RANDOLPH E. SCHMID
Associated Press Writer
|
Preliminary reports rated the Maryland tornado at F5 on the Fujita scale for twisters, with winds of about 250 mph, making it the strongest tornado in the state in 75 years, according to Susan Weaver of the National Weather Service.
|
|
Tornadoes are rated from F0 to F5 on the Fujita scale, based on wind speed and damage. The scale was developed by Theodore Fujita of the University of Chicago.
- FO: Light damage, winds under 73 mph. Some damage to chimneys, branches broken off trees, shallow-rooted trees pushed over, sign boards damaged.
- F1: Moderate damage, winds 73-112 mph. Surfaces peeled off roofs, mobile homes pushed off foundations or overturned, moving autos blown off road.
- F2: Considerable damage, winds 113-157 mph. Roofs torn off frame houses, mobile homes demolished, boxcars overturned, large trees snapped or uprooted, light objects become missiles.
- F3: Severe damage, winds 158-206 mph. Roofs and some walls torn off well-constructed houses, trains overturned, most trees in forest uprooted, heavy cars lifted off ground and thrown.
- F4: Devastating damage, winds 207-260 mph. Well-constructed houses leveled, structures with weak foundations blown off some distance, cars thrown, large missiles generated.
- F5: Incredible damage, winds 261-318. Strong frame houses lifted off foundations and swept away, automobile-sized missiles fly through the air more than 100 yards, trees debarked.
|
|
WASHINGTON (AP) -- Tornadoes concentrate vicious winds, up to 300 mph, focusing their twisting forces on small areas where they can destroy everything in their path.
That power was evident Sunday when a series of storms struck from Missouri to Maryland, claiming a half-dozen lives and ruining homes and businesses.
As many as 1,000 tornadoes occur each year in the United States, yet each is uniquely dangerous and frightening when it arrives, moving quickly and often accompanied by rain, hail, thunder and lightning.
Unlike hurricanes, usually reported days before they threaten, the storms that spawn tornadoes can erupt rapidly.
Only a few decades ago tornado warnings didn't go out until a twister was on the ground, but thanks to radar and satellites the conditions that create these storms can now be better monitored. Last year the National Weather Service delivered warnings, on average, 11.5 minutes before a tornado struck.
In the storm that claimed three lives in Maryland, for example, warning of a severe storm with possible tornado was issued at 6:45 p.m. It was upgraded to a tornado warning at 7:02 p.m. Approximately eight minutes later the tornado hit La Plata and 39 minutes later it reached Calvert County.
Preliminary reports rated the Maryland tornado at F4 on the Fujita scale for twisters, with winds of about 250 mph, making it the strongest tornado in the state in 75 years, according to Susan Weaver of the National Weather Service. The scale goes from F0 to F5.
No state is exempt from tornadoes; Missouri and Illinois each average 26 twisters annually. In a normal year Kentucky faces nine tornadoes and there are four in Maryland.
Little seemed normal to those experiencing the storms, which formed along a massive front moving from west to east across the country, mixing warm and cool, wet and dry air and stirring up a witches' brew of winds.
``They appear to have been fairly prolific producers of tornadoes,'' said Harold Brooks, a research meteorologist at the government's National Severe Storms Laboratory in Norman, Okla. He said there were at least 35 twisters over the weekend.
Many were supercells, powerful storms in which the whole cloud mass rotates, wrapping the storm in rain.
It isn't that giant rotation that forms the tornado, Brooks said. ``You don't directly take that rotation 10,000 feet above the ground and somehow move that down to the ground.''
But the rotating cloud mass helps create a favorable environment for a tornado to form, he said, offering a low cloud base and strong wind shear.
Wind shear means that winds are blowing in one direction at one level and another direction above, causing the air in between to rotate.
Warm and moist air is lighter than air that is cold and dry, so the lighter stuff tends to rise upward. As the rising air cools the moisture begins to condense, producing clouds and rain.
When the warm, moist air rising from below reaches that spinning area in the wind shear it tilts the rotation and the funnel cloud forms.
Getting a better understanding of how that rotation reaches down to the ground is a major focus of research, Brooks added.
When it does reach the ground, forces in the twister become unbalanced and air is drawn in at the base of the tornado.
If conditions are just right, winds speed up as they near the center of the tornado, like a figure skater spins faster when she pulls her hands in close to her body.
While tornadoes have occurred in every state, they are most common in the center of the country, especially in Tornado Alley, a band running north from Texas through Oklahoma and Kansas.
In most cases the formation process is the same.
``If you get a good environment to make a tornado the atmosphere doesn't care where it is. It's just more common to get those environments out here,'' Brooks said in a telephone interview from his Oklahoma office.
Related:
