Science Puzzle
How Can Ice Fall from the Sky in Summer?
On a hot summer afternoon, with the temperature at 30 °C, a thunderstorm rolls in and suddenly the ground is bouncing with balls of ice. Cut one of these hailstones in half and you'll often find rings inside it, like the layers of an onion.
How can solid ice fall from the sky on a hot day, and what made the rings?
The Answer
The ice is made high up inside the storm cloud, where it's bitterly cold even in summer. Each ring is a trip up and down through the cloud.
Air cools as it rises. A thunderstorm cloud can tower 10 to 15 kilometres high. No matter how hot it is on the ground, the top of the cloud can be colder than -40 °C. Inside the cloud are powerful updrafts, currents of rising air that can blow upwards at more than 100 kilometres per hour.
Raindrops are swept up above the freezing level and freeze into tiny ice pellets. As a pellet is tossed around, it collides with droplets of water that are still liquid even though they're below 0 °C, and they freeze onto it. Sometimes a layer freezes slowly into clear ice; sometimes it freezes fast, trapping air bubbles, into cloudy white ice. Each trip through different parts of the cloud adds another ring.
The hailstone keeps growing until it's too heavy for the updraft to hold up. Then it falls, so fast that it reaches the ground before it has time to melt. The stronger the updraft, the bigger the hail it can support, which is why the most violent summer storms produce stones bigger than a golf ball.
The principle: Updrafts and the freezing level. Storm clouds reach far above the height where the air is below freezing, and strong rising air keeps ice pellets aloft long enough to grow into hailstones, layer by layer.