Montana

Last night Michael was in Sheridan, Montana.


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The group wanted to get a bit further west, but they were caught by a big thunderstorm. Michael said the storm started with golf ball sized hail and then there was a lot more marble sized hail following that. I believe they all got to shelter in time not to be too injured, but the whole town had hail damage and the motel that they stayed at last night had broken windows and damaged siding from the hail. In addition, Michael's bicycle fell over during the storm and lots of his stuff got wet in the rain (somehow it was in a small flood or something), so he was hoping it would all dry in the night.

Hail the size that they experienced is relatively uncommon and suggests that the storm that passed over was very large with a strong updraft area. In order to bring the air and water droplets high enough to be frozen the cloud has to extend high into the troposphere where temperatures are very low.

Hail is created when a drop of water turns into an ice pellet. It gets caught in an updraft of the thunderstorm and collides with water droplets which then stick to it and make it grow. It eventually falls again and can either fall to the ground in this state or be caught by another updraft where it can accumulate more volume. The hail can be tossed around in the cloud for a long while and grow in size, thus leading to larger and more damaging hail. As a result the hail grows in layers and if you take a cross section of hail you can see these layers:


The larger the number of layers the larger the hail and more severe the storm. You can imagine that only a very strong storm with powerful updrafts could sustain large hail. As the hail grows in size it becomes more massive and the force of gravity on them increases. If it remains airborne long enough to grow to the large sizes then the updrafts must have been very powerful to prevent the hail from falling. However, at some point gravity always wins and the hail falls to earth.

Marble sized hail is the most common size and that hail is most often found near the borders of Wyoming, Colorado, and Nebraska. I also read (in my handy atmos handbook) that hail is more common over mountain ranges because the mountains force air upward, and this intensifies the updrafts in the storm. Since Michael is right in the middle of the Rockies up there I wouldn't be surprised if this played into the hail.