Fri., Apr. 11 notes

Friday Apr. 11, 2008

It looked for a moment like the quizzes wouldn't all be graded in time to be returned in class today, but they were. There was a lot of work done in a short amount of time before class, so be sure to carefully check the grading of your quiz.

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 Monday (Apr. 14)

Today's class notes were assembled quickly late Friday afternoon and might have a lot of typos.

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 in class on Monday. 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 following picture may be redrawn for improved clarity.

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.