|HAIL SIZE AND HAIL DAMAGE
METEOROLOGIST JEFF HABY
Hail is spheroid particles of ice that fall from storms. To be hail it must fall from a convective storm and the
particle size must be greater than or equal to ~0.2 inches. The official definition of severe hail that is used by the
National Weather Service is hail that is 1 inch or greater. The 1" is a magic number only because it is
defined as such. Hail smaller than 1" can still do damage. To some structures, hail greater than 1" will
not cause damage. Some in the severe weather field will consider 1/2" or greater as the size to alert on
while others do not feel an alert is warranted until the size is 1" or greater. Below are a variety of my
thoughts on the hail size issue.
WIND: The wind speed will enhance the damage that is caused by hail. For example, hail being blown by
a 70 mile per hour wind is capable of doing more damage than hail falling in light wind. In a severe storm
situation the hail size can often be greater than 1" combined with severe winds. This type of situation
can cause severe damage from hail. A hail size of less than 1" that is blown by very strong wind can
still do significant damage to crops. To farmers, any hail size is too big.
WHAT IS IT YOU WANT TO PROTECT?: The answer to the question will greatly determine the hail
size that is considered severe. If it is a car, window, aircraft or crops that need protection, hail size
smaller than 1" can still do damage especially in a wind blown situation. Since
plants are very sensitive, even small hail below the severe threshold can do significant damage to them.
If it is a sturdy building, small hail is
unlikely to do much damage. The magnitude of damage tends to be much less with the small hail when it comes
to buildings and structures. Once hail gets over one inch then the damage to cars, windows and roofs and
structures goes up significantly. The force impact of hail goes up exponentially with hail size. For example,
1.5 inch hail will not just do twice as much damage as 0.75 inch hail, it can do many times more the damage.
VARIABILITY OF HAIL SIZE: The hail falling from a storm will not be all of one size. There will be a great
variability in hail size. Warnings are issued for the largest hail size falling from the storm thus the
hail size experienced will often be smaller than the reports of largest hail size falling from the storm. For example,
if a storm is reported to be severe due to 1" hail size it is likely most will experience smaller hail than
this unless the storm dynamics change. There
is not an exact way to know the hail size any one place will experience. One place could have golf ball size hail
while a mile away there is just marble size hail. Because the hail size is variable, the damage will be
variable also. The damage will be much more extreme where the hail sizes are the largest. When a warning is
issued from a storm for large hail you could end up getting large hail, small hail or no hail from the storm
depending on where exactly you are in relation to the storm.
WHAT MAGNITUDE OF DAMAGE WILL YOU ACCEPT?: The smaller hail size will result in less damage. However, one
person's light damage can be another person's severe damage. One person may be able to overlook very small and
hard to see dents on a car while another person would consider it very bad damage. Some minor crop damage can
be considered very severe to a farmer if they were counting on harvesting all of the crop. Those that
can not tolerate any damage will tend to go with a smaller threshold (i.e. 1/2" for severe hail) while those able to
tolerate minor damage will go with a higher threshold (i.e. 1" for severe hail).
NUMBER OF WARNINGS: The number of severe storm warnings will be much higher if the hail threshold size is
smaller (i.e. 1/2"). This will produce an excess of warnings in which the public will not take them as seriously.
Also, as mentioned, when a storm is warned on for hail almost all people will experience a smaller hail size than
what the storm is warned for. Partially because of this, the threshold was moved from 3/4" up to
1" for severe hail. This will reduce the number of warnings and people will take the hail warnings more
seriously. The downside though is that the smaller hail can still produce damage.
SHOULD SEVERE HAIL THRESHOLD BE CHANGED?: For general purposes the 1" threshold is a good one in my
opinion. It is pretty much just right.. not too big and not too small of a threshold. However, as mentioned
the threshold you consider severe is going to depend on what you are trying to protect, what you
consider significant damage and on other factors that go along with the hail.