Hail -- defined as precipitation of balls or irregular lumps of ice produced by storm clouds -- forms when liquid raindrops freeze at altitudes above a threshold called the "freezing level height," researchers said. If drops cannot be formed above the freezing level, precipitation remains in a liquid state.

The scientists from the University of Hawaii and the Beijing School of Physics wanted to determine whether precipitation trends in hail have changed on regional scales. To accomplish that, they studied data from more than 753 meteorological stations across China.

Their analysis suggests that while there is no trend in the mean number of average hail days between 1960 and early 1980, the number of such days diminished thereafter from about two days to less than one day each year.

The researchers posit the drop in hail precipitation is due to a rising trend in freezing level height.

The study by Baoguo Xie and Qinghong Zhang of the School of Physics and Yuqing Wang of the University of Hawaii is reported in the journal Geophysical Research Letters.