Middle latitude storms	Similarities	Hurricanes
1	form at middle latitudes  (30 to 60 degrees latitude)	both types of storms have low pressure centers (cyclone refers to winds blowing around low pressure)	from in the subtropics (5 to 20 degrees latitude)
2	can form over land or water	upper level divergence is what causes both types of storms to intensify	only form over warm ocean water
3	bigger (might cover half the US)		not as big (might fill the Gulf of Mexico)
4	fronts separate warm and cold air masses		just warm moist air
5	at middle latitude the prevailing westerly winds move these storms from west to east		the trade winds move hurricanes from east to west
6	strongest in the winter into early spring		strongest in late summer into fall (when the ocean is warmest)
7	produce rain, snow, sleet, graupel		mostly just rain (lots of rain)
8	usually no name		get names

Hurricanes receive names (when they reach tropical storm strength).  The names now alternate male and female.  The names of particularly strong or deadly hurricanes (such as Katrina) are retired, otherwise the names repeat every 6 years. 

Hurricanes form between 5 and 20 degrees latitude, over warm ocean water, north and south of the equator.  The warm layer of water must be fairly deep to contain enough energy to fuel a hurricane and so that turbulence and mixing don't bring cold water up to the ocean surface.  The atmosphere must be unstable so that thunderstorms can develop.  Hurricanes will only form when there is very little or no vertical wind shear (changing wind direction or speed with altitude).  Hurricanes don't form at the equator because there is no Coriolis force there (the Coriolis force is what gives hurricanes their spin and it causes hurricanes to spin in opposite directions in the northern and southern hemispheres).

Note that more tropical cyclones form off the west coast of the US than off the east coast.  The west coast hurricanes don't generally get much attention, because they move away from the coast and usually don't present a threat to the US (except occasionally to the state of Hawaii).  The moisture from these storms will sometimes be pulled up into the southwestern US where it can lead to heavy rain and flooding.

Some kind of meteorological process that produces low level convergence is needed to initiate a hurricane.  One possibility, and the one that fuels most of the strong N. Atlantic hurricanes, is an "easterly wave."  This is just a "wiggle" in the wind flow pattern.  Easterly waves often form over Africa or just off the African coast and then travel toward the west across the N. Atlantic.  Winds converge as they approach the wave and then diverge once they are past it .  The convergence will cause air to rise and thunderstorms to begin to develop. 

In an average year, in the N. Atlantic, there will be 10 named storms (tropical storms or hurricanes) that develop during hurricane season.  2005 was, if you remember,  a very unusual year.  There were 28 named storms in the N. Atlantic in 2005.  That beat the previous record of 21 names storms that had been set in 1933.  Of the 28 named storms, 15 developed into hurricanes.

In some ways winds blowing through an easterly wave resembles traffic on a multi-lane highway.  Traffic will back up as it approaches a section of the highway with a closed lane.  Once through the "bottleneck" traffic will begin to flow more freely.

Another process that causes surface winds to converge is a "lee side low."

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This figure tries to explain how a cluster of thunderstorms can organize and intensify into a hurricane.

1.  Converging surface winds pick up heat and moisture from the ocean.  These are the two mains sources of energy for the hurricane.

2.   Rising air expands, cools, and thunderstorm clouds form.  The release of latent heat during condensation warms the atmosphere.  The core of a hurricane is warmer than the air around it.

3.   Pressure decreases more slowly with increasing altitude in the warm core of the hurricane.  The result is that pressure at the top center of the hurricane is higher than the pressure at the top edges of the hurricane (pressure at the top center is still lower than the pressure at the bottom center of the hurricane).  Upper levels winds diverge and spiral outward from the top center of the hurricane (you can sometimes see this on satellite photographs of hurricanes).

4.   The upper level divergence will cause the surface pressure at the center of the hurricane to decrease.  The speed of the converging surface winds increases and the storm intensifies.  The converging winds pick up additional heat and moisture which warms the core of the hurricane even more.  The upper level high pressure and the upper level divergence increase.  The increased divergence lowers the surface pressure even more.

Here's another view of hurricane development and intensification (not shown in class)

In the figure at left the upper level divergence is stronger than the surface convergence.  Divergence is removing more air than is being added by surface convergence.  The surface low pressure will decrease.  The decrease in surface pressure will cause the converging surface winds to blow faster.

In the middle picture, the surface low pressure is lower, the surface convergence is stronger.  The upper level divergence has also been strengthened a little bit.  The upper level divergence is still stronger than the surface convergence so the surface so the surface low pressure will decrease even more.

In the right figure the surface low pressure has decreased enough that the surface convergence now balances the upper level divergence.  The storm won't strengthen any more.

Generally speaking the lower the surface pressure at the center of a hurricane the stronger the storm and the faster the surface winds will blow.  The following figure (not shown in class) shows this


This figure tries to show the relationship between surface pressure and surface wind speed.  The world record low sea level pressure reading, 870 mb, was set by Typooon Tip off the SE Asia coast in 1979.  Sustained winds in that storm were 190 MPH.  Three 2005 Atlantic hurricanes: Wilma, Rita, and Katrina had pressures in the 880 mb to 900 mb range and winds ranging from 170 to 190 MPH.

The stages of storm development that lead up to a hurricane are shown at the bottom of p. 143a in the photocopied ClassNotes.

A tropical disturbance is just a localized cluster of thunderstorms that a meterologist might see on a satellite photograph.  But this would merit observation because of the potential for further development.  Signs of rotation would be evidence of organization and the developing storm would be called a tropical depression.

In order to be called a tropical storm the storm must organize a little more, and winds must increase to 35 knots.  The storm receives a name at this point.  Finally when winds exceed 75 MPH (easier to remember than 65 knots or 74 MPH) the storm becomes a hurricane.


That was all the new material we had time to cover in class because a 20 minute segment from a NOVA program (PBS network) on hurricanes was shown.  A film crew was on board a NOAA reconnaissance plane as it flew into the narrow eye of hurricane GILBERT.  Gilbert set the record low sea level pressure reading for the Atlantic ocean (888 mb).  That record stood until the 2005 hurricane season when WILMA set a new record of 882 mb.  The world record low sea level pressure, 870 mb, was set in a SE Asian typhoon in 1979.

Here's a simplified version of the Saffir-Simpson scale used to rate hurricane strength or intensity.

And with that we came to
THE END
of what we will be able to cover in NATS 101 this semester.  In class on Wednesday we will begin the  review for next week's Final Exam.