Mon., Apr. 18 notes

This seemed like a reasonable picture of the day (from the University Corporation for Atmospheric Research)
A tornado (probably a relatively weak one) dropping down from the edge of a wall cloud.

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.

I got a little carried away with the colored pencils. In the spring, cold dry air can move all the way from Canada and collide 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). You'll find this map on p. 161 in the photocopied ClassNotes)

Here are some basic tornado characteristics.

1. About 2/3rds of tornadoes are F0 or F1 tornadoes (we'll learn moare about the Fujita scale used to rate tornado intensity later today and on Wednesday) and have spinning winds of about 100 MPH or less. Microburst winds can also reach 100 MPH. Microbursts are much more common in Tucson in the summer than tornadoes and can inflict the same level of damage.

2. A very strong inwardly directed pressure gradient force is needed to keep winds spinning in a circular path. 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 such a short distance. The PGF is much stronger than the Coriolis Force (CF) and the CF can be neglected.

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

4, 5, 6. Tornadoes usually last only a few minutes, leave a path on the ground that is a few miles long, and move at a few 10s of MPH. There are exceptions, we'll look at one shortly.

7, 8. Most tornadoes move from the SW toward the NE. This is because tornado-producing thunderstorms are often found just ahead of a cold front. Winds ahead of a cold front often blow from the SW. Most tornadoes have diameters of tens to a few hundred yards 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.

The figure traces out the path of the 1925 "Tri-State Tornado" . The tornado path (note the SW to NE orientation) was 219 miles long, the tornado lasted 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 in the United States.

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 (see the weather map below). 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.

Tornadoes the past several days qualify as an outbreak. According to this source, 50 confirmed tornadoes (the total will probably exceed 200) have killed at least 38 people. North Carolina seems to have been particularly hard hit (here is a link with lots of photographs from the Charlotte Observer newspaper).

The two tables above are on p. 161 in the photocopied ClassNotes. At the present time about 75 people are killed every year in the United States. This is about a factor of ten less than a century ago due to improved methods of detecting tornadoes and severe thunderstorms. Modern day communications also make easier to warm people of dangerous weather situations. Lightning and flash floods (floods are the most serious severe weather hazard) kill slightly more people. Hurricanes kill fewer people on average than tornadoes.

We watched a short segment of video from Tornado Video Classics vol. 1 (see the summary 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 second column above is the Fujita scale rating of each tornado. That provides an idea of the intensity of the tornado.

Here's an easy to remember version of the Fujita Scale used to rate tornado strength and damage potential. It is very hard to actually measure the speed of the rotating winds in a tornado. Researchers usually survey the damage caused by the tornado to come up with a Fujita Scale rating. The Fujita Scale has recently been revised. The newer scale is called the Enhanced Fujita Scale to differentiate it from the original scale introduced in 1971.

Tornadoes begin in and descend from a thunderstorm. You would usually 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. We saw an example of this in a video tape of a tornado in Laverne Oklahoma.

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 (though still a dangerous) 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 is just like the cloud that forms when air rises (and moves into lower pressure and expands).

This seemed like a good place to briefly discuss supercell thunderstorms.

Here is a relatively simple drawing showing some of the key features on a supercell thunderstorm. In a supercell the rotating updraft (shown in red above) is strong enough to penetrate into the stratosphere. This produces the overshooting top or dome feature above. A wall cloud and a tornado are shown at the bottom of the mesocyclone. In an ordinary thunderstorm the updraft is unable to penetrate into the very stable air in the stratosphere and the upward moving air just flattens out and forms an anvil. The flanking line is a line of new cells trying to form alongside the supercell thunderstorm.

Here is a second slightly more complicated drawing of a supercell thunderstorm. A typical air mass thunderstorm (purple) has been drawn in for comparison.

A short segment of video was shown at this point. The video first showed some good quality video of a close tornado. This was followed by photographs of a distant supercell thunderstorm and photographs of the bases of nearby supercell thunderstorms. Here you could see the spectacular wall cloud that often forms at the base of these storms. Finally a computer simluation showed some of the complex motions that form inside supercell thunderstorms, particularly the tilted rotating updraft.

A wall cloud can form a little bit below the rest of the base of the thunderstorm. The figure (shown but not really discussed in class) above tries to explain why that is true. Clouds normally form when air rises, expands, and cools as shown above at left. The rising air expands because it is moving into lower pressure surroundings at higher altitude.

At right the air doesn't have to rise to as high an altitude to experience the same amount of expansion and cooling. This is because it is moving into the core of the rotating updraft where the pressure is a little lower than normal for this altitude. Cloud forms a little bit closer to the ground.