Hail, a form of solid precipitation, consists of ice pellets that fall from convective storm clouds, specifically thunderstorms. It seems counterintuitive that this icy weather event is statistically far more frequent and severe during the warm late spring and summer months, rather than in the depths of winter. The reason for this seasonal difference lies not in the air temperature at the ground, but in the atmospheric mechanics required to build a massive chunk of ice high above us.
The Essential Ingredients for Hail Formation
Hailstone formation requires specific atmospheric conditions inside a towering cumulonimbus cloud. The process needs sufficient moisture, providing the water droplets and supercooled liquid water necessary for the hailstone to grow. This growth must take place high in the atmosphere, typically between 20,000 and 30,000 feet, where temperatures are below freezing. The most important requirement, however, is a strong, sustained vertical current of air, known as an updraft. This forceful column holds the embryonic ice pellet suspended against gravity, cycling it through layers of supercooled water to allow repeated growth.
The Role of Strong Summer Convection
Summer conditions provide the engine for generating the powerful updrafts needed to create large hailstones. Intense solar heating of the Earth’s surface causes the air near the ground to become warmer and less dense than the air above it, leading to thermal instability. This instability fuels deep, powerful convection, which is the process of warm, moist air rising rapidly into the atmosphere. The resulting vertical wind currents can reach speeds exceeding 100 miles per hour, acting like a high-speed elevator within the storm cloud.
These powerful updrafts lift water droplets and small ice embryos well above the freezing level, keeping them suspended for several minutes. As the stone is held aloft, it collides with numerous supercooled water droplets that instantly freeze upon contact, adding layer upon layer of ice. The strength of the updraft allows the hailstone to grow large enough to survive the fall through the warmer air near the ground without melting. While the freezing level is higher in summer, the intensity and depth of the convection easily overcome this, enabling the production of damaging hail.
Why Winter Conditions Limit Hail Growth
While the air temperature in winter is cold enough for ice to form, the lack of intense solar heating limits the energy available to drive strong convection. The sun’s angle is lower and the days are shorter, which results in weaker surface heating and shallow vertical air movement. The updrafts that do form are not strong enough or deep enough to lift water droplets high into the cloud and suspend the forming ice for the time needed to grow into significant hailstones.
Furthermore, the overall instability in the atmosphere during winter is much lower than in summer. Precipitation that freezes aloft will usually fall quickly to the ground as snow, ice pellets, or sleet, which are products of different, less violent formation processes. True hail requires the vigorous vertical development of a severe thunderstorm, an atmospheric structure that is rarely supported by the weak, shallow convection of a typical winter day. Hailstones that do form in winter are generally quite small because the updrafts cannot support them long enough for substantial growth.