DEVELOPMENTAL STAGES COLD FRONT OCCLUSION
CHARACTERISTICS Initial - when a bend forms in the polar front Air ahead of the cold front is slightly warmer than the air behind the
Low pressure cell cold front. Slightly warmer air rises up the occluded front along with
Mature - bend in the polar front deepens and the pressure gradient the warm air from the warm sector.
Isobar pattern and winds converging
becomes steeper. Winds blow more strongly. As the air rises, it cools and condensation occurs, resulting in
Clockwise – Southern Hemisphere
Occluded - cold front gradually overtakes the warm front nimbostratus cloud.
Anticlockwise – Northern Hemisphere
Degeneration - low pressure cell disappears and the isobars even out. Rain falls. On the ground, the cold, wet eather associated with a cold
Mid-latitude cyclones develop in the middle latitudes from 35°–70° north, and 35°–70° front occlusion is very similar to the weather associated with a cold
south of the equator along the region known as the polar front. Weather conditions are cold with clouds and rain clearing.
front.
1 500 and 3 000 km in diameter
Move West - East
HOW TO IDENTIFY
max temp ahead of the cold front should be higher than the max
temp. behind the cold front since you are moving from the warm sector to
the cold sector.
The cloud cover should increase along and behind the cold front since this is
where the cumulonimbus cloud forms
Mid Latitude Cyclones WARM FRONT OCCLUSION
Air ahead of the cold front is colder than the air behind the
cold front. This results in the air behind the cold front and the
COLD FRONT warm air in the warm sector rising up the occluded front.
As the air rises, it cools and condensation occurs, resulting in
nimbostratus cloud.
LIRC – Low Inwards Rising Clockwise Rain falls. On the ground, the weather ass. with a warm front
occlusion is very similar to the weather associated with a warm
front.
ASSOCIATED WEATHER
COLD FRONT WARM FRONT
Decrease temp. Increase temp.
Decrease pressure Increase pressure
Decreases humidity Increase humidity
Thunderstorms & rain First cirrus clouds appear at a high level above
Wind direction changes ground, and then altostratus clouds. Later,
nimbostratus clouds appear, bringing soft, soaking
rain.
,Ass. Weather in Winter MOISTURE FRONT & LINE THUNDERSTORMS COASTAL LOW PRESSURE SYSTEMS
Affects SA all year round – warm dry most of the year Hot interior = trough of LP Move around the coast
HP belt moves north Winds drawn into trough from SAHP & SIHP West coast -> East coast
Clear skies, no rain Warm moist air from SIHP Onshore flow brings fog and clouds
Freezing in the am -> Hot in pm Cooler air from SAHP AHEAD of coastal low is ANOTHER onshore flow with warm
Often causes drought Meet at moisture front in trough weather = fire hazard
Cause by Kalahari anticyclone Warm air unstable, rises = thunder storms
Only occur if KHP has risen to allow moist air to reach interior
Cold air above the plateau. Not the same as the convection storms that occur in the interior during hot
Dry air warms up as it subsides. weather – can occur at night/day
Above the plateau there is a layer of air where the temp increases with height. LINE THUNDERSTORM HAZARDS
This is the inversion layer. Rising air cannot penetrate the inversion layer. Flooding + Hail+ Lightening + Strong Winds
Warm moist air from the Indian Ocean cannot blow onto the plateau because it is
blocked by the inversion layer. This is why in winter there is a drought in the
interior of SA BERG WIINDS
Coasties experience strange weather
Ass. Weather in Summer
HP belts move south
Anticyclones Hot dry windless weather –> cold wet windy weather
Caused by berg wind followed by cold front
Kalahari “disappeared”
Very hot inland = LP on surface
HP CELLS A cold front approach CT = strong pressure gradient between the high
pressure of the Kalahari anticyclone over the interior and the
Inversion layer rises with the HP allowing moist air from the Indian approaching cold front.
Ocean to move inland = rain in summer.
Winds blow along this pressure gradient from HP to LP
Sun is directly overhead in this hemisphere = heat distribution and
HASO – High Anticlockwise Sinking Outwards
pressure belts to shift southwards As the winds blow down from the plateau to the coast, they warm up
adiabatically and can cause temperatures of over 30 °C at coastal towns
during winter.
‘ridging in’ means the anticyclone extends
sideways into a ridge shape
CUT OFF LOW SOUTH EASTER AKA CAPE DOCTOR
From strong ridge behind cold front In summer the SAHP moves south, and elongates to form a
ridge south of the country = south easterly winds over the
“blocking "or “cutting off”
Cape Peninsula and southern Cape.
Cold moist air flow in Winds reach land and funnelled btwn Mt.ranges, down valleys
Very cold, heavy rain and snow on Mtn. and streets – sometimes causing gale force winds.
Can reach from CT to JHB
, CHARACTERISTICS FORMATIVE
Very strong up draughts and the low pressure intensifies, although it may not fall below 1 000
Name - The first letter of the name tells us how many cyclones hPa at this stage.
have occurred that year.
TC is not very large, and there is no clear eye.
Strong winds
Winds spiralling & are already gale force in the forward left-hand quadrant (quarter circle).
Huge waves created by the strong winds
In this part of the cyclone, the speed of the wind is increased due to the fact that the whole
30° N and 30° S latitude system is moving in the same direction as the winds are blowing.
Move East to West
IMMATURE
Hurricanes in Gulf of Mexico region, typhoons in Japan and Pressure is now below 1 000 hPa in the centre and continuing to fall. Winds are now hurricane
eastern China, cyclones in southern Africa and willy-willies in Aus
strength (150 km/h) in the area 30–50 km from the eye.
FACTORS NEEDED FOR DEVP. MATURE
Hot moist air Pressure gradient is very strong, and wind speeds reach hurricane strength
Devp. ITCZ where hot, moist air from the tropical easterlies converges. The calm, clear eye is well developed & air pressure is <950 hPa.
Devp. over the sea where the temp H20 is over 28 ° TC moves south westerly direction from the equator, and then turns southeast at about 20° S.
intenselow pressure cell
Low pressure + fast wind over the ocean cause a storm surge (bulge of sea water), which leads to
The hot air rising forms an intense LP cell on the surface. The tropical jet stream in the upper further flooding of the coastal areas
air causes an upper-air LP cell and this intensifies the LP cell on the surface. Air is sucked PREVENTION & MANAGING
into the LP cell.
Coriolis force
TROPICAL CYCLONES Close watch on satellite images
Warn coastal areas at risk of flooding
How does TC keep energy?
Large scale condensation releases latent heat into the atmos, gives energy- needed to sustain the Predictions are based on models of atmospheric circulation & the tracks of previous hurricanes
cyclone. Aid from other countries helps manage supplies & emergency support
Air more unstable and contributes to more air rising, causes cumulonimbus to form, and torrential Insurance important management especially for more devp. countries
rain. If warnings prove wrong there are high economic losses, for eg. businesses close and people
Air pressure is so low in the eye that some air is sucked down, forming calm, cloudless conditions can’t get to work. Too many false alarms = ignore future warnings
where the air is warmer due to adiabatic heating.
Short-term effects – what are the issues now?
Strong winds and flooding in coastal areas = destroy homes & buildings.
Burst pipes = the spread of diseases such as cholera.
Bridges and roads damaged = cutting off access for emergency personnel.
Power lines and water supplies may be cut off.
Flooding can cause the loss of crops and livestock.
Long-term effects – what are the issues ahead?
Water supplies contaminated when sewage systems are destroyed by floods
Large-scale soil erosion can occur because floods remove valuable topsoil
Business may struggle as the close & need to rebuild
