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Name: John William E.
Status: Other
Age: 30s
Location: N/A
Country: N/A
Date: July 30, 2003


Question:
I recently made a visit to Carlsbad Caverns in New Mexico, which I understand is approximately one mile deep. When you enter a cave it is always cooler than the outside air temperature. As you go deeper into the earth toward the core, I understand that for every 60 feet of depth the temperature rises one degree. Is that a true statement? If that is true, then why at a depth of one mile is it cold at the bottom of Carlsbad Caverns in New Mexico?



Replies:
Although caves in the vicinity of Carlsbad Caverns are over 1800 feet deep, the caverns themselves are roughly 800 feet deep. This is substantially less than one mile. Additionally, the geothermal gradient, which is the rate of temperature change with depth, can vary significantly with location. A discussion of cave meteorology can be found at the web site of Wind Cave National Park in South Dakota. The link is:

www.nps.gov/wica/Cave_Meteorology.htm.

An excerpt of their discussion follows. I recommend that you visit their site for more information. The average annual temperature surface temperature at Carlsbad Caverns is approximately 62.5 degrees F. Since the temperature in the caverns is lower than the average surface temperature, it can be inferred that the geothermal gradient is low in that area.

"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 us 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!"

Andy Johnson


John,

First, a cave is not always cooler than the air outside. In winter, in most places, the temperature in the cave is greater than outside. The cave temperature is influenced not only by depth, but greatly by the outside temperature. Cave temperature is usually very close to the annual average outside temperature, as the cave has entrances through which outside air penetrates. Because of the large mass of the cave rock, over time the cave temperature has equilibrated to the average outside temperature (human intervention, such as lighting or expansion/creation of entrances can upset this balance).

A cave in a warm climate, like New Mexico, will have a higher temperature than one in Maine.

I do not know the depth at which the warming effect from the Earth's core begins to dominate the climate effect from above, but I suspect that depth is great. Hopefully a geologist will answer with that information.

David R. Cook
Atmospheric Research Section
Environmental Research Division
Argonne National Laboratory



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