Question Answered by Tim Axford, Meterologist, National Weather Service-Pocatello, ID.
Usually occurs during the spring and summer but could occur during all seasons.
When thunderstorms form, air moves from near the ground up into the sky, sometimes as high as 50,000 ft up! This air moving upwards will carry drops of water with it. As the water moves higher and higher, it gets colder and colder and begins to freeze and cling to the cold water around it, becoming ice. This ice will then fall back to the ground as hail. However, if the storm is strong enough, the rising air within it will be strong enough to push the hail back up, adding a new coating of ice as it circulates through the storm. This cycling allows the hail to grow bigger and bigger, eventually becoming too heavy and falling to the ground. Hail can be clear, or appear cloudy and have layers where new ice has formed.
Hail is usually an inch or less in diameter and compared in size to peas or coins, but bigger “hailstones” can reach the size of sports balls (ping pong balls, baseballs, softballs, etc.). Hail of this size can cause significant damage to crops, cars, and buildings and can even injure humans and animals. In general, hail size can be a sign of how strong a storm is.
Fun fact: The largest hailstone ever recorded occurred in Vivian, SD in 2010 and measured 7.9 inches in diameter and weighed 1.94 pounds!
Usually occurs in the winter months.
Sleet begins in much the same way as snow does. A cloud below freezing begins to produce snowflakes several thousand feet above the ground. Following these snowflakes on their downward journey, if the air remains cold enough, they remain snowflakes all the way to the ground. If the snowflakes fall down into warm air below, they partially or completely melt, turning into raindrops. Sometimes another layer of cold air exists below the warm air. This unique balance allows these newly formed raindrops to re-freeze into little round balls of ice called sleet. Sleet is typically the size of a pea or smaller and can be pretty clear. While the impact of sleet won’t damage surfaces like large hail does, it can make for very slick walking and driving conditions and be just as dangerous!
Fun fact: If the right conditions exist, sleet can accumulate like snow and can be measured in inches the same way!
Create a Hailstone
Hailstones are created when frozen droplets collide with other droplets and moisture, growing in size as they collide. Once they are too heavy for the storm’s wind and energy to keep aloft, they fall through the clouds to the Earth.
Let’s make a hailstone!
Materials: ice cube, bowl, cold water, plate sprayed with non-stick spray, string, freezer, tape measure
- Tie a string around the ice cube and measure your ice cube around the middle. Record the size.
- Dip the cube into the bowl of cold water.
- Place the ice cube on the plate and put it into the freezer. Leave it for 3 hours until it is re-frozen.
- Take the cube out of the freezer and remeasure it. Did it grow?
- Repeat 3 and 4 for several days. After re-dipping your ice cube, measure it again and calculate how much it has grown. Have an adult help you cut the hailstone in half. What does it look like inside? Can you see the layers?