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NOAA - Meteorology glossary Category: Sciences > Meteorology Date & country: 14/10/2013, USA Words: 668

ETA
Eta (from Greek) model generated every 12 hours by NCEP

Evaporation
the process of a liquid changing into a vapor or gas.

Excessive Heat Warning
Issued within 12 hours of the onset of the following conditions: heat index of at least 105 degrees Fahrenheit for more than 3 hours per day for 2 consecutive days or heat index more than 115 degrees Fahrenheit for any period of time.

Excessive Heat Watch
Issued for the potential of the following conditions within 12 to 36 hours: heat index of at least 105 degrees Fahrenheit for more than 3 hours per day for 2 consecutive days or heat index more than 115 degrees Fahrenheit for any period of time.

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.

Extended Outlook
a basic forecast of general weather conditions three to five days in the future.

Extratropical cyclone
A storm that forms outside the tropics, sometimes as a tropical storm or hurricane changes. See table below for differences between extratropical and tropical cyclones.

Eye
The low pressure center of a tropical cyclone. Winds are normally calm and sometimes the sky clears.

Eye wall
The ring of thunderstorms that surrounds a storm's eye. The heaviest rain, strongest winds and worst turbulence are normally in the eye wall.

Fahrenheit
the standard scale used to measure temperature in the United States; in which the freezing point of water is thirty-two degrees and the boiling point is two hundred and twelve degrees.

Fair
describes weather in which there is less than 4/10ths of opaque cloud cover, no precipitation, and there is no extreme visibility, wind or temperature conditions.

Fall Wind
a strong, cold, downslope wind.

Feeder Bands
Lines or bands of low-level clouds that move (feed) into the updraft region of a thunderstorm, usually from the east through south (i.e., parallel to the inflow). This term also is used in tropical meteorology to describe spiral-shaped bands of convection surrounding, and moving toward, the center of a tropical cyclone.

Fetch
The area in which ocean waves are generated by the wind. Also refers to the length of the fetch area, measured in the direction of the wind.

Few
A cloud layer that covers between 1/8th and 2/8ths of the sky.

Flanking Line
A line of cumulus connected to and extending outward from the most active portion of a parent cumulonimbus, usually found on the southwest side of the storm. The cloud line has roughly a stair step appearance with the taller clouds adjacent to the parent cumulonimbus. It is most frequently associated with strong or severe thunderstorms.

Flash Flood
A flood that occurs within a few hours (usually less than six) of heavy or excessive rainfall, dam or levee failure or water released from an ice jam.

Flash Flood Warning
Issued to inform the public, emergency management, and other cooperating agencies that flash flooding is in progress, imminent, or highly likely.

Flash Flood Watch
Issued to indicate current or developing hydrologic conditions that are favorable for flash flooding in and close to the watch area, but the occurrence is neither certain or imminent.

Flood
a condition that occurs when water overflows the natural or artificial confines of a stream or river; the water also may accumulate by drainage over low-lying areas.

Flood Crest
The highest stage or flow occurring in a flood.

Flood Stage
The stage at which water overflowing the banks of a river, stream or body of water begins to cause damage.

Flood Warning
Issued when there is expected inundation of a normally dry area near a stream, other water course; or unusually severe ponding of water.

Flurries
Light snow falling for short durations. No accumulation or just a light dusting is all that is expected.

Foehn
A warm dry wind on the lee side of a mountain range. The heating and drying are due to adiabatic compression as the wind descend downslope.

Fog
Water that has condensed close to ground level, producing a cloud of very small droplets that reduces visibility to less than one km (three thousand and three hundred feet).

Fogbow
A rainbow that has a white band that appears in fog, and is fringed with red on the outside and blue on the inside.

Forecast
A forecast provides a description of the most significant weather conditions expected during the current and following days. The exact content depends upon the intended user, such as the Public or Marine forecast audiences.

Fractus
Ragged, detached cloud fragments

Freeze
Occurs when the surface air temperature is expected to be 32 degrees Fahrenheit or below over a widespread area for a significant period of time.

Freeze Warning
Issued during the growing season when surface temperatures are expected to drop below freezing over a large area for an extended period of time, regardless if frost develops or not.

Freezing
The change in a substance from a liquid to a solid state.

Freezing Drizzle
Drizzle that falls in liquid form and then freezes upon impact with the ground or an item with a temperature of 32 degrees Fahrenheit or less, possibly producing a thin coating of ice. Even in small amounts, freezing drizzle may cause traveling problems.

Freezing fog
A suspension of numerous minute ice crystals in the air, or water droplets at temperatures below 0 Celsius, based at the Earth's surface, which reduces horizontal visibility; also called ice fog.

Freezing Level
The altitude in the atmosphere where the temperature drops to 32F.

Freezing Nuclei
Particles suspended in the air around which ice crystals form.

Freezing Rain
Rain that freezes on objects such as trees, cars and roads, forming a coating or glaze of ice. Temperatures at higher levels are warm enough for rain to form, but surface temperatures are below 32 degrees Fahrenheit, causing the rain to freeze on impact.

Freshet
the annual spring rise of streams in cold climates as a result of snow melt; freshet also refers to a flood caused by rain or melting snow.

Frog Storm
the first bad weather in spring after a warm period.

Front
The boundary or transition zone between two different air masses. The basic frontal types are cold fronts, warm fronts and occluded fronts.

Frost
The formation of thin ice crystals on the ground or other surfaces. Frost develops when the temperature of the exposed surface falls below 32 degrees Fahrenheit and water vapor is deposited as a solid.

Frost Advisory
Issued during the growing season when widespread frost formation is expected over an extensive area. Surface temperatures are usually in the mid 30s Fahrenheit.

Frost Point
When the temperature to which air must be cooled to in order to be saturated is below freezing.

Frozen Dew
When liquid dew changes into tiny beads of ice. The change occurs after dew formation and then the temperature falls below freezing.

Fujita Scale
System developed by Dr. Theodore Fujita to classify tornadoes based on wind damage. Scale is from F0 for weakest to F5 for strongest tornadoes.

Fujiwhara effect
The Fujiwhara effect describes the rotation of two storms around each other.

Funnel Cloud
A rotating, cone-shaped column of air extending downward from the base of a thunderstorm but not touching the ground. When it reaches the ground it is called a tornado.

Gale
Sustained wind speeds from 34 to 47 knots (39 to 54 mph).

Gale Warning
A marine weather warning for gale force winds from a non tropical system.

Geostationary Satellite
A satellite positioned over the equator that rotates at the same rate as the earth, remaining over the same spot.

Glaze
a layer or coating of ice that is generally smooth and clear, and forms on exposed objects by the freezing of liquid raindrops.

GOES-8
One of the Geostationary Operational Environmental Satellites. They are owned and run by the National Oceanic and Atmospheric Administration (NOAA), while NASA designs and launches them.

Gradient
The time rate or spatial rate of change of an atmospheric property.

Graupel
Small pellets of ice created when supercooled water droplets coat, or rime, a snowflake. The pellets are cloudy or white, not clear like sleet, and often are mistaken for hail.

Gravity Wave
A wave disturbance in which buoyancy acts as the restoring force on parcels displaced from hydrostatic equilibrium. Waves on the ocean are examples of gravity waves.

Greenhouse Effect
The warming of the atmosphere by the trapping of longwave radiation (heat) being radiated to space. The gases most responsible for this effect are water vapor and carbon dioxide.

Ground Fog
Shallow fog (less than twenty feet deep) produced over the land by the cooling of the lower atmosphere as it comes in contact with the ground. Also known as radiation fog.

Growing Degree Day
A form of degree day to estimate the approximate dates when a crop will be ready to harvest. one growing degree day occurs when the daily mean temperature is one degree above the minimum temperature required for the growth of that specific crop.

Growing Season
The period of time between the last killing frost of spring and the first killing frost of autumn.

Gust
A brief sudden increase in wind speed. Generally the duration is less than 20 seconds and the fluctuation greater than 10 mph.

Gust Front
The leading edge of the downdraft from a thunderstorm. A gust front may precede the thunderstorm by several minutes and have winds that can easily exceed 80 mph.

Gustnado (or Gustinado)
Gust front tornado. A small tornado, usually weak and short-lived, that occurs along the gust front of a thunderstorm. Often it is visible only as a debris cloud or dust whirl near the ground. .

Hail
Precipitation in the form of balls or irregular lumps of ice produced by liquid precipitation, freezing and being coated by layers of ice as it is lifted and cooled in strong updrafts of thunderstorms..

Halo
A ring or arc that encircles the sun or moon. Halos are caused by the refraction of light through the ice crystals in cirrus clouds.

Hard Freeze
freeze where vegetation is killed and the ground surface is frozen solid.

Harmattan
a hot, dry, and dusty northeasterly or easterly wind that occurs in West Africa north of the equator and is caused by the outflow of air from subtropical high pressure areas.

Haze
Fine dust or salt particles in the air that reduce visibility.

Heat Advisory
Issued within 12 hours of the onset of the following conditions: heat index of at least 105 degrees but less than 115 degrees for less than 3

Heat Balance
The equilibrium existing between the radiation received and emitted by a planetary system.

Heat Index
An index that combines air temperature and humidity to give an apparent temperature (how hot it feels). Here is a heat index formula originally from Weatherwise magazine. It gives valid results above 70 deg. F.

Heat Island
A dome of elevated temperatures over an urban area caused by the heat absorbed by structures and pavement.

Heat Lightning
Lightning that can be seen, but is too far away for the thunder to be heard.

Heating Degree Day
A form of degree day used to estimate the required energy for heating. One heating degree day occurs for each degree the daily mean temperature is below 65 degrees Fahrenheit.

Heavy snow
Depending on the region of the USA, this generally means that four or more inches of snow has accumulated in 12 hours, or six or more inches of snow in 24 hours.

Heavy Snow Warning
Older terminology replaced by winter storm warning for heavy snow. Issued when 7 or more inches of snow or sleet is expected in the next 24 hours. A warning is used for winter weather conditions posing a threat to life and property.

Heavy Surf
the result of large waves breaking on or near the shore resulting from swells or produced by a distant storm.

Helicity
A property of a moving fluid which represents the potential for helical flow (i.e. flow which follows the pattern of a corkscrew) to evolve. Helicity is proportional to the strength of the flow, the amount of vertical wind shear, and the amount of turning in the flow (i.e. vorticity).

High
An area of high pressure, usually accompanied by anticyclonic and outward wind flow. Also known as an anticyclone.

High Risk (of severe thunderstorms)
Severe weather is expected to affect more than 10 percent of the area.

High Wind Warning
Issued when sustained winds from 40 to 73 mph are expected for at least 1 hour; or any wind gusts are expected to reach 58 mph or more.

High Wind Watch
Issued when conditions are favorable for the development of high winds over all of or part of the forecast area but the occurrence is still uncertain. The criteria of a high wind watch are listed under the high wind warning and should include the area affected, the reason for the watch and the potential impact of the winds.

Hodograph
A plot representing the vertical distribution of horizontal winds, using polar coordinates. A hodograph is obtained by plotting the end points of the wind vectors at various altitudes, and connecting these points in order of increasing height.

Hook Echo
A radar pattern sometimes observed in the southwest quadrant of a tornadic thunderstorm. Appearing like a fishhook turned in toward the east, the hook echo is precipitation aloft around the periphery of a rotating column of air 2-10 miles in diameter.

Horse Latitudes
Subtropical regions where anticyclones produce settled weather.

Humidity
The amount of water vapor in the atmosphere.

Hurricane
A severe tropical cyclone with sustained winds over 74 mph (64 knots). Normally applied to such storms in the Atlantic Basin and the Pacific Ocean east of the International Date Line.

Hurricane Warning
Warning issued when sustained winds of 74 mph (64 knots) or more are expected within 24 hours. This implies a dangerous storm surge.

Hydrologic Cycle
The composite picture of the interchange of water substance between the earth, the atmosphere and the seas which includes the change of state and vertical and horizontal transport.

Hydrology
The study of the waters of the earth with relation to the effects of precipitation and evaporation upon the water in streams, rivers, lakes, and its effect on land surfaces.

Hydrosphere
The totality of water encompassing the Earth, comprising all the bodies of water, ice, and water vapor in the atmosphere.

Hygrometer
An instrument used to measure humidity.

Ice age
Periods in the history of the earth characterized by a growth of the ice caps towards the equator and a general lowering of global surface temperatures, especially in temperate mid-latitudes. The most recent ice age ended about 10,000 years ago. Ice advances in this period are known to have altered the whole pattern of global atmospheric circulation.

Ice Crystals
A barely visible crystalline form of ice that has the shape of needles, columns or plates. Ice crystals are so small that they seem to be suspended in air. Ice crystals occur at very low temperatures (around zero degrees F and colder) in a stable atmosphere.

Ice Fog
A suspension of numerous minute ice crystals in the air, or water droplets at temperatures below 0 Celsius, based at the Earth's surface, which reduces horizontal visibility. Usually occurs at -20F and below.

Ice Jam
An accumulation of broken river ice caught in a narrow channel that frequently produces local floods during a spring break-up.

Ice pellets
Precipitation of transparent or translucent pellets of ice, which are round or irregular, rarely conical, and which have a diameter of 0.2 inch (5

Ice Storm
Liquid rain falling and freezing on contact with cold objects creating ice build-ups of 1/4th inch or more that can cause severe damage.

Ice storm warning
Older terminology replaced by winter storm warning for severe icing. Issued when 1/2 inch or more of accretion of freezing rain is expected. This may lead to dangerous walking or driving conditions and the pulling down of power lines and trees. A warning is used for winter weather conditions posing a threat to life and property.

Indefinite ceiling
The ceiling classification applied when the reported ceiling value represents the vertical visibility upward into surface-based obscuration.

Indian Summer
An unseasonably warm period near the middle of autumn, usually following a substantial period of cool weather.