How Does Hail Form?

Tuesday, June 10, 2014


All inhabitants of Western Canada are familiar with hail. Still, few of us know where it comes from and how it is formed.

  • Hail is not frozen raindrops, as many people believe. When raindrops freeze due to the low temperature in the atmosphere, it occurs too close to the ground to form such large stones.

  • Hailstones form in large cumulonimbus thunderstorm clouds. When drops of water are about to fall from such a cloud, but an updraft of wind drives them back into the freezing upper regions of the cloud, they freeze together in lumps.

  • The size of a Hailstone depends on the strength of the updrafts that drive them back into the cloud, or on the weight that such a cloud is able to support. If the stone becomes too large for the wind to lift, or too large for the cloud to hold, then it drops to earth as a hailstone. 

  • Western Canada is part of an area called Hail Alley. The atmospheric level where the temperature drops far below freezing is much lower here than in most other parts of the world, which means that the hailstones do not have enough time to melt when they drop from the clouds. This is the greatest disadvantage of living in the elevated area around the Rockies.

  • When you cut a hailstone in half, you will see that some rings of ice inside are milky white while others are clear. This shows the two ways by which hailstones grow larger: wet and dry growth. 

  • Wet growth occurs when temperatures in the clouds are not too far below freezing. The drops of water collide and freeze slowly, like water freezes in your freezer, for example. During this slow freezing, air bubbles escape and forms a layer of clear ice.

  • Dry growth occurs when the temperature in the clouds are far below freezing. When the water drops collide, they freeze immediately and the air bubbles freeze in place. This causes the ice to be milky white.

  • The temperature in the upper regions of clouds is lower than in the lower regions. This means that you can test how many times a hailstone moved up and down in the cloud. Cut a stone open and count the layers. A stone with many layers of both clear and cloudy ice was formed in both the upper and lower regions, while one with few or no layers was balanced in the updraft and formed at one height.

If you can do so safely, dash out quickly and collect some hailstones for the family to study. Hail should not only be something that scares us, but also that fascinates.