COLORADO, USA — Colorado is often referred to as the "hail capitol" of the country, and we've got plenty of it lately, but we rarely hear about large hail in the mountains.

9NEWS viewer Clark White noticed the same thing, and wrote to us asking if it was just due to a lack of observations there, or if there is something in the atmosphere that prevents it. 

Well the answer is, of course, a little of both, but it is meteorologically more difficult for large hail to form in the mountains. Large or damaging hail is one-inch in diameter or larger, and that size hail rarely falls above 9,000 feet due to the lack of rotating supercell thunderstorms that most large hail falls from. 

However, it is possible and has happened on occasion.

Large hail forms high in a thunderstorm cloud, usually at 25,000 feet or higher, so it’s not elevation that prevents large mountain hail, it has more to do with storm mechanics.

In order for hailstones to grow large, they must be suspended in a storm for an extended period of time by rising air.

The key to having a powerful, long-lasting updraft is for it to be rotating. That’s called a supercell thunderstorm.

Supercells are very rare over the mountains because the environment lacks two key ingredients: wind shear and low-level moisture, while there is plenty of instability.

The poor wind shear is created by chaotic wind patterns that usually prevent the organized wind shear or counterclockwise turning, needed to maintain rotation. That chaos is created by winds shifting in valleys and canyons, and along steep slopes.

The lack of moisture is mainly a result of the mountains blocking most of it, as it comes in from the east Pacific or Gulf of Mexico.

No supercells means no large hail. There is one area, high in the Colorado Rockies, that has been known to produce many supercells: South Park.

An opening just south and also to the north of the Wet Mountains, allows low-level moisture to flow in with southeasterly flow, and the relatively flat terrain allows thunderstorms time to organize and power up.

While there has never been a report of large hail in Park County above 9,000 feet, it leaves the backside of the Pikes Peak Massif vulnerable to drifting supercells.

Ping pong ball size hail was reported in Divide just last summer, and there have been 23 large hail reports in Teller County above 9,000 feet since 1993.

There are four counties in Colorado that have never even had a hail report of any size: Hinsdale, Jackson, Mineral, and San Juan counties.

One of the most telling statistics though, is that the highly populated Summit County has only had one large hail report since 1950, which occurred on June 12, 1988. 

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