I recently visited Wind Cave National Park in South Dakota, which protects a beautiful expanse of the Northern Great Plains as well as one of the largest and most complex cave systems in the world. While well known for its geology, the park’s namesake feature is also an excellent example of the science behind a basic weather phenomenon – wind.
Wind, which is air in motion, is the result of differences in atmospheric pressure. These pressure differences are caused by the temperature differences created by the uneven heating of the Earth’s surface by the Sun. Several factors contribute to this unbalanced process, including cloud cover, large bodies of water, topography, and vegetation.
As the surface warms, air heats and rises, creating an area of low pressure. To fill that void, air from an area of relatively higher-pressure rushes in, creating a flow of air that we recognize as wind. The greater the pressure differences between these two areas, the stronger the breeze.
Atmospheric pressure conditions at the cave entrance during my visit. Credit: Melissa Fleming
At Wind Cave, given its vast size, the air pressure inside the cave is constantly working to equalize with that above ground. Therefore, when there is an area of high pressure at the surface, the wind will blow into the cave. If there is an area of low pressure on the surface, the wind will blow out of the cave. For this reason, the cave is described in the oral histories of the Lakota – a Native American tribe who consider it scared – as “the hole that breathes cool air”.
Park Ranger demonstrates the flow of air coming out of the small cave entrance with a ribbon. Credit:RVDreamLife
While other large cave systems can generate barometric winds, those at Wind Cave are more noticeable because of the small size of its entrance. As the Venturi Effect shows, when space is constricted, air will flow faster. Legend says that the first non-native settlers to discover the cave – two brother named Jesse and Tom Bingham – did so by accident when the wind from its entrance blew the hat off one of their heads in 1881.
According to the NPS, winds at the cave’s natural entrance have reached up to 25-mph.
Wind Cave National Park, SD. Credit: Melissa Fleming