Wed., Apr. 18 notes

Wednesday Apr. 18, 2007

The last group of Assignment #3 1S1P reports was collected today. We'll get those graded and returned to you as quickly as possible.

The Expt. #4 revised reports are due next Monday as is the Optional Assignment.

The Quiz #4 Study Guide is now available online.

There are a few things to mention about thunderstorms and then we will get started on the tornado section.

We have talked about the shelf cloud and gust front (think dust front) features in the top picture before. The dust storms that thunderstorm winds stir up can cause a sudden drop in visibility and are a serious risk to automobile traffic on the interstate highway.

A narrow intense downdraft is called a microburst. At the ground microburst winds will sometimes reach 100 MPH (over a limited area); most tornadoes have winds of 100 MPH or less. Microburst winds can damage homes, uproot trees, and seem to blow over a line of electric power poles at some point every summer in Tucson. Microbursts are a serious threat to aircraft especially when they are close to the ground during landing or takeoff (see Fig. 10.10 in the text).

Falling rain could warn of a (wet) microburst. In other cases, dangerous (dry) microburst winds might be invisible (the virga, evaporating rain, will cool the air, make the air more dense, and strengthen the downdraft winds).

A simple demonstration can give you an idea of what a microburst might look like.

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.

The demonstration was followed with a short time lapse video showing a microburst that occurred over the Santa Catalina mountains. Cold air and rain suddenly fell out of a thunderstorm sank to the ground and then spread out sideways. The surface winds could well have been strong enough to blow down a tree or two.

The winds are increasing in speed with increasing altitude in the figure above. This is vertical wind shear (changing wind direction with altitude is also wind shear).

The thunderstorm will move to the right more rapidly than the air in the thunderstorm updraft which originates at the ground. Rising air that is situated at the front bottom edge of the thunderstorm will find itself at the back edge of the storm when it reaches the top of the cloud. This produces a tilted updraft.

Remember that an ordinary air mass thunderstorm will begin to dissipate when the downdraft grows horizontally and cuts off the updraft. In a severe storm the updraft is continually moving to the right and staying out of the downdraft's way. Severe thunderstorms can get bigger, stronger, and last longer than ordinary air mass thunderstorms. The strong updraft winds can keep hailstones in the cloud longer which will allow them to grow larger.

We will find that sometimes the tilted updraft will begin to rotate. A thunderstorm with a rotating updraft is capable of producing tornadoes.

This figure (found on p. 161 in the photocopied Class Notes) has some general information about tornadoes. The US has more tornadoes than any other country in the world. This is illustrated in the next figure.

The central US has just the right mix of meteorological conditions. In the spring cold air collides with warm moist air from the Gulf of Mexico to make strong thunderstorms.

Within the US, tornadoes are most frequent in the central plains.

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." 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.

Tornadoes are of course a serious weather hazard and kill around 80 people every year. The statistics below show that the death toll has been steadily decreasing since the early 1900s. This is due to better prediction and warning of conditions that might cause tornadic thunderstorms to form.

There are about 1000 tornadoes per year in the US. Most tornadoes went unobserved a century ago.

We looked at a portion of a video tape with a whole series of 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.
47	F2	Minneapolis MN	Jul. 18, 1986	Tornado cloud appears and disappears.