It has long been thought that caves possess constant temperatures and humidity; never changing, even during periods of extreme temperature fluctuation on the surface. Although this may be true in very remote parts of large cave systems, most caves exhibit enough variation in this respect to warrant serious investigation. Such an investigation took place at Wind Cave during 1984 and 1985. What follows is a summary of what was learned in this study, together with some information on the role cave climate plays in the overall cave environment.
Weather can be defined as the present state of the atmosphere in an area with respect to heat, wind, pressure, and moisture. Of these, heat is the most important since changes in heat quite often bring about changes in the others. On the surface, weather is driven by the sun, which heats some areas of the earth more than others. Temperature differences lead to pressure differences which lead to winds and precipitation. But what are changes in the weather of a dark, sunless cave caused by? To understand, we must first look at how heat enters the cave.
Significant amounts of heat can enter a cave in four possible ways. Heat may enter from the overlying rock, from the underlying rock, from air flowing into the cave, or from water flowing into the cave. We will look at each of these four possibilities, and discuss how each affects the weather in Wind Cave.
First, let's look at how heat enters the cave from the rock above it. The flow of heat through the sandstone and limestone overlying Wind Cave's passages is extremely slow. Temperature fluctuations of over 60°F between day and night are not uncommon on the surface, but if we were to monitor the temperature only two feet below the surface on such a day, the fluctuation in temperature would be only about 1°F. Therefore, it does not stay warm long enough during the day, nor does it stay cool long enough during the night, to significantly change the temperature of the rock only two feet underground. The same principle holds true for seasonal temperature fluctuations as well, although the depth at which temperatures begin to stabilize is greater. Seasonal temperature fluctuations of 80°F are reduced to only l°F at a depth of about 50 feet. So if a cave's only source of heat was from the overlying rock, its temperature would approximate the mean annual surface temperature for its area. It appears this is rarely the case. At Wind Cave for instance, the mean annual surface temperature is 47°F, yet the temperature in most parts of the cave (away from the tour routes) is 55°F. Where is all this extra heat coming from? The answer appears to be from below!
Since the interior of the earth is a tremendously hot, molten mass, one would naturally assume that temperature would increase with depth below the surface. This change in temperature with depth is known as the geothermal gradient. In areas where the geothermal gradient is low, cave temperature is influenced mostly by the mean annual surface temperature. In areas with high geothermal gradients, cave temperature is influenced by the mean annual surface temperature and by heat from below. Caves in areas such as this tend to be warmer than the mean annual surface temperature. This is apparently the case at Wind Cave. The same geothermal gradient which is responsible for the warm waters of Hot Springs is heating the cave!
Caves with no connections to the outside world have no further sources of heat. Since the heat supplied by the rock is provided very evenly, and since changes in the amount of heat reaching the cave can only occur over very long periods of time, the weather in such caves is very constant. But caves with open connections to the surface have two more possible sources of heat which can alter the weather inside the cave: airflow and water. Both are capable of transporting outside conditions into the cave environment.
The effects of water on the cave temperature at Wind Cave are very slight. Water dripping into the cave normally has reached rock temperature by the time it reaches the cave atmosphere, so little heat exchange can take place. But in caves with active streams flowing in from the surface, large seasonal fluctuations in temperature can occur very deep into the cave, since it takes a long time for the water to heat or cool to cave temperature.
The real weather (i.e. daily changes in the cave's atmospheric conditions) of Wind Cave is driven by the wind. Few caves experience the volume of airflow which Wind Cave exhibits. On average, almost 1,000,000 cubic feet of air enter or leave the cave per hour when the Walk-In Entrance is open! That's enough air to completely fill a cave 10 feet wide, 10 feet tall, and almost 2 miles long! Since it takes a long time for a volume of air this large to warm or cool to cave temperature, temperature changes can occur surprisingly deep into the cave on days when the cave is inhaling. The Wind Cave Climate Study of 1984-85 showed that with the Walk-In Entrance open in the winter, temperatures could fluctuate by over 12°F as far into the cave as the Post Office (over 500 feet from the entrance). The entire Half-Mile Tour route is almost always cooler than the 55°F deep cave temperature, mostly due to cold air brought into the cave during the winter. So much for constant temperatures at Wind Cave!
Not only does inflowing air affect cave temperature, but it also affects cave humidity. The same study showed that humidity was lower whenever the cave inhaled. Natural cave humidity is probably about 95%-100%. Cold air is usually dry, and when it enters the cave environment and warms it becomes even drier. Some areas showed humidity as low as 60% on days when the cave inhaled. Humidity did not stabilize until the Temple was reached, over 2000 feet into the cave. This drier air evaporates water from the cave environment, further cooling the cave since evaporation requires large amounts of heat.
Cave climate is concerned not with the present conditions of the cave atmosphere, but with the average conditions over a period of time. At one time (prior to 1890) the only significant opening to the surface at Wind Cave was the blowhole near the Walk- In Entrance. The construction of man-made entrances allowed for a huge increase in airflow into and out of the cave, bringing with it changes in the cave climate. What follows are a few examples of how the fate of both the cave and its fragile ecosystem are intertwined with changes in the climate of the cave.
The greatest harm to the cave itself may come from the evaporation which takes place in the winter. Many of the cave's speleothems are directly dependent upon the amount of water available. Stalactite growth may be slowed or even stopped when less dripping water is available. There is also considerable evidence which shows that aragonite (present in Wind Cave in the form of popcorn or frostwork) tends to form in preference to calcite in areas with high evaporation rates. Thus, a change in cave climate can change the very chemical structure of the speleothems themselves!
Cave fauna will also be disturbed by a change in the cave climate. Animals which have evolved in the cave's environment over thousands of years will probably have little tolerance for major temperature changes. Many of these animals live on moist surfaces. When evaporation takes place on these surfaces it can become remarkably cool. Different species of bats prefer different environments in the cave for roosting and they also could be disturbed by a change in the cave climate.
Charles Dudley Warner once said, "Everybody talks about the weather, but nobody does anything about it". Unfortunately, he was right, at least as far as outside weather is concerned. But we really can do something about the weather in a cave. It is simple for us to alter it, and it is possible for us to restore it to its natural state. Our actions will have remarkably profound effects on the future of the cave!