How hail is made
- View
Wet growth, dry growth, and the updraft that has to hold a stone up long enough to build it. Plus the two pieces of hail folklore worth unlearning first.
In this lesson you will
- Explain the difference between wet growth, which makes clear ice, and dry growth, which makes cloudy ice.
- Relate hail size to the updraft that supports it, and state the NSSL fall-speed bands.
- Locate the hail core relative to supercell structure.
- Evaluate green storm color as a hail cue, and state what it actually indicates.
One stone, two kinds of ice
A hailstone starts as something small carried above the freezing level, and grows by collecting supercooled water that freezes onto it. How fast that water freezes decides what the ice looks like, and NSSL puts the distinction plainly: water that freezes instantly on contact traps air and makes cloudy ice; water that freezes slowly lets the air escape and makes clear ice.
Hailstones do not bob up and down through the storm, adding a layer per trip. NSSL corrects this explicitly: the layering comes from the stone's horizontal passage across and near the updraft, meeting different water content and temperature each time. Counting layers does not count trips.
How fast it falls, and what that costs the updraft
For a stone to grow, the updraft has to hold it up. That is the whole relationship between hail size and storm strength, and the fall speeds are the reason it is so demanding at the top end.
| Stone size | Fall speed |
|---|---|
| Under 1 inch | 9 to 25 mph |
| 1 to 1.75 inches, typical severe | 25 to 40 mph |
| 2 to 4 inches | 44 to 72 mph |
NSSL figures. Higher numbers circulate for golf balls and softballs, and they come from secondary sources rather than NSSL; this course does not use them.
So a storm producing four-inch hail is holding up something falling at seventy miles an hour, and doing it long enough to build the stone. The configuration that manages it consistently is the broad, persistent, rotating, tilted supercell updraft from module 4. Tables that convert a specific updraft speed to a specific hail size are not NWS figures and are not reproduced here.
Where the hail comes down is structure, not chance: the hail core sits near and forward of the updraft, on the forward flank, which is the same part of the storm module 4 called the FFD. Between 40 and 70% of what forms aloft can melt before it reaches the ground, which is a footnote here and the whole story in lesson 8.
Green sky
Green storms are real. Gallagher, Beasley and Bohren measured them, and Bohren and Fraser worked out the optics. What the research did not find is any reliable relationship between the color and hail size, or between the color and tornadoes.
That the storm is deep. That is the whole of it. The color is a property of light traveling a long way through a very tall cloud, and it is independent of the ground underneath it, so the version about green fields reflecting upward is wrong too. A deep storm is worth watching. It is not a hail measurement.
Check yourself
A hailstone cut in half shows six alternating clear and cloudy layers. What can you conclude?
- That it passed repeatedly through regions with different water content and temperature
- That it was carried up and down through the storm six times
- That the updraft was at least six thousand feet deep
- That the storm was producing severe hail somewhere
Hint
One of these is the folklore version, and NSSL corrects it by name.
Answer
That it passed repeatedly through regions with different water content and temperature
The layers record conditions, not trips. NSSL is explicit that layering comes from horizontal passage across and near the updraft, not from cycling up and down, so counting layers tells you nothing about how many times the stone went round.