How Hail Forms

Most of us have seen little balls of ice fall from the sky; this can be an entertaining occurrence, a painful experience or a disgusting incident. The latter of these often used to be witnessed on occasion by people in the wrong place at the wrong time, often falling out of a clear blue sky.

You’re walking along on a bright sunny day, minding your own business, maybe glancing skyward to observe the formation of a con trail overhead when suddenly balls of ice begin to crash to earth all around you. One or two may strike you on the head and face; some may even make it into your mouth before you realize, with dawning horror, that the little balls of ice are a pale yellow color and have a horrible and somewhat familiar flavor. As you put the pieces together in your mind, you finally realize that what you experienced was not hail, but discharge from an airplane passing high overhead as someone flushed a urinal. This is when you run screaming all the way home and spend the next half hour gargling with hydrogen peroxide.

Fortunately, airplanes don’t dump frozen urine onto unsuspecting people anymore, so if you see balls of ice falling from the heavens, most likely what you see is hail.

Hail is formed in large thunderclouds, or cumulonimbus clouds, usually in the spring or summer because warm air is crucial to the formation of hailstones. As the thunderstorm marches across the plains it is met by warm updrafts that race from ground level up the forward face of the storm, as well as into the leading edge of it. These updrafts are what create the “anvil”, or the mushroom-shaped dome of the clouds. They also carry whatever is falling through the cloud—such as frozen raindrops—back up into the cloud, where the temperature is well below freezing.

Hail begins with a nucleus, which can be any infinitesimal speck of matter from a dust particle to a grain of sand or even a bug. Super-cooled water strikes the nucleus and sticks, forming the graupel, the center of the hailstone. Gravity tries to draw the graupel down to the ground, but those updrafts I mentioned earlier don’t make that easy; they push the proto-hailstone back into the top of the cloud, where more super-cooled water drops accrete onto the stone. As it drops again, more water sticks and freezes into new layers, the updraft pushes the hailstone back up into the upper reaches of the cloud and this process repeats until gravity wins over the force of the updraft.

This explains why hail usually falls near the front edge of storms, where the updrafts are strongest, as opposed to the backside of the storm where downdrafts prevail. It also explains why the largest hailstones usually fall where the weather is hottest, in the southern states, for the hotter the air, the stronger the updrafts. Naturally, as the hail falls through increasingly warm air, some of the ice melts. If enough ice melts that the hailstone’s inertia is not enough to overcome the powerful updrafts, up it goes again.

Hail forms more often than you think; in most storms, it simply doesn’t make it all the way to the ground before melting.

So you can rest assured that when ice falls from the sky, it isn’t frozen pee, not anymore. However, before you start munching down hailstones, keep in mind that you don’t know what the nucleus that formed the stone was; for all you know, it might have been a fly that had previously enjoyed unspeakable things for lunch.