Lab 11 Objectives *

Lab 11 Key Concepts *

Lab 11 Reading Resources *

Lab 11 Activities Guidance *

Laboratory 11 Activities *

• be able to list and describe the large scale and small local scale factor that play a role in tornado formation and development
• understand and be able to describe safety actions for both inside and outside of your home
• understand the type of destruction associate with the tornadoes
• understand and be able to describe the climatology of tornado around the world.
• forecast the area of the likely development of a tornado

1. Main ingredients for potential tornado formation include:

• warm, moist air near ground level
• an approaching cold front, upper-level trough, or surface low-pressure system
• a layer of warm, dry air near altitudes of about 2 kilometers (6,500 feet)
• the presence of significant vertical wind shear

2. Counterclockwise circulation starting within a supercell thunderstorm is called a mesocyclone. A mesocyclone that is "stretched" vertically causes an increased rate of spin, helping to sometimes develop it into a tornado. You can see this process in Figure 10.1 in the course content module 10.

3. The Fujita scale of wind speed and damage is the scheme used to classify tornado strength. The scale ranges from F0 (light) to F5 (incredible).

4. Tornadoes occur throughout the world, but the United States experiences the most tornadoes, averaging over 900 annually.

5. A fairly reliable method for pinpointing a tornado area associated with a low-pressure system is called Larko's triangle.

Tornadoes are rotating columns of wind that form beneath some severe thunderstorms. The strongest tornadoes typically form from what is called a "supercell" thunderstorm. The most immediate cause of a tornado is the tilting of a horizontal rotating column of air within the thunderstorm to an upright position and then a stretching of the column (vortex stretching). However to get to the point where a tornado will form, many ingredients are needed in order to first form a strong thunderstorm.

For diagrams of tornado formation see Lesson 10 Fig. 10.1 in course content module #3.

(Go directly to this location if link doesn't work)

Tornado formation: http://tychousa2.umuc.edu/NSCI170/0302/Modules/M3-Module_3/S4-Lesson_10.html

For a PowerPoint presentation on tornadoes with good diagrams of conceptual models of tornadoes go to one of the sites below:

Lec 11 Tornadoes

Lab 11 Tornaadoes (alternate site)

Time of Year: Best time to observe/chase is mid-late spring (May-June). See figure 14.4 on page 390.

Time of Day: Mid to late afternoon is peak. This is the most likely time that the inversion lid can be broken.

Location: In general the U.S. has by far the most tornado activity of anywhere in the world. The greatest concentration of tornado activity is in Central U.S. from Texas - Oklahoma - Nebraska - Iowa and Illinois Indiana - secondary max. (Note max in Louisiana and Florida) is in part due to Hurricane induced tornadoes.) See Figure 14.2b page 389 in WoW.

Favorable Synoptic Pattern: See Figure 14.3 a and b page 389 in WoW.

Favorable factors in the low levels are a strong mid latitude low pressure system cyclone and cold front, warm and moist mT air in the warm sector of the low, and a "low level jet".

Favorable factor in the mid troposphere is dry continental air that moves over the warm moist air in the low levels. See figure 13.8 page 370 in text.

Favorable factors in the upper levels are cold air aloft. Also anything that will cause divergence over the area of the warm moist air are positive factors that included a strong trough to the west of the surface low, a strong polar front jet stream that follows the upper trough moving south over the low pressure system then back north, with the jet max (streak) right over the region of the low. A subtropical jet that moves up from the south over the low and then curves back south.

Seasonal Adjustment to Favorable Synoptic Pattern: The location of the maximum thunderstorm activity will begin in the southern regions of "Tornadoes Ally" in early spring then move north in to the middle part of the country then finally into the Upper Plains by early summer.

Once the synoptic scale ingredients have formed severe thunderstorms it takes a small scale force to actually produce the tornado. As described earlier, tornadoes are rotating columns of wind that form beneath some severe thunderstorms. The actual tornado is formed when a horizontal rotating column of air within the thunderstorm is "tilted" to an upright position and then the column is stretched in the vertical. Click here to see conceptual mode of this process.

The stretching of the column of rotation are is called vortex stretching. When vortex stretching occurs the column of air that is initially rotating slowly is stretched along the axis of rotation. As the column of air stretches it also becomes skinnier. Because of conservation of angular momentum, as the column becomes skinnier it begins to spin faster and faster. Vortex stretching occurs in your bathtub or sink, and is why a whirlpool often forms in a draining bathtub. · This vortex stretching can occur in a thunderstorm updraft, and can cause the vortex to rotate at incredible speeds (over 250 mph!).

· So the process of tornado formation begins when a small tornado forms when the vertical wind shear near the ground produces a horizontal rotating column of air. So initially, the rotation will be along a horizontal axis. If there is little tilting and stretching only small tornadoes will form. But if the thunderstorm updraft tilts the rotation strongly to the vertical, and the vortex stretching occurs the tornado will become much stronger.

· We still don't understand everything about the formation of tornadoes. Horizontal wind shear may also produce some tornadoes, but these are likely very weak tornadoes. Tornadoes that form from both vortex tilting and stretching in an updraft are the strongest tornadoes. You may be surprised to know that when tornadoes are formed in this manner, they can spin either clockwise or counterclockwise. Because Coriolis is NOT an important direct factor in the balance of forces causing tornadoes, even thought the tornado is a low pressure center it can spin either clockwise or counter clockwise. When a low pressure center can spin either clockwise or counter clockwise it is called a cyclostrophic system.

The larger and most devastating tornadoes form from supercell thunderstorms. · The updrafts in supercell thunderstorms are already slowly spinning in a counterclockwise (cyclonic) direction. This is often referred to as a mesocyclone. · The tornado, and vortex stretching, occurs within the mesocyclone. · Tornadoes formed in this manner always rotate counterclockwise. So it turns out the very strongest tornadoes spin cyclonically, not because of the direct influence of Coriolis, but because the parent circulation is rotating cyclonically.

· A tornado may have multiple suction vortices. · Although the visible part of the tornado usually extends from the cloud and moves toward the ground, the suction in the tornado is actually upwards. · The cloud forms in the vortex because pressures are low enough to cause the air to adiabatically cool to saturation. · In order to be classified as a tornado, the vortex must reach the ground. Otherwise, it is just reported as a funnel cloud. Water Spouts are tornadoes that occur over water.

The conditions of wind shear, tilting and stretching needed to produce tornadoes are most likely to be found with a super cell thunderstorm. The most likely part of a line of thunderstorms to produce a supercell is on the right usually southwest corner of a moving line of thunderstorms.

Click here for example.

The tornado itself usually forms in the rain free area underneath the base of the thunderstorm cumulonimbus cloud. Click here for diagrams of:

• The interacting up and downdrafts that produce the shear that produced shear thunderstorm
• Supercell thunderstorm structure
• Location of the tornado within a supercell thunderstorm
• Overshooting tops

· About 70 - 80% of tornadoes spin counterclockwise, 20 - 30 % spin clockwise. · Coriolis force does not directly act upon the tornado because tornadoes are too small to directly feel the effects of the earth's rotation. Rotating systems that don't feel the effects of Coriolis are called "cyclostrophic". Why is there a preference for counterclockwise rotation? The midlatitude low-pressure system, which is large enough to feel the effects of Coriolis, has to a tendency to transfer its direction of spin to the mesocyclones and then to the tornadoes.

Conventional Radar only measures the amount of energy returned to the radar, so it can only estimate storm intensity based upon the intensity of the strength of the returned signal. . A Doppler capable radar has both a conventional radar capability but also can use the principle that radiation reflected off of an object will change frequency if the object is in motion. If the object is moving toward the receiver the frequency will be higher, and if moving away it will be lower.

• · Plan where you will take shelter at work or home in the event of a tornado.
• · Practice your plan!
• · Stock designated shelter with emergency supplies.
• · Pay attention to the TV or radio when thunderstorms are in your area, and listen for the issuance of Tornado Watches or Tornado Warnings.
• · Keep a battery operated AM or FM radio available.
• · You may want to purchase a weather-band radio:
• The National Atmospheric and Oceanic Administration (NOAA), the parent organization of the National Weather Service, operates radio stations that continually broadcast weather information.
• Radios that can receive these broadcasts are relatively inexpensive, and can be bought at most electronic stores.
• Some of these radios even have alarms on them that will sound if a tornado watch or warning has been issued.
• Tornado Watches and Warnings

• If you are outside or a tornado chaser you should keep aware of the surrounding situation at all times.
• If you see a wall cloud then this is an indication that a tornado may form quickly.
• Overshooting tops are also a sign of a possible tornado formation.
• If a strong storm approaches the best advice is to seek shelter immediacy. If shelter is not available move perpendicular to the movement of the tornado. Beware that a tornado can take sharp sudden turns from the direction of movement and be prepared to take quick action.
• See pages 398 - 400 in the WOW textbook for more safety tips.
• The following sites can provide useful safety information on tornadoes when you are outside:

• Tornado intensity is measured on a scale created by Dr. Ted Fujita, a noted tornado researcher. It is referred to as the Fujita or F-scale.
• Since tornado winds cannot be measured directly, the F-scale is based upon the damage caused by the tornado.
• The F-scale goes from F0 to F5, with F0 being light damage, and F5 being incredible damage. See Table 14.1 page 398 of WoW. of each tornado
• When evaluating the strength of tornadoes you have to use some common sense. For example if 99% of the very strong well constructed buildings of a town are completely destroyed, but there are a couple of unexplained houses still left standing it probably had F5 intensity winds

Tornadoes can and do occur during any month of the year in many parts of the world. In fact tornadoes have been reported in every state of the U.S. including Alaska. However, the Central U.S. and parts of southern Canada is by far the most active region for tornado formation anywhere in the world. See pages 388 - 389 in your WoW test book for more information and diagrams.

Even in the U.S. there is a preferable time for tornadoes to form. Most tornadoes occur during the Spring over the Great Plains. This is because the conditions for forming severe thunderstorms (the collision of mT air with cP air) often occur in this region at this time of year. The location and orientation of the Rocky Mountains plays a huge role in creating the general conditions that favor the formation of tornadoes in the central U.S. and Canada. The Rocky Mounts act to focus the collision of the mT and cP air on the eastern side of the Rockies as it blocks most of the more moderate mP air from moving east from the Pacific. Another thing the Rockies do is to enhance the upper level divergence east of the Rockies when the strong winds are forced up and over the mountain chain.

· Since most tornadoes that form in association with synoptically forced thunderstorms form in the mT air ahead of a cold front they tend to move from the southwest to northeast. This is because the prevailing flow is typically southwesterly ahead of a cold front. But always keep in mind that this is just the typical observed movement based upon statistics. Localized short-lived conditions in the wind flow can cause the tornado to move in any direction very quickly. Remember, statistical rules don't cause tornado movement, the laws of physics do.

Another way tornadoes form occurs when mature tropical cyclones (hurricane and typhoons) make land fall. The sudden increase in friction near the surface increases wind shear allowing vortex tilting and stretching to take place. So you will see the chances of tornadoes increased anywhere mature tropical cyclones have a tendency to make land fall.

1. Knowing the area of a region of interest, and total annual occurrence of tornadoes of the area, find the annual occurrence for the standardized unit area of "per 10,000 mi^2". Using the standardized unit area allows for a fair comparison of tornado likelihood for any given region regardless of the size of the region. thus

State X with a size of 40,000 mi² has 10 tornadoes on average each year.

So the annual occurrence per 10,000 mi², would be

2. Knowing the standardized frequency of occurrence for a region and the area of a region of interest, you can calculate the total average annual occurrence of tornadoes for that region.

Annual total tornado occurrence:

State Y with a size of 40,000 mi² has a standardized occurrence of 1.5 tornadoes/10,000 mi² on average each year.

So the total average annual occurrence would be

If you were trying to estimate the total number of tornadoes for a long period you would just multiply the tornadoes per year by the numbers of years in the period of interest.

For example for state y for the period from 1981 to 2000 (a twenty year period) you could estimate that about 6 x 20 or 120 tornado should have occurred.

Make sure you use common sense when interpreting the statistical data. For example, if the average annual frequency is any non zero number no matter how small the decimal fraction there must have been at least 1 tornado occurrence for that region.

As described tornadoes are rotating columns of air that form beneath some severe thunderstorms. The strongest tornadoes have a greater tendency to form in the warm sector of a mid-latitude cyclone defined by "Larko's Triangle" (see pg. 372, Fig. 13.12 in WoW text). Larko's triangle is defined as the "triangle" created in the warm sector of a mid-latitude cyclone by the warm front, cold front and the first closed isobar around the center of the low pressure system. One must use common sense when applying this rule and expand to the second or even third isobar area for very strong systems with strong isobaric gradient.

The Larkos' triangle is a forecasting rule, so it doesn't always apply, but there are reasons why it usually works. Larko's triangle is typically the location for tornado formation for the following reasons:

Please continue to try to make the connection between the concepts we are learning in the course and how they can help you understand your local weather.

This week, think about how weather at your location is related to the possibility of severe thunderstorm formation and tornadoes.

In your brief forecast discussion for this week, focus on the potential of severe thunderstorms and tornadoes over the next few days.

Go to this link "help for weather observations and forecasts" for more help on topics relevant to taking observations and making forecasts.

You've been given the role of "tornado chaser" for a very important tornado research project. Answer the following question.

• Time of year:
• Time of day:
• Part of the country:
• Favorable synoptic (large scale) pattern (both low level and upper levels):
• Comment on Seasonal Adjustment to the best location to find tornadic thunderstorms.

• What part of a line of thunderstorms would be most likely to produce a tornado?
• What type of cloud would you look for?
• If you had access to a visible and IR satellite images what would you look for?
• If you had access to a conventional radar what would you look for?
• If you had access to a Doppler radar what would you look for?
• How does rotation, tilting and stretching with a mesocyclone help form the tornado?
• Why do some tornadoes rotate clockwise (anticyclonically)?

3a. Estimate the average annual number of tornadoes in:

• Pennsylvania (area 45,308 mi²)
• Oklahoma (area 69,956 mi²)
• Florida (area 58,664 mi²)
• California (area 158,706 mi²)

3b. Estimate the total number of tornadoes that occurred in Alaska (area 591,004 mi²) during the 38 year period from period 1953-1990.

3c. Of the four Southern Hemisphere countries for which information is given, which experiences the most tornadoes per year, on average. Land areas for the countries are:

• Uruguay (area 68,037 mi²)
• Argentina (area 1,068,302 mi²)
• Australia (area 2,966,151 mi²)
• New Zealand (area 103,883 mi²)

4a. Using the surface map for 18Z on March 12, 1976 at one of the sites below,

Your analysis should be similar to the one in the links below:

4b. Would Chicago be in this watch area?

Using the instruments found in your "weather kit," or any other source of local weather information, take a weather observation (obs) of the weather elements indicated below. Record your obs in a simple text format. Think about and briefly how the weather at your location at the times of the observation is related to the possibility of severe thunderstorm and tornado formation.

Go to this link "help for weather observations and forecasts" for more help on topics relevant to taking observations and making forecasts.

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