is Hail
What hail?
Hail is a ball or irregular clump of ice Hail is different from winter precipitation!
Not sleet (raindrops that have frozen) Not freezing rain Not snow Not graupel
Hail forms only in convective processes
Hail cannot exist without a thunderstorm
Hailstones range in size from that of a pea to that of a grapefruit (more precisely 5 mm)
Dr. Christopher M. Godfrey University of North Carolina Asheville ATMS 103 Fall 2008 ATMS 103 Fall 2008
Oh, Hail!
World Record Hailstone
Aurora, Nebraska, 22 June 2003
ATMS 103 Fall 2008 ATMS 103 Fall 2008
An Aurora resident with a handful of hail
The old record holder (but still the heaviest!)
Coffeyville, Kansas, 3 September 1970
ATMS 103 Fall 2008 ATMS 103 Fall 2008
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My biggest hail stone
U.S. Hail Climatology
# Days per year with hail 0.75 inches in diameter
C. Godfrey 29 May 2003
ATMS 103 Fall 2008
# Days per year with hail 2.0 inches in diameter
Hail causes nearly $1 billion in damage to crops and property annually
ATMS 103 Fall 2008
Hail Damage
Hail Formation
Above ~15C within a cumulonimbus cloud, supercooled droplets freeze on contact with ice nuclei (the frozen equivalent of condensation nuclei)
At the surface, the equivalent process produces rime ice For rime ice accretion, the feathers of the crystals point in the direction of the wind (i.e., where the wind is coming from)
Amarillo, TX, 21 June 2004
ATMS 103 Fall 2008
Photo: Derek Brown
ATMS 103 Fall 2008
Rime Ice
Hail Formation
Supercooled drops collide with and coalescence onto a frozen particle, forming a hail embryo Hail embryos in the strongest part of the updraft are carried upward to the anvil Embryos on the edges of the updraft fall back supercooled into water and grow to form graupel (diameter ~15 mm).
ATMS 103 Fall 2008
ATMS 103 Fall 2008
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Hail Formation
Some graupel particles grow into hail Largest hail forms when a graupel particle grows within the turbulent updraft in a rich environment of supercooled water Largest hail falls nearest to the main updraft Hail eventually falls out of the thunderstorm because the updraft can no longer sustain the weight of the hailstone
Hail Formation
Wet Growth
So many supercooled drops freeze onto the hailstone that the resulting latent heat release raises the surface temperature so that the water briefly remains unfrozen Liquid water fills the porous regions, removing air bubbles
Dry Growth
Supercooled water freezes on contact and leaves air bubbles (like rime ice)
ATMS 103 Fall 2008
ATMS 103 Fall 2008
Wet vs. Dry Growth
Hail Embryo Wet Growth Dry Growth
Hail Growth in a Supercell
ATMS 103 Fall 2008
ATMS 103 Fall 2008
Forecasting Hail
Strong updrafts
Measured by convective available pote...