Hail
Large hail is not a killer, but does considerable damage
Falling from 20,000 feet or higher, large hailstones pack quite a punch!
Large hail does significant damage to cars.
Average annual number of days with hail
This is the Aurora
hailstone, the
largest one at the
time. It fell at
Aurora, NE on
June 22, 2003.
The Aurora hailstone was 7 inches in diameter. Imagine that falling
on your head from 20,000 feet up!
This is the previous record-holder, the Coffeyville hailstone with
some props (egg, hand) for comparison.
The Coffeyville hailstone cut into sections. Notice the layers
This stone fell on Coffeyville, KS on Sept 3, 1970.
The rings are caused by different growth regimes. Clear ice is
deposited slowly and contains few air bubbles. Cloudy ice occurs
when water freezes quickly, trapping the air bubbles.
The current record-holder is the Vivian (SD) hailstone, July 23, 2010.
(http://www.noaa.gov/features/02_monitoring/hailstone.html)
Hail is often categorized by its size in relation to everyday objects
Marble
Sizes of hail and the everyday objects used as adjectives
(e.g., baseball-sized hail is 2.75” in diameter)
Oneonta hail, June 15, 2009.
This is marble-sized
Those are small pine cones for comparison.
A schematic supercell radar echo with a hook echo.
Tornadic thunderstorms often also have large hail.
The hailfall area is close to the tornado.
Ingredients for Hail Growth
In a basic sense, 3 Main “ingredients” are required:
1. Adequate updraft to keep hailstone aloft for an
appropriate amount of time, -10 to -30C
2. Sufficient supercooled water near the
hailstone to enable growth during transit
through an updraft
3. A piece of ice or snow (embryo) for it to grow
upon
Vertical cross section of a supercell hailstorm.
Forecasting Hail
Deep, Moist Convection (DMC) has 3
Ingredients:
1. Sufficiently deep low level moisture
2. Steep lapse rates (related to instability)
3. Sufficient lifting from LCL to LFC
 Once DMC identified as possibility, look
for hail signal
Hail Ingredients
1. Strong
updrafts
necessary but
not sufficient
 High CAPE in
hail growth
zones
Hail Ingredients (continued)
2. Storm scale
winds:
Speed/Direct-ional
shear is important.
Directional shear helps
create a rotating
updraft. Rotation can
enhance updraft
strength by a factor of
2 or more, above that
of an updraft created
by buoyancy alone.
12 hours earlier
The concept of the Wet-bulb Zero
Wet-bulb Zero (WBZ): This is the level where evaporative cooling reduces a parcel
temperature to 0C. It correlates well with large hail when the altitude of the WBZ is
between 2200 meters and 2800 meters.
If the WBZ is higher than 2800 meters, hailstones must fall through a large layer which
is above 0C and usually do not reach the ground in frozen form.
If the WBZ is lower than 2200 meters, the lower atmosphere is relatively cold and stable
so the large updrafts needed for hail formation don’t exist.
The exception to the above guidelines is when the ground level is significantly above
sea level. That usually reduces the thickness of the low-level warm layer and hail
becomes more common. This is the reason for the hail maximum in the High Plains and
especially east of the Colorado Rockies.
Source: A World of Weather by Lee Grenci and Jon Nese (2001)
Example of Skew-T Log P determination of WBZ
WBZ
729 mb 2788m
Tennessee
Colony
SHIP - Significant Hail Parameter
Strengths
• Based on large number of soundings
and is simple to calculate
•High POD of SIG HAIL (>90%)
•Statistically separates SIG from NON
SIG hail
• Widely available: (SPC Mesoanalysis
page, SFCOA, NMAP, NSHARP)
Weaknesses
• Only forecasts SIG Category, not
actual hail size
(values > 1 favorable for SIG)
•Conditional on >0.75” hail
“going to occur”
•Based on MUCAPE, thus may exhibit
noise at times
• Again, dependent upon accurate
thermodynamic profile forecast
• Will instability be realized? Forcing is
not parameterized
SHIP = MUCAPE * MUMR * 7-5LR * 500T* 0-6 SHR / Constant
MUMR = Mixing Ratio of Most Unstable Parcel
Next: The most common type of severe
weather: Downbursts