Weather Resources - Glossary

Wondering what “graupel” is? Or how about “virga”? You have come to the right place. This section contains the definitions of several hundred weather related terms.

Please contact us if you have a suggestion for additional terms.

We have also developed a special glossary section with winter weather terms only.

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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.

El Nino/ La Nina - It is a pattern of warm or cold water in the equatorial Pacific has ended for the time being. Sea surface temperatures have returned to a slight warm, indicating that an El Nino event is occurring once again. It is still too early to know if this will become a strong, long lasting El Nino, or just slightly warmer than normal sea surface temperatures, so the effects of El Nino may still not be a major factor in the long range forecast.
El Nino winters and early springs often bring heavy storms into the southern and central parts of California and eventually into southwestern Colorado. The really heavy snows hit in the San Juan Mountains and favor Telluride, Durango and Wolf Creek.
La Nina tends to favor the northern mountains of Colorado with an abundance of snow. Steamboat enjoys a wonderful season during La Nina events, but the rest of the high country can often use more snow.
Along the Front Range, La Nina winters tend to be dry and windy ones - indeed 1999-2000 proved to be that way. El Nino winters can bring a few good storms, for instance 1997 was an El Nino year, but for the most part, the eastern plains do not feel the biggest effects from the El Nino/La Nina phenomena.
When there are near normal ocean temperatures in the Pacific, other factors enter into the long-range forecast equation. These include temperature fluctuations in the northern Atlantic and surface pressure oscillations in the arctic. Other factors include the amount of snow pack over the plains of northern and central Canada, southward into the northern Great Plains of the United States. All of these factors are important, but they tend to be much weaker indicators of the type of winter we will have - especially compared to strong indicators like El Nino or La Nina.

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 Fujita Scale (or EF Scale) - The scale for rating the strength of tornadoes estimated by the damage they cause. The EF scale replaced the original Fujita scale (or F Scale) on February 1, 2007. The scale was revised to better align the estimated wind speeds with the actual damage observed by storm damage survey crews.

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, (rare) total destruction.

All tornadoes are assigned a single number from this scale according to the most intense damage caused by the storm. Approximately 89% of all tornadoes in Colorado since 1950 have been classified as EF0 or EF1.

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.

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.

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.

Eta Model
(now known as the NAM model) - One of the operational numerical forecast models run at NCEP. The NAM is run four times daily, with forecast output out to 84 hours or 3.5 days.

Evaporation - A process that occurs when a liquid becomes a gas. It is a cooling process and is the opposite of condensation.

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.

Eye - The center of a hurricane or tropical storm where the wind is light and skies are variably cloudy. It's diameter ranges from under a mile to several miles across.

Portions of the 9NEWS Weather Glossary were taken from the second edition of the Glossary of Meteorology published by the American Meteorological Society (AMS). © 2009 American Meteorological Society