Hail Formation and Effects
Hail is one of the many weather phenomena that a storm chaser faces.
When we're in pursuit of a storm, and catch some hail, there are a
multitude of thoughts; most of which begin with the phrase, "Uh oh."
Uh oh. How big is it going to get?
Uh oh. How long is it going to last?
Uh oh. We screwed our angle of approach.
Uh oh. We're closer than we thought to the danger zone.
Uh oh. We're in the danger zone.
Uh oh. Do I have glass coverage?
Mid-latitude thunderstorms can grow to be 16-18 km (10-11 miles) tall.
Although the surface may be fairly warm, the freezing level (0°C or 32°F)
typically occurs at about 4 km (2.5 miles) up... meaning that the upper
75% of the storm is maintained at or below freezing temperatures. The
strong updrafts in a developing thunderstorm or supercell can reach 160kph
(100mph), bringing large amounts of moisture and debris into the storm.
The debris (aerosols, dust, dirt, insects) act as nuclei for the water to
collect on, creating a tiny raindrop. As the new raindrop travels upward
in the updraft, it collects more water droplets... a process called
coalescence. The larger the drop becomes, the more efficiently it
collects other drops.
So far, only raindrops have been produced. To produce ice, the
droplets must continue traveling upward beyond the freezing level. At
this level, it is possible to have solid particles (snowflakes or ice
crystals), spongy ice-water particles (graupel), or supercooled water
droplets (below freezing, but still liquid). With one of these particles
as a nucleus, other frozen particles can collect on it and form larger
frozen particles. For example, snowflakes, ice crystals, or other graupel
might adhere to the walls of a graupel particle, making it bigger. As
the new particle continues traveling upward, the environment becomes even
colder, forcing all remaining water to freeze.
At some point, the mass of the ice particle and the gravitational pull
on it are balanced and the ice particle reaches the top of its journey...
it falls back toward the ground, still collecting more mass on its way.
The return trip downward is a good point to introduce the two types of
hail growth: dry growth and wet growth. If the hailstone (an ice
particle larger than about 1mm or 0.04") is still in the very cold
portion of the cloud (colder than -40°C or -40°F), it will undergo dry
growth. This means that the hail stone itself has a hard, dry, icy
coating and grows by collecting other "dry" particles... this process is
not very efficient.
However, in the region of the thunderstorm where temperatures are
between -40°C (-40°F) and 0°C (32°F), the hailstone can undergo wet
growth. Wet growth involves the latent heat of fusion. To explain this,
start with ice. To melt the ice, heat must be extracted from the
environment and added to the ice. This added energy turns the ice into
water. On the other hand, when water freezes and becomes ice, energy is
released to the environment. The amount of energy required to make this
transformation is called the latent heat of fusion. So, back to the
hailstone. If it collects a supercooled water droplet and the droplet
freezes on contact, and this layer becomes clear if the freezing is very
fast and somewhat opaque if the freezing is slow (as oxygen bubbles
incorporated into the layer). It releases energy to the environment...
in this case, to the hailstone. Every supercooled droplet the stone
collects releases energy, and eventually,
effect is large enough that the hailstone's surface begins to melt! Now
the hailstone has a wet coating, it collects other particles (water,
supercooled water, snowflakes, ice crystals, graupel) very efficiently.
If the percentage of liquid in the hailstone becomes too great, some
water will shed off of the stone (because of a balance of air drag and
surface tension) leaving a smaller, lighter stone in its place. At this
point, the hailstone (whether it shed water or not) could still be light
enough to be caught by the updraft again and start the growth process all
over. This repeats until the hailstone is too heavy for the updraft to
support and it falls out of the cloud. As it falls, it travels into
air, allowing it to melt slightly. Even so, the largest hailstones can
reach 15cm (6") in diameter, weigh 3.5 pounds, and hit the ground at
(110mph). All of this from an updraft and a piece of dust. If you slice a
hailstone in half, you can see the layers that chart its lifespan...many
times clear layers altering with frostier, more opaque ones. I believe
record count of layers was recorded by NSSL as 24. There are other
much more dramatic.
Hail accounts for the destruction of 1% of the worlds crops. Damage
be severe enough to kill livestock and create millions of dollars worth of
damage to homes. Hail can also kill humans as well as damage cars and
vehicles in the path. The appearance of hail in a storm can be of a
tint to the gray cloud mass of the storm. In the US, there are about 4800
hailstorms every year, which, combined with property damage, equates to 1
billion dollars a year in this country alone. The largest hailstone
in this country was in Coffeyville, Kansas, and was 17.5 inches in
circumference. When you consider that quarter sized hail require an
of nearly 40 MPH to sustain it, this is a rather impressive size.
the heaviest recorded hailstone was in northern India. In was born from
March 10, 1939 storm and weighed a full 7.5 pounds.
Just as there is a tornado alley, there is a hail alley. This covers
a portion of our country including E. Colorado, Nebraska, and Wyoming.
Other areas of high hail instances are northern India. The common thread
seems to be midlatitude areas on the downwind side of a mountain chain.
The lowest instances of hail in this country are on the Pacific coast,
and in Florida.
Knowing the dynamics of a storm can help you to estimate where a hail
shaft may exist in a particular storm. It is usually between the sections
of the storm known as the downdraft and the updraft. When using computer
programs to track storms, dbZ, A logarithmic expression for reflectivity
factor, is often used to determine the probability of hail within a storm.
Z is the amount of radar beam energy that is back scattered by a target and
detected as a signal (or echo). The higher the z, the more energy is being
scattered back by the target, or the greater the intensity of the
precipitation. So many other factors can affect reflectivity, such as
distance, angle of beam, width of beam, the size and type of the
precipitation; that dbZ cannot verify conclusively the presence of size of
hail in a storm. Values of 50 or above usually do occur when hail is
Despite the fact that hail is extremely dangerous and damaging, it has
been used by chaser as a known precursor of an even more danger severe
weather event...the tornado. The hail shaft, as mentioned, most often
in the portion of the storm very near to where a tornado would form.
how one severe weather event can be the warning bell for yet another.
However, penetration of a hail shaft has been enough to send many seasoned
chasers into either an about face, or searching for immediate shelter.
If you have the capability of tracking the storm, and understand the
dynamics of thunderstorm development, a seasoned chaser can almost always
avoid the hail shaft. A NOAA weather radio can be a helpful tool also to
prevent yourself from getting caught. Should all else fail, all you can
do is seek shelter under a sturdy shelter until the storm passes.