EPSC 185 Lecture Notes - Lecture 4: Cold Front, Middle Latitudes, Intertropical Convergence Zone

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Where do hurricanes occur:
Note that tropical cyclones do not form near the equator
Also, storms tend to curve to the north and east as they interact with westerlies
Six important points about worldwide hurricane occurrences:
1) About 66% occur in the northern hemisphere
2) Hurricanes never originate within 5 degrees of the equator
3) Hurricanes rarely originate poleward of 25 degrees (North or South)
4) Of the 80 tropical storms that develop annually around the globe; between 50 and
70% develop into hurricanes (or cyclones or typhoons)
5) The western Pacific in the Northern Hemisphere produces the largest number of
tropical cyclones (winter and summer)
6) Hurricanes form over all tropical oceans except the south Atlantic and the southeast
pacific oceans
-In addition to the season cycle in tropical cyclone frequency, there are significant ‘interannual’
and longer-period variabilities
-16 major (cat 3 or higher) hurricanes between 1980 and 1990
-40 major (cat 3 or higher) hurricanes between 1996 and 2006
-List of names is alphabetical, alternates male and female names and uses names from
the French, English, and Spanish languages. With the exception of retired names, the list
repeats itself every six years… 2023 will have the same list as for 2017.
Tropical cyclone development (the necessary environment conditions)
-There are at least three main requirements for an environment to allow tropical cyclogenesis:
—> Enough Coriolis force to develop a low pressure centre
—> Sufficiently warm sea surface temperatures
—> Approximately 27 degrees c at least 60 m deep
—> Low vertical wind shear (upper level winds need to be weak)
-These conditions are necessary but not sufficient conditions for the formation of
tropical cyclones
-So why is the Coriolis force so crucial to the rotation seen in hurricanes?
-By looking downward onto the ocean (or land) surface in the Northern Hemisphere, consider
the following winds:
—> First they go towards the left and later as the Coriolis force deflects, the winds go towards
the right
-Southern hemisphere: clockwise
-Northern hemisphere: counter clockwise
-The rotation of the earth is cyclonic
Intensity
-So why do we need warm sea surface temperatures?
-Consider the same schematic that we saw earlier:
1) Warm moist air moves over the ocean
2) Water vapour rises into the atmosphere
3) As the water vapour rises, it cools and condenses into liquid droplets
4) Condensation releases heat into the atmosphere making the air lighter
5) The warmed air continues to rise with moist air from the ocean taking its place creating more
wind
-There is a direct relationship between the intensity (central pressure) of the most intense
hurricanes and the temperature of the sea-surface over which the storms are moving
-Note from the following table that the six strongest hurricanes all occurred in the western
Pacific where the sea-surface temperatures are warmest
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Document Summary

Note that tropical cyclones do not form near the equator. Also, storms tend to curve to the north and east as they interact with westerlies. In addition to the season cycle in tropical cyclone frequency, there are signi cant interannual" and longer-period variabilities. 16 major (cat 3 or higher) hurricanes between 1980 and 1990. 40 major (cat 3 or higher) hurricanes between 1996 and 2006. List of names is alphabetical, alternates male and female names and uses names from the french, english, and spanish languages. With the exception of retired names, the list repeats itself every six years 2023 will have the same list as for 2017. There are at least three main requirements for an environment to allow tropical cyclogenesis: > enough coriolis force to develop a low pressure centre. > low vertical wind shear (upper level winds need to be weak) These conditions are necessary but not suf cient conditions for the formation of tropical cyclones.

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