Thunderstorms can occur in Missouri any time of the year but close to 60 percent of them rumble across the state between May and August. Lightning, thunder, and rain are the usual ingredients of a storm but occasionally other elements such as hail and tornadoes will accompany it. Fortunately, most of us will never witness a tornado in our lifetime, but we will likely experience hail.
Hail develops in strong thunderstorms when rapidly rising currents of air, called updrafts, carry raindrops to a height where they freeze. The vertical currents of air cause the hailstone to move up and down in the cloud which causes each layer of ice to melt a little during every descent and acquire a new sheathing of ice during each ascent into freezing temperatures. Droplets of supercooled water continue to freeze onto the hailstone giving it a stratified interior structure that resembles the rings of an onion. Stronger updrafts will produce larger hailstones and they will eventually fall out of the sky when the updraft can no longer support them.
Missouri's wet and unsettled weather pattern this year has also translated to an above normal severe weather season. Already, some locations in the Show-Me state have witnessed hail during three separate occasions which is about average on an annual basis. Typically, any location in Missouri will experience hail 2-3 times during the year with most events occurring during the spring. On most occasions it is smaller than a pea (1/4") and it will cause little or no damage. Occasionally, however, the hail can be bigger. The National Weather Service considers a thunderstorm severe if hail is ¾" in diameter (penny size) or larger. Reports of large hail are more common in the Great Plains with the highest frequency of large hail reports over central Oklahoma. Hail is also a frequent visitor to our neighbors in the western Great Plains and along the Colorado Front Range. The area where Colorado, Wyoming, and Nebraska meet is known as "Hail Alley." On average, hail falls over this region between 7-9 days a year.
The maximum diameter a hailstone can grow is about 4.5 inches, or about the size of a softball. These larger hailstones can fall at speeds greater than 100 mph! Fortunately, it is rare to experience hail larger than 0.75 inch in diameter because hail swaths are usually narrow and short lived. On June 22, 2003 a world record hailstone fell on Aurora, Nebraska surpassing a former record-setting hailstone that fell over Coffeyville, KS in 1970. The Aurora hailstone measured 7 inches in diameter and 17.5 inches in circumference. The largest documented hailstone to fall in Missouri occurred on May 24, 2004 near the community of Meadville, MO in Linn County. The hailstone was 6 inches in diameter and had a circumference of 16.5 inches.
Property and crop hail losses are estimated to exceed 2 billion dollars annually, and represent between 1 and 2 percent of the annual crop value. Wind and hail can be a destructive combination, shredding corn, soybeans and other plants in minutes. Some regions are more vulnerable to hail losses, with hail damage representing 1 to 2 percent of the crop value in the Midwest and 5 to 6 percent of the crops produced in the High Plains.
Hailstorms that occur in Missouri during April and May are usually responsible for minor crop yield losses since corn has not yet emerged or is too small to suffer significant damage. Corn becomes more vulnerable to hail damage about 3 weeks after emergence and continues through the tasseling stage, which is the most critical period. Numerous information resources are available for assessing corn and soybean damage to hail, including potential yield loss information. Some example Web resources include: http://www.agry.purdue.edu/ext/corn/news/timeless/HailDamageYoungCorn.html and http://www.extension.iastate.edu/nwcrops/hail_soybean.htm.
Additional replant decision guides can be found at the following link: http://muextension.missouri.edu/explore/agguides/crops/g04091.htm.
REVISED: October 2, 2015