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

CAPE (Convective Available Potential Energy) - 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.

Carbon Dioxide - CO₂; a colorless and odorless gas which is the fourth most abundant constituent of dry air

Cb - short for Cumulonimbus cloud; known colloquially as a "thunderhead"

CC - cloud-to -cloud lightning

Cc - short for Cirrocumulus

CCL - acronym for convective condensation level

Ceiling - 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 obscuration and not classified "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 line 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

CG - a cloud-to -ground lightning flash

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.

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

Ci- abbreviation for cirrus clouds

CIN (Convective INhibition) - a measure of the amount of energy needed to initiate convection; values of CIN typically reflect the strength of the cap

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

Cirrostratus - a high-level cloud which is composed mostly of ice crystals and has the appearance of a whitish veil which may totally cover the sky

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

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.

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

Climatology - the science that deals with climates and their phenomena

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

Closed Low - a low pressure area with a distinct center of cyclonic circulation which 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 - 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 (water droplets, ice crystals, etc.)

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 the 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

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, permiting target velocities to be calculated using the Doppler effect.

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

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 water droplets

Cold Front - an advancing edge of a cold 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 constructed by overlaying critical values of atmospheric parameters analyzed 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 - a particle in the atmosphere, either liquid or solid, upon which condensation of water vapor begins

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 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

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

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

Continental Tropical Air Mass - an air mass characterized by warm or hot dry air

Contour Line - generally, a line of constant value; in meteorology, 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 Cloud - a cloud which 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 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 degrees Fahrenheit; one cooling degree-day is given for each degree that the day's average temperature is above the base temperature

Coriolis Force - an apparent force that as a result of the earth's rotation 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 it shines through a thin cloud

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

Cumulonimbus - 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

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

Cumulus Congestus (or simply Congestus) - same as towering cumulus

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.

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

Cyclone - an atmospheric circulation that rotates counter-clockwise in the Northern Hemisphere and clockwise in the Southern Hemisphere, that usually has a diameter of 2000 to 3000 kilometers

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