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,CLIMATOLOGY
1. MID-LATITUDE CYCLONES
* CHARACTERISTICS/ AREA/ CONDITIONS FOR FORMATION:
• mid-latitude cyclone: low pressure system that develops in the westerlies and travels W—>E
• Coriolis force: causes winds to deflect left (SH) or right (NH)
• convergence: air comes together
• divergence: air moves apart
• general characteristics:
-cold polar & warm subtropical air masses meet —> 2 fronts (warm & cold front)
-pressure at centre: 996hPa (lower than 1000)
-diameter: 1500-3000km
-occur in families (form one after another and travel closely at 50-60km/hour)
-last 4-14 days
• areas where they form:
-40 °-60 ° N/S (warm air in tropics meets cold air in polars)
-travel in westerly winds W—>E
-in winter: pressure belts & wind systems follow migration of the sun therefore mid-lat cyclones move
North —> cold fronts pass southern cape
-in summer: pressure belts & wind systems move South therefore cold fronts pass south of South Africa
-cold fronts (cold & wet) —> SW of SA get wet winters & dry summers // interior of SA gets cold
snaps during winter
• conditions necessary for formation of mid-lat cyclone:
-mass of warm moist air meets mass of cold dry air
-upper air divergence (removes air ascending from below and makes space)
-something to trigger development of low-pressure centre
* STAGES OF DEVELOPMENT & RELATED WEATHER CONDITIONS:
• cyclogenesis: development & strengthening of cyclone (stage 1 & 2)
• mid-lat cyclones go through 4/5 stages
• STAGE 1: the initial stage
-warm tropical air meets cold polar air at polar front (big difference in temps and winds on both sides blow
in opposite directions)
-front doesn’t move therefore stationary front
,• STAGE 2: the developing stage
-disturbance in fast moving upper air (jet stream)/ mountain range disrupts air motion air —> wave
develops at polar front —> affects air movement on surface
-small mass of warm air extends into cold air
-rising air causes pressure to decrease and forms low pressure centre
• STAGE 3: the mature stage
-warm air moves further into cold air —> wave increases and cold/warm sectors are established
-2 fronts develop making boundaries between cold & warm air masses
-warm front: leading / eastern edge of warm sector (ahead of warm air)
-cold front: leading edge of cold sector following warm sector
-whole system moving W—>E
-air rises along warm front (warm air rises above colder air ahead of front)
-air rises along cold front as heavy cold air wedges under warm air ahead
-rising air along both fronts —> condensation/ cloud formation/ rain/ pressure in centre drops further
-air-flow around centre is cyclonic (clockwise in SH)
• STAGE 4: the occluded stage
-cold air behind cold front travels faster than warm sector air —> cold front catches up with warm front
-warm air forced to rise —> small warm sector on ground
-cold & warm fronts joined & form occluded front
-widespread rain because warm air rises along occluded front, cools & condenses
• STAGE 5: the dissipated stage
-entires warm sector is above ground (no warm air on surface)
-no longer 2 masses of air therefore no warm/cold fronts
-isobars return to regular patterns
-light gusts of cold air on ground
,* WEATHER PATTERNS ASSOCIATED WITH WARM/ COLD/ OCCLUDED FRONTS:
• warm fronts:
-gentle angle therefore covers large area
-warm moist air rises over cold air ahead —> forms high-level cirrus; altostratus; nimbostratus; stratus
-gentle uplift of air along front —> soft rain over large area ahead of front
-changing clouds indicate to person ahead that warm front is approaching
-temperature increases
-pressure decreases
-warm air can hold more water therefore humidity increases as warm front passes over
-wind changes direction in anti-clockwise direction
• cold fronts:
-tall cumulonimbus clouds form —> heavy rain & thunderstorms
-steeper angle covers smaller area
-temperature decreases
-pressure increases in colder air
-cold air holds less water therefore humidity decreases as cold front passes over
-wind backs: changes from NW —> SW
• occluded fronts:
-cold front catches up to warm front —> pushes warm air upwards —> cold air behind cold front meets
cold air ahead of warm front —> 2 cold masses different temperatures —> occlusion
-cold front occlusion: cold air behind cold front colder than cold air ahead of warm front (cold, wet
conditions)
-warm front occlusion: cold air ahead of warm front colder than cold air behind cold front (cool, wet
conditions)
* READING & INTERPRETING SATELLITE IMAGES & SYNOPTIC WEATHER MAPS:
• mid-lat cyclones in satellite images:
-cold front: band of clouds
-comma (,) shape of cyclone indicates cold & leads to centre of cyclone
• mid-lat cyclones on synoptic maps:
-cold air is behind cold front line // cloud cover & rain behind front and just ahead —> thunderstorms
-temp will be higher on east of front because cold air behind front has not yet arrived
-front moves eastwards —> temperatures expected to drop
-wind backs: NE—> NW—>SW
, 2. TROPICAL CYCLONES:
• low pressure system that develops over warm tropical water & moves with Tropical Easterlies
* CHARACTERISTICS/ AREAS/ FACTORS NEEDED FOR FORMATION:
• general characteristics of tropical cyclones:
-systems of wind blowing around LP centre (airflow is clockwise in SH)
-isobars around LP centre are almost circular
-pressure at centre very low (950hPa)
-diameter: 600-1000km
-develop in warm moist air masses above tropical oceans
-have no fronts
-in tropical easterlies & travel westwards E—>W at 10-20km/hr
-mature cyclone develops —> lasts a week, less if it reaches land
-form in late summer (feb/march)/ early autumn (when water surface is warmest/climate change impacts)
• areas where tropical cyclones form:
-tropical latitudes & driven by tropical easterlies
-over oceans 5-20° N/S
-very rare in south Atlantic ocean (water not warm enough)
-form on eastern side of continents (warm currents)
-move E—>W but later curve N/S towards poles
-different areas have different names (hurricane/cyclone/typhoon/willi-willies)
-cyclone is named after wind speed exceeds 65km/hour (named alphabetically//list restarts each season)
• factors necessary for formation of tropical cyclone:
-warm sea surface of +27°C —> water evaporates & rises rapidly —> initiates LP centre (energy used
for evaporation is released as latent heat when water vapour condenses) —> provides energy for tropical
cyclone to continue developing —> warms air to rise further to reduce LP —> further surface
convergence & uplift of air)
-Coriolis force needed for rotation of air around LP centre
-upper air divergence to remove rapidly rising air —> encourages further rising & subsidence in eye
,CLIMATOLOGY
1. MID-LATITUDE CYCLONES
* CHARACTERISTICS/ AREA/ CONDITIONS FOR FORMATION:
• mid-latitude cyclone: low pressure system that develops in the westerlies and travels W—>E
• Coriolis force: causes winds to deflect left (SH) or right (NH)
• convergence: air comes together
• divergence: air moves apart
• general characteristics:
-cold polar & warm subtropical air masses meet —> 2 fronts (warm & cold front)
-pressure at centre: 996hPa (lower than 1000)
-diameter: 1500-3000km
-occur in families (form one after another and travel closely at 50-60km/hour)
-last 4-14 days
• areas where they form:
-40 °-60 ° N/S (warm air in tropics meets cold air in polars)
-travel in westerly winds W—>E
-in winter: pressure belts & wind systems follow migration of the sun therefore mid-lat cyclones move
North —> cold fronts pass southern cape
-in summer: pressure belts & wind systems move South therefore cold fronts pass south of South Africa
-cold fronts (cold & wet) —> SW of SA get wet winters & dry summers // interior of SA gets cold
snaps during winter
• conditions necessary for formation of mid-lat cyclone:
-mass of warm moist air meets mass of cold dry air
-upper air divergence (removes air ascending from below and makes space)
-something to trigger development of low-pressure centre
* STAGES OF DEVELOPMENT & RELATED WEATHER CONDITIONS:
• cyclogenesis: development & strengthening of cyclone (stage 1 & 2)
• mid-lat cyclones go through 4/5 stages
• STAGE 1: the initial stage
-warm tropical air meets cold polar air at polar front (big difference in temps and winds on both sides blow
in opposite directions)
-front doesn’t move therefore stationary front
,• STAGE 2: the developing stage
-disturbance in fast moving upper air (jet stream)/ mountain range disrupts air motion air —> wave
develops at polar front —> affects air movement on surface
-small mass of warm air extends into cold air
-rising air causes pressure to decrease and forms low pressure centre
• STAGE 3: the mature stage
-warm air moves further into cold air —> wave increases and cold/warm sectors are established
-2 fronts develop making boundaries between cold & warm air masses
-warm front: leading / eastern edge of warm sector (ahead of warm air)
-cold front: leading edge of cold sector following warm sector
-whole system moving W—>E
-air rises along warm front (warm air rises above colder air ahead of front)
-air rises along cold front as heavy cold air wedges under warm air ahead
-rising air along both fronts —> condensation/ cloud formation/ rain/ pressure in centre drops further
-air-flow around centre is cyclonic (clockwise in SH)
• STAGE 4: the occluded stage
-cold air behind cold front travels faster than warm sector air —> cold front catches up with warm front
-warm air forced to rise —> small warm sector on ground
-cold & warm fronts joined & form occluded front
-widespread rain because warm air rises along occluded front, cools & condenses
• STAGE 5: the dissipated stage
-entires warm sector is above ground (no warm air on surface)
-no longer 2 masses of air therefore no warm/cold fronts
-isobars return to regular patterns
-light gusts of cold air on ground
,* WEATHER PATTERNS ASSOCIATED WITH WARM/ COLD/ OCCLUDED FRONTS:
• warm fronts:
-gentle angle therefore covers large area
-warm moist air rises over cold air ahead —> forms high-level cirrus; altostratus; nimbostratus; stratus
-gentle uplift of air along front —> soft rain over large area ahead of front
-changing clouds indicate to person ahead that warm front is approaching
-temperature increases
-pressure decreases
-warm air can hold more water therefore humidity increases as warm front passes over
-wind changes direction in anti-clockwise direction
• cold fronts:
-tall cumulonimbus clouds form —> heavy rain & thunderstorms
-steeper angle covers smaller area
-temperature decreases
-pressure increases in colder air
-cold air holds less water therefore humidity decreases as cold front passes over
-wind backs: changes from NW —> SW
• occluded fronts:
-cold front catches up to warm front —> pushes warm air upwards —> cold air behind cold front meets
cold air ahead of warm front —> 2 cold masses different temperatures —> occlusion
-cold front occlusion: cold air behind cold front colder than cold air ahead of warm front (cold, wet
conditions)
-warm front occlusion: cold air ahead of warm front colder than cold air behind cold front (cool, wet
conditions)
* READING & INTERPRETING SATELLITE IMAGES & SYNOPTIC WEATHER MAPS:
• mid-lat cyclones in satellite images:
-cold front: band of clouds
-comma (,) shape of cyclone indicates cold & leads to centre of cyclone
• mid-lat cyclones on synoptic maps:
-cold air is behind cold front line // cloud cover & rain behind front and just ahead —> thunderstorms
-temp will be higher on east of front because cold air behind front has not yet arrived
-front moves eastwards —> temperatures expected to drop
-wind backs: NE—> NW—>SW
, 2. TROPICAL CYCLONES:
• low pressure system that develops over warm tropical water & moves with Tropical Easterlies
* CHARACTERISTICS/ AREAS/ FACTORS NEEDED FOR FORMATION:
• general characteristics of tropical cyclones:
-systems of wind blowing around LP centre (airflow is clockwise in SH)
-isobars around LP centre are almost circular
-pressure at centre very low (950hPa)
-diameter: 600-1000km
-develop in warm moist air masses above tropical oceans
-have no fronts
-in tropical easterlies & travel westwards E—>W at 10-20km/hr
-mature cyclone develops —> lasts a week, less if it reaches land
-form in late summer (feb/march)/ early autumn (when water surface is warmest/climate change impacts)
• areas where tropical cyclones form:
-tropical latitudes & driven by tropical easterlies
-over oceans 5-20° N/S
-very rare in south Atlantic ocean (water not warm enough)
-form on eastern side of continents (warm currents)
-move E—>W but later curve N/S towards poles
-different areas have different names (hurricane/cyclone/typhoon/willi-willies)
-cyclone is named after wind speed exceeds 65km/hour (named alphabetically//list restarts each season)
• factors necessary for formation of tropical cyclone:
-warm sea surface of +27°C —> water evaporates & rises rapidly —> initiates LP centre (energy used
for evaporation is released as latent heat when water vapour condenses) —> provides energy for tropical
cyclone to continue developing —> warms air to rise further to reduce LP —> further surface
convergence & uplift of air)
-Coriolis force needed for rotation of air around LP centre
-upper air divergence to remove rapidly rising air —> encourages further rising & subsidence in eye
