Science 10
Weather & Climate Unit
Hurricane Notes
Anatomy of a Hurricane
• Hurricane: An intense storm of tropical origin, with sustained winds >(120 km/hr)
• Other Names: Typhoon: Western North Pacific
• Cyclone: In India and Australia
• International agreement: Tropical cyclone
• Tropics: region between 23.5° north – 23.5° south; noon sun is always high in the sky, and
so diurnal and seasonal changes in temperature are small; daily heating of the surface &
high humidity favor the development of cumulus clouds & afternoon thunderstorms.
Tropical Weather
• Most of tropics are marked by seasonal differences in amount of precipitation, as
temperature remains warm all year long
• Greatest cloudiness & precipitation occur during high-sun period (when the ITCZ moves into
the region)
• Winds blow from the east, northeast or southeast – the trade winds
Hurricane Anatomy
• Average diameter of a hurricane ~500 km
• The eye is ~ 40 km wide; within the eye, the winds are light and clouds are mainly broken
• Observations when we cross a hurricane (W to E):
– When we approach, sky is overcast with cirrostratus clouds;
– Pressure drops slowly at first & then more rapidly as we move closer to the center
– When we approach the eye, winds increase speed and heavy rain showers
• Inside the eye: air temperature rises, winds slacken, rainfall ceases & sky brightens.
• As we move away from the eye wall, pressure rises, winds diminish, rain decreases, &
eventually sky clears.
Hurricane Formation and Dissipation
• Hurricanes form over tropical waters where winds are light, surface water temp is warm
(>26.5°C) over a vast area with high humidity.
• These conditions prevail over tropical and subtropical North Atlantic and North Pacific
oceans during summer and early fall (June-November).
• The surface winds must converge for a mass of unorganized thunderstorms to develop into
a hurricane; such as surface winds along ITCZ.
• Tropical disturbance: A thunderstorm with a slight wind circulation
• Tropical depression (TD): When Tropical disturbance becomes TD when winds speeds
reach 37-64 km/h & several closed isobars appear about its center in a surface weather
map
• Tropical Storm: When TD wind speed reaches 65-120 km/h & when the isobars are packed
together
• Hurricane: When tropical storm wind speeds exceed 120 km/h.
• Storm surge is produced by a combination of high winds, high waters and the net transport
of water toward the coast – affects low-lying areas and turns beachfront homes into piles of
splinters
Naming Hurricanes
• A name is assigned when a storm reaches tropical storm strength
• Beginning 1953 (-1977), the National Weather Service began using female names
• Beginning 1978-1979, alternately male and female names were assigned.
• Refer to the naming hurricanes handout for more information.