Dr. Universe: How does hail form? -Emilio, 11, San Diego, California
 
Dear Emilio,
 
During a thunderstorm, there are often lots of tiny water droplets in the clouds that form precipitation like water, snow or hail. But that precipitation doesn’t always fall right to the ground. 
 
Sometimes a falling raindrop will get swept back up in a current of air. The air current can carry the raindrop to higher parts of the thunderstorm cloud where temperatures are below freezing.
 
Under these super cold temperatures, a raindrop will freeze. Then, other water droplets will start clinging to the frozen droplet. This is how hail, or a hailstone, begins to form.
 
That’s what I found out from my friend Jonathan Contezac, a field meteorologist with AgWeatherNet at Washington State University.
 
“As long as the updraft within the storm is strong enough to keep this hailstone suspended in the atmosphere, it’ll continue to grow. If it gets too heavy, it will fall to the earth, or if the updraft weakens, it will fall to the earth,” Contezac said.
 
While it may not hail very often in places like San Diego, there are some regions that experience really intense hailstorms.
 
According to the National Severe Storms Laboratory, Florida has the most thunderstorms, but Nebraska, Colorado and Wyoming get the most hailstorms.
 
In the summer, when humidity and warmth fuels thunderstorms, the region can experience anywhere from seven to nine days of hail. It’s no wonder this part of the country has even been given the name “hail alley.”
 
Often hailstones are about the size of a pea, but sometimes they can grow to the size of grapefruit. Contezac told me that the size of a hailstone depends on how long it stays up in the storm. As the hailstone gets tossed around, new layers of ice can form around it. 
 
In fact, if we look at a hailstone cut in half, we might just be able to observe some icy rings. They would look sort of similar to the rings you might find if you sawed open a tree trunk.   
 
The way the icy rings look can tell us a bit about the hailstone’s journey through the storm. A white, cloudy ring of ice means the water droplets froze very quickly as they met the hailstone. The water froze so fast, some air bubbles were even left in the water which made it appear cloudy.
 
When we see a ring of clear ice, it tells us that droplets were freezing more slowly onto the hailstone. There was enough time for the air bubbles to escape before the water froze, so the ice looks clear.   
 
While some hailstones make the journey to Earth’s surface, there are other hailstones that simply melt away on their journey down from the atmosphere.
 
Hail might make us take cover, but it sure makes me curious, too. Who knows, maybe one day you will become a meteorologist and help us learn more about our planet’s incredible weather.
 
Sincerely,
Dr. Universe