1. General Atmospheric Circulation and Weather Systems

2. General or average atmospheric flow
Polar High
Cold polar air
Sub-polar Low
Sub-tropical High
Warm tropical air
Equatorial Trough
Inter tropical convergence zone (ITCZ)
Warm tropical air
Sub-tropical High
Sub-polar Low
Cold polar air
Polar High
Document Reference: RTC-PRE Date of last revision: 21 August 2017

3. High and low pressure cells
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4. High and low pressure cells Sub-tropical or mid latitude lows
Surface trough
Tropical Cyclone
Anticyclone
Anticyclone
Sub-tropical or mid latitude lows
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5. Frontal systems - only in mid latitude cyclones
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6. Southerly onshore and inland flow
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7. Ridging high behind cold front
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8. Coastal Lows 1. Usually occurs in winter.
2. Dry, warm to hot winds from the land
ahead of the low.
3. Cool winds from the sea behind the low.
4. Moves from the west coast and propagates
along the SA coast to dissipate near
Mozambique.
5. Usually indicates an approaching cold front.
Document Reference: RTC-PRE Date of last revision: 21 August 2017

9. Cut off low (COL) and Upper air trough
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10. Pressure systems and resulting winds
Wind is the movement of air caused by differences in surface temperatures and air pressure.
The greater the difference, the faster the air moves (strong winds).
Always blows from an area of a high pressure to an area of a low pressure.
Document Reference: RTC-PRE Date of last revision: 21 August 2017

11. Measurement Averaged quantities: for Forecasting: 10 min average
for Climatological statistics: 60 min average
Highest gust:
– the maximum observed wind speed over a specified
time interval
Wind has a direction and speed
– Direction is from where the wind is coming (either
geographic [NW, SSE etc] or degrees)
– Speed is how fast/strong the wind is blowing – indicated
in either m/s, knots or km/hr
0.5ms-1 = 1 kt = 1.85 km/hr
Document Reference: RTC-PRE Date of last revision: 21 August 2017

12. Wind direction
Compass
direction
Exact
equivalent
in degrees
Calm -
N by E
11.25
NNE
22.5
NE by N
33.75 NE 45
NE by E
56.25
ENE
67.5
E by N
78.75 E 90
E by S
101.25
ESE
112.5
SE by E
123.75 SE
135
SE by S
146.25
SSE
157.5
S by E
168.75 S
180
Compass
direction
Exact
Equivalent
in degrees
S by W
191.25
SSW
202.5
SW by S
213.75 SW
225
SW by W
236.25
WSW
247.5
W by S
258.75 W
270
W by N
281.25
WNW
292.5
NW by W
303.75 NW
315
NW by N
326.25
NNW
337.5
N by W
348.75 N
360
Variable
Document Reference: RTC-PRE Date of last revision: 21 August 2017

13. Subsiding air over the plateau
Local winds
Berg Wind Formation
Subsiding air over the plateau
Surface winds flow down towards the sea
Air descends and warms at DALR
Escarpment
Berg wind
Subsidence
Berg winds
Winds blowing from the interior to the coast warm up and reach the coastal regions as hot, dry and windy.
This can cause the highest temperature reached during winter along the coastal regions.
Temperature increases by 1oC / 100m
18oC
28oC
23oC
1000m
100m
700m
400m
300m
200m
500m
900m
800m
600m
22oC
19oC
21oC
25oC
26oC
27oC
24oC
Sea level
Document Reference: RTC-PRE Date of last revision: 21 August 2017

14. Local winds Anabatic winds Katabatic winds
During the day, these are upslope winds driven by the warmer surface temperatures on a mountain slope than the surrounding air column which causes the air to rise.
Katabatic winds
During the night, these are downslope winds driven by the colder surface temperatures on a mountain slope than the surrounding air column which causes the air to move down.
Document Reference: RTC-PRE Date of last revision: 21 August 2017

15. Local winds Sea and Land Breeze
During the day, the warms up faster than the sea.
The wind then blows from the sea to the land as a sea breeze.
At night, the land cools faster than the sea.
The wind then blows from the land to the sea as a land breeze.
Document Reference: RTC-PRE Date of last revision: 21 August 2017

16. Winds in the vertical Altitude (Height) Wind – length represents speed
Ocean – lowest resistance
Rural area – moderate resistance
Urban area – highest resistance due to tall buildings
Wind – length represents speed
Altitude (Height)
Document Reference: RTC-PRE Date of last revision: 21 August 2017

17. The End