Weather is short term atmospheric conditions while climate describes average conditions over 30 years. Various instruments are used to measure weather elements like temperature, humidity, pressure, wind, and precipitation. Climate is influenced by both long term geographic factors like elevation, ocean currents, as well as short term factors like seasons and cloud cover. Different air masses with distinctive temperature and moisture characteristics affect weather and climate in Southern Africa.

1. Weather and climate
Weather: Is the hour to hour, day
by day state of the earth’s
atmosphere. It is short and often
localised in area. Includes elements
such as temperature, humidity,
precipitation, wind and sunshine
Climate: The average weather
conditions as measured and recorded
over an extended period of at least
30 years. While the study of weather
is – usually applied to large areas –
can be studied in the global context.

2. Weather instruments
 Stevenson Screen
 Stevenson Screen must be placed in an
open, grassy area away from any
obstacles such as buildings, trees or walls
– designed to hold instruments such as
thermometers and hygrometers – keeps
instruments away from direct sunlight.
 Painted white – reflect sunlight. Louvred
or slatted – air movement. Door southern
- light

3. Six’s Thermometer (min& max)
 Measures minimum and maximum
temperatures usually 24 hours. U tube
contains mercury and alcohol.
 As the temperature rises, alcohol expands
and pushes the mercury and metal index
up one side of the tube
 As temperature falls, the alcohol contracts.
The mercury moves along the tube in
opposite direction leaving the metal index
to mark maximum temperature.
 The metal index in the other tube marks
the minimum temperature, highest point
mercury moved. Reset by pulling the index
with magnet.

4. The Hygrometer/Wet and Dry Bulb
 Two identical mercury thermometers –
one has a muslin wick hanging in a
container of water – the drier the air, the
greater the amount of evaporation from
the wick – the lower the temperature
shown by the wet-bulb thermometer.
Subtract the wet reading from the dry-
bulb reading to get the depression
 – use the table to find the relative
humidity – the amount of moisture in the
atmosphere as a percentage of the
amount the air can hold at that
temperature.

5. Barometer
 Measures atmospheric pressure, 1000 mb
– a small drum with a partial vacuum –
contracts when air pressure rises – moves
a needle on a dial - pressure is expressed
in HetcoPascals or Millibars – may be kept
in a Stevenson Screen.
 Barograph: Records atmospheric
pressure – small drum with a partial
vacuum – attached to a stylus or pen –
draws a line on a sheet of graph paper
wrapped around a revolving drum – gives
a continuous record of the changes in
atmospheric pressure for a week.

6. Anemometer
 Measures wind speed – has three or four
arms with cups on the end – wind catches
cups – causes the anemometer to spin
around an axis – digital counter at the
base shows wind speed in knots, meters
per second or kilometres per hour. The
system must be located in a clear area
(away from buildings, trees), meters
above the ground. Wind speed can also
be indicated on the Beaufort scale.

7. Wind Vane
 Shows wind direction – an arrow
points in the direction from which
the wind is blowing – has four
pointers indicating the four main
compass directions – north, south,
east and west – must also be
located away from any obstacles
e.g. trees and buildings, in a clear
area. Wind –sock. Windrose
diagram

8. Rain Gauge
 measures the amount of rainfall in
millimetres – should be made of
copper or plastic – placed in an
open area, away from any
obstruction – may be above ground
1m – or sunk into grass with 30cm
protruding – reduces evaporation –
grass prevents splashing. Bar
Graphs

9. The Sunshine Recorder /
Pyronometer
 Records the number of hours of
sunshine during a day – glass ball
focuses sunlight on a piece of graph
paper – burns a track along the
paper whenever the sun shines –
must be placed where sun will not
be obscured by trees or buildings.

10. Cloud cover:
 Measures by estimating the amount
of cloud cover in the sky – use eyes
– recorded in one-eight fractions, or
octas – eight-eights showing full
overcast.

11. Elements, instruments & units
 Precipitation - Rain gauge – mm
 Temperature – Min. Max. Thermometers - °C
 Humidity – Hygrometer (W&D thermo) - %
 Atmospheric Pressure –Barometer - Mlb/Hpl
 Wind Direction - Wind Vane - Compass Points
 Wind Speed – Anemometer - Km/h
 Cloud Cover - Using eyes-Eighth or Octas
 Hours of Sunshine -Sunshine Recorder- Hours

12. Using and analysing climatic data
 Remember:
 Total: Add all the values
 Average: Add all the values and divide by
the number of values
 Range: Subtract the lowest values from the
highest value.
 Median: Write all the values in ascending
order. The middle value is the median.
 Mode: The number that appears most in
your list of values.

13.  Isolines: Lines drawn on a map to
connect places with the same value
 Isotherms: Lines connecting
places with the same temperature
 Isobars: Lines connecting places
with the same atmospheric pressure
 Isohyets: Lines connecting places
with the same rainfall (pronounced
iso-hites)

14. Factors influencing temperature
Long-term factors
 Height above sea level: temperature drops 1°C for
every 100m.
 Angle of the sun: when the sun is vertically overhead
the rays of the sun strike the atmosphere at a near-
vertical angle and are less likely to be lost due to
reflection or scattering.
 The presence of water or distance from the sea:
water allows sunlight to penetrate deeper than land.
Water heats up slower than land – retains its heat longer
than the land – Places near the coast experience Maritime
climates with more moderate temperatures – inland
places have Continental climate with more extreme
conditions.
 Ocean Currents: Warm ocean currents move warm
water to the polar regions and raise the temperatures –
cold currents carry cold water towards the equator –
lower the temperatures
 Prevailing winds: winds that blow over cold oceans
cause low rainfall – winds blowing from interior of
continent will be dry.

15. Short-term factors
 The influence of the seasons: summer in the
southern hemisphere – sun directly over Tropic of
 Capricorn (23,5°S) – the southern hemisphere receives
maximum insolation – winter in the southern
hemisphere – sun directly over the Tropic of Cancer
(23,5°N) – southern hemisphere receives more oblique
rays – less insolation.
 Aspect of a slope: Southern hemisphere – slopes
facing north receive the greatest amount of sunlight
and are warmer.
 Land use: land cleared of vegetation heats up and
dries out more – tar and cement absorb more heat
than vegetation and tend to be warmer.
 Cloud cover: The greater the amount of cloud cover
the greater the amount of insolation reflected back into
the atmosphere – less heat to earth – cloud cover acts
as insulation – prevents terrestrial radiation from
escaping – higher temperatures.

16. Why are temperatures in urban areas are
usually higher?
 Tar and cement absorb more heat than
natural surfaces
 Cities produce artificial heat e.g. air
conditioners, office equipment
 Tall buildings receive sunlight for longer
periods than flat lands
 Pollution absorbs heat and retains it over
the city
 Most water in cities is drained away so
cities are drier and hotter

17.  Fog and precipitation are common in urban
areas because there is more pollution – more
hygroscopic particles for condensation
 Temperature Inversions: when temperature
rises with increasing altitude
 Coriolis Force: if you stand with your back to
the high pressure in the southern hemisphere, air
is moving to a low pressure in front of you will be
deflected to the left. North of the equator the
direction of deflection will be to the right – winds
blow parallel to isobars – Coriolis force acts at
right angles to pressure gradient force to create a
geostrophic wind.

18. Air Masses
 Air mass: a body of air with
particular characteristics e.g. warm,
dry, moist, cool or combinations of
these.
 Front: Narrow zone of contact
between two different air masses –
they do not mix.

19. The most important air masses
influencing Southern Africa
 Tropical Continental (Tc): Originate over
Central Africa-Warm and dry may pick up
moisture from the ITCZ during summer.
 Tropical Maritime (Tm): Originate over the
Indian Ocean. Warm moist
 Subtropical Maritime (STm): Originate over the
Atlantic Ocean. Cool moist air – is stable at first
– may become less stable as it moves over
warmer water.
 Sub polar Maritime (SPm): Originate over the
South Atlantic. Cold and dry.
 Polar Maritime (Pm): Originate over the sea
surrounding the Antarctic. Very cold and very
dry at first – may pick up moisture as it moves
over warmer water.

20. Convergence Zones
 Found where air masses move towards each other-
lighter, less dense air mass will be lifted and form
clouds and rain with a low-pressure system.
 Intertropical Convergence Zone (ITCZ): air from
the tropical the tropical easterlies converges in the
equatorial region.
 Indian Ocean Convergence Zone and Cloud Band
over the Indian Ocean southeast of Southern Africa –
links the tropical and the temperate latitudes to move
large amounts of moist unstable air towards SA in
summer
 Depressions: Cold fronts that form along the Polar
Front and move towards SA – from West to East –
bring rain and cold to Southern Africa.

21. Tropical Cyclones
 Required conditions:
 Warm oceans – average surface water
temperature exceeds 27 C
 Late summer to early autumn when sea
temperatures are highest
 In the belt of the Tropical Easterlies –
surface winds heat as they blow towards
the equator
 Between latitudes 5° and 20° north or
south of the equator – always move east
to west.

22. The Hydrological Cycle
 Evaporation: water changes from a
liquid to water vapour
 Sublimation: ice changes to water
vapour, or vice versa
 Transpiration: plants give off water
vapour that evaporates
 Evapotranspiration: water from plants
and surfaces turns to water vapour
 Dew point temperature: the
temperature at which condensation
begins
 Condensation: when air becomes
saturated with moisture and turns to
liquid on tiny hygroscopic condensation
nuclei

23. Reasons for condensation
 Radiation cooling: the earth radiates the heat it
receive from the sun – if temperatures drop low
enough moisture in the air will condense – usually
on cold surfaces
 Advection cooling: when warm, moist air moves
over a cold land or sea surface.
 Orographic uplift: when warm, moist air is forced
up a physical obstacle like a mountain rising air
cools.
 Frontal uplift: when war, moist air is forced to rise
over heavier, denser, colder and drier air, the rising
air will cool and condense
 Convectional uplift: when air is heated during the
daytime and is forced to rise in thermals, or rising
air currents, it cools and condenses

24.  Stable air: Usually dry – forced to
rise – cools faster than surrounding
air – colder than surrounding air –
returns to old height – clear skies –
no rain
 Unstable air: Usually moist air –
forced to rise – cools slower than
surrounding air – warmer than
surrounding air – continues rising –
condensation, clouds and rain