How Hailstones Formed in a Hot Night in Austin


On Sunday night, September 25, 2023, a severe thunderstorm hit the Austin area, producing hailstones as large as baseballs that caused widespread damage to cars, windows, and roofs. Many residents were shocked and frightened by the sudden and violent hailstorm, which occurred in a hot and humid night. How did such large hailstones form in such high temperatures? Here is an explanation of the science behind this phenomenon.

The Ingredients for Hail Formation

Hail is a form of solid precipitation that consists of balls or lumps of ice. Hail forms when raindrops are carried upward by strong updrafts of air into the upper atmosphere, where the temperature is below freezing. There, the raindrops freeze and become hailstones. As the hailstones fall, they may encounter more updrafts that lift them up again, adding more layers of ice. The more trips the hailstones make up and down, the bigger they get. Eventually, the hailstones become too heavy for the updrafts to support, and they fall to the ground.

The main ingredients for hail formation are moisture, instability, and lift. Moisture is the source of the raindrops that freeze into hailstones. Instability is the difference in temperature between the warm and moist air near the ground and the cold and dry air aloft. The greater the instability, the stronger the updrafts. Lift is the mechanism that triggers the upward movement of air, such as a cold front, a mountain range, or a thunderstorm.

How Hailstones Formed in a Hot Night in Austin

The Conditions for Sunday Night’s Hailstorm

According to the National Weather Service (NWS), the conditions for Sunday night’s hailstorm were favorable for hail formation. The NWS reported that the temperature at the Austin-Bergstrom International Airport was 88°F at 9 p.m., while the dew point was 75°F, indicating high humidity. The NWS also said that the atmosphere was very unstable, with a lifted index of -8°C, which measures the difference in temperature between a parcel of air lifted from the surface and the surrounding air at 500 millibars (about 18,000 feet). A negative lifted index means that the parcel of air is warmer and less dense than the surrounding air, and therefore rises rapidly. The NWS also noted that there was a strong jet stream aloft, which enhanced the lift and the wind shear, or the change in wind speed and direction with height. Wind shear helps to organize the thunderstorms and sustain the updrafts.

All these factors combined to create a powerful thunderstorm that produced large hailstones. The NWS estimated that the hailstones reached a maximum size of 2.75 inches in diameter, or about the size of a baseball. The NWS also said that the hailstorm was very localized, affecting mainly the central and eastern parts of Austin, and lasting for about 15 minutes.

The Impacts and the Advice for Hail Damage

The hailstorm caused significant damage to many properties and vehicles in the affected areas. Some residents reported broken windows, dented roofs, and smashed windshields. One resident, Gary Lansford, said that he and his wife had minor cuts from the glass after hailstones shattered multiple windows of their car while they were driving on I-35. He said that their car was a total loss, but they were grateful that it was not worse.

The Texas Department of Insurance (TDI) advised the residents who suffered hail damage to file an insurance claim if the damage was more than their deductible. The TDI also recommended taking pictures of the damage, covering the openings with plastic to prevent further damage, and checking the contractor references before hiring them for repairs. The TDI warned against throwing away anything until after discussing it with the insurance company, and against falling for any scams or frauds by people who offer to check the roofs or do the repairs.


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