BOZEMAN, Mont. - A line of storms packing rain and strong winds caused Gallatin County residents to pull out their cameras Monday morning. The menacing cloud formations prompted waves of pictures and videos to appear on social media throughout the day. The storm brought rain and wind, with wind gusts between 30 and 40 miles per hour recorded during the storm's passing.
The formation that captivated many in Bozeman is known as a shelf cloud. It is a result of warm air and cool air colliding. In this case, a cold front across Southwest Montana provided the difference in temperature and moisture needed for the clouds to form.
Cooler air behind the front and accompanying rain causes what's known as a cold pool beneath the line of showers. This cold pool is essentially a bubble of air cooler than the surrounding air, situated underneath the storm.
As the cold front pushed the showers and it's cold pool towards Bozeman, warmer air ahead of the storm was pushed upward. A side effect of this interaction is that moisture in the warmer air is released as it rises, forming an ominous, low hanging cloud where the warm and cool air meet. Since the warm air and cool air are going in opposite directions, they interact with each other in complex ways. In the meteorology world, this process is known as wind shear. This process can create a vortex, where the clouds directly behind the shelf cloud appear to roll. Several rolling vortices are visible in video captured of this storm.
This is a case where the bark of this type of storm is worse than the bite. This type of cloud is often mistaken for a wall cloud. Wall clouds are common in supercell thunderstorms, and produce almost all tornadoes; and shelf clouds often are mistakenly identified as a sign that a tornado is close by. A shelf cloud does signal that heavy rain and strong winds are on the way, and should not be ignored.