Hurricanes
Certain atmospheric and ocean conditions seem to be at the root of their formation. The following develops this ‘formation’ theme. Wind spins around an eye of 25 to 50 km this decreases in size as hurricanes speed up.
The effects of hurricanes include:
- Generally these intense low-pressure systems (less than 920 Mb is common) form between 30° N and S, in an area called the Intertropical Convergence Zone. With Coriolis Effect at its greatest in the tropics the systems’ rotation can be easily initiated (winds in the order of 119 to 300 km/hr are common). The ITCZ is where convergent air meets near the equator. This area is intermittent and can vary its position.
- The sea in this area is well heated, and exceeds the minimum 26° to 27°C necessary (between August and October) for latent heat to be released (because of condensation of water vapour) to strengthen the hurricane (relative humidity of 60% is common). The ocean then provides the initiating and sustaining energy for hurricanes. They quickly ‘die’ once they move onto the land. The high levels of moisture held also point to a sea origin (typically in the order of 10 to 25 cm/day).
- Towering (15 km is common) cumulo-nimbus clouds form around the central eye in highly unstable conditions. These clouds further fuel the hurricane as latent heat energy exchanges moisture from gas to liquid. Spinning weathersub-systems develop all around the main hurricane mass.
The effects of hurricanes include:
- storm surges – it is this phenomenon that potentially is the biggest killer. Hurricanes distribute energy polewards.
- damage to property (the power of the hurricane is such it turns property instantly into matchwood. e.g. Hurricane Andrew, in Florida, left 250000 homeless)
- agricultural damage
- huge insurance claims – when Hurricane Andrew struck in 1992, 25% of damage was paid for through insurance (some $5.6 billion), the federal government picked up the other $19 billion bill! Between 1900 and 1991 there were 152 direct hurricane hits on the USA.
- refugees – the 250 000 that Hurricane Andrew left homeless moved temporarily into other areas of Florida and into neighbouring states
- property – if all the debris from the properties that hurricane Andrew destroyed were to be piled up it would have towered 300 stories high!
- tourism – for Florida, hurricanes create a unique situation. It is a major tourist destination. Florida has suffered three of the five deadliest hits in the USA this century. Tourists are rightly wary of holidaying during the hurricane season in this state.
Tornadoes
These are a concentration of cyclonically spinning air (about 160 km across), found overland, rather than over water. Most often a visible cloud forms from a large cumulo-nimbus origin. The tremendous rotations are initiated by rapid convergence at the base of the cumulo-nimbus cloud, as rapid updraughts develop. .
N.B. Tornadoes may be a by-product of hurricane development.
N.B. Tornadoes may be a by-product of hurricane development.