National Weather Service United States Department of Commerce

Weather Glossary: C's

An acronym for Cold Air Advection.
C-Band Radar
A radar operating in the 3,900 to 6,200 megahertz range whose wavelength is generally accepted as 5 centimeters.
A condition when no air motion is detected.
Cap or Cap Strength

It measures the ability of stable air aloft (a layer of relatively warm air) to inhibit low-level parcel ascent. Empirical studies show that a cap greater than 2°C often precludes thunderstorms in the absence of a strong dynamical or forced lift.

This occurs even when the instability is excessive. A strong cap prevents widespread convection from occurring; thus, it allows low level heat and moisture to increase over a period of time. This in turn increases the amount of potential instability.

Also, the air above it can cool, which also increases potential instability. This delay in the onset of convection increases the severe potential for a limited number of cells that manage to punch through the cap or reach the boundary separating capped from uncapped region.

The cap is an important ingredient in most severe thunderstorm episodes, as it serves to separate warm, moist air below and cooler, drier air above. Meanwhile, when there is no cap, either process (warming/moistening at low levels or cooling aloft) results in a faster release of available instability - often before instability levels become large enough to support severe weather development. As a result, convection tends to be widespread, but less intense.

This is a result of developing thunderstorms competing for a limited amount of available moisture. Therefore, severe storms often form along these lid boundaries, where the release of potential instability is favored. As a result, thunderstorms showing rapid growth within or very near a strongly capped region become severe. This is also called a lid.

An acronym for Convective Available Potential Energy.
Capillary Fringe
The soil area just above the water table where water can rise up slightly through the cohesive force of capillary action. This layer ranges in depth from a couple of inches, to a few feet, and it depends on the pore sizes of the materials. The capillary fringe is also called the capillary zone.
Capillary Potential
The work required to move a unit mass of water from the reference plane to any point in the soil column.
Capillary Zone
The soil area just above the water table where water can rise up slightly through the cohesive force of capillary action. This layer ranges in depth from a couple of inches, to a few feet, and it depends on the pore sizes of the materials. The capillary zone is also called the capillary fringe.
Catchment Area
An area having a common outlet for its surface runoff (also see Drainage Area or Basin, Watershed).
A National Weather Service precipitation descriptor for a 80%, 90%, or 100% chance of measurable precipitation (0.01"/0.25 mm). See Precipitation Probability (PoP).
An acronym for cumulonimbus.
An acronym for Convective Condensation Level.
The height of the lowest layer of broken or overcast clouds.
Ceiling Balloon
A small balloon used to determine the height of the cloud base. The height is computed from the balloon's ascent rate and the time required for its disappearance into the cloud.
Ceiling Light
A type of cloud-height indicator that uses a focused light to project vertically a narrow beam of light onto a cloud base.
A device used to evaluate the height of clouds or the vertical visibility into a surface-based obscuration.
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. A typical thunderstorm consists of several cells (see multi-cellular thunderstorm). The term "cell" also is used to describe the radar echo returned by an individual shower or thunderstorm. Such usage, although common, is technically incorrect.
Celsius (°C)
A temperature scale in which 0° is the freezing point of water and 100° is the boiling point.
The vertical axis or core of a tropical cyclone. It is usually determined by cloud vorticity patterns, wind, and/or pressure distributions.
Center/Vortex Fix
The location of the center of a tropical or subtropical cyclone obtained by reconnaissance aircraft penetration, satellite, radar, or synoptic data.
Central North Pacific Basin
The region north of the Equator between 140°W and the International Dateline. The Central Pacific Hurricane Center (CPHC) in Honolulu, HI is responsible for tracking tropical cyclones in this region.
The center of mass of a convective cell (storm) or other precipitation system.
Fear of being struck by lightning. See Astraphobia, Astrapophobia, Brontophobia, Keraunophobia, and Tonitrophobia.
CFS (Cubic Feet per Second)
The flow rate or discharge equal to one cubic foot (of water, usually) per second. This rate is equivalent to approximately 7.48 gallons per second. This is also referred to as a second-foot.
CFS (Cubic Feet per Second) Day
The volume of water discharged in twenty four hours, with a flow of one cubic foot per second is widely used; 1 cfs-day is 24 x 60 x 60 = 86,000 cubic feet (2,435 cubic meters), 1.983471 acre-feet, or 646,317 gallons (2,938,215 liters). The average flow in cubic feet per second for any time period is the volume of flow in cfs-days.
Small strips of metal foil, usually dropped in large quantities from aircraft or balloons. Chaff typically produces a radar echo which closely resembles precipitation. Chaff drops once were conducted by the military in order to confuse enemy radar, but now are conducted mainly for radar testing and calibration purposes.
A National Weather Service precipitation descriptor for 30%, 40%, or 50% chance of measurable precipitation (0.01"). When the precipitation is convective in nature, the term scattered is used. See Precipitation Probability (PoP).
Channel (watercourse)
An open conduit either naturally or artificially created which periodically, or continuously contains moving water, or forms a connecting link between two bodies of water. River, creek, run, branch, anabranch, and tributary are some of the terms used to describe natural channels. Natural channels may be single or braided. Canal and floodway are some of the terms used to describe artificial channels.
Channel Inflow
Water, which at any instant, is flowing into the channel system form surface flow, subsurface flow, base flow, and rainfall that has directly fallen onto the channel.
Channel Lead
An elongated opening in the ice cover caused by a water current.
The tendency of the wind to follow the axis of a channel or be steered by sloping land, resulting in a change in its direction.
Channel Routing
The process of determining progressively timing and shape of the flood wave at successive points along a river.
The modification of a natural river channel; may include deepening, widening, or straightening.
Chart Datum
Lake Height Above Sea Level
(feet / meters)
Erie 569.2 / 173.5
Huron 577.5 / 176.0
Michigan 577.5 / 176.0
Ontario 243.3 / 74.2
St. Clair 572.3 / 174.4
Superior 601.1 / 183.2

A plane of reference, established by the National Ocean Survey (NOS), as a mean low water level for each of the Great Lakes. These are expressed in feet above mean water level at Point-au-Pere (Father Point), Quebec, which is considered the Mean Sea Level (MSL) datum for the Great Lakes.

All depths and clearances shown on NOS charts refer to Chart Datum. Actual water levels are reported by the Coast Guard in inches above or below Chart Datum. The following table shows the Chart Datums for Great Lakes in feet above sea level.

The fear of cold. See also Cheimatophobia, Cryophobia, Sychrophobia, Psychorophobia.
The fear of cold. See also Cheimaphobia, Cryophobia, Sychrophobia, Psychorophobia.
Fear of snow.
Chinook Wind
A warm, dry wind that descends the eastern slope of the Rocky Mountains. The warmth and dryness of this wind can quickly melt and evaporate snowcover. Another name for this type of wind is "foehn".
An acronym for Convective Inhibition.
Circulation Cell
A "package" of air with a distinct circulation pattern, i.e., a lake breeze.
They are thin clouds, the individual elements which appear as small white flakes or patches of cotton, usually sowing brilliant and glittering quality suggestive of ice crystals. They form at altitudes between 16,500 to 45,000 feet (5,000 to 13,700 meters) above ground.
Cirrocumulus Standing Lenticular (CCSL)
These clouds are formed on the crests of waves crested by barriers in the wind flow. The clouds show little movement hence the name standing. Wind, however, can be quite strong blowing through the cloud. They are characterized by their smooth, polished edges. They may also form on wave crests. They are ice crystal clouds and generally are whiter than ACSL. These clouds from between 16,500 and 45,000 feet (5,000 to 13,700 meters).
Cirrostratus (Cs)
They are thin, whitish cloud layers appearing like a sheet or veil. They are diffuse sometimes partially striated or fibrous. Due to their ice crystal makeup, these clouds are associated with halos - large, luminous circles or arcs of circles surrounding the sun or moon. The layer frequently is the edge of a frontal shield. They form at altitudes between 16,500 to 45,000 feet (5,000 to 13,700 meters) above ground.
Cirrus (Ci)
They are thin, feather like clouds composed entirely of ice crystals. They form at altitudes between 16,500 to 45,000 feet (5,000 to 13,700 meters) above ground. Thunderstorm anvils are a form of cirrus cloud, but most cirrus clouds are not associated with thunderstorms.
Civil Emergency Message (CEM)
These National Weather Service statements are issued when a local or state official wants a warning disseminated regarding nuclear accidents, spills of toxic material, and other similar situations.
Classic Supercell
See supercell.
Clear Ice
It is a glossy, clear or translucent ice formed by the relatively slow freezing of large supercooled droplets. The large droplets spread out over the airfoil of an airplane before complete freezing, forming a sheet of clear ice.
Clear Skies
Skies are clear when no clouds or obscurations are observed or detected from the point of observation.
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.
Client Agency
As used in connection with reimbursable National Weather Service (NWS) fire weather services, a public fire service or wildlands management agency, Federal or non-Federal, which requires and uses NWS fire and forestry meteorological services.
Cliff Effect
The dramatic alteration in direction of an onshore wind by a cliff face. The offshore equivalent is called the lee effect.
Climatological Data (CD)

This National Centers for Environmental Information (NCEI) publication, also produced monthly and annually, contains daily temperature and precipitation data for over 8,000 locations.
Monthly editions contain station daily maximum and minimum temperatures and precipitation. Some stations provide daily snowfall, snow depth, evaporation, and soil temperature data.

Each issue also contains monthly summaries for heating and cooling degree days (65°F / 18.3°C base). The July issue also contains monthly heating degree days and snow data for the preceding July through June. The Annual issue contains monthly and annual averages of temperature, precipitation, temperature extremes, freeze data, soil temperatures, evaporation, and a recap of monthly cooling degree days. The CD is published by state or region (New England), with a total of 45 issues produced each month.

Climate Prediction Center (CPC)
The CPC is one of nine national centers that comprises the National Centers for Environmental Prediction (NCEP). Their mission is to maintain a continuous watch on short-term climate fluctuations and to diagnose and predict them. These efforts are designed to assist agencies both inside and outside the federal government in coping with such climate related problems as food supply, energy allocation, and water resources.
Closed Basin
A basin draining to some depression or pond within its area, from which water is lost only by evaporation or percolation. A basin without a surface outlet for precipitation falling precipitation.
Closed Basin Lake Flooding
Flooding that occurs on lakes with either no outlet or a relatively small one. Seasonal increases in rainfall cause the lake level to rise faster than it can drain. The water may stay at flood stage for weeks, months, or years.
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. Closed lows aloft typically are partially or completely detached from the main westerly current, and thus move relatively slowly (see cutoff low).
A visible aggregate of minute water droplets or ice particles in the atmosphere above the Earth's surface.
Cloud-Air Lightning (CA)
Streaks of lightning which pass from a cloud to the air, but do not strike the ground.
Cloud-Cloud Lightning (CC)
Streaks of lightning reaching from one cloud to another.
Cloud-Ground Lightning (CG)
Lightning occurring between cloud and ground.
Cloud Height
The height of the base of a cloud or cloud layer above the surface of the earth.
Cloud Layer
An array of clouds whose bases are at approximately the same level.
Cloud Seeding
An experimental process used to weaken hurricanes or make rain in dry areas.
Cloud Streets
Rows of cumulus or cumulus-type clouds aligned parallel to the low-level flow. Cloud streets sometimes can be seen from the ground, but are seen best on satellite photographs.
Cloud Tags
Ragged, detached cloud fragments; fractus or scud.
Cloud-Water Lightning (CW)
Lightning occurring between cloud and water.
When the predominant/average sky condition is covered completely by opaque (not transparent) clouds. In other words, 8 octants of the sky is covered by opaque clouds.
Radar echoes that interfere with observation of desired signals on the radar display.
The process by which water droplets in a cloud collide and come together to form raindrops.
Coastal Convergence
The convergence or running together of land and sea winds, creating a stronger band of wind near the shore. Factors such as the shape of the shoreline and the angle between the wind and the shore determine the severity of this effect.
Coastal Flood Statement
This National Weather Service product keeps the public and cooperating agencies informed of the status of existing coastal flood watches and warnings as well as provides an update on local conditions. It is also used to cancel a Coastal Flood Watch or a Coastal Flood Warning.
Coastal Flood Warning
This National Weather Service product alerts residents along the Atlantic, Pacific, and Gulf Coasts that coastal flooding is either imminent or occurring.
Coastal Flood Watch
This National Weather Service product alerts residents along the Atlantic, Pacific, and Gulf Coasts to the possibility of coastal flooding.
Coastal Flooding
Flooding that occurs from storms where water is driven onto land from an adjacent body of water. These can be hurricanes, "nor'easters," or tropical storms, but even a severe winter storm or thunderstorm can cause this type of flooding.
Coastal Marine Forecasts (CWF)
This National Weather Service marine product is designed to serve the needs of the widest variety of maritime activities in the coastal waters of Atlantic Ocean, Pacific Ocean, and Gulf of Mexico. Coastal water traffic ranges from numerous small and weather-sensitive craft, many of which do not venture far from land, to the largest ocean-going vessels. Another important activity is the offshore energy vessels that includes mobile drill ships and fixed platforms.
Coastal Waters
The marine area, including bays, harbors, and sounds, from a line approximating the mean high water mark (average height over a 19-year period) along the mainland or near-shore islands out to as much as 100 nautical miles offshore.
Coherent Radar
A radar that utilizes both signal phase and amplitude to determine target characteristics (e.g., velocity, spectrum width).
Cold Air Advection
Transport of cold air into a region by horizontal winds.
Cold Core 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 Core Low
A low-pressure area which is colder at its center than at its periphery. Mid-latitude cyclones exhibit this temperature pattern. 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
The leading edge of a relatively colder airmass which separates two air masses in which the gradients of temperature and moisture are maximized. In the northern hemisphere winds ahead of the front will be typically southwest and shift into the northwest with frontal passage.
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.
Collar Cloud
A generally circular ring of cloud that may be observed on rare occasions surrounding the upper part of a wall cloud. This term sometimes is used (incorrectly) as a synonym for wall cloud.
Columnar Ice
Ice consisting of columnar shaped grain. The ordinary black ice is usually columnar-grained.
Combined Seas
Generally referred to as "SEAS". It is used to describe the combination or interaction of wind waves and swells. In some prediction techniques, its height is the square root of the sum of the squares of the wind wave and swell heights. It is generally equal to the height of the swell plus 1/3 the height of the wind waves.
Combined Shear (CS)
This WSR-88D radar product displays a combined radial and azimuthal shear of the mean radial velocity. It is available for all elevation angles. It is used to identify low-level wind shear associated with gust fronts, downbursts, and mesoscale rotational phenomena. Aviation interests and operational researchers primarily use this radar product.
Combined Shear Contour (CSC)
This WSR-88D radar product is a contoured version of Combined Shear (CS) that is displayable alone or as an overlay on reflectivity or velocity products.
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.
Comma Echo
A thunderstorm radar echo which has a comma-like shape. It often appears during latter stages in the life cycle of a bow echo.
Complex Gale/Storm
In the National Weather Service High Seas Forecast, an area for which gale/storm force winds are forecast or are occurring, but for which no single center is the principle generator of these winds.
Composite Hydrograph
A stream discharge hydrograph which includes base flow, or one which corresponds to a net rain storm of duration longer than one unit period.
Composite Reflectivity (CR)

This WSR-88D radar product displays the maximum reflectivity for each resolution grid box for all elevation angles in a volume scan.

Available with combined attribute table which provides valuable information concerning storm characteristics, such as storm tops, maximum radial velocity and reflectivity, and possible existence of hail and mesocyclones. It is used to observe the highest reflectivity in a storm from any scanned elevation angle; determine intensity trends; and generate cross section through maximum reflectivity.

Composite Reflectivity Contour (CRC)
This WSR-88D radar product is a line contoured image of composite reflectivity (CR). Contour intervals and number of contours are changed at the User Control Processor. There is also a combined attribute table available for this product. It is used to view a contoured image of higher reflectivity values; examine storm structure features such as overhang, tilt, Weak Echo Regions (WER), and Bounded Weak Echo Regions (BWER); estimate height of higher dBZ's and echo tops; and locate the bright band (where snow is melting and becoming rain).
Concentric Rings
These are common in the most intense hurricanes. They usually mark the end the period of intensification. These hurricanes then maintain quasi-constant intensity or weaken. When the inner eye is completely dissipated, more intensification may occur.
The process by which a gas or vapor changes into a liquid.
Condensation Funnel
A funnel-shaped cloud associated with rotation and consisting of condensed water droplets (as opposed to smoke, dust, debris, etc.). Compare with debris cloud.
Any substance or object which carries electricity.
Conditional Symmetric Instability (CSI)
Fundamentally, CSI results from a combination of forces acting simultaneously along different planes of the atmosphere.

As is well known, an uneven distribution of gravitational forces in the vertical can give rise to convective instability. Similarly, horizontal inertial instability can develop in strongly anticyclonically-sheared regimes, due to an imbalance of centrifugal forces. CSI environments typically exhibit weak convective and inertial stability for strictly vertical or horizontal motions.

However, air parcels displaced along certain sloped, or "slantwise" trajectories may attain positive buoyancy due to a unique combination of gravitational and centrifugal forces.

While convective available potential energy (CAPE) values within these environments are typically much smaller than those associated with upright convection, sufficient energies are frequently present to support the formation and maintenance of thunderstorm cells. There is growing evidence supporting the role CSI plays in the development of some types of thunderstorms, particularly those occurring within strongly baroclinic regimes.

Confined Ground Water
Ground water held under an aquiclude or an aquifuge called artesian if the pressure is positive.
A pattern of wind flow in which air flows inward toward an axis oriented parallel to the general direction of flow. It is the opposite of difluence. Confluence is not the same as convergence. Winds often accelerate as they enter a confluent zone, resulting in speed divergence which offsets the (apparent) converging effect of the confluent flow.
Congressional Organic Act of 1890
The act that assigned the responsibility of river and floor forecasting for the benefit of the general welfare of the Nation's people and economy to the Weather Bureau, and subsequently the National Weather Service.
Conservation Storage
Storage of water for later release for usual purposes such as municipal water supply, power, or irrigation in contrast with storage capacity used for flood control.
Considerable Cloudiness
When the predominant/average sky condition is covered by more than half, but not completely covered by opaque (not transparent) clouds. In other words, 5/8 to 7/8 of the sky is covered by opaque clouds. Same as mostly cloudy.
Consolidated Ice Cover
Ice cover formed by the packing and freezing together of floes, brash ice and other forms of floating ice.
The volume of water in a reservoir. Unless otherwise indicated, reservoir content is computed on the basis of a level pool and does not include bank storage.
Continental Shelf (CONSHELF)
The zone bordering a continent and extending to a depth, usually around 100 fathoms (600 feet / 182 meters), from which there is a steep descent toward greater depth.
Continental Slope
The area of descent from the edge of the continental shelf into greater depth.
Control Points
Horizontal and Vertical: Small monuments securely embedded in the surface of the dam. Any movement of the monument indicates a movement in the dam itself. Movements in the dam are detected by comparing control points location to location of fixed monuments located off the dam using accurate survey techniques.
An acronym for Continental United States.

Generally, transport of heat and moisture by the movement of a fluid. In meteorology, the term is used specifically to describe vertical transport of heat and moisture, especially by updrafts and downdrafts in an unstable atmosphere.

The terms "convection" and "thunderstorms" often are used interchangeably, although thunderstorms are only one form of convection. Cumulonimbus (Cb), towering cumulus clouds, and Altocumulus Castellanus (ACCAS) clouds all are visible forms of convection.

However, convection is not always made visible by clouds. Convection which occurs without cloud formation is called dry convection, while the visible convection processes referred to above are forms of moist convection.

Convective Available Potential Energy (CAPE or B+)

It defines the vertically integrated positive buoyancy of an adiabatically rising air parcel on a sounding. This is proportional to the amount kinetic energy that the air parcel gains while it is warmer that its surrounding environment.

As a result, CAPE provides the best measure of the potential instability available in the atmosphere. Increasing values of CAPE generally lead to progressively vigorous convection. However, severe thunderstorms can form in environments showing weak to moderate CAPE, especially if the Storm Relative Helicity values are high.

Convective Clouds

The vertically developed family of clouds are cumulus and cumulonimbus. The height of their bases range from as low as 1,000 feet (300 meters) to a bit more than 10,000 feet (3,000 meters). Clouds with extensive vertical development are positive indications of unstable air.

Strong upward currents in vertically developed clouds can carry high concentrations of supercooled water to high levels where temperatures are quite cold. Upper portions of these clouds may be composed of water and ice.

Convective Condensation Level (CCL)
It is the height to which a parcel of air, if heated sufficiently from below, will rise adiabatically until it is just saturated (condensation starts). It approximates the base height of cumuliform clouds which are, or would be, produced by surface heating.
Convective INhibition (CIN or B-)

It represents the cumulative effect of atmospheric layers the are warmer than the parcel moving vertically along the adiabat. Low level parcel ascent is often inhibited by such stable layers near the surface. If natural processes fail to destabilize the lower levels, an input of energy from forced lift (a front, an upper level shortwave, etc.) will be required to move the negatively buoyant air parcels to the point where they will rise freely.

Since CIN is proportional to the amount of kinetic energy that a parcel loses to buoyancy while it is colder than the surrounding environment, it contributes to the downward momentum.

Convective Outlook (SWO)

A forecast containing the area(s) of expected thunderstorm occurrence and expected severity over the contiguous United States, issued several times daily by the Storm Prediction Center SPC in Norman, Oklahoma. They are sent out as both a narrative and a graphic covering a period of up to 52 hours in advance.

This product serves as guidance to the local National Weather Service Office for use in the preparation of forecast products issued; to advise the public, media, and other interests of the possibility of severe weather; and to assist with preliminary staffing should severe weather be anticipated.

The terms marginal risk, slight risk, enhanced risk, moderate risk, and high risk are used to describe severe thunderstorm potential. Local versions sometimes are prepared by local NWS offices.

Storm Prediction Center's risk categories
Convective Rain
Rain associated with convective or cumuliform clouds characterized by vertical development in the form of rising mounds, domes, or towers.
Convective Rings and Bands
Like "Stratiform Rings and Bands" they occur outside the eye wall of the hurricane. They exhibit a VIP Level 2 or greater reflectivity and occasionally display the bright band aloft. When they pass over a location, the wind speed increases by as much as 50%, accompanied by a significant increase in the rainfall rate. When tornadoes and downbursts occur, they are likely to come from convective rings and bands.
Convective SIGMETs
These NWS aviation products are issued in the conterminous U.S. for any of the following:
  • Severe thunderstorm due to:
    • surface winds greater than or equal to 50 knots (58 mph / 93 km/h)
    • hail at the surface greater than or equal to 1" (2.54 cm) in diameter,
    • tornadoes,
    • Embedded thunderstorms,
    • Line of thunderstorms, or
    • Thunderstorms greater than or equal to VIP level 4 affecting 40% or more of an area at least 3000 square miles (4,800 square kilometers).
Any Convective SIGMET implies severe or greater turbulence, severe icing, and low-level wind shear. A Convective SIGMET may be issued for any convective situation which the forecaster feels is hazardous to all categories of aircraft.
Convective Temperature
It is the surface temperature that must be reached to start the formation of convective clouds by solar heating of surface-air layer. Calculation of the convective temperature involves many assumptions, such that thunderstorms sometimes develop well before or well after the convective temperature is reached (or may not develop at all). However, in some cases the convective temperature is a useful parameter for forecasting the onset of convection.
A contraction of a vector field; the opposite of divergence. Convergence in a horizontal wind field indicates that more air is entering a given area than is leaving at that level. To compensate for the resulting "excess," vertical motion may result: upward forcing if convergence is at low levels, or downward forcing (subsidence) if convergence is at high levels. Upward forcing from low-level convergence increases the potential for thunderstorm development (when other factors, such as instability, are favorable). Compare with confluence.
Convergence Line
A horizontal line along which horizontal convergence of the airflow is occurring. Common forms of convergence lines are sea-breeze fronts, cold-air outflow from thunderstorms, and synoptic fronts.
Conveyance Loss
The loss of water from a conduit due to leakage, seepage, evaporation, or evapotranspiration.
Cooling Degree Day
see Degree Day.
Cooperative Observer
An individual (or institution) who takes precipitation and temperature observations-and in some cases other observations such as river stage, soil temperature, and evaporation-at or near their home, or place of business. Many observers transmit their reports by touch-tone telephone to an NWS computer, and nearly all observers mail monthly reports to the National Climatic Data Center to be archived and published.
Coordinated Universal Time (UTC)
The time in the zero-degree meridian time zone. In order to convert to Eastern Standard Time, subtract 5 hours (Eastern Daylight Time subtract 4 hours). For example, 0900 UTC is 4:00 AM EST or 5:00 AM EDT.
Core Punch
Slang for a penetration by a vehicle into the heavy precipitation core of a thunderstorm. Core punching is not a recommended procedure for storm spotting.
Coriolis Effect
The effect caused by the Earth's rotation which deflects air moving between two places. It causes an object to move to the right in the Northern Hemisphere and to the left in the Southern Hemisphere.
Corn Snow Ice
Rotten granular ice.
Corner Effects
A small-scale convergence effect that can be quite severe. It occurs around steep islands and headlands.
A measure of the similarity between variables or functions.
County Warning Area (CWA)
All the counties or parishes assigned to a specific National Weather Service Forecast Office for the purpose of warnings issuance and hazard awareness responsibility.
Adjacent maxima of radial velocities of opposite signs.
A separation formed in an ice cover of floe that does not divide it into two or more pieces.
A small stream of water which serves as the natural drainage course for a drainage basin of nominal, or small size. The term is a relative one as to size, some creeks in the humid section would be called rivers if they occurred in the arid portion.
  1. The highest stage or level of a flood wave as it passes a point.
  2. The top of a dam, dike, spillway, or weir, to which water must rise before passing over the structure.
Crest Gage
A gage used to obtain a record of flood crests at sites where recording gages are installed.
Crest (Top) of Dam
The elevation of the uppermost surface of a dam excluding any parapet walls, railings, etc.
Crest Width (Top thickness)
The thickness or width of a dam at the level of the crest (top) of the dam. The term "thickness" is used for gravity and arch dams and "width" for other types of dams.
Critical Depth
The depth of water flowing in an open channel or conduit, partially filled, corresponding to one of the recognized critical velocities.
Critical Flow
A condition of flow where the mean velocity is at one of the critical values; ordinarily at Belanger's critical depth and velocity. Another important usage is in reference to the Reynolds' critical velocities which define the point at which the flow changes from streamline or nonturbulent to turbulent flow.
Critical Rainfall Probability (CRP)
The Probability that the actual precipitation during a rainfall event has exceeded or will exceed the flash flood guidance value.
Cross Seas
Steep waves with short, sharp wave crests. They form when two or more wave trains moving in different directions run together.
Cross Section
See radar cross section.
Cross-Sectional Area
Area perpendicular to the direction of flow.
CRP (Critical Rainfall Probability)
The Probability that a given rainfall will cause a river, or stream to rise above flood stage.
The science of the physical aspects of snow, ice, hail, and sleet and other forms of water produced by temperatures below Zero degrees Celsius.
Fear of extreme cold, ice, or frost.
An acronym for Conditional Symmetric Instability.
An acronym for cumulus.
Cubic Feet Per Second (CFS)
A unit expressing rates of discharge. One cubic foot per second is equal to the discharge through a rectangular cross section, 1 foot wide by 1 foot deep, flowing at an average velocity of 1 foot per second. It is also approximately 7.48 gallons per second.
Descriptive of all clouds with vertical development in the form of rising mounds, domes, or towers.
Cumuliform Anvil
A thunderstorm anvil with visual characteristics resembling cumulus-type clouds (rather than the more typical fibrous appearance associated with cirrus). A cumuliform anvil arises from rapid spreading of a thunderstorm updraft, and thus implies a very strong updraft. See anvil rollover, knuckles, and mushroom.
Cumulonimbus Cloud (Cb)
They are the ultimate manifestation of instability. They are vertically developed clouds of large dimensions with dense "boiling" tops often crowned with thick veils of dense cirrus (anvil). This is also called a "thunderstorm cloud". It can produce very heavy precipitation, lightning, large hail (greater than 1"), damaging winds, and tornadoes.
Cumulonimbus Mammatus Cloud (CBMAM)
It is associated with a cumulonimbus cloud. It indicates extreme instability. This cloud is characterized by hanging festoons or protuberances underneath the anvil of the Cumulonimbus Cloud (Cb). The festoons may be at any level of the cloud from the underside of the anvil to the base of the cloud.
Cumulus Cloud (Cu)
These clouds form in convective currents and are characterized by relatively flat bases and dome-shaped tops. Fair-weather cumulus do not show extensive "towers" or vertical development and do not produce precipitation. A cumulus cloud may, however, be the initial stage in the development of towering cumulus or cumulonimbus (Cb). More often, fair-weather cumulus indicate a relatively shallow layer of instability.
Cumulus Congestus
Same as towering cumulus. Sometimes referred to just as congestus.
Current Meter
Device used to measure the water velocity or current in a river.
Curtain Drain
A drain constructed at the upper end of the area to be drained, to intercept surface or ground water flowing toward the protected area from higher ground, and carry it away from the area. Also called an Intercepting Drain.
The reciprocal of the radius of a circle; the rate of change in the deviation of a given arc from any tangent to it.
Cutoff Low
A closed low which has become completely displaced (cut off) from 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 (i.e., retrogression). "Cutoff low" and "closed low" often are used interchangeably to describe low pressure centers aloft. However, not all closed lows are completely removed from the influence of the basic westerlies. Therefore, the recommended usage of the terms is to reserve the use of "cutoff low" only to those closed lows which clearly are detached completely from the westerlies.
Cyclic Storm
A 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. A storm which undergoes only one cycle (pulse), and then dissipates, is known as a pulse storm.
Development or intensification of a low-pressure center (cyclone).
An area of low atmospheric pressure that has a closed circulation. Cyclones (or more commonly called "low pressure areas") rotate counter-clockwise in the Northern Hemisphere and clockwise in the Southern Hemisphere. They usually bring about clouds and precipitation.
Cyclonic Circulation (or Cyclonic Rotation)
Circulation (or rotation) which is in the same sense as the Earth's rotation, i.e., counterclockwise (in the Northern Hemisphere) as would be seen from above. Nearly all mesocyclones and strong or violent tornadoes exhibit cyclonic rotation, but some smaller vortices, such as gustnadoes, occasionally rotate anticyclonically (clockwise). Compare with anticyclonic rotation.