SYNOPTIC WEATHER MAPS WHAT IS A SYNOPTIC WEATHER MAP? It is a map that gives a summary of the weather conditions for a specific time and place WHAT IS ATMOSPHERIC PRESSURE? DEFINISIE/DEFINITION It is the force exerted by the weight of the atmosphere on a surface DIFFERENCE BETWEEN A HIGH PRESSURE AND LOW PRESSURE CELL LOW pressure/ HIGH pressure/ DIFFERENCE BETWEEN HIGH AND LOW PRESSURES Anti-clockwise DESCENDING/SINKING AIR Clockwise ASCENDING/RISING AIR SURFACE/UPPER AIR N HEMISPHERE CONVERGENCE DIVERGENCE AND Upper air divergence Upper air convergence Surface air divergence Surface convergence LOW PRESSURE HIGH PRESSURE Isobars are circular Isobars are oval shaped Associated with rising (ascending) air; Associated with sinking (descending) air; Air circulates clockwise Air circulatesanticlockwise Highest pressure (SH) • Lowest pressure reading is in the centre; (SH) reading is in the centre; Cloudy conditions and the possibility if rain occur (unstable weather conditions Air converges on the Air diverges in the surface upper air Fair weather conditions occur (stable weather conditions) Air diverges on the surface from this pressure cell Air converges in the upper air COMPONENTS ON A WEATHER MAP Date indicates season Isobar- line joining places with the same atmospheric pressure Tropical cyclone (L) 1017-1019hpa Pressure reading THERMAL OR HEAT LOW (L) Isobaric interval = 4hpa SOUTH ATLANTIC HP SOUTH INDIAN HP Weather station Mid-latitude cyclone (L) HOW DOES WIND DEVELOP? AIR ALWAYS MOVES FROM A HP TO LP PRESSURE GRADIENT A force that determines the speed at which air flows from a HP to a LP across the isobars (a)This wind is called a gradient wind that moves ACROSS isobars (b) It influences WIND SPEED/VELOCITY (c) Is used to describe the DIFFERENCE IN WINDSPEED on weather maps Animation by: Hal GENTLE OR WEAK PRESSURE GRADIENT = LIGHT TO MODERATE WIND STEEP PRESSURE GRADIENT = STRONG WINDS Pressure gradient on weather maps When isobars are far apart = pressure gradient is described as weak – light to moderate wind When isobars are closer to each other = pressure gradient is described as steep – strong wind TYPE OF HIGH PRESSURE CELLS KALAHARI HIGH SAH SIH SOUTH ATLANTIC HP KALAHARI OR CONTINENTAL HIGH • Located off the west Found over the interior coast of South Africa of South Africa; • It produces clear, dry It dominates over the and stable weather land in winter but conditions weakens in summer Reasons: Causes dry conditions • Winds that diverge out of over interior during this system are dry since winter there is little evaporation over the cold oceans; • Cold air over the Atlantic ocean is associated with strong subsidence that prevents air to rise and cool to produce rain • Main types of precipitation is fog and mist SOUTH INDIAN HP • Located off the east coast of South Africa • During summer it is far away from the land and further south; • On-shore winds carry moisture over the east coast and the interior. • During winter it is closer to the land and further north. • The wind has a small fetch and carries little moisture onto the east coast – area is then drier during winter TYPE OF CYCLONES/LOW PRESSURE CELLS 1. KUSLAAGDRUK/COASTAL LOW MIS/FOG MOTREËN DRIZZLE L L L Develops along west coast and moves around the coast to the east coast If present along west coast If the coastal low is present along the EAST coast is causes unstable conditions FOGGY OR MISTY DRIZZLE 2. MID LATTITUDE CYCLONES: CONCEPTS COLD FRONT MID-LATITUDE CYCLONE ON SATELLITE IMAGE Mid-latitude cyclones (frontal or temperate depressions) Areas where the occur (30° - 60° N and S) Move from WEST to EAST Steered by the westerlies in the westerly wind belt Mid-latitudes MID-LATITUDES MID-LATITUDE CYCLONES: CHARACTERISTICS COLD FRONT ISOBAR COLD SECTOR WARM SECTOR L WARM FRONT CLOCKWISE WIND CIRCULATION COLD SECTOR SATELITE IMAGE COLD FRONT CLOUDLESS WEATHER CHANGES: COLD FRONT COASTAL LOW Cold front NW Wind SW Wind Warm front Weather changes: Mid-latitude cyclone © Western Cape Government 2012 | WEATHER CHANGES: COLD FRONT Movement of cyclone •SW-wind •Low temp •High cloud cover •NW-wind •High temp •Few/no clouds •No precipitation •HP •Precipitation •LP LP CHANGE IN WEATHER REASON FOR CHANGE Drop in temperature As a result of cold air behind the cold front Sudden, gusty winds (increase Strong pressure gradient in wind velocity Change in wind direction from Northwest to south westerly winds…backing occurs Clockwise spiralling air Dense cloud cover Due to strong uplift of warm air and result in heavy rainfall/thunderstorms/hail Atmospheric pressure will be increase Due to the cold, dense air following (behind) the cold front Decreasing humidity Due to cold air being more dense Snowfalls may occur Dewpoint temperature is reached below freezing point POSITIVE IMPACT Increased rainfall fills up dams, rivers; Enables irrigation of winter crops which provides enough food Low temperatures ideal for crops that thrive in cold conditions Cold conditions can kill pest that eat crops Improves pasturage and water for livestock NEGATIVE IMPACT Heavy downpours are associated with serious flooding that damage properties, infrastructure and crops The formation of snow on the mountains damage crops and kills livestock People are left homeless and displaced especially in lowlying areas; Poor visibility on roads results in accidents/traffic congestion PRECAUTIONARY MEASURES Avoid development in low- Farmers should have lying areas sufficient grain and fodder to minimise An efficient stormwater loss of livestock drainage system to reduce flooding Secure fishing vessels in harbours and keep Livestock should be placed track of weather via in a kraal or shed TV/radio Stock up on essentials (batteries, food, medication) Important to keep updated on weather as electricity blackouts , conditions in winter loss of communication can before planning occur outdoor activities 3. TROPICAL CYCLONES Tropical cyclone Eline Symbol of tropical cyclones TROPICAL CYCLONES Equator/Ewenaar From EAST to WEST EYE WINDLESS, CLOUDLESS, NO RAIN DESTRUCTION •Storm winds •Torrential rain •Flooding Turns EAST at 30° DISSIPATES •No Moisture •No warm air •Friction How are they named? Names according to the ALPHABET ANEL BRANDON CARLYLE David Ester FATIMA Kaylin/ Kirsty TROPICAL CYCLONES Develop between 5 ⁰ and 25 ⁰ N and S Move from EAST TO WEST Steered by the TROPICAL EASTERLIES OR TRADE WINDS Verspreiding van Tropiese Siklone Distribution of Tropical Cyclones Willy Willies HURRICANES C TYPHOONS TROPICAL CYCLONES B D WILLY WILLIES A TROPICAL CYCLONES (Grade 12) NAME Alphabet Andrew Bonny Carl Debora IN EYE No Wind No Clouds No Rain ORIGIN In tropics OCEANS Not nearer than 5° from equator WEATHER Before eye During eye After eye MOVEMENT DESTRUCForm E to W TION Away from Equator Turns east at 30° Storm winds Very hard rain Flooding DISSIPATION MANAGED Sand bags Early warning No Moisture Track storm No Warm air Services Friction Inform people Evacuation WEATHER IN EYE CLOUDLESS WINDLESS NO RAIN STORM WINDS GREAT SWELLS & WAVES FLOODING DESTRUCTION OF TROPICAL CYCLONE ELOISE 26 JANUARY 2021 MPUMALANGA, SOUTH AFRICA STORM SURGES Cloud cover Air temperature 23 ● Precipitation 20 Dew point temp Wind speed 1 feather=10knots knots 1knot = 1,8km/h WINTER WEATHER MAP DATE ON MAP WINTER MAP 1. 2. 3. 4. 5. 6. 7. Date on the map will help determine the season; The mid-latitude cyclone has a cold front passing over the continent – placed further north Kalahari HP is dominant over the interior; There may be a coastal low but it is ahead of the cold front; Low air temperatures with low dew point temperature over the interior SAH and SIH are closer to the land Cloudless conditions over the interior 1. Strong Kalahari High pressure in interior during winter Sterk Kalahari Hoogdruk oor binneland gedurende winter Anti clockwise circulation Antikloksirkulasie H SUMMER WEATHER MAP THUNDER STORM HIGH PRESSURES FURTHER SOUTH DATE ON MAP
