The Black Hills are similar in structure to an island, basically a region of terrain elevated 3000 to 4000 feet above the surrounding prairie. They are elliptical in shape oriented north to south with a length of 65 miles and a width of 30 miles. There is a striking contrast in average annual precipitation, ranging from over 30 inches on the north end to less than 15 inches on the surrounding plains. The main reason for this difference is due to upslope formed or enhanced precipitation events. The following set of diagrams help explain the complexities of air circulations and how they affect the weather around the black hills.

Conceptual Model for Upslope Precipitation

When Precipitation Cannot Form

Downslope Winds

Lee Side Convergence

There are tools that have been developed to help forecasters determine when upslope precipitation may or may not occur, and also what type of flow to expect around the Black Hills. One of these is called the Froude number. In the previous images, it was shown how the stability of the atmosphere can effect the ability to develop precipitation. The speed of the wind also plays a major factor. The Froude number is a non dimensional value based on a combination of the mean wind speed, height of the obstacle, and the buoyancy.  The basic concept is that values of Froude number will indicate the type of flow that can be expected. In other words, it helps a forecaster determine if upslope will be a factor in producing precipitation.

When the Froude number has a value less than 1, the flow can be expected to be around the obstacle or terrain. When it is greater than 1, the air flow will go over the obstacle and upslope flow will result. If the value is within 0.2 of either side of 1, then a combination will often be observed. All this leads to a complex pattern of eddies and waves that form when the wind blows over western South Dakota, and dramatically affects precipitation patterns.

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