Why do hailstones grow big in summer, the hottest time of year?
After you watchWhy do hailstones grow big in summer, the hottest time of year?
The short answer
Big hail falls in summer because it needs a violent thunderstorm with a powerful updraft, and hot summer afternoons are what build those storms. The updraft is a strong column of rising air that catches a small ice pellet and flings it back up to the freezing top of the cloud again and again. Each trip the pellet collects a layer of water that freezes onto it, so it grows bigger and bigger until it is too heavy for the wind to hold and drops as a large hailstone.
Try this next
- What if you keep the storm strong but let the pellet start out heavier? Drop a bigger, heavier pellet into the strong storm and predict first: will the updraft still be able to fling it back up, or will it fall out sooner with fewer layers?
- What if a storm had a really strong updraft but the freezing part of the cloud was tiny? Picture shrinking the freezing zone at the top. Predict how many layers a pellet could collect before it runs out of cold cloud to loop through.
Now you — bend it
- What if What if you nudge the updraft just past the speed where a stone's fall finally beats the wind?A stone drops the instant its terminal velocity overtakes the updraft. Predict whether a hair more wind makes the final hailstone bigger, smaller, or the same size before you slide it.
- What if What if the storm's freezing level sat much higher up, so the cold zone the pellet loops through is a thin sliver near the very top?Each loop only adds a layer while the pellet is in below-freezing air. Predict how a shorter freezing zone changes how many rings a stone can stack per trip.
- What if What if you launched a pellet that was already heavy and many-layered into the strongest updraft you can make?Lift has to beat the stone's growing weight to recycle it. Predict whether even a violent blast can fling a baseball-sized stone back up, or whether it falls out on its first pass.
Can you prove it?A hailstone only grows while the updraft is faster than the stone's terminal (falling) speed; the moment the stone's fall speed wins, it drops — so bigger hail demands a faster updraft. — Watch the strong-storm run and count: the stone climbs and gains a ring each loop, but the layers make it heavier, so its fall speed creeps up every trip. Predict the loop where its fall speed should finally beat the wind, then watch it fall on exactly that loop. Re-run with a weaker wind and check the stone drops out on an earlier loop, at a smaller final size — falling speed crossing the wind speed is what ends growth, not the cold.
Design your own test:Before you release the pellet, predict the exact updraft setting below which it falls straight out as a tiny speck after one trip — and how many extra layers each notch of stronger wind buys you above that threshold.
Explain it to a 6-year-old: A strong wind inside the cloud keeps tossing a little ice ball back up into the freezing part, and each toss adds a new frozen coat until it's too heavy and falls.
The whole story
How it works
A tall summer storm cloud is hot and sticky near the ground but far below freezing high up. Strong sunshine heats the ground, which heats the air, and that warm air shoots upward as a fast updraft through the middle of the cloud. A tiny ice pellet gets carried up into the freezing region, where it bumps into cold water droplets that freeze onto it as a new layer. It then falls, but a strong updraft catches it and lifts it back up to freeze on another layer, looping over and over. Only when the hailstone is too heavy for the updraft to hold does it finally fall to the ground.
What people get wrong
Many people think hail means the weather is cold, or that hail just falls straight down out of a cold cloud. Really the air at the ground on a hail day is usually hot. The ice forms high up where the cloud is freezing, and what makes the hailstone grow big is not the cold but the strength of the storm's rising wind. A fierce, hot-day storm has the strongest updraft, so it grows the biggest hail.
The catch
A strong updraft can recycle a pellet dozens of times and grow hail the size of a marble or even a baseball, but only the most violent summer storms make a wind that strong, so giant hail is rare and usually signals a dangerous storm. A weak updraft is far gentler and much more common, but it cannot hold a growing pellet up, so it can only make a few tiny specks of ice, or just rain.
Questions kids ask
Why does hail fall in hot weather instead of cold weather?
Hail forms high inside a storm cloud where it is always below freezing, even on a hot day. What makes a big hailstone is a strong updraft, the rising wind inside the storm, and hot summer afternoons are exactly what build storms with the strongest updrafts. So the heat at the ground is what powers the storm that grows the ice.
How does a hailstone get its layers?
Each time the updraft flings the pellet back up to the freezing part of the cloud, it picks up a coat of water that freezes onto it as a new layer. The more loops it makes, the more layers it stacks up, which is why if you slice a big hailstone you can see rings like an onion.
Why does a stronger storm make bigger hail?
A stronger updraft can hold up a heavier hailstone, so it keeps catching the pellet and lifting it back to the freezing top many more times. Each loop adds another frozen layer, so the stone grows larger before it finally becomes too heavy for the wind to hold and falls.
Why doesn't it hail in winter?
Winter storms have weak rising winds and the freezing level sits very low, so ice pellets fall out before they can loop and grow. Instead the cloud's ice simply drifts down gently as snow. Big hail needs the powerful updraft of a strong, warm-season thunderstorm.
Talk about it
- It is hot out and ice is falling from the sky — guess first, where do you think that ice came from?
- If we sliced a big hailstone in half like an onion, what do you think we would see inside, and why?
- Two storms pass over us on the same day. What might make one drop hail and the other just rain?
For grown-ups
Intense summer surface heating fuels powerful convective updrafts (tens of metres per second). A small ice pellet called graupel is carried into the cloud's sub-freezing region, where it collects supercooled water droplets that freeze onto it (a process called accretion). The updraft recirculates the pellet, so it gains a fresh ice layer on each pass, which is why a sliced hailstone shows concentric rings. It falls only when its terminal velocity exceeds the updraft speed, so a stronger updraft suspends a heavier, larger stone before dropping it. Winter storms have weak updrafts and a low freezing level, so they produce snow rather than large hail.
Keep going
What else makes you wonder?
- What is the biggest hailstone that has ever fallen, and how many trips up the cloud did it take to grow that big?
- If you could see inside a storm cloud, how fast would the updraft feel if you were riding it?
- Why do some summer storms make hail while others right next door only make rain?