Air Mass-A body of air that extends hundreds or thousands of kilometers horizontally and is relatively uniform in temperature and moisture content (see continental arctic, continental polar, continental tropical, maritime polar, and maritime tropical air masses); because all thunderstorms are associated with some type of forcing mechanism, synoptic-scale or otherwise, the term is somewhat controversial and should be used with discretion.
Altimeter-An instrument for measuring the altitude with respect to a fixed level.
Anemometer-Instrument for measuring the speed of the wind.
Anticyclone-System of high pressure in which wind blows in a clockwise direction in the Northern Hemisphere and counterclockwise in the Southern Hemisphere.
Atmosphere-Gases surrounding the surface of a planet, moon or star.
Average-An arithmetic mean.
Axis-An imaginary line that passes through the poles of a body, like the Earth, around which it spins or rotates.
Barometer-Weather instrument for measuring atmospheric pressure.
Beaufort Scale-A scale of wind force that includes a description of the wind, its range of velocity and a classification of the wind from force 0 to force 12.
Bermuda High-The semipermanent atmospheric subtropical anticyclone (high pressure system) over the North Atlantic Ocean, so name especially when it is located in the western part of the ocean, near Bermuda (near 30° N).
Bernoulli's Principle-Air flowing over an airfoil results in an increase in flow speed over the upper curved surface. Since a velocity increase in fluid flow results in a corresponding pressure decrease, the increased airflow over the upper surface of the airfoil produces a lift on the airfoil because of lower pressure exerted on the upper surface. Named for Daniel Bernoulli (1700 -1782), a Swiss physicist who discovered the effect.
Boyle's Law-The empirical generalization that for many so-called perfect gases, the product of pressure and volume is constant in an isothermal process. Named for Robert Boyle (1627-1691), a British chemist who formulated this relationship.
Buoyancy-The tendency of a body to float or to rise when submerged in a fluid; the power of a fluid to exert an upward force on a body placed in it.
Carbon Dioxide-CO2; a colorless and odorless gas which is the fourth most abundant constituent of dry air.
Celsius Scale-Temperature scale on which the interval between the freezing point and the boiling point of water is divided into 100 degrees, with 0 degrees representing the freezing point and 100 degrees the boiling point.
Charles' Law-In a gaseous system at constant pressure, the temperature increase and relative volume increase are proportionally the same for all perfect gases. Named for Jacques Charles (1746 - 1823), a French chemist.
Climate-The average weather conditions of an area over a long time period.
Cold Front-The leading edge of a cold air mass. Cold frontal passages are usually associated with sharp wind shifts, pressure rises, showery or stormy conditions, and rapid temperature drops.
Condensation-The process by which a gas turns into a liquid or solid.
Condensation Nuclei-Small Particles on which molecules of water vapor may condense to form cloud droplets of ice particles.
Conduction-The transfer of energy by molecular motion from warmer to colder regions through a substance or between objects in direct contact, and without any net external motion.
Conservation of Energy-A law of physics that states that energy can not be created or destroyed only converted from one form to another.
Conservation of Mass-A law of physics that states that mass can not be created or destroyed only transferred from one volume to another.
Conservation of Momentum-A law of physics that states that an object in motion will stay in motion until acted upon by an outside force; an object at rest will remain at rest until acted upon by an outside force.
Contour Line-A line on a map or chart that connects points of equal elevation.
Convection-Transfer of heat through a liquid or gas by the movement of currents from hotter to cooler regions.
Coriolis Force-A force resulting from the Earth's rotation that causes the motion of particles to deflect towards the right in the Northern Hemisphere and the left in the Southern Hemisphere. The force is weakest around the Equator and its important in the formation of cyclones.
Current-The rate at which electric charges move through a given area.
Cycle-A sequence of changing states that upon completion produces a final state that is identical to the original one.
Cyclone-A disturbance in the atmosphere in which winds rotate around a low pressure center; the winds blow counterclockwise in the Northern Hemisphere and clockwise in the Southern Hemisphere; associated with stormy weather.
Density-The mass of a substance per unit volume.
Dew Point-The temperature at which moist air becomes saturated and dew forms.
Differential-Pertaining to the difference between two or more motions, forces. etc.
Doppler Effect-A change in the wavelength or frequency of electromagnetic radiation due to the relative motion between the observer and the emitting source.
Doppler Radar-A weather radar that uses the Doppler effect to determine whether a storm and/or its components are moving toward or away from the radar.
Doppler Shift-The change in frequency with which energy from a given source reaches an observer when the source and the observer are in motion relative to each other.
Downdrafts-A relatively small-scale current of air with marked downward motion.
Drought-A period of abnormally dry weather sufficiently prolonged for the lack of water to cause a serious shortages of water for agriculture and other needs in the affected area.
Dryline-A boundary separating warm, dry air from warm, moist air, typically across parts of New Mexico, Texas, Oklahoma, or Kansas. It typically lies north-south across the central and southern high Plains states during the spring and early summer, where it separates moist air from the Gulf of Mexico (to the east) and dry desert air from the southwestern states (to the west).
Dust Devils-A small whirlwind, usually of short duration, that is not associated with a thunderstorm and contains dust, sand, and debris picked up from the ground.
Energy-The ability to do work.
El Niño-Unusual warming of the Eastern Pacific, disrupting fishing off the coast of Ecuador and Peru; noticed around Christmas and named El Niņo, Spanish for "the Christ child," by local residents. Now known to impact global weather conditions.
Electromagnetic Spectrum-The entire range of wavelengths of electromagnetic radiation; light (the visible spectrum) is one small segment of this much broader spectrum.
Elliptical-Shaped like an ellipse or an oval.
Equinox-The two days of the year (spring and fall) when day and night are equally long, with the Sun appearing over the equator.
Evapotranspiration-The transfer of water from land surfaces into the atmosphere through the processes of evaporation and transpiration.
Fahrenheit Scale-A temperature scale on which the boiling point of water is at 212 degrees above the zero of the scale and the freezing point of water is at 32 degrees above zero.
Flash Floods-A local flood of great volume and short duration generally resulting from heavy rainfall in the immediate vicinity.
Fog-A cloud with its base in direct contract with the ground.
Forecast-Prediction of the weather using principles of physics, computer models and statistical and empirical techniques.
Front-The transition zone between two distinct air masses.
Frost-The condition which exists ice is formed on the earth's surface when the temperature falls below freezing.
Frostbite-The freezing or local effect of partial freezing of some part of the body.
Fujita Scale-A scale for classifying tornadoes according to the damage they cause; the tornado's rotational wind speed is inferred from an analysis of the wind damage.
F0 Weak 40 - 72 mph Light damage
F1 Weak 73 - 112 mph Moderate damage
F2 Strong 113 - 157 mph Considerable damage
F3 Strong 158 - 206 mph Severe damage
F4 Violent 207 - 260 mph Devastating damage
F5 Violent 261 - 318 mph Incredible damage
Funnel Cloud-A rotating cloud column or inverted cloud cone extending downward from a cloud base that is not in contact with the ground.
Gradient-In general, the spatial change of a physical quantity (e.g., temperature).
Gravity-In particular, the force imparted by the earth which tends to draw all bodies in the earth's sphere of influence toward the center of the earth.
Greenhouse Effect-The process through which some of the heat radiated from the surface of the Earth is trapped by gases in the atmosphere and re-radiated back to Earth.
Greenhouse Gases-The gases that absorb terrestrial radiation and contribute to the greenhouse effect; the main greenhouse gasses are water vapor, methane, CO2 and ozone.
Groundwater-Water That exists below the surface of the Earth as a result of rain percolating through the ground.
Gulf Stream-The warm, swift ocean current that flows along the East coast of the US.
Hail-Precipitation in the form of rounded balls of ice, always formed in convective clouds, nearly always thunderstorms.
Heat-A form of energy transferred between objects or systems as a result of a difference in temperature; not the same as temperature.
Heat Index-A value that represents the temperature it actually feels at a given air temperature and relative humidity.
High Pressure-A region of high pressure, marked as "H" on a weather map. A high is usually associated with fair weather.
Humidity-A term that refers to the water vapor content in the air.
Hurricane-A tropical cyclone with winds exceeding 66 knots, generally accompanied by rain, thunder, and lightning.
Hydrologic Cycle-The water cycle; the movement and exchange of water between the Earth, the oceans, and the atmosphere.
Hydrosphere-The water on or surrounding the Earth, including the oceans and water in the atmosphere.
Hygrometer-An instrument that measures the water vapor content in the air.
Infrared-Heat radiation with a wavelength that is longer than visible light and shorter than microwave and radio waves, in the range of about 0.75 to 1000 micrometers.
Inversion-A condition in which air temperature increases with height.
Isobar-Line connecting points of equal pressure.
Isotherm-A line connecting points of equal temperature.
Intertropical Convergence Zone-The boundary zone separating the northeast trade winds of the Northern Hemisphere from the southeast trade winds of the Southern Hemisphere.
Kelvin Temperature Scale (K)-An absolute temperature scale in which a change of 1 Kelvin equals a change of 1 degree Celsius; 0K is the lowest temperature on the Kelvin scale. A temperature scale with the freezing point of +273 degrees K (Kelvin) and the boiling point of +373 degrees K. It is used primarily for scientific purposes. Also known as the Absolute Temperature Scale. Proposed in 1848 by William T. Kelvin, 1st Baron of Largs (1824-1907), Irish-born Scottish physicist and mathematician.
Kinetic Energy-Energy that a body has as a result of its motion. Mathematically, it is defined as one-half the product of a body's mass and the square of its speed.
La Niņa-La Niņa is a periodic cooling of a large area of the tropical Pacific Ocean. It can alternate with an ocean warming called El Niņo. Both can have impacts on weather worldwide.
Lake/Land Breeze-A lake breeze occurs when prevailing winds blow off the water, while a land breeze indicates winds blowing from land to sea. Both are caused by the difference in surface temperature (heating) of the land and water. As a result, a lake breeze occurs during the day while a land breeze happens at night.
Lake Effect Snow-Snow showers that are created when cold dry air passes over a large warmer lake, such as one of the Great Lakes, and picks up moisture and heat.
Latent Heat-The heat released or absorbed by a substance during a phase change. Examples include condensation and sublimation.
Latitude-The measurement of distance north or south of the Equator of a spherical body such as the Earth. Latitude is measured in degrees, minutes and seconds of arc.
Longitude-The measurement of distance east or west of the prime meridian of a spherical body such as the Earth. Longitude is measured in degrees, minutes and seconds of arc.
Low Pressure System-An area of a relative pressure minimum that has converging winds and rotates in the same direction as the earth. This is counterclockwise in the Northern Hemisphere and clockwise in the Southern Hemisphere. Also known as an cyclone, it is the opposite of an area of high pressure, or a anticyclone.
Mesocyclone-A vertical column of cyclonically rotating air within a thunderstorm.
Mesoscale-Size scale referring to weather systems smaller than synoptic-scale systems but larger than single storm clouds. Horizontal dimensions generally range from around 50 miles to several hundred miles. Squall lines are an example of mesoscale weather systems.
Microwave-A form of electromagnetic radiation with a high frequency and long wavelength.
Molecules-A collection of atoms held together by chemical forces.
NEXRAD (NEXt-Generation Weather RADar)-Network of high-resolution Doppler radars operated by the NWS; NEXRAD units are known as WSR-88D.
Nitrogen-A colorless, tasteless, odorless gas that constitutes 78% of the atmosphere by volume and occurs as a constituent of all living tissues in combined form.
Occluded Front-The front formed by a cold front overtaking a warm or stationary front and lifting the warm air above the earth's surface.
Orbit-The path followed by a planet, satellite or star around a more massive body in its gravity induced motion.
Oxygen-In its free form, a colorless, tasteless, and odorless gas in the atmosphere of which it forms about 21% by volume.
Ozone-A pungent-smelling, slightly bluish gas which is a close chemical cousin to molecular oxygen. About 90% of the earth's ozone is located in a natural layer far above the surface of the globe, in a frigid region of the atmosphere known as the stratosphere. Here in this outer region it protects the earth and its inhabitants from the harmful effects of ultraviolet radiation from the sun.
Poles of Rotation-Rotation of a body around its axis, e.g. Earth rotates around the axis formed by its North and South poles.
Precipitation-Any form of water particles that fall from the atmosphere and reaches the ground.
Pressure-When a force is exerted on a surface, the ratio of this force to the area of the surface.
Pressure Gradient-The change in pressure over a given distance at a given time.
Prism-A solid object made of glass or clear plastic with triangular ends that are equal and parallel that disperses white light into visible light (the colors of the rainbow).
Psychrometer-An instrument consisting of a dry bulb and wet bulb thermometer that is used to measure the water vapor content of the air.
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.
Radiation-The process by which radiated energy moves through space or material media; (2) energy propagated through space or through material media in the form of an advancing disturbance in electric and magnetic fields (e.g., visible light, x-rays, microwaves, radio waves, infrared radiation, ultraviolet waves, cosmic rays, etc.).
Radiational-Relating to the transfer of energy in the form of electromagnetic radiation.
Radiosonde-The instrument package carried by weather balloons to measure the temperature, humidity and pressure of the atmosphere at multiple levels (up to at least 30000 feet in altitude).
Radome-A dome-shaped device that houses a radar antenna.
Refraction-The bending of a light beam due to variations in atmospheric density.
Relative Humidity-A measure of the water vapor content of the air at a given temperature; the amount of moisture in the air as compared with the amount that the air could contain at the same temperature, expressed as a percentage.
Respiration-The process by which living things take in oxygen and use it to produce energy.
Revolution-The orbital motion of one object around another.
Rotation-The spin of an object around its own axis.
Saturation-Where the amount of actual water vapor is at the maximum theoretically possible at a given temperature and atmospheric pressure.
Sea Breeze-A cooling breeze blowing generally in the daytime inland from the sea, caused by the temperature difference when the sea surface is cooler than the adjacent land.
Severe Thunderstorm-A thunderstorm with wind gusts of 50 knots (58 mph) or greater, hail at least three-quarters of an inch in diameter, and/or a tornado or funnel cloud.
Sleet-Frozen or partly frozen falling rain; ice pellets.
Smog-A natural fog made heavier and darker by smoke and chemical fumes.
Solstice-The longest and shortest days of the year when the Sun appears to "stand still" at its northernmost and southernmost positions as seen from Earth.
Southern Oscillation-The reversal of typical surface air pressure patterns across the tropical Pacific that occurs during a major El Niņo event.
Spectrum of Visible Light-The band of colors, in order of increasing wavelength from red to violet that can be seen when white light travels through a diffracting medium such as a prism.
Squall Line-Any line or narrow band of active thunderstorms which is not directly along a frontal boundary.
Static Electricity-The accumulation of positive or negative charges on an object.
Stationary Front-A surface boundary between air masses (such as a cold or warm front) that has stalled and has little or no motion.
Storm Surge-A rise above the normal water level along a shore caused by strong onshore winds and/or reduced atmospheric pressure. The surge height is the difference of the observed water level minus the predicted tide. Most hurricane deaths are caused by the storm surge. It can be 50 or more miles wide and sweeps across the coastline around where the hurricane makes landfall. The maximum rises in sea-level move from under the storm to the right of the storm's track, reaching a maximum amplitude of 10 to 30 feet at the coast. The storm surge may even double or more in height when the hurricane's track causes it to funnel water into a bay. The storm surge increases substantially as it approaches the land because the normal water depth decreases rapidly as it approaches the beaches. The moving water contains the same amount of energy; thus, resulting in an increase of storm surge. Typically, the stronger the hurricane, the greater the storm surge.
Supercell-The largest thunderstorms that approach a steady-state internal structure for a few hours and are able to produce large hail and tornadoes. The NWS defines supercells as thunderstorms that have a persistent, rotating updraft as part of their structure - these rotating updrafts have the potential to spawn deadly tornadoes.
Temperature-Degree of hotness or coldness measured according to a scale on a thermometer.
Thermal Equilibrium-The state in which two bodies in physical contact with each other have identical temperatures.
Thunderstorms-or thundershower.. A local storm, produced by a cumulonimbus cloud, and accompanied by thunder and lightning.
Topographical-Referring to the relief features or surface configurations of an area.
Tornado-A tornado appears as a violent funnel-shaped wind vortex in the lower atmosphere with upward spiraling winds of high speeds - spawned by severe thunderstorms. The tornado usually appears from a bulge in the base of a cumulonimbus cloud. It has a typical width of tens to hundreds of meters and a lifespan of minutes to hours. In area, it is one of the least extensive of all storms, but in violence, it is the world's most severe. More tornadoes occur in the United States than in any other country. In Canada, when they do occur, it is mainly in the Prairies and southern Ontario.
Tornado Alley-"Tornado Alley" is the informal name given to that region of the mid-United States, from Texas in the South, to Illinois in the North, where weather conditions mean tornadoes are, on average, found each year in the greatest numbers and intensity. "Tornado Alley" includes Oklahoma (home of NOAA's NSSL), where tornadoes are found in large numbers almost every spring and summer. But tornadoes can, in fact, be found in any state, at any time of year when conditions (high humidity, unstable air and a boundary layer separating wet and dry air) are right.
Trace Gases-Gases present in the atmosphere in small quantities (compared to nitrogen and oxygen) including water vapor, carbon dioxide, methane and ozone.
Transpiration-The process by which moisture is released from leaves and other parts of plants into the atmosphere.
Trade Winds-The winds that occupy most of the tropics and blow from subtropical highs to the equatorial low.
Tropical Depression-A stormy region in the tropics more intense than a tropical disturbance but less intense than a tropical storm. Wind speeds in a tropical depression are less than 39 mph.
Tropical Storms-It is a warm-core tropical cyclone that has maximum sustained surface wind speed (using the U.S. 1-minute average) ranges from 34 kt (39 mph or 63 kph) to 63 kt (73 mph or 118 kph).
Tropical Wave-Trough or cyclonic curvature maximum in the trade wind easterlies and it is not classified as a tropical cyclone. The wave may reach maximum amplitude in the lower middle troposphere.
Troposphere-The layer of the atmosphere closest to Earth, where clouds and weather occur and temperature decreases with increasing altitude.
Updrafts-A small-scale current of rising air. If the air is sufficiently moist, then the moisture condenses to become a cumulus cloud or an individual tower of a towering cumulus or Cb.
Vortex/Vorticity-In meteorology any rotating flow in the atmosphere.
Warm Front-The leading edge of a warm air mass. Warm fronts are usually associated with temperature rises, cloudy and wet weather. Clouds in a warm front are generally layered, and less convection (thunderstorm activity) is seen along warm fronts, as compared to cold fronts.
Warning-(as in hurricane or tornado warning, as used by NOAA/NWS) Conditions are expected in the specified area, usually within 24 hours.
Watch-(as in hurricane or tornado warning, as used by NOAA/NWS)Conditions are possible in the specified area, usually within 36 hours.
Water Vapor-Water in its gaseous state.
Waterspouts-In general, a tornado occurring over water. Specifically, it normally refers to a small, relatively weak rotating column of air over water beneath a Cb or towering cumulus cloud.
Wavelength-The distance between successive crests and troughs in a wave.
Wet-Bulb Thermometer-A thermometer with a cloth-covered bulb that is used to evaluate relative humidity.
Whirlwinds-A small, rotating column of air; may be visible as a dust devil.
