As the frequency and severity of US thunderstorms get worse, Dr Peter S Dailey and Dr Ioana Dima from AIR Worldwide examine the causes and consequences of these phenomena.
century: more than 300 people were killed that day. As reported by ISO’s Property Claim Service (PCS), 2011 already has set a record for severe thunderstorm losses—by the beginning of October, total insured losses were nearly $23 billion.
E
12 15
0 3 6 9
Insured losses from severe thunderstorms, 1991-2010, adjusted to 2011 dollars. (Source: PCS)
Even such a brief overview raises several questions: what is driving the apparent increase in severe thunderstorm losses? Are severe thunderstorms becoming more frequent or more severe—or possibly both at once? What is the role of climate change in these possible trends? Does the apparent increase in losses perhaps reflect different factors entirely, such as an increasing number and value of properties at risk?
WHAT CAUSES SEVERE
THUNDERSTORMS? The US National Weather Service describes a severe thunderstorm as a
thunderstorm that is capable of producing at least one of three destructive phenomena: wind gusts of 58 miles per hour or greater (straight-line
The graph below shows annual losses from severe thunderstorms from
1991 to 2010 expressed in constant (2011) dollars, as compiled by PCS. In addition to the apparent increasing trend, particularly in recent years, the graph clearly demonstrates a volatility in losses from year to year.
xactly 753 confirmed tornadoes touched ground in the US in April this year, the most to strike in a single month ever. April 27 became the deadliest day for tornadoes in over half a
winds), hail of at least 0.75 inches in diameter or tornadoes—which can achieve wind speeds of 250 mph and cut paths of destruction a mile wide and 50 miles long. The most violent type of severe thunderstorm— “tornadic supercell” thunderstorms—have been observed around the world, but they occur most frequently in the central US between the Rocky Mountains and the Mississippi River.
SEVERE THUNDERSTORM FORMATION At least three specific meteorological conditions are required for severe
thunderstorms to form: (1) atmospheric instability that accompanies the vertical movement of warm air from the surface; (2) sufficient moisture content (atmospheric water vapour); (3) the presence of a lifting mechanism, such as a frontal boundary (where two air masses of different densities meet), an upslope wind or a “dry line” (a boundary marking two regions of distinctively different moisture content).
Tornadic supercell thunderstorms require one additional and important condition: sufficient wind shear (the wind changing in speed and direction with height). Wind shear is necessary for a thunderstorm to develop a rotating updraft, the mechanism that completes the storm’s transition to a tornadic supercell.
MACRO-SCALE INFLUENCES The distinctive geography of the North American Great Plains—an
almost level unbroken surface stretching for thousands of miles from the Arctic to the Gulf of Mexico—ensures that there is no significant physical barrier to the flow and interaction of air masses up and down and across the centre of the continent.
A second contributory factor is an abundance, especially in the spring,
of low-level atmospheric moisture that originates in the Gulf of Mexico (and to a lesser degree, the Atlantic Ocean). This warm, moist air clashes with the cooler, drier air from the north, thereby setting in play the local conditions noted above that are necessary for severe thunderstorms to form.
Third is the polar jet stream. When the jet stream is situated above a
developing thunderstorm, its intense winds act to “vent” the storm, pulling its rising inner air up and away, thereby helping the rotating updraft to strengthen and its speed to increase.
November 2011 | INTELLIGENT INSURER | 45
1991 1992 1993 1994 1995 1996 1997 1998 1999 2000 2001 2002 2003 2004 2005 2006 2007 2008 2009 2010
$ BILLIONS
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