Glossary
Echo
energy back-scattered from a target (precipitation, clouds, etc.) and received by radar and displayed on a radar screen.
EchoTops
the height above ground of the center of the radar beam using the tilt, or scan, that contains the highest elevation where reflectivities greater than 18 dBZ can be detected
Effective Terrestrial Radiation
the difference between upwelling infrared or terrestrial radiation emitted from the earth and the downwelling infrared radiation from the atmosphere
El Niño
a warming of the ocean current along the coasts of Peru and Ecuador that is generally associated with dramatic changes in the weather patterns of the region; a major El Niño event generally occurs every 3 to 7 years and is associated with changes in the weather patterns worldwide
El Niño Southern Oscillation (ENSO)
a combination of ocean current warming and a "see-saw" of pressure changes. Along the coasts of Peru and Ecuador, sea surface temperatures warm, changing the weather patterns over the Pacific Ocean and influencing the weather across the world. The Southern Oscillation is characterized by simultaneously opposite sea level pressure anomalies at Tahiti and Darwin, Australia.
Electric Field
an invisible force created by the attraction and repulsion of electrical charges. It is measured in Volts per meter (V/m).
Electromagnetic Spectrum
the ordered series of all known types of electromagnetic radiation, arranged by wavelength ranging from the short cosmic rays through gamma rays, X-rays, ultraviolet radiation, visible radiation, infrared radiation, microwaves, to the long wavelengths of radio waves
Elevated Convection
convection that occurs within an elevated layer, i.e., a layer in which the lowest portion is based above the earth’s surface. This convection often occurs when air near the ground is relatively cool and stable, but an unstable layer of air is present aloft. Severe weather is possible, but is less likely than it is with surface-based convection.
Enhanced Fujita Scale (EF Scale)
an improved tornado damage rating scale. It takes into account structures and vegetation, as well as structural integrity of buildings.
|
EF0 |
Weak |
65 - 85 mph |
Light damage |
|
EF1 |
Weak |
86 - 110 mph |
Moderate damage |
|
EF2 |
Strong |
111 - 135 mph |
Considerable damage |
|
EF3 |
Strong |
136 - 165 mph |
Severe damage |
|
EF4 |
Violent |
166 - 200 mph |
Devastating damage |
|
EF5 |
Violent |
Over 200 mph |
Incredible damage |
Ensemble Forecast
multiple predictions from an ensemble of slightly different initial conditions and/or various versions of models. The goal is to improve the accuracy of the forecast by averaging the various forecasts--this eliminates non-predictable components. Forecasters use this tool to measure the likelihood of a forecast.
Entrainment
the capture or mixing of environmental air into a preexisting air current or cloud. For example, mixing in the atmosphere can bring drier air into a cloud (the dry air is entrained or brought into a particular area). Dry air entrainment (bringing dry air in) can help "destroy" clouds by reducing the amount of moisture in a particular area.
Entropy
the amount of energy that is not available for work during a certain process. It is sometimes described as a measure of how disorganized a system is. The world tends toward increasing entropy (less organization, which is often irreversible). Entropy is an important part of the Second Law of Thermodynamics and is measured in units of energy per temperature (joules per kelvin).
Environmental Modeling Center (EMC)
one of 9 NCEP centers. Develops and improves numerical weather, climate, hydrological, and ocean predictions. It is located in Camp Springs, Maryland.
Environmental Temperature
the temperature that is measured by a radiosonde and plotted on a Skew-T diagram. It is also the temperature outside of an imaginary parcel.
Equation of State
the state of an ideal gas can be shown to be pa=RT, where p is the pressure; a is the specific volume; R is the specific gas constant; and T is the absolute temperature
Equilibrium Level (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 environmental temperature. 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.
Equilibrium Vapor Pressure
the vapor pressure of a system in which vapor and liquid (or vapor, liquid and solid) coexist in equilibrium (i.e., at saturation)
Equivalent Potential Temperature (or Theta-e)
the temperature a parcel of air would have if a) it was lifted until it became saturated, b) all water vapor was condensed out, and c) it was returned adiabatically (i.e., without transfer of heat or mass) to a pressure of 1000 millibars. Theta-e, which typically is expressed in Kelvins, is directly related to the amount of heat and moisture present in an air parcel. Thus, it is useful in diagnosing atmospheric instability.
Eta Model (ETA)
one of the operational numerical forecast models run at NCEP, now known as the North American Mesoscale model (NAM). It is run four times daily (00, 06, 12, 18 UTC), with forecast output out to 84 hours.
European Center for Medium-Range Weather Forecasting Model (ECMWF)
a medium-range numerical forecast model that runs out to 10 days.
Evaporation (or Mixing) Fog
when moist air mixes with cold, dry air, it can form this type of fog. The evaporation of water raises the dew point of the adjacent air. One example is steam fog, which forms when cold air blows over warm water, such as a lake. The warm, moist air rises into the cold, dry air, giving the appearance of "steam".
Exosphere
the uppermost layer of the earth’s atmosphere; the only layer where atmospheric gases can escape into outer space
Extratropical Cyclone
a cyclone in the middle and high latitudes, often 2000 kilometers in diameter and usually containing a cold front that extends toward the equator for hundreds of kilometers
Eye
the roughly circular area of a tropical cyclone marked by only light winds or completely calm with no precipitation
