Cyclones

Topic 11: Cyclones - Nature’s Deadliest Storms

Definition of Tropical Cyclones

  • Tropical Cyclone: A cyclonic low-pressure system occurring in the tropics with sustained winds of 119 km/h (74 mph) or greater.

    • Characterized by counterclockwise rotation around the region of low pressure (in the Northern Hemisphere).

Names for Tropical Cyclones

  • Different names based on geographic location:

    • Hurricane in North America.

    • Typhoon in Southeast Asia.

    • Cyclone in countries bordering the Indian Ocean.

    • Tropical Cyclone in Australia.

Page 2: Air Movement within a Hurricane

  • Eye: Center of the hurricane, characterized by the lowest pressure.

  • Eyewall: Region around the eye with the fastest winds and heaviest rain.

    • Contains the most severe weather conditions.

  • Rain Bands: Spiraling bands of thunderstorm activity surrounding the eyewall.

Page 3: Hurricane Strength

  • **Key Factors Influencing Strength: **

    • Amount of water vapor condensing to liquid; condensation releases latent heat, creating positive feedback.

    • Requires warm water (about 26°C) for evaporation.

    • Little to no wind shear is needed to prevent disintegration.

Stages of Hurricane Growth

  • Progression of tropical cyclones:

    • Tropical Disturbance → Tropical Depression → Tropical Storm → Hurricane.

  • Tropical Storm: Defined as a tropical cyclonic storm with sustained winds between 63 and 118 km/h.

  • Categories of hurricanes based on wind speed.

Page 4: Saffir–Simpson Hurricane Wind Scale

  • Ranks hurricanes based on highest wind speed averaged over a one-minute interval, measured at 10 m above the surface.

Page 5: Tropical Cyclone Geography

  • Location: Hurricanes are confined to tropical oceans.

    • Subtropical Highs: Guide the paths of hurricanes.

  • Restricted from forming within about 5° latitude of the equator due to lack of Coriolis force.

Page 6: Hurricane Hazards

  • Impact: Hurricanes are the leading meteorological killer worldwide.

  • Coastal Storm Surge: Most dangerous aspect of a hurricane, characterized by a rise in sea level due to strong winds and low atmospheric pressure.

    • Particularly threatens the Indian Ocean region due to high coastal populations and poverty limiting evacuation.

Page 7: Extratropical Cyclones

  • Midlatitude Cyclone: A large cyclonic system occurring at mid to high latitudes, also known as depressions or low-pressure systems.

Anatomy of a Midlatitude Cyclone

  • Composed of:

    • Cold Front: Cold air advances on warm air.

    • Warm Front: Warm air flows over cooler, denser air.

Page 8: Cyclogenesis

  • Definition: The process of midlatitude cyclone formation occurring mainly from fall through spring in areas between approximately 30° and 70° N/S.

  • Key Locations for Formation:

    • Downwind of mountain ranges.

    • Areas with warm water downwind of cold water or land surfaces.

Page 9: Effects on Weather

  • Midlatitude cyclones cause stormy conditions in regions like the U.S. and Canada from fall to spring.

  • Weather Associations:

    • Warm Fronts: Linked to steady precipitation and nimbostratus clouds.

    • Cold Fronts: Associated with short bursts of rainfall and cumulonimbus clouds, leading to severe weather events.

Page 11: Lifecycle of a Midlatitude Cyclone

  • Growth, maturation, and dissipation cycle lasts 1–2 weeks.

  • After passing over Great Lakes, can induce lake-effect snow:

    • Lake-effect Snow: Heavy snowfall when cold air moves over warm waters.

    • Nor’easter: A specific type of midlatitude cyclone famous for its blizzard conditions impacting the Mid-Atlantic states and New England.