Detailed notes that summarise the Geography IEB syllabus of Climate and Weather. Included in these notes are important definitions and concepts to learn as well as diagrams that make the section more understandable.
High Pressure Cells: (HASOS)= SH
• Anticlockwise
• Sinking
• outwards/diverging
• stable= clear and dry
Tropical Cyclones
A rapidly rotating storm system characterised by a low pressure centre, strong winds and
a spiral arrangement of thunderstorms that produce heavy rain. They last 3-8 days.
Latitude • They occur between 5° and 20° N&S
• Not 0°- 5° N&S= Coriolis force to weak to create cyclonic action (Right- NH & Left- SH)
• 20° or more= Oceans are too cold for dramatic evaporation rate/ uplift
Pressure • Intense LP system where the eye can reach at least 960 hPa
Isobars • Isobars are circular and close together= very steep pressure gradient & gale force winds
Oceans • They form over tropical oceans where temperatures are greater than 27℃
Winds • Moved by the tropical easterlies (E-W), curving away from the equator
• A ect the east coasts (NH= anticlockwise & SH= clockwise)
Season • They occur in late summer- when tropical oceans are warmest (can’t form over sea that is
too cold)
• SH (11/04) ; NH (06/11)
Names • Hurricanes- North Atlantic
• Typhoons- North Paci c
• Tropical Cyclones- South Indian and South Paci c
• They are named alphabetically per season
Weather Strong gale force winds, heavy torrential rain, hail, lightning
Conditions
Life Cycle of a Tropical Cyclone
Latent heat (“hidden heat”)- heat energy that is released when water changes state.
Directions:
Step 1: Initial Stage- (tropical low/depression)
• Convergence of warm, moist air towards a developing LP= clockwise.
• Pressure is ± 1002 hPa ; winds are ± 60Km/hr.
• Clouds and no movement.
ff fi fi
, Step 2: Developing Stage- (Tropical Storm)
• Surface area convergence continues. Air rises in the LP centre- intensi es the storm.
• Upper air divergence occurs- drawing more air into the system.
• Pressure is ± 996 hPa ; winds are ± 120Km/hr (gale force).
• In the centre of the LP a distinct ‘eye’ forms, surrounded by huge Cumulonimbus clouds
(vortex).
• Small area.
Step 3: Mature Stage- (Tropical cyclone- given a name)
• Pressure is ± 950 hPa ; winds are >120Km/ hr.
• The eye has clear, cloudless condition (eye= 50KM).
• Well developed vortex with a diameter of up to 600Km.
• Heavy rain, Huge Cumulonimbus clouds, lightning and strong winds= hurricane strength
torrential rain.
• Large area.
Step 4: Decaying Stage
• The cyclone enters temperature latitudes and cools down, increasing pressure.
• If it moves inland the moisture supply is cuto and surface friction slows it down.
• No more moisture= over land= friction= wind speed slows down= cold ocean.
Ingredients:
> 5° N/S of the equator
• The Coriolis force de ects winds towards the LP centre causing cyclonic movement of
air.
Tropical ocean water (27℃)
• Results in increased evaporation and high humidity = sustained convection.
• A smooth surface reducing frictional drag.
Rapid cooling & increased condensation of humid air
• To release more latent heat, which fuels the system.
Low wind shear (light, variable winds) & a period of calm weather
• High shear winds disrupt circulation into the storm.
• Strong prevailing winds stop vortex formation as they disturb the disruption of water
vapour and cool surface waters.
fl ff fi
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