Lecture 3 Notes
Outline of this lecture
• Part I Hurricane Primer
• Part II Storm Research – Hurricanes (please read the papers to know the detail)
• Steenhof and Gough (2007)
• Ralph and Gough (2007)
1. Part 1. Hurricane Primer
Five elements will be covered: definitions, dynamics, distribution and forecasting,
Hurricane – North American term from the Taino language of Central America,
meaning “god of evil”
Typhoon – Term used in the Western Pacific from the Chinese word “taifung”
meaning „big wind‟
Cyclone – Term used in Australia and in the Indian Ocean
Tropical Cyclone – Term used in the scientific community
What is a tropical storm? A tropical storm is defined as a storm occurring in the
tropical region (20oN to 20oS) in which the sustained winds range from 18 to 33
What is a hurricane? It is a tropical storm in which the sustained winds range
from 33 m/s to 50 m/s.
What is a major hurricane? It is a tropical storm which has sustained winds which
exceed 50 m/s.
Another way to classify and quantify hurricanes is the Saffir-Simpson Scale. It is
a rating scale spanning from 1 to 5 and is based on wind speed and the central
pressure of the hurricane. For context the average air pressure at sea level is 1013 mb.
Relatively small variations about this value (less than 10%) characterize the high and
low pressures on the planet. All storms involve low pressures.
Category One Hurricane
• 119-153 km/h (33 – 42.5 m/s), > 980 mb
Category Two Hurricane
• 154-177 km/h (42.5 - 49 m/s), 965 – 979 mb
Category Three Hurricane
• 178-209 km/h (49 – 58 m/s), 945 – 964 mb
Category Four Hurricane
• 210-249 km/h (58 – 69 m/s), 920 – 944 mb
Category Five Hurricane
• Greater than 249 km/h (> 69 m/s), < 920 mb
How do hurricanes form?
Tropical storms are fueled by sea surface temperatures and latent heat release.
Observationally it has been noted that 26.5C is the threshold for tropical storm
formation. This is a necessary but not sufficient condition for tropical storms to form.
Also needed is a relatively quiescent atmosphere (low winds) initially.
What is latent heat?
Latent heat is the energy released due to a change of phase of matter. Heat is
released when matter changes from gas to liquid and from liquid to solid. To make
the opposite conversion heat is required. Heat is released when a gas condenses to a
liquid. So when water vapour becomes water droplets, heat is released. Once a storm
has formed this latent heat release provides more fuel for the storm.
Tropical Storm Development
Tropical storms begin as a tropical wave in the Intertropical convergence zone
(ITCZ) near the equator. The storms spawn between latitudes 5 – 20 in either of the
northern and southern hemispheres. Groupings of thunderstorms become organized
and self-sustaining and form a tropical cyclone that can persist up to two weeks.
Requirements for Tropical Cyclones
For a tropical storm to occur, convergence must occur at the surface. This is a
common feature of the ITCZ. Convergence is the focusing of surface winds. Above
the surface divergence must occur in order to balance the surface convergence
(otherwise the storm will not be self-sustaining). Rising air releases latent heat which
causes the upper atmosphere to warm and expand resulting in divergence. The eye of
the storm occurs at the point of upper level divergence and is characterized by
subsidence (sinking air).
What affects the length and strength of a tropical cyclone?
The sea surface temperatures both affect genesis, strength and length of tropical
storms. Warmer surfaces result in the more frequent and stronger and longer storms.
Upper wind structure plays an important role. Strong upper level winds inhibit
tropical cyclone longevity, by cutting off development and growth of the storm. El
Nino (ENSO), an interannual variability of Pacific sea surface temperatures and
accompanying atmospheric circulation, affects winds in both the Eastern Pacific (EP)
and the Atlantic (Atl). It enhances EP hurricanes, suppresses Atl hurricanes. The
QBO (quasi-biennial oscillation of the winds in the stratosphere) also introduced in
lecture 2, affects hurricanes in a similar fashion by changing the upper wind structure.
Landfall inhibits tropical cyclones by cutting the storm off from both sources of
energy (SSTs and water vapour) and increases surface roughness.
Why do the storms occur where they do?
Tropical cyclones occur in the North Atlantic, the northeastern Pacific, the Indian
Ocean and both sides of Australia. Why do they not occur in the South Atlantic and
the southern Eastern Pacific?
The temperature distribution of the world ocean indicates in that these two areas
have summer temperatures less than the tropical cyclone 26.5C threshold.
However, in 2004, the South Atlantic experienced its first recorded hurricane off
the Brazilian coast, named Catarina. In 1991, a tropical storm formed off the coast of
Africa. It is an open question whether Catarina can be linked to global warming.
The eastern provinces of Canada occasionally are hit by hurricanes, as far west as
the Great Lakes. It is not an issue for the Western provinces. In Canada there is a
Canadian Hurricane Centre in Halifax, Nova Scotia. Hurricanes are felt in the Great
Lakes region once in about every five years, once in every two years in the Eastern
Emergency Preparedness Canada has analyzed natural disasters. Tropical storms
accounts for the 8th costliest natural disaster in Canada. There have been 19 tropical
storms hitting Canadian soil from 1900 to 1999. There have been 136 fatalities and
1868 evacuated. Damage has totaled 1.15 billion dollars of damage
Outline of this lecture: part i hurricane primer, hurricanes, part ii storm research hurricanes (please read the papers to know the detail, steenhof and gough (2007, ralph and gough (2007, part 1. Five elements will be covered: definitions, dynamics, distribution and forecasting, climate change. Hurricane north american term from the taino language of central america, meaning god of evil . Typhoon term used in the western pacific from the chinese word taifung meaning big wind . Cyclone term used in australia and in the indian ocean. Tropical cyclone term used in the scientific community. A tropical storm is defined as a storm occurring in the tropical region (20on to 20os) in which the sustained winds range from 18 to 33 m/s. It is a tropical storm in which the sustained winds range from 33 m/s to 50 m/s. It is a tropical storm which has sustained winds which exceed 50 m/s.