directly or indirectly from precipitation. Snow Flurries - Light snow showers, usually of an intermittent nature and short duration with no measurable accumulation. Snow Intensity - In the US, intensity of snowfall is characterized by visibility through the falling precipitation as follows: • Light Snow - visibility of 1 km (1,100 yards) or greater. • Moderate Snow - visibility between 1km (1,100 yards) and 0.5 km (550 yards). • Heavy Snow - visibility of less than 0.5 km (550 yards). Snow Shower - Snow falling at varying intensities for brief periods of time. Some accumulation is possible. Snowfall - The depth of new snow that has accumulated since the previous day or since the previous observation. Station Pressure - The pressure that is read from a barometer but is not adjusted to sea level. Atmospheric pressure. Temperature-humidity-sun-wind (THSW) index - Like Heat Index, the THSW Index uses humidity and temperature to calculate an apparent temperature. In addition, THSW incorporates the heating effects of solar radiation and the cooling effects of wind (like wind chill) on our perception of temperature. Warm Front - A narrow transitions zone separating advancing warmer air from retreating cooler air. The air behind a warm front is warmer and typically more humid than the air it is replacing. Warning - Forecast issued when a particular weather or flood hazard is "imminent" or already occurring (e.g., tornado warning, flash flood warning). A warning is used for conditions posing a threat to life or property. Wind - Air in motion relative to the surface of the earth. Wind Chill - The additional cooling effect resulting from wind blowing on bare skin. The wind chill is based on the rate of heat loss from exposed skin caused by the combined effects of wind and cold. The (equivalent) wind chill temperature is the temperature the body "feels" for a certain combination of wind and air temperature Wind Direction - The direction from which the wind is blowing, the compass direction from which the wind is coming from. Wind Speed - The rate at which air is moving horizontally past a given point. It may be a 2 -minute average speed (reported as wind speed) or an instantaneous speed (reported as a peak wind speed, or gust).

More Weather Terms & Definitions (Updated 20210808) COMBINED SOURCE WX GLOSSARY https://w1.weather.gov/glossary/ http://www.selstonweather.info/wxglossary.php

Air Quality Index (AQI) - The United States Environmental Protection Agency (EPA) has developed an Air Quality Index that is used to report air quality. This AQI is divided into six categories indicating increasing levels of health concern. An AQI value over 300 represents hazardous air quality and below 50 the air quality is good. The AQI is based on the five "criteria" pollutants regulated under the Clean Air Act: ground-level ozone, particulate matter, carbon monoxide, sulfur dioxide, and nitrogen dioxide. The EPA has established National Ambient Air Quality Standards (NAAQS) for each of these pollutants in order to protect public health. An AQI value of 100 generally corresponds to the level of the NAAQS for the pollutant. The Clean Air Act (USA) (1990) requires the EPA to review its National Ambient Air Quality Standards every five years to reflect evolving health effects information. The Air Quality Index is adjusted periodically to reflect these changes. (Wikipedia) Clear - In terms of octants, with the sky having 1/8th or less of opaque cloud coverage. Cloud - A visible cluster of tiny water and/or ice particles in the atmosphere. Cloud Base - For a given cloud or cloud layer, it is the lowest level in the atmosphere where cloud particles are visible Cloudy - The state of the sky when When 7/8ths to 8/8ths of the sky is covered by clouds. Aviation forecasts utilize the term OVC (“Overcast”) with 8/8ths of opaque sky coverage. Cold Front - A narrow transition zone separating advancing colder air from retreating warmer air. The air behind a cold front is cooler and typically drier than the air it is replacing. Dense Fog - a fog in which the visibility is less than one- quarter mile. Drizzle - Small, slowly falling water droplets, with diameters between 0.2 and 0.5 millimeters. Precipitation consisting of numerous minute droplets of water less than 0.5 mm (500 micrometers) in diameter. Fair - describes weather in which there is less than 4/10ths of opaque cloud cover, no precipitation, and there is no extreme visibility, wind or temperature conditions. Flurries - Snow flurries are an intermittent light snowfall of short duration (generally light snow showers) with no measurable accumulation (trace category). Freezing Drizzle - A drizzle that falls as a liquid but freezes into glaze or rime upon contact with the cold ground or surface structures. Freezing Rain - Rain that falls as a liquid but freezes into glaze upon contact with the ground. Rain that freezes on objects such as trees, cars and roads, forming a coating or glaze of ice. Temperatures at higher levels are warm enough for rain to form, but surface temperatures are below 32 degrees Fahrenheit, causing the rain to freeze on impact. Front - The boundary or transition zone between two different air masses. The basic frontal types are cold fronts, warm fronts and occluded fronts. Occluded fronts usually form around mature low pressure areas. There are two types of occlusion, warm and cold: In a cold occlusion, the cold air mass overtaking the warm front is colder than the cool air ahead of the warm front, and plows under both air masses. In a warm occlusion, the cold air mass overtaking the warm front is warmer than the cool air ahead of the warm front, and rides over the colder air mass while lifting the warm air. Gust - A brief sudden increase in wind speed. Generally the duration is less than 20 seconds and the fluctuation greater than 10 mph. Heating Degree Day - A form of degree day used to estimate the required energy for heating. One heating degree day occurs for each degree the daily mean temperature is below 65 degrees Fahrenheit. Hail - Showery precipitation in the form of irregular pellets or balls of ice more than 5 mm in diameter, falling from a cumulonimbus cloud. Heavy snow - Depending on the region of the USA, this generally means that four or more inches of snow has accumulated in 12 hours, or six or more inches of snow in 24 hours. This generally means... • snowfall accumulating to 4" or more in depth in 12 hours or less; or • snowfall accumulating to 8" or more in depth in 24 hours or less In forecasts, snowfall amounts are expressed as a range of values, e g.. "8 to 12 inches." However, in heavy snow situations where there is considerable uncertainty concerning the range of values, more appropriate phrases are used, such as "...up to 12 inches..." or alternatively "...8 inches or more...". Isobar - A line of equal barometric pressure on a weather map. Jet Stream - Strong winds concentrated within a narrow band in the upper atmosphere. It normally refers to horizontal, high -altitude winds. The jet stream often "steers" surface features such as front and low pressure systems. Knot - A measure of speed. It is one nautical mile per hour (1.15 mph). A nautical mile is one minute of one degree of latitude. Low - An area of low pressure, usually accompanied by cyclonic and inward wind flow. Also known as a cyclone. METAR - A weather observation near ground level. It may include date and time, wind, visibility, weather and obstructions to vision, sky condition, temperature and dew point, sea level pressure, precipitation amount and other data used for aircraft operations. Millibar - A metric unit of atmospheric pressure. 1 mb = 100 Pa (Pascal). Normal surface pressure is approximately 1013 millibars. Mostly Clear - When 1/8th to 3/8ths of the night sky is covered by with opaque (not transparent) clouds. The sky is referred to as “Mostly Sunny” if this condition is present during daylight hours. Aviation terminology for this sky condition is FEW (“trace”) Mostly Cloudy - When 5/8ths to 7/8ths of the night time sky is covered by with opaque (not transparent) clouds. Daylight hours would us the term “Considerable Cloudiness.” Aviation terminology for this degree of sky coverage is BKN (“broken”). Overcast - Aviation term for a sky condition having 9/10 or more of the sky covered with opaque clouds. Abbreviated as OVC. An official sky cover classification for aviation weather observations, when the sky is completely covered by an obscuring phenomenon. This is applied only when obscuring phenomenon aloft are present—that is not when obscuring phenomenon are surface-based, such as fog. Corresponds with NWS definition of “Cloudy,” with 8/8ths coverage. Partly Cloudy - When the night time sky is covered with from 3/8ths to 5/8ths of opaque (not transparent) clouds. The daytime term used to describe this sky condition is “Partly Sunny.” Aviation sky conditions are described as SCT (“scattered”) with sky coverage from 3/8ths to 4/8ths. Partly Sunny - Similar to partly cloudy. Used to emphasize daytime sunshine. Precipitation - Liquid or solid water that falls from the atmosphere and reaches the ground. Pressure - The force exerted by the interaction of the atmosphere and gravity. Also known as atmospheric pressure. Relative Humidity - The amount of water vapor in the air, compared to the amount the air could hold if it was totally saturated. (Expressed as a percentage). Sky Condition - The state of the sky in terms of such parameters as sky cover, layers and associated heights, ceiling, and cloud types. Sky Cover - The amount of the sky which is covered by clouds or obscurations in contact with the surface. Sky Condition - Used by NWS in a forecast to describe the predominant average sky condition based upon octants (eighths) of the sky covered by opaque (not transparent) clouds. Sky Condition Cloud Coverage Clear / Sunny 0/8 to 1/8 Mostly Clear/Mostly Sunny 1/8 to 3/8 Party Cloudy/Partly Sunny 3/8 to 5/8 Mostly Cloudy/Considerable Cloudiness 5/8 to 7/8 Cloudy 7/8 to 8/8 Fair (mainly for night) Less than 4/10 opaque clouds, no precipitation, no extremes of visibility/ temperature/wind Sleet - (PL) - Sleet is defined as pellets of ice composed of frozen or mostly frozen raindrops or refrozen partially melted snowflakes. Transparent or translucent precipitation in the form of pellets of ice that are smaller than 5 mm (.2 in). These pellets of ice usually bounce after hitting the ground or other hard surfaces. “Heavy” sleet is a relatively rare event and can be defined as an accumulation of ice pellets covering the ground at a rate of 0.30”/hr or more. Snow - Frozen precipitation composed of ice particles in complex hexagonal patterns. Snow forms in cold clouds by the direct transfer of water vapor to ice. Snow Depth - The vertical height of frozen precipitation on the ground. For this purpose, frozen precipitation includes ice pellets, glaze, hail, any combination of these, and sheet ice formed directly or indirectly from precipitation. Snow Flurries - Light snow showers, usually of an intermittent nature and short duration with no measurable accumulation. Snow Shower - Snow falling at varying intensities for brief periods of time. Some accumulation is possible. Snowfall - The depth of new snow that has accumulated since the previous day or since the previous observation. Station Pressure - The pressure that is read from a barometer but is not adjusted to sea level. Atmospheric pressure. Temperature-humidity-sun-wind (THSW) index - Like Heat Index, the THSW Index uses humidity and temperature to calculate an apparent temperature. In addition, THSW incorporates the heating effects of solar radiation and the cooling effects of wind (like wind chill) on our perception of temperature. Warm Front - A narrow transitions zone separating advancing warmer air from retreating cooler air. The air behind a warm front is warmer and typically more humid than the air it is replacing. Warning - Forecast issued when a particular weather or flood hazard is "imminent" or already occurring (e.g., tornado warning, flash flood warning). A warning is used for conditions posing a threat to life or property. Wind - Air in motion relative to the surface of the earth. Wind Chill - The additional cooling effect resulting from wind blowing on bare skin. The wind chill is based on the rate of heat loss from exposed skin caused by the combined effects of wind and cold. The (equivalent) wind chill temperature is the temperature the body "feels" for a certain combination of wind and air temperature Wind Direction - The direction from which the wind is blowing, the compass direction from which the wind is coming from. Wind Speed - The rate at which air is moving horizontally past a given point. It may be a 2 -minute average speed (reported as wind speed) or an instantaneous speed (reported as a peak wind speed, or gust).