Friday Apr. 18

Bicycle drawings II (from the T Th section)

Here's a scary thought

The Quiz #4 Study Guide has  made its appearance in a very preliminary form.

Any interest in an in-class Optional Assignment?  The answer to that question was yes, so a relatively long in class optional assignment was handed out in class and collected at the end of the period.  The assignment will be returned next week and answers will be posted somewhere online.
Any interest in extra Office Hours next week?
  I actually forgot to ask that question.  So if you're reading this and think that is a good idea, just send me an email ( weidman@atmo.arizona.edu )



Remembering what you learned about cloud-to-ground lightning in class on Wednesday, can you make any sense out of the following figure?

Which of these two discharges came first?  What is the name of that initial discharge?  Click here for the answer.



I believe we mentioned positive lightning in class on Wednesday.
A positive cloud-to-ground discharge begins with a downward moving positively charged leader.
That is followed by a very powerful return stroke (5 to 10 times more current than negative cloud-to-ground lightning)




Lightning sometimes starts at the ground and travels upward.  Upward lightning is generally only initiated by mountains and tall objects such as a skyscraper or a tower of some kind.  These discharges are initiated by an upward leader.  This is followed by a more normal downward leader and an upward return stroke.

Scientists are able to trigger lightning by firing a small rocket up toward a thunderstorm.  The rocket is connected by a thin wire to the ground.  When the rocket gets 50 to 100 m above the ground upward lightning will develop off of the top of the wire.

Scientists are able to take closeup photographs and make measurements of lightning currents using triggered lightning.  Triggered lightning can also be used to test the operation of lightning protection devices.  A short video showing rocket triggered lightning experiments was shown in class.

Near the end of the tape you will some cases where the lightning didn't follow the wire all the way to the ground (this is one reason why you need to be very careful doing experiments of this type).  When the lightning strikes the sandy soil (instead of striking instruments on the ground) it sometimes will leave behind a fulgurite (the spelling in the figure is incorrect)
.




This is a drawing of a science fair project I once saw (I don't think it won a prize).  If 10 to 20 Amps will cook a hotdog, imagine what the 10,000 to 30,000 Amps in a lightning return stroke can do to a person.  It is best to try not to let that happen to you.



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.


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 people in Florida that were triggering lightning were inside a metal trailer and were perfectly safe).  The rubber tires really don't play any role at all.



You shouldn't use a corded phone and electrical appliances during a lightning storm because lightning currents can follow wires into your home.  Cordless phones and cell phones are safe.

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.

The latest lightning safety recommendation is the 30/30 Rule.  You 'll see that explained below.


The 30/30 rule
30/30 Rule graphicPeople should seek shelter if the delay in seconds 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.

(for further information see  Jetstream An Online School for Weather )


We had a little extra time for a little introductory information on the next topic we will be covering - hurricanes.


The figure above (a copy was distributed in class) shows the two types of large synoptic scale storm systems: middle latitude storms (extra tropical cyclones) and hurricanes (tropical cyclones).  The word cyclone is telling you that both types of storms have at least one characteristic in common, winds spinning around a surface center of low pressure.  The winds spin counterclockwise around low pressure in the northern hemisphere.  The winds reverse direction and spin clockwise around low pressure in the southern hemisphere.  The Coriolis force is what causes this change.  I hope to cover the Coriolis force at least briefly before the end of the semester.

You may already have been to the southern hemisphere or you may go there one day.  You'll probably hear about how the Coriolis force or the Coriolis effect causes water to spin in a different direction in the southern hemisphere when it is draining out of a sink and toilet bowl (it's not true).  That's another reason for covering the Coriolis effect in NATS 101. 

We'll compare middle latitude storms and hurricanes in a little more detail next Monday.  But notice that middle latitude storms generally move from west to east in both the northern and southern hemispheres.  Hurricanes, on the other hand move from east to west, again in both hemispheres.  The next figure shows why this is true.

Winds blow from the west at middle latitudes (30 to 60 degrees latitude).  These are called the prevailing westerlies.  The winds blow from the southwest in the northern hemisphere and from the NW in the southern hemisphere.  There is very little land at middle latitudes in the southern hemisphere.  There is less friction when winds blow over ocean water and the prevailing westerlies can get very strong in the southern hemisphere especially in the winter.  This is what the names "roaring 40s" and "ferocious 50s" are referring to.

In the tropics (30 N to 30 S) winds blow from the east, form the NE in the northern hemisphere and from the SE in the southern hemisphere.  These are the trade winds.  The trade winds converge at the equator and cause air to rise.  This is referred to as the Intertropical Convergence Zone (ITCZ).  Since the air is warm and moist clouds form and this is a rainy part of the globe.  Surface winds weaken at the equator and back when ships depended on sails to move about the ships would sometimes get stuck near the equator.  This is the origin of the name "doldrums" used to describe this part of the globe.