Weather Glossary: C's

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

• 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).

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.