It's midsummer, so why is ice falling from the sky?
― Lifted again and again, the ice thickens bit by bit
On an afternoon over 30°C, white pellets of ice can suddenly hammer the roof. It sounds strange, but a few kilometres up, the air is below freezing even in midsummer. Inside a towering storm cloud, a powerful updraft carries small ice pellets upward again and again. Each time they're carried up, water around them freezes onto them. Layer upon layer builds up until the ice becomes too heavy to stay aloft and falls. That, it's believed, is hail.
On a summer afternoon, the sky suddenly darkens and heavy rain begins to fall. Before long, something white and hard starts falling along with the rain. A dry clattering sound comes from the roof and balcony.
Looking outside, you see small pellets of ice bouncing off the ground. Place one on your palm and it melts into water almost at once. The temperature is above 30°C — and yet, it's ice.
Where was this ice made, and how did it survive the summer heat all the way down without melting completely?
There are two main reasons
Air temperature drops the higher you go. Even when the ground is 30°C, the air at about 5 kilometres up is already below 0°C. A "place where ice can form" sits over our heads all summer long.
Inside a well-developed storm cloud, air is rushing forcefully upward. A small ice pellet that tries to fall gets pushed back up. During these back-and-forth trips, water freezes onto it, and layer by layer it grows bigger.
Only when both of these come together can large ice pellets form. Let's look at each in turn.
The "ice room" high above never closes, even in summer
The higher you go from the ground, the colder the air gets — roughly 0.6°C for every 100 metres of altitude. If the ground is at 30°C, the air at around 5 kilometres up is close to 0°C.
A storm cloud can push far past this freezing layer, sometimes reaching more than 10 kilometres high. Near the top of the cloud, temperatures can be −20°C or −40°C. In other words, the summer sky always has somewhere ice can form. The reason it doesn't normally reach the ground is that it melts on the way down.
Being lifted "again and again" is what makes the pellet grow
A small ice pellet, left alone, would simply melt as it fell and become ordinary rain. But inside a storm cloud there can be updrafts of tens of metres per second. Before the pellet finishes falling, it gets pushed back up into the freezing layer again.
Within the updraft float many water droplets that stay liquid even below 0°C. These are called supercooled droplets. When an ice pellet collides with one, the droplet freezes instantly on contact. As the pellet is carried up and falls back down, over and over, the ice builds up in layers like an onion. Take a look at Figure 1.
The air is below freezing up high even in winter, but weaker updrafts mean large pellets rarely get the chance to grow. From spring into summer, when strong sunshine heats the ground, the force pushing up into the clouds is stronger. In Japan, statistics show it's especially common in early summer.
Under Japan's meteorological definitions, pellets under 5 millimetres across are called "graupel," and those 5 millimetres or larger are called "hail." The way they form is the same process — the difference is simply how far they've grown.
So what should you actually do?
- If you hear a hard clattering sound, get indoors right awayIce pellets fall fast. Being under a roof is the safest place.
- Move away from windowsLarge pellets can crack glass. Stay near an interior wall.
- Don't go outside while it's fallingYou may want to go pick some up — wait until it stops.
Storm clouds that produce hail can also bring strong winds and lightning at the same time. Stay away from trees and utility poles. The basic advice is to take shelter in a sturdy building and wait it out. If someone is injured — say, from a blow to the head — or has collapsed, don't hesitate to call emergency services. The advice here is only a general guide; the safest action always depends on the actual situation.
Summary
Ice falls in summer because a freezing layer always sits high in the sky. A powerful updraft then carries ice pellets back and forth between that layer and the warmer air below. Each pass, supercooled droplets freeze onto the pellet, adding a layer and making it bigger. When the updraft can no longer hold it up, the pellet suddenly falls. Large hailstones falling are themselves evidence of just how powerful the updraft was.
Hail isn't ice that simply fell from the sky.
It's ice that travelled up and down the sky many times, thickening bit by bit.
For more on the cloud that grows hail, see "Why do towering storm clouds grow so tall?" For how ice and water droplets stay afloat in the sky, see "Clouds are made of water — so why don't they fall?" And for how ice takes its shape, see "Why are snowflakes six-sided?"
- After hail has stopped falling, pick up a pellet and measure its diameter with a ruler. 5 millimetres or more counts as "hail."
- Place a pellet on a chilled plate and hold it up to bright light. You may see clear patches and cloudy white patches arranged in layers.
- Compare how ice water in an ice-cube tray looks when frozen slowly versus frozen quickly. Ice that freezes fast turns cloudier white. Hail's white layers form for the same reason.
Only pick up pellets once the hail has fully stopped and it's safe to go outside.
Want to go deeper? ― terms, formulas, and how this fits the curriculumEach item is labelled by level, from middle-school science through university-level specialty courses
- Middle Schoolcovered in middle-school science
- High Schoolcovered in high-school "Earth Science Basics / Physics Basics"
- High School+advanced high-school content, or textbook sidebar material
- Universitynot covered in high school — university-level specialty course content (meteorology, cloud physics)
- Researchnot yet settled even at university level — something researchers are actively investigating
Middle SchoolTerms: this phenomenon has names
- Supercooled droplet: a tiny water droplet that stays liquid even below 0°C. Given something to collide with, it freezes suddenly on the spot.
- Riming: an ice pellet capturing and freezing supercooled droplets, growing outward. This is the main way hail grows.
- Terminal velocity: the speed a falling object reaches when air resistance balances gravity and it stops accelerating. Bigger pellets fall faster.
- Lapse rate: the rate at which temperature drops with altitude — roughly 0.6°C per 100 metres.
Middle SchoolHigh SchoolChecking with a formula: how fast does hail actually fall?
The falling speed of hail is roughly determined by the pellet's diameter. Here we use approximate figures and convert to km/h to get a feel for it. The symbols and units used are shown in the table below.
| Symbol d | Meaning: diameter of the hail pellet / unit: centimetres |
| Symbol v | Meaning: terminal velocity while falling / unit: metres per second |
| Symbol V | Meaning: terminal velocity converted to km/h / unit: kilometres per hour |
| In symbols | v = √( 8 × ρice × g × r ÷ ( 3 × ρair × Cd ) ) |
| In words | Terminal velocity = the square root of (8 × ice density × gravitational acceleration × pellet radius) divided by (3 × air density × drag coefficient) |
| Where it comes from | It's the balance point where the pellet's weight (proportional to volume) and air resistance (proportional to cross-sectional area and the square of speed) cancel out, so it stops accelerating. |
| Symbol ρice (density of hail) | taken as approx. 900 kg/m³ |
| Symbol ρair (density of air aloft) / Symbol Cd (drag coefficient for a near-spherical pellet) | taken as approx. 1.0 kg/m³ / approx. 0.6 |
| Symbol g (gravitational acceleration) / Symbol r (radius) | 9.8 m/s² / 0.005 m |
| ρice × g | 900 × 9.8 = 8820 |
| Multiply by radius r | 8820 × 0.005 = 44.1 |
| Multiply by 8 (numerator) | 44.1 × 8 = 352.8 |
| Denominator 3 × ρair × Cd | 3 × 0.6 = 1.8 |
| Divide numerator by denominator (v squared) | 352.8 ÷ 1.8 = 196 |
| Square root gives terminal velocity v | the square root of 196 is 14, so approx. 14 metres per second |
The value from the formula matches well with the observed benchmark used in ① below (about 14 metres per second). If the radius grows 5-fold, speed grows by the square root of 5, about 2.2 times.
| Falling speed of a 1-centimetre pellet | approx. 14 metres per second |
| Falling speed of a 5-centimetre pellet | approx. 33 metres per second |
| Conversion factor, m/s to km/h | 3.6 |
| Convert the 1-centimetre pellet to km/h | 14 × 3.6 = 50.4 |
| Convert the 5-centimetre pellet to km/h | 33 × 3.6 = 118.8 |
| How many times faster is the larger pellet? | 33 ÷ 14 ≒ 2.4 |
Even a 1-centimetre pellet falls at roughly 50 km/h — about the speed of a car in city traffic. A 5-centimetre pellet falls at roughly 119 km/h. And for a pellet to grow that large, the updraft inside the cloud must have been blowing at roughly that same speed.
High SchoolHigh School+Why does it form layers?
High SchoolWhen water freezes, it releases what's called latent heat of fusion. Whether that heat can escape into the surroundings determines how the ice looks. Freeze slowly, and air gets pushed out, leaving clear ice. Freeze quickly, and air gets trapped, leaving cloudy white ice.
High School+When a pellet passes through a region rich in supercooled droplets, the water spreads over the surface and freezes slowly, forming a clear layer. In colder regions with fewer droplets, the water that hits freezes almost instantly and traps air, forming a white layer. The stacked layers are thought to be a record of the path the pellet travelled.
UniversityThe force balance holding up the pellet
A falling pellet experiences gravity and air resistance. Air resistance is roughly proportional to the pellet's cross-sectional area and the square of its speed. The mass behind gravity is proportional to volume — that is, to the cube of the diameter. Working out the balance between the two shows that terminal velocity is roughly proportional to the square root of the diameter — a fairly gentle relationship: even a 25-fold increase in diameter only multiplies the speed by about 5. This relationship is covered in cloud physics textbooks, formalized through terminal velocity and drag (drag coefficient).
📖 For the derivation and further reading: Terminal velocity (Japanese Wikipedia) / Drag (Japanese Wikipedia)
ResearchWhat's still not fully understood
- Does the number of layers really equal the number of up-and-down trips? It's been pointed out that the layer count doesn't necessarily match the number of round trips one-for-one — several layers can sometimes form during a single rise and fall.
- Size is hard to predict. Even when the formation of a storm cloud can be forecast, pinning down in advance exactly where and how large the falling pellets will be remains a difficult problem.
- Its relationship to a changing climate. As temperatures rise, the freezing layer sits higher, lengthening the distance a pellet must fall through warm air and melt. At the same time, updrafts may strengthen — so the net effect isn't settled.
In other words, even the content of this article reflects "what's understood so far." As observation networks get finer, some of it may need to be rewritten.
How this connects to the curriculum (by level)
| Level | Subject / Unit | Where in this article |
|---|---|---|
| Middle School | Science — weather and how it changes | Temperature dropping with altitude; how clouds form |
| High School | Earth Science Basics — atmospheric structure / Physics Basics — falling bodies and drag | Lapse rate and terminal velocity |
| High School+ | Chemistry Basics (advanced) — phase change and latent heat | Heat released on freezing and how it changes the ice's appearance |
| University | Meteorology / cloud physics | Deriving terminal velocity from a force balance |
| Research | Mesoscale meteorology / climate-impact assessment | The "what's still not understood" section |
| ― | Everyday relevance | Deciding to take shelter indoors on hearing a hard clattering sound |
- Japan Meteorological Agency, "Forecasting Terminology — Precipitation (Definitions of Graupel and Hail)," JMA website
- Yoshimitsu Ogura, General Meteorology (2nd revised edition), University of Tokyo Press
- Meteorological Society of Japan (ed.), Encyclopedia of Meteorological Science, Tokyo Shoseki
- NOAA National Severe Storms Laboratory, "Severe Weather 101: Hail"
- Japan Meteorological Agency, "Protecting Yourself from Lightning and Hail," JMA website
※This article is a general-audience science explainer. The figures given are approximate values meant to aid understanding of the mechanism. For what to do during severe weather, follow announcements from the Japan Meteorological Agency and instructions from fire authorities and local government.