Hurricane: an organized tropical storm system featuring vigorous convection and sustained winds in excess of 64 knots (74 mph)

Hurricane: an organized tropical storm system featuring vigorous convection and sustained winds in excess of 64 knots (74 mph)

The storms have different names, depending on where they form: Western Hemisphere: hurricane Eastern Hemisphere: typhoon Southern Hemisphere: tropical cyclone

As a tropical storm system strengthens it goes through different categories: mass of thunderstorms: tropical disturbance winds exceeding 20 knots: tropical depression winds exceeding 34 knots: tropical storm (gets a name) storm disturbance hurricane depression

Hurricane structure: A hurricane consists of vigorous convection (or cumulus clouds) surrounding a calm central eye The convection is strongest in the region immediately outside the eye This band of strong convection just outside the storm's eye is called the eyewall

Beyond the eyewall, the air swirls counter-clockwise into the center (Except at upper levels but we'll get back to that) The fastest winds in the storm are found just outside the eyewall And as the air swirls into the center, it organizes into spiral rainbands (next page) wind arrows and wind speeds for Hurricane Dora

Satellite image of Hurricane Katrina, showing spiraling bands of convection spiral rainbands Composite radar image of Katrina, showing rainbands in cross-section

Hurricane formation: A hurricane is effectively a giant heat engine, with latent heating in the cumulus clouds providing the heat source - the fuel for the engine is the, moist air feeding into the storm So how does one of these things get started?

200 mb 400 mb 650 mb 990 mb We start with relatively weak winds over a ocean surface.

200 mb 400 mb 650 mb 990 mb Some pre-existing disturbance comes along and causes a weak pressure drop...

200 mb 400 mb 650 mb 990 mb Some pre-existing disturbance comes along and causes a weak pressure drop...just enough to organize some clouds and convection at the storm center.

PGF H PGF 200 mb 400 mb 650 mb 990 mb Latent heating causes the center of the storm to heat up, which in turn causes the air column to expand upward. The higher heights of the pressure surfaces aloft then produce an outward directed PGF at upper levels.

H 200 mb 400 mb 650 mb 990 mb This outward PGF produces divergence at upper levels...

H L 200 mb 400 mb 650 mb 990 mb This outward PGF produces divergence at upper levels...which in turn causes the surface pressure to drop. And as the surface pressure drops, the winds get stronger...and away we go.

Schematic 3D structure of a hurricane, showing inflow (convergence) at the bottom and outflow (divergence) at the top. Note the counter-clockwise flow at the bottom (like a low pressure system), and clockwise at the top (like a high).

Recipe for a Hurricane Thermodynamic ingredients: Lots and lots of water vapor to feed the clouds, which in turn means ocean water to encourage evaporation A conditionally unstable environment to allow clouds to form Mechanical ingredients: Relatively weak wind shear, so the system stays vertically aligned Some sort of pre-existing disturbance, to get the clouds going The Coriolis force, to get the storm rotating - Hurricanes are rare near the equator (between roughly 5 on and 5 os)

Where do hurricanes form? Hurricanes form in regions with ocean waters (typically 80 of or greater) and weak wind shear But not too near the equator Sea surface temperatures (orange regions show > 80 of)

Where do hurricanes form? Hurricanes form in regions with ocean waters (typically 80 of or greater) and weak wind shear But not too near the equator Regions where hurricanes tend to form

When do hurricanes form? Hurricanes form in late summer and early fall, when the ocean temperatures are est Number of hurricanes in past the 100 years for the Atlantic Ocean, Carribean and Gulf of Mexico region