While the nation is fixated on the punishing heat and drought gripping the United States, parts of the country are still coping with losses from another blast of extreme weather that battered their cities, towns and farms this spring: hailstorms.
The Storm Prediction Center in Norman, Okla., said there were about 500 more reports of one-inch diameter or larger hail the first half of this year than the 2005-2011 average. Property damage from just two intense hailstorms that hit large cities could exceed $1.5 billion.
Is this year's hail onslaught a sign of things to come in a warming world? Much uncertainty remains about climate change's impact on future hailstorms, with some scientists saying there could be fewer of them, and others predicting even more damage in the future.
To call some of this year's hailstorms "extreme" might be putting it mildly. Here's a recap of just a few of them:
— On April 12, pea-sized hail pelted northern Texas near Amarillo. Nothing remarkable there—except the volume. There was so much, mixed with rain, that snowplows had to be called out to clear 4-foot-high drifts.
— Two days later, another high-volume storm, the likes of which long-time residents said they'd never seen before, inundated Norfolk, Neb., leaving 3-foot drifts in spots.
— On April 28, baseball- to softball-sized hail pummeled the St. Louis area. About 50,000 cars were damaged. Individual insurance claims, including for home roofs and sidings, could top 80,000 in Missouri, according to the property claims services unit of Verisk Analytics. Insured losses are estimated at $450 million, but the total loss should be much higher when uninsured damage is included.
— On June 13, what appears to have been the largest hail of the year fell on Dallas. Early estimates put damages at $900 million or more, which would make the hailstorm the fifth most costly in Texas history. Damage claims totaled about 130,000.
Reports of two-inch diameter hail, the size of a hen egg, have been steadily increasing since the 1950s, when the average was 150 to 200 events per year. By the early 2000s, the average had risen to 400 to 500. From 2008 to 2011, the average was 695, and last year the figure topped 1,000 for the first time.
But Greg Carbin, the warning coordination meteorologist at the Storm Prediction Center, told InsideClimate News he's cautious about reading too much into those numbers.
"Part of the [bigger] number is realistic, and it suggests we're seeing more hail events, but there are non-meteorological factors accounting for some of it," he said. "The statistics are there, but you should be very careful in filtering it."
Much of the increase in reported hailstorms can be attributed to a larger population, the presence of more trained weather spotters and improvements in radar.
In terms of hail crop damage, last year was the worst on record in the United States. In 2011, premiums for hail insurance reached an all-time high of $841.7 million, according to Crop Insurance Today. And for only the second time since 1948, paid losses, which hit $962.2 million, exceeded the premiums.
Although there have been more reports of large hail so far this year, there may not be a corresponding increase in hail crop damage. Last year, most of the damaging hail came later in the summer, when crops were further along.
Hail and Global Warming
For a storm to produce hail, it must have strong updrafts. (Here's an animated explanation of the mechanics of a hailstorm.) The storm doesn't have to be a thunderstorm to produce small hail, but very large hail does form in severe thunderstorms.
What the future holds for hail is unclear.
"NOAA's position is that extreme precipitation events are increasing, and that does appear to be a result of climate change," Carbin said. "But we can't tell you that there's necessarily a correlation between a warmer climate and more hailstorms."
He said two critical atmospheric ingredients are needed to create severe thunderstorms, the kinds of storms that also produce tornados: instability and wind shear. The National Weather Service describes instability as the "tendency for air parcels to accelerate when they are displaced from their original position; especially, the tendency to accelerate upward after being lifted." The greater the instability, the greater the storm's precipitation potential.
A warmer world should have more instability, Carbin said, and that means there should be more storms.
"But you also need shear involvement," he said. "That may be even more important than instability. You may not have more shear. You may have less."
Shear is a change in wind speed or direction as the height changes. It's based on the temperature differential over an area, basically with cold air from the north and warm air from the south.
The more intense that temperature gradient, the more intense the shear. In a warmer world, scientists aren't sure what happens to that temperature gradient, Carbin said.