Glossary of Weather Terms - Beginning with "E"

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Searching for what is El Nino, the difference between El Nino vs La Nina, how the Enhanced Fujita Scale works, the difference between a hurricane eye and eyewall, or what an extratropical cyclone is? This page defines meteorological terms beginning with “E”, in plain English, with guidance on what each means and how it affects your weather.

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Weather Terms Beginning with “E”

El Nino

El Nino is a periodic warming of the central and eastern tropical Pacific Ocean sea surface temperatures that recurs every 2-7 years and disrupts normal global weather patterns. It is officially declared when sea surface temperatures in the Nino 3.4 region of the Pacific are at least 0.5°C above average for five consecutive overlapping three-month periods. In the United States, El Nino typically brings warmer and drier winters across the northern tier states, wetter and cooler winters across the South and Southwest (often helpful in alleviating California droughts), and reduced Atlantic hurricane activity due to increased upper-level wind shear over the tropical Atlantic. Globally, El Nino can cause severe droughts in Australia and the western Pacific and catastrophic flooding along the Pacific coast of South America. A typical El Nino event lasts 9-12 months before conditions return to neutral or flip to the opposite La Nina phase.

La Nina

La Nina is the opposite phase of El Nino, a cooling of the central and eastern tropical Pacific Ocean sea surface temperatures at least 0.5°C below the long-term average. Where El Nino weakens the atmospheric circulation over the Pacific, La Nina strengthens it, enhancing the jet stream and intensifying temperature contrasts between air masses. In the United States, La Nina winters typically bring cooler and wetter conditions to the northern tier states (often with above-normal snowfall across the northern Plains and Great Lakes), while the Southwest and Southeast tend to be warmer and drier than normal. La Nina also significantly enhances Atlantic hurricane activity by reducing wind shear over the tropics. Unlike El Nino events that usually last one season, La Nina events can persist for one to three years, making their long-range impacts particularly significant for drought and water resource planning.

ENSO (El Nino-Southern Oscillation)

ENSO is the climate pattern describing the coupled ocean-atmosphere cycle in the tropical Pacific that drives El Nino and La Nina events. The “Southern Oscillation” component refers to the seesaw in atmospheric pressure between the eastern and western Pacific, when pressure is high in the east, it is low in the west, and vice versa. The Walker Circulation, a vast loop of rising air over the western Pacific, upper-level flow toward the east, sinking air over the eastern Pacific, and return flow near the surface, is strengthened and enhanced during La Nina, and weakened or reversed during El Nino. ENSO phases are measured using the Oceanic Nino Index (ONI), which categorizes current conditions as Neutral, El Nino, or La Nina based on a rolling three-month average of sea surface temperature anomalies in the Nino 3.4 region.

El Nino vs La Nina, Effects on US Weather

Region / Factor	El Nino Winter/Spring	La Nina Winter/Spring
Northern US (Great Plains to Great Lakes)	Warmer, drier than normal	Colder, snowier than normal
Southern US (Gulf Coast, Southeast)	Wetter, cooler than normal	Drier, warmer than normal
Pacific Northwest	Warmer, drier than normal	Wetter, cooler than normal
Southwest (California, Arizona)	Wetter, good for drought relief	Drier, drought conditions likely
Atlantic Hurricane Season	Below-normal activity	Above-normal activity
Tornado Alley (Spring)	Mixed; varied activity	Often more active spring severe season

Enhanced Fujita Scale (EF Scale)

The Enhanced Fujita Scale is the updated tornado intensity classification system adopted by the National Weather Service in February 2007, replacing the original Fujita Scale developed by Dr. Ted Fujita in 1971. It rates tornado damage on a six-level scale from EF0 (65-85 mph estimated winds, causing minor damage such as broken branches and damaged shingles) through EF5 (200 mph or more, causing complete destruction of well-constructed frame homes with swept-clean foundations). The EF Scale is based on 28 standardized Damage Indicators, from one-family residences to large shopping malls to hardwood trees, that are evaluated by trained NWS meteorologists during post-storm damage surveys. EF0 and EF1 tornadoes account for roughly 80% of all tornadoes but only about 5% of tornado-related deaths; EF4 and EF5 tornadoes make up less than 1% of all tornadoes but are responsible for roughly 70% of all tornado fatalities. See also: The Fujita Tornado Damage Scale, FAQ and Criteria for a Tornado Warning.

Equinox

An equinox is either of the two moments each year when the sun crosses the celestial equator and day and night are of approximately equal length everywhere on Earth. The Vernal (Spring) Equinox occurs around March 19-21 and marks the astronomical beginning of spring in the Northern Hemisphere, after which days grow progressively longer; the Autumnal (Fall) Equinox occurs around September 22-23 and marks the start of astronomical fall, after which nights grow longer. The word derives from the Latin “aequinoctium,” meaning “equal night.” The precise date of each equinox can shift by a day from year to year because Earth’s actual solar year is approximately 365.25 days, requiring the calendar to be adjusted by a leap day every four years to keep the seasons aligned.

Evaporation

Evaporation is the phase change process by which liquid water at the surface converts to water vapor and enters the atmosphere, driven by the absorption of heat energy that breaks the hydrogen bonds holding liquid water molecules together. The rate of evaporation is governed by three main factors: solar energy input (which provides the energy for phase change), relative humidity (drier air has more capacity to absorb water vapor), and wind (which removes moisture-saturated air from the surface and replaces it with drier air). Evaporation is a critical component of the hydrological water cycle and is directly responsible for evaporative cooling, the reason wet skin feels cold in a breeze. Evapotranspiration, combining evaporation from ground and water surfaces with transpiration from plants, is a key variable in drought forecasting and agricultural planning, as it determines how rapidly soil moisture is depleted even during periods of adequate precipitation.

Extratropical Cyclone

An extratropical cyclone is a large, low-pressure weather system that forms in the mid-latitudes (roughly 30-60 degrees north and south of the equator) along frontal boundaries, deriving its energy from temperature contrasts between adjacent air masses rather than from ocean heat. Also called a “mid-latitude cyclone” or “frontal cyclone,” these systems are responsible for the vast majority of significant weather events across the continental United States, including nor’easters, Plains blizzards, and the powerful Pacific storms that regularly batter the West Coast. Wind patterns circulate counterclockwise around the low-pressure center in the Northern Hemisphere, drawing in warm air ahead of the system and cold air behind it. A mature extratropical cyclone can span more than 1,000 miles in diameter and persist for several days as it tracks across the continent, simultaneously producing rain, snow, strong winds, and severe thunderstorms in different sectors of the storm.

Eye (of a Hurricane)

The eye is the roughly circular area of calm winds, sinking air, and often clear or partly cloudy skies at the center of a mature tropical cyclone. Eyes typically range from 20-40 miles in diameter but can be as small as 5 miles or as large as 200 miles; the smallest, most compact eyes are often associated with the most intense storms. Atmospheric pressure reaches its absolute lowest value at the center of the eye, which is why a sudden dramatic rise in barometric pressure following the passage of the eye signals the approaching eyewall on the far side of the storm. The calm conditions inside the eye are dangerously deceptive, a common and fatal mistake during land-falling hurricanes is for residents to venture outside during the eye passage, not realizing the violent eyewall is only minutes away on the opposite side. An eyewall replacement cycle can also cause the eye to temporarily expand and weaken before a new, tighter eyewall contracts and the storm re-intensifies. See also: What is a Tropical Cyclone? and Know the Hurricane Hazard Terms.

Eyewall

The eyewall is the ring of towering thunderstorm cells that immediately surrounds the eye of a tropical cyclone, and it is the location of the storm’s most extreme conditions, maximum sustained winds, heaviest rainfall rates, and cloud tops reaching 40,000-50,000 feet into the atmosphere. Wind speeds within the eyewall define the storm’s category on the Saffir-Simpson Hurricane Wind Scale, and the storm surge, the dome of ocean water pushed ashore by the storm, is at its most catastrophic directly beneath the eyewall at the moment of landfall. During an eyewall replacement cycle, a new outer ring of convection forms, contracts inward, and ultimately replaces the original inner eyewall; this process typically causes a temporary period of weakening as the original eyewall collapses, followed by potential re-intensification as the new, larger eyewall tightens. Understanding eyewall structure and replacement cycles is essential to accurate hurricane intensity forecasting.

Evaporative Cooling

Evaporative cooling is the temperature reduction that occurs when water evaporates from a surface, removing latent heat energy in the process and lowering the temperature of whatever the water was in contact with. This principle explains why perspiration is such an effective cooling mechanism for the human body in dry, breezy conditions, but why it fails almost completely on hot, humid days when the air is already near saturation and sweat cannot evaporate efficiently, creating the dangerous “feels like” temperatures measured by the heat index. In thunderstorm meteorology, evaporative cooling plays a critical role in downdraft and downburst formation, when rain falls into a dry sub-cloud layer, the rapid evaporation chills the air, making it denser and causing it to sink violently toward the surface. In arid climates, this process can be so complete that all the precipitation evaporates before reaching the ground, producing a “dry microburst”, a blast of cold, gusty wind at the surface with no accompanying rainfall.

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Frequently Asked Questions, “E” Weather Terms

What is El Nino and how does it affect US weather?

El Nino is a periodic warming of the central and eastern tropical Pacific Ocean that disrupts normal weather patterns across the globe every 2-7 years. In the US, El Nino winters typically bring warmer, drier conditions across the northern Plains and Great Lakes, while bringing wetter and cooler weather to the South and Southwest, often helpful in breaking California droughts. El Nino also suppresses Atlantic hurricane activity by increasing upper-level wind shear. Events typically last 9-12 months.

What is the difference between El Nino and La Nina?

El Nino (a warming of the tropical Pacific) and La Nina (a cooling of the tropical Pacific) produce nearly opposite effects on US weather. El Nino brings warmer, drier winters to the northern US and wetter winters to the South and Southwest, while suppressing Atlantic hurricane seasons. La Nina brings cooler, snowier winters to the northern US and warmer, drier conditions to the South and Southwest, while enhancing Atlantic hurricane seasons. Both are measured against a neutral baseline using the Oceanic Nino Index.

What is the Enhanced Fujita Scale?

The Enhanced Fujita (EF) Scale is the classification system used in the US to rate tornado intensity based on damage caused. EF0 represents the weakest tornadoes with wind speeds of 65-85 mph causing minor damage such as broken branches and damaged shingles. EF5 represents the strongest tornadoes with winds above 200 mph causing complete destruction of well-built homes. The EF Scale replaced the original Fujita Scale in 2007 with improved damage indicators for more accurate wind speed estimates.

What is the difference between a hurricane’s eye and eyewall?

The eye is the calm, clear center of a hurricane, a roughly circular area of sinking air where winds are light and skies may be partially clear. The eyewall is the ring of intense thunderstorms immediately surrounding the eye, where the strongest winds, heaviest rain, and most dangerous conditions occur. If the eye passes directly overhead, you will experience sudden calm conditions followed by the eyewall on the opposite side. Never assume a hurricane has passed when the eye arrives, the worst winds are still coming.

What is an extratropical cyclone?

An extratropical cyclone is a low-pressure storm system that forms in the mid-latitudes along boundaries between warm and cold air masses. Unlike tropical cyclones (hurricanes) which are powered by ocean heat, extratropical cyclones are driven by temperature contrasts between air masses. They are responsible for most of the significant weather in the continental United States, including nor’easters, blizzards, and major Pacific storms. Extratropical cyclones can span over 1,000 miles and produce rain, snow, strong winds, and severe thunderstorms simultaneously.

What is an equinox?

An equinox is one of the two moments each year when the sun crosses the equator and the length of day and night is approximately equal everywhere on Earth. The Spring (Vernal) Equinox occurs around March 20-21 and marks the astronomical start of spring in the Northern Hemisphere; the Fall (Autumnal) Equinox occurs around September 22-23. After the Spring Equinox, days grow longer in the Northern Hemisphere; after the Fall Equinox, nights grow longer. The equinoxes mark the transition between seasons and have been used to track time since ancient civilizations.

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Glossary of Weather Terms – Beginning with “E”