Is climate change making hailstones larger?

In addition to several deadly tornadoes, severe storms brought heavy rain, gusty winds and damaging hail to parts of Iowa Tuesday. Two-inch-diameter hail was reported in Pella, a town southeast of Cedar Rapids, after hail was observed in Kansas and Colorado on Monday.

Statistics show an increase in the number of large hailstones in the United States over the past two years, which has caused meteorologists and researchers to wonder: Is hail getting larger, and, if so, is climate change to blame?

Hail develops when rising air in a thunderstorm, known as the updraft, lifts water droplets high into the atmosphere where temperatures are below freezing.

Before hailstones become too heavy to fall to the ground, the updraft pushes them up repeatedly, freezing more ice around them. The stronger the storm, the more powerful the updraft, resulting in larger hailstones.

Hail is classified by diameter, from pea size, one-quarter of an inch across, to softball size, 4 inches across. One hailstone that fell in Texas on April 28, 2021, set a new state record at 6.4 inches in diameter, and Colorado set its state record with a 5.25-inch stone that fell on Aug. 8, 2023.

A hailstone that fell in Vivian, South Dakota, on July 23, 2010, holds the United States record for the largest size with a diameter of 8 inches and heaviest weight at 1.94 pounds.

As of May 21, the Storm Prediction Center (SPC) has tallied 317 storm spotter reports in the United States of hail larger than 2 inches in diameter (a golf ball is 1.75 inches). If that sounds like a lot of large hail, it is. May isn't over yet, and the month has already claimed the second-highest count for May since 2006.

SPC storm spotter reports for hail two inches in diameter or larger, by month, from 2004 to 2024. (NOAA/SPC/Iowa Environmental Mesonet)

This comes on the heels of a blockbuster hail season in the summer of 2023, which had 540 reports in June and 305 in July, both new records for those months that exceeded previous records by more than 200 percent. The top six days with the most large hail reports since 2006 have occurred in 2023 and 2024, with 106 reports on March 14, 2024, a new record.

These stats have meteorologists asking whether large hail is becoming more common.

Large and extreme hailstones have always fallen on the Plains but were not well documented until the mid-2000s. One important thing to remember about spotter network data is that more spotters lead to an increase in reports over time. More storm chasers trying to document the largest hailstones could account for some of the recent upswing, but last June remains an outlier.

Because documenting hail size is difficult, only nine states maintain official records on hailstones. Other inconsistencies can also call the data into question.

Hail can melt quickly before a measurement can be taken. Spotter reports can be estimated. Although oblong conglomerations of hailstones can have a large diameter, they may weigh less than other round stones of a similar diameter.

(NOAA)

Because hail formation is a complicated process, few studies have examined whether a warming world produces larger hailstones or more days with hail. The IPCC warns that there is "low confidence in observed trends in tornadoes and hail because of data inhomogeneities and inadequacies in monitoring systems."

One of the only studies on this subject was conducted in 2017 and was based on a computer model forecast in a warming climate.

The paper's hypothesis was that increasing temperatures should lead to more thunderstorms with strong updrafts, which would lead to more-or larger-hailstones. However, they found that this was not necessarily true because higher temperatures could also melt small hail before it reaches the ground.

Hail size changes for 2041-2070 vs. 1971-2000 in spring and summer. (Nature: 10.1038 / NCLIMATE3321)

The paper concluded that hail size is expected to increase, noting, "Although fewer hail days are expected over most areas in the future, an increase in the mean hail size is projected, with fewer small hail events and a shift toward a more frequent occurrence of larger hail."

The study's figures show that the 30 years between 2041 and 2070 would produce fewer hail days and smaller hailstones compared to the 1971-2000 period for states on the East Coast of the United States. However, both measures would increase in the central Plains, Rockies and Canada.

Change in hail days, 2041-2070 vs. 1971-2000, for Spring and Summer. (Nature: 10.1038 / NCLIMATE3321)