24/7 Weather Glossary -- E Words
A | B | C | D | E | F | G | H | I | J and K | L | M | N | O | P | Q and R | S | T | U | V | W | X, Y and Z 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.EARTHLIGHT (EARTHSHINE)
The faint illumination of the dark part of the moon's disk produced by sunlight reflected onto the moon from the earth's surface and atmosphere.
EARTHQUAKE
A sudden, transient motion or trembling of the earth's crust, resulting from the waves in the earth caused by faulting of the rocks or by volcanic activity.EASTERLIES
Usually applied to the broad patterns of persistent winds with an easterly component, such as the easterly trade winds.EASTERLY WAVE
An inverted, migratory wave-like disturbance or trough in the tropical region that moves from east to west, generally creating only a shift in winds and rain. The low level convergence and associated convective weather occur on the eastern side of the wave axis. Normally, it moves slower than the atmospheric current in which it is embedded and is considered a weak trough of low pressure. It is often associated with possible tropical cyclone development and is also known as a tropical wave.ECHO
The energy return of a radar signal after it has hit the target.ECLIPSE
The obscuring of one celestial body by another.ECLIPTIC
The sun's apparent path across the sky that tracks a circle through the celestial sphere.ECOLOGY
The study of the relationships between living organisms and their environment.EDDY
A small disturbance of wind in a large wind flow, which can produce turbulent conditions. They can also be areas of warmer air north of the main westerlies or colder air south of the westerlies. In oceanic circulation, it is a circular movement of water usually formed where currents pass obstructions, between two adjacent currents flowing counter to each other, or along the edge of a permanent current.ELECTROMAGNETIC RADIATION
Also called radiation, it is waves of energy propagated though space or through a material media.ELECTROMAGNETIC SPECTRUM
The band of electromagnetic radiation with components that are separated into their relative wave lengths. The portion of the spectrum that the human eye can detect is called visible light, between the longer infrared waves and the shorter ultraviolet waves. The various types of energy comprising the spectrum are (from longest to shortest) radio, infrared, visible, ultraviolet, x-rays, gamma rays, and cosmic rays.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.ELEVATION
The measure of height with respect to a point on the earth's surface above mean sea level. Sometimes referred to as station elevation.EL NINO
The cyclical warming of East Pacific Ocean sea water temperatures off the western coast of South America that can result in significant changes in weather patterns in the United States and elsewhere. This occurs when warm equatorial waters move in and displace the colder waters of the Humbolt Current, cutting off the upwelling process.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.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.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.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.ENVIRONMENT
The sum total of all the external conditions that effect an organism, community, material, or energy.EQUATOR
The geographic circle at 0 degrees latitude on the earth's surface. It is equal distance from the North and South Poles and divides the Northern Hemisphere from the Southern.EQUATORIAL TROUGH
The quasi-continuous area of low pressure between the subtropical high pressure areas in both the Northern and Southern Hemisphere.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.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.EQUINOX
The point at which the ecliptic intersects the celestial equator. Days and nights are most nearly equal in duration. In the Northern Hemisphere, the vernal equinox falls on or about March 20 and the autumnal equinox on or about September 22.EROSION
The movement of soil or rock from one area to another by the action of the sea, running water, moving ice, precipitation, or wind.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.EVAPORATION
The physical process by which a liquid, such as water is transformed into a gaseous state, such as water vapor. It is the opposite physical process of condensation.EVAPOTRANSPIRATION
The total amount of water that is transferred from the earth's surface to the atmosphere. It is made up of the evaporation of liquid or solid water plus the transpiration from plants.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.EXOSPHERE
This region is considered the very outer limits of the earth's atmosphere. Its lower boundary is often called the critical level of escape, where gas atoms are so widely spaced that they rarely collide with one another and have individual orbits. It is estimated to be some 400 plus miles (640 kilometers) above the surface.EXTRATROPICAL CYCLONE
Any cyclone not of tropical origin. Generally considered to be a migratory frontal cyclone found in the middle and high latitudes.EYE
The center of a tropical storm or hurricane, characterized by a roughly circular area of light winds and rain-free skies. An eye will usually develop when the maximum sustained wind speeds exceed 78 mph. It can range in size from as small as 5 miles to up to 60 miles, but the average size is 20 miles. In general, when the eye begins to shrink in size, the storm is intensifying.EYE WALL
An organized band of convection surrounding the eye, or center, of a tropical cyclone. It contains cumulonimbus clouds, intense rainfall and very strong winds. A | B | C | D | E | F | G | H | I | J and K | L | M | N | O | P | Q and R | S | T | U | V | W | X, Y and Z
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.EARTHLIGHT (EARTHSHINE)
The faint illumination of the dark part of the moon's disk produced by sunlight reflected onto the moon from the earth's surface and atmosphere.
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A sudden, transient motion or trembling of the earth's crust, resulting from the waves in the earth caused by faulting of the rocks or by volcanic activity.EASTERLIES
Usually applied to the broad patterns of persistent winds with an easterly component, such as the easterly trade winds.EASTERLY WAVE
An inverted, migratory wave-like disturbance or trough in the tropical region that moves from east to west, generally creating only a shift in winds and rain. The low level convergence and associated convective weather occur on the eastern side of the wave axis. Normally, it moves slower than the atmospheric current in which it is embedded and is considered a weak trough of low pressure. It is often associated with possible tropical cyclone development and is also known as a tropical wave.ECHO
The energy return of a radar signal after it has hit the target.ECLIPSE
The obscuring of one celestial body by another.ECLIPTIC
The sun's apparent path across the sky that tracks a circle through the celestial sphere.ECOLOGY
The study of the relationships between living organisms and their environment.EDDY
A small disturbance of wind in a large wind flow, which can produce turbulent conditions. They can also be areas of warmer air north of the main westerlies or colder air south of the westerlies. In oceanic circulation, it is a circular movement of water usually formed where currents pass obstructions, between two adjacent currents flowing counter to each other, or along the edge of a permanent current.ELECTROMAGNETIC RADIATION
Also called radiation, it is waves of energy propagated though space or through a material media.ELECTROMAGNETIC SPECTRUM
The band of electromagnetic radiation with components that are separated into their relative wave lengths. The portion of the spectrum that the human eye can detect is called visible light, between the longer infrared waves and the shorter ultraviolet waves. The various types of energy comprising the spectrum are (from longest to shortest) radio, infrared, visible, ultraviolet, x-rays, gamma rays, and cosmic rays.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.ELEVATION
The measure of height with respect to a point on the earth's surface above mean sea level. Sometimes referred to as station elevation.EL NINO
The cyclical warming of East Pacific Ocean sea water temperatures off the western coast of South America that can result in significant changes in weather patterns in the United States and elsewhere. This occurs when warm equatorial waters move in and displace the colder waters of the Humbolt Current, cutting off the upwelling process.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.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.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.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.ENVIRONMENT
The sum total of all the external conditions that effect an organism, community, material, or energy.EQUATOR
The geographic circle at 0 degrees latitude on the earth's surface. It is equal distance from the North and South Poles and divides the Northern Hemisphere from the Southern.EQUATORIAL TROUGH
The quasi-continuous area of low pressure between the subtropical high pressure areas in both the Northern and Southern Hemisphere.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.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.EQUINOX
The point at which the ecliptic intersects the celestial equator. Days and nights are most nearly equal in duration. In the Northern Hemisphere, the vernal equinox falls on or about March 20 and the autumnal equinox on or about September 22.EROSION
The movement of soil or rock from one area to another by the action of the sea, running water, moving ice, precipitation, or wind.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.EVAPORATION
The physical process by which a liquid, such as water is transformed into a gaseous state, such as water vapor. It is the opposite physical process of condensation.EVAPOTRANSPIRATION
The total amount of water that is transferred from the earth's surface to the atmosphere. It is made up of the evaporation of liquid or solid water plus the transpiration from plants.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.EXOSPHERE
This region is considered the very outer limits of the earth's atmosphere. Its lower boundary is often called the critical level of escape, where gas atoms are so widely spaced that they rarely collide with one another and have individual orbits. It is estimated to be some 400 plus miles (640 kilometers) above the surface.EXTRATROPICAL CYCLONE
Any cyclone not of tropical origin. Generally considered to be a migratory frontal cyclone found in the middle and high latitudes.EYE
The center of a tropical storm or hurricane, characterized by a roughly circular area of light winds and rain-free skies. An eye will usually develop when the maximum sustained wind speeds exceed 78 mph. It can range in size from as small as 5 miles to up to 60 miles, but the average size is 20 miles. In general, when the eye begins to shrink in size, the storm is intensifying.EYE WALL
An organized band of convection surrounding the eye, or center, of a tropical cyclone. It contains cumulonimbus clouds, intense rainfall and very strong winds. A | B | C | D | E | F | G | H | I | J and K | L | M | N | O | P | Q and R | S | T | U | V | W | X, Y and Z
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