What Causes Hail in the Summer?

Hail, the irregularly shaped lumps of ice that tumble out of the sky during thunderstorms, is a puzzling precipitation type. It's made of ice and common during the spring and summer months, yet it resembles winter's sleet and graupel. The explanation of how this is possible lies overhead: Although outdoor temperatures might be 70, 80, or 90 degrees F outside your door, tens of thousands of feet aloft, temperatures are typically freezing, 32 degrees F and below.

Although most convective thunderstorms produce hail, not all thunderstorms drop hailstones on the ground, according to NOAA’s National Severe Storms Laboratory (NSSL). Still, hailstorms have inflicted between $8 to 14 billion in property damage every year in the United States for the past decade. 

How Does Hail Form?

Hail is born deep inside the belly of towering cumulonimbus clouds, which can extend 40,000 to 60,000 feet into the atmosphere. (To get a feeling for how high that is, most commercial airliners cruise at altitudes of 31,000 to 38,000 feet.) The lower regions of storm clouds contain warm, humid air; however, their mid-regions are generally where freezing levels are found. Updrafts within the thunderstorm can whisk raindrops up into a freezing region, causing them to transform into ice crystals. These seeds of ice then grow into a hailstone by colliding with neighboring ice crystals and supercooled cloud droplets that freeze onto its surface.

With each collision that occurs above the cloud’s freezing level, a new coating of ice is added to the mini hailstone, expanding its size. If temperatures are near the freezing mark, water freezes slowly around the growing hailstone. This allows air bubbles time to escape, and a layer of clear ice results. If the environment is sub-freezing, however, supercooled water droplets freeze almost instantaneously onto the growing hailstone, trapping air bubbles into place and yielding cloudy ice. (If you’ve ever looked closely at a hailstone and seen striations resembling layers of an onion, this is why.) 

Lift a hailstone up too high—to the top levels of a thunderstorm where cloud temperatures can easily measure around minus 60 degrees F, for example—and it won't grow. That’s because at temperatures that cold, all liquid water, even supercooled water, will have frozen into ice. And hail needs liquid water or a water-ice blend in order to aggregate.

A hailstone’s collision-accumulation cycle can repeat multiple times, but typically not for longer than 30 minutes, since thunderstorms typically don’t live much longer than this. 

At What Speed Does Hail Fall?

Once a hailstone’s mass becomes too heavy for the updraft to lift, gravity wins out, and the chunk of ice falls earthward.

How fast a hailstone falls varies depending on the hailstone’s size and shape, the frictional force between it and the surrounding air, the degree of melting during its journey, and local wind conditions. According to NSSL, hail’s terminal fall speed (the maximum speed it can reach before gravity’s acceleration balances air resistance) ranges from about 10 mph for very small hailstones to 100 mph for baseball-size and larger hailstones.

What Determines Hailstone Size?

A hailstone’s size ultimately depends on the updraft strength of its parent thunderstorm. The stronger the updraft, the longer the hailstone remains suspended in the storm cloud where it will undergo multiple collisions and thereby, grow.

According to the National Weather Service, updraft speeds of around 24 mph are needed to sustain even some of Mother Nature’s smallest hailstones, such as pea-size hail. As for the 8-inch diameter, 1.93-pound hailstone that fell in Vivian, South Dakota, in June 2010, and ranks as the widest and heaviest hailstone in the United States, meteorologists estimate it was supported by a 160 to 180 mph-strong updraft.

Melting also plays a role in determining what size a hailstone will be once it hits the ground. Once hail drops below a cloud’s freezing level (this altitude varies per cloud, time of year, and geographical location) it’ll begin to melt. According to the National Weather Service office in Louisville, Kentucky, if a hailstone falls through a layer of warm air that's 11,000 feet or thicker, it typically won’t survive its journey to the ground and will instead arrive at the surface as what it began: a raindrop.