Weather Terms and Definitions

This page provides some basic weather terms and definitions which may appear on our site. At the end of some terms are links to additional information.

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Weather FAQ:
CA - Cloud-to-Air lightning

  • Cloud-to-Air lightning

Weather FAQ:
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.
  • Air parcels rising into this layer become cooler than the surrounding air, which inhibits their ability to rise further.
  • As such, the cap often prevents or delays thunderstorm development even in the presence of extreme instability.
  • However if the cap is removed or weakened, then explosive thunderstorm development can occur.
  • See CIN and Figure 6.
  • 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.
  • With the cap in place, air below it can continue to warm and/or moisten, thus increasing the amount of potential instability. Or, air above it can cool, which also increases potential instability.
  • But without a 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.

Weather FAQ:
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.
  • However, as with other indices or indicators, there are no threshold values above which severe weather becomes imminent.
  • CAPE is represented on a sounding by the area enclosed between the environmental temperature profile and the path of a rising air parcel, over the layer within which the latter is warmer than the former. (This area often is called positive area.)
  • See also CIN and Figure 6.

Weather FAQ:
Cb - Cumulonimbus Cloud

  • Cumulonimus cloud, characterized by strong vertical development in the form of mountains or huge towers topped at least partially by a smooth, flat, often fibrous anvil.
  • Also known colloquially as a "thunderhead."

Weather FAQ:
CC - Cloud-to-Cloud Lightning.

  • Cloud-to-Cloud lightning.

Weather FAQ:

  • 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.

Weather FAQ:
CG - Cloud-to-Ground Lightning Flash

  • Cloud-to-Ground lightning flash.

Weather FAQ:

  • 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.

Weather FAQ:
CIN - Convective INhibition

  • A measure of the amount of energy needed in order to initiate convection.
  • Values of CIN typically reflect the strength of the cap.
  • They are obtained on a sounding by computing the area enclosed between the environmental temperature profile and the path of a rising air parcel, over the layer within which the latter is cooler than the former. (This area sometimes is called negative area.)
  • See CAPE and Figure 6.

Weather FAQ:

  • High-level clouds (16,000 feet or more), composed of ice crystals and appearing in the form of white, delicate filaments or white or mostly white patches or narrow bands.
  • Cirrus clouds typically have a fibrous or hairlike appearance, and often are semi-transparent.
  • Thunderstorm anvils are a form of cirrus cloud, but most cirrus clouds are not associated with thunderstorms.

Weather FAQ:
Classic Supercell

Weather FAQ:
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.

Weather FAQ:
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).

Weather FAQ:
Cloud Height

  • The cloud height on this site is an estimate of cumulus clouds using a formula based on temperature and dew point. Actual measurements of cloud height are made with a ceilometer. This device fires a laser into the sky and measures the backscattered signal. Costs for such a device are beyond the scope of weather hobbyists.

Weather FAQ:
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.

Weather FAQ:
Cloud Tags

Weather FAQ:
Cold Advection

  • Transport of cold air into a region by horizontal winds.

Weather FAQ:
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.

Weather FAQ:
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.

Weather FAQ:
Collar Cloud

  • A generally circular ring of cloud that may be observed on rare occasions surrounding the upper part of a wall cloud.
  • See Figure 7.
  • This term sometimes is used (incorrectly) as a synonym for wall cloud.

Weather FAQ:
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.

Weather FAQ:
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 (see Figure 1).

Weather FAQ:
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.

Weather FAQ:

  • 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.

Weather FAQ:

Weather FAQ:

  • Generally, transport of heat and moisture by the movement of a fluid.
  • The circulatory motion that occurs at nonuniform temperatures due to gravity and density variation, resulting in the transfer of heat.
  • 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.
  • Cbs, towering cumulus clouds, and 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.

  Wikipedia: Atmospheric/Barometric Pressure

Weather FAQ:
Convective Outlook

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

Weather FAQ:
Convective Temperature

  • 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.

Weather FAQ:

  • 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.

Weather FAQ:
Core Punch

  • [Slang] A penetration by a vehicle into the heavy precipitation core of a thunderstorm.
  • Core punching is not a recommended procedure for storm spotting.

Weather FAQ:
Cosine Response

  • The output of a solar radiation sensor based on a given input. The absorption of radiation is proportional to the cosine of the angle between an oblique ray and one that strikes perpendicularly.

Weather FAQ:
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, mushroom.

Weather FAQ:

  • Detached clouds, generally dense and with sharp outlines, showing vertical development in the form of domes, mounds, or towers.
  • Tops normally are rounded while bases are more horizontal.
  • See Cb, towering cumulus.

Weather FAQ:
Cumulus Congestus

Weather FAQ:
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.

Weather FAQ:
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.

Weather FAQ:

  • Development or intensification of a low-pressure center (cyclone).

Weather FAQ:
Cyclonic Circulation

  • 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.


Weather FAQ:

  • Nondimensional "unit" of radar reflectivity which represents a logarithmic power ratio (in decibels, or dB) with respect to radar reflectivity factor, Z.
  • The value of Z is a function of the amount of radar beam energy that is backscattered by a target and detected as a signal (or echo). Higher values of Z (and dBZ) thus indicate more energy being backscattered by a target.
  • The amount of backscattered energy generally is related to precipitation intensity, such that higher values of dBZ that are detected from precipitation areas generally indicate higher precipitation rates.
  • However, other factors can affect reflectivity, such as width of the radar beam, precipitation type, drop size, or the presence of ground clutter or AP.
  • WSR-88D radars can detect reflectivities as low as -32 dBZ near the radar site, but significant (measurable) precipitation generally is indicated by reflectivities of around 15 dBZ or more.
  • Values of 50 dBZ or more normally are associated with heavy thunderstorms, perhaps with hail, but as with most other quantities, there are no reliable threshold values to confirm the presence of hail or severe weather in a given situation.
  • See VIP for threshold dBZ values associated with each VIP level.

Weather FAQ:
Debris Cloud

  • A rotating "cloud" of dust or debris, near or on the ground, often appearing beneath a condensation funnel and surrounding the base of a tornado.
  • This term is similar to dust whirl, although the latter typically refers to a circulation which contains dust but not necessarily any debris.
  • A dust plume, on the other hand, does not rotate.
  • Note that a debris cloud appearing beneath a thunderstorm will confirm the presence of a tornado, even in the absence of a condensation funnel.

Weather FAQ:
Degree Day

  • A measure of the departure of the mean daily temperature above or below a given standard. A ten-degree difference for one day equals ten degree days, as does a one-degree difference for ten days.

  Wikipedia: Degree Day

Weather FAQ:
Delta T

  • A simple representation of the mean lapse rate within a layer of the atmosphere, obtained by calculating the difference between observed temperatures at the bottom and top of the layer.
  • Delta Ts often are computed operationally over the layer between pressure levels of 700 mb and 500 mb, in order to evaluate the amount of instability in mid-levels of the atmosphere.
  • Generally, values greater than about 18 indicate sufficient instability for severe thunderstorm development.

Weather FAQ:

  • Derecho (pronounced deh-REY-cho), a widespread and usually fast-moving windstorm associated with convection.
  • Derechos include any family of downburst clusters produced by an extratropical MCS, and can produce damaging straight-line winds over areas hundreds of miles long and more than 100 miles across.

Weather FAQ:
Dew Point

  • Dew Point (or Dew-point Temperature)
  • A measure of atmospheric moisture.
  • A measure of humidity stated in terms of the temperature at which the air would be saturated and dew would begin to form if the amount of water vapor in the air were held constant. The dew point is an important measurement used to predict the formation of dew, frost, and fog. If dew point and temperature are close together in the late afternoon when the air begins to turn colder, fog is likely during the night. Dew point is also a good indicator of the air's actual water vapor content, unlike relative humidity, which takes the air's temperature into account. High dew point indicates high vapor content; low dew point indicates low vapor content. In addition a high dew point indicates a better chance of rain and severe thunderstorms. You can even use dew point to predict the minimum overnight temperature. Provided no fronts or other weather pattern changes are expected overnight, the afternoon's dew point gives you an idea of what minimum temperature to expect overnight.
  • It is the temperature to which air must be cooled in order to reach saturation (assuming air pressure and moisture content are constant).

  Wikipedia: Dew Point

Weather FAQ:
Differential Motion

  • Cloud motion that appears to differ relative to other nearby cloud elements, e.g. clouds moving from left to right relative to other clouds in the foreground or background.
  • Cloud rotation is one example of differential motion, but not all differential motion indicates rotation. For example, horizontal wind shear along a gust front may result in differential cloud motion without the presence of rotation.

Weather FAQ:

  • Difluence (or Diffluence)
  • A pattern of wind flow in which air moves outward (in a "fan-out" pattern) away from a central axis that is oriented parallel to the general direction of the flow.
  • It is the opposite of confluence.
  • Difluence in an upper level wind field is considered a favorable condition for severe thunderstorm development (if other parameters are also favorable).
  • But difluence is not the same as divergence. In a difluent flow, winds normally decelerate as they move through the region of difluence, resulting in speed convergence which offsets the apparent diverging effect of the difluent flow.

Weather FAQ:
Directional Shear

  • The component of wind shear which is due to a change in wind direction with height, e.g., southeasterly winds at the surface and southwesterly winds aloft.
  • A veering wind with height in the lower part of the atmosphere is a type of directional shear often considered important for tornado development.

Weather FAQ:

  • Daytime, or daily, or during the day
  • Related to actions which are completed in the course of a calendar day, and which typically recur every calendar day (e.g., diurnal temperature rises during the day, and diurnal falls at night).

Weather FAQ:

  • The expansion or spreading out of a vector field; usually said of horizontal winds. It is the opposite of convergence.
  • Divergence at upper levels of the atmosphere enhances upward motion, and hence the potential for thunderstorm development (if other factors also are favorable).

Weather FAQ:
Doppler Radar

  • Radar that can measure radial velocity, the instantaneous component of motion parallel to the radar beam (i.e., toward or away from the radar antenna).

Weather FAQ:

  • A strong downdraft resulting in an outward burst of damaging winds on or near the ground.
  • Downburst winds can produce damage similar to a strong tornado.
  • Although usually associated with thunderstorms, downbursts can occur with showers too weak to produce thunder.
  • See dry and wet microburst.

Weather FAQ:

  • A small-scale column of air that rapidly sinks toward the ground, usually accompanied by precipitation as in a shower or thunderstorm.
  • A downburst is the result of a strong downdraft.

Weather FAQ:

  • In the same direction as a stream or other flow, or toward the direction in which the flow is moving.

Weather FAQ:
Dry Adiabat

Weather FAQ:
Dry Line

  • A boundary separating moist and dry air masses, and an important factor in severe weather frequency in the Great Plains.
  • It typically lies north-south across the central and southern high Plains states during the spring and early summer, where it separates moist air from the Gulf of Mexico (to the east) and dry desert air from the southwestern states (to the west).
  • The dry line typically advances eastward during the afternoon and retreats westward at night.
  • However, a strong storm system can sweep the dry line eastward into the Mississippi Valley, or even further east, regardless of the time of day.
  • A typical dry line passage results in a sharp drop in humidity (hence the name), clearing skies, and a wind shift from south or southeasterly to west or southwesterly.
  • (Blowing dust and rising temperatures also may follow, especially if the dry line passes during the daytime; see dry punch).
  • These changes occur in reverse order when the dry line retreats westward. Severe and sometimes tornadic thunderstorms often develop along a dry line or in the moist air just to the east of it, especially when it begins moving eastward.
  • See LP storm.

Weather FAQ:
Dry-Line Bulge

  • A bulge in the dry line, representing the area where dry air is advancing most strongly at lower levels (i.e., a surface dry punch).
  • Severe weather potential is increased near and ahead of a dry line bulge.

Weather FAQ:
Dry-Line Storm

  • Generally, any thunderstorm that develops on or near a dry line.
  • The term often is used synonymously with LP storm, since the latter almost always occurs near the dry line.

Weather FAQ:
Dry Microburst

  • A microburst with little or no precipitation reaching the ground; most common in semi-arid regions.
  • They may or may not produce lightning. Dry microbursts may develop in an otherwise fair-weather pattern; visible signs may include a cumulus cloud or small Cb with a high base and high-level virga, or perhaps only an orphan anvil from a dying rain shower.
  • At the ground, the only visible sign might be a dust plume or a ring of blowing dust beneath a local area of virga.
  • Compare with wet microburst.

Weather FAQ:
Dry Punch

  • [Slang] A surge of drier air; normally a synoptic-scale or mesoscale process.
  • A dry punch at the surface results in a dry line bulge. A dry punch aloft above an area of moist air at low levels often increases the potential for severe weather.

Weather FAQ:
Dry Slot

  • A zone of dry (and relatively cloud-free) air which wraps east or northeastward into the southern and eastern parts of a synoptic scale or mesoscale low pressure system.
  • A dry slot generally is seen best on satellite photographs.

Weather FAQ:
Dust Devil

  • A small atmospheric vortex not associated with a thunderstorm, which is made visible by a rotating cloud of dust or debris (dust whirl).
  • Dust devils form in response to surface heating during fair, hot weather; they are most frequent in arid or semi-arid regions.

Weather FAQ:
Dust Plume

Weather FAQ:
Dust Whirl

Weather FAQ:

  • Generally, any forces that produce motion or affect change.
  • In operational meteorology, dynamics usually refer specifically to those forces that produce vertical motion in the atmosphere.


Weather FAQ:
ECMWF - European Center for Medium-Range Weather Forecasting

  • Operational references in forecast discussions typically refer to the ECMWF's medium-range forecast model.
  • See MRF, UKMET.

Weather FAQ:
Elevated Convection

  • Convection occurring within an elevated layer, i.e., a layer in which the lowest portion is based above the earth's surface.
  • Elevated convection often occurs when air near the ground is relatively cool and stable, e.g., during periods of isentropic lift, when an unstable layer of air is present aloft.
  • In cases of elevated convection, stability indices based on near-surface measurements (such as the lifted index) typically will underestimate the amount of instability present.
  • Severe weather is possible from elevated convection, but is less likely than it is with surface-based convection.

Weather FAQ:
Energy Helicity Index (or EHI)

  • An index that incorporates vertical shear and instability, designed for the purpose of forecasting supercell thunderstorms.
  • It is related directly to storm-relative helicity in the lowest 2 km (SRH, in m2/s2) and CAPE (in j/kg) as follows:
  • EHI=(CAPE x SRH)/160,000.
  • Thus, higher values indicate unstable conditions and/or strong vertical shear.
  • Since both parameters are important for severe weather development, higher values generally indicate a greater potential for severe weather.
  • Values of 1 or more are said to indicate a heightened threat of tornadoes; values of 5 or more are rarely observed, and are said to indicate potential for violent tornadoes.
  • However, there are no magic numbers or critical threshold values to confirm or predict the occurrence of tornadoes of a particular intensity.

Weather FAQ:
Enhanced V

  • A pattern seen on satellite infrared photographs of thunderstorms, in which a thunderstorm anvil exhibits a V-shaped region of colder cloud tops extending downwind from the thunderstorm core.
  • The enhanced V indicates a very strong updraft, and therefore a higher potential for severe weather.
  • Enhanced V should not be confused with V notch, which is a radar signature.

Weather FAQ:
Enhanced Wording

  • An option used by the SPC in tornado and severe thunderstorm watches when the potential for strong/violent tornadoes, or unusually widespread damaging straight-line winds, is high.
  • The statement "THIS IS A PARTICULARLY DANGEROUS SITUATION WITH THE POSSIBILITY OF VERY DAMAGING TORNADOES" appears in tornado watches with enhanced wording. Severe thunderstorm watches may include the statement "THIS IS A PARTICULARLY DANGEROUS SITUATION WITH THE POSSIBILITY OF EXTREMELY DAMAGING WINDS," usually when a derecho event is occurring or forecast to occur.
  • See PDS watch.

Weather FAQ:
Entrance Region

  • The region upstream from a wind speed maximum in a jet stream (jet max), in which air is approaching (entering) the region of maximum winds, and therefore is accelerating.
  • This acceleration results in a vertical circulation that creates divergence in the upper-level winds in the right half of the entrance region (as would be viewed looking along the direction of flow).
  • This divergence results in upward motion of air in the right rear quadrant (or right entrance region) of the jet max.
  • Severe weather potential sometimes increases in this area as a result.
  • See also exit region, left exit region.

Weather FAQ:
Equilibrium Level (or EL)

  • On a sounding, the level above the level of free convection (LFC) at which the temperature of a rising air parcel again equals the temperature of the environment.
  • See Figure 6
  • The height of the EL is the height at which thunderstorm updrafts no longer accelerate upward.Thus, to a close approximation, it represents the height of expected (or ongoing) thunderstorm tops.
  • However, strong updrafts will continue to rise past the EL before stopping, resulting in storm tops that are higher than the EL. This process sometimes can be seen visually as an overshooting top or anvil dome.
  • The EL typically is higher than the tropopause, and is a more accurate reference for storm tops.

Weather FAQ:
Erythema Action Spectrum

  • The range of wavelengths of light responsible for erythema, which is the reddening of the skin due to capillary congestion. Sunburn is among the most common forms of erythema.

Weather FAQ:
Eta Model

  • One of the operational numerical forecast models run at NCEP.
  • The Eta is run twice daily, with forecast output out to 48 hours.

Weather FAQ:

  • The amount of water transferred from the earth to the atmosphere due to the combined effects of evaporation and transpiration.
  • Transpiration is the process by which plants release water vapor into the air.

  Wikipedia: EvapoTranspiration

Weather FAQ:
Exit Region

  • The region downstream from a wind speed maximum in a jet stream (jet max), in which air is moving away from the region of maximum winds, and therefore is decelerating.
  • This deceleration results in divergence in the upper-level winds in the left half of the exit region (as would be viewed looking along the direction of flow).
  • This divergence results in upward motion of air in the left front quadrant (or left exit region) of the jet max. Severe weather potential sometimes increases in this area as a result.
  • See also entrance region, right entrance region.


Weather FAQ:
F Scale

Weather FAQ:
Feeder Bands

  • Lines or bands of low-level clouds that move (feed) into the updraft region of a thunderstorm, usually from the east through south (i.e., parallel to the inflow).
  • Same as inflow bands.
  • This term also is used in tropical meteorology to describe spiral-shaped bands of convection surrounding, and moving toward, the center of a tropical cyclone.

Weather FAQ:
Flanking Line

  • A line of cumulus or towering cumulus clouds connected to and extending outward from the most active part of a supercell, normally on the southwest side.
  • The line normally has a stair-step appearance, with the tallest clouds closest to the main storm, and generally coincides with the pseudo-cold front.
  • See Figure 3 and Figure 7.

Weather FAQ:
Fosberg Index

  • (Also called Fosberg Fire Weather Index or Fire Weather Index.)  A measure that reflects expected flame length and fuel drying based on wind speed, temperature and humidity. High values indicate high flame lengths and rapid drying.
  • Fosberg Fire Weather Index uses the current temperature, relative humidity, and 10 minute average wind speed, where the Chandler Burning Index only uses temperature and relative humidity.
  • The FWI (Fire Weather Index) is defined by a quantitative model that provides a nonlinear filter of meteorological data which results in a linear relationship between the combined meteorological variables of relative humidity and wind speed, and the behavior of wildfires. Thus the index deals with only the weather conditions, not the fuels. Several sets of conditions have been defined by Fosberg (Fosberg, 1978) to apply this to fire weather management. The upper limits have been set to give an index value of 100 if the moisture content is zero and the wind is 30 mph. Thus, the numbers range from 0 to 100 and if any number is larger than 100, it is set back to 100. The index can be used to measure changes in fire weather conditions. Over several years of use, Fosberg index values of 50 or greater generally appear significant on a national scale. The SPC fire weather verification scheme uses the Fosberg Index, but with a check for both temperature (60F) and adjective fire danger rating (3-High, 4-Very High, 5-Extreme).
  • Fosberg index values are displayed in increments of 10 starting at 50 through 100 with the color pink indicating values of 50 or 60, dark orange indicating values of 70 or 80, while values of 90 and 100 are shown in bright orange.
  • The Fosberg Index, orginially called the Fire Weather Index (Fosberg, 1978), was created to meet management needs for timeliness of weather information and for a meaningful interpretation of the short time and close space weather impacts on fire management. It is a non-linear filter of meteorological data developed by first transforming temperature and relative humidity to equilibrium moisture content, then transforming the equilibrium moisture content to combustion efficiency.
  • The index is approximated by F = D((Rate of Spread) (Energy Release)) ^0.46

Weather FAQ:
Forward Flank Downdraft

Weather FAQ:

  • A boundary or transition zone between two air masses of different density, and thus (usually) of different temperature.
  • A moving front is named according to the advancing air mass, e.g., cold front if colder air is advancing.

Weather FAQ:

  • Ragged, detached cloud fragments; same as scud.

Weather FAQ:
Freezing Rain

  • Made of supercooled raindrops. The rain falls in liquid form but freezes when it hits the ground or an exposed object, creating a coating of ice known as glaze.

  Wikipedia: Freezing Rain

Weather FAQ:
Fujita Scale (or F Scale)

  • A scale of wind damage intensity in which wind speeds are inferred from an analysis of wind damage:
  • F0 (weak):    40-72 mph, light damage.
  • F1 (weak):    73-112 mph, moderate damage.
  • F2 (strong):  113-157 mph, considerable damage.
  • F3 (strong):  158-206 mph, severe damage.
  • F4 (violent):  207-260 mph, devastating damage.
  • F5 (violent):  261-318 mph, (rare) incredible damage.
  • All tornadoes, and most other severe local windstorms, are assigned a single number from this scale according to the most intense damage caused by the storm.

Weather FAQ:
Funnel Cloud

  • A condensation funnel extending from the base of a towering cumulus or Cb, associated with a rotating column of air that is not in contact with the ground (and hence different from a tornado).
  • A condensation funnel is a tornado, not a funnel cloud, if either a) it is in contact with the ground or b) a debris cloud or dust whirl is visible beneath it.


Weather FAQ:
Ground Clutter

  • A pattern of radar echoes from fixed ground targets (buildings, hills, etc.) near the radar.
  • Ground clutter may hide or confuse precipitation echoes near the radar antenna.

Weather FAQ:

  • [Slang] Anything in the atmosphere that restricts visibility for storm spotting, such as fog, haze, precipitation (steady rain or drizzle), widespread low clouds (stratus), etc.

Weather FAQ:

  • Gusts are short-lived increases in the strength of the wind. Conversely, when the wind decreases to less than the average wind speed, this is known as a lull. Gusts can be very brief, or can last several seconds or more. A squall is a sudden increase in wind speed which is typically associated with active weather, such as rain showers, thunderstorms, or heavy snow, but which lasts much longer than a gust. Squalls refer to an increase in the non-sustained winds over an extended time interval, as there may be lower gusts during a squall event.

  Wikipedia: Wind

Weather FAQ:
Gust Front

Weather FAQ:
Gustnado (or Gustinado)


Weather FAQ:
Heat Index

  • The Heat Index is a measure of relative discomfort due to combined heat and high humidity. It was developed by R.G. Steadman (1979) and is based on physiological studies of evaporative skin cooling for various combinations of ambient temperature and humidity. As temperatures climb above 90 °F and humidity goes above 40 percent, conditions are ripe for heat-related illnesses.

Heat Index Chart

  Wikipedia: Heat Index

Weather FAQ:

  • A property of a moving fluid which represents the potential for helical flow (i.e. flow which follows the pattern of a corkscrew) to evolve.
  • Helicity is proportional to the strength of the flow, the amount of vertical wind shear, and the amount of turning in the flow (i.e. vorticity).
  • Atmospheric helicity is computed from the vertical wind profile in the lower part of the atmosphere (usually from the surface up to 3 km), and is measured relative to storm motion.
  • Higher values of helicity (generally, around 150 m2/s2 or more) favor the development of mid-level rotation (i.e. mesocyclones).
  • Extreme values can exceed 600 m2/s2.

Weather FAQ:
High Risk

Weather FAQ:

  • A plot representing the vertical distribution of horizontal winds, using polar coordinates.
  • A hodograph is obtained by plotting the end points of the wind vectors at various altitudes, and connecting these points in order of increasing height.
  • Interpretation of a hodograph can help in forecasting the subsequent evolution of thunderstorms (e.g., squall line vs. supercells, splitting vs. non-splitting storms, tornadic vs. nontornadic storms, etc.).

Weather FAQ:
Hook (or Hook Echo)

  • A radar reflectivity pattern characterized by a hook-shaped extension of a thunderstorm echo, usually in the right-rear part of the storm (relative to its direction of motion).
  • A hook often is associated with a mesocyclone, and indicates favorable conditions for tornado development.
  • See Figure 2 and Figure 7.

Weather FAQ:
HP Storm (or HP Supercell)

  • High-Precipitation storm (or High-Precipitation supercell).
  • A supercell thunderstorm in which heavy precipitation (often including hail) falls on the trailing side of the mesocyclone (Figure 3).
  • Precipitation often totally envelops the region of rotation, making visual identification of any embedded tornadoes difficult and very dangerous.
  • Unlike most classic supercells, the region of rotation in many HP storms develops in the front-flank region of the storm (i.e., usually in the eastern portion).
  • HP storms often produce extreme and prolonged downburst events, serious flash flooding, and very large damaging hail events.
  • Mobile storm spotters are strongly advised to maintain a safe distance from any storm that has been identified as an HP storm; close observations (e.g., core punching) can be extremely dangerous.
  • See bear's cage.

Weather FAQ:

  • Hectopascal, a measurement of barometric pressure. One hPA equals 100 pascals or one millibar. One pascal is equal to a force of one newton over an area of one square meter.

  Wikipedia: Hectopascal

Weather FAQ:

  • The humidex is a number used by Canadian meteorologists to reflect the combined effect of heat and humidity - a way to describe what the temperature feels like when it's very hot and very humid. It differs from the heat index used in the United States in using dew point rather than relative humidity.
  • When the temperature is 30°C (86°F) and the dew point is 15°C (59°F), the humidex will be 34 (note that humidex is a dimensionless number, but that the number indicates an approximate temperature in °C). If the temperature remains 30°C but the dew point rises to 25°C (77°F), the humidex will rise to 42. The humidex tends to be higher than the U.S. heat index at equal temperature and relative humidity.
  • The humidex was introduced in 1965 by what is now the Meteorological Service of Canada. The current formula for determining the humidex was developed by J.M. Masterton and F.A. Richardson of Canada's Atmospheric Environment Service in 1979. The term humidex is widely used during the summer months in weather reports.
  • The Weather Service makes the following assessment of how unpleasant to how dire humidex ratings can seem:
Humidex Table - Legend
Humidex Degree of Comfort
20 - 29 No discomfort.
30 - 39 Some discomfort.
40 - 45 Great discomfort. Avoid exertion
Above 45 Dangerous. Probable heat stroke
Above 54 Heat stroke imminent
  • The humidex formula is as follows:
  • e = 6.11 * exp [ 5417.7530 * ( ( 1 / 273.16 ) - ( 1 / dew point in kelvins ) ) ]
  • h = ( 0.5555 ) * ( e - 10.0 )
  • humidex = air temperature (in °Celsius ) + h
  • The complete formula:
  • humidex = air temperature + 0.5555 * (6.11 * 5417.7530 * (( 1 / 273.16) - (1 / dew point)) - 10.0)

    Also check the Humidex Tables

  Wikipedia: Humidex

Weather FAQ:

  • Generally, a measure of the water vapor content of the air.
  • Humidity or relative humidity measures the amount of water vapor in the air relative to the temperature.
  • It is important in weather because humidity affects how humans feel.
  • A hot, humid day feels hotter because we cannot sweat as effectively. A cool, dry day feels colder because moisure evaporates more easily.
  • Popularly, it is used synonymously with relative humidity.

  Wikipedia: Humidity

Most of the definitions on this page were borrowed from El Dorado Weather

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Some of the definitions on this page were borrowed from

Some of the definitions on this page were borrowed from Davis Instruments