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Planetary Atmospheres, the Environment and Life (ExCos2Y)
Topic 8: Storm Systems
Chris Parkes
Rm 455 Kelvin Building
7. Water
The effects of Waters
Heat Capacity of water
Wind – water interaction
Ocean Circulation
El Nino
Revision
Hurricanes
Tropical Cyclones
classification according to wind speed
< 17m/s tropical depression
17 – 33m/s tropical storm
> 33m/s hurricane
typhoon
severe tropical cyclone
severe tropical storm
(Atlantic)
(Eastern Pacific)
(Western Pacific)
(Indian Ocean)
Hurricane Formation Conditions• Warm Ocean (> 26.5ºC) to 50m depth
– Provides energy for convection• Coriolis Force
– Formation of vortex, at > 5º latitude• High Humidity
– Heat of condensation powers hurricane• Suitable Upper Winds
– Winds in different directions at different heights would disrupt vortex formation
• Unstable Air– Cools fast with height
• Pre-existing disturbed weather– Allows cyclone to start
Typical radius 450 Km
Hurricane formation and Atmospheric “waves”
Wave – disturbance in lower troposphere (0-5km)
Due to African easterly jet
Period 3~4 days; wavelength 2~2.5k km; speed ~20km/h
~60 per year; these waves start ~85% intense hurricane
Hurricane Life-Cycle
• Formation– “Seeded” in tropical disturbance– Thunderstorm activity – liberation of heat stored in warm
waters– Heat transfer warm water to humid air– Release of latent heat when vapour condenses– Energy drives convection
• Decay– moves inland: friction lessens winds, source of moisture
removed – or over cooler water– System stops moving, sea surface cooled– move to higher latitudes – cooler water– upper wind patterns disrupt vortex
Hurricane Structure
• Central Eye:
• ~30-60 km diameter– relatively calm– Slowly sinking air
• Eyewall– Circle of thunderstorms, strongest winds
• Outer spiral rain bands
Hurricane structure
storm surge – rise of water level associated with low pressure system
•Eye•Eyewall•Rainbands
Hurricane air circulation
Release of “latent heat” of water vapour
increase convection
Massive outflow at troposphere fall in surface pressure
Low surface pressure (950mb) large inflow at surface
Coriolis force causes inflow to rotate
Hurricane classification
Saffir-Simpson scale:
Wind speed is max. sustained speed
Max. at right front quadrant (in Northern hemisphere)
Global warming may be increasing intensity
Category Wind speed (m/s)
Storm surge (m)
1 33-42 1.0-1.7
2 43-49 1.8-2.6
3 50-58 2.7-3.8
4 59-69 3.9-5.6
5 70+ 5.7+
Tornado
• Also known as Twisters
• Not the same as hurricanes !– Only 75m across– Only last minutes to hours– Form on ground
• Very Strong Wind speeds– 100 Km/hr
• Formed by
“super-cell” thunderstorm
• Hurricanes can spawn Tornados
Tornado alley
Other storm features – Water Spouts• Tornado-like over water
• Wind speeds lower than typical tornado
• Funnel is condensation– Does not ‘suck’ up water
Other storm features – dust devilDust devil – heating from ground
not associated with a supercell thunderstormtypically lower intensity than tornados
Dust Devil on Mars
Dust devil on Mars, photographed by the Mars rover Spirit
Martian dust devils can be fifty times as wide and ten times as high as Earth dust devils
Mars - dust storm• Direction of winds
changes with seasons– Extreme summer in
southern hemisphere
• Winds initiate huge dust storms
Venus –polar vortex
• Polar Vortex– hurricane-like storms – four times larger than Earth. – Each vortex has two "eyes"
© The Planetary Society
South PoleVenus Express, 2006
Example exam questionsQ1. List the conditions necessary for the formation
of hurricanes.
Q2. What are the main features of a tropical cyclone?
Q3. Sketch a diagram to explain the supercell thunderstorm structure.
Q4. Why are tropical cyclones not observed in equatorial regions?
Next topic – Climate Change