02. 2013年9月07日 16:48:15
: niiL5nr8dQ
実は平年の半分程度しか発生していない 2013年のアメリカの竜巻の数
下は、 NOAA が発表しているアメリカの平年の竜巻の発生数と、今年2013年（ 9月2日まで）の竜巻の発生数の比較です。上が平年の平均値で、下の赤いグラフが今年です。
tornado-2013-01.jpg ▲ 原寸のグラフのリンクはこちらです。 ちょっと縮小すると見づらいですので、数字の部分を拡大します。 tornado-big1.png
グラフは、８月あたりまで上昇して、そこからグラフはあまり動きませんが、多くの竜巻は５月から８月くらいまでの間に発生するからのようです。つまり、今のこの時期の竜巻の数が、大体、その年の全体の竜巻の発生数の目安となるようです。
そして、さらには下のグラフ。
これは、2005年から１年ごとのアメリカの竜巻の発生数をあらわしたものです。 tornado-2005-2013-02.png ▲ 原寸のグラフのリンクはこちらです。 上のグラフの年間ごとの色分けと数字は以下の通りとなっています。 tornado-small2.png
確かに2013年の発生件数は最近では非常に少ないようです。
では、「威力」のほうはどうか？。
竜巻の威力も 2013年は低いレベル 実はアメリカの 2013年は、威力のある竜巻の発生回数に関しても少ない年なのでした。 これを示すグラフも NOAA にあります。 竜巻のレベルを表す単位として、アメリカでは改良藤田スケールという等級を使用しているのだそうで、大ざっぱに書けば、上から強い順番で、 EF0
EF1
EF2
EF3
EF4
EF5 となります。 先日の埼玉の竜巻は、下から２番目の EF4 程度のものだと思われます。 NOAA に「1954年からの EF1 以上の竜巻の発生回数」を示したグラフがあります。
つまり、最も強いレベルの竜巻の発生回数のグラフです。 全体だとグラフが大きいですので、1970年からといたします。それでも縮小すると非常に見づらいですが、下のグラフとなります。 ef1-2012.png ▲ 原寸のグラフのリンクはこちらです。
最強レベルである EF1 以上の竜巻が 1970年以降でもっとも少なかったのは 2002年の「 311回」で、次が 1987年の「 316回」だそう。
今年 2013年は 9月 2日までに 340回の EF1 以上の竜巻が発生しているということですので、少なくとも過去数十年で最低ということはないですが、しかし、９月を過ぎると巨大な竜巻の発生数は一気に減るということを考えますと、 2013年は、全体の数と共に、威力もそれほど強くはない年だったようです。 もちろん、５月のオクラホマでの竜巻のように大きな被害が出ている竜巻は起きてはいるわけですけれど、全体としては今年のアメリカの竜巻の発生状況は、グラフの通り「穏やかな2013年」だったということのよう。感覚としてはちょっと信じがたい感じの事実ではあります。
oklahoma_tornado.jpg
▲ 2013年5月にアメリカ中西部のムーアという町が竜巻で壊滅した後の光景。廃墟と化した自分の町の中に座る男性。
私などは、今年のアメリカは竜巻の発生した回数やその規模も「絶対に増加している」と勝手に思いこんでいたわけですけれど、実際には、ムチャクチャ減っていたという、意外といっていいのかどうかわからないですが、そういうことを NOAA の発表で知ったのでした。
どんなことでも感覚や思い込みはよくないですね。 ただ、これは、先日の記事の、 ・この夏すでに聞こえていた小氷河期の足音 ： アメリカのこの夏は記録的な「低温」が圧倒していたことが判明
2013年08月27日 での「寒かったアメリカの夏」ということも関係しているかもしれないですね。 実は、 NOAA の発表の中に、さらに興味深いデータがありました。 それは「南極の氷の面積」に関してのものなのですが、次回になるかどうかわからないですが、そちらもご紹介できる時にご紹介したいと思っています。
http://oka-jp.seesaa.net/article/373910578.html
Timing
Because most tornadoes are related to the strength of a thunderstorm, and thunderstorms normally gain most of their energy from solar heating and latent heat released by the condensation of water vapor, it is not surprising that most tornadoes occur in the afternoon and evening hours, with a minimum frequency around dawn (when temperatures are lowest and radiation deficits are highest). However, tornadoes have occurred at all hours of the day, and nighttime occurrences may give sleeping residents of a community little or no warningTornado Occurrence by hour of day for the United States
U.S. Tornado Occurrence by Hour
Tornado Occurrence by hour by region of the United States
Tornado Alley
U.S. Tornado Occurrence by Hour Dixie Alley
U.S. Tornado Occurrence by Hour Northeast Climate Region
U.S. Tornado Occurrence by Hour
Southeast Climate Region
U.S. Tornado Occurrence by Hour Central Climate Region
U.S. Tornado Occurrence by Hour East North Central Climate Region
U.S. Tornado Occurrence by Hour
South Climate Region
U.S. Tornado Occurrence by Hour West North Central Climate Region
U.S. Tornado Occurrence by Hour Southwest Climate Region
U.S. Tornado Occurrence by Hour
Northwest Climate Region
U.S. Tornado Occurrence by Hour West Climate Region
U.S. Tornado Occurrence by Hour
In addition, tornadoes occur throughout the year. Because a tornado may occur at any time of the day or year somewhere in the U.S., there really is no national tornado "season" (as there is with Atlantic hurricanes). Instead, each region may experience increased tornadic potential at different times of the year. Like with the diurnal pattern, for the United States (and hemisphere) as a whole, the months in which tornadoes are most likely correspond to the times of year with increased solar heating and strong frontal systems. Regionally, the frequency of tornadoes in the United States is closely tied with the progression of the warm season when warm and cold air masses often clash. Most of the early spring tornadoes in the U.S. tend to occur in the Southeast and South Central regions. Gulf States, such as Mississippi and Louisiana are the frequent recipients of tornadoes from February to April. Late spring tornadoes generally spread a bit farther north, often into Kansas, Nebraska and the Tennessee Valley region. By mid-summer, most of Tornado Alley is active and tornadoes may occur throughout the United States. Late summer tends to bring some of the stronger tornadoes into the upper Midwest and Ohio valleys, and the pattern shifts back southward into the late autumn. The fewest tornadoes are documented during the winter months. Although rare, deadly winter outbreaks do occurAverage tornado frequency by month of year 1991-2010
U.S. Tornado Occurrence by Month
Top of Pagetop of the page Historical Records and Trends One of the main difficulties with tornado records is that a tornado, or evidence of a tornado must have been observed. Unlike rainfall or temperature, which may be measured by a fixed instrument, tornadoes are ephemeral and very unpredictable. If a tornado occurs in a place with few or no people, it is not likely to be documented. Unfortunately, much of what we know as tornado alley was very sparsely populated until the 20th century, and so it is possible that many significant tornadoes may never have made it into the historical recordMuch early work on tornado climatology in the U.S. was done by John Park Finley in his book Tornadoes, published in 1887. While some of Finley's safety guidelines have since been refuted as dangerous practices, the book itself remains a seminal work in tornado research. The University of Oklahoma has created a pdf copy of the entire book and made it accessible at: John Finley's 'Tornadoes' Today, nearly all of the United States is reasonably well populated, or at least covered by NOAA's Doppler weather radars. Even if a tornado is not actually observed, modern damage assessments by NWS personnel can discern if a tornado caused the damage, and if so, how strong the tornado may have been. This disparity between tornado records of the past and current records contributes a great deal of uncertainty regarding questions about the long-term behavior or patterns of tornado occurrence. Improved tornado observation practices have led to an increase in the number of reported weaker tornadoes, and in recent years the number of EF-0 tornadoes have become more prevelant in the total number of reported tornadoes. In addition, even today many smaller tornadoes still may go undocumented in places with low populations or inconsistent communication facilitiesWith increased national Doppler radar coverage, increasing population, and greater attention to tornado reporting, there has been an increase in the number of tornado reports over the past several decades. This can create a misleading appearance of an increasing trend in tornado frequency. To better understand the variability and trend in tornado frequency in the U.S., the total number EF1 and stronger, as well as strong to violent tornadoes (EF3 to EF5 category on the Enhanced Fujita scale) can be analyzed. These are the tornadoes that would have likely been reported even during the decades before Doppler radar use became widespread and practices resulted in increasing tornado reports. The bar charts below indicates there has been little trend in the frequency of the stronger tornadoes over the past 55 years EF1-EF5 Tornado Counts EF3-EF5 Tornado Counts Top of Page top of the page Occurrence Maps Annual statewide and national tornado maps from 1950 through 2010 are available through NOAA's Storm Prediction Center
Annual Averages by State
(1991-2010 averaging period) Map
Overall Average U.S. Tornado Occurrence by Hour
Overall Average
per 10,000 Square Miles U.S. Tornado Occurrence by Hour
Strong-Violent (EF3-EF5) Average U.S. Tornado Occurrence by Hour
Strong-Violent (EF3-EF5) Average
per 10,000 Square Miles U.S. Tornado Occurrence by Hour Monthly Tornado Occurrence Maps [January| February| March| April| May| June| July| August| September| October| November| December] Top of Page top of the page Deadliest Tornadoes Most tornadoes do not result in death, and of those that do, most claim only a few lives. Also, as a result of improved forecasting and early warning systems, the death toll from tornadoes has dropped significantly over the years, despite increasing populations in tornado-prone areas. Unfortunately, super-violent tornadoes are still documented, some with exceptional death tolls. Interestingly, a number of these devastating tornadoes have occurred outside of Tornado Alley, and several at times of day or year not normally associated with violent tornadoes. Although all of most deadly tornadoes occurred prior to the invention of the Fujita Scale, historical records of their damage have led them to be classified as either F4 or F5The ten deadliest documented tornado events
Rank Date Estimated intensity* State Injuries (Deaths) Remarks
1 March 18, 1925 F5 MO, IL, IN 2027 (695) Tri-State Tornado
Wikipedia Entry
Tornado Project Page
2 May 6, 1840 Unkn. LA, MS 109 (317) hit Nachez, MS
NWS information
Wikipedia Information
Tornado Project Page
3 May 27, 1896 F4 MO, IL 1000 (255) The Great St. Louis Tornado
Tornado Project Page
The Great Cyclone. SIU Press
St. Louis American Local History Network
4 April 5, 1936 F5 MS 700 (216) hit Tupelo, MS
NWS information
Tornado Project Page
5 April 6, 1936 F4 GA 1600 (203) hit Gainesville, GA
Tornado Project Page
About N. Georgia
6 April 9, 1947 F5 TX, KS, OK 970 (181) Tornado Project Page
7 May 22, 2011 EF5 MO 1,000 (158) Hit Joplin, MO
NWS Summary
8 April 24, 1908 F4 LA, MS 770 (143) hit Amite, LA and Purvis, MS
Tornado Project Page
9 June 12, 1899 F5 WI 200 (117) hit New Richmond, WI
Tornado Project Page
10 June 8, 1953 F5 MI 844 (116) hit Flint, MI
Tornado Project Page Top of Page top of the page Special Reports
U.S. Tornado Overview - 2011
U.S. Tornado Overview - 2010
U.S. Tornado Overview - 2009
U.S. Tornado Overview - 2008
spring 2011 Tornado Outbreaks
Super Tornado Outbreak of April 3-4, 1974
Worldwide Weather and Climate Events (historical reports by month) Links to additional information
NOAA Storm Prediction Center Tornado FAQ
Storm Prediction Center Tornado Data
Tornado Climatology from NOAA NSSL
NSSL Tornado Basics
Tornadoes in Bangladesh and East India
Historical Tornado Cases for North America
The Tornado Project
http://www.ncdc.noaa.gov/oa/climate/severeweather/tornadoes.html#overview |
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