Unit 5: Weather Vocabulary

from http://www.bom.gov.au/info/wwords/

Description of Phenomena
Fog: Suspension of very small water droplets in the air, reducing visibility at ground level to less than a kilometer.

Smog: Smog ( contraction for 'smoke fog') is a fog in which smoke or other forms of atmospheric pollutant have an important part in causing the fog to thicken, and have unpleasant and dangerous physiological effects.

Mist: Similar to fog, but visibility remains more than a kilometre.

Frost: Deposit of soft white ice crystals or frozen dew drops on objects near the ground; formed when surface temperature falls below freezing point.

Precipitation: Any or all of the forms of water particles, whether liquid (e.g. rain, drizzle) or solid (e.g. hail, snow), that fall from a cloud or group of clouds and reach the ground. (See Drizzle, Rain)

Drizzle: Fairly uniform precipitation composed exclusively of very small water droplets (less than 0.5 mm in diameter) very close to one another.

Rain: Precipitation of liquid water drops greater than 0.5 mm in diameter. In contrast to showers, it is steadier and normally falls from stratiform (layer) cloud.

Showers: Usually begin and end suddenly. Relatively short-lived, but may last half an hour. Often, but not always, separated by blue sky.

Blizzard: Violent and very cold wind which is laden with snow, some part, at least, of which has been raised from snow covered ground.

Thunderstorms: Thunderstorms are one or more convective clouds in which electrical discharge can be seen as lightning and heard as thunder by a person on the earth's surface.
A severe thunderstorm produces one or more of :-

hail at the ground with diameter of 2 cm or more;
wind gusts at the ground of 90 km/h or more;
tornadoes; or
very heavy rain likely to cause flash flooding.
Tornado: A tall, rapidly rotating column of air between 5 and 1000 metres in diameter which is attached to the base of a cumulonimbus or large cumulus cloud and which is capable of producing damage at the earth's surface.

Precipitation
Duration
Brief: Short duration.

Intermittent: Precipitation which ceases at times.

Occasional: Precipitation which while not frequent, is recurrent.

Frequent: Showers occurring regularly and often.

Continuous: Precipitation which does not cease, or ceases only briefly.

Periods of Rain: Rain is expected to fall most of the time, but there will be breaks.

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Intensity
Slight or Light:

Rain: Individual drops easily identified, puddles form slowly, small streams may flow in gutters.

Drizzle: Can be felt on the face but is not visible. Produces little run off from roads or roofs. Generally visibility is reduced, but not less than 1000 m.

Snow: Small sparse flakes. Visibility generally reduced but not less than 1000 m.

Hail: Sparse hailstones of small size, often mixed with rain.

Moderate:

Rain: Rapidly forming puddles, down pipes flowing freely, some spray visible over hard surfaces.

Drizzle: Window and road surfaces streaming with moisture. Visibility generally between 400 and 1000 m.

Snow: Large numerous flakes and visibility generally between 400-1000 m.

Hail: Particles numerous enough to whiten the ground.

Heavy:

Rain: Falls in sheets, misty spray over hard surfaces, may cause roaring noise on roof.

Drizzle: Visibility reduced to less than 400 m.

Snow: Numerous flakes of all sizes. Visibility generally reduced below 400 m.

Hail: A proportion of the hailstones exceed 6 mm diameter.

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Distribution of Showers(or other weather phenomena)
Few: Indicating timing, not an area.

Isolated: Showers which are well separated in space during a given period.

Local: Restricted to relatively small areas.

Patchy: Occurring irregularly over an area.

Scattered: Irregularly distributed over an area. Showers which, while not widespread, can occur anywhere in an area. Implies a slightly greater incidence than isolated.

Sporadic: Scattered or dispersed in respect of locality or local distribution. Characterised by occasional or isolated occurrence.

Widespread: Occurring extensively throughout an area.

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Wind Terms
The wind is a continuous succession of gusts and lulls associated with equally rapid changes of direction over a range which may exceed 30°. The mean wind speed over a period of time is therefore the mean of many gusts and lulls. Usually only the mean wind is forecast, unless the gusts are expected to be a significant feature. For instance, Fresh, gusty southwest winds indicates that the mean wind speed will be between 17 and 21 knots and the mean wind direction will be from the southwest, but that there will also be gusts to speeds significantly higher than the mean.

Gust: A gust is any sudden increase of wind of short duration, usually a few seconds.

Squall: A squall comprises a rather sudden increase of the mean wind speed which lasts for several minutes at least before the mean wind returns to near its previous value. A squall may include many gusts.

Wind descriptions (derived from the Beaufort Wind Scale) Wind speeds are given as the equivalent speed at a standard height of 10 metres above open flat ground