METEOROLOGIST JEFF HABY
Hail is both destructive to vegetation and manmade structures. Hail is classified as severe by the National
Weather Service if it is equal to or greater than 3/4" in diameter.
Strong winds make these darting spheres
of ice even more damaging. It is difficult to pin point where exactly a
large hail shaft will strike just
as it is difficult to predict where
tornadoes will exactly occur. However, the general region where hail
can be expected is very predictable. Hail occurs in association with thunderstorms, particularly
supercell thunderstorms. Below are factors to consider when trying to forecast for the likeliness and size of hail.
Higher elevation areas are closer to the cold layers of the upper atmosphere. When a hail stone falls,
it rapidly begins to melt when the environmental temperature rises above freezing. If the hailstone has
to fall through a deep layer of warm air, it will melt from the outside in, turning into non-damaging
raindrops or decreasing significantly in size. Mountainous regions and the High Plains of the United
States have the highest number of hail days per year. Small hail which would normally melt before
reaching the surface in a low elevation area reaches the surface in high elevation area. Storms do
not need to be as severe in the lee of the Rockies as in lower elevation areas for hail
to reach the surface.
2. FREEZING LEVEL:
The freezing level determines the depth of the atmosphere that is above freezing. If the freezing level is
high in the atmosphere, hailstones will have more time to melt than if the freezing level is close to the
surface. A high freezing level also decreases the vertical depth in which hailstone formation and growth is possible.
The freezing level depends on elevation, the season, and the temperature profile of the atmosphere.
High elevation areas will have relatively low freezing levels in all seasons. For low elevation areas a
general rule to follow is: If the freezing level is closer to the surface than 650 millibars, strong
thunderstorms have a good probability of producing hail that will reach the surface. The freezing level can
be found readily by examining the morning or afternoon Skew-T Log-P plot or forecast sounding.
3. WET BULB ZERO LEVEL:
The wet bulb zero level is defined as the freezing level that will result due to
evaporative cooling. The freezing
level will lower if there is dry air in the mid-levels of the atmosphere. This occurs due to evaporative cooling
of environmental air that entrains into a thunderstorm. This same entrainment can also produce strong and
gusty surface winds. Dry mid-levels are common in the Great Plains. This is another factor that leads to many hail
days in this region of the U.S.
4. CONVECTIVE AVAILABLE POTENTIAL ENERGY
This is the most important factor in determining hail size. CAPES under 1000 J/kg generally produce borderline
severe hail (near 3/4" or less) while CAPES over 2000 J/kg can produce very large hailstones. High CAPES lead
to high upward vertical velocities within a thunderstorm. High UVV's can suspend hailstones and add layers of
ice onto already developed hailstones. The amount of CAPE can be approximated by modifying the morning Skew-T
sounding for that day. In many cases this is executed by changing the surface temperature and
dewpoint to fit
current observations. Forecast model soundings can also be examined for changes in CAPE during the day.
5. SUPERCELL THUNDERSTORM (HIGH WIND SHEAR):
Strong upper level winds allow CAPE to be maximized to its fullest potential. Strong upper level winds tilt the
updraft of developing thunderstorms. This allows the
updraft and downdraft to be separated from each other. This
produces higher UVV's in the updraft.
6. PRECIPITABLE WATER:
The weight of moisture and water will influence the strength of the updraft. High moisture soundings result in water
loading. CAPE is reduced with water loading since the force of
gravity pushes down on the liquid water drops.
Precipitable water values of less than 1.0" will not be nearly as influenced by water loading than if precipitable
water values are above 1.5". Lower precipitable water values have the potential to produce large hailstones when
significant CAPE is present. Low precipitation supercells are notorious for producing large hail. In
the lee of the Rockies, PW is climatologically low, adding to the hail potential.
----- consensus -----
Hailstone size is maximized by high elevation, low freezing levels, low PW, dry mid-level air, high CAPE, and large wind
shear. The region of the country that these factors come together the most are in
the High/Great Plains of the US.
Hailstone size is minimized by low elevation, high freezing levels, water loading (high PW), moist mid-levels,
low CAPE, and weak wind shear.