Weather Definitions

Weather SA

GLOSSARY OF WEATHER RELATED TERMINOLOGY - source: SAWS

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 an 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 of the air to below its dew point. As happens along the Cape west coast.   

Aircraft Icing (Airframe icing)

The accumulation of ice on the exposed surfaces of aircraft when flown through super cooled 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:

Beaufort

Number

Wind speed

(m/s)

Wind speed

(km/h)

Designation

Description

0

<0.3

<2

Calm

Smoke rises vertically, trees do not move

1

0.3 - 1.5

2-5

Light air

Smoke drift indicates wind direction

2

1.6 - 3.3

6-11

Light breeze

Weather vane moves, leaves rustle

3

3.4 – 5.4

12-19

Gentle breeze

Leaves and twigs in constant motion

4

5.5 – 7.9

20-29

Moderate breeze

Dust and loose paper raised, small branches move

5

8.0 -10.7

30-38

Fresh breeze

Small trees sway

6

10.8 – 13.8

39-50

Strong breeze

Large branches move, wind whistles wires

7

13.9 – 17.1

51-61

Near gale

Whole trees move, walking affected

8

17.2 – 20.7

62-74

Gale

Twigs break of trees, walking difficult

9

20.8 – 24.4

75-86

Strong gale

Slight structural damage occurs, branches break

10

24.5 – 28.4

87-100

Storm

Trees uprooted, considerable structural damage

11

28.5 -32.6

101-118

Violent storm

Widespread damage

12

32.7 +

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 vapour 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 layman’s terms got left behind. The Jet Stream shifted to fast to pick up the low pressure system. So it is just cut-off 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 cut-off 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-vapour 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 phenomenon) 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. 

Dry line

A narrow zone of extremely sharp moisture gradients. Thundershowers usually develop just to the east of the dry line 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)

 

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 spreads along the ground ell in advance of the parent thunderstorm cell; a mesoscale cold front. A shelf or roll cloud may accompany the gust front.

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 exists 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 behaviour 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, call signs 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 dew point. This fog generally forms in the early morning and dissipate when he 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.

Rain shadow

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 cause 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.    


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