Lecture 3: Hurricanes

• Typical hurricanes are ~300 miles wide
• Sustained wind speeds >74 mph


  cyclone or hurricane or typhoon
• Sustained wind speeds <74 mph (but >39 mph)


  tropical storm
• Tropical storm


  A low-pressure weather system with high winds and heavy rainfall
• Hurricane


  A very severe tropical storm (winds >74 mph) characterised by a rotating organised system of clouds and a warm 'core'
• Form over oceans. Not all make landfall
• Seven 'basins' where tropical cyclones form

  • North Atlantic Ocean (inc. Gulf of Mexico, Caribbean sea)
  • Northeast Pacific
  • Norwest Pacific
  • North Indian
  • Southwest Indian
  • Southeast Indian/Australian
  • Australian/Southwest Pacific
• Hazard level


  Risk based on this
• Not all tropical storms mature into hurricanes
• They form at preferred times when environmental conditions are favourable
• Hurricanes often begin off the coast of Africa and as develop steered east by trade winds
• Major affected regions include southern USA states, Mexico and parts of central America
• Form at latitudes between 5-30°N/S
• Ingredients for hurricane formation

  • A low pressure zone (inward converging winds, counter clockwise in NH)
  • Warm oceans (>27°C)
  • High relative humidity
  • Atmospheric instability
  • Location more than 5° from equator (sufficient Coriolis force)
  • Low vertical wind shear
• Hurricanes are powered by humid air and subsequent latent heat energy released from condensation. Once over land they weaken.
• Formation


  1. Wind blows over warm ocean

  2. Warm moist air rises

  3. Sucks up heat and water vapour

  4. Water vapour condenses to form clouds

  5. Increases wind speed

  6. Decreases air pressure at the surface

  7. Causes air to expand and rise even more

  8. Condensation releases latent heat
• Take several hours or days to form. May last for many days or even weeks.
• Travel several thousands of miles – sometimes in a very erratic path ("steered" by winds)
• Decay after passing over cooler water or land
• Important to differentiate between speed of rotating winds and speed of forward propagation
• The 'eye' (1) is the central innermost area (20-40 miles across) with calm, clear skies, low air pressure, low wind speed, low rain, dry air descending.
• The 'eyewall' (2) is a zone 10-25 miles wide, with dense thunderstorms. Rainfall and wind speeds at a maximum
• Outer rainbands (3) are zones of intense cloud and rainfall extending several hundred miles from eye. Wind speeds progressively decrease towards edge of storm.
• Hurricane wind velocity


  • Categories 1-5 on Saffir-Simpson scale
  • Minimum 74 mph (below this: Beaufort Scale 0-11)
  • Maximum > ~160 mph
• Storm centre velocity


  • Speed of entire storm – typically 15–20 mph
  • Can reduce to near 0 mph if hurricane stalls
  • Can exceed 60 mph, particularly at high latitudes
• Intensity is strongest where storm centre velocity and hurricane wind velocity are in the same direction.
• For a northward travelling hurricane in the northern hemisphere, highest intensity is on the eastern side.
• Saffir–Simpson scale


  Classifies tropical cyclones based on wind speed (specifically maximum sustained wind) and the resulting impact
• Outer rainbands


  Zones of intense cloud and rainfall extending several hundred miles from eye. Wind speeds progressively decrease towards edge of storm.
• Hurricane structure in Northern Hemisphere

  • Zone 10-25 miles wide, with dense thunderstorms. Rainfall and wind speeds at a maximum
• Hurricane wind velocity


  Speed of rotating winds within hurricane. Categories 1-5 on Saffir-Simpson scale. Minimum 74 mph (below this: Beaufort Scale 0-11). Maximum > ~160 mph
• Storm centre velocity


  Speed of entire storm – typically 15–20 mph. Affected by wider atmospheric flows that steer the storm system. Can reduce to near 0 mph if hurricane stalls. Can exceed 60 mph, particularly at high latitudes
• Intensity


  Strongest where storm centre velocity and hurricane wind velocity are in the same direction. For a northward travelling hurricane in the northern hemisphere, highest intensity is on the eastern side.
• Saffir–Simpson scale


  Classifies tropical cyclones based on wind speed (specifically maximum sustained wind) and the resulting impacts. Other impacts may not directly related to wind speed (e.g. torrential rainfall, storm surges)
• Saffir-Simpson scale categories


  • Category 1: 74-95 mph, Very dangerous winds. Some damage
  • Category 2: 96-110 mph, Extremely dangerous winds. Extensive damage
  • Category 3 (major): 111-129 mph, Devastating damage will occur
  • Category 4 (major): 130-156 mph, Extreme damage will occur
  • Category 5 (major): >157 mph, Catastrophic damage will occur
• Hazards of tropical cyclones


  • Extreme wind speeds
  • Storm surges
  • Torrential rainfall and flooding
  • Rip currents (even when miles from shore)
  • Tornadoes (next lecture!)
  • Loss of life from other causal factors (e.g. disease transmission)
• Storm surge


  A change in sea level caused by high winds and low pressure. Flooding and heavy waves can damage coastal areas 6-12 hrs before landfall.
• Tropical cyclones can produce copious amount of rainfall leading to destructive flooding (6-12 inches or more) +/- 6 hours from landfall
• Slower moving (<10 mph or "stalled") and larger storms


  Produce the most rainfall
• Flooding in Port Arthur, Texas on August 31 2017