see Cold Air Damming
see Cold Front
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.
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).
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
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.
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.
a warm, dry wind that descends the eastern slopes of the Rocky Mountains
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.
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.
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.
in aviation, sudden severe turbulence occurring in cloudless regions that causes violent buffeting of aircraft
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.
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.
the statistical collection of weather conditions at a place over a period of years
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.
the science that deals with climates and their phenomena
an instrument that measures angles of inclination; used to measure cloud ceiling heights
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.
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.
approximately 0.05 mm (~ 0.002 in) in diameter, about 100 times smaller than a typical rain droplet.
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)
a group of clouds, not necessarily of the same type, that has cloud bases at the same altitude
any technique carried out to introduce artificial substances into the cloud with the intent of altering the natural development of that cloud
streaks of lightning that pass from a cloud to the air, but do not strike the ground.
streaks of lightning reaching from one cloud to another.
streaks of lightning that touch both a cloud and the ground.
converge (see Convergence)
convective (see Convection)
see Carbon Dioxide
when large rain droplets overtake and collide with smaller droplets in their path, they may merge or stick together (coalesce).
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.
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.
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 theinstability is the greatest. At night, the clouds and precipitation usually diminish significantly.
an advancing edge of a cold air mass that is replacing a warmer air mass
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.
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
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.
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.
a funnel-shaped cloud associated with rotation and consisting of condensed water droplets (as opposed to smoke, dust, debris, etc.)
same as Towering Cumulus
a law of physics that states that energy cannot be created or destroyed. It can only be converted from one form to another
a law of physics that states that mass cannot be created or destroyed. It can only be transferred from one volume to another
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
an air mass that starts over land and is dry
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)
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
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.
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 Lifting Condensation Level (LCL).
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
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.
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
the set of colored rings around the sun (or moon) created when the light source shines through a thin cloud
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.
same as County Warning Area.
the alternating bands of light and dark (rays and shadows) seen at the earth’s surface when the sun shines through clouds
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.
see Cumulus Fractus
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".
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
same as Towering Cumulus
looks like a ragged or shredded cumulus cloud
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.
Cyclonic Vorticity Advection; see Positive Vorticity Advection (PVA)
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.
the development or intensification of a low-pressure center (cyclone)
(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).
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
see Positive Vorticity Advection (PVA)