Ohio Weather Library

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   Absolute Zero -  The zero point on the Kelvin temperature scale equal to -273.15 degrees  Celsius  or  -459.67 degrees Fahrenheit.

   Accretion  -  Growth of an ice particle  by collision with cloud drops whose temperature is below freezing but which have not frozen (see supercooled).  These supercooled cloud drops then freeze entirely or partially upon contact with the ice particle, thus adding to the ice particle.

   Advection  -  The horizontal  transport of a mass of air (warm air, cold air, moist air, etc.).

   Advection Fog  -  Fog which results from the horizontal transport (advection) of moist air over a cold surface and the resultant cooling of the moist air to below its dewpoint

    African Jet  -  A low-level  easterly jet stream  occurring during the summer over the Sahara Desert in North Africa.  Disturbances, known as easterly or African waves, draw energy from this  jet stream  and progress  westward into the Atlantic Ocean.  Some of these waves may grow into hurricanes.

    Air Discharge  -  A form of lightning  where the lightning channel moves from the thunderstorm cloud into apparently clear air where it ends.

    Air Mass  -  Generally, a large, or widespread, body of air with fairly uniform temperature and moisture characteristics.

    Air Parcel  -  A rather small, imaginary volume of Airmass Shower/Thunderstorm  -  A shower or thunderstorm not associated with any front but occurring in a moist, unstable air mass and formed as a result of daytime heating producing large cumulus or cumulonimbus clouds.

   Airmass Source Region  -  Place where air masses originate and acquire their  temperature and moisture characteristics.

    Alberta Clipper  -  A fast-moving cyclone, or low pressure area, originating in or near the Province of Alberta, Canada and named after the fast clipper ships of sailing days.air with all the properties of the air mass in which it originates.

   Altocumulus - a principal cloud type with bases between 6,500 and 10,000 feet above the ground.  Although "alto" means high and "cumulus" means pile, heap or accumulated; altocumulus clouds are not among the highest clouds but are classified as middle level clouds.  There are 13 different kinds of altocumulus clouds, and they  are composed mostly of water drops, but may be partially composed of ice crystals.  Altocumulus clouds are usually seen in patches, layers or rolls.  



                                                 A typical display of altocumulus clouds.

   Anabatic Wind - This is a wind which blows up the surface of a mountain or hillside slope due to surface heating under fair weather conditions.  The warm slope surface heats a column of air above it, and that air is warmer than air at the same level over the valley locations.  This means that the warmer air over the slope is also less dense and therefore has a lower pressure than that over the valley.  Since air always flows from higher to lower pressure, wind begins to blow up the slope of the mountain or hillside (see upslope wind).

   Anafront - A front along which the warm air is rising up the frontal surface to great altitudes.

   Anemometer - An instrument used in measuring wind speed.                                    

   Aneroid    barometer - An instrument used in measuring air pressure.          

   Anticrepuscular rays - These are extensions of crepuscular rays which go across the sky toward the antisolar point.  




Anticrepuscular Rays - photo from Flickr


   Anticyclone - An area of high pressure (a High) around which winds are flowing clockwise in the Northern Hemisphere and counterclockwise in the Southern Hemisphere.

   Anticyclonic - The direction of winds around a high pressure area and rotation about the local vertical opposite the earth's rotation - clockwise in the Northern Hemisphere and counterclockwise in the Southern Hemisphere.

   Antisolar point - The point directly opposite the sun from the observer on a line from the sun through the observer.

    Anvil - Popular name for the flattened top of a thunderstorm (see incus).

   Arc cloud - A line of cumulus type clouds which forms when there is local convergence of air along the boundary which separates low-level thunderstorm outflow from the surrounding environment.

   Arctic air - An air mass which is very cold and dry and forms over the ice and snow surfaces of the Arctic Ocean.

   Arctic high - A very cold area of high pressure originating over the Arctic Ocean.

   Atmosphere - The envelope of gases which surrounds Earth (or any other planet).  Earth's atmosphere  is composed mostly of nitrogen and oxygen.

  Atmospheric pressure - The pressure which the atmosphere exerts on a column of air lying directly above any one point.  This is also called air pressure or barometric pressure.

   Aurora - The occasional emission of electromagnetic radiation from the upper atmosphere in the form of colorful streamers or arches in the night sky over the middle and high latitudes.

   Aurora australis - The aurora of southern latitudes which is also known as the southern lights.

   Aurora borealis - The aurora of northern latitudes which is also known as the northern lights.

                           Aurora Borealis (Northern Lights) Wetaskiwin, Canada August 22, 2006           Photo by NovusPluto

     Austru - Southeast or east winds in Rumania.

     Autan - In south-central France, especially Gascony and the upper Garonne River, a strong southeast wind which reaches speeds of 30 mph with gusts to 45-50 mph at Toulouse.

    Autumnal equinox - The date in September when the sun crosses the equator from the Northern Hemisphere to the Southern Hemisphere and day and night are everywhere of equal length.

   Azores high - A semi permanent subtropical high pressure area located over the North Atlantic Ocean named after the Azores Islands near which it is frequently located.

    Back door cold front - A cold front which moves toward the south and southwest rather than the normal west to east or northwest to southeast movement.

    Back-building thunderstorm - A thunderstorm which develops on the upwind side ( usually the west or southwest) so that the storm appears to be nearly stationary or develop in a backward direction. 

    Backing wind - A wind which rotates in a counterclockwise direction in the Northern Hemisphere with increasing height or a wind which shifts in a counterclockwise direction (such as from southwest to southeast) at any one location.

    Baguio - A name given to a severe tropical cyclone in the Philippine Islands.

    Ball lightning - A fairly rare form of lightning usually in the shape of a glowing ball or globe with a radius of about 5 to 20 inches which may float in the air, roll around a room, or even bounce on a road.  Usually, ball lightning is orange or reddish in color, but it can be blue, green black, or multicolored like a soap bubble.  It may or may not give off a hissing sound, and it usually disappears with a bang.  Most of the time, ball lightning is seen in the area of a thunderstorm and many of its appearances come after a particularly powerful lightning discharge.

    Banded structurePrecipitation echoes arranged in long lines or bands on radar

    Barber - At sea, a severe storm in which precipitation and spray freeze onto rigging and decks of ships.  Also, in the Gulf of St. Lawrence a blizzard in which wind-driven ice particles nearly cut the skin from a person's face. 

    Barograph - A recording barometer.

    Barometer  -  An instrument which measures atmospheric pressure.

    Barotropic system - A weather  system  in  which  there  is  no  temperature  gradient (temperature is uniform) on a constant pressure surface.

    Bead lightning - A form of ordinary lightning in which the lightning channel appears to break up into glowing segments or beads. 

    Beaufort Wind Scale - A wind scale developed by Admiral Beaufort of the British Navy  which ranges from 0 for calm to 12 for hurricane.

    Berg wind - A squally, hot and dry wind blowing from the interior plateau of South Africa mainly in winter when an anticyclone covers that plateau, and winds flow out from it across the South African coastal sections.

    Bermuda High - A semipermanent subtropical high located in the North Atlantic ocean getting its name from the Island of Bermuda near which it is often located.  This high is often an extension of the Azores high.

    Bise - Cold winds from the north, northeast  or east following the passage of a front in the Swiss Middleland and parts of eastern France. 

    Black frost - Freezing of vegetation without the presence of hoarfrost - a dry freeze which is always a killing frost.

    Black ice - On roadways, a thin sheet of ice which is fairly dark in appearance. 

    Blinter - a gust of wind in Scotland.

    Blirty - Gusts of wind accompanied by rain, or changeable weather in Scotland.

    Blizzard - A weather condition whose requirements are a wind of 35 mph or greater and enough snow in the air to reduce visibility to one quarter mile or less.  Previous definitions included low temperatures, but these requirements have been dropped. 

    Blocking high - An anticyclone, or high pressure area, which is nearly stationary or is very slow moving compared to west-to-east motion; thus, effectively blocking the eastward movement of low pressure systems, or cyclones

    Blue flash - A flash of blue light seen on or apparently adjacent to the upper rim of the sun at sunrise or sunset - similar to the green flash.

    Blue jets -  Blue colored, luminous, upward propagating discharges originating from the tops of thunderstorms often after powerful, positively charged cloud-to-ground lightning strokes.  Blue jets may reach to the ionosphere. 

    Bomb - A surface extratropical cyclone whose central pressure falls at least 1 millibar (about .03 inch) per hour for 24 hours.

    Bora - A type of fall wind whose source is so cold that when it reaches the lowlands dynamic warming has not been able to elevate its temperature to the normal temperature, so it is a cold wind.

    Boundary layer -  Usually, this refers to the planetary boundary layer, or atmospheric boundary layer, which is the lower portion of the troposphere in contact with the earth's surface (about the lower 1.25 miles).



                                            Bounded Weak Echo Region (BWER)                                                                                             from Sioux Falls, South Dakota NWSFO.


    Bounded Weak Echo Region (BWER) - A thunderstorm signature on radar marked by minimum reflectivity at lower levels and extending upward into the thunderstorm.  This area of minimum reflectivity is surrounded by higher reflectivities and marks a strong updraft. 



                                                       Bow Echo May 20, 1998                                                                                                     as seen on Louisville, Kentucky NWS radar.


    Bow echo - A bow-shaped line of thunderstorms (squall line) which has strong straight-line winds and possibly weak tornadoes. 

    Breaks in overcast - In weather observing practice in the U.S., a condition in which the cloud cover is greater than 0.9 but less than 1.0.

    Breeze - Generally speaking, a light wind, and on the Beaufort Wind Scale a wind which covers numbers 2-6 (about 4-12 mph).

    Brickfielder - An obsolete term for a southerly burster which blew a lot of red dust over Sydney, Australia.

    Brisa - Spanish for breeze but usually referring to northeasterly winds blowing off the ocean.  In Brazil and Venezuela, it is used in reference to the northeast trade winds, while at Montevideo, Uruguay it refers to a strong breeze.  It refers to the northeast monsoon in the Philippines and a damp, light breeze in Columbia.  In northern Puerto Rico, the northeast trade winds are deflected to the east by a mountain range, and such winds there are called brisa.



                   Cloudbow, or fogbow, with Brocken Spectre in center - photo from Flickr


   Brocken Spectre - The shadow of an observer cast upon a cloud most often occurring when the observer is located on a ridge or mountain with a low sun casting the shadow of the observer on fog or clouds in a valley below.

    Broken clouds - Clouds covering between 0.6 and 0.9 of the sky.

    Bruma - A layer of haze appearing in the afternoon along the coast of Chile when sea air is transported inland.

    Bubble high - A mesoscale high pressure area between approximately 50 and 300 miles in diameter usually resulting from rain-cooled air from a thunderstorm or thunderstorm complex.  These short-lived high pressure areas are complete with anticyclonic circultation and are often seperated from the surrounding air by a gust front or outflow boundary.

    Bulls eye-squall - Named for the characteristic appearance of the isolated cloud which marking the storm's vortex, this squall forms in fair weather  over the ocean off the South African coast.

    Buran - In Russian and central Asia, a strong northeast wind most frequent in winter when it is similar to a blizzard as it picks up snow from the ground and blows it around.

    Burga - In Alaska, a storm from the northeast which brings sleet or snow.

    Burster - In southeastern Australia, especially on the coast of New South Wales in near Sydney in summer, a sudden wind shift to the southeast and south.



     Copyright 2006 Ronald Hahn. All Rights Reserved














                                                                            Copyright 2006 Ronald Hahn. All Rights Reserved