Easterlies - any winds with components from the east

Eccentricity - a dimensionless quantity describing the elliptical shape of a planet's orbit

Echo Tops - 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

ECMWF - European Center for Medium-Range Weather Forecasting

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

Electromagnetic Radiation - see radiation

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 energy

Energy - the capacity for doing work and overcoming resistance

Entropy - the amount of Energy that is not available for work during a certain process

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 - a state of balance in a system where opposing forces or fluxes balance

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.

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 degrees Kelvin, is directly related to the amount of heat and moisture present in an air parcel. Thus, it is useful in diagnosing atmospheric instability.

Equinox - the time when the sun crosses the equator and day and night are everywhere of equal length

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 process by which a liquid is transformed into a gas

Evapotranspiration - loss of water from the soil both by evaporation and by transpiration from the plants growing thereon

Exosphere - the upper most 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 being 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

Eye Wall- the collection of thunderstorms which generally surround the eye of a hurricane