It seems crazy that softball-sized chunks of ice would hurtle through the sky on a midsummer’s afternoon. So what makes summer hail storms so severe? And what causes hail in the first place?
Hail, in short, is basically a game of tug-of-war between rising winds and gravity. Warm, moisture-rich air is the main fuel source for thunderstorms. The warm air gets absorbed into a storm through the rapidly rising wind, known as an updraft, which powers the whole system.
The rapid-rising motion from an updraft forces water molecules (rain) and water vapor to get sucked up to heights where the temperature is freezing, turning water into ice. In between those updrafts, a hail stone will drop down and absorb more water around it, before another updraft pushes it back up to the freezing level and adds another ring of frozen water to the hail stone.
This up-and-down motion continues until the hail grows large enough that the updraft can’t support it anymore. That’s when gravity takes over, forcing the hailstone to fall to the ground. It takes an exceptionally strong updraft to get a hail stone to the size of a golf ball, grapefruit or softball.
In order to get a hailstone to the size of a softball, it takes an updraft of about 98 miles per hour, according to figures from the National Weather Service (NWS). To put that into perspective, that’s equivalent to the wind speed found in an EF-1 tornado or a Category 2 hurricane. It’s extremely rare, even here in Colorado, and it is typically caused by an unusually high amount of energy fueling a storm.
| Hailstone size | Measurement (inches) | Updraft speed (mph) |
|---|---|---|
| BB | < 1/4 | < 24 |
| Pea | 1/4 | 24 |
| Marble | 1/2 | 35 |
| Dime | 7/10 | 38 |
| Penny | 3/4 | 40 |
| Nickel | 7/8 | 46 |
| Quarter | 1 | 49 |
| Half Dollar | 1 1/4 | 54 |
| Walnut | 1 1/2 | 60 |
| Golf Ball | 1 3/4 | 64 |
| Hen egg | 2 | 69 |
| Tennis Ball | 2 1/2 | 77 |
| Baseball | 2 3/4 | 81 |
| Tea Cup | 3 | 84 |
| Grapefruit | 4 | 98 |
| Softball | 4 1/2 | 103 |
A massive hail storm rocked the Colorado Springs metro area in August 2018, smashing car windshields throughout much of the city’s south side and injuring at least eight people.
Temperatures rose to near 80 degrees in Colorado Springs on that Monday afternoon, a warm but not exceptionally hot reading for the heart of summer. That sort of heat alone won’t serve as a trigger for a severe hail storm. What stood out about the conditions on that day in the Colorado Springs area were unusually high dew points – a measure of the amount of moisture in the atmosphere. Thanks to a moisture-rich easterly upslope wind, dew points rose to around 60 degrees that day, a figure only seen a handful of times a year in the normally dry Front Range.
The forecast from the NWS office in Pueblo that day warned Convective Available Potential Energy (CAPE) values could top 2,000 joules/kilogram, showing an unusually high degree of energy for the area. These figures also helped cause the Storm Prediction Center, the governing body for severe weather, to bump up the Front Range’s severe weather outlook to a “slight risk,” noting in its discussion “a mixture of supercells and multicells capable of large hail and damaging winds” in its Colorado discussion.
Finally, the relatively cool conditions, at least for summer standards, in turn meant freezing levels above the surface were only hovering at around 11,000 to 12,000 feet, a low-ish figure for the middle of summer. That meant updrafts had a shorter distance to get water droplets to the freezing level and less distance between the freezing level and the ground for the hail to melt in.
From a broad perspective here in Colorado, we’re extra-prone to large hail for one simple reason: Our elevation. Denver’s mile-high altitude places it 5,280 feet closer to the freezing level than a sea level location, giving us somewhat of a head start in getting those hail stones to the freezing level and making us one of the most susceptible locations in the world to large hail.
Additionally, since we’re closer to where hail actually forms, it also gives it less time to melt before it reaches the ground. It’s like eating an ice cream cone right away as opposed to a minute or two minutes after you order it.
That combination of broad, natural geographic conditions paired with an unusually high energy supply makes Colorado susceptible to destructive hail storms that can go down in the history books.
