Kirk's Weather Zone Definitions

Term	Definition
Acid Rain	Cloud droplets or raindrops combining with gaseous pollutants, such as oxides of sulfur or nitrogen, to make falling rain (or snow) acidic (pH less than 5.6). If fog droplets combine with such pollutants it becomes acid fog.
Advection	The horizontal transfer of any atmospheric property (hot air, cold air, moist air, etc.) by the wind. "Warm Air Advection" is the term used to describe warmer air blowing into a region.
Advection Fog	Occurs when warm, moist air moves over a cold surface and the air cools to below its dew point... condensing the air's moisture and creating fog.
Aerosols	Tiny suspended solid particles (dust, smoke, etc.) or liquid droplets that enter the atmosphere from either natural or human (anthropgenic) sources, such as the burning of fossil fuels. Aerosols can affect the way the skies appear... for example aerosols from a volcanic eruption or a forest fire can turn the sunrises and sunsets extraordinarily red and orange.
Aerovane	A wind instrument that indicates or records both wind speed and wind direction.
Air Density	The ratio of the mass of a substance to the volume it occupies. In oceanography, it is equivalent to specific gravity and represents the ratio of the weight of a given volume of sea water to that of an equal volume of distilled water at 4.0 degrees Celsius or 39.2
Air Mass	A large body of air that has similar horizontal temperature and moisture characteristics.
Air Mass Weather	A persistent type of weather that may last for several days (up to a week or more). It occurs when an area comes under the influence of a particular air mass.
Air Pressure	The pressure exerted by the weight of the air above a given point, usually expressed in millibars (mb) or inches of mercury (Hg). High pressure usually indicates stable weather... while low pressure (as in a hurricane) indicates unstable and stormy weather.
Altimeter	An instrument that indicates the altitude of an object above a fixed level. Pressure altimeters use an aneroid barometer with a scale graduated in altitude instead of pressure. Altimeters are mainly used in aircraft... for hiking and orienteering.
Altocumulus	A middle cloud, usually white or gray. Often occurs in layers or patches with wavy, rounded masses or rolls.
Altocumulus Castellanus	An altocumulus cloud showing vertical development. Individual cloud elements have tower-like tops, often in the shape of tiny castles.
Altostratus	A middle cloud composed of gray or bluish sheets or layers of uniform appearance. In the thinner regions, the sun or moon usually appears dimly visible.
Analysis	The drawing and interpretation of the patterns of various weather elements on a surface or upper-air chart.
Anemometer	An instrument designed to measure wind speed and direction.
Aneroid Barometer	An instrument designed to measure atmospheric pressure. It is usually a small box-like object that is completely sealed. It expands and contracts with varying pressure. The aneriod's changes in size are measured to calculate pressure.
Anitcyclone	An area of high pressure around which the wind blows clockwise in the Northern Hemisphere and counterclockwise in the Southern Hemisphere.
Arcus Cloud	See Roll Cloud.
Arid Climate	An extremely dry climate. Drier than the semi-arid climate. Often referred to as a "true desert" climate. I.e. Antarctica, the Mojave and Sonoran deserts in the Southwest U.S., and the Sahara Desert in Africa.
Atmosphere	The envelope of gases that surround a planet and are held to it by the planet's gravitational attraction. The earth's atmosphere is mainly composed of Nitrogen and Oxygen.
Atmospheric Models	Computer simulations of the atmosphere's behavior by mathematical equations or physical models. These models help forecasters make their predictions.
Atmospheric Stagnation	A condition of light winds and poor vertical mixing (no rising air) that can lead to a high concentration of pollutants. Air stagnations are most often associated with fair weather, an inversion, and the sinking air of a high-pressure area.
Aurora	Glowing light display in the nighttime sky caused by excited gases in the upper atmosphere giving off light. In the Northern Hemisphere it is called the "aurora borealis" (northern lights); in the Southern Hemisphere, the "aurora australis" (southern lights). Also a city near Denver.
Autumnal Equinox	The equinox at which the sun approaches the Southern Hemisphere and passes directly over the equator. Occurs around September 23.
Average	This station is set up for Daily Average beginning at midnight and lasting 24 hours.
Backdoor Cold Front	A cold front moving South or Southwest... opposite of its "normal" Easterly movement.
Ball Lightning	A rare form of lightning that may consist of a reddish, luminous ball of electricity or charged air.
Barometric Pressure	The pressure exerted by the atmosphere as a consequence of gravitational attraction exerted upon the "column" of air lying directly above the point in question. The measurement can be expressed in several ways. One is in millibars. Another is in inches or millimeters of mercury (Hg). Also known as atmospheric pressure.
Beaufort Wind Scale	A system of estimating and reporting wind speeds. It is based on the Beaufort Force or Number, which is composed of the wind speed, a descriptive term, and the visible effects upon land objects and/or sea surfaces. The scale was devised by Sir Francis Beaufort (1777-1857), hydrographer to the British Royal Navy.
Cirrocumulus	A high cloud that appears as a white patch of clouds without shadows. It consists of very small elements in the form of grains or ripples.
Cirrus Clouds	A high cloud composed of ice crystals in the form of thin, white, feather like clouds in patches, filaments, or narrow bands.
Cloud Base	For a given cloud or cloud layer. The lowest level in the atmosphere at which the air contains a perceptible quantity of cloud particles. The instrumentation used for this site (Virtual Weather Station) estimates the cloud base on temperature and humidity readings, using the following equation: Cloud Base (ft) = 250(Temperature - Dew Point)
Contrail	A cloud like streamer frequently seen forming behind aircraft flying in clear, cold, humid air.
Cooling Degree Day	A cooling degree day is given for each degree that the daily mean temperature departs above the baseline of 75 degrees a given temperature It is used to estimate the energy requirements, and is an indication of fuel consumption for air conditioning or refrigeration. Refer to degree day or heating degree day.
Corona	A series of colored rings concentrically surrounding the disk of the sun or moon. Smaller than the halo, the corona is caused by the diffraction of light around small water droplets of uniform size.
Crepuscular Rays	Alternating light and dark bands of light that appear to fan out from the sun's position, usually at twilight.
Cumulonimbus	An exceptionally dense and vertically developed cloud, often with a top in the shape of an anvil. The cloud is frequently accompanied by heavy showers, lighting, thunder and sometimes hail. It is also known as a thunderstorm. In Colorado... dangerous lightning, strong winds and hail are the most common types of weather associated with a thunderstorm.
Cumulus	A cloud in the form of individual, detached domes or towers that are usually dense and well defined. It has a flat base with a bulging upper part that often resembles cauliflower. Cumulus clouds of fair weather are calls "cumulus humilis". Those that exhibit much vertical growth are called "cumulus congestus" or "towering cumulus"... which can form thunderstorms or "cumulonibus" clouds.
Cutoff Low	A cold upper-level low that has become displaced out of the basic westerly flow of the jet stream. It usually lies to the south of the jet... creating a slow-moving area of unstable weather.
Cyclone	An area of low pressure around which the winds blow counterclockwise in the Northern Hemisphere and clockwise in the Southern Hemisphere.
Density Altitude	The altitude at which a given density is found in the standard atmosphere. Used in aviation, it is computed from the station pressure at takeoff and the virtual temperature at the particular altitude under consideration.
Deposition	A process that occurs in subfreezing air when water vapor changes directly to ice without becoming a liquid first.
Dew	Water that has condensed onto objects near the ground when their temperatures have fallen below the dew point of the surface air.
Dew Point	The temperature to which a sample of air must be cooled, while the mixing ratio and barometric pressure remain constant, in order to attain saturation by water vapor. When this temperature is below O�C, it is sometimes called the frost point. This reading is taken every second and updated on the Weather Zone every 15 minutes. Current Temperature and Dew Point Graph
Diffraction	The bending of light around objects, such as cloud and fog droplets, producing fingers of light and dark or colored bands.
Dispersion	The separation of white light into its different component wavelengths. I.e. creating a rainbow by passing sunlight through a prism.
Divergence	An atmospheric conditions that exists when the winds cause a horizontal net outflow of air from a specific region.
Doppler Radar	Weather radar that measures direction and speed of a moving object, such as drops of precipitation, by determining whether atmospheric motion is horizontally toward or away from the radar. Using the Doppler effect, it measures the velocity of particles. Named for J. Christian Doppler, an Austrian physicist, who in 1842 explained why the whistle of an approaching train had a higher pitch than the same whistle when the train was going away
Downburst	A severe localized down draft of wind that can be experienced beneath a severe thunderstorm.
Drizzle	Small water drops between 0.2 and 0.5 millimeters in diameter that fall slowly and reduce visibility more than light rain.
Drought	A period of abnormally dry weather sufficiently long enough to cause serious effects on agriculture and other activities in the affected area.
Dry Line	A boundary that separates warm dry air from warm moist air. It usually represents a zone of instability along which thunderstorms form. The boundary is most often found between Pueblo and Lamar during the summertime.
Dust Devil	Also called a "whirlwind". A small but rapidly rotating wind made visible by the dust, sand and debris it picks up from the surface. It develops best on clear, dry, hot afternoons. It extends from the ground up but does NOT connect with a cumulus clouds. A Dust Devil is often confused with a tornado... whose vortex extends from the ground INTO a cumulus cloud.
El Nino	An extensive ocean warming that begins along the coast of Peru and Ecuador. Major El Nino events occur once every 3 to 7 years as a current of nutrient-poor tropical water moves southward along the coast of South America. El Nino is typically followed by an effect called "La Nina".
Evaporation	The process by which a liquid changes into a gas.
Extratropical Cyclone	A cyclonic storm that most often forms along a front in the middle and high latitudes. Also called a "middle latitude storm", a "depression", and a "low". It is NOT a tropical storm or hurricane.
Eye	A region in the center of a hurricane (or tropical storm) where the winds are light and skies are clear to partly cloudy.
Eye Wall	A wall of dense thunderstorms that surrounds the eye of a hurricane.
Fahrenheit Scale	A temperature scale where 32 is assigned to the temperature where water freezes and 212 to the temperature where water boils (at sea level).
Flash Flood	A flood that rises and falls quite rapidly with little or no advance warning, usually as the result of intense rainfall over a relatively small area.
Fog	A cloud with its base at the earth's surface. It reduces visibility below 1.5 miles. In Colorado the most typical type of fog is known as "up slope fog".
Freeze	A condition occurring over a widespread area when the surface air temperature remains below freezing for a sufficient time to damage certain agricultural crops. A freeze most often occurs as cold air is advected into a region, causing freezing conditions to exist in a deep layer of surface air. Also called "advection frost"
Freezing Rain and Freezing Drizzle	Rain or drizzle that falls in liquid form and then freezes upon striking a cold object or ground. Both can produce a coating of ice on objects which is called "glaze".
Front	The transition zone between two distinct air masses Types of fronts include: Cold, Warm, Stationary and Occluded.
Frost	A covering of ice produced by deposition on exposed surfaces when the air temperature falls below the frost point.
Frostbite	The partial freezing of exposed parts of the body, causing injury to the skin and sometimes to deeper tissues.
Fujita Scale	A scale developing by T. Theodore Fujita for classifying tornadoes according to the damage they cause and their rotational wind speed. An "F0" is a weak tornado... an "F5" is a super-tornado capable of wiping a town off the map. Ex: Oklahoma City
Funnel Cloud	A rotating cone like cloud that extends downward from the base of a thunderstorm. It DOES NOT touch the ground. When it reaches the surface it is called a "tornado".
Glory	Colored rings that appear around the shadow of an object.
Graupel	Ice particles between 2 and 5 millimeters in diameter that form in a cloud often by the process of accretion. Snowflakes that become rounded pellets due to riming are called "graupel" or "snow pellets".
Greenwich Mean Time	The name of the twenty-four hour time scale which is used throughout the scientific and military communities. Standard Time begins at Greenwich, England, home of the Royal Observatory which first utilized this method of world time. This is also the Prime Meridian of Longitude. The globe is divided into twenty-four (24) time zones of 15 degrees of arc, or one hour in time apart. To the east of this meridian, time zones are number from 1 to 12 and prefixed with a minus (-), indicting the number of hours to be subtracted to obtain Greenwich Time (GMT). To the west, the time zones are also numbered 1 through 12, but are prefixed with a plus (+), indicating the number of hours to be added to obtain GMT
Gust	A sudden significant increase in or rapid fluctuations of wind speed. For this station peak wind must reach at least 16 knots (18 miles per hour) and the variation between peaks and lulls is at least 10 knots (11.5 miles per hour). The duration is usually less twenty seconds.
Gust Front	A boundary that separates a cold down draft of a thunderstorm from warm, humid surface air. On the surface its passage resembles that of a cold front. It is not a "true" frontal boundary.
Hail or Hailstones	Transparent or partially opaque particles of ice that range in size from that of a pea to that of golf balls. Hail is one of the most common elements of a thunderstorm in Colorado. A thunderstorm producing hail 3/4 inch in diameter is called a "Severe Thunderstorm".
Halos	Rings or arcs that encircle the sun or moon when seen through an ice crystal cloud or a sky filled with falling ice crystals. Halos are produced by the refraction of light.
Haze	Fine dry or wet dust particles dispersed through a portion of the atmosphere. Individually these are not visible but cumulatively they will diminish visibility. Haze can be a result of poor air quality and air pollution.
Heat Index	The combination of air temperature and humidity that gives a description of how the temperature feels. This is not the actual air temperature.
Heating Degree Day	One heating degree day is given for each degree that the daily mean temperature is below 65 degrees a given temperature. It is used as an indication of fuel consumption. Refer to degree day or cooling degree day.
Heat Lightning	Heat lightning refers to lightning that illuminates the sky, but is too far away for its thunder to be heard. It is not a true form of lighting, but rather a way to describe the sheet-like flash that is observed.
Heatstroke	A physical condition induced by a person's overexposure to high temperatures, especially when accompanied by high humidity.
Humidity	The amount of water vapor in the air. It is often confused with relative humidity or dew point. Types of humidity include absolute humidity, relative humidity, and specific humidity.
Hurricane	A severe tropical cyclone having winds in excess of 64 knots (74 miles per hour).
Hygrometer	An instrument designed to measure the air's water vapor content.
Hypothermia	The deterioration in one's mental and physical condition brought on by a rapid lowering of human body temperature. Hypothermia can be fatal.
Ice Fog	A type of fog composed of tiny suspended ice particles that forms at a very low temperature.
Indian Summer	An unseasonably warm spell with clear skies near the middle of Autumn. Usually follows a substantial period of cool weather.
Inversion	An increase in air temperature with height. A "normal" situation would have air temperature decreasing with height. Inversions are one of the main reasons the Colorado Front Range is plagued with poor air quality. Inversions can be caused by a general downward movement of cooler air from the upper layers of the atmosphere... trapping pollutants near the surface.
Ionosphere	An electrified region of the upper atmosphere where fairly large concentrations of ions and free elections exist.
Iridenscence	Brilliant spots or borders of colors, most often red and green, observed in clouds up to about 30 degrees from the sun.
Isobar	A line connecting points of equal pressure.
Isotach	A line connecting points of equal wind speed.
Isotherm	A line connecting points of equal temperature.
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Knot	A unit of speed equal to 1 nautical mild per hour. 1 knot equals 1.15 mile per hour.
Lake-Effect Snows	Localized snowstorms that form on the downwind side of a lake. Such storms are common in late fall and early winter near the Great Lakes as cold, dry air picks up moisture and warmth from the unfrozen bodies of water... and deposits great amounts of snow on the immediate shore.
Latitude	The location north or south in reference to the equator, which is designated at ze ro (0) degrees. Parallel lines that circle the globe both north and south of the equator. The poles are at 90 degrees North and South latitude.
Leeside Low	Storm systems that form on the downwind (lee) side of a mountain chain. In the United States, leeside lows frequently form on the eastern side of the Rockies and the Sierra Nevada.
Lenticular Cloud	A cloud in the shape of a lens. Lenticular clouds are indicative of strong and fast moving winds in the upper atmosphere. They have been confused for flying saucers and U.F.O.'s.
Lightning	A visible electrical discharge produced by thunderstorms. The majority of lightning strikes occur within a cloud... while only about 20% or so occur between the cloud and the ground. Lightning heats the surrounding air close to 54,000 F. This rapid and extreme heating causes the air to expand explosively, thus initiating a shock wave that becomes a booming sound wave... or thunder (a sonic boom)... that travels outward in all directions. Lightning is a major danger in Colorado, especially during the summertime.
Longitude	The location east or west in reference to the Prime Meridian, which is designated as zero (0) degrees longitude. The distance between lines of longitude are greater at the equator and smaller at the higher latitudes, intersecting at the earth's North and South Poles. Time zones are correlated to longitude. See Greenwich Mean Time.
Macroburst	A strong down draft (down burst) greater than 4 km wide that can occur beneath a thunderstorm. A down burst less than 4 km is called a "microburst". Thunderstorm winds that exceed 58 miles per hour are a feature of a "Severe Thunderstorm".
Mammatus Clouds	Clouds that look like pouches hanging from the underside of a thundercloud. Indicative of severe weather, especially hail.
Mesocyclone	A vertical column of cyclonically rotating air (rotating counterclockwise in the Northern Hemisphere) within a severe thunderstorm.
METAR	Acronym for METeorological Aerodrome Report. It is the primary observation code used in the United States to satisfy requirements for reporting surface meteorological data. Minimum reporting requirements includes wind, visibility, runway visual range, present weather, sky condition, temperature, dew point, and altimeter setting.
Meteorology	The study of the atmosphere and atmospheric phenomena as well as the atmosphere's interaction with the earth's surface, oceans, and life in general.
Microburst	A strong localized down draft (down burst) less than 4 km wide that occurs beneath thunderstorms. A strong down burst greater that 4 km across in called a "macroburst". Thunderstorm winds that meet or exceed 58 miles per hour are a feature of a "Severe Thunderstorm".
Monsoon or Monsoon Wind System	A wind system that reverses direction between winter and summer. Usually the wind blows from land to sea in winter and from sea to land in summer. In Colorado, a monsoonal flow blows moisture in from the Pacific Ocean near Baja California. It usually begins around July and can last as late as September... bringing flooding rains to the region.
Moon Phase	The moon phase is caused by sun rays reflecting off the moon's surface while it moves around the earth. The sun illuminates half of the moon at any time while the moon orbits around the earth. The variation in the angle made by the earth- moon line with respect to the earth-sun line causes changing phase of the moon. The moon completes one revolution around the earth in 27.322 days with respect to the background stars. This is called the SIDERIAL period of the moon. During this same time the earth moves about 27 degrees along its orbit around the sun. As a result, the moon takes about two extra days to complete the cycle with respect to the sum-earth line. This longer cycle of the moon that takes about 29.57 days is called SYNDONIC period of the moon. The longer cycle is considered as Lunar month.
Mountain and Valley Breeze	A local wind system of a mountain valley that blows downhill (mountain breeze) at night and uphill (valley breeze) during the day.
Nexrad	Nexrad stands for "Next Generation Radar". (See Radar). It refers to the powerful radar system in use around the country. Each NEXRAD site operates in one of two modes, "Clear Air" mode or "Precipitation" mode. Clear air mode (CAM), the normal operational mode, is used during periods of little or insignificant precipitation. The radar is very sensitive when operating in CAM. As an example, cold, light snow is best detected in CAM. During significant precipitation events, the radar will change to Precipitation Mode (PM). PM provides higher resolution for strong echoes.
Nimbostratus Cloud	A flat, dark, gray cloud characterized by more or less continuously falling perception. It is not accompanied by lightning, thunder or hail.
Noctilucent Clouds	Wavy, thin, bluish-white clouds that are best seen at twilight in polar latitudes. They form at altitudes of 80 to 90 km above the surface.
Occluded Front	A complex frontal system that ideally forms when a cold front overtakes a warm front. When the air behind the front is colder than the air ahead of it, the front is called a "cold occlusion". When the air behind the front is milder than the air ahead of it, it is called a "warm occlusion".
Permafrost	A layer of soil beneath the earth's surface that remains frozen throughout the year.
Pileus Cloud	A smooth cloud in the form of a cap. Occurs above, or is attached to, the top of a cumuliform cloud. You may see this cloud at the very top of a thunderstorm as winds blow up, and over the anvil.
Pollutants	Any gaseous, chemical, or organic matter that contaminates the atmosphere, soil or water.
Precipitation	Any form of water particles, liquid or solid that falls from the atmosphere and reaches the ground.
Pressure Altitude	Atmospheric or barometric pressure expressed in terms of altitude which corresponds to that pressure in the standard atmosphere.
Pressure Tendency	The rate of change of atmospheric pressure within a specified period of time, most often three hours.
Psychrometer	An instrument used to measure the water vapor content of the air. It consists of two thermometer (dry bulb and wet bulb). After whirling the instrument, the dew point and relative humidity can be obtained with the aid of tables.
Quasi-Stationary Front	QUASI-STATIONARY FRONT A front which is nearly stationary or moves very little since the last synoptic position.
Radar	An electronic instrument used to detect objects (such as falling precipitation) by their ability to reflect and scattered microwaves back to a receiver. In Colorado, the National Weather Service operates a radar systems called Nexrad, Doppler, or the WSR-88D). It is the most powerful and accurate radar in the region.
Radiation Fog	Fog produced over land when radiational cooling reduces the air temperature to or below its dew point. It is also known as "ground fog" and "valley fog".
Radiation Inversion	An increase in temperature with height due to radiational cooling of the Earth's surface. Also called a "nocturnal inversion". See inversion.
Radiational Cooling	The process by which the earth's surface and adjacent air cool by emitting infrared radiation. In other words... when skies clear overnight, the day's warmth radiates from the ground into the atmosphere at night. This allows Colorado to cool down significantly.
Rain	Precipitation in the form of liquid water droplets greater than 0.5 mm. If widely scattered, the drop size may be smaller. It is reported as "R" in an observation and on the METAR. The intensity of rain is based on rate of fall. "Very light" (R--) means that the scattered drops do not completely wet a surface. "Light" (R-) means it is greater than a trace and up to 0.10 inch an hour. "Moderate" (R) means the rate of fall is between 0.11 to 0.30 inch per hour. "Heavy" (R+) means over 0.30 inch per hour.
Rain Gauge	An instrument designed to measure the amount of rain that falls during a given time interval.
Rain Shadow	The region on the lee-side of a mountain where the precipitation is noticeably less than on the windward side.
Rainbow	An arc of concentric colored bands that spans a section of the sky when rain is present and the sun is positioned at the observer's back.
Rate of Change	The derivative or change in a parameters value with respect to time. This instrumentation (Virtual Weather Station) calculates the rate of change by calculating the derivative of a parameter, and then filtering it over one hour. Thus, the rate of change equation factors all of the measurements taken in the last hour, and may not exactly match the change in one hour.
Reflection	The process whereby a surface turns back a portion of the radiation that strikes it.
Refraction	The bending of light as it passes from one medium to another. I.e. Light passing through a prism undergoes refraction... causing white light to be bent into a "rainbow" of colors. A true rainbow, however, is the result of a combination between reflection and refraction of light within raindrops and drizzle.
Relative Humidity	The ratio of the amount of water vapor actually in the air compared to the amount of water vapor the air can hold at that particular temperature and pressure. The ratio of the air's actual vapor pressure to its saturation vapor pressure.
Return Stroke	The luminous lightning stroke that travels upward from the Earth to the base of the cloud.
Ridge	An elongated area of high atmospheric pressure.
Rime Ice	A white, granular deposit of ice formed by the freezing of water drops when they come in contact with an object. Rime Ice can be found at times on buildings and walls on the summit of Pikes Peak.
Roll Cloud	A dense, roll-shaped cloud attached to the lower front part of the main cloud. It often forms with thunderstorms along the leading edge of a gust front. Also called an "arcus cloud".
Rotor Cloud	A turbulent cumuliform type of cloud that forms on the leeward side of large mountains ranges. The air in the cloud rotates about an axis parallel to the range.
Rotors	Turbulent eddies that form downwind of a mountain chain, creating hazardous flying conditions.
Saffir-Simpson Scale	A scale relating a hurricane's central pressure and winds to the possible damage it is capable of inflicting. Categories range from 1 to 5. Category 1 is a weak hurricane with winds blowing 74 to 95 miles per hour. It is dangerous, but not necessarily deadly. Category 5 is a super-hurricane with winds in excess of 155 miles per hour. Severe damage to structures is likely along with severe flooding.
Sand Dunes	A hill or ridge of loose sand shaped by the winds. Sand Dunes are found on the windward side of the Sangre de Cristo Mountains in the San Luis Valley. The dunes are recognized as a national monument. (Great Sand Dunes National Monument).
Santa Ana Wind	A warm, dry wind that blows into southern California from the east off the elevated desert plateau. Its warmth is derived from compressional heating... just like warming Chinook winds in Colorado.
Sea Level Pressure	The atmospheric pressure at mean sea level either directly measured by stations at sea level or empirically determined from the station pressure and temperature by stations not at sea level. Used as a common reference for analyses of surface pressure patterns.
St. Elmo's Fire	A bright electric discharge that is projected from objects (usually pointed) when they are in a strong electric field, such as a thunderstorm.
Sunrise	The daily appearance of the sun on the eastern horizon as a result of the earth's rotation. In the United States, it is considered as that instant when the upper edge of the sun appears on the sea level horizon. In Great Britain, the center of the sun's disk is used instead. Time of sunrise is calculated for mean sea level. See sunset for comparison.
Sunset	The daily disappearance of the sun below the western horizon as a result of the earth's rotation. In the United States, it is considered as that instant when the upper edge of the sun just disappears below the sea level horizon. In Great Britain, the center of the sun's disk is used instead. Time of sunset is calculated for mean sea level. See sunrise for comparison.
Temperature	The measure of molecular motion or the degree of heat of a substance. It is measured on an arbitrary scale from absolute zero, where the molecules theoretically stop moving. It is also the degree of hotness or coldness. In surface observations, it refers primarily to the free air or ambient temperature close to the surface of the earth.
Thunder	The sound emitted by rapidly expanding gases along the channel of a lightning discharge. Over three-quarters of lightning's electrical discharge is used in heating the gases in the atmosphere in and immediately around the visible channel. Temperatures can rise to over 10,000 °C in microseconds, resulting in a violent pressure wave, composed of compression and rarefaction. The rumble of thunder is created as one's ear catches other parts of the discharge, the part of the lightning flash nearest registering first, then the parts further away.
Upslope	The cooling of an air flow as it ascends a hill or mountain slope. If there is enough moisture and the air is stable, stratiform clouds and precipitation may form. If the air is unstable, there might be an increased chance of thunderstorm development.
Vapor Pressure	The pressure exerted by water vapor molecules in a given volume of air.
Virga	Precipitation that falls from a cloud but evaporates before reaching the ground. A common occurrence in Colorado
Virtual Temperature	Virtual temperature is a fictitious temperature that takes into account moisture in the air. The formal definition of virtual temperature is the temperature that dry air would have if its pressure and specific volume were equal to those of a given sample of moist air. Virtual temperature allows meteorologists to use the equation of state for dry air even though moisture is present.
Wind Chill	The calculation of temperature that takes into consideration the effects of wind and temperature on the human body. Describes the average loss of body heat and how the temperature feels. This is not the actual air temperature.
Wind Direction	The direction from which the wind is blowing. For example, an easterly wind is blowing from the east, not toward the east. It is reported with reference to true north, or 360 degrees on the compass, and expressed to the nearest 10 degrees, or to one of the 16 points of the compass (N, NE, WNW, etc.).
Wind Run	The distance or length of flow of the air past a point during a given interval of time.
Wind Speed	The rate of the motion of the air on a unit of time. It can be measured in a number of ways. In observing, it is measured in knots, or nautical miles per hour. The unit most often used in the United States is miles per hour.
X-Rays	The portion of the electromagnetic spectrum that has a very short wave length. It has a wave length longer than gamma rays, yet shorter than visible light. X-rays can penetrate various thickness of all solids, and when absorbed by a gas, can result in ionization.
Yellow Snow	Snow that is given golden, or yellow, appearance by the presence of pine or cypress pollen in it.
Zonal Flow	The flow of air along a latitudinal component of existing flow, normally from west to east.
Zulu Time	One of several names for the twenty-four hour time which is used throughout the scientific and military communities.