| A |
| |
| Adiabatic Process |
A thermodynamic change in the state of a system in which there is no transfer
of heat or mass across the boundaries of the system. In a adiabatic process
compression always results in warming, expansion in cooling. i.e. subsidence
will result in warming and rising of air will result in cooling. |
| |
| Advection |
The horizontal movement of an air mass that causes changes in the physical properties
of the air such as temperature and moisture. |
| |
| Advection Fog |
A type of fog caused by the advection of moist air over a cold surface and the
consequent cooling cooling of the air to below its dewpoint. As happens along
the Cape west coast. |
| |
| Aircraft Icing ( Airframe icing) |
The accumulation of ice on the exposed surfaces of aircraft when flown through
supercooled water droplets (cloud or precipitation). |
| |
| Air Drainage |
General term for gravity-induced, downslope flow of relatively cold air. |
| |
| Air Mass |
It is a widespread body of air that is nearly homogeneous in its horizontal extent,
particularly with reference to temperature and moisture distribution; is addition
the vertical temperature and moisture variations are approximately the same
over its horizontal extend. |
| |
| Atmospheric pressure (barometric pressure) |
The pressure exerted by the atmosphere as a consequence of gravitational attraction
exerted upon the "column" of air lying directly above the point in
question. |
| |
| B |
| |
| Barometer |
Instrument for measuring atmospheric pressure. |
| |
| Beaufort scale |
A numeric scale used to estimate the force of the wind when no instruments are
available:
|
| Wind speed (km/h) |
Designation |
Description |
| < 2 |
calm |
smoke rises vertically, trees do not move |
| 2-5 |
light air |
smoke drift indicates wind direction |
| 6-11 |
light breeze |
weather vane moves, leaves rustle |
| 12-19 |
gentle breeze |
leaves and twigs in constant motion |
| 20-29 |
moderate breeze |
dust and loose paper raised, small branches
move |
| 30-38 |
fresh breeze |
small trees sway |
| 39-50 |
strong breeze |
large branches move, wind whistles wires |
| 51-61 |
moderate gale |
whole trees move, walking affected |
| 62-74 |
fresh gale |
twigs brake of trees, walking difficult |
| 75-86 |
strong gale |
slight structural damage occurs, branches break |
| 87-100 |
whole gale |
trees uprooted, considerable structural damage |
| 101-118 |
storm |
widespread damage |
| 119+ |
hurricane |
severe and extensive damage |
| |
| Berg Wind |
A hot dry wind blowing off the interior plateau of South Africa, roughly at right
angles to the coast. Occur mainly in winter when there is a low pressure system
south of the country and a strong high over the country |
| |
| Black Frost |
A dry freeze with respect to its effects upon vegetation, that is, the internal
freezing of vegetation unaccompanied by the protective formation of hoarfrost. |
| |
| Blocking High |
Any high that remains nearly stationary or moves slowly , so that it effectively "blocks" the
movement of migratory lows (cyclones) across its latitudes. |
| |
| Buster |
A sudden shift in wind direction behind a coastal low from north-east to south-west.
The buster is well known for its sudden onslaught with winds going from calm
to 40 knots+ in a matter of minutes. |
| |
| C |
| |
| CAT |
Clear air turbulence. Turbulence experienced by aircraft as it is flying in cloudless
conditions |
| |
| Cloudburst |
Any sudden and heavy fall of rain, almost always of the shower type. Most of
the times it is associated with thunderstorms. |
| |
| Cold Front |
Any non-occluded front that moves so that the colder air replaces the warmer
air; i.e. the leading edge of a cold air mass. |
| |
| Condensation |
The physical process by which water vapor becomes liquid or solid. |
| |
| Cut-off Low |
A cold low which has become displaced out of the basic westerly current, and
lies to the south of this current.
It is a low pressure system that to put it in laymans terms got left
behind. The Jet Stream shifted to fast to pick up the low pressure
system. So it is just cutoff and by itself. To picture this. Have you
ever tried to pick up something with a magnet, and as long as you do it slowly it held the object, however if you made any fast movements the
object fell back down. Well this is about what happens with a cut off
low. Thus the low has nothing to move or guide it. Cut off lows can sit
in the same place for days or longer until it is picked up by the Jet
stream again. Because of this cutoff low just sitting in one place
flooding can become a big issue. The Laingsburg flood (January 1981) is a good example of flood caused by a cut-off low. |
| |
| Cyclone (low or depression) |
An area of low pressure with a cyclonic flow. ( clockwise in the Southern hemisphere). |
| |
| D |
| |
| dBZ |
| dBZ is an indication of the reflectivity from the cloud drops, ice and hail. The colour scale on the radar maps thus indicates the amount of reflectivity recorded with the radars. The higher the dBZ value the more likely it is that the cloud will have rain and or hail |
|
| Dew Point |
The temperature to which a given parcel of air must be cooled at constant pressure
and constant water-vapor content in order for saturation to occur. |
| |
| Discomfort Index |
During summer season, or in very hot periods anyway, the human organism makes use of perspiration
to maintain its temperature within proper physiological limits. The sweat, which is most made of water,
evaporates (endothermal phenomenum) taking away heat so as to have a cooling effect on the skin.
A high humidity level in the surrounding environment may obstruct this process limiting the
evaporation. The human body so can't eliminate the excessive heat (compared to its own physiological limits)
receiving a sensation of a higher temperature.
|
| |
| Doppler Weather Radar |
A new Weather Surveillance Radar system developed in 1988. This powerful and
sensitive Doppler system generates many useful products for meteorologists,
among them: standard reflectivity 'echoes', wind 'velocity' or atmospheric
air motion pictures, and Arial 1-hour, 3-hour , or storm-total precipitation
images. |
| |
| Downburst (microburst) |
A strong downdraft, initiated by a thunderstorm, that includes an outburst of
damaging winds on or near the ground. Downbursts may last for anywhere from a
few minutes in small scale microbursts on up to 20 minutes in lager , longer
lived microbursts. One example of a downburst, called straight-line winds, can
reach speeds of 176km to 240km, or squarely in the range of a strong tornado.
Downbursts are further detailed as either: Microburst: a convective downdraft
with an affected outflow area of less than 4 km wide and peak winds lasting less
than 5 minutes. They can create dangerous vertical/horizontal wins shears which
can adversely affect aircraft performance and cause property damage. |
| |
| Dryline |
A narrow zone of extremely sharp moisture gradients. Thundershowers usually develop
just to the east of the dryline in South Africa. |
| |
| Dust Devil |
A well developed dust whirl; a small but vigorous whirlwind usually of short
duration, rendered visible by dust, sand and debris picked up from the ground.
Diameters range from about 3m to 30m; their average height is about 200m but
a few has been observed as high as 2000m. |
| |
| E |
| |
| El Niño |
Significant warming of the waters in the eastern Pacific Ocean, usually off the
coast of South America, which results in shifts of world-wide weather
patterns. Can cause prolonged periods of drought or floods. In South
Africa, El Niño
is associated with prolonged droughts. |
| |
| F |
| |
| Fetch |
An area from which waves are generated by a wind that is nearly constant in direction
and speed. |
| |
| FDI |
Fire Danger Index. |
| |
| Flash Flood |
A flood that rises and falls quite
rapidly with little or no advance warning,
usually as the result of intense rainfall
over a relative small area. |
| |
| Freezing Level |
In aviation terminology, then lowest altitude in the atmosphere over a given
location , at which the air temperature is 0 degrees C; the height of the 0
C constant temperature surface |
| |
| Fujita-Pearson Scale |
A scale used to classify tornadoes based on wind damage and was developed by
Theodore Fijita( university of Chicago)
| F scale |
Wind Speed (km/h) |
Damage |
|
F0
|
64-115
|
light
|
|
F1
|
116-179
|
Moderate
|
|
F2
|
180-251
|
Major
|
|
F3
|
252-329
|
Severe
|
|
F4
|
330-416
|
Devastating
|
|
F5
|
417-508
|
Incredible
|
|
| |
| G |
| |
| Greenhouse Effect |
The heating effect exerted by the atmosphere upon the earth by virtue of the
fact that the atmosphere absorbs and remits infrared radiation. |
| |
| Gust Front |
The leading edge of a mass of relatively cool, gusty air that flows out of the
base of a thunderstorm cloud and and spreads along the ground ell in advance
of the parent thunderstorm cell; a mesoscale cold front. A shelf or roll cloud
may accompany the gustfront. |
| |
| H |
| |
| Halo |
Rings or arcs that encircle the sun or moon which are caused by the refraction
of light through ice crystals that make up high level clouds. |
| |
| Heat Thunderstorm |
A thunderstorm of the air mass type which develops near the end of a hot, humid,
summers day. |
| |
| Heat Wave |
A heat wave exist when for 3 days the maximum temperature is 5 degrees
higher than the mean maximum for the hottest month. |
| |
| High ( anti-cyclone) |
An area of high pressure with a anticyclonic circulation. ( anti clock wise in
the southern hemisphere) |
| |
| Hoarfrost |
A deposit of interlocking ice crystals formed by direct sublimation on objects.
Most of the frost experienced in winter on the high lying areas of South Africa
is hoarfrost. |
| |
| Hurricane |
See tropical cyclone. |
| |
| I |
| |
| Instability |
A state of the atmosphere in which the vertical distribution of temperature allows
rising warm air to continue to rise and accelerate.. This kind of motion is
conducive for thunderstorm development. |
| |
| Inversion |
A situation where the temperature increases with height instead of decreasing.
It is quite common in the winter and because there is no upward motion of warm
gases it results in severe pollution of the lower layers. |
| |
| Isobar |
A line of equal barometric pressure as shown on a weather map. |
| |
| J |
| |
| Jet Stream |
A narrow band of strong winds in the atmosphere that controls the movement of
high and low pressure systems and associated fronts. Wind speeds can reach
380 km or higher in certain cases. Jet streams are usually found at 30 to 40
000 ft above the surface. It owes it existence to the large temperature contrast
between the polar and equatorial regions. |
| |
| K |
| |
| Knot |
Unit of speed used in aviation and marine activities to measure the speed of
the wind. It is equal to about 1.15 statue mile ore 1.84 km per hour. |
| |
| L |
| |
| Land Breeze |
A coastal breeze blowing from land to sea, caused by temperature difference when
the sea surface is warmer than the adjacent land. Normally occurs in the early
mornings. |
| |
| La Niña |
La Niña is characterised
by unusually cold ocean temperatures in the
eastern equatorial Pacific. It is the opposite
of El Niño. La Niña is associated
with above normal rain over the summer rainfall
areas of South Africa. |
| |
| M |
| |
| Mesoscale |
Dimensions of an atmospheric layer that ranges from a few kilometres to some
tens of kilometres horizontally and, vertically from the ground to the top
of the friction layer. |
| |
| N |
| |
| Numerical Forecasting |
The forecasting of the behavior of the atmospheric disturbances by the numerical
solution of the governing fundamental equations of hydrodynamics, subject to
observed initial conditions; computers and sophisticated computational models
are required. |
| |
| O |
| |
| Orographic Lifting |
The lifting of an air current caused by its passage up and over mountains or
escarpments.As the air is forced upwards it cools and if moist enough clouds
can form and additional cooling results in rain. It can cause extensive cloudiness
and increased amounts of precipitation in higher terrain. |
| |
| Ozone |
A nearly colourless (but faintly blue) gaseous form of oxygen, with a characteristic
odour like that of weak chlorine. It is usually found in trace amounts in the
atmosphere, but is primarily found at 30 000 to 150 000 feet. Its production
results from photochemical process involving ultraviolet radiation. Because
it absorbs harmful ultraviolet radiation at those heights, it is a beneficial
gas. |
| |
| P |
| |
| Precipitation |
Any or all of the forms of water particles, whether liquid or solid, that fall
from the atmosphere and reach the ground. |
| |
| Q |
| |
| Q Code |
The Q code is a set of three-letter code signals to be used in radiotelegraphy and amateur radio communications. It was developed and instituted in 1912 as a way to facilitate communication between maritime radio operators of different nationalities. For this reason, callsigns never begin with a Q.
|
| |
| QNH |
| QNH is a Q code used by pilots, air traffic control (ATC) and low frequency weather beacons to refer to the current mean air pressure over a given region at sea level (if there is no sea, this is a virtual value by adjusting the value at the ground for its elevation), the "regional pressure setting". This value is used by pilots to calibrate the altimeter on board the aircraft, to ensure that the pilot is accurately aware of his actual flying height. The QNH allows a pilot to know his altitude with respect to mean sea level |
| |
| Quasi-stationary Front |
A front which is stationary or nearly
so. |
| |
| R |
| |
| Radiation Fog |
It is fog that form over land due
to heat loss through radiation during the
night and resulting in the cooling of the
air to below its dewpoint. This fog generally
form in the early morning and dissipate when
the sun is warms up the ground. |
| |
| Rainbow |
An arc that exhibits the concentric
bands of colours of the spectrum and is formed
opposite the sun by refraction and reflection
of the sun's rays in raindrops.
|
| Rainshadow |
Areas on the leeward side of a mountain
or mountain range which often receive less
rain than the windward side. The Klein Karoo
is a good example of this. |
| |
| Relative
humidity |
The ratio of the amount of moisture
in the air to the amount which the air could
hold at the same temperature and pressure
if it were saturated; usually expressed in
percent. |
| |
| Ridge |
An elongated area of high pressure
in the atmosphere: the opposite of a trough. |
| |
| Roll Cloud |
A turbulent cloud formation that
resembles a roller. This cloud can be found
in the lee of some mountains. The air in
the cloud rotates around a axis parallel
to the range of mountains. It is also sometimes
found along the leading edge of a thunderstorm,
formed by the rolling action in the wind
shear region between cool downdrafts and
warm updrafts. |
| |
| S |
| |
| Severe
Thunderstorm |
A thunderstorm that produces either
of the following: damaging winds of 93 km/h
or greater, hail 1.9 centimetre in diameter
or larger, or a tornado. Severe thunderstorms
can result in the loss of life and property. |
| |
| Sleet |
Describes the solid grains of ice
formed by the freezing of raindrops or the
refreezing of largely melted snowflakes. |
| |
| Smog |
A natural fog contaminated by industrial pollutants, literally, a mixture of
smoke and fog. |
| |
| Snow |
A steady fall of snowflakes for
several hours over the same area. |
| |
| Southern
Oscillation |
A periodic, large scale atmospheric
oscillation of the large scale distribution
of sea level pressure, and air and water
temperature that originates over the southern
hemisphere. Consequently, there is an associated
change in the surface winds, and some storms
become stronger than normal. This oscillation
is on the scale of a year or a few years,
and has global implications such as widespread
drought or flooding. Oceanic fishing is also
disrupted. |
| |
| Squall Line |
A broken or solid line of thunderstorms
that may extend across several hundred kilometres. |
| |
| Subsidence |
A descending motion of air in the atmosphere, usually with the implication that
the condition extends over a rather broad area. |
| |
| T |
| |
| Thermal |
A relatively small-scale, rising
air current produced when the earth's surface
is heated. Thermals are a common source of
low level turbulence for aircraft. |
| |
| Tornado |
A violently rotating column of air,
usually pendant to a cumulonimbus, with circulation
reaching the ground. The visible cloud may
not reach the ground, but if the lower circulation,
marked by dust, dirt, and/or debris, reaches
the ground, it is classified as a tornado.
It nearly always starts off as a funnel cloud
and may be accompanied by a loud roaring
noise. Tornadoes are classified into 3 main
groups: weak- wind speeds up to 170 km/h:
strong- wind speeds of 170- 330 km/h; violent-
wind speeds of 340 to perhaps 500 km/h. |
| |
| Tropical
Cyclone |
A cyclone originating over
tropical or subtropical waters with
organized convection and a definite
cyclonic surface wind circulation.
Tropical cyclones are large and span
areas of 1000 of kilometres. They
can causes a great deal of damage
when they make landfall. A lot of
damage is caused by the storm surge
that result in widespread flooding
Tropical cyclones get their energy
from the warm oceans and therefore
dissipate rapidly as they move in
over land. |
| |
| Tropical
or Subtropical Depression |
Cyclones that have maximum
sustained winds of 33 knots or less. These
are referred to as low pressure systems in
public advisories and statements. |
| |
| Tropical
Disturbance |
An area of organized convection which originates
in the tropics or subtropics and maintains it identity
for 24 hours or more. In successive stages of intensification,
it may be subsequently classified as a tropical wave, tropical
depression, tropical storm or tropical cyclone. |
| |
| Tropical
Storm |
Tropical cyclone that has
maximum sustained winds from 34 to
63 knots inclusive. |
| |
| Trough |
An elongated area of low pressure
in the atmosphere, the opposite of a ridge. |
| |
| U |
| |
| Upper-Level
Disturbance |
| A disturbance of the flow pattern in the
upper atmosphere, which is usually associated with clouds
and precipitation.. This disturbance is characterised by
distinct cyclonic flow, a pocket of cold air; and sometimes,
a jet streak. These features make the air aloft more unstable
and conducive to clouds and precipitation. |
| |
| UVB |
A biologically effective portion of solar ultraviolet radiation that reaches
the earth's surface; in the wavelength range of 280 to 320 nanometres; responsible
for sunburn and skin cancers. |
| |
| V |
| |
| Virga |
Wisps or streaks of rain or snow
falling out of a cloud, but evaporating before
reaching the ground. |
| |
| W |
| |
| Waterspout |
Very similar to a tornado with the
difference that a waterspout occurs over
a body of open water. |
| |
| Wet
Bulb Temperature |
The temperature an air parcel would
have if cooled to saturation at a constant
pressure by evaporation of water into it. |
| |
| Wind Chill |
An apparent temperature
that describes the combined effect
of wind and low air temperature on
exposed skin. |
| |
| Z |
| |
| Zonal flow |
The flow of air along a latitude circle; more specifically the latitudinal (east
or west) component of existing flow. |
| |
| |