Hurricanes - Chapter 16

what is a hurricane?

intense storm that forms over tropical waters

• organized mass of thunderstorms

• strong circular flow around low pressure (CCW in NH) composed of convective clouds with air ascending in spiral motion

• winds are > 74 mph

• form over north atlantic & pacific

• ~500km diameter, 40 km eye

  • June-November for Atlantic hurricanes

what is the first stage of a hurricane?

tropical disturbance: mass of thunderstorms with slightly converging winds

what is the second stage of hurricane formation?

tropical depression: classsified if winds increase to between 20-34 knots and several closed isobars appear on the center

what is the third stage of hurricane formation?

tropical storm: isobars are packed closer together, winds are between 35-63 knots

• if sustained winds reach 74 miles/hour —> hurricane

what is the eye & eyewall of a hurricane?

eye - relatively clear area at the center

• winds are light, clouds are broken, low surface air pressure

• clouds align in spiraling bands that swirl in towards the storm’s center, wrapping themselves around the eye

• surface winds increase in speed as they blow counterclockwise and inward toward this center

eyewall - surrounds the eye, a ring of intense thunderstorms that extend upward to almost 18km

• heaviest precipitation, strongest winds

where does hurricane energy come from?

• rising air releases latent energy, resulting in high pressure aloft, inducing sinking in the eye

• sinking air warms by compression, reducing density and pressure near the surface

• warm ocean provides a continuous heat source

what are conditions of hurricanes?

• Sea-surface temperatures > 26.5 degrees Celsius

• no wind shear, light winds

• high humidity

• 5-20 degrees latitude, Coriolis Force has an effect

• Convergence

  • Tropical wave over Africa, ICTZ, midlatitude front extending to Tropics

  • dissipates over cold water or land

what is the difference between hurricanes and mid-latitude storms?

mid-latitude storms are driven by horizontal pressure gradients

• cold core lows: intensify with height

• low aloft is to west of surface low (rising air)

hurricanes are driven by warm water and heat of condensation

• warm core lows: warm all the way up

• low at surface, high aloft (eye sinking)

  • surface low weakens with height

• no fronts associated

which side has stronger winds? (east/west)?

hurricane moving northward will have higher sustained winds on eastern side

• maximum wind speed is on eastern side

• storm surge on western side