Are tornadoes getting stronger and more frequent?

By: Dr. Jeff Masters , 4:24 PM GMT on February 26, 2008

Are tornadoes and severe thunderstorms getting more numerous and more extreme due to climate change? To help answer this question, let's restrict our attention to the U.S., which has the highest incidence of tornadoes and severe thunderstorms of any place in the world. At a first glance, it appears that tornado frequency has increased in recent decades (Figure 1).

Figure 1. The number of EF-0 (blue line) and EF-1 and stronger tornadoes (maroon diamonds) reported in the U.S. since 1950. There is not a decades-long increasing trend in the numbers of tornadoes stronger than EF-0, implying that climate change, as yet, is not having a noticeable impact on U.S. tornadoes. However, statistics of tornado frequency and intensity are highly uncertain. Major changes in the rating process occurred in the mid-1970s (when all tornadoes occurring prior to about 1975 were retrospectively rated), and again in 2001, when scientists began rating tornadoes lower because of engineering concerns and unintended consequences of National Weather Service policy changes. According to Brooks (2013), "Tornadoes in the early part of the official National Weather Service record (1950-approximately 1975) are rated with higher ratings than the 1975 - 2000 period, which, in turn, had higher ratings than 2001 - 2007." Also, beginning in 2007, NOAA switched from the F-scale to the EF-scale for rating tornado damage, causing additional problems with attempting to assess if tornadoes are changing over time. Image credit: Kunkel, Kenneth E., et al., 2013, "Monitoring and Understanding Trends in Extreme Storms: State of Knowledge," Bull. Amer. Meteor. Soc., 94, 499–514, doi:

However, this increase may be entirely caused by factors unrelated to climate change:

1) Population growth has resulted in more tornadoes being reported.

2) Advances in weather radar, particularly the deployment of about 100 Doppler radars across the U.S. in the mid-1990s, has resulted in a much higher tornado detection rate.

3) Tornado damage surveys have grown more sophisticated over the years. For example, we now commonly classify multiple tornadoes along a damage path that might have been attributed to just one twister in the past.

Given these uncertainties in the tornado data base, it is unknown how the frequency of tornadoes might be changing over time. The "official word" on climate science, the 2007 United Nations IPCC report, stated it thusly: "There is insufficient evidence to determine whether trends exist in small scale phenomena such as tornadoes, hail, lighting, and dust storms." Furthermore, we're not likely to be able to develop methods to improve the situation in the near future.The current Doppler radar system can only detect the presence of a parent rotating thunderstorm that often, but not always, produces a tornado. Until a technology is developed that can reliably detect all tornadoes, there is no hope of determining how tornadoes might be changing in response to a changing climate. According to Doswell (2007): I see no near-term solution to the problem of detecting detailed spatial and temporal trends in the occurrence of tornadoes by using the observed data in its current form or in any form likely to evolve in the near future.

Are strong tornadoes increasing?
Stronger tornadoes (greater than EF-0 on the Enhanced Fujita Scale, or F0 on the pre-2007 Fujita Scale) are more likely to get counted, since they tend to cause significant damage along a long track. Thus, the climatology of these tornadoes may offer a clue as to how climate change may be affecting severe weather. Unfortunately, we cannot measure the wind speeds of a tornado directly, except in very rare cases when researchers happen to be present with sophisticated research equipment. Tornadoes are categorized using the Enhanced Fujita (EF) scale, which is based on damage (note that the EF scale to rate tornadoes was adopted in 2007, but the transition to this new scale still allows valid comparisons of tornadoes rated, for example, EF-5 on the new scale and F-5 on the old scale.) So, if a strong tornado happens to sweep through empty fields and never destroy any structures, it will never be rated as a strong tornado. Thus, if the number of strong tornadoes has actually remained constant over the years, we should expect to see some increase in these twisters over the decades, since more buildings have been erected in the paths of tornadoes. However, if we look at the statistics of U.S. tornadoes stronger than EF-0 or F-0 since 1950, there does not appear to be any increase in their number. Not surprisingly, a study accepted for publication in Environmental Hazards (Simmons et al., 2012) found no increase in tornado damages from 1950 - 2011, after normalizing the data for increases in wealth and property (note, though, that I am suspicious of studies that normalize disaster data, since they are prone to error, as revealed by a 2012 study looking at storm surge heights and damages.)

The future of tornadoes
An alternate technique to study how climate change may be affecting tornadoes is look at how the large-scale environmental conditions favorable for tornado formation have changed through time. Moisture, instability, lift, and wind shear are needed for tornadic thunderstorms to form. The exact mix required varies considerably depending upon the situation, and is not well understood. However, Brooks (2003) attempted to develop a climatology of weather conditions conducive for tornado formation by looking at atmospheric instability (as measured by the Convective Available Potential Energy, or CAPE), and the amount of wind shear between the surface and 6 km altitude. High values of CAPE and surface to 6 km wind shear are conducive to formation of tornadic thunderstorms. The regions they analyzed with high CAPE and high shear for the period 1997-1999 did correspond pretty well with regions where significant (F2 and stronger) tornadoes occurred. The authors plan to extend the climatology back in time to see how climate change may have changed the large-scale conditions conducive for tornado formation. Riemann-Campe et al. (2009) found that globally, CAPE increased significantly between 1958 - 2001. However, little change in CAPE was found over the Central and Eastern U.S. during spring and summer during the most recent period they studied, 1979 - 2001. A preliminary report issued by NOAA’s Climate Attribution Rapid Response Team in July 2011 found no trends in CAPE or wind shear over the lower Mississippi Valley over the past 30 years. However, preliminary work by J. Sander of Munich Re insurance company, presented at the December 2011 American Geophysical Union meeting in San Francisco, found that the number of days with very high CAPE values over the eastern two-thirds of the United States between 1970 and 2009 did increase significantly.

Del Genio et al.(2007) used a climate model with doubled CO2 to show that a warming climate would make the atmosphere more unstable (higher CAPE) and thus prone to more severe weather. However, decreases in wind shear offset this effect, resulting in little change in the amount of severe weather in the Central and Eastern U.S. late this century. The speed of updrafts in thunderstorms over land increased by about 1 m/s in their simulation, though, since upward moving air needed to travel 50-70 mb higher to reach the freezing level. As a result, the most severe thunderstorms got stronger. In the Western U.S., the simulation showed that drying led lead to fewer thunderstorms, but the strongest thunderstorms increased in number by 26%, leading to a 6% increase in the total amount of lighting hitting the ground each year. If these results are correct, we might expect more lightning-caused fires in the Western U.S. late this century, due to enhanced drying and more lightning.

Using a high-resolution regional climate model (25 km grid size) zoomed in on the U.S., Trapp et al. (2007) and Trapp et al. (2009) found that the decrease in 0-6 km wind shear in the late 21st century would more than be made up for by an increase in instability (CAPE). Their model predicted an increase in the number of days with high severe storm potential for almost the entire U.S., by the end of the 21st century. These increases were particularly high for many locations in the Eastern and Southern U.S., including Atlanta, New York City, and Dallas (Figure 3). Cities further north and west such as Chicago saw a smaller increase in the number of severe weather days.

Figure 3. Number of days per year with high severe storm potential historically (blue bars) and as predicted by the climate model (A2 scenario) of Trapp et al. 2007 (red bars).

We currently do not know how tornadoes and severe thunderstorms may be changing due to changes in the climate, nor is there hope that we will be able to do so in the foreseeable future. At this time, it does not appear that there has been an increase in U.S. tornadoes stronger than EF-0 in recent decades. Preliminary research using climate models suggests that we may see an increase in the number of severe storms capable of producing tornadoes over the U.S. late this century. However, this research is just beginning, and much more study is needed to confirm these findings.

Brooks, H.E., 2013, "Severe thunderstorms and climate change," Atmospheric Research, Volume 123, 1 April 2013, Pages 129–138,

Brooks, H.E., J.W. Lee, and J.P. Craven, 2003, "The spatial distribution of severe thunderstorm and tornado environments from global reanalysis data", Atmospheric Research Volumes 67-68, July-September 2003, Pages 73-94.

Doswell, C.A., 2007, "Small Sample Size and Data Quality Issues Illustrated Using Tornado Occurrence Data", E-Journal of Severe Storms Meteorology Vol 2, No. 5 (2007).

Del Genio, A.D., M-S Yao, and J. Jonas, 2007,
Will moist convection be stronger in a warmer climate?, Geophysical Research Letters, 34, L16703, doi: 10.1029/2007GL030525.

Kunkel, Kenneth E., et al., 2013, "Monitoring and Understanding Trends in Extreme Storms: State of Knowledge," Bull. Amer. Meteor. Soc., 94, 499–514, doi:

Marsh, P.T., H.E. Brooks, and D.J. Karoly, 2007, Assessment of the severe weather environment in North America simulated by a global climate model, Atmospheric Science Letters, 8, 100-106, doi: 10.1002/asl.159.

Riemann-Campe, K., Fraedrich, K., and F. Lunkeit, 2009, Global climatology of Convective Available Potential Energy (CAPE) and Convective Inhibition (CIN) in ERA-40 reanalysis, Atmospheric Research Volume 93, Issues 1-3, July 2009, Pages 534-545, 4th European Conference on Severe Storms.

Simmons, K.M., Dutter, D., and Pielke, R., 2012, "Normalized Tornado Damage in the United States: 1950-2011," DOI: 10.1080/17477891.2012.738642

Trapp, R.J., N.S. Diffenbaugh, H.E. Brooks, M.E. Baldwin, E.D. Robinson, and J.S. Pal, 2007, Severe thunderstorm environment frequency during the 21st century caused by anthropogenically enhanced global radiative forcing, PNAS 104 no. 50, 19719-19723, Dec. 11, 2007.

Trapp, R. J., Diffenbaugh, N. S., & Gluhovsky, A., 2009, "Transient response of severe thunderstorm forcing to elevated greenhouse gas concentrations," Geophysical Research Letters, 36(1).

Jeff Masters

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243. Patrap
8:20 AM CST on February 29, 2008
120 hour GOM SSt model

Member Since: July 3, 2005 Posts: 449 Comments: 140098
242. Patrap
8:16 AM CST on February 29, 2008
One can find their Local 7 day forecast by inserting your Location or Zip Code in the Box on top Left of this page. Forecasts for any location worldwide.
Member Since: July 3, 2005 Posts: 449 Comments: 140098
241. yamil20
2:08 PM GMT on February 29, 2008
good morning everyone,what are the chances of south florida getting severe weather early next week?
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240. surfmom
2:03 PM GMT on February 29, 2008
$4.00 for Gas! Glad I have a VW diesel, never the less, makes those surf safari trips to the east coast rather expensive.

The people I really feel for are the farmers - for me in the horse industry (POLO) it is now starting to really hurt. Feed, hay, alfalfa flakes, shavings,these prices are escalating every month - some times weekly. The drought intensifies this further. Many people have been serious hurt as they figured barn fee contracts based on numbers that have gone through the roof. Polo is a wealthy enclave, up until this time I have noticed that most of these people have been insulated (and very callous & unaware) until recently. Now I am observing people who are normally clueless about the world outside their comfort zone start to feel what I have been feeling every time I go to buy groceries. Maybe now they will understand what the worker is getting paid in regard to what it costs to fill my tank. The salary is not keeping up with my diesel tank or my grocery bill. Very interesting times ahead.
Member Since: July 18, 2007 Posts: 30 Comments: 26536
239. weathermanwannabe
8:55 AM EST on February 29, 2008
Good Morning All......For our friends in the SE/Along the Gulf, enjoy the nice weather over the weekend, but, as Storm has noted (and I find it interesting that local weathermen in North Florida are already talking about the possibility of a "bumpy ride" on Monday-Tuesday), SPC is predicting a high 30% probability of a severe weather outbreak as the next front crosses LA-AL-FL-GA starting on Monday......As always, keep a NOAA radio handy and it might be a good idea (if you live in a sturdy structure)to let friends and family who might live in moble homes know that they would be welcome to "stay over" on Monday if the need arises.....We'll probably have a pretty good idea of what is coming (and things can change)by mid-Monday and I'm sure that the Blog will be busy on Monday.....Take Care All and I'll See You Next Week.......
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238. surfmom
1:49 PM GMT on February 29, 2008
Aloha & Gmorning. Woke to 47 degrees here in SRQ, cranky cause I am tired of being COLD - WFL surfers look to have an exciting week coming up. Get your boards ready. The front on tues should deliver some great midweek waves. Monday starts the building (1-2ft)Tuesday promises bigger waves, but expect that w/windy conditions and squally weather, Now if I understand this correctly.... We are looking at 6ft. on wednesday --still hard for me to see that in the Gulf w/out Wilma, but that seems to be what they are calling for. (i am going to check this further) a steady drop after that with waves expected to be 2-3ft. Check (as I will) Magic seaweed to verify wave heights and plan your skip school/skip work excuses accordingly. Presently the Gulf is 64 degrees - cooled off three degrees since this last front.
Member Since: July 18, 2007 Posts: 30 Comments: 26536
235. BahaHurican
11:03 PM EST on February 28, 2008
Evening everybody.

Just checking in. The cold front went through here last night. It was really cold last night after the leading edge passed. However this morning it was surprisingly warm, perhaps because there was relatively little wind. It's supposed to warm up considerably by Saturday, and I can see that as quite a reasonable happening.

More tomorrow. G'night!
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4:05 AM GMT on February 29, 2008
just scaning atlantic basin and near 27n 32.9w nice little spin nothing of coarse but odd
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233. tiggeriffic
3:51 AM GMT on February 29, 2008
everyone got quiet all of a sudden
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232. tiggeriffic
3:44 AM GMT on February 29, 2008

I was in Charleston, SC during Hugo. It was like nothing I have ever seen nor wish to see again.
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231. hahaguy
10:30 PM EST on February 28, 2008
god 23 the sound of that wind on that video is bone chilling. i get the same sounds when high i get high winds. In between my and my neighbors house it is exactly like a wind tunnel
Member Since: August 12, 2007 Posts: 2 Comments: 2838
2:55 AM GMT on February 29, 2008
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229. kmanislander
2:51 AM GMT on February 29, 2008
Well I am off for tonight.Time to catch up with the news ( like there is anything other than politics right !! )

Look forward to chatting with you all again real soon
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228. kmanislander
2:48 AM GMT on February 29, 2008
Hi Pottery

Great to see so many on tonight during the " off season ". Nice and cool and windy here. So unlike summer. By late march we are in the peak of the drought with climbing temps. Have to enjoy the next 3 weeks or so
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227. kmanislander
2:44 AM GMT on February 29, 2008
Sorry Keeper I was off looking at something online. I am waiting on price quotes for the Andersen 400 series window that has different specs depending on the design pressure tolerance of the glass ( which comes in varying thicknesses ). There are so many to choose from now, like CGI, PGT etc

Suffice it to say that I will be buying the strongest available !
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2:28 AM GMT on February 29, 2008
imo more frequent cat 5's are goin to occur and they will become quicker in developing when they do occur and we have already been a witness to that lets see if pattern continues over the coming seasons
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225. pottery
10:29 PM AST on February 28, 2008
Good evening. Good to see you Kman.
Great weather here in Trinidad. Dry season setting in slowly, and still occasional showers to keep the fires away.
Hoping for a tranquil Caribbean Hurricane season come June and beyond.......
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224. hurricane23
9:34 PM EST on February 28, 2008
One of my favorite videos off all time recorded from a good friend.Peak winds of 135mph are observed along with gusts to 165.

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223. hurricane23
9:28 PM EST on February 28, 2008
The great Hugo was the highlight of the year.The flooding from allison and Chantal were significant events also.
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2:20 AM GMT on February 29, 2008
how thick is that glass kman
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220. hurricane23
9:21 PM EST on February 28, 2008
Kman check your mail!Thanks
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219. kmanislander
2:18 AM GMT on February 29, 2008
I do recall earlier seasons where cold front activity was down prior to the hurricane season and even though water temps were up overall activity was down. Perhaps there is a correlation between reduced cold front activity and a pre season La Nina coupled with reduced hurricane activity.

Will be interesting to see how this plays out this yr.
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2:10 AM GMT on February 29, 2008
waters off of africa are warming as if a show of a warning along east as well sst's from ny all the way to southern waters newfoundland are high which i find strange so late in winter
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217. kmanislander
2:13 AM GMT on February 29, 2008
Hi H23

Good to see you. I agree. Last year was unusual with Dean and Felix.
I was commenting earlier that we have not had more than a couple of cold fronts in the NW Caribbean this "winter"
Those fronts take water temps down with relatively cold temps as well as from some degree of upwelling from high winds.

By the end of March we typicaaly have had all the fronts that we will see so that bears some watching for SST's
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216. hurricane23
9:15 PM EST on February 28, 2008
A strong pre-season La Nina has never resulted in a high activity tropical season.About Ninety percent of high activity seasons occur under ENSO neutral pre-season conditions.We'll see what the coming months bring.A good 6 months for the meat of the season.
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215. hurricane23
9:13 PM EST on February 28, 2008
Lots of different views on this seasons activity but the vibe iam getting is a pretty normal season number wise with similar areas being affected.Rather cool MDR as of right now.
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214. hurricane23
9:10 PM EST on February 28, 2008
Doubt you'll see 2 Cat 5's rolling through the caribbean this season.
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213. kmanislander
2:07 AM GMT on February 29, 2008
Hi there keeper

I know what you mean. Luckily all my windows are long and narrow (90 x 24 and 78 x 24 inch wide )so they are naturally rigid to begin with.Looking at Anderson windows for replacements

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2:01 AM GMT on February 29, 2008
kman dont be fooled by that so called hurricane proof stuff they also claimed the titanic unsinkable better be at least 200mph + rated
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211. kmanislander
2:02 AM GMT on February 29, 2008
I don't normally blog off season but do lurk from time to time until June. Thought I would check in and say hi. Soon the " pre season " analysis will begin and it will be prediction time LOL

Sure don't want to see 2 CAT 5's treking through here again this year

Will check back from time to time. Watch out for those tornados. What a year its been for those. At least we get days of warning for tropical systms
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210. kmanislander
1:55 AM GMT on February 29, 2008
I have shutters but they are recessed ply wood with dead bolts into the side of the sill.

With a 2 storey home I need help with ladders and putting them up requires relatively calm winds. Too much work and somewhat dangerous on ladders if the wind is kicking up.

Plus Ivan ruined my windows with salt water so now is a good time to change them out
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209. auburn (Mod)
7:58 PM CST on February 28, 2008
Are tornadoes getting stronger and more frequent?

Nope ..just mother nature doing her thing I think...LOL
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208. hahaguy
8:59 PM EST on February 28, 2008
i guess they're not worried about the buoys
Member Since: August 12, 2007 Posts: 2 Comments: 2838
207. kmanislander
1:52 AM GMT on February 29, 2008
I was checking the buoys to the S of the Caymans and would you believe that the two that were out of service from around may last year are STILL out.This is almost a year now that they have not been scheduled for maintenance

You would think that two buoys in the Central and NW Caribbean would be priority items
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206. hahaguy
8:52 PM EST on February 28, 2008
lol hey thank god shutters arent that hard to put up
Member Since: August 12, 2007 Posts: 2 Comments: 2838
205. kmanislander
1:49 AM GMT on February 29, 2008

Well at least I just ordered hurricane impact resistant windows for my home. Now I can sit inside and look out while I am panicing LOL
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204. hahaguy
8:46 PM EST on February 28, 2008
kmanislander , another worry to worry about for a few months lol
Member Since: August 12, 2007 Posts: 2 Comments: 2838
203. kmanislander
1:41 AM GMT on February 29, 2008
Hi all

Nice cold front in the NW Caribbean today. We have not had many this year and if this pattern holds the Caribbean will start the 08 hurricane season with above average water temps.
Hard to believe we are only 3 months away from June 1 st !
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7:48 PM EST on February 28, 2008
201. severstorm
Hope you brought the Zephyrs inside.
Member Since: October 13, 2005 Posts: 4 Comments: 2156
201. severstorm
7:32 PM EST on February 28, 2008
Hey all, Just checking the weatherstation and to my surprise it got down to 28.6 here in zephyrhills FL.last night Brrrr.
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200. atmoaggie
10:40 PM GMT on February 28, 2008
Hello gang, havent been here since Hurricane Season ended. I was curious as to what have been the Pacific and Atlantic ocean temps thus far. Are we above or below average?

The current anomaly relative to the 1985-1993 (omitting 1991 & 1992) climatology shows a warm GoM, a cool Caribbean and a cold Pacific except for some of Central America. I personally think the anomaly-relative-to-some-other-measurement does not get enough play here and elsewhere. "Anomalous relative to what?" should cruise through your minds every time the anomalous word comes up. I am certain I could build a dataset that show a cool anomaly in the GoM right now.

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199. Floodman
10:36 PM GMT on February 28, 2008
193. groundswell 9:25 PM GMT on February 28, 2008
OK-climate change. But with a worldwide population explosion continuing, there would seem to be little hope of bringing emissions under control. Everybody wants a nice, comfortable life, and that costs.

Research, gs...we've knwon about these issues for a very long time. We should have been researching alternatives this whole time...oh, well *SIGH*
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198. Floodman
10:34 PM GMT on February 28, 2008
Howdy, shen...NEWx, only logic will see us through the times ahead
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197. weathermanwannabe
4:56 PM EST on February 28, 2008
Hey All........Finally been getting some frosty shots of air around the Gulf areas to keep the local waters cool a bit longer............Dropping back down to the upper 20's tonight in the Florida Panhandle...See Yall Tommorow.........
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196. NEwxguy
9:53 PM GMT on February 28, 2008
Flood,I miss your voice of logic.
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195. pottery
5:25 PM AST on February 28, 2008
Howdy all.
Some good posts recently, and some bad ones too. The GW 's and the Status Quo keepers.
Its all good in the end.
By the way, realy nice weather here at 11n 61w, with dry season setting in. Cool, breezy days with occasional showers and all the trees flowering, and losing leaves.
Cutting fire traces and topping up watertanks in preparation for April and May, when the fires rage.
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194. ShenValleyFlyFish
4:09 PM EST on February 28, 2008
Hi Floodman longtime no see.


I'm beginning to believe that research might show that any upward trends in recorded temperatures might be demonstrated to correlate with the membership trends on WU. It seems that whenever someone rubs the GW/CC chestnut this site starts emitting more heat than light. Now I am keeping in mind that my first research methods drummed it into our heads that "correlation is not causation" but it may "Call For Further Research.
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193. groundswell
9:22 PM GMT on February 28, 2008
OK-climate change. But with a worldwide population explosion continuing, there would seem to be little hope of bringing emissions under control. Everybody wants a nice, comfortable life, and that costs.
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Dr. Masters co-founded wunderground in 1995. He flew with the NOAA Hurricane Hunters 1986-1990. Co-blogging with him: Bob Henson, @bhensonweather

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