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. 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 NIÑO 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. 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. 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. 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. 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.