Severe Thunderstorm element: HAIL

By Rick Swierczynski, N9TPY, Associate Director and Vice President MASA Inc.

Thunderstorm hail is not normally a direct threat to life, but hail stones are one of the more costly severe weather elements in its damage to crops, roofs, livestock, automobiles and other outdoor objects. There are several requirements for a storm to be classified as severe. The hail requirement: 3/4 inch stones or larger.

Roughly 20% of all severe weather events in Wisconsin are hail events in which the hailstones are a least 3/4 inch in diameter. Serious hail storms (with hailstones 1.5 inches or larger in diameter) are not common in Wisconsin and rarely is a person injured or killed by large hail in Wisconsin. The peak hail season in this area is April through August, although hail has been reported with thunderstorms in every month of the year. One of Wisconsin's worst hail storms hit the city of Madison (60 miles West of Milwaukee) on April 13, 2006 resulting in damage to over 200 autos and other home structures with stones up to 4 inches in diameter.

Meteorologically, hail forms in the updraft of thunderstorms. The size of the hail is related to the updraft strength. Weak updrafts will produce smaller hail, while well organized storms with strong updrafts often produce larger hailstones.

If a severe storm is approaching you directly, you may experience the following sequence: light rain changing to heavy rain, then small hail becoming larger and larger. Any combination is possible, but any hail event should immediately increase the spotters situational awareness. If large hail falls at your location you are often in the most dangerous area of the storm.

The larger (and heavier) hailstones often fall very close to the updraft or tornado developing area, while the smaller hail and rain are blown further downwind in the direction of storm movement by upper level winds before falling back to the ground.

For the Mobile Skywarn spotter the automobile will offer adequate protection from most upper Midwest hailstorms, however hailstones larger then 2 inches seem to be a threshold when car windows start to smash. Never drive willfully into a hail shaft. What may start out as pea sized hail can quickly become golf ball or larger in just a few seconds. Pull off the road, or seek shelter under the nearest overpass provided no tornadic storms. Our best spotters practice safety first!!

When viewed from a distance some approaching hailstorms appear as a curtain or sheet of grey to white extending from cloud base to ground level. Green and Turquoise sky colors have often been associated with some hail and tornado producing storms--and this will sometimes alarm the uninformed public. In storms with very large hail, an audible rumble is sometimes heard. Not all hailstones are round in shape, some may be egg-shaped, while others have sharp protruding points along many directions.

Ideally, ALL HAIL REPORTS SHOULD BE MEASUREMENTS. Sending in a report with a specific numerical size leaves no room for confusion.

Marble size hail..... Since marbles come in different sizes, this report would require a clarification. A better approach would be a hail conversion chart and ruler (or calipers) with the spotter and at the relaying station. In 2006, spotters in southeast Wisconsin may use the term "Small Hail" to indicate 1/2 inch stones (only).

Numerical hail reports (estimated or measured) assist radar operators to more precisely adjust the radar's hail detecting software programs for more accurate forecasts regarding hail producing storms.

SERIOUS Spotters should obtain a ruler or calipers for measurement accuracy, making measurements only when it safe to do so. Typically the largest or most common sized hailstones are reported. For odd shaped stones measure along the stones longest axis. If a stone measures 2x3 inches, report it as 3 inches. 2x4 inches, report it as 4 inches. etc. If the hail covers the ground in sufficient quantities and depth, report that as well. If you observe real-time large hail damage occurring report it immediately, along with injuries or other significant damage.

Most people tend to over estimate hailstone sizes, and this condition is worse during night spotting. When making estimates keep in mind as the distance between you and the stone increase YOUR accuracy decreases!

ALL reports should be qualified as either "MEASURED" or "ESTIMATED" in inches.

HAIL SIZE CHART

Pea Size	.25	1/4	Inch
Small Marble	.50	1/2	"
Penny	.75	3/4	"
Nickel	.88	7/8	"
Quarter	1.00	15/16	"
Half Dollar	1.25
Ping Pong	1.50
Golf Ball	1.75
Lime	2.00
Tennis Ball	2.50
Base Ball	2.75
Large Apple	3.00
Grapefruit	4.00
CD or DVD	5.00

Copyright and Updated 3/22/98, 3/3/02, 3/3/03, 4/19/04, 5/6/06, 11/4/08, 2/7/09. Ask permission to use this material before you copy and paste. We go after plagiarizers. Violators have been prosecuted.

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