South African Weather Service - Educational Questions Sun, 23 Sep 2018 15:07:18 +0000 Joomla! - Open Source Content Management en-gb (MFI SAWS) Explanation of Fisher-Tippett distribution for the calculation The return periods of extreme rainfall (or temperature) are calculated using the Fisher-Tippett Type I probability distribution. This is a statistical calculation of extreme values based on existing time-series of data. A very simple explanation of this will now be attempted:

In order to calculate the extreme high values of say 24 hour rainfall, the daily rainfall data for a particular station is extracted. The highest 24-hour rainfall for each month, for each year is extracted and ordered in chronological date order. All the data for say January is then fit to a probability curve. Based on this unique curve, a value is determined which will meet the condition of only occurring every 25 years, or 100 years. This value itself may not exist in the initial time series from which the curve was drawn, but is calculated from the time series and uses the trends and characteristics in the time series to come up with a value. An extreme value calculated at a return period of 1:100 years will correspond on the probability curve to a probability of 0.99, 1:25 years to a probability of 0.96 and so on (Probability = 1-1/(Return Period in years)).

In simple terms, a 1:100 rainfall value of say 120mm in January implies that rainfall close to 120 mm in 24 hours can only be expected to occur in January once every 100 years. It is possible, however, for 120mm of rainfall to fall this year and then again in 5 years time, but the likelihood is that this rainfall will then not be experienced again for at least another 150 to 200 years.

The entire process is repeated for each month of the year. The values for the year are calculated by taking the highest 24 hour rainfall for each year and fitting the data to the probability curve. The return values are always higher for the whole year than for any of the individual months because the time series of annual extremes consists of higher values than any of the individual months.

Educational Questions Fri, 13 Sep 2013 11:21:17 +0000
Guidelines are for learners wanting to do a weather project Step 1: Choose your topic carefully

Try to pick a topic you know something about or that you are interested in. Remember to consider where and how you are going to obtain or collect your data for your project before deciding on a topic.

Step 2 : Focus your study

Be very careful not to make your question/hypothesis too broad e.g. Climate change (or global warming) and its influence on South Africa. This is a huge topic, think of all the sectors of society that may be affected by climate change or global warming e.g. Agriculture, mining, fisheries, forestry, health the list is endless. Other topics to be avoided are: drought or floods in South Africa, ozone hole and its influence on South Africa.

Limit your topic to a specific place or time e.g. The 1981 Laingsburg floods. Another way to limit your study is to look in the newspaper for an article which you can use as the basis for your study. Either investigate the issue further or more in depth or investigate what role the atmospheric condition had on the reported event/s.

You may want to consider doing a case study type project. Data for this type of study can be obtained from the Climate Information and Publications Office at telephone number 083 233 8686 or

Step 3 : Draw-up a study plan

This plan will give you an idea of where to start with your study. Remember to give yourself enough time to complete each activity in your project. It’s no good contacting people you have identified to supply you with data the day before your project has to be handed in. Plan carefully as your study topic or focus may change as you collect your data.

Step 4 : Write-up your project

Possible outline for your study (your educator may give you an outline that you must follow)
Question to be considered or hypothesis
Background to study
Research method

Educational Questions Fri, 13 Sep 2013 11:20:48 +0000
How do I calibrate a barometer? Barometers are not the easiest instrument to set. The best way to do this is to set the instrument using the pressure reading from a barometer which is already calibrated and this instrument should be at the same height above sea level as the instrument you are setting.

Educational Questions Fri, 13 Sep 2013 11:20:14 +0000
How do I interpret maps, charts and diagrams? What is a wind rose and how is it interpreted?

A wind rose is a graphical representation of the wind speed and wind direction for a specified period for a particular location. The wind rose can be representative of the wind for a day, month, year or a long-term average by month or year. There are two graphics for each wind rose. One graphic depicts the average wind speed by wind direction in meters per second. The other graphic represents the percentage of frequency by wind direction.


How do I interpret a synoptic chart?

When looking at a synoptic chart the first thing to take note of are the isobars. These are lines joining places of equal pressure. Areas with high numbers are known as areas of high pressure and low-pressure areas are indicated by lower numbers. Wind blows from high-pressure areas to low-pressure areas. However, due to Coriolis force wind doesnt blow in a straight line from high- to low-pressure areas but blow in a clockwise direction around a low-pressure area and in an anti-clockwise direction around a high-pressure area (in the Southern Hemisphere). Winds thus tend to blow sub-parallel to the isobars and from the isobar patterns on a synoptic chart it is possible to estimate from which direction the wind will be blowing at any location. The closer the isobars are together, the steeper the gradient between areas of high and low pressure and the stronger the wind. By studying the patterns shown by isobars, forecasters can make predictions about how the weather conditions will develop. 

Air does not just move horizontally but also vertically. Descending (or sinking) air is associated with high-pressure areas and this produces clear skies and generally fair weather. Ascending (or rising) air on the other hand is associated with low-pressure areas causing the formation of clouds with possibly precipitation. Troughs of low pressure and ridges of high pressure can also be identified. Ridges are areas of high pressure that generally result in dry conditions in their immediate vicinity. A ridge of high pressure may be associated with coastal showers when it brings onshore winds along the east coast in advance of the ridge itself. These onshore winds can produce widespread coastal showers. The zone of interaction of the ridge with nearby areas of low pressure or troughs can be unstable and produce storms or rain in any area. Troughs are regions of relatively low pressure which often precede a cold front. These areas of relatively low pressure are unstable and tend to have high moisture associated with them. Consequently, they are good sources of thunderstorms.

Temperatures from a large number of weather stations are also plotted on the synoptic chart. This information is used to determine the location of fronts. By seeing where temperature changes significantly across a small area, it is possible to locate the position of these fronts. Cold fronts have triangles along the line indicating the position of the front and warm fronts have half-circles. Fronts occur at the boundaries of converging air masses which come together from different parts of the world. Since air masses usually have different temperature, they cannot mix together immediately owing to their different densities. Instead, the lighter, warmer air mass begins to rise above the cooler, denser one.

Fronts are usually associated with regions of low pressure, also known as depressions. As the sector of warm air is forced to rise, the cold air begins to engulf it. The leading edge of the warm air is marked by the warm front. The cold front marks the rear edge of the warm air and the leading edge of the ensuing cold air. When the warm air is completely lifted off the ground and is no longer in contact with the surface of the earth, this may be marked on a synoptic chart by an occluded front. Fronts are usually accompanied by clouds of all types, and very often by precipitation. Precipitation is usually heavier although less prolonged at cold fronts than at warm fronts, since the uplifting of warm air is more vigorous due to the undercutting of cold air, resulting in increased atmospheric instability.

Educational Questions Fri, 13 Sep 2013 11:19:42 +0000
What are heating degree days and cooling degree days? Heating degree days are indicators of household energy consumption for space heating. One heating degree-day unit is given for each degree that the mean daily temperature departs below the base of 18 °C. One cooling degree-day unit is given for each degree that the mean daily temperature departs above the base of 18 °C.

How heating and cooling degree days are computed?

Take the highest (Tmax) and lowest temperature (Tmin) for the day, and average them. If this number is greater than 18 °C, then we have (Average temperature - 18) cooling degree days. If the average temperature is less than 18 °C, then we have (18 - Average temperature) heating degree days. Running totals are kept for these units over a time period of a year so fuel distributors and power companies can assess average demands.

Heating Degree Days in equation form:
HDD = Tbase - Ta if Ta is less than Tbase
HDD = 0 if Ta is greater or equal to Tbase
Where: Tbase = temperature base
Ta = average temperature, Ta = (Tmax + Tmin) / 2

Cooling Degree Days in equation form:
CDD = Ta - Tbase if Ta is greater than Tbase
CDD = 0 if Ta is less than or equal to Tbase
Where: Tbase = temperature base
Ta = average temperature, Ta = (Tmax + Tmin) / 2

Educational Questions Fri, 13 Sep 2013 11:17:59 +0000
What does the wind direction in degrees mean? Wind direction is measured in degrees, similar to reading a compass, and is reported as the direction from where the wind is blowing.
South - 180 degrees 
West - 270 degrees 
North - 360 degrees

For example, if the wind direction is 45 degrees, the winds are coming out of the northeast and blowing towards the southwest. This would be called a north-easterly wind.

Educational Questions Fri, 13 Sep 2013 11:17:20 +0000
What is a Growing Degree Day (GDD)? This is an index used to express crop or insect maturity. Growing degree days are those days necessary for crops or insects to complete their growth and development. The basic concept is that development will only occur if the temperature exceeds some minimum development threshold, or base temperature (TBASE). The base temperatures are determined experimentally and are different for each organism.

Reported Base Temperatures for GDD Computations
23,4 °C - sunflower, potato 
32,4 °C - sweet corn, corn, sorghum, rice, soybeans, tomato

To calculate GDD, you must first find the average temperature for the day. The average temperature is found by adding the maximum and minimum temperatures for the day and dividing the sum by two. If the average temperature is at or below TBASE, then the Growing Degree Day value is zero. If the average temperature is above TBASE, then the Growing Degree Day amount equals the average temperature minus TBASE. The values that are greater than zero are added to determine the weekly, monthly or yearly GDD

Accumulated GDD can also be used to:
estimate the growth-stages of crops or life stages of insects; 
predict maturity and cutting dates of forage crops; 
estimate the heat stress on crops; 
help estimate the yields of cereals 
as a planning tool for spacing planting dates to separate harvest dates in vegetable production

Educational Questions Fri, 13 Sep 2013 11:16:43 +0000
What is air density and absolute humidity? The density of air depends on its temperature, its pressure and how much water vapor is in the air.

Air Density Calculations

To begin to understand the calculation of air density, consider the ideal gas law:
(1) P*V = n*R*T
V = volume
n = number of moles
R = gas constant
T = temperature

Density is simply the number of molecules of the ideal gas in a certain volume, in this case a molar volume, which may be mathematically expressed as:
(2) D = n / V:
n = number of molecules
V = volume

Then, by combining the previous two equations, the expression for the density becomes:
(3) D = P / (R * T)
P = pressure in Pascals (multiply mb by 100 to get Pascals)
R = gas constant, (J/(kg*degK) = 287.05) for dry air
T = temperature, (degK = deg C + 273.15)

The density of a mixture of dry air molecules and water vapour molecules may be expressed as:
(4) D = (Pd / (Rd * T))+(Pv / (Rv * T))
Pd = pressure of dry air in Pascals
Pv= pressure of water vapour in Pascals
Rd = gas constant for dry air (J/(kg*degK) = 287.05)
Rv = gas constant for water vapour (J/(kg*degK) = 461.495)
T = temperature (degK = deg C + 273.15)

To determine the density of the air, it is necessary to know is the actual air pressure (also known as absolute pressure, or station pressure), the water vapour pressure, and the temperature.

Vapor Pressure

A very accurate, albeit quite odd looking, formula for determining the saturation vapour pressure is a polynomial developed by Herman Wobus
(5) Es = Eso / p ^ 8
p = (c0+T*(c1+T*(c2+T*(c3+T*(c4+T*(c5+T*(c6+T*(c7+T*(c8+T*(c9)))))))))) 
T = temperature, deg C
c0 = 0.99999683
c1 = -0.90826951*10-2
c2 = 0.78736169*10-4
c3 = -0.61117958*10-6
c4 = 0.43884187*10-8
c5 = -0.29883885*10-10
c6 = 0.21874425*10-12
c7 = -0.17892321*10-14
c8 = 0.11112018*10-16
c9 = -0.30994571*10-19

For situations where a slightly less accurate formula is acceptable, the following equation offers good results, especially at the higher ambient air temperatures where the saturation pressure becomes significant for the density altitude calculations.
(6) Es = C0 * 10 ^ ((C1 * Tc)/(C2 * Tc))
Tc = temperature, deg C
c0 = 6.1078
c1 = 7.5
c2 = 237.3

What is air pressure?

The air's pressure is the weight of the air molecules pressing down on the Earth surface below. Since the pressure depends on the amount of air above the point where you're measuring the pressure, the pressure falls at higher altitudes.

Educational Questions Fri, 13 Sep 2013 11:16:05 +0000
What is an isopleth? This is a line of equal value (a Greek word iso - equal; pleth - value). Examples of different types of isopleths:
Isoneph Cloudiness
Isopycnic Density
Isodrosotherm - Dew point
Isohume Humidity
Isohyet Precipitation
Isobar Pressure
Isallobar - Pressure tendency
Isohel Sunshine
Isotherm Temperature
Isogon - Wind direction
Isoshear - Wind shear
Isotach - Wind speed

Educational Questions Fri, 13 Sep 2013 11:15:20 +0000
What is climate variables are reordered by South African Weather Service stations? The South African Weather Service maintains a network of various types of weather stations throughout South Africa. The country is divided into different areas and a Weather Office is responsible for maintaining the weather stations in its area of responsibility.

Types of station

First order stations: Visual observations are done which include cloud identification, visibility, dew, frost and present- and past weather conditions. Synoptic reports are sent after each observation. First order stations do three-hourly observations from 05:00 to 20:00.

Second order stations: Limited visual observations are done and no synoptic reports.

Third order stations: Only maximum and minimum temperatures and rainfall are done. Some observers at third order stations undertake to submit further elements voluntarily.

Rainfall stations: These stations are equipped with a 127 mm rain gauge only. Rainfall is measured daily at 08:00.

Sea-surface stations: The sea-surface temperatures are measured daily and the data is processed by selected Weather Offices.

Observation times (SAST)
14:00 - only first and second order stations
20:00 - only first order stations

Educational Questions Fri, 13 Sep 2013 11:04:45 +0000