Monday, Apr. 22, 2019

Willie Nelson "I Never Cared for You" (5:01), "Still is Still Moving to Me" (3:07), "Stay a Little Longer" (3:25), "Nothing I Can Do About it Now" (2:49)

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 (bottom of figure). 

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.



Photograph (source) showing (from left to right) Hurricanes Katia (making landfall in the Mexican state of Veracruz), Irma (approaching Cuba), and Jose on Sept. 8, 2017.
This photograph was taken by NOAA's Suomi NPP Polar Orbiting satellite.





This is a Astronaut Serena Aunon-Chancellor's photograph of the eye of Hurricane Michael taken from the International Space Station (photo source).  Hurricane Michael, which made landfall near Mexico Beach on the Florida Panhandle on Oct. 10, 2018 was the 3rd strongest hurricane to make landfall in the US (after the Labor Day Hurricane of 1935 and Hurricane Camille in 1969). 

Here's a short video that follows one of NOAA's "Hurricane Hunter" aircraft as is passes through the eye wall and into the eye of Hurricane Emily (2005).  Here is a time-lapse video that recorded the eye of Hurricane Irma pass over Cane Garden Bay (British Virgin Islands) on Sept. 6, 2017.  Notice how the winds calm down starting about 2:00 minutes into the video.  A couple of people can be seen on the balcony at about 2:50 before winds begin to blow again (from a different direction) as the other side of the eye wall moves in.  The battery in the camera died shortly after this.

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 (*I don't remember having mentioned the Beaufort and Richter scales in class).
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 Richelieu Apartments in Pass Christian, Mississippi, before and after being destroyed by a storm surge produced by Hurricane Camille in August, 1969 (source of these pictures: https://www.nola.com/hurricane/2014/08/hurricane_camille_party_what_h.html

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 click on the Notable Surge Events link in the contents list).  Here's an interesting "Fast Draw" explanation of storm surge from the National Weather Service.

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.

In the same way you might expect to find the strongest winds on the SE side of a counterclockwise spinning tornado moving from the SW toward the NE.