A look at how spring storms can produce Texas-size hail
Hail, that frozen marvel that can range from pea-size to baseball-size behemoths, is formed within thunderstorms of considerable strength. Understanding the process behind hail formation reveals the raw power and dynamics of this atmospheric phenomenon.
Where the journey begins
First, it should be noted that not all thunderstorms are capable of producing large hail. Factors such as atmospheric instability, severe wind shifts and moisture content at the surface and aloft play crucial roles in determining a storm’s potential for hail formation.
Hail starts as tiny water droplets or ice crystals within a thunderstorm’s updraft. Updrafts are powerful currents of rising air within a storm, fueled by the intense heat and moisture at the surface. As these droplets are carried aloft by a thunderstorm’s updraft, they encounter supercooled water droplets.
What is supercooled water? It’s simply pure water droplets that are able to remain liquid despite being in a below-freezing environment. These supercooled droplets quickly freeze upon contact with the tiny water droplets or ice crystals, creating small hailstones.
How does hail get so big?
The hailstones then begin to grow as they are repeatedly carried up and down through the storm’s updrafts. Each time they ascend, they gather more moisture, which freezes onto their surface, adding layers and increasing their size.
The strength and duration of the updrafts determine how large the hailstones can grow. In particularly strong updrafts, hailstones can be propelled high into the storm, where they may encounter multiple layers of supercooled water, resulting in even larger hailstones.
Big updrafts, large hail
To create hail to an inch in diameter, which is the size required to be considered severe weather, updrafts within a thunderstorm need to be around 30 mph. The stronger the updrafts are, the more time the hailstones spend growing in the upper reaches of a thunderstorm.
A number of hail reports of 2 to 3 inches were observed near Needville, southwest of Houston, before dawn on March 16. To get hail to this size, thunderstorm updrafts must be between 70 and 85 mph.
Supercell thunderstorms, which are characterized by rotating updrafts known as mesocyclones, are the most likely to produce large hail because of their intense and sustained updrafts.
Updraft speeds within supercells can exceed 100 mph, producing hailstones larger than baseballs or even softballs.
How often does San Antonio see hail larger than 2 inches?
The Storm Prediction Center, an arm of the National Weather Service, considers a severe thunderstorm significant when it is capable of producing hail 2 inches in diameter or larger. Since 1993, South Texas — which we’re counting as Atascosa, Bexar, Comal, Guadalupe, Kendall, Medina and Wilson counties — has reported 1,026 hail events, including 474 days with large hail. During the same time frame, a reported 145 hail events dropped hail larger than 2 inches in diameter on 58 days
South Texas is most likely to see large hail events between March and May, with the overwhelming majority of large hail events taking place in the afternoon and evening.