Weather Glossary - E

Easterly wave

A migratory wave like trough or disturbance in tropical regions that moves from east to west. Normally it moves slower then the winds in which it is embedded. It is often associated with tropical cyclone development.

ECMWF

European Centre for Medium-Range Weather Forecasting. Operational references in forecast discussions typically refer to the ECMWF's global forecast model, which provides forecasts out to 7 days.

Ekman spiral

An idealised description of the way wind-driven ocean currents vary with depth. In the atmosphere, it represents the way the wind varies from the surface upwards in the planetary boundary layer.

Ekman transport is the net water transport due to the Ekman spiral. In the southern hemisphere, net mass transport is 90° to the left of the surface wind direction. Ekman transport away from the eastern Australian coast (caused by prolonged northerly winds) results in upwelling near the coast.

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. Also called high-based convection.

Energy Helicity Index (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=(CAPEx 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. Vaues 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, as with most indices, there are no magic numbers or critical threshold values to confirm or predit the occurence of tornadoes of a particular intensity.

ENSO

El Niño/Southern Oscillation. The condition in the tropical Pacific Ocean where the reversal of surface air pressure at opposite ends of the Pacific induces westerly winds, a strengthening in the equatorial countercurrent and extensive ocean surface warming.

Equatorial trough

The zone of relatively low pressure, which lies between the subtropical anticyclones of the two hemispheres.

Equivalent Potential Temperature

The temperature a parcel of air would have if

• it is lifted until it became saturated,
• all water vapour is condensed out, and
• returned adiabatically (i.e., without transfer of heat or mass) to a pressure of 1000 millibars.

Evapotranspiration

The total amount of water that is transferred from the earth’s surface to the atmosphere. It is made up of evaporation and transpiration.

Extratropical cyclone

A cyclone (low pressure system) that possesses a cold core. Any low pressure system that forms outside the tropics is extratropical. A tropical cyclone will become extratropical if it drifts into temperature regions and becomes cold cored.

Eye wall

The wall of dense thunderstorms that surrounds the eye of a Tropical Cyclone. Generally around 50-100km in diameter and the region of strongest winds and heaviest rainfall.