CA - California

CA - see Cloud-to-Air Lightning

cA - see Continental Arctic Air Mass

CAA - see Cold Air Advection

CAD - see Cold Air Damming

C AMS - see Continental Air Mass

Cap (or Capping Inversion) - a layer of relatively warm air aloft (usually several thousand feet above the ground) that suppresses or delays the development of thunderstorms

CAPE - see Convective Available Potential Energy

Carbon Dioxide (CO2) - a colorless and odorless gas that is the fourth most abundant constituent of dry air

CAT - see Clear Air Turbulence

Cb - see Cumulonimbus

CC - see Cloud-to-Cloud Lightning

Cc - see Cirrocumulus

CCL - see Convective Condensation Level

CCN - see Cloud Condensation Nuclei

CD - cold

CDFRNT - see Cold Front

CDT - Central Daylight Time

Ceiling (CIG) - the height above the earth’s surface given to the lowest cloud layer or obscuring phenomena when the sky cover is reported as broken, overcast, or obscured and not classified as "thin" or "partial." See Vertical Visibility.

Ceilometer - an automatic, recording, cloud-height indicator. A light is projected upward onto the cloud base; the reflected light is detected by a photocell, and the height is determined by triangulation (the unique point where three lines meet).

Cell - convection in the form of a single updraft, downdraft, or updraft/downdraft couplet, typically seen as a vertical dome or tower as in a cumulus or towering cumulus cloud

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

Centrifugal Force - an apparent force that appears to pull an object in a circular path outward. See Centripetal Force for the actual force involved.

Centripetal Force - an inward force that keeps an object in a circular motion. Without this force, the object would follow a straight path (see Newton’s First Law of Motion). For example, without centripetal force, a ball on a string swung in a circular motion could fly out of the circle and keep moving in a straight line until some other force stops it. To keep the ball in a circular path, one must exert a force towards the center (where the hand is holding the string). Since the ball is accelerating, there is no balance of forces (Newton’s First Law). What is referred to as Centrifugal Force is actually a Lack-of-Centripetal Force. If enough centripetal force is applied, an object will continue in a circular path.

CFCs - see Chlorofluorocarbons

CG - see Cloud-to-Ground Lightning

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.

CHC - chance

CHG - change

CHGS - changes

Chinook Wind - a warm, dry wind that descends the eastern slopes of the Rocky Mountains

Chlorofluorocarbons (CFCs) - manufactured substances used as coolants and computer-chip cleaners. When these products break down, they destroy stratospheric ozone, creating the Antarctic Ozone Hole in the Southern Hemisphere spring (Northern Hemisphere autumn). While no longer in use, their long lifetime will lead to a very slow removal from the atmosphere.

Ci - see Cirrus

CIG - see Ceiling

CIN - see Convective INhibition

Cirrocumulus (Cc) - a high-level cloud that is composed mostly of ice crystals and has the appearance of a thin, white patch of rippled cloud

Cirrostratus (Cs) - a high-level cloud that is composed mostly of ice crystals and has the appearance of a whitish veil. It may totally cover the sky

Cirrus (Ci) - a high-level cloud (16,000 feet or more) that is composed mostly of ice crystals and has the appearance of white, delicate filaments in patches or narrow bands

Classic Supercell - radar characteristics often (but not always) include a hook echo, bounded weak echo region (BWER), V-notch, mesocyclone, and sometimes a TVS. Visual characteristics often include a rain-free base (with or without a wall cloud), flanking line, overshooting top, and back-sheared anvil, all of which normally are observed in or near the right rear or southwest part of the storm. Storms exhibiting these characteristics often are called classic supercells. See Supercell.

Clausius-Clapeyron Equation - the equation showing the relationship between pressure and temperature where two phases of a substance (liquid water and water vapor) are in equilibrium. Named for Rudolph Clausius (1822 -1888), a German physicist, and Benoit-Pierre-Emile Clapeyron (1799 - 1864), a French engineer.

CLD - see Cloud

Clear-air Mode - a highly sensitive operational mode of a WSR-88D radar in which the antenna scans slowly, obtaining only 5 elevation slices in 10 minutes. This slow scan speed allows the radar to sense echoes from "clear-air" (i.e., no precipitation). These echoes can be from dirt, insects, smoke, and changes in the air density.

Clear Air Turbulence (CAT) - in aviation, sudden severe turbulence occurring in cloudless regions that causes violent buffeting of aircraft

Clear Slot - a local region of clearing skies or reduced cloud cover, indicating an intrusion of drier air; often seen as a bright area with higher cloud bases on the west or southwest side of a wall cloud. A clear slot is believed to be a visual indication of a rear flank downdraft.

Climate - the statistical collection of weather conditions at a place over a period of years

Climate Change - a non-random change in climate that is measured over several decades or longer. It usually refers to human-induced causes, but is sometimes used to include both human-induced and natural causes.

Climate Prediction Center (CPC) - monitors and forecasts short-term climate variations and provides guidance information on the long-term global effects climate patterns can have on the nation. It is located in Camp Springs, Maryland.

Climatology - the science that deals with climates and their phenomena

Climometer - an instrument that measures angles of inclination; used to measure cloud ceiling heights

Clipper - a fast-moving low-pressure system. See Alberta Clipper.

Closed Low - a low-pressure area with a distinct center of cyclonic circulation that can be completely encircled by one or more isobars or height contour lines. The term usually is used to distinguish a low-pressure area aloft from a low-pressure trough.

Cloud (CLD) - a visible mass of minute water and/or ice particles in the atmosphere suspended above the earth’s surface

Cloud Base - the lowest level in the atmosphere that contains cloud particles (liquid cloud droplets, ice crystals, etc.)

Cloud Condensation Nuclei (singular: nucleus) or CCN - tiny particles (around 0.0002 mm) on which water vapor condenses and eventually forms cloud droplets. Some CCNs include particles of dust, clay, soot, and sea salt. Without these particles, relative humidities of several hundred percent would be required before condensation could begin.

Cloud Droplet - approximately 0.05 mm (~ 0.002 in) in diameter, about 100 times smaller than a typical rain droplet.

Cloud Height - the altitude of the cloud base above the local terrain or the difference in height between the cloud top and the cloud base; (sometimes called "thickness" or "depth" of the cloud)

Cloud Layer - a group of clouds, not necessarily of the same type, that has cloud bases at the same altitude

Cloud Seeding - any technique carried out to introduce artificial substances into the cloud with the intent of altering the natural development of that cloud

Cloud-to-Air Lightning (CA) - streaks of lightning that pass from a cloud to the air, but do not strike the ground.

Cloud-to-Cloud Lightning (CC) - streaks of lightning reaching from one cloud to another.

Cloud-to-Ground Lightning (CG) - streaks of lightning that touch both a cloud and the ground.

Clutter - radar echoes that interfere with observation of desired signals on the radar display. See Ground Clutter.

CLR - clear

CLRG - clearing

CLSE - close

CMPLT - complete

CMPLX - complex

CNTR - center

CNTRL - central

CNVG - converge (see Convergence)

CNVTV - convective (see Convection)

CO - Colorado

CO2 - see Carbon Dioxide

Coalescence - when large rain droplets overtake and collide with smaller droplets in their path, they may merge or stick together (coalesce).

Coherent Radar - a radar in which the phase of the transmitted radiation is known. A coherent radar compares the phase of transmitted and received pulses, permitting target velocities to be calculated using the Doppler effect.

Cold Advection (or Cold Air Advection - CAA) - the transport of cold air into a region by horizontal winds

Cold Air Damming (CAD) - a phenomenon in which a low-level cold air mass is trapped by topography (e.g., mountains). Effects on the weather include cold temperatures, freezing precipitation (if temperatures are cool enough), and extensive cloud cover.

Cold-Air Funnel - a funnel cloud or (rarely) a small, relatively weak tornado that can develop from a small shower or thunderstorm when the air aloft is unusually cold (hence the name). They are much less violent than other types of tornadoes.

Cold Cloud - a cloud comprised of ice particles or a mixture of ice particles and liquid droplets

Cold Core Low - a low-pressure area that is colder at its center than at its edges. Mid-latitude cyclones are usually cold core lows. They usually produce much of their cloud cover and precipitation during the daytime when the instability is the greatest. At night, the clouds and precipitation usually diminish significantly.

Cold Front (CDFRNT) - an advancing edge of a cold air mass that is replacing a warmer air mass

Cold Pool - a region of relatively cold air, represented on a weather map analysis as a relative minimum in temperature surrounded by closed isotherms. Cold pools aloft represent regions of relatively low stability, while surface-based cold pools are regions of relatively stable air.

Comma Cloud - a synoptic-scale cloud pattern with a characteristic comma-like shape, often seen on satellite photographs associated with large and intense low-pressure systems

Composite Chart - a map created by overlaying critical values of atmospheric parameters. It may be used to assess severe weather potential. A composite chart might indicate the position of low level moisture axes, a surface temperature ridge, a 300 mb jet stream, and a 500 mb height trough.

Composite Reflectivity - the maximum reflectivity in a vertical column. This product is obtained by comparing several individual tilts, or scans, of the radar, each one successively looking at different elevations in the atmosphere.

Condensation - the physical process by which a gas becomes a liquid; the opposite of evaporation

Condensation Funnel - a funnel-shaped cloud associated with rotation and consisting of condensed water droplets (as opposed to smoke, dust, debris, etc.)

Condensation Nucleus - see Cloud Condensation Nuclei

Conditional Symmetric Instability (CSI) - a type of instability that can be conducive to the formation of single- and multiple-banded clouds.

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

Cone-of-Silence - an area directly above and surrounding the radar where the radar does not sample the atmosphere. This is an artifact of the particular VCP that is used by the radar.

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

Congestus (or Cumulus Congestus) - same as Towering Cumulus

Conservation of Energy - a law of physics that states that energy cannot be created or destroyed. It can only be converted from one form to another

Conservation of Mass - a law of physics that states that mass cannot be created or destroyed. It can only be transferred from one volume to another

Conservation of Momentum - a law of physics that states that the total momentum of a system cannot change, unless outside forces act upon on the object. 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

Continental Air Mass (C AMS) - an air mass that starts over land and is dry

Continental Arctic Air Mass (cA) - an air mass characterized by extremely cold, dry air

Continental Polar Air Mass (cP) - an air mass characterized by cold, dry air

Continental Tropical Air Mass (cT) - an air mass characterized by warm or hot, dry air

Contour Line - generally, a line of constant value; in meteorology, it typically refers to a line of constant elevation above a specified reference level (usually mean sea level)

Contrail (or Condensation Trail) - streaks of condensed water vapor created in the air behind aircraft flying in clear, cold, humid air

Convection - in general, the transport and mixing of the properties of a fluid (e.g., heat, moisture, etc.) by means of mass motion within the fluid; in meteorology, atmospheric motions generally are divided into those in the horizontal, or advection, and those in the vertical, or convection; convection typically results from surface heating and the subsequent rising of warm air

Convective Available Potential Energy (CAPE) - a measure of the amount of energy available for convection. CAPE is directly related to the maximum potential vertical speed within an updraft; thus, higher values indicate greater potential for severe weather. Observed values in thunderstorm environments often may exceed 1,000 Joules per kilogram (J/kg), and in extreme cases may exceed 5,000 J/kg. See Positive Area.

Convective Boundary Layer - the unstable boundary layer that forms at the surface and grows upward through the day as the ground is heated by the sun and convective currents transfer heat upwards into the atmosphere.

Convective Cloud - a cloud that develops vertically by convection

Convective Condensation Level (CCL) - the level in the atmosphere to which an air parcel, if heated from below, will rise dry adiabatically, without becoming colder than its environment just before the parcel becomes saturated. See Lifted Condensation Level (LCL).

Convective Inhibition (CIN) - a measure of the amount of energy needed to initiate convection; values of CIN typically reflect the strength of the cap. See Negative Area.

Convective Outlook (sometimes called AC) - a forecast containing the area(s) of expected thunderstorm occurrence and expected severity over the contiguous United States, issued several times daily by the SPC

Convective Temperature - the approximate temperature that the air near the ground must warm to in order for surface-based convection to develop

Conventional Weather Radar - a weather radar that measures only the intensity of returned radiation, or reflectivity.

Convergence - the net inflow of air into a region, typically caused by horizontal wind motion; the opposite of divergence

Cooling Degree-Day - a type of degree day used for estimating energy requirements for cooling the indoor environment to a base temperature, generally to 65°Fahrenheit; one cooling degree-day is given for each degree that the day’s average temperature is above the base temperature

COOP - see Cooperative Observing Program

Cooperative Observing Program (COOP) - a national weather and climate observing network of, by, and for the people. More than 11,000 volunteers take observations (daily maximum and minimum temperatures, snowfall, and 24-hour precipitation totals) from farms, in urban and suburban areas, National Parks, seashores, and mountaintops. The National Weather Service collects the data from the volunteers.

Core-Punching - slang for when a vehicle goes into the heavy precipitation core of a thunderstorm. The low visibility makes it difficult to see a possible tornado, so core-punching is not recommended.

Coriolis Force - an apparent force that results from the earth’s rotation. It deflects objects moving above the earth’s surface to the right in the Northern Hemisphere and to the left in the Southern Hemisphere

Corona - the set of colored rings around the sun (or moon) created when the light source shines through a thin cloud

Cosmic Rays - high-energy particles that bombard the Earth from anywhere beyond its atmosphere at extremely high speeds. Most are believed to come from supernovas, although some are created in solar flares.

County Warning Area (CWA) - all of the counties or parishes assigned to a specific National Weather Service Forecast Office for the purpose of issuing warnings and hazard awareness.

County Warning and Forecast Area (CWFA) - same as County Warning Area.

Couplet - adjacent maxima of radial velocities of opposite signs. For example, if a strong away (away from the radar) velocity is right next to a strong toward (toward the radar) velocity, it is a couplet.

cP - see Continental Polar Air Mass

CPC - see Climate Prediction Center

Crepuscular Rays - the alternating bands of light and dark (rays and shadows) seen at the earth’s surface when the sun shines through clouds

Critical Thickness - the thickness that separates rain and snow. Areas north of a critical thickness line are likely to experience snow if precipitation falls, areas south of the line will likely experience rain, and along the line mixed precipitation could fall (sleet, freezing rain, snow, and rain). This line, along with other critical thicknesses, gives a good first guess at precipitation type.

CSI - see Conditional Symmetric Instability

CT - Connecticut

cT - see Continental Tropical Air Mass

Cu - see Cumulus

CUFRA - see Cumulus Fractus

Cumulonimbus (Cb) - exceptionally dense and vertically developed cloud type, occurring both as isolated clouds and as a line or wall of clouds, and generally accompanied by heavy rain, lightning, and thunder. Also known as a "thunderhead".

Cumulus (Cu) - cloud type in the form of individual, detached elements which are generally dense, have well-defined outlines, and show vertical development in the form of domes, mounds, or towers

Cumulus Congestus (or simply Congestus) - same as Towering Cumulus

Cumulus Fractus (CUFRA) - looks like a ragged or shredded cumulus cloud

Cup Anemometer - a device that measures wind speed only. Air flow past the four cups in any horizontal direction turns the cups in a way that is proportional to the wind speed.

Cutoff Low - a closed low which has become completely displaced (cut off) from the basic westerly current, and moves independently of that current. Cutoff lows may remain nearly stationary for days, or on occasion may move westward -- opposite to the prevailing flow aloft.

CVA - see Cyclonic Vorticity Advection

CWA - see County Warning Area

CWFA - see County Warning and Forecast Area

CYC - see Cyclone

CYCLGN - see Cyclogenesis

Cyclic Storm - thunderstorm that undergoes cycles of intensification and weakening (pulses) while maintaining its individuality. Cyclic supercells are capable of producing multiple tornadoes (i.e., a tornado family) and/or several bursts of severe weather.

Cyclogenesis (CYCLGN) - the development or intensification of a low-pressure center (cyclone)

Cyclone (CYC) - (1) an atmospheric circulation that rotates counter-clockwise in the Northern Hemisphere and clockwise in the Southern Hemisphere, which usually has a diameter of 2000 to 3000 kilometers. (2) Colloquial term for a tornado. (3) Shortened name for a tropical system (see Tropical Cyclone).

Cyclonic Rotation - rotation in the same sense as the earth’s rotation (i.e., counter-clockwise in the Northern Hemisphere as would be seen from above); the opposite of anticyclonic rotation

Cyclonic Vorticity Advection (CVA) - see Positive Vorticity Advection (PVA)