Tornadoes are possible anywhere in the United States, but are most common in the central plains east of the Rocky Mountains and west of the Appalachians. They usually occur in the late afternoon and evening during the spring and summer, being most common in the south in early spring and in the northern tier of states in the summer. However, tornadoes can occur on any day of the year and at any hour.

Of the 1000 tornadoes that occur each year, about 2% of them are rated F4 or F5. That means that there are as many as 20 devastating tornadoes each year. It is possible that meteorologists have underestimated the number of violent tornadoes that occur each year. Tornadoes are rated only by damage they do to man-made structures. Therefore, if a tornado doesn't hit a structure of some kind, we cannot estimate its strength.

There is no particular temperature at which tornadoes form. It is more about what the surface temperature is in relation to the temperature higher up in the atmosphere. Even if it is cold near the surface, as long as it is colder higher up, the winds are right to set up low-level wind shear, along with other necessary ingredients, a tornado is possible.

Yes, but the U.S. has more tornadoes and stronger tornadoes than any other nation. The only Web source we've found with information about tornadoes outside the U.S. is the British Tornado and Storm Research Organisation site. It has links to information about tornadoes not only in the United Kingdom, but also other parts of the world. Click here to go to this site.

Tornadoes can develop at any time of the year. The most tornadoes occur in April, May and June. Tornadoes are most likely to appear between 3 p.m. and 9 p.m. but they can occur at all hours of the day or night.

For the U.S., May averages the most tornadoes, but over the years, April has had more deadly twisters. But they can happen just about any time of the year. Tornadoes are likely in the Southeastern states in the late winter and early spring. By mid-April they are more likely to occur on the southern and central Plains and on the northern Plains later in the spring and early summer.

Varicose veins are most commonly found on the legs of women and men who spend a lot of time on their feet. Birth control pills, hormone therapy, obesity, weight fluctuation, and constipation can also add to the development of varicose veins.

This is a natural process. If it occurs more often, with equal or bigger icebergs, perhaps then we can say more about what is causing this that isn't the natural part of the hydrological cycle, which is for snow to fall over Antarctica, and the continually forming icecap to slowly increase, move out to sea, and calve off icebergs.

Tornadoes can last from a few seconds to more than an hour. They might remain on the ground for just a few yards or for more than 100 miles. On average, tornadoes move from the southwest to the northeast at about 30 mph. However, as the killer Jarrell, Texas, tornado of May 27, 1997, showed, tornadoes can defy these averages. The "F-5" tornado that killed 27 people when it churned through a Jarrell subdivision moved southwest at only 10 mph.

In an average year, about 1,000 tornadoes are reported across the United States, resulting in 80 deaths and over 1,500 injuries. A tornado is a violently rotating column of air extending from a cumuliform cloud and in contact with the ground. A tornado usually (but not always) is visible due to condensation and dust and debris within the rotating column.

Once thunderstorms develop, the National Weather Service's WSR-88D Doppler radar is used to evaluate severe weather and tornadic radar signatures in order to make short-term predictions of tornado occurrence. If a tornado is suspected, a tornado warning is issued for the affected area. However, while Doppler radar can interrogate severe storms very well, it cannot always detect the small-scale processes that actually lead to tornadogenesis.

You should consult your doctor. Usually, side effects are well tolerated. In some cases, your physician will direct you to use less RENOVA, decrease the frequency of application, increase the use of moisturizers, discontinue use temporarily or discontinue use altogether. RENOVA should only be used under medical supervision as part of a total skin care and sun protection program.

The overwhelming majority of abortions are performed in the first trimester of pregnancy. The Centers for Disease Control estimates that 58 percent of legal abortions occur within the first eight weeks of gestation, and 88 percent are performed within the first 12 weeks (based on the most recent data from 2000). Just over 10 percent are performed between 13 and 20 weeks. Less than one-half-of-one percent occur after 24 weeks.

CWD is relatively rare. In Colorado, for example, fewer than 200 cases, mostly in mule deer, have been documented since 1981. In checking hunter-harvested animals, less than 6% of deer and 1% of elk in north-central and northeastern Colorado were found to have CWD. Similar surveillance in southeastern Wyoming also indicated that less than 7% of deer had CWD. Deer and elk checked outside these areas in Colorado and Wyoming did not have CWD.

There are certain conditions in the urine including those caused by not drinking enough water, that make crystals which can tribute to stone formation. Some patients form stone because of anatomical malformations of the urinary tract, while other patient may have a genetic predisposition, environmental or nutritional risk factors.

Influenza pandemics are known to have occurred several times each century since the Middle Ages. There were three influenza pandemics in the 20th century, in 1918, 1957 and 1968. Experts believe we are overdue for the next influenza pandemic. No one can predict when a pandemic might occur, but many scientists from around the world are watching the H5N1 avian (bird) influenza situation in Asia and Europe very closely.

A "seizure" is a transient event caused by abnormal excitability of all or part of the brain. The principal cells of the brain, or "neurons", use several different types of chemicals (called neurotransmitters) to communicate with each other. These chemical messages are translated into electrical signals, which allow neurons in different parts of the brain to work together and perform tasks: to move your arm, read a book, or tell a story.

Herpes symptoms come and go, but the virus will remain in your body for life. Some people may never show symptoms, some may only show symptoms once or twice in their lives, and others may have symptoms many times each year.