HURRICANE DEVELOPMENT What conditions are required? Water temperatures > 26.5°C Convergence of surface winds Upper air divergence Organized mass of thunderstorms Coriolis force (none at equator)

Spring: Too cold, ITCZ too far south

The First South Atlantic Hurricane

Tropical Wave

What causes surface winds to converge? Low pressure over ITCZ East side of tropical wave (in North Atlantic, these waves are often remnants of convective storms from continental Africa) Front from middle latitudes Trade wind inversions can prevent storm development even if surface conditions are ideal !

Upper level conditions: 1.Low shear (shear disrupts convection and disperses heat and moisture; El Niño causes shear over North Atlantic, enhances activity over Pacific) 2.Upper level divergence

Strong, upper level divergence Hurricane Mitch

Organized Convection Theory 1.Thunderstorms form over tropical wave, near ITCZ, or at remnant of midlatitude front (latent heat confined to limited area) 2.If air is much cooler aloft, strong convection occurs 3.Upper air rapidly warms due to condensation

4.Air pressure rises aloft, enhancing divergence 5.Surface pressure drops 6.Surface air spins counter-clockwise and moves more quickly near the centre (conservation of angular momentum) 7.Rough seas enhance convection due to friction and increase surface area Positive feedback; strength only limited by latent and sensible heat