The Importance of Physical Geography

There are several unique geographic characteristics of the Black Hills that play a role in the regions weather and climate. When viewed from space, the Black Hills stand out as an oblong patch of dark green rising out of a sea of tan in the summer, or a dark patch on a white background in the winter.

With over 90% of its surface area covered by ponderosa pine, the Black Hills formation rises like a dark mass some three to four thousand feet above the surrounding prairies. The formation presents an impressive obstacle to winds blowing across the plains.
Imagine a large rock in the middle of a stream. Some of the water in the current flows around the rock, and some flows over it. In the same way, as wind blows against the Black Hills, some is deflected around the formation and some over the top. These winds then come together on the side of the Black Hills formation opposite the direction from which the wind is coming called the leeward side, or lee side for short. Winds blowing into each other are called convergence, and where air converges it is also forced upward. Upward motion to air is a key element to producing clouds, and eventually, precipitation.

Getting back to our rock for a moment, you would notice swirls or eddies developing on the down-stream side of the rock where the water flowing around it comes together. As winds blow at the Black Hills from different directions, eddy currents of circulating air can develop near the downwind edges of the formation just like the water flowing around our rock. This, in turn, can cause small scale areas of low pressure and areas of convergence on lee side of the Black Hills, as shown in the illustration at the right. For precipitation to develop, the four key ingredients needed are lift, instability, moisture, and a trigger to set things in motion. An unstable atmosphere is one that will not resist or inhibit vertical lifting of air, and moisture is the amount of water vapor present in the atmosphere. The lee side convergence zone is a zone of colliding, lifting air that may also act as a trigger mechanism for precipitation. Let's look at an example and explain a situation where this could impact the weather on a particular summer day.

Suppose we start out with a sunny day in late June. As the day progresses, winds increase from the southeast at 20 to 30 mph with occasional higher gusts. Dewpoint temperatures rise into the upper 50s as the actual air temperature rises into the low 80s.
Other weather ingredients are present that favor the development of afternoon thunderstorms, namely instability and moisture, but where will they form first? We are beginning to learn that the above scenario would favor the formation of the "lee convergence zone" described in the previous paragraph somewhere in a triangle that runs from Rochford to Spearfish to Sturgis and back to Rochford. Therefore, a place for area meteorologists to watch for thunderstorm development would be eastern Lawrence and western Meade counties in the northern Black Hills. It is no secret to long time residents of the northern Black Hills that there seems to be a severe weather hotspot in the region, with thunderstorms frequently forming between Gillette, Wyoming and Spearfish, South Dakota and appearing to follow Interstate 90 through Whitewood and Sturgis before breaking to the east near Piedmont. One possible explanation for this would be that the majority of our severe thunderstorm days in this region begin with a fairly strong south or southeast wind flow at the surface that favor the lee side convergence zone's formation in these areas. As the storms grow, they become strong enough to no longer depend on this zone for intensification and they then break eastward off the hills.

Yet another example of how the geography of the Black Hills formation impacts the region's weather would be the effect of "upslope flow". Two prime examples of this would be snowfall in the northern Black Hills and fog formation on the eastern foothills. Let's begin with the snow.
In winter storms, the heaviest area of snow is north and northwest of the storm's center. The kind of winter storm that brings the Black Hills the heaviest snow is the "Colorado Low", a strong low pressure system that forms on the eastern foothills of the Rockies and tracks northward toward central or eastern South Dakota. Warm moist air is drawn into the system from the south. Wind flow around low pressure is counter clockwise in the northern hemisphere, so this moisture is drawn up and around the storm center into colder Canadian air coming down from Alberta, usually with strong northerly winds. In a normal atmospheric situation, the higher up you go from the ground, the colder it gets. As moist air wraps around low pressure and is pushed against the northern Black Hills, it is forced upward into cooler air. The process of saturation, condensation, and precipitation is then accelerated as the air is lifted and snowfall is intensified where the northerly winds are forced upward. This is one reason why snowfall amount forecasts in the northern Black Hills can run 2 or 3 times the amounts predicted for the adjacent plains or other parts of the Black Hills, a fact not lost on the operators of downhill skiing slopes who prudently chose Lawrence county for a location.
The formation of fog around the hills follows similar principles in an upslope situation. For the Rapid City area, a moist east wind is a big fog producer in the spring and fall. As the moist air blows westward from the Badlands toward the Black Hills, it is lifted and cooled. Condensation results and fog forms on the eastern slopes of the Black Hills formation. If sufficient moisture is present, a steady light rain or drizzle will also result. The odd thing about this situation is that all of the moisture can be trapped close to the ground in the lowest levels of the atmosphere. Skies over the Black Hills themselves can be clear as a bell, while Rapid City residents endure a cold, foggy mist.

These are but a few examples of how the physical geography of the Black Hills formation plays a role in the region's weather. As more sophisticated weather computer models and equipment becomes available, we will no doubt find even more ways in which the lay of the land influences weather.