Weather Terms

 Air Mass: A large body of air with the uniform temperature and humidity of its source region.
 Anticyclone: A large area of high atmospheric pressure, characterized by outward-spiraling winds. A high.
 Atmospheric Pressure: The force exerted by the atmosphere's weight on a surface of unit area.
 Climate: The long-term average weather of a region.
 Cyclone: A large area of low atmospheric pressure, characterized by inward-spiraling winds. A low.
 Front: The surface between two different air masses.
 High: The center of an area with high atmospheric pressure.
 Humidity: The amount of water vapor in the air.
 Isobar: A line through points of equal atmospheric pressure.
 Low: The center of an area with low atmospheric pressure.
 Polar Front: The surface between polar and tropical air masses, along which cyclonic disturbances form.
 Synoptic Chart: A map depicting the weather in an area at a given moment.
 Wave Cyclone: A storm or low-pressure center that moves along a front.

Anti -Trades Winds – In Trade-Winds regions at a height of 6000 ft (1800 m) or more above the surface the wind dir-
rection is sometimes reversed, giving, for example, a SW wind on the Peak of Tenerife. These winds are believed to be the return currents carrying the air of the trade – winds back to higher latitudes, hence they are termed “anti-trades”,
or “counter-trades”, but they are not regularly developed.

Aurora – Bright steamers of light, ascending from the hori-
zon towards the zenith, or luminous arcs, which are manifes-
tations of electrical energy in the upper atmosphere. The au-
rora is seen in both hemispheres, in high and sometimes in
medium latitudes. In the northern hemisphere it is known as
Aurora Borealis, in the southern as Aurora Australis.

Backing – A change in the direction of the wind, in anti-clockwise direction.

Blizzard – A high wind accompanied by great cold and drifting or falling snow.

Col – The saddle-backed region occurring between two anti-
cyclones and two depressions, arranged alternatively.

Convection – In convection, heat is carried from one place to another by the bodily transfer of the matter containing it.
In particular, this is method by which heat raises the tempe-rature of a fluid mass. That part in close contact with the heating agent expands and rises, moving away from it, while
colder fluid moves in to take its place. This action in the atmosphere gives rise to convectional  currents which may
produce cumulus or cumulonimbus cloud.

Corona (ae) – A series of coloured rings round the sun or moon caused by diffraction of the light by water-drops, chiefly of altoclouds. The innermost rings is usually a brow-
nish red and this is often the only one visible. Within is there
is a clear space, which, with this inner ring, forms the “aure-
ole”. When the corona is fully developed there is a violent
ring outside the brownish red ring, followed by blue, green,
yellow, and red rings outwards. Additional series of coloured
rings in the same order, are occasionally seen on the outside of the first set, forming a double or triple corona. (See Halo)

Corposants – Luminous brush discharges of electricity,
sometimes observed at the mastheads and on projecting parts
of ships during electrical storms. Also known as St. Elmo’s
Fire. Due to atmospheric electricity.

Cyclone – A name given to the tropical cyclones of the Bay
of Bengal and the Arabian Sea. Sometimes used as general
term for tropical cyclones of all oceans, or in the form “Tro-
pical Cyclone”. Depression of the Temperate Zones were formerly often referred to as cyclones but “depression” or “low” is now used to distinguish them from the tropical storms. The term “cyclonic depression” is still sometimes used for a depression, as also is “extra-tropical cyclone.

Dangerous Quadrant – The forward quadrant of the dange-
rous semi-circle of a cyclone, which before recurvature is nearer  the pole (in both hemispheres)

Dew – Water drops deposited by condensation of water vapour from the air, mainly on horizontal surfaces cooled
by nocturnal radiation.

Dew Point – The temperature to which air can be cooled without causing condensation.

Diffraction – The diversion of a ray of light from the
straight path by a material obstacle. Thus light may be dif-
fracted by water drops, producing coronae (q.v.).

Diurnal Variation – This term is used to indicate the chan-
ges, in the course of an average day, in the magnitude of a meteorological element. The most striking example of this is the diurnal variation of barometric pressure in the tropics, the chief component of which has a 12-hourly period. The maxima of this variation are about 10 a.m. and 10 p.m., the
minima about 4 a.m. and 4 p.m. local time.

Doldrums – The equatorial oceanic region of calms and
light variable winds, accompanied by heavy rains, thunder-
storms, and squalls. These belts are variable in position and extent, and as a whole move north, and south with the annual changes of the sun’s declination.

Eddy – A name given to the deviation from steady motion
which occurs in any viscous fluid which flows past an obs-
tacle, or in which neightbouring steams flow past or over each other. Air and water eddies are formed over and around
 a ship as she moves along.

Eye of Storm – The calm, central area of a tropical cyclone.
The most noticeable feature of this area is the sudden drop
in wind from hurricane force to light unsteady breezes or
even to a complete calm, with more or less cloudless sky and
absence of rain. Over the ocean the sea in the eye of the storm is usually very high and turbulent.

Frontogenesis – The development or marked intensification of a front.

Frontolysis – The disappearance or marked weakening of a
front. Subsidence is the most important factor in causing
frontolysis.

Gust – A comparatively rapid fluctuation in the strength of
the wind, characteristic of the winds near the surface of the
earth. Gusts are mainly due to the turbulence or eddy motion
arising from the friction offered by the ground to the flow of
the current of air.(See Squall).

Hail – Hard pellets of ice, of various shapes and sizes, and
more or less transparent, which fall from cumulonimbus clouds and are often associated with thunderstorms.

Halo – Halo phenomena constitute a large group of pheno-
mena produced by the refraction or reflection of the light of
the sun or moon by the ice crystals composing cirrus of cir-
rostratus cloud. The components is the halo of about 22 deg.
radius round the sun or moon; other fairly common forms are
mock suns (or moons), sun pillars, the halo of 46 deg. radius,
etc. (See Corona)

Horse Latitudes – The belts of calms, light winds and fine,
clear weather between the trade-wind belts and the prevailing
westerly winds of higher latitudes.

Hurricane – A name given to the tropical cyclone of the West Indian region. Also applied to force 12 in the Beaufort
scale, whatever its cause.

Intertropical Convergence Zone – The zone of separation
between the winds circulations proper to the northern and
southern hemispheres. Over the Atlantic and Pacific Oceans,
where it is closely related to the Doldrums, it is the boundary
between the north-east and south-east trade-winds.

Inversion – An abbreviation for “inversion of temperature
gradient”. The temperature of the air, generally decreases with increasing height but occasionally the reverse in the case; when the temperature thus increases with height there is said to be an inversion. When an inversion exists at lower levels fog often occurs.

Isallobars – are lines drawn upon a chart through place at
which equal changes of pressure have occurred in some  pe-
riod of time. Lines of equal change, or isallobars, are drawn on enclose regions of rising or of falling pressure.
Isobars – Lines drawn through positions having the same barometric pressure, when reduced to sea level.

Looming – The vertical extension of an object due to abnor-
mal refraction, making it appear unusually tall. The word
“loom” is also used in such expression as “the loom of a light” or “the loom of a land” when the light or the land cannot be seen directly, but is presence is inferred from ref-
lection seen in the sky, or from other optical effects.

Orographic Rain – Rain caused by the interference of rising
land in the path of moisture-laden air. A horizontal air cur-
rent striking a mountain slope is deflected upwards and the
consequent dynamical cooling associated with the expansion
of the air produces cloud and rain, if the air contains suffi-
cient aqueous vapour. Banner clouds, such as the “Table Cloth” over Table Mountain, “Tursui” over mount Fuji, and
the cloud over the Rock of Gibraltar during the Levanter,
are examples of orographic clouds.

Rainbow – An arch of coloured light in the sky, made by
raindrops breaking up the white light of the sun into its component colour. The colours of the rainbow are, from the outer to the inner edge; red, orange, yellow, green, blue, indi-
go and violet. The radius of the bow is 42 deg. One or more
narrow, faint supernumerary bows may be seen inside the  
main bow, touching it. A secondary bow, less bright than the
primary bow, and with colours reversed, is often seen about
9 deg. outside it.

Recurvature of storm – This expression refers to the recurvature of the track of a tropical cyclone, which is a typical feature of great majority of these phenomena. It is also known as the “recurve”. In the northern hemisphere a tropical cyclone, after proceeding in a more or less westerly
direction, recurves and normally takes a north-easterly direc-
tion; in the southern hemisphere the final direction is normal-
ly south-easterly.

Ridge – An extension of an anticyclone or high-pressure are shown on a pressure chart, corresponding with a ridge run-
ning out from the side of a mountain.

Roaring Forties – A nautical expression for the region of
westerly winds in south temperate latitudes, which reach
their greatest development south of 40S. A general term for
the prevailing westerly winds in the temperate latitudes of
both hemispheres is Brave West Winds.

Saturation – A given volume of ordinary air which is expo-
sed to a plane surface of water or ice has for a given tempe-
rature a definite saturation pressure of water- vapour; this
saturation pressure increases rapidly with increasing tem-
perature. A fall of temperature would lead to condensation
of some of this water-vapour, while a rise of temperature
would make the air unsaturated and therefore able to take up
more water-vapour.

Scud – A word used by sailors to describe ragged fragments
of cloud drifting rapidly in a strong wind, often underneath
rain cloud. The meteorological term is Stratus fractus. 

Secondary Depression or “Secondary” – The isobars around a depression are frequently not quite symmetrical; they some times show bulges or distortions which are accompanied by marked deflections in the general circula-
tion of the wind in the depression; such distortions are called secondaries; they may appear merely as sinuosities in the isobars, but at other times they enclose separate centres of low pressure and show separate wind circulations from that of the parent depression.

Stratosphere – The region of the atmosphere immediately
above the troposphere (q.v.). In the lower stratosphere tem-
perature may continue to decrease with increase of height
(but more slowly than in the troposphere) or may remain practically constant, or may increase with height. The tran-
sition from troposphere , to stratosphere, judged by change
of temperature with height, is not always abrupt.
    At greater heights are other regions with special characte-
ristics, e.g.
(a)    the ozonosphere, where the concentration of ozone gas       
      is greatest, centred at height of about 20 miles;
(b)    the ionosphere, the highly electrical conducting region
of ionized gases, extending upwards from the height of 50 or 60 miles. This region plays an important part in
radio propagation. The main subdivisions of this region in order of increasing height are usually ref-
fered to as  the D.E. (or Kennelly-Heaviside), F (or
Appeleton) region or layers.

Subsidence – Descent of air over a wide area, associated with
a developing ridge or anticyclone. The subsiding air warms up, its relative humidity falls, and fine weather is the usual
accompaniment of subsidence, through fog may occur under
certain conditions.

Troposphere – The lower region of the atmosphere through-
out which temperature in general decreases as height increa-
ses, and within which occur practically all clouds and the va-
rious other phenomena normally styled “weather”. The upper
boundary of the region is known as the tropopause. The height of the tropopause varies with the latitude from an ave-
rage of about 5 ½ miles in polar region to about 11 miles at
the equator, but the height also varies from summer to winter
and with the general meteorological situation.

Trough – The trough line of a circular depression is the line,
through the centre, perpendicular to the line of advance of the
centre. During the passage of a depression over any given place the pressure at first falls and later rises; the through line
passed over the place during the period of transition from the
falling to the rising barometer. The word through is also used in a more general sense for any “valley” of low pressure, and
is thus the opposite of a “ridge” of high pressure.

Typhoon – A name given to the tropical cyclones of the Chi-
na Sea and the west part of the North Pacific Ocean.

Waterspout – An air whirl, normally with a funnel-shaped cloud projecting downwards from a cumulonimbus cloud,
accompanied by an agitation of the sea surface beneath it, and
the formation of a cloud or spray. The complete waterspout
is formed when the funnel-shaped cloud has descended far
enough to join up with the cloud of spray. The spout then
assumed the appearance of a column of water.