Back radiation / Counter-radiation

Atmospheric radiation which reaches the Earth’s surface.

Backdoor cold front

Cold front that moves from east to northeast and leads a cold air mass toward the south and southwest, along the Atlantic seaboard of the United States.

Back-bent occlusion / Bent-back occlusion

Occluded front which is sometimes present in the rear quadrant of a depression, associated with the movement of the depression along the original occluded front, or with the formation of a new center near the point of occlusion. The direction of movement of a back-bent occlusion is generally toward the south or the southeast (in the Northern Hemisphere) compared with a general east to northeast movement of the original occluded front.

Backing wind

Counter-clockwise change of wind direction, in either hemisphere.


A “mist” that occurs in China and Japan in spring and autumn. Clouds of loose dust are raised by the wind to great heights and, after becoming moist aloft, fall to earth as a colored mist which gives a fine, thick, yellow deposit.

Bai-u / Plum or Mould rains

The name given to the season of heaviest rainfall in southern Japan and in parts of China. These rains are also called Plum rains or Mould rains.

Band lightning / Fillet or Ribbon lightning

Ordinary streak lightning which appears to be spread horizontally into a ribbon of parallel luminous streaks when a very strong wind is blowing at right angles to the observer’s line of sight. Successive strokes of the flash are then displaced by small angular amounts and may appear to the eye or camera as distinct paths.

Bands of rain

Banner cloud

Stationary orographic cloud which forms in the neighborhood of a mountain crest or peak and takes the shape of a banner streaming downwind from the mountain peak. This type of cloud must not be confused with snow which is blown off a mountain summit and carried downwind.


Unit of pressure equal to 105 Pascals.


1. A severe storm during which spray and precipitation freeze onto the decks and rigging of ships.
2. In the Gulf St. Lawrence, a local form of blizzard in which the wind-borne ice particles almost cut the skin from the face.

Baroclinic disturbance / Baroclinic wave

Low-pressure system which forms in a region of the atmosphere characterized by a strong horizontal gradient of temperature and a strong thermal wind. Wave disturbances on the polar front are examples of these.

Baroclinic instability

Hydrodynamic instability of a fluid (such as the atmosphere) in which there exists a horizontal temperature gradient and hence a vertical shear in the flow. This instability results in a conversion of the available potential energy of the mean flow into the kinetic energy of the baroclinic disturbance.

Baroclinicity / Barocliny

Atmospheric state in which the constant pressure surfaces intersect with those of constant density.


Instrument for measuring atmospheric pressure. The two most common types are the mercury barometer and the aneroid barometer.

Barometric tendency

Barotropic disturbance / Barotropic wave

Perturbation of the basic flow in a barotropic atmosphere. The energy of the disturbance is derived from the horizontal shear of the basic flow.

Barotropic instability

Hydrodynamic instability arising from certain distributions of vorticity in a two-dimensional non-divergent flow.


Atmospheric state in which the surfaces of constant pressure are parallel to those of constant density.


Body of water that has no horizontal communication with open ocean at all depth.

Beaded lightning / Pearl necklace lightning or Pearl lightning

Particular aspect of a normal lightning flash occasionally seen when a number of segments of the irregular channel are viewed end-on and hence give an impression of higher intensity at a series of points along the channel.

Beaufort wind scale

System of estimating wind speed using a numerical scale from 0 –calm- to 12 –hurricane-. It has been modified since its invention; in its present form it equates: Beaufort force or number, wind speed, descriptive term and visible effects upon objects on land or sea surface.

Beaumont period

Period of 48 consecutive hours, in at least 46 of which the hourly reading of temperature and relative humidity at a given place have not been less than 68° F (20° C) and 75% respectively; the occurrence of this period has been widely used as a criterion for issuing warnings of the onset of potato blight.

Bergeron process

Process by which precipitation-sized particles are produced in a mixed cloud that contains ice crystals and supercooled water, through evaporation and deposition. In Bergeron-Findeisen theoretical explanation of the process, the ice crystals would grow by sublimation at the expense of the liquid drops which would lose mass through evaporation. Once the ice crystals become big and heavy enough, they fall to the surface as snow.

Biennial wind oscillation / Quasi-biennial oscillation

Alternation of easterly and westerly wind regimes in the stratosphere, within about 12° of the Equator, with a period varying from about 24 to 30 months. A new regime starts above an altitude of 20 miles (30kms) and propagates downward at a rate of about 3300 feet (1 km) per month, the amplitude decreasing below about 14 miles (23km).


Transition zone between earth and atmosphere, where most forms of life are found.


Cold and fairly dry north to northeast wind which blows often in the mountainous regions of France and Switzerland.

Bistatic radar

Radar system configuration that has the receiver and the transmitter located in different places.

Black frost

Dry freeze, with respect to its effects on vegetation, which suffers internal freezing and has a blackened appearance.

Black ice

1. Thin ice on a fresh or salt water body which appears dark in color because of its transparency.
2. A mariner’s term for a dreaded form of icing sometimes sufficiently heavy to capsize a small ship.


Severe weather condition characterized by high wind blowing snow, which reduces visibility noticeably. Popularly, the term refers to any heavy snowstorm that presents high winds.


Rain of a reddish color caused by dust particles containing iron oxide picked up by the raindrops.

Blowing dust / Blowing sand

Dust (sand) raised by the wind to moderate heights above the ground sensibly reducing the horizontal visibility at eye level.

Blowing snow

Snow lifted from the surface by the wind to a height of 2 meters or more, reducing horizontal visibility at eye level.

Blue moon or green moon / Blue sun or greensun

Phenomenon caused by the presence of large quantities of suspended particles in the atmosphere which selectively remove the longer lunar or solar visible wavelength more than the blue or green wavelengths.


Strong, cold squally downslope wind on the Dalmatian coast of Yugoslavia in winter.

Bouguer’s halo / Ulloa’s ring

An infrequently observed faint, white circular arc or complete ring of light with a radius of about 39° and centered on the antisolar point. It is usually in the form of a separate outer ring around an anticorona. A strong theoretical argument can be advanced to suggest that this phenomenon is a true fogbow.

Bow shock

Supersonic shock wave that forms when the solar wind interacts with the most external layer of a planet´s magnetosphere.

Boyden index

Numerical index of the likelihood of thunderstorms occurrence, derived from a sounding. The index is given by: Z – T700, where Z is the 1000-700 hPa thickness in decameters and T700, the 700 hPa temperature in °C. Thunderstorms become more likely as the index increases above the threshold of 94.

Brave west wind

Strong and rather persistent westerly winds which blow in all seasons between about 40° and 65° South latitude over oceanic and adjacent regions.

Breakup season

Ensemble of the phenomena associated with the disappearance of the ice-pack due to climatic (temperature, wind) and hydrological (waves, currents, tides) factors.


Light or moderate wind. In the Beaufort wind scale, a wind between 4 knots –lower limit of “light breeze”- and 27 knots –upper limit of “strong breeze”-.

Brewster’s point

Neutral point situation 15 to 20° directly below the Sun.

Brocken bowl / Anti-corona

A glory which may surround the Brocken spectrum.

Brückner cycle

Hypothetical climatic cycle of about 35 years in temperature, precipitation, lake levels, etc.


Text message composed of meteorological information, identifying type of bulletin, point of origin and issue time.


Upward force exerted on a body by the fluid in which it is immersed. It is the property that allows an object to float on a liquid surface or to ascend and remain suspended in the atmosphere.

Burst of the monsoon

1. At a specific place, the sudden arrival of the air mass associated with the summer monsoon.
2. Sometimes, a sudden intensification of the atmospheric conditions associated with the summer monsoon.

Butterfly Effect

Hypothetical example of the sensitivity of a system to initial conditions: the flap of a butterfly wings in a place might affect weather patterns worldwide.