Naked Science Forum
Life Sciences => The Environment => Topic started by: stuartr on 27/10/2008 17:32:42
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Hurricanes in the Northern hemisphere spin counter clockwise because of the coreolis effect.Does the same apply to smaller storms i.e tornadoes.At what scale do the twisters/dust devils etc rotate either way?
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Q. How are tornadoes in the northern hemisphere different from tornadoes in the southern hemisphere?
A. The sense of rotation is usually the opposite. Most tornadoes -- but not all! -- rotate cyclonically, which is counterclockwise in the northern hemisphere and clockwise south of the equator. Anticyclonic tornadoes (clockwise-spinning in the northern hemisphere) have been observed, however -- usually in the form of waterspouts, non-supercell land tornadoes, or anticyclonic whirls around the rim of a supercell's mesocyclone. There have been several documented cases of cyclonic and anticyclonic tornadoes under the same thunderstorm at the same time. Anticyclonically rotating supercells with tornadoes are extremely rare; but one struck near Sunnyvale, CA, in 1998. Remember, "cyclonic" tornadoes spin counter-clockwise in the northern hemisphere, and clockwise.
http://www.spc.noaa.gov/faq/tornado/
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The direct in which a tornado spins is not really down to the coriolis force. The force is noticable on the large scale, high and low pressure area's but not in the small scale. What does get a tornado spinning is wind shear.
Wind sheer
Directional wind shear is the change in wind direction with height. In the image (right), the view is looking north. The wind near the surface is blowing from the southeast to the northwest.
As the elevation increases the direction veers (changes direction in a clock-wise motion) becoming south, then southwest, and finally, west.
Speed shear is the change in wind speed with height. In the illustration below, the wind is increasing with height. This tends to create a rolling affect to the atmosphere and is believed to be a key component in the formation of mesocyclones which can lead to tornadoes.
Strong vertical shear is the combination of a veering directional shear and strong speed shear and is the condition that is most supportive of supercells.
Whe dealing with(typical) supercell storms, A updraft lifts a rotating column of air that has created by the speed shear. This provides two different rotations to the supercell; cyclonic and anti-cyclonic. Once the column of air has been lifted and rotation is taking place, the cyclonic rotation is amplified by the directional shear and thus reduces the effect of the ant cyclonic rotation. This is your typical tornado.
Some termss you may wish to look up are:
Advection
The horizontal movement of an air mass that causes changes in the physical properties of the air such as temperature and moisture. Commonly used with temperatures, i.e., "warm air advection", or moisture, i.e.,"moisture advection".
Negative Vorticity Advection
A region of negative vorticity usually several hundred of kilometers wide on a upper level chart that moves with the general wind flow. It aids in weather prediction by showing where regions of sinking air. This is typically associated with clear skies.
Positive Vorticity Advection
A region of positive vorticity usually several hundred of kilometers wide on a upper level chart that moves with the general wind flow. It aids in weather prediction by showing where regions of rising air. This usually results in clouds and precipitation.
Helicity
A property of a moving fluid which represents the potential for helical flow (i.e. flow which follows the pattern of a corkscrew) to evolve. Helicity is proportional to the strength of the flow, the amount of vertical wind shear, and the amount of turning in the flow (i.e. vorticity). Atmospheric helicity is computed from the vertical wind profile in the lower part of the atmosphere (usually from the surface up to 3 km), and is measured relative to storm motion.
The helicity is the area on a hodograph that is enclosed by a line from the tip of the storm motion vector to the surface wind vector, then following the hodograph curve to 3 km level, then back to storm motion vector. This value allows the forecaster to determine the rotational tendency of a thunderstorm. Higher values of helicity (generally, around 150 m2/s2 or more) favor the development of mid-level rotation (i.e. mesocyclones). Extreme values can exceed 600 m2/s2. It is dependent on the local environmental wind profile in which a thunderstorm develops and the thunderstorm motion.
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From the link provided by RD
Was tornado forecasting once banned in the U.S.? Yes. Before 1950, at various stages of development of the Weather Bureau, the use of the word "tornado" in forecasts was at times strongly discouraged and at other times forbidden, because of a fear that predicting tornadoes may cause panic. This was in an era when very little was known about tornadoes compared to today, by both scientists and the public at large. Tornadoes were, for most, dark and mysterious menaces of unfathomable power, fast-striking monsters from the sky capable of sudden and unpredictable acts of death and devastation. As the weather patterns which led to major tornado events became better documented and researched, the mystery behind predicting them began to clear -- a process which still is far from complete, of course. In 1950, the Weather Bureau revoked the ban (PDF) on mentioning tornadoes in forecasts.
Anyone care to guess which 9 states have not had a tornado since 1950 (the year the beather bureau / national weather service starting using the name "tornado" in forecats?
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Here are my 9 guesses!
Alaska; Hawaii; Washington; Vermont; Rhode Island; Delaware; Maine; New Hampshire; District of Columbia
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and what's a forecat? Somewhere you go to buy second hand moggies?
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All tornadoes spinn the same way, but sometimes you have to look at them from above and sometimes from underneath.
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Here are my 9 guesses!
Alaska; Hawaii; Washington; Vermont; Rhode Island; Delaware; Maine; New Hampshire; District of Columbia
Five out of nine ( alaska, hawaii, new hampshire, rhode island and vermont) well done, Stuart. Do you want to know the other four, or have one more guess?
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I wasn't doing the guessing but since you ask
nevada ?
California ?
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Ok, the complete list is:
alaska, hawaii, new hampshire, rhode island, nevada, California, idaho, vermont and oregon.
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I wasn't doing the guessing but since you ask
nevada ?
California ?
I am also a Stuart.....But I was Stuart the First!
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California hasn't had a tornado for 58 years? I'd have bet a stack on that (and obviously lost...)
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Paul - I was so surprised that California hadn't had a tornado, that I looked up a site (below). It seems to list not only California, but also the other 8 states listed. Where did you get your info from?
http://www.tornadoproject.com/alltorns/worstts.htm#CA
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I got the data from the National Climatic Data Center (http://www.ncdc.noaa.gov). Im running a programme at the moment that blocks these (all gov. sites) from being loaded! When i have finished this "task" i will look for the right page to link to.
Ps, Happy Birthday for Tomorrow.
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thanks Paul for the link and THANKS PAUL FOR THE BIRTHDAY WISH!