The science behind the formation of hail

We’re now at a point where snowstorms should be behind us and we’re shifting our attention to other types of storms, including thunderstorms.

Thunderstorms can occur year-round but are most common in the late spring and summer when conditions are more favourable. These storms can produce intense rain, wind and lightning, and sometimes hail. But just how does hail form and what determines its size?

Every thunderstorm has an updraft and downdraft, and the updraft is a key component to the development of a thunderstorm. The updraft sucks up rain droplets that can be carried beyond the freezing level within the cloud. These liquid droplets freeze and the subsequent droplets then freeze and form hailstones. The longer the hail is suspended within the freezing layer, the larger the hailstone. Once the weight of the hailstone exceeds the force of the updraft, it falls to the surface through the thunderstorm’s downdraft.

We most often experience pea, marble, or coin-sized hail here in Atlantic Canada and when hail exceeds two centimetres in diameter, it is considered severe. Damaging hail sometimes frequents other parts of the country and the U.S., where hail can sometimes be the size of golf balls or, in extreme cases, baseball and grapefruit sized.

But not every thunderstorm produces hail. It all comes down to the strength of the updraft and the location of the freezing layer, which can vary based on the time of year and the storm structure. Remember, hail is only the result of thunderstorms. Precipitation types such as ice pellets or graupel resemble hail, but the formation processes are completely different.

Allister Aalders is the weather specialist for the Saltwire Network, providing forecasts and analysis for Atlantic Canada. #Askallister