FUNNEL CLOUDS CAN BE DANGEROUS! Even if a funnel cloud does not reach the ground, it can still cause significant damage. Even if it appears to be above the ground. Don't go near it.
What is a waterspout?
A waterspout is a tornado that passes over water and picks up a column of water and mist. It is a beautiful natural phenomenon.
Where do tornados occur?
Although tornadoes occur in many parts of the world, these destructive forces of nature are found most frequently in the United States east of the Rocky Mountains during the spring and summer months. In an average year, 800 to 1,000 tornadoes are reported nationwide, resulting in 80 deaths and over 1,500 injuries. No other place in the entire world has that amount of tornados.
Why does the United States have more tornados than any other part of the world?
It is mostly due to the unique geography of the country which breeds many strong, long-lived storms, especially in the region that has become known as "tornado alley".
Where is "tornado alley"?
The peak of the tornado season is April through June and more tornadoes strike the central United States than any other place in the world. This area has been nicknamed "tornado alley."
What is it about tornado alley that creates so many tornados?
Tornado alley is mostly flat farm land. During tornado season, weather generally travels east. As it passes over the Rocky Mountains it cools down, then heats up quickly over tornado alley where it has unobstructed opportunity to develop into powerful thunder cells.
When do tornados occur?
Thunderstorms develop in warm, moist air in advance of eastward-moving cold fronts. These thunderstorms often produce large hail, strong winds, and tornadoes. Tornadoes in the winter and early spring are often associated with strong, frontal systems that form in the Central States and move east. Occasionally, large outbreaks of tornadoes occur with this type of weather pattern. Several states may be affected by numerous severe thunderstorms and tornadoes.
During the spring in the Central Plains, thunderstorms frequently develop along a "dryline," which separates very warm, moist air to the east from hot, dry air to the west. Tornado-producing thunderstorms may form as the dryline moves east during the afternoon hours.
Along the front range of the Rocky Mountains, in the Texas panhandle, and in the southern High Plains, thunderstorms frequently form as air near the ground flows "upslope" toward higher terrain. If other favorable conditions exist, these thunderstorms can produce tornadoes.
Tornadoes occasionally accompany tropical storms and hurricanes that move over land. Tornadoes are most common to the right and ahead of the path of the storm center as it comes onshore.
Tornadoes often form during the early stages of rapidly developing thunderstorms. Tornadoes may appear nearly transparent until dust and debris are picked up. Occasionally, two or more tornadoes may occur at the same time.
What is the source of power for tornados?
Tornadoes are powered by a combination of the sun and gravity. In fact, those are the two primary and constant forces in all of nature.
Hot air weighs less than cold air, so when both hot and cold air are in the same place, the cold air sinks and the hot air rises. This is why a hot-air balloon floats. The balloon is filled with hot air, and hot air goes up as cold air pushes it out of the way.
Technically, because of gravity, it is more correct to say, "cold air falls," than to say, "hot air rises." Gravity pulls the heavier cold air down forcing the hotter air out of the way.
Of course, it is the sun that heats up the air, so without both natural forces, air would not move, we would not have the rain cycle, and life would cease to exist. Both forces are critical to our existence, and storms are just one of the byproducts of these two natural power sources.
Before we go any further about the power source of tornados, let's consider air thermals.
A thermal, or "updraft" is a column of warm air that rises into the sky.
How does a thermal work?
Let's assume for a minute that there are no clouds, no moisture in the sky, and no wind. But the sun is out, and doing what it always does, heating things up.
As the sun heats up the earth and objects on the earth, those objects give up their heat to the colder air around them. Just as we described above, when cold and hot air mix, the heavier cold air is drawn to the earth by gravity forcing the warmer air out of its way. The warmer air can only go one way--up. As it rises, it meets more cold air which continues to push it up. This is a thermal. It continues until the temperatures mix together and equalize. Thermals occur continuously all day long as the sun performs its daily role.
You can actually see hot air rising off the roof of a car. It looks like wavy vapor, almost like a liquid rising up off the car. This is especially visible above a black car because black absorbs more heat from the sun. But all objects are heated by the sun, and all objects, including the earth itself, give off heat.
You can experience this at home in a number of ways. The next time you have a birthday, look closely at the air just above the candles. You can actually see it dancing. And of course, the flame itself is going "up". You can also see heat rising off the burner on your stove or out of your toaster. Just get down to eye-level and look closely.
Experiment: Pour a cup of water into a pan and bring it to a boil. Observe the steam coming out of the pan. This is "heat rising" and carrying moisture with it. Hold a pan or skillet upside down about a foot above the steaming water. You will notice that as the steam collects (condenses) in the upside-down pan, it eventually gets to heavy and starts dripping back down into the boiling water.
Now you know how moisture from the earth gets into clouds. As the earth warms, moist air rises and collects into clouds. When the clouds become saturated, they "drip" which we call rain!
That is a description of what happens just from the warming sun. Now, let's get back to tornados, and the thunderstorm conditions that supercharge the thermals.
Since it is colder in the sky than on the ground, thunderclouds generate massive amounts of even colder air which sinks all around and forces the warmer surface air out of the way.
Once it begins rising, it finds the vacuum left vacant from the falling rain and cold air, and accelerates its journey up through the cell. Thunder cells can be 50,000 or more feet tall, and that is a long wind-tunnel...over 8 miles of violent super-fast rising wind. Imagine the suction at the ground as this hungry 8-mile tall monster MUST be fed.
Now that we have a better understanding of thermals and thunderclouds, let's learn how they contribute making tornados.
How do tornados form?
Most tornadoes follow a recognizable life cycle. The cycle begins when a strong thunderstorm develops a rotating system a few miles up in the atmosphere which becomes a supercell or thunder cloud cell. As rainfall in the cell increases, it drags with it an area of quickly descending air. This downdraft accelerates as it approaches the ground, and drags the rotating system towards the ground with it.
As it approaches the ground, a visible condensation funnel descends from the base of the storm. As the funnel descends, the rotation speed increases and it becomes a tornado when it touches the ground and dramatically increases its speed and power as it now feeds on the warmer surface air.
Initially, the tornado has a good source of warm, moist inflow to power it, so it grows until it reaches the mature stage. During its mature stage, which can last a few minutes to more than an hour, a tornado often causes the most damage.
What causes a tornado to dissolve and stop its destruction?
Weather systems, like all natural phenomena, have a beginning and an end. Thankfully, tornados are not an exception.
As the warm air powers up through the center of the tornado and continues up through the thunder cell, the descending cold air moves toward the outside of the thundercell and eventually wraps around the rotating system, cutting off the inflow of warm air which feeds the tornado.
This cold air completely wraps around and chokes off the tornado's air supply, and the tornado begins to weaken, becoming thin and rope-like. This is the dissipating stage, and the tornado often dissolves within minutes. During the dissipating stage, the tornado becomes highly influenced by the direction of surface winds, and can be blown into fantastic flexible patterns.
Are all tornados destructive?
No. Some are small and harmless. They form on the ground, and lack the tall horizontal thunder cell system to develop into a dangerous tornado.
Many harmless tornados begin their life horizontally.
As wind blows across the ground, it meets resistance due to a hill, fallen tree, or other object. Naturally, this forces the wind to rise over the obstruction.
Similarly to how air travels over the wing of an airplane, it has to go faster over the top of the path because that is a longer distance than the air closer to the obstruction. As the faster wind catches up with the slower wind on the other side of the obstruction, it "rolls" into a horizontal spinning effect illustrated above.
If you could see the air, you would see several of these horizontal "rolling" phenomena as the wind rushes over mounds, rocks, and man-made obstructions like houses. Most of them dissolve quickly, but when thermals occur over the top of a horizontal roller, a fascinating thing happens.
Rising air within the thermal tilts the horizontally rotating air to vertical and it begins to take the familiar funnel shape of a tornado.
Because thermals occur all over, it is common to see multiple small "wisps" of mini-tornados that form in a few seconds and dissipate just as quickly. They are easier to see over a dirt field or an empty parking lot since they suck up dust. But even when you can't see them, these innocent mini-tornados are very common.
What direction do tornados rotate?
Large-scale storms always rotate counterclockwise in the northern hemisphere because of the "Coriolis force" based on the rotation of the earth. This is discussed more fully in the Whirlpool section. Tornados usually rotate the same way. However, they are on too small a scale to always be directly affected by the rotation of the earth, so they sometimes (rarely) are influenced by other surrounding physical forces such as mountains or large structures that shift wind direction. Because the large weather system is rotating counterclockwise, a tornado that "goes against the grain" is usally short-lived and does not become a major tornado.
What is a tornado F-Scale?
An F-Scale is named for Dr. T.T. (Ted) Fujita. It is a classification model that equates tornado wind speeds with potential damage. It is similar to the "category" ratings of hurricanes.
Speed and Width
Very Weak Tornado
|Light: Damages chimneys or TV antennae; breaks branches off trees; pushes over shallow-rooted trees; old trees with hollow insides break or fall; sign boards are damaged.
|Moderate: Peels off roofs; breaks windows; pushes or overturnes trailer houses; uproots trees on soft ground; some trees snapped; damages automobiles.
100-200 yards wide
|Considerable: Roofs torn off frame houses leaving strong upright walls standing; weak structures or outbuildings demolished; trailer houses demolished; railroad boxcars pushed over, large trees snapped or uprooted; light-objects turned into missiles; cars blown off highway; block structures and walls badly damaged.
200 yards to 1/4 mile across
|Severe: Roofs and some walls torn off well-constructed fame houses; some rural buildings completely demolished or flattened; trains overturned; steel framed hangar-warehouse type structures torn; cars lifted off ground and may roll some distance; most trees in a forest uprooted, snapped, or leveled; block structures often leveled.
1/4 to 1 mile across
|Devastating: Well constructed frame houses leveled, leaving piles of debris; structures with weak foundations lifted, torn, and blown some distance; trees debarked by small flying debris; sandy soil eroded and gravel is very destructive.
F5 Incredible Tornado
1/2 to 2 miles across
|Incredible: Strong frame houses lifted clear off foundation and carried considerable distance; steel reinforced concrete structures badly damaged; automobile-sized missiles fly for 100 yards or more; trees left standing are completely debarked.
F6 Inconceivable Tornado
|Inconceivable: In fact, there is reason to believe that there is a "thermodynamic speed limit," and winds faster than 300 mph cannot occur in a tornado.
The following pie charts show that the vast majority of tornados (74%) are weak and cause relatively less damage than what we see on television. Thankfully, they also cause very little death (4%). On the other end of the scale, notice that violent tornados are only 1% of all tornados, yet they produce 67% of all deaths.
Pictures of Tornado Damage
This is a picture of damage caused by an F3 tornado with speeds from 158-206 miles per hour . Houses are demolished, cars are lifted off the ground tossed around, trees are uprooted and snapped.
The damage is severe, and there are two higher ratings that cause even more damage.
You don't want to be anywhere near a tornado.
General conditions for tornados are fairly easy to predict. Forecasters usually err on the side of caution and issue "Tornado Warnings' whenever these conditions exist, whether any tornados have actually been spotted. Part of the reason for this caution is that they can form in a matter of seconds, so it is better to be prepared.
However, when the radar shows the "classic hook" it is almost certain to spawn a tornado. Notice the shape of the dark red (heavy rain and hail) are on this image.
The following illustration shows the same development with the wind-direction added.
When they see this pattern, it is almost certain to generate a tornado, and a siren warning.
What is the shape of a tornado?
A tornado is shaped like a funnel, also known as a vortex. It has a small bottom and wide top. This shape is the natural result of a fast-spinning body of fluid or air.
The centrifugal force in a tornado vortex forces air and everything it is carrying out to the perimeter where some of it is thrown long distances, illustrated by the arrows.
This same shape occurs in other natural phenomena, most notably a whirlpool which is very fascinating. We have created another page to answer all your questions about whirlpools and provide some exciting experiments you can do at school or in your home. Learn which way water circles a drain and why it is different above or below the equator. Click here for more about whirlpools.