This document discusses hurricanes, including their structure, formation, and naming conventions. It notes that hurricanes are a type of tropical cyclone that forms over warm ocean waters and features heavy rains and strong winds that spiral inward. The document outlines the typical stages of a hurricane's life cycle from formation to maturity to decay. It also describes the vertical structure of hurricanes and notes key factors like sea surface temperature that enable their development. Finally, the document indicates that meteorological organizations name hurricanes to identify and track them, classifying storms on scales of intensity.

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Introduction Structure of a Hurricane Formation and life cycle of cyclones Naming of cyclones and their impacts
ENV 111: Introduction to Meteorology
Lecture 8
Hurricanes
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Introduction Structure of a Hurricane Formation and life cycle of cyclones Naming of cyclones and their impacts
Presentation outline
1 Large scale cyclonic storms
Classification of cyclonic storms
Regions and tracks of cyclones
2 Structure of a Hurricane
Horizontal extent of a cyclone
Vertical structure
3 Formation and life cycle of cyclones
Formation theory of cyclones
Life cycle of Hurricanes
4 Naming of cyclones and their impacts
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Introduction Structure of a Hurricane Formation and life cycle of cyclones Naming of cyclones and their impacts
Cyclonic storms
Convective and revolving storms
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Introduction Structure of a Hurricane Formation and life cycle of cyclones Naming of cyclones and their impacts
Cyclonic storms
Figure 2 : Tropical cyclone Giovana in Indian Ocean, February 9, 2012
A hurricane is one of the tropical revolving storms whose generic term
is Tropical cyclone
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Introduction Structure of a Hurricane Formation and life cycle of cyclones Naming of cyclones and their impacts
Cyclonic storms
Cyclonic systems can be classified using either the Beaufort scale of
wind intensity or the pressure deficit:
System BF No. Vortex speed Pressure deficit
Depression 5 17 − 21 5 mb
” 6 22 − 27
” 7 28 − 33
Storm 8 34 − 40 15 mb
” 9 41 − 47
” 10 48 − 55
” 11 56 − 63
Cyclone 12 64− 30 mb
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Introduction Structure of a Hurricane Formation and life cycle of cyclones Naming of cyclones and their impacts
Cyclonic storms
Other local names used for tropical cyclones include:
⇒ Cyclone
⇒ Typhoon, from Chinese word ”Taifung” meaning ”big wind”
⇒ Hurricane, from Taino word ”hurucan” meaning ”god of evil”
⇒ Willy willy
⇒ Cordanzo
Cyclones occur mostly during summer (late), though they can form at
any time of the year; often in western North Pacific
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Introduction Structure of a Hurricane Formation and life cycle of cyclones Naming of cyclones and their impacts
Tropical cyclones
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Table 1 : Frequency of occurrence of tropical cyclones
Region Frequency Season
%
Western North Pacific 30 June - October
North Indian Ocean 15 April - June;
October - November
South Western Indian 14 December - April
West Atlantic Ocean 12 July - October
East Pacific (north of equator) 11 July - October
South Pacific 11 December - March
North and West of Australia 7 December - February
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Introduction Structure of a Hurricane Formation and life cycle of cyclones Naming of cyclones and their impacts
Regions and tracks
Figure 3 : Tracks of cyclones in regions of occurrence
Cyclones originate over
eastern part of tropical
oceans where trade winds
had long passage
Controls of tracks are:
⇒ basic flow in upper
troposphere,
⇒ Coriolis force,
⇒ outflow jet, and
⇒ terrain features e.g. islands
or mountains
ndettoel@2016 ENV 111: Introduction to Meteorology

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Introduction Structure of a Hurricane Formation and life cycle of cyclones Naming of cyclones and their impacts
Structure of a Hurricane
Figure 4 : Hurricane Elena over the Gulf of Mexico
Hurricane Elena, as one of the average storm had its central pressure
of 955 mb, and sustained winds of 105 knots
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Introduction Structure of a Hurricane Formation and life cycle of cyclones Naming of cyclones and their impacts
Structure of a Hurricane
Dimension: 200 − 1000 km radius; 10 − 15 km high
The diameter is smallest when the cyclone is closest to the equator,
and it increases as the cyclone recurves poleward
The cyclones originate in the doldrums over the ocean between 6 ◦
and 20 ◦
North and South, when the doldrums is farthest away from
the equator
However, no cyclones occur in the South Atlantic Ocean since in this
ocean, the doldrums (ITCZ) never cross the equator southwards
ndettoel@2016 ENV 111: Introduction to Meteorology

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Introduction Structure of a Hurricane Formation and life cycle of cyclones Naming of cyclones and their impacts
Structure of a Hurricane
The storm’s thickest clouds cover an area of about 500 km in diameter
An eye is the centre of the storm with a relatively clear area:
⇒ Winds are light and clouds are mainly broken
⇒ Surface air pressure is lowest, around 955 mb
⇒ The width of the eye is almost 40 km
The eye-wall is a ring of intense thunderstorms that whirl around the
storm’s centre
⇒ It may extend upward to almost 18 km above sea level
⇒ Heaviest precipitation and the strongest winds exist here
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Introduction Structure of a Hurricane Formation and life cycle of cyclones Naming of cyclones and their impacts
Vertical structure
Figure 5 : A model depicting a vertical structure of a cyclone
The vertical view of a hurricane shows that the storm is composed of
an organized mass of thunderstorms
ndettoel@2016 ENV 111: Introduction to Meteorology

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Introduction Structure of a Hurricane Formation and life cycle of cyclones Naming of cyclones and their impacts
Formation and life cycles of cyclones
The Conditional Instability of the Second Kind (CISK) summarize
the processes responsible for the formation of a tropical cyclone:
A depression with falling pressure leads to horizontal inflow and vertical
motions. The motion leads to condensation of moisture and formation of
cumulonimbus clouds which trigger release of latent heat. Energy release
leads to further drop of pressure and the cycle continues
A tropical cyclone undergoes four life stages of development:
Formative stage:
⇒ disorganised squall type of clouds and rainfall;
⇒ low winds (about 34 kt);
⇒ surface pressure of 1000 mb
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Introduction Structure of a Hurricane Formation and life cycle of cyclones Naming of cyclones and their impacts
Formation and life cycles of cyclones
Immature stage:
⇒ wind speed at hurricane force (64 kt);
⇒ clouds and rainfall spiralling inward
Mature stage:
⇒ Low pressure (900 mb or less);
⇒ more steady hurricane wind force;
⇒ torrential weather
Decaying stage:
⇒ frictional dissipation;
⇒ disruption of vortexes;
⇒ low moisture; move to colder areas
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Introduction Structure of a Hurricane Formation and life cycle of cyclones Naming of cyclones and their impacts
Formation and life cycles of cyclones
Conditions necessary for the formation
⇒ SST of about 26.5 ◦
C. Warm ocean water favours convection instability
⇒ Enough moisture to provide latent heat of condensation
⇒ Pre-existing region of low level convergence e.g. monsoon lows, easterly
waves and ITCZ
⇒ Coriolis force large enough to provide vortex
⇒ Vertical instability aloft for formation of Cbs
⇒ Weak wind shears to maintain warm air and Cbs
⇒ Upper level divergence to enhance vertical motion and low level convergence
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Introduction Structure of a Hurricane Formation and life cycle of cyclones Naming of cyclones and their impacts
Hurricane naming and Impacts (Homework)
1 Meteorologists have devised a means to name the hurricane whenever
they occur. Your are advise to read and find out why hurricanes are
named and what is the system used for naming. Further find out, at
which stage will a hurricane be given a name.
2 Hurricane can also be classified as we have seen in the case for
tornadoes. Then find out the system used for classifying hurricanes.
3 Hurricane is one of the most severe weather systems hence it can
cause a lot of damage. However, it is also possible that a hurricane
can be of benefit somehow. therefore, discuss the possible impacts
(positive and negative) of hurricanes in different sectors. Support your
discussion with vivid examples.
ndettoel@2016 ENV 111: Introduction to Meteorology