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When we think of weather, we tend to think of what happens in the sky above us. But if we consider weather as the physical state of the air in the natural environment, then weather can happen hundreds of feet beneath us, in caves. Wind, fog, and rain occur, even under the ground. Hi I'm Dave Thurlow and this is The Weather Notebook.

One of the basic factors that cause our weather above ground ­ changes in air pressure ­ leads to changing weather in caves too. As high pressure moves over the region above, air is pushed into the cave. And when the high pressure yields to low pressure, air is literally sucked out of the cave. The resulting in and out air movement causes the cave winds.

Cave fog occurs when cool, dry air sinks into a cave entrance. The sinking air mixes with warm, moist air rising from the cave. When the air parcels meet and mix, some of the moisture condenses into fog.

And I'll bet you don't think that it would rain in a cave, but it can. The rain that falls on the ground above a cave may slowly seep into the cave, falling through the ground over days, weeks, or even months, before dripping to the cave floor.

Cave temperatures don't change much year-round, since a cave contains a relatively small air mass surrounded by a very large, temperature moderating ground mass. So caves can be great places to escape the extremes of summer heat and winter cold.

So, while you might not need your umbrella or sunglasses, the next time you visit a cave, keep an eye out for the weather.

Special thanks today to Paul Burger, Hydrologist at Carlsbad Caverns National Park and to writer Peter Crane. The Weather Notebook is underwritten by Subaru, with major support from the National Science Foundation.