South African Weather Service - Climate Questions http://www.weathersa.co.za/learning/climate-questions Fri, 20 Apr 2018 02:43:48 +0000 Joomla! - Open Source Content Management en-gb damien.millard@mfi.fr (MFI SAWS) What is the Greenhouse Effect? http://www.weathersa.co.za/learning/climate-questions/45-what-is-the-greenhouse-effect http://www.weathersa.co.za/learning/climate-questions/45-what-is-the-greenhouse-effect The gases in our atmosphere allow the sun’s energy (shortwave radiation) to pass through and heat the earth’s surface. The earth in turn radiates the energy back to the atmosphere in the form of long-wave infrared radiation. This process causes the net warming of the earth and atmosphere and is sometimes referred to as the natural greenhouse effect. The glass of a greenhouse works in a similar way, letting in short-wave radiation but preventing long-wave radiation from escaping and thus the air in the greenhouse is warmer that the air outside. The major greenhouse gases are Water Vapor (H2O), Carbon Dioxide (CO2), Methane (CH4), Ozone (O3) and Nitrous Oxide (N2O). This natural greenhouse effect enables the earth to maintain an average of 15 degrees Celsius; without this effect the surface temperature would drop drastically to -18 degrees Celsius and the earth would be unable to support life as we know it. Humans have however altered the chemical composition of the atmosphere by releasing large quantities of greenhouse gases - primarily carbon dioxide, methane, and nitrous oxide. These gases are trapping more energy and reflecting it back to earth. This is having an effect on global atmospheric temperatures. This is often referred to as the human-induced greenhouse effect.

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Climate Questions Fri, 13 Sep 2013 10:25:19 +0000
What is the difference between weather and climate? http://www.weathersa.co.za/learning/climate-questions/43-what-is-the-difference-between-weather-and-climate http://www.weathersa.co.za/learning/climate-questions/43-what-is-the-difference-between-weather-and-climate Climate Questions Fri, 13 Sep 2013 10:21:15 +0000 What is Climate Change? http://www.weathersa.co.za/learning/climate-questions/42-what-is-climate-change http://www.weathersa.co.za/learning/climate-questions/42-what-is-climate-change Climate Questions Fri, 13 Sep 2013 10:20:33 +0000 What is a drought? http://www.weathersa.co.za/learning/climate-questions/41-what-is-a-drought http://www.weathersa.co.za/learning/climate-questions/41-what-is-a-drought Climate Questions Fri, 13 Sep 2013 10:18:38 +0000 What drought indices are currently in use at SAWS? http://www.weathersa.co.za/learning/climate-questions/40-what-drought-indices-are-currently-in-use-at-saws http://www.weathersa.co.za/learning/climate-questions/40-what-drought-indices-are-currently-in-use-at-saws Climate Questions Fri, 13 Sep 2013 10:16:54 +0000 What are the temperature, rainfall and wind extremes in SA? http://www.weathersa.co.za/learning/climate-questions/39-what-are-the-temperature-rainfall-and-wind-extremes-in-sa http://www.weathersa.co.za/learning/climate-questions/39-what-are-the-temperature-rainfall-and-wind-extremes-in-sa The highest worldwide temperature was recorded in AlAziziya, Libya measuring 57.7 ºC on 13 September 1922 and in South Africa at Dunbrody (Sundays River Valley in Eastern Cape) measuring 50.0 ºC on 3 November 1918. The hottest place in South Africa is Letaba (Limpopo Province) with a mean annual temperature of 23.3 ºC and an average annual maximum temperature of 35.0 ºC. The lowest worldwide temperature was recorded in Vostok, Antarctica at -89.2 ºC on 21 July 1983 and in South Africa at Buffelsfontein near Molteno (Eastern Cape) measuring -18.6 ºC on 28 June 1996.

The coldest place in South Africa is Buffelsfontein near Molteno (Eastern Cape) with a mean annual temperature of 11.3 ºC and an average annual minimum temperature of 2.8 ºC. The highest worldwide monthly rainfall occurs at Cherrapunji, India (9300 mm) and in South Africa at Matiwa in the Limpopo Province (1510 mm measured in January 1958). The highest worldwide 24 hour rainfall was measured at Fac Fac, Reunion Island (1825 mm) and in South Africa at St Lucia in KwaZulu-Natal (597 mm measured on 31 January 1984). This was rainfall associated with the passage of tropical cyclone Domoina. In South Africa the highest ever rainfall in one year was measured at Woodbush in the Limpopo Province (4359 mm in 2000). The wettest place in South Africa is Matiwa with an average annual rainfall of 2004 mm (calculated over a 60-year period). The driest Place in South Africa is Alexander Bay in the Northern Cape with an average annual rainfall of only 46 mm. 

The windiest place in South Africa is Cape Point (Western Cape) which experiences only 2% of all hours in the year with calm conditions. The average wind speed is 6.9 m/s with 42.1% of the wind speeds greater than 8 m/s. The strongest wind gust ever in South Africa occurred at Beaufort West (Western Cape) on 16 May 1984 and measured 186 km/h. Which is the windiest place Cape Town or Port Elizabeth? This is a difficult question to answer. What magnitude of wind is more noticeable than others? Possibly we can resolve the issue by referring to the Beaufort Wind Scale. If one uses the Beaufort Wind Scale, wind only really becomes felt by human beings when the wind speed exceeds 1.5 m/s. Cape Town experiences wind of 1.6 m/s or more on 95.6% of the days of the year and Port Elizabeth on 95.7% of the days of the year. The Beaufort Wind Scale classifies any wind greater than 8 m/s as a Fresh Breeze.

In Cape Town a fresh breeze is experienced on 18.7% of the days in the year (68 days of the year) and in Port Elizabeth on 20.2% of the days in the year (73 days of the year). How often, and during which months, does the South Easter usually blow in Cape Town? South Easterly winds are experienced throughout the year, but are not that frequent when compared to winds blowing from other directions. South Easterly winds blow about 3% of the time in Cape Town, mainly in August (average wind speed 5 m/s) and October (average wind speed 6 m/s). The strongest South Easterly winds occur in the summer months of December and February. During these months the average speed of the South Easterly winds is 7 m/s, but they occur less frequently during these months. How windy is South Africa compared with the windiest place on earth? The windiest place on earth is the coastal part of Antarctica. Cape Denison on Commonwealth Bay in Antarctica has an average wind speed of 20 m/s. The windiest place in South Africa is Cape Point with an average wind speed of 6.9 m/s.

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Climate Questions Fri, 13 Sep 2013 10:16:20 +0000
What are the coldest months around the world ? http://www.weathersa.co.za/learning/climate-questions/38-what-are-the-coldest-months-around-the-world http://www.weathersa.co.za/learning/climate-questions/38-what-are-the-coldest-months-around-the-world This is a quick summary of the average data by month for a large number of cities as found in the publication The World Weather Guide, E A Pearce and C G Smith, Hutchinson and CO Publishers, 1984

  • Africa south of the Equator - July
  • Africa north of the Equator - January
  • Canada - January followed very closely by February
  • USA - January
  • Mexico - January and December
  • Central America - January
  • South America north of the Equator - January
  • South America south of the Equator - July
  • Asia - January
  • Japan - January
  • Australia - July
  • New Zealand - July
  • Europe - January
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Climate Questions Fri, 13 Sep 2013 10:11:07 +0000
What are averages and normals? http://www.weathersa.co.za/learning/climate-questions/37-what-are-averages-and-normals http://www.weathersa.co.za/learning/climate-questions/37-what-are-averages-and-normals What is the difference between averages and normals?

A normal of temperature, precipitation or any other weather element is defined as the arithmetic average of the observed values of that element. A normal is strictly for 30 years, whereas an average can be computed over any time span. There is an international agreement that normals should be from a 30-year period from 1961 to 1990.

To calculate the normal daytime maximum temperature for Pretoria on say April 15, we would add together all the daily maximum temperatures on this day over the 30-year period from 1961 to 1990, divide the total by 30, to get the normal. Normal values are supposed to be representative, not necessarily what should occur.

   

    What are period averages?

Period averages are averages of climatological data computed for any period of at least ten years starting on 1 January of a year ending with digit 1.

 

    What are climatological standard normals?

Climatological standard normals are averages of climatological data computed for the following consecutive periods of 30 years:
1 January 1931 to 31 December 1960
1 January 1961 to 31 December 1990

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Climate Questions Fri, 13 Sep 2013 10:10:24 +0000
What kind of droughts does South Africa experience? http://www.weathersa.co.za/learning/climate-questions/36-what-kind-of-droughts-does-south-africa-experience http://www.weathersa.co.za/learning/climate-questions/36-what-kind-of-droughts-does-south-africa-experience Introduction

The rainfall climate of South Africa is one of great variability. Seasonal rainfall percentage deviations since 1960 demonstrate wide fluctuations about the long-term average and it is in this context that large rainfall deficits must be assessed. Between July of 1960 and June of 2004, there have been 8 summer-rainfall seasons where rainfall for the entire summer-rainfall area has been less than 80% of normal. A deficit of 25% is normally regarded as a severe meteorological drought but it can be safely assumed that a shortfall of 20% from normal rainfall will cause crop and water shortfalls in many regions accompanied by social and economic hardship.

   

   Discussion

All but the south-western and southern regions of South Africa rely on summer rainfall, which normally falls between October and March, the summer season. Rainfall is heaviest in the east and decreases westward. For convenience the rainfall season is taken to run from July until June of the following year, but rainfall outside of the summer season is usually insignificant.

The consequence of rainfall being confined to six months of the year is that most crops can only be grown during this period. Similarly, the recharging of water resources is also confined to these crucial six months. When the seasonal rainfall is seriously below normal, crop yields are poor and ground and dam water levels fall dangerously low. Should these conditions occur in swift succession, as in the periods from 1964 to 1970, 1991 to 1995 and again from 2002 to 2005, there is insufficient time for natural resources and the economy to recover from each rainfall-deficit period.



Graph showing percentage-of-normal rainfall for Gauteng - bad rainfall years have a medium term effect

Simultaneous to low rainfall are cloud-free skies and high temperatures. The effect of abnormally high temperatures is an increase in evapotranspiration as well as stress on plants whilst further depleting surface-water reserves through evaporation.

 

    Impacts

The most serious impact, other than dwindling water supplies, is the effect on staple crops and, ultimately, commercial crops. In 1992/1993, undoubtedly one of the most widespread droughts of the last 45 years, maize had to be imported to South Africa (as well as the rest of southern Africa). The knock-on effect of crop failure could be seen in the population drift from rural areas into the cities, farm labour lay-offs and farm closures as well as an increasing indebtedness in the agricultural sector.



Graph showing percentage-of-normal rainfall for Freestate - maize crop impact

Other serious impacts brought about by drought are the devastating veld fires which destroy large areas of grazing at a time when grass is in short supply. Commercial timber and orchards are also prone to damage at such times. In 1992 there were several huge fires which destroyed thousands of hectares of grassland. In one of the worst events, during August, at least nine people perished. In 1994, a combination of unusually strong winds and very dry conditions saw large areas of grazing and timber destroyed. Six people died in one such fire in July of that year. Again, in July of 2002, Mpumalanga was devastated by fires that destroyed 24,000 ha of pasture and left four people dead and damages amounting to more than R32 million.

 

    Severity of Recent Droughts

It is very difficult to look at the entire summer-rainfall region and deduce that drought affected all of these areas equally. On the contrary, some of the provinces in South Africa appear to suffer more harshly than others at times of rainfall deficit.

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Climate Questions Fri, 13 Sep 2013 10:09:46 +0000
How does ENSO affect South Africa? http://www.weathersa.co.za/learning/climate-questions/35-how-does-enso-affect-south-africa http://www.weathersa.co.za/learning/climate-questions/35-how-does-enso-affect-south-africa The impact of ENSO on South Africa

 

Although the southern part of Africa generally receives below-normal rainfall during El Nino years and La Nina usually brings normal or above-normal rainfall, it cannot be accepted as a rule. Southern Africa can be divided into numerous rainfall regions, each region having a different correlation with ENSO. Also, ENSO explains only approximately 30% of the rainfall variability, which means that other factors should also be taken into account when predicting seasonal rainfall. For example: The 1997-98 El Nino was the strongest on record, but not all of South Africa received below-normal rainfall. Some regions had an abundance of rain because of moist air that was imported from the Indian Ocean. One should be careful not to make a general rule for rainfall and temperature changes in ENSO years over southern Africa.

 

Does El Nino always cause drought in South Africa?

 

No. Although most El Nino years have been associated with below-normal rainfall, the impact of El Nino is often reduced by the sufficient groundwater and soil moisture content carried over from previous seasons.

 

Can ENSO be forecast?

 

Yes. SSTs are used to measure the state of the ocean (and ENSO) and can be forecast up to 9 months ahead with good skill. Computer models are used for this and the first indication of ENSO influencing the October-to-March (summer) rainfall season can be forecast as early as the preceding May. IMPORTANT: An El Nino/La Nina forecast is NOT a rainfall forecast.

If you would like to know more about ENSO please follow these links to other ENSO sites:

World Meteorological Organisation (WMO)

Climate Prediction Centre ENSO
Climate Prediction Centre Cold Impact
Climate Prediction Centre Warm Impact
International Research Institute for Climate Prediction

 

Glossary

Anomaly: The deviation from the mean. To calculate SST anomalies, the long-term mean for a specific point in the ocean is subtracted from the current value. A negative value indicates that the current value is cooler (smaller) than usual, while a positive value indicates that the current value is warmer (larger) than usual.

For example: The Nino 3.4 value for December 2003 is 26.9 °C. The long-term mean for the Nino 3.4 region is 26.5 °C
Anomaly = current value – mean
Anomaly = 26.9 °C - 26.5 °C = 0.4 °C

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Climate Questions Fri, 13 Sep 2013 10:08:53 +0000