By Paul Homewood
It seems that tornadoes, or the lack of them, are back in the news at the moment.
American Meteorological Society President, Dr Marshall Shepherd, seems to think it is unfair to mention low tornado numbers, saying it is an “abuse”.
So I thought it worthwhile to take another look at the stats at the end of 2012.
Figure 1
Figure 2
Figure 1 shows all tornadoes above EF1. (See here, why EF1’s are excluded.) The 10-Year Trend is significantly below the level consistently seen up to 1991, although the high totals in 2011 have inevitably caused a small upwards blip.
We see a similar pattern with the stronger EF3+ tornadoes.
I do not claim to know what will happen to tornado numbers in coming years. And anyone who does is lying.
NOAA sums up the situation neatly in their FAQ.
Does “global warming” cause tornadoes? No. Thunderstorms do. The harder question may be, “Will climate change influence tornado occurrence?” The best answer is: We don’t know. According to the National Science and Technology Council’s Scientific Assessment on Climate Change, “Trends in other extreme weather events that occur at small spatial scales–such as tornadoes, hail, lightning, and dust storms–cannot be determined at the present time due to insufficient evidence.” This is because tornadoes are short-fused weather, on the time scale of seconds and minutes, and a space scale of fractions of a mile across. In contrast, climate trends take many years, decades, or millennia, spanning vast areas of the globe. The numerous unknowns dwell in the vast gap between those time and space scales. Climate models cannot resolve tornadoes or individual thunderstorms. They can indicate broad-scale shifts in three of the four favorable ingredients for severe thunderstorms (moisture, instability and wind shear), but as any severe weather forecaster can attest, having some favorable factors in place doesn’t guarantee tornadoes. Our physical understanding indicates mixed signals–some ingredients may increase (instability), while others may decrease (shear), in a warmer world. The other key ingredient (storm-scale lift), and to varying extents moisture, instability and shear, depend mostly on day-to-day patterns, and often, even minute-to-minute local weather. Finally, tornado recordkeeping itself also has been prone to many errors and uncertainties, doesn’t exist for most of the world, and even in the U. S., only covers several decades in detailed form.
It is worth reemphasizing just how short the record period is. Given that ocean cycles are around 60 years long, it is difficult to see how we can properly monitor trends with less than at least a century worth of data.
All we can reasonably say is that the long term trend, as measured over a 10-Year average, is lower than it has been for much of the period since 1970.
References
As usual, all data is from NOAA’s Storm Prediction Centre.
http://www.spc.noaa.gov/wcm/#data
Related articles
- Severe Weather Drought: Tornadoes drop to a new all time record low, major hurricane absence is setting a new record every day (wattsupwiththat.com)
- Tornado drought: only 3 U.S. tornadoes during May’s first week (wunderground.com)
- Lingering Winter Chill Suppressing Tornadoes in 2013 (livescience.com)
- Warmists Fret: Don’t Mention Low Tornado Numbers – It’s Not Fair!!! (climatedepot.com)
- U.S. Sees Record-Low Tornadoes and Tornado Deaths (climatecentral.org)
If I can throw in my amateur’s 2 cents:
Having lived in the Midwest most of my life (though not the Great Plains) I’ve paid a good bit of attention to tornadoes and storms over the years. I’ve seen many a weather map with the colliding cold air masses and the warm mist air coming up from the Gulf of Mexico. They collide often on that front from around central Texas on a line more or less NNE up through OK and KS. Sometimes that line is east a bit, sometimes west. When it is east, the collision of air masses puts the twisters into MO, IL, KY, IN and OH.
In my mind, that last key ingredient – storm-scale lift – is the trigger for tornadoes. The other three are the underlying conditions that, by themselves cannot create a tornado, only storms. Actually, the instability and wind shear are to a large degree, themselves IMHO part of the storm-scale lift. Thus the four ingredients are not, in themselves, the fundamental ingredients. The TWO fundamental ingredients are the warm, moist air mass and the cold, dense air mass. Those two are more basic than the four ingredients listed and what provide the four, more specific, ingredients.
We all know that the tornadoes are birthed at the southern end of what we call “the comma” – at its tip. I argue that there is a reason for this, and one that has not been addressed in storm science yet:
A cold air mass is a bubble that pushes under a warm air mass. In the center of that bubble the lift is more or less straight upward. However, on the edges of the bubble it is sloped, and there is a sloped component/vector where the bubble reaches down to the ground: The lift is not totally UP, but also at an angle. In the center of the air mass the lift makes for a rolling of the air into a horizontal tube and it never reaches the ground (but it may in fact be the reason for wind shear). But at the edge of the bubble the rolling tube starts out angled (visible in almost every photo and video of tornadoes), thus giving it the chance to reach down to the ground. The rolling tube starts out farther up, where the lift magnitude is greatest, but if there is enough rotational force being generated, it can entrain the air farther down and reach the ground. Many a tornado has begun higher up and then petered out before reaching the ground. The main point here is that the rolling tube that becomes a tornado forms ONLY at the sloping edge of the air mass. Its axis of rotation MUST have a tilt to it in order to reach downward into becoming a tornado. (The rolling tube in the center of the air mass cannot reach downward.)
Consistent with all that, what could be happening when warmer climate produces LESS tornadoes? My thinking is that the cold air mass is the necessary trigger, so if the cold air masses are less cold (less dense), they cannot provide enough storm-scale lift.
Steve Garcia
@Paul Homewood –
In your linked blog post you say:
Yes, but it is not just tornadoes. The ENTIRE skeptical effort is to “set the record straight” that has been skewed out of all proportions by the claims of CAGW activists (which includes the IPCC). Had the alarmists not made such bogus claims – including tornadoes – and created the entire global warming scare, the skeptics would have no reason to even exist.
Skepticism, pretty much by definition, is a reaction to weak or unfounded claims. Duh.
Steve Garcia
Glad you’ve made it harder to twist the twister data.