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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.
Cave
Weather
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
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!
Wind
Cave Temperature Fluctuation Study
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