Friday, April 29, 2016

Gaelic Storm "Before the Night is Over" (4:53), "Samurai Set" (3:50), "Black is the Colour" (5:32),  De Dannan "Hibernian Rhapsody" (6:48)



A tropical disturbance is just a localized cluster of thunderstorms that a meteorologist 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 strengthen 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.  You don't need to remember all these names, just try to remember the information highlighted above.


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.



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 Typhoon 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.  I will have to update this figure at some point to include hurricane Patricia from Fall 2015.

Hurricane features: eye, eye wall, spiral rain bands

A crossectional view of a mature hurricane (top) and a picture like you might see on a satellite photograph (below). 

Sinking air in the very center of a hurricane produces the clear skies of the eye, a hurricane's most distinctive feature.  The eye is typically a few 10s of miles across, though it may only be a few miles across in the strongest hurricanes.  Generally speaking the smaller the eye, the stronger the storm.

A ring of strong thunderstorms, the eye wall, surrounds the eye.  This is where the hurricane's strongest winds are found. 

Additional concentric rings of thunderstorms are found as you move outward from the center of the hurricane.  These are called rain bands.  These usually aren't visible until you get to the outer edge of the hurricane because they are covered by high altitude layer clouds.


Hurricane Katrina making landfall on Aug. 29, 2005. (source)


The Saffir Simpson Scale is used to rate hurricane intensity (just as the Fujita Scale is used for tornadoes).  The scale runs from 1 to 5.  Remember that a hurricane must have winds of 74 MPH or above to be considered a hurricane.  Category 3,4, and 5 hurricanes are considered "major hurricanes" (in other parts of the world the term super typhoon is used for category 4 or 5 typhoons).

Here's an easy-to-remember version of the scale

Pressure decreases by 20 mb, wind speeds increase by 20 MPH, and the storm surge increases by 5 feet with every change in level on the scale.

Caution: don't get the various scales mixed up
Scale
 Phenomenon
Beaufort
 Wind speed
Fujita
 Tornado intensity
Kelvin
 Temperature
Richter
 Earthquakes
Saffir-Simpson
 Hurricane intensity

Storm surge
The storm surge listed above is a rise in ocean level when a hurricane makes landfall.  This causes the most damage and the greatest number of fatalities near a coast.



The converging surface winds associated with a hurricane sweep surface water in toward the center of a hurricane and cause it to pile up.  The water sinks and, in deeper water, returns to where it came from.  This gets harder and harder to do as the hurricane approaches shore and the ocean gets shallower.    So the piled up water gets deeper and the return flow current gets stronger.

The National Weather Service has developed the SLOSH computer model that tries to predict the height and extant of a hurricane storm surge (SLOSH stands for Sea, Lake, and Overland Surges from Hurricanes).  You can see some animations of SLOSH predictions run for hurricanes of historical interest (including the Galveston 1900) hurricane at a National Hurricane Center website (http://www.nhc.noaa.gov/surge)

While watching the animations, you might notice the storm surge is generally larger on the right hand side of the approaching hurricane.  This is something than can be explained fairly easily.



In this figure a hurricane with 100 MPH winds is traveling from east to west at a speed of 15 MPH.


On the north side of the hurricane, the spinning winds and the motion of the hurricane are in the same direction and add together.  This is where you would expect to find the strongest winds and the highest storm surge.