What conditions are needed for hurricane formation ? (6 marks)
Define tropical storm: A large, low-pressure system characterised by high wind speeds and heavy rain. Prerequisite conditions:
- Location: Between 8o-20o North and South of the equator so that Coriolis force takes effect (Coriolis force = 0 at Equator, thus hurricanes do not form at the Equator)
- Temperature: 26o C at a depth of 70 metres for evaporation to occur, as warm, humid air is the driving force of hurricanes
- Pre-existing storm conditions: Hurricane formation requires rotating winds over the ocean's surface that can pick up warm, humid air that has evaporated so that a regular storm/cluster of storms becomes a hurricane. Formation:
- When a pre-existing storm passes over warm waters (26oC at a depth of 70m), water evaporates and warm air rises.
- When air rises higher in the atmosphere, it cools and condenses, releasing latent heat.
- Water droplets coalesce into clouds, eventually falling as rain.
- Rising warm air creates low pressure below, and air from surrounding areas of high pressure is pushed into the low pressure area. It is warmed, and rises as well.
- The Coriolis force deflects wind, causing it to rotate. Rising warm air is balanced by descending cool air in the centre.
- In order to be classified as a tropical storm (rather than a tropical depression), the system must reach wind speeds of at least 63km/h.
- The eye of the storm is a relatively calm area of high pressure, surrounded by the eye wall, which is an area of low pressure and has the highest wind speeds.
- The system travels over the ocean, and is strengthened by warm, humid air.
- When system makes landfall, it dissipates quickly as it loses its source of energy; water.
