Tue., Apr. 15 notes

Tuesday Apr. 15, 2008

Quiz #3 was returned in class today. Also a few Expt. #4 reports have been graded and were returned. There are still several people that haven't returned their materials or turned in their report.

The not-to-surprising results from the poll are in and we will be covering tornadoes first in this last segment of the course. This will be followed by lightning and then hurricanes. Some of the material on Newton's laws of motion, and forces that produce wind may be inserted as we make our way through this material. As a matter of fact here's a Short Optional Assignment on Newtons' Laws of Motion. The assignment is due at the beginning of class next Thursday (Apr. 17)

Today is April 15, the deadline to file your income taxes. Many of you probably don't get the chance to try to read through and understand the instructions and complete your own return. With that kind of experience in mind, here's a worksheet and instructions that will allow you to estimate your current grade in this class.

Something relatively unusual happened this past weekend (and has continued into the early part of this week)
It would probably be best if I just told you what it was, but that often goes in one ear and out the other.
I tried something different, went to the grocery store and bought the following card.

Have a close look at the cat, can you get any sense of what happened over the weekend?
No? Well the message (the message is a little different from the one shown in class) that was inside the card is shown above at right.
Now look back at the cat. Doesn't the cat look like he(she?) was a little hot and maybe thirsty?

The United States has more tornadoes in an average year than any other country in the world (over 1000 per year). The central US has just the right mix of meteorological conditions.

In the spring, cold dry air from Canada collides with warm moist air from the Gulf of Mexico to form strong cold fronts and thunderstorms.

Tornadoes have been observed in every state in the US, but tornadoes are most frequent in the central plains, a region referred to as "Tornado Alley" (highlighted in red, orange, and yellow above).

The figure above is found on p. 161 in the photocopied class notes.

1. About 2/3rds of tornadoes are F0 or F1 tornadoes (see below) and have spinning winds of about 100 MPH or less. Microburst winds can also reach 100 MPH. We'll learn more about microbursts later today or on Thursday. They are a lot more common in Tucson in the summer than tornadoes.

2. A very strong inwardly directed pressure gradient force is needed to keep winds spinning in a circular path. The PGF is much stronger than the Coriolis Force (CF) and the CF can be neglected. The pressure in the center core of a tornado can be 100 mb less than the pressure in the air outside the tornado. This is a very large pressure difference in a short distance.

3. Tornadoes can spin clockwise or counterclockwise, though counterclockwise rotation is more common.

The Coriolis force is what causes surface winds to spin counterclockwise around low pressure and clockwise around high pressure in the northern hemisphere. The direction of spin reverses in the southern hemisphere.

4, 5. Tornadoes usually last only a few minutes and leave a path on the ground that is a few miles long. We will look at an exception below.

6, 7, 8. Most tornadoes move from the SW toward the NE at a few 10s of MPH. Most tornadoes have diameters of hundreds of feet, but tornadoes with diameters over a mile have been observed.

9, 10. Tornadoes are most frequent in the Spring. The strongest tornadoes also occur at that time of year. Tornadoes are most common in the late afternoon when the atmosphere is most unstable.

At the present time about 75 people are killed every year in the United States. Lightning and flash floods (floods are the most serious severe weather hazard) kill slightly more people. Hurricanes kill fewer people on average than tornadoes. Heat in the summer and cold in the winter kill many more people than floods, tornadoes, lightning, and hurricanes.

Most tornadoes last only a few minutes and leave a path a few miles long on the ground. There are of course exceptions. One is discussed below.

The path of the 1925 "Tri-State Tornado" is shown above. The tornado path (note the SW to NE orientation) was 219 miles long, the tornado last about 3.5 hours and killed 695 people. The tornado was traveling over 60 MPH over much of its path. It is the deadliest single tornado ever.

Tornadoes often occur in "outbreaks." The paths of 148 tornadoes during the April 3-4, 1974 "Jumbo Tornado Outbreak" are shown above. Note the first tornadoes were located in the upper left corner of the map. The tornadoes were produced by thunderstorms forming along a cold front. During this two day period the front moved from the NW part toward the SE part of the figure. Note that all the tornado paths have a SE toward NE orientation.

Before looking at the first of several segments of tornado video, here's an easy to remember version of the Fujita Scale used to rate tornado intensity. Because it is so hard to make measurements of tornado wind speeds, intensity estimates are usually based on an examination of the damage caused by the tornado.

The Fujita Scale normally runs from F0 to F5 but there have been a F6 few tornadoes where winds might have exceeded 300 MPH.

We looked at a portion of a video tape with several different tornadoes. The tornadoes, Fujita scale ratings, and comments are given in the table below:

54a	F3	Grand Island, NE	Mar. 13, 1990	tornado cloud is pretty thick and vertical
61f	F3	McConnell AFB KS	Apr. 26, 1991	this is about as close to a tornado as you're ever likely to get. Try to judge the diameter of the tornado cloud. What direction are the tornado winds spinning?
52	F5	Hesston KS	Mar. 13, 1990	Watch closely, you may see a tree or two uprooted by the tornado winds
51	F3	North Platte NE	Jun. 25, 1989	Trees uprooted and buildings lifted by the tornado winds
65	F1	Brainard MN	Jul. 5, 1991	It's a good thing this was only an F1 tornado
57	F2	Darlington IN	Jun. 1, 1990	Tornado cloud without much dust
62b	F2	Kansas Turnpike	Apr. 26, 1991	It's sometimes hard to run away from a tornado. Watch closely you'll see a van blown off the road and rolled by the tornado. The driver of the van was killed!
47	F2	Minneapolis MN	Jul. 18, 1986	Tornado cloud appears and disappears.

The tornado life cycle (don't worry about learning the names of the various stages). Tornadoes begin in and descend from a thunderstorm. You might see a funnel cloud dropping from the base of the thunderstorm. Spinning winds will probably be present between the cloud and ground before the tornado cloud becomes visible. The spinning winds can stir up dust at ground level. The spinning winds might also be strong enough at this point to produce some minor damage.

In Stage 2, moist air moves horizontally toward the low pressure in the core of the tornado. This sideways moving air will expand and cool just as rising air does (see figure below). Once the air cools enough (to the dew point temperature) a cloud will form.

Tornadoes can go from Stage 2 to Stage 3 (this is what the strongest tornadoes do) or directly from stage 2 to stage 5. Note a strong tornado is usually vertical and thick as shown in Stage 3. "Wedge tornadoes" actually appear wider than they are tall.

The thunderstorm and the top of the tornado will move faster than the surface winds and the bottom of the tornado. This will tilt and stretch the tornado. The rope like appearance in Stage 5 is usually a sign of a weakening tornado.

The tornado cloud forms when moist air moves into lower pressure in the core of the tornado. The air expands and cools to the dew point and a cloud forms.

This was the first part of Tuesday's class. The remainder will be added after my MWF 2:00 class.

Dust being stirred up by a gust front (dust front). The gust front is moving from right to left. A dust storm like this is sometimes called a Haboob.

A large plastic tank was filled with water, the water represents air in the atmosphere. Then a colored mixture of water and glycerin, which is a little denser than water, is poured into the tank. This represents the cold dense air in a thunderstorm downdraft. The colored liquid sinks to the bottom of the tank and then spreads out horizontally. In the atmosphere the cold downdraft air hits the ground and spreads out horizontally. These are the strong microburst winds that can reach 100 MPH.