tornadoes

[engly-saxo]

Tornadoes come from thunderstorms, right?

Considering this, why do tornadoes largely exist in North America? Thunderstorms are present in other areas of the world, yet I seldom here of tornadoes causing damage outside of North America.

Also, is it true that the maximum "possible" speed of a tornado is 300 mph?

[EsoCyn]

Tornadoes come from when warm, moist masses of air from the South rest on top of cold, dry masses of air from the North (or may be the other way around, I get that part confused.) Now, that's enough to just get a strong thunderstorm... what's unique about what happens in tornado alley is there is a jet stream that comes through and literally stirs things up in the masses... once you get the jet stream in there, thats when you can get a tornado and thats why it mostly happens in the US more often than other places.

That and our geographical plains allow it more often than most other places in the world.

Also, there is no "maximum" possible speed... it's just that there has been no tornado thats been recorded at exceeding 320 MPH so far. As we've seen history repeat time and time again, strange shit can happen and it's never a good idea to rule out that sort of "super" tornado.

A tornado in Moore, OK got up to 318 MPH. http://www.distant.ca/UselessFacts/fact.asp?ID=51

Forgot to add this, too; The UK actually is the most "prone" to tornados when speaking of tornados per unit area.

ETA, once more; I have to apologize... even though I have a strong interest in weather, my interests have been drifting more towards lightning and hurricanes more than tornadoes lately... soooooo.... the first part of my post, it is actually the other way around. The cold, dry air sits on top of the war, most air and thats what develops a strong thunderstorm... like I've said though, what makes the storm tornadic is when you have the jetstream in there "stirring" thing sup.

[MBS]

Tornadoes come from thunderstorms, right? Considering this, why do tornadoes largely exist in North America?


Yes, but not all thunderstorms are able to produce tornadoes. The most destructive tornadoes tend to come from supercell thunderstorms.

A typical thunderstorm is formed when a large bubble of warm moist air (updraft) rises from the surface through cooler layers above. Though the updraft cools as it rises, as long as it remains warmer than the surrounding air it continues to rise. A typical thunderstorm rises up to a certain height at which point it produces a heavy shower which chokes off the original updraft by cooling the air underneath the storm. This causes the storm to dissipate. This is a single cell storm.

Often, instead of forming directly under the dissipating storm, new storms will form near the edge of dissipating storm where they can tap into air that hasn't been rain cooled yet. This would be a multi-cell storm (the most common type).

However, if there is strong wind shear (wind increasing with height), storms will tend to tilt so that most of the precipitation falls to the side of the updraft. Then rather than being choked off by rain, the updraft continues to grow and strengthen. If on top of that there is a clockwise turning (veering) of the wind direction with height in the first 3 km (for example a SE wind at ground level and a SW wind at 3 km above ground), you will get a large rotating updraft. This is a supercell, the mother of all thunderstorms. These monsters (which can be 18 km high and have 100 mph (150 kph) updrafts) almost exclusively produce the incidents of large damaging hail and killer tornadoes in the U.S.

Tornadoes exist largely in the plains and Midwest of the North America because these areas are exceptionally conducive to the formation of supercell thunderstorms. Why? There are two main geographical ingredients that come together. First, you have the Gulf of Mexico, a tropical body of water which is a source of abundent heat and humidity. Second you have the Rockie Mountains. These mountains tend to partially block the normal west-to-east low level wind flow that is common in the mid-latitudes and this tends to generate low pressure regions to the lee of the mountains. These low pressure areas often draw in warm humid air from the Gulf of Mexico in the form of a low level jet. The low level jet comes up from the south or southeast and is a source of warm humid air and also tends to create a veering wind shear profile in the first 3 km. Often a strong upper level jet stream is also associated with developing low pressure areas and this contributes the vertical shear needed for supercell development.

Sorry if that was a little long. It is the simplest accurate explanation I could come up with (believe me, it gets even more complicated).


Thunderstorms are present in other areas of the world, yet I seldom here of tornadoes causing damage outside of North America.


Actually, smaller tornadoes that aren't produced by supercell thunderstorms are relatively common in most parts of the world. However, because these storms tend to be very localized and quite weak, they aren't reported as much.

Also, there are other tornado hot spots in the world besides the U.S. In May of 1996 a huge tornado in NW Bangladesh took 800 lives. I've also heard reports of very large and destructive tornadoes occurring in both South Africa and Australia.


Also, is it true that the maximum "possible" speed of a tornado is 300 mph?

Wind speeds above 300 mph are exceedingly rare, though I don't know if there is a theoretical limit. A hurricane specialist from MIT once proposed that if the ocean reached a certain critical temperature (I think it was something like 50 deg C), you would get runaway hurricanes (called hypercanes) which would continue to intensify until they reached supersonic speeds!

[Vortex]

Considering this, why do tornadoes largely exist in North America? Thunderstorms are present in other areas of the world, yet I seldom here of tornadoes causing damage outside of North America.
Well, MBS did a pretty good job of explaining this bit — it's the geography of N. America. It allows enough of the right ingredients to meet to form supercell thunderstorms, which then spawn tornadoes. However, Bangladesh gets tornadoes with some frequency, because it has similar geography: flat land, mountains (the Himalayas) to the N, a source of warm, moist air (the Bay of Bengal) to the S. In fact, the deadliest tornado to date occurred there on April 26, 1989; it killed 1,300 people around Shaturia (or Salturia) and Manikganj. However, tornadoes also occur — just with less frequency — in Europe, E Asia, South Africa, South America (especially Argentina), and Australia. Violent (F4/F5) tornadoes have been observed in France, Holland, Belgium, Switzerland, and Australia. Possible violent tornadoes have occurred in Russia.


Also, is it true that the maximum "possible" speed of a tornado is 300 mph?
Yes, but they don't last very long — maybe a second or two, and all the recorded windspeeds in the vicinity of 300 mph have been well above ground level. Until we develop remote sensing devices (like portable Doppler radar) that can measure where in the tornado they're measuring wind values — actually measured off debris and other things being carried on the wind rather than the wind itself — we'll never know if these winds can affect us. We might also never know if they do, because the damage they cause may very well be indistinguishable from F5 damage, which is classified as the complete destruction of a well-constructed frame home, leaving no walls standing and sweeping all debris off the foundation. Only a detailed engineering study may reveal faster winds, and nowadays, that's an increasingly unlikely prospect (unfortunately).

[Mickie]

Tornadoes come from thunderstorms, right?

I have a friend from Austrialia that said one of the things he had to get used to when he moved here (Pennsylvania then Ohio) was the spring (tornado season) thunderstorms. The conversation started over talking about storm doors and how, at first, he really didn't understand the need for that second door. (Ohio isn't in tornado alley but we get our share of severe thunderstorms and tornados.)

http://www.loweslink.com/llmain/images/install_storm_door.jpeg
storm door

Spring is tornado season because is the time of year where is it most common to have the weather conditions already mentioned.

[EsoCyn]

Wind speeds above 300 mph are exceedingly rare, though I don't know if there is a theoretical limit. A hurricane specialist from MIT once proposed that if the ocean reached a certain critical temperature (I think it was something like 50 deg C), you would get runaway hurricanes (called hypercanes) which would continue to intensify until they reached supersonic speeds!

Observers say that a hypercane would be close to what you see with The Great Red Spot on Jupiter.

http://en.wikipedia.org/wiki/Great_Red_Spot

[flintknapper]

I live in the Midwest and have seen my share of tornados. One of the important indicators I look for as to whether a coming storm might produce a tornado is the wind direction. The ground wind must be from the SE, and fairly strong. AS MBS described, without this SE wind, there is less chance for rotation to start.
Another indicator is the change in the color of the sky/light. When you look out the window and notice everything has the so-called "puke green" tint, that is an indicator of high ozone. (O-3) I'm not sure what causes this, or how it relates to tornados, but this color is sometimes seen before a tornado hits.

[Yelnik McGwawa]

I live in the Midwest and have seen my share of tornados. One of the important indicators I look for as to whether a coming storm might produce a tornado is the wind direction. The ground wind must be from the SE, and fairly strong. AS MBS described, without this SE wind, there is less chance for rotation to start.
Another indicator is the change in the color of the sky/light. When you look out the window and notice everything has the so-called "puke green" tint, that is an indicator of high ozone. (O-3) I'm not sure what causes this, or how it relates to tornados, but this color is sometimes seen before a tornado hits.

Oh that green. I live in central Wisconsin and have seen 4 tornadoes live and in person. 3 went through a field behind our family's house in the same summer. Every one went the same way, the storm dies down, the sky turns green, the hail starts then debris starts flying.

[Amen-Moses]

Thunderstorms are present in other areas of the world, yet I seldom here of tornadoes causing damage outside of North America.



Tornados are very common here in the UK, in fact we are the world number one when it comes to sheer number per year but that might be because we have vastly more weather stations to pick them up than almost anywhere else in the world.

As to why they are less destructive here as opposed to say Kansas, it is mainly down to distance from the equator and geography; storms are stronger closer to the equator and longer lived where large flat plains exist. The longer lived a storm is the more power it can put into tornado formation.

When I lived in East Anglia it was quite common for large storms to result in the destruction of farm buildings or damage to crops due to tornado formation but they were usually short lived phenomena, down here on the coast the tornados form out in the Solent and in the Channel and only cause damage if they come ashore. One did come ashore a few years ago and decimated a local town.

Here is a link to the UK tornado study site with photos:

http://www.torro.org.uk/TORRO/php/gallery.php?category_path=tornados&start=0

Amen-Moses

[webwide]

I live in the Midwest and have seen my share of tornados. One of the important indicators I look for as to whether a coming storm might produce a tornado is the wind direction. The ground wind must be from the SE, and fairly strong. AS MBS described, without this SE wind, there is less chance for rotation to start.
Another indicator is the change in the color of the sky/light. When you look out the window and notice everything has the so-called "puke green" tint, that is an indicator of high ozone. (O-3) I'm not sure what causes this, or how it relates to tornados, but this color is sometimes seen before a tornado hits.
The green is sunlight refracted through hail.

And while SE winds are ideal, it really has to do with the angle between surface winds and the next layer up. So even when winds are out of the South if a storm cell turns from a NE direction to due East or ESE this creates the same difference of angle as a NE storm with SE surface winds. So if all the storms are moving NE and one of them turns to the right, that's the one to watch out for. Or, if you're a storm chaser, that's the one to HEAD for. :D

[Amen-Moses]

From my link above:

"Some tornadoes form out to sea as strong waterspouts (q.v.) which sometimes cross the coast, so a waterspout may become a tornado as the twisting funnel moves from land to sea (and vice-versa). A recent powerful and well-documented example is that of Selsey on the south coast of England on the night of 7 to 8 January 1998. When the waterspout made landfall, it carved a trail of damage a kilometer wide through the town as it damaged hundreds of buildings in less than ten minutes."

That was only a few miles down the coast from me and I remember that night well.

Amen-Moses

[Amen-Moses]

Tornadoes come from thunderstorms, right?



Actually they can form without there being any thunder as such.

When you pull out the plug in your bath the water spirals as it exits, this is the coriolis effect caused by the earths spin. Up here in the almost artic this effect creates a tight spiral whereas down near the equator it forms a looser spiral, that is why Hurricanes near the equator have large "eyes" which tighten up as they travel north and why tornados down in the southern US states can form huge "eyes" whereas up here they are smaller.

Basically what happens is that a large amount of air is suddenly driven up or suddenly drops down and as it does so it spirals just like the bathwater. The driving force is simply air pressure differences (which can be caused by lightning) as blocks of air at different temperatures collide. If the air contains a lot of moisture (or dust/sand) then the resulting spiral formed can have a lot of mass and hence a lot of energy, they are the dangerous ones.

Amen-Moses

[flintknapper]

The green is sunlight refracted through hail.

And while SE winds are ideal, it really has to do with the angle between surface winds and the next layer up. So even when winds are out of the South if a storm cell turns from a NE direction to due East or ESE this creates the same difference of angle as a NE storm with SE surface winds. So if all the storms are moving NE and one of them turns to the right, that's the one to watch out for. Or, if you're a storm chaser, that's the one to HEAD for. :D


Thanks! I'll keep that in mind, and amend my 'SE only' observation. :thumbs:

[Mageth]

When you pull out the plug in your bath the water spirals as it exits, this is the coriolis effect caused by the earths spin. Up here in the almost artic this effect creates a tight spiral whereas down near the equator it forms a looser spiral,

Umm, nope. The coriolis effect caused by the earth's spin only comes into play at a much larger scale. It has no effect on the water draining from your bath. In other words, the water draining from my bath here in the temperate zone does so with the same spiral as yours in the arctic zone. (It's also a myth that the coriolis effect causes baths to drain one way in the southern hemisphere and the opposite way in the northern hemisphere).

...that is why Hurricanes near the equator have large "eyes" which tighten up as they travel north and why tornados down in the southern US states can form huge "eyes" whereas up here they are smaller.

That may be true for hurricanes (that the coriolis effect "tightens up" hurricanes as they move north), because hurricanes are on a large enough scale, but I seriously doubt that the coriolis effect effects tornadoes in that way at all. Tornadoes are simply of too small a scale to be effected in such a way by the coriolis effect.

[Amen-Moses]

Umm, nope. The coriolis effect caused by the earth's spin only comes into play at a much larger scale. It has no effect on the water draining from your bath. In other words, the water draining from my bath here in the temperate zone does so with the same spiral as yours in the arctic zone. (It's also a myth that the coriolis effect causes baths to drain one way in the southern hemisphere and the opposite way in the northern hemisphere).


Well you should tell the BBC then as they demonstrated the effect on a documentary (Pole to Pole IIRC), they demonstrated that just north of the equator the water spiraled one way and just south of it it went the other, then they sat right on the equator and showed it went straight down with no spiral.

It really was quite a cool demonstration.

Amen-Moses

[Mageth]

Well you should tell the BBC then as they demonstrated the effect on a documentary (Pole to Pole IIRC), they demonstrated that just north of the equator the water spiraled one way and just south of it it went the other, then they sat right on the equator and showed it went straight down with no spiral.

It really was quite a cool demonstration.

Amen-Moses

I'm sure it was, but it's a myth nonetheless.

http://www.straightdope.com/classics/a1_161

http://www.loc.gov/rr/scitech/mysteries/coriolis.html

http://www.madsci.org/posts/archives/dec96/843673964.Es.r.html

http://itotd.com/index.alt?ArticleID=498

Here's an interesting discussion on tornadoes and the coriolis effect:

http://www.newton.dep.anl.gov/newton/askasci/1993/general/GEN016.HTM

[Amen-Moses]

I'm sure it was, but it's a myth nonetheless.

http://www.straightdope.com/classics/a1_161

http://www.loc.gov/rr/scitech/mysteries/coriolis.html

http://www.madsci.org/posts/archives/dec96/843673964.Es.r.html

http://itotd.com/index.alt?ArticleID=498

Here's an interesting discussion on tornadoes and the coriolis effect:

http://www.newton.dep.anl.gov/newton/askasci/1993/general/GEN016.HTM

OK, I'll amend that to say:

"With perfectly still water in your sink and a perfectly smooth sink and a perfectly circular drain when you pull out the plug ... etc etc". :Cheeky:

Presumably the reason a tornado spins in a particular direction is because the entire air block is turning in that direction to begin with, maybe it's more like the classic ice skater with the outstretched arms analogy.

Amen-Moses

[Loren Pechtel]

Well you should tell the BBC then as they demonstrated the effect on a documentary (Pole to Pole IIRC), they demonstrated that just north of the equator the water spiraled one way and just south of it it went the other, then they sat right on the equator and showed it went straight down with no spiral.

It really was quite a cool demonstration.

Amen-Moses

You mean they fell for that scam? I have a memory of some travel show debunking it.

Yes, they start out with water that's been allowed to settle so there's no spinning. It's sitting at the equator. The guy picks up the containers of water and positions them for the demonstration. With the northern hemisphere container he *TURNS* north. Likewise with the southern hemisphere container he *TURNS* south.

What happens when you turn? You start the water spinning!

[youngalexander]

Tornadoes come from thunderstorms, right?

Considering this, why do tornadoes largely exist in North America? Thunderstorms are present in other areas of the world, yet I seldom here of tornadoes causing damage outside of North America.
There has been much excellent discussion re the generation of tornadoes etc., but little addressing this question. As has been pointed out such events are widespread. The reason that you do not hear much about tornadoes outside NAmerica is that they are called either hurricanes or cyclones in other regions of earth.

Also, there are cyclones and cyclones. Generally speaking cyclones are low pressure systems rotating CW in southern hemisphere and ACW in north. Theoretically however the rotation may be opposite to this. Evidentally this does occur (rarely) as has been mentioned.

Altho cyclonic events can cause severe storms, the really nasty systems such as Katrina are generated near the equator and above warm moist sea water. These are called Tropical Cyclones in Australia. We had one pass thru Darwin the capitol of Northern Territory a quarter of a century ago. Completely flattened it requiring a total rebuild.

[Vortex]

There has been much excellent discussion re the generation of tornadoes etc., but little addressing this question. As has been pointed out such events are widespread. The reason that you do not hear much about tornadoes outside NAmerica is that they are called either hurricanes or cyclones in other regions of earth.

Also, there are cyclones and cyclones. Generally speaking cyclones are low pressure systems rotating CW in southern hemisphere and ACW in north. Theoretically however the rotation may be opposite to this. Evidentally this does occur (rarely) as has been mentioned.

Altho cyclonic events can cause severe storms, the really nasty systems such as Katrina are generated near the equator and above warm moist sea water. These are called Tropical Cyclones in Australia. We had one pass thru Darwin the capitol of Northern Territory a quarter of a century ago. Completely flattened it requiring a total rebuild.
This is not true. Tornadoes are an entirely different kind of storm than tropical cyclones (the general meteorological name for the storms called hurricanes or typhoons regionally). The big difference is, tornadoes are local storms — typically a few hundred feet wide, or up to a mile or two at most. Tropical cyclones are huge storms several hundred miles across, whose formation and structure are quite different to tornadoes.

It is probably because of tornadoes' size that more aren't reported around the world — or indeed in the U.S.; they can easily pass through an uninhabited area and go completely undetected and unreported. Most countries don't have storm spotter networks, which have — in the U.S. — seen the number of tornadoes observed every year increase from 5–600 in the 1950s to 1,400 or more now.

[youngalexander]

This is not true. Tornadoes are an entirely different kind of storm than tropical cyclones (the general meteorological name for the storms called hurricanes or typhoons regionally). The big difference is, tornadoes are local storms — typically a few hundred feet wide, or up to a mile or two at most. Tropical cyclones are huge storms several hundred miles across, whose formation and structure are quite different to tornadoes.
Agreed, but I thort that the original post was refering to Katrina and not the more localised events. Perhaps I was wrong. Also neglected to mention typhoons.

[Amen-Moses]

You mean they fell for that scam? I have a memory of some travel show debunking it.

Yes, they start out with water that's been allowed to settle so there's no spinning. It's sitting at the equator. The guy picks up the containers of water and positions them for the demonstration. With the northern hemisphere container he *TURNS* north. Likewise with the southern hemisphere container he *TURNS* south.

What happens when you turn? You start the water spinning!

From what I remember of Pole to Pole there was no moving or turning involved, that was inferrred as the only way he could of faked it.

When I tried it for myself last night it didn't make a blind bit of difference which way I started the water moving, sometime it would spiral in that direction and others it would go the other way but I did prove to myself that it didn't always go in one direction so I reckon that a small basin or sink is too prone to chaos to detect any bias. What I might do when I have the time is leave a large bathful of water to stand for a few hours and then pull the plug, if I repeat that experiment a number of times and note the direction (I'll arrange for the egress to have only a single circular hole as well) then if there is a bias I'd be hard pressed to discount the coriolis effect as being a contributing cause of the bias.

Amen-Moses

[Mageth]

What I might do when I have the time is leave a large bathful of water to stand for a few hours and then pull the plug, if I repeat that experiment a number of times and note the direction (I'll arrange for the egress to have only a single circular hole as well) then if there is a bias I'd be hard pressed to discount the coriolis effect as being a contributing cause of the bias.

Amen-Moses

You'd have to do that experiment with a statistically significant number of tubs, though. Yours might be biased to spin one way or the other.

In any case, the coriolis effect simply is not significant enough at such a micro level. Period.

[Sarpedon]

Elsewhere they are often referred to as 'cyclones'.

[MBS]

Hurricanes, typhoons, cyclones are all the same phenomena. The universal scientific term for these storms is "tropical cyclone" because these storms derive thier energy from warm tropical oceans.

The comma shaped storms which circle the poles and produce your typical gales, rainstorms, and snowstorms are called "extratropical cyclones" or "shortwaves", though they more commonly refered to as just "lows" or "storm systems" by TV metoerologists. They are several times larger in scale (but usually much less intense) than the average tropical cyclone. These storms derive their energy from temperature contrasts between the arctic regions and the subtropical regions. They are also stronger and more common in winter and spring, while tropical cyclones are stronger and more common during summer and fall.

Tornadoes, on the other hand are totally different phenomena, 1000-100,000 times smaller in scale. They derive their energy from much more localized atmospheric disturbances.

These are the three most recognized types of cyclonic whirls that form in the atmosphere. There are other less common types of storms such as polar cyclones (these are similar in structure to tropical cyclones but form over the polar oceans). Also some smaller tornadoes do not bear much resemblance in structure or genesis condition to the much larger/taller supercell spawned tornadoes, and thus should probably be classified in thier own groups. I'm talking mostly about waterspouts and dust devils (not sure what they call these in other parts of the world, they're large whirls of dust which form over arid regions).

[MBS]

You'd have to do that experiment with a statistically significant number of tubs, though. Yours might be biased to spin one way or the other.

In any case, the coriolis effect simply is not significant enough at such a micro level. Period.

I think a swimming pool is the smallest body of water in which the coriolos force has been statistically measured. You would have to waste a heck of a lot of water if you were to try it yourself though.

[Vortex]

Presumably the reason a tornado spins in a particular direction is because the entire air block is turning in that direction to begin with, maybe it's more like the classic ice skater with the outstretched arms analogy.
This probably has more to do with the airflow patterns around and beneath (i.e. on the ground) the updraft base of the parent supercell — things like inflow, outflow from the precipitation, wind shear. A good example of how this may work is shown by a tornado family at Grand Island, NE, on June 3, 1980. There, the supercell unusually stood still while it produced 7 tornadoes. The tornadoes couldn't just move in a fairly straight line with the supercell in whatever direction, and instead they took all manner of crazy paths (http://www.hprcc.unl.edu/nebraska/twistermap-grandisland-gii.html) — and, three of them were anticyclonic. Storm chasers have observed small anticyclonic vortices forming around cyclonic tornadoes, too.