Monday Apr. 25, 2011
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A couple of selections from the Blue Man Group The Complex Rock Tour Live DVD ("Above", "Drumbone", and "Shadows Part 2").  The Blue Man Group will be in Tucson this coming weekend.

Pt. 1 and Pt. 2 of the Quiz #4 Study Guide have been finalized (points totals for the various sections has been added).  Reviews will be held this afternoon and Tuesday afternoon.

If you are planning on turning in two reports as part of 1S1P Assignment #3, you should have turned in at least one report today.  If you're only doing one report or plan to do a second report you can have until Friday to turn those in.  A few more names have been added to the list of students that have earned 45 1S1P pts.

The Experiment #3 revised reports have been graded and were returned today.




Today's picture of the day.  The French caption says "A portable lightning rod or the umbrella-lightning rod of Barbeu-Dubourg."  It was an idea he gave to Benjamin Frankllin who was won international renown for his lightning experiments.  Something like this wouldn't provide any protection from lightning.

Lightning kills just under 100 people every year in the United States.  Here are some lightning safety rules that you should keep in mind during thundery weather.


Stay away from tall isolated objects during a lightning storm.  You can be hurt or killed just by being close to a lightning strike even if you're not struck directly.  Lightning currents often travel outward along the surface of the ground (or in water) rather than going straight down into the ground.  Just being close to something struck by lightning puts you at risk.

An automobile with a metal roof and body provides good protection from lightning.  The lightning current will travel through the metal and around the passengers inside.  The rubber tires really don't play any role at all.  The people in Florida that were triggering lightning with rockets (shown on a video last week) were inside a metal trailer and were perfectly safe.  All of the connections made to equipment outside the trailer were done using fiber optics, there were no metal wires entering or leaving the trailer. 

You shouldn't use a corded phone or electrical appliances during a lightning storm because lightning currents can follow wires into your home.  Cordless phones and cell phones are safe.  It is also a good idea to stay away from plumbing as much as possible (don't take a shower during a lightning storm, for example).  Vent pipes that are connected to the plumbing go up to the roof of the house which puts them in a perfect location to be struck.



To estimate the distance to a lightning strike count the number of seconds between the flash of light and when you first hear the thunder.  Divide this by 5 to get the distance in miles. 


For example, a delay of 15 seconds between the flash of light and the sound of thunder would mean the discharge was 3 miles away.  Research studies have shown that about 95% of cloud to ground discharges strike the ground within 5 miles of a point directly below the center of the storm.  That's a 10 mile diameter circle and covers the area of a medium size city.

The latest lightning safety recommendation is the 30/30 Rule.

 

The 30/30 rule
People should seek shelter if the delay between a lightning flash and its  thunder is 30 seconds or less.

People should remain under cover until 30 minutes after the final clap of thunder.

The following information won't be on this week's quiz
We spent a few minutes looking at some unusual upper atmospheric phenomena that are sometimes produced by lightning.  The figure (source: Wikipedia) below gives you an idea of where sprites, elves, and blue jets are found and sort of what they look like.  They're very faint and don't last very long so they difficult to see.




You find some good actual pictures of sprites (mainly) at this sky-fire.tv website.  A site maintained by the New Mexico Institute of Mining and Technology (NM Tech) also has some good photographs, video, and more information about these phenomena.

We spent the rest of class today on Hurricanes.  This will be the final topic that we cover this semester.  This new material won't be on this week's quiz.

A good place to begin is to compare hurricanes (tropical cyclones) with middle latitude storms (extratropical cyclones).  They are the two large storm systems found on the earth.

Satellite photographs and sketches of the two types of storm system are shown below.



Next we'll list some of the similarites (first table below) and differences (second table, the left column applies to middle latitude storms, the right most column to hurricanes) between these storms.


Similarities
both types of storms have low pressure centers
(the term cyclone refers to winds blowing around low pressure)
upper level divergence is what causes both types of storms to intensify
(intensification means the surface low pressure gets even lower)


Differences
1. Middle latitude storms are bigger,
perhaps 1000 miles in diameter (half the US)
 1. Hurricanes are smaller,
100s of miles in diameter (fill Gulf of Mexico)
2. Formation can occur over land or water
 2. Can only form over warm ocean water
weaken rapidly when they move over land
3. Form at middle latitudes 30o to 60o
 3. Form in the sub tropics, 5o to 20o latitude
4. Prevailing westerlies move these storms
from West to East
 4. Trade winds move hurricanes
from East to West
5. Storm season winter to early spring
 5. Storm season late summer to fall
(when ocean water is warmest)
6. Air masses of different temperatures collide along fronts
 6. Single warm moist air mass
7. All types of precipitation: rain, snow, sleet freezing rain
 7. Mostly just lots (a foot or more) of rain
8. Only an occasional storm gets a name
("The Perfect Storm", "Storm of the Century", etc.)
 8. Tropical storms & hurricanes gets names



The figure above shows the relative frequency of tropical cyclone development in different parts of the world.  The name hurricane, cyclone, and typhoon all refer to the same type of storm (tropical cyclone is a generic name that can be used anywhere).  In most years the ocean off the coast of SE Asia is the world's most active hurricane zone.  Hurricanes are very rare off the east and west coasts of South America.

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.

We'll come back to this topic on Friday.