South African Weather Service - Weather Questions http://www.weathersa.co.za/learning/weather-questions Thu, 19 Jul 2018 23:22:22 +0000 Joomla! - Open Source Content Management en-gb damien.millard@mfi.fr (MFI SAWS) What is a heat wave? http://www.weathersa.co.za/learning/weather-questions/346-what-is-a-heat-wave http://www.weathersa.co.za/learning/weather-questions/346-what-is-a-heat-wave There is a specific criterion that the South African Weather Service applies to each town or city, in order to determine whether heat wave conditions are met or not.

The criterion reads as follows: If the maximum temperature at a particular town is expected to meet or exceed 5 degrees C above the average maximum temperature of “the hottest month” for that particular place, as well as persisting in that mode for 3 days or more, then a heat wave may be declared. For ease of use by SAWS meteorologists, a detailed map of the “heat wave threshold” has been compiled (see figure 1 below), using the latest climatological data to hand. This threshold is thus the maximum temperature which needs to be met or exceeded, for a heat wave to occur.

Heat wave temperature thresholds over South Africa

 heatwave

Heat wave precautions

 

 

a. Stay indoors in a well-ventilated or air conditioned room.
b. If working outside, wear protective clothing (head-gear, etc.) and take breaks at regular intervals.
c. Avoid playing strenuous sports or engaging in excessive manual labour, as one runs the risk of heat exhaustion or sunstroke.
d. Dress in cool, lightweight clothes which are not constricting.
e. The wearing of hats (preferably with a wide brim) as well as the generous application of sunblock should be encouraged if one is outdoors for any length of time.
f. Remain adequately hydrated by drinking plenty of liquids (not alcohol).
g. Outside playtime for children should ideally be restricted or prohibited between 10:00 and 16:00.
h. The aged as well as infants are particularly vulnerable to dehydration and/or heatstroke and should be afforded extra care and attention during prolonged spells of hot weather.

 

 

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jeethend.ranjeeth@weathersa.co.za (Saws Super Admin) Weather Questions Mon, 12 Dec 2016 08:44:23 +0000
How are the dates of the four seasons worked out? http://www.weathersa.co.za/learning/weather-questions/82-how-are-the-dates-of-the-four-seasons-worked-out http://www.weathersa.co.za/learning/weather-questions/82-how-are-the-dates-of-the-four-seasons-worked-out The Seasons

One finds disagreement on the starting dates of the seasons at both the scientific and the lay level. There are however three basic ways in which starting dates may be assigned. South Africa does not really experience four distinct seasons. Throughout South Africa the transitional seasons of Autumn and Spring tend to be very short. Most analysis of climate is done using the assumption that January is mid-summer and July min-winter.

Astronomical basis

The instances at which the solstices and equinoxes occur can be accurately calculated. Earlier astronomical textbooks often defined the four seasons as starting on the dates of the corresponding equinoxes and solstices. But more recent books avoid defining the seasons in any way: two new editions of earlier books have in fact deleted their previous definitions. Moreover, expressions such as spring equinox and summer solstice are no longer used in astronomy. Instead, the four astronomical instances are identified as the ascending and descending equinoxes in March and September respectively, and the northern and southern solstices in June and December. One reason for these changes is to avoid the inevitable confusion of northern and southern hemisphere seasons: the ascending equinox in March is the spring equinox in the northern hemisphere, but the autumnal equinox in the southern hemisphere.

Thus the earlier astronomical definitions of the starting dates of the seasons are no longer relevant, and their use should be discouraged.

Climatological basis

A further failing of the earlier astronomically defined seasons is that they simply did not describe the real seasons as actually experienced. As one of the contributors to this article expressed it, summer does not start four days before Christmas. A climatologically definition of the seasons would obviously be more realistic. In the temperate latitudes of Europe and North America the climatological seasons are conventionally defined as shown in Table I. The equivalent seasons in the southern hemisphere are of course six months out of phase with those in the northern hemisphere, and are also given in Table I.

The use of intervals of exactly three calendar months for the conventional temperate latitude seasons is a matter of convenience rather than climatological reality. For example, in England the latter part of November is wintry rather than autumnal, with cold, foggy days occurring fairly frequently. Elsewhere in the world the disparity becomes even worse. Climatologists therefore ignore the conventional seasonal nomenclature and use labels which are more appropriate to the climate of a particular region: for example hot season, cold season, post-rainy season, etc. The durations of these seasons depend on the climate of the region, and have no direct relationship to either the astronomical seasons or the calendar months. Thus September is spring-like in Gauteng, with cool mornings and warm afternoons, whereas it is still winter-like in the Western Cape, with the possibility of snow on the Eastern Cape mountains. Unfortunately the lay public would find it too confusing if a different set of seasonal dates was adopted for different parts of the country in order to encompass this variability.

Phenological basis

Phenological phenomena (this is, those relating to the natural seasonal behaviour of plants and animals) are the most fundamental markers of the changing seasons. This can be seen from the etymology of the names of the seasons in various languages. Thus in English, spring, from Anglo-Saxon for rise or burst forth, is the season when sap rises and plants put out buds. Autumn, from early Latin for ripen, is the season when crops reach maturity and can be harvested.

Unfortunately, an appeal to the phonological seasons merely confounds the confusion. In parts of Europe the phonological seasons are taken to occur one month earlier than the conventional climatic season, where in other parts and in the USA the two systems coincide. In any case, May Day (1st May) rather than the first day of spring however that may be chosen seems to be the preferred date for celebrating this season in Europe.
In South Africa, the wide range of climatic regions and phenotypes (compare coastal KwaZulu-Natal with the south-western Cape) also adds to the difficulty of defining a clear-cut seasonal calendar based on phenology.

Conclusion

Apart from rejecting the astronomically-based seasons in compliance with modern astronomical usage, there are no firm grounds for choosing one set of dates rather than another for the starting dates of the seasons. There is certainly no official designation of the starting dates. On broad climatological and sociological grounds, however, choosing the dates in Table I would have the advantage of conformity established conventions.

On the basis of these conclusions, the following recommendations are suggested:
Adopt the seasonal calendar given in Table I for the southern hemisphere.
Emphasize strongly that these are conventional or traditional dates, and that an official calendar does not exist.

seasons

Table I

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Weather Questions Fri, 13 Sep 2013 12:44:11 +0000
How is rainfall measured? http://www.weathersa.co.za/learning/weather-questions/81-how-is-rainfall-measured http://www.weathersa.co.za/learning/weather-questions/81-how-is-rainfall-measured Rainfall is measured at 08:00 SAST. The rain accumulated in the bucket of the rain gauge is poured into a specially calibrated measuring glass and the millimetre reading is recorded, whereafter the water is discarded. The rainfall measured between 08:00 SAST yesterday and 08:00 SAST today is recorded against yesterdays date on the database. What is meant by 1 mm of rainfall is rainfall equivalent to 1 litre of water in a 1 square meter box with no runoff, infiltration or evaporation.

The South African Weather Service also has a number of electronic rain gauges called Tipping Bucket rain gauges. These gauges consist of a large cylinder set into the ground. At the top of the cylinder is a funnel that collects and channels the precipitation. The precipitation falls into one of the two small buckets which are balanced side by side. As soon as it starts raining one of the buckets fills with water. After an amount equal to 0.2 mm falls into the bucket the bucket tips and an electrical signal is sent to a recorder. The next bucket is then in place to collect precipitation. The two buckets seesaw up and down collecting and recording the rainfall. The instrument is thus able to record not only the amount of rain that falls but when it started and stopped raining.

Different rainfall intensities are described in the image below.

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Weather Questions Fri, 13 Sep 2013 12:42:52 +0000
Is a Tropical Cyclone and Tornado the same thing? http://www.weathersa.co.za/learning/weather-questions/80-is-a-tropical-cyclone-and-tornado-the-same-thing http://www.weathersa.co.za/learning/weather-questions/80-is-a-tropical-cyclone-and-tornado-the-same-thing No these phenomena are not the same thing. A tropical cyclone is an intensely developed low-pressure cell that usually occurs over warm oceans. Its diameter can range between 200 and 2 000 km. Tropical cyclones can last for a few days.

A tornado is a violent rotating column of air that extends from a thunderstorm and is much smaller in diameter, typically less than a few hundred meters, but some are larger than 1 km. They usually occur over land. Tornados are generally short lived and can be on the ground for a few seconds to minutes. Some have been know to be on the ground for over an hour but these are rare. If a tornado occurs over water is called a water spout.

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Weather Questions Fri, 13 Sep 2013 12:42:03 +0000
What can you tell me about lightning? http://www.weathersa.co.za/learning/weather-questions/79-what-can-you-tell-me-about-lightning http://www.weathersa.co.za/learning/weather-questions/79-what-can-you-tell-me-about-lightning What are my chances of being struck by lightning?

Lightning occurs with every thunderstorm and must be expected as soon as thunderstorms form. Lightning results from the build-up and discharge of electrical energy between positively and negatively charged areas in the atmosphere and clouds. The most lightning occurs between clouds but it is the ground strikes that are dangerous. Your chances of being struck by lightning are estimated to be 1 in 350,000 but could be reduced by following lightning safety rules.

Precautions: Stay indoors during a lightning storm and if you are travelling, stay in the vehicle. When indoors stay away from windows, do not hold any metal object nor use any electrical appliance. Do not use the telephone. Do not take a bath or shower during the lightning storm. If you are caught in the open, seek shelter in a building. Avoid hilltops and do not shelter under lone trees nor in isolated sheds. Keep distance from fences, telephone or power lines and steel structures such as pylons and windmills. Do not swim during a lightning storm and seek shelter if you are in a boat. If you are in the open playing sport, seek shelter especially if you are fishing, or playing golf, soccer or rugby.

Note that lightning can occur when it is not raining. Lightning often strikes outside of heavy rain and may occur as far as 16 km away from any rainfall. Rubber-soled shoes and rubber tires provide NO protection from lightning. However, the steel frame of a hard-topped vehicle provides increased protection if you are not touching metal. Although you may be injured if lightning strikes your car, you are much safer inside a vehicle than outside.

Do people who have been struck by lightning carry an electrical charge?

Lightning-struck victims carry no electric charge and should be attended to immediately.

What is heat lightning?

Heat lightning is actually lightning from a thunderstorm too far away for thunder to be heard. However, the storm may be moving in your direction so beware.

How often does lightning occur over the earths surface at any one time?

Over the entire earth, estimates are that 100 lightning flashes occur each second.

How much electricity can one lightning bolt provide?

The resultant electric current from a single lightning bolt can provide enough energy to light a city of 200 000 people for one minute.

Can lightning strike the same place twice?

Yes, lightning can strike the same place twice. The Empire State Building in the USA gets hit about two dozen times per year! One bad storm saw lightning strike the building fifteen times in twenty minutes.

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Weather Questions Fri, 13 Sep 2013 12:41:16 +0000
What interesting weather phenomena occur in South Africa? http://www.weathersa.co.za/learning/weather-questions/78-what-interesting-weather-phenomena-occur-in-south-africa http://www.weathersa.co.za/learning/weather-questions/78-what-interesting-weather-phenomena-occur-in-south-africa What is a jet stream?

It is 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/h or higher in certain cases. Jet streams are usually found at 9 - 12 km (30 000 - 40 000 ft) above the surface. It owes it existence to the large temperature contrast between the polar and equatorial regions.

What is an urban heat island?

An urban heat island is an urban area which is significantly warmer than its surroundings. This occurs because roof and paving materials have a low reflectivity and absorb more of the sun's rays than natural surfaces, causing both surface temperature and overall ambient air temperature in an urban area to rise. In addition, there are fewer trees and other natural vegetation to shade buildings, block solar radiation and cool the air by evapotranspiration in urban areas. The urban heat island effect is characterized by city centres commonly having temperatures six degrees higher than that of the surrounding environment.

What is a berg wind?

A berg wind is a hot, dry wind blowing off the interior plateau of South Africa, roughly at right angles to the coast. Berg winds usually occur in winter when there is a low-pressure system along the coastal areas with a high-pressure system over the continent.

What is frost?

Frost is formed in the same way as dew, but occurs when the temperature of the air in contact with the ground is below the freezing point of water (0 ºC). The water vapour changes directly from a gas to a solid state to form tiny ice crystals. Like dew, frost forms during calm, clear nights when the ground and objects near it cool rapidly because the earth’s heat is lost to space.

In South Africa light frost can be expected when the minimum temperatures reach 3 ºC. This is because on a calm night, the ground, where the frost forms, is usually several degrees cooler than the level at which temperature is measured (about 1,2 m above the ground) and thus the ground will probably be at, or below, freezing.

What is black frost?

Black frost occurs when the temperature is below zero but there is not enough moisture in the air to produce visible frost. Its name is derived from the resulting blackened appearance of affected vegetation.

What is dew?

Dew forms when water vapour condenses on objects and solid surfaces on or near the ground because these have cooled during the night time below the dew-point temperature of the air around them. For dew to form, however, there must be moisture in the air and the night must be clear (cloudless) and fairly windless so that the earth can radiate its heat energy out into space.

What is fog?

Fog is a suspension of usually very small water particles, but also sometimes ice particles, that reduces visibility at the surface to distances less than 1 km. Although fog is not usually considered to be a cloud, it has essentially the same properties. The two phenomena differ only in extend that the base of fog is at the ground, whereas clouds are above the ground.

What is smog?

Smog is a mixture of smoke and fog which contains large amounts of soot, sulphur dioxide and other pollutants which act as nuclei around which water condenses.

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Weather Questions Fri, 13 Sep 2013 12:40:30 +0000
What are the main cloud types? http://www.weathersa.co.za/learning/weather-questions/77-what-are-the-main-cloud-types http://www.weathersa.co.za/learning/weather-questions/77-what-are-the-main-cloud-types The recognition of a cloud by having a name and being in a certain class is a useful indication of the nature of a cloud and the atmospheric movements and processes which are present. Classification is based on heights, appearance, whether the cloud consists of liquid water or ice, on the processes which lead to the formation of the clouds. Clouds are classified as low, middle or high level according to their base heights. There are three cloud forms, namely stratiform, cumuliform and cirriform cloud. Stratiform cloud are layers of cloud formed by widespread ascent and cumuliform cloud form as air rises by convection, while fibrous cloud composed of ice crystals are called cirriform cloud. There are then basic cloud types:

Cirrus (Ci) 

cirrus jrvdm

Formation
High-level ascent
Shape due to wind shear

Characteristics
White, without shading
Delicate filaments
Patches of narrow bands
Fibrous or silky appearance
Hook-shaped feathery filaments
Composed of ice crystals

Precipitation
None

Cirrocumulus (Cc)

cirrocumulus epopich

Formation
Convection
Develops from cirrus of cirrostratus

Characteristics
Thin white patch, sheet or layer
Very small elements - grains, ripples
Width of less than one finger held at arm's length
Merged or separate
More or less regularly arranged

Precipitation
None

Cirrostratus (Cs) 

cirrostratus jrvdm

Formation
Widespread upper-level ascent

Characteristics
Transparent whitish veil
Fibrous or smooth
May cover whole sky
Commonly produces halo

Precipitation
None

Altocumulus (Ac)

altocumulus

Formation
Convection
Wave flow near mountains

Characteristics
Grey, featureless sheet or layer cloud
Usually waved or in lumps or layers
May be lens-shaped near mountains or islands
White or grey or both white and grey
Smallest elements have apparent width of two fingers when held at arm's length

Precipitation
Virga
Light showers occasionally

Altostratus (As)

altostratus epopich

Formation
Widespread ascent

Characteristics
Grey, featureless sheet or layer cloud
Can be fibrous or uniform
Covers whole or part of sky
Sun shines weakly (as if through ground glass)
Great horizontal extent

Precipitation
Rain

Nimbostratus (Ns)

nimbostratus epopich

Formation
Widespread ascent

Characteristics
Dark grey cloud layer
Generally covering the sky
Dense and thick enough to hide sun or moon
Base indistinct as a result of continuous rain or snow
Base often lower than 2 500 meters

Precipitation
Rain or snow - almost continuous

Cumulus (Cu)

cumulus epopich

Formation
Convection
Surface heating
Instability

Characteristics
Detached cloud
Develops vertically upwards
In the form of turrets, towers
Tops domed or cauliflower-shaped
Shaped outlines
Sunlit parts are bright white
Base nearly horizontal, relatively dark
Size depends on stage of development

Precipitation
Showers of rain
Snow from large cumulus

Cumulonimbus (Cb)

cumulonimbus

Formation
Convection
Surface heating
Instability

Characteristics
Bulging, dense cloud masses
Huge cumulus cloud
Fibrous top, often anvil-shaped or plume shaped
Base dark and stormy looking
Thunder and lightning common
Low tattered clouds below base
Associated with gusts and squalls

Precipitation
Showers of rain, snow - may be heavy
Hail

Stratocumulus (Sc)

stratocumulus popich

Formation
Spreading out of cumulus (usually evenings)
Turbulent mixing under stable layer

Characteristics
Grey or white layer with darker areas
Often regular undulations
Elements have apparent width of three or more fingers when extended at arms length

Precipitation
Occasional light rain
Drizzle

Stratus (St)

stratus popich

Formation
Low-level ascent or cooling
Due to 
Precipitation that has saturated the air
Lifting fog or mist

Characteristics
Grey uniform layer
May be continuous or patchy
Resembling fog, but not on the ground
May appear as shreds of fragments below nimbostratus
May cover tops of mountains or hills

Precipitation
Drizzle

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Weather Questions Fri, 13 Sep 2013 12:39:45 +0000
What is a cold front and what weather is associated with it? http://www.weathersa.co.za/learning/weather-questions/76-what-is-a-cold-front-and-what-weather-is-associated-with-it http://www.weathersa.co.za/learning/weather-questions/76-what-is-a-cold-front-and-what-weather-is-associated-with-it A cold front occurs where a large mass of cold air meets a mass of warmer air, and the cold air advances on the warmer air. The cold air undercuts the warm air pushing it upwards. Cumulonimbus clouds form a well-defined line along the boundary between the air masses. As the cold front passes, the clouds roll by and the air temperature may become noticeably cooler, with temperatures dropping by 5 °C or more within the first hour. Rain, gusty winds, and, sometimes, thunderstorms occur with the passage of the cold front.

On synoptic (weather) charts a cold front is represented by a solid line with triangles along the front pointing towards the warmer air and in the direction of movement. On coloured weather maps, a cold front is drawn in blue.

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Weather Questions Fri, 13 Sep 2013 12:31:10 +0000
When and where do tornadoes occur in South Africa http://www.weathersa.co.za/learning/weather-questions/75-when-and-where-do-tornadoes-occur-in-south-africa http://www.weathersa.co.za/learning/weather-questions/75-when-and-where-do-tornadoes-occur-in-south-africa Occurrence in South Africa

Tornadoes can occur basically anywhere where a thunderstorm is possible. From an analysis of the occurrence of South African tornadoes it became clear that most of them have been observed in Gauteng, the Free State, KwaZulu-Natal (along a line from Pietermaritzburg to Ladysmith) and the northern region of the former Transkei. In figure 1 the eastern part of the country is depicted, showing the more significant events (F2 and F3) from 1905 to 1997.

Some 65% of the South African tornadoes are classified as F0 or F1 (light damage), while more than 90% are classified as F0, F1 or F2 (considerable damage) or less. The tornado which occurred at Harrismith on 15 November 1998 was classified as F2 and the Mount Ayliff tornado which occurred in the Eastern Cape on 18 January 1999 was classified as F4.

Tornadoes can occur basically anywhere where a thunderstorm is possible. From an analysis of the occurrence of South African tornadoes it became clear that most of them have been observed in Gauteng, the Free State, KwaZulu-Natal (along a line from Pietermaritzburg to Ladysmith) and the northern region of the former Transkei. There seems to be a preference to mountainous areas.

tornado locations

Figure 1

Seasonal Distribution

The seasonal distribution of South African tornadoes is given in figure 2. Most of the events occur in mid-summer from November to January, although a large number of tornadoes have occurred in spring and early summer (September and October) and in the late summer and autumn (February to May). It is also worth mentioning that most tornado events (for which the time of the day were available) occurred in the late afternoon or early evening, typically between 16:00 and 19:00.

tornado distribution

Figure 2

Forecasting Tornadoes

Meteorologists rely on weather radar to provide information on developing storms. Currently in the USA, only a 20-minutes before touchdown prediction is possible by identifying the so-called vortex signature of the tornado on radar.
Storm structure can provide clues about the existence of a tornado, clearly shown in the radar images for the Harrismith tornado. The Bow echo shape in the Harrismith area is typical of severe storms.

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Weather Questions Fri, 13 Sep 2013 12:30:23 +0000
What is a Tornado? http://www.weathersa.co.za/learning/weather-questions/74-what-is-a-tornado http://www.weathersa.co.za/learning/weather-questions/74-what-is-a-tornado Forecasting Tornadoes

A tornado, from the Latin tornare (to turn), is a violent rotating column of air extending from a thunderstorm. Tornadoes are amongst the most violent and destructive of all weather phenomena, but despite the significant amount of research into the origin and prediction thereof, the phenomenon is still not fully understood or predictable. Field research on tornadoes is particularly difficult due to their short lifetime, the small area affected by them and the low probability of occurrence at a particular point.

 

Where do they come from?

 

Tornadoes in South Africa are typically associated with very hot air masses and severe thunderstorms. To date scientists do not know exactly how tornadoes are formed, but all the evidence suggests that they develop as a combination of (1) strong spinning effects inside a thunderstorm or in the air surrounding the storm and (2) accelerated strong updrafts (wind moving vertically in an upward direction).

One of the more recent theories explains it as follows: Before thunderstorms develop, a change in wind direction takes place. The wind speed also increases with height. These two factors combined give rise to an invisible, horizontal spinning effect in the lower atmosphere. The rising air within the thunderstorm updraft tilts the rotating air from horizontal to vertical (figure 1). An area of rotation now extends through much of the storm. Most strong and violent tornadoes form within this area of strong rotation.

tornado funnel

Figure 1

There are a few meteorological conditions necessary for tornado formation. Because of the scientific nature of this, the conditions are only listed and not further discussed here.

A deep layer of mid-atmospheric dry air above a moist surface layer
Steep moisture and temperature gradients
High surface temperatures
Low level convergence and upper level divergence
Vertical wind shear (change in wind direction and speed with height)
Atmospheric instability (air continues to rise once it starts rising)

Characteristics and Classification

Damage Path: Not all tornadoes are strong enough to extend down to the surface of the earth. If a tornado touches the ground the damage path depends on the position within the thunderstorm where the tornado was generated and the topography. The length and width of the path also depends on the speed of the thunderstorm - when the thunderstorm slows down, the path of the tornado widens.

Wind Speed: It is very difficult, if not impossible, to obtain a direct and reliable wind speed record from a tornado. Even in those cases in which standard wind-measuring devices have happened to be present in the tornadic path, the actual wind speeds have been well above the limits of these devices and, besides this, the wind-speed anemometers have been damaged and/or displaced. The only actual measurements currently available are for USA tornadoes. Indirect methods of estimating the wind speeds have generally been used. One such estimate made for a South African event suggested a wind speed in the order of 350 km/h.

Classification: There are several different methods of classifying tornadoes. The most commonly used is the Fujita-Pearson scale classification. This system classifies tornadoes in six intensities, ranging from F0 (no damage) to F5 (incredible damage). The intensity is based on the apparent damage to structures, the extent of the path and other descriptors from which wind speeds are then inferred. 65% of the South African tornadoes are classified as F0 or F1 (light damage), while more than 90% are classified as F0, F1 or F2 (considerable damage) or less. The tornado which occurred at Harrismith on 15 November 1998 was classified as F2 due to the severity of the damage.

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Weather Questions Fri, 13 Sep 2013 12:28:39 +0000