Paul has just returned from two months at McMurdo station in Antarctica, a place as alien as you can get and still be on our planet.

"When I was standing beside the crater at the top of Mount Erebus, Antarctica's active volcano, I felt like a character in a science fiction movie," Paul reports. "I was completely encased in protective clothing, including goggles and mask—I might as well have been wearing a spacesuit. When I looked down into the volcanic crater, I saw a bubbling lava lake. Incandescent lava glowed through cracks in the black surface. The crater walls were covered with ice. Steaming fumaroles made tubes of ice that clung to the walls of the crater, each one big enough to swallow a person. I kept waiting for a giant extraterrestrial ice termite to crawl out of a tube."

Pat, on the other hand, is just back from a visit to the rain forest of Belize, an exotic jungle worthy of an Edgar Rice Burroughs novel. No giant ice termites there—but plenty of heat and howler monkeys and vampire bats, army ants and poisonous snakes and blood-sucking insects.

Extreme environments—from the overheated jungles of Burroughs to the ice planet Hoth in Star Wars—have long been a staple of science fiction. With that in mind, we decided to dedicate this column to exploring one aspect of these extreme environments—temperature extremes and how the human body experiences them.

For Paul, staying warm was a matter of life or death. Standing on the crater rim of Mount Erebus, Antarctica's active volcano, Paul had reason to remember the scene in The Empire Strikes Back where Luke Skywalker nearly freezes to death on the ice planet Hoth, surviving only by taking shelter in the warm entrails of a giant dead lizardlike Tauntaun.

On Mount Erebus, the temperature was -30^ocelsius (-22^oF) and the wind was blowing at 30 miles per hour. In these conditions, exposed flesh would freeze within 5 minutes. Of course, being a physicist, Paul noted that the real problem wasn't just the temperature. It was the heat flow.

In the jungle, Pat was never in real danger from the heat. The abundance of poisonous snakes was a problem, and the lack of beer at the research station was decidedly uncivilized, but the heat was not life threatening. Still, while Pat was trekking through the jungle, wearing long pants and a long sleeved shirt to keep off ticks, dengue-fever-carrying mosquitoes, bot flies, and other vermin, she was pickling in her own sweat. And she had plenty of time to realize that the problem was not just the temperature. It was the heat flow.

In this column, we'll tell you a little about how your body experiences temperature, then talk about the pickiness of physicists and the difference between temperature and heat flow. Finally, we'll discuss wind chill and the heat index, both attempts to correlate environmental conditions with your body's experience of heat or cold.

Maybe you think that you can learn a lot about the temperature of an object just by touching it. If you believe that, consider the last time you stepped out of the shower onto a tile floor. Your bare feet probably felt chilled. If you had covered that same tile floor with a cotton bath mat, your feet would have felt just fine.

The tile and the bath mat are the same temperature. But the tile makes your feet feel cold and the bath mat doesn't. So what's the deal?

The temperature-sensitive nerve endings in your skin detect the difference between your inside body temperature and your outside skin temperature. When you touch something and your skin cools down, your temperature-sensitive nerves tell you that the object you are touching is cold.

But temperature alone does not determine whether your skin cools when you touch something. To cool your skin, an object must meet two conditions: it must be colder than your hand, and it must carry your body heat away.

That second condition is the tricky one. The tile floor and the bath mat are the same temperature. Though both are colder than your warm feet, they don't feel equally cold. That's because they carry heat away from your feet at different rates.

The cotton bath mat is an insulator, a poor conductor of heat. When you stand on the bath mat, heat flows from your bare feet to the bath mat and warms the cotton surface. Because this heat is not conducted away quickly, the surface of the bath mat soon becomes as warm as your feet. After this happens, little or no additional heat leaves your skin. Since there is no difference in temperature between the inside of your body and the outside of your skin, the temperature-sensitive nerves detect no difference in temperature and the bath mat feels warm.

Unlike the bath mat, the tile of the floor is a good conductor of heat. Heat flowing from your feet into the tile is conducted rapidly away. This leaves the surface of the tile and your skin surface relatively cool—and therefore the tile feels cold.

To feel even more dramatic proof of the fallibility of your temperature sensors, here's a simple experiment to try. Take three tall glasses. Fill one with hot tap water, one with ice and cold water and one with room-temperature water.

Grab the cold glass with one hand, completely encircling it with your palm. Grab the hot glass in the same way with the other hand. Hold both glasses for a full minute. After the minute is up, release those glasses and grab the room-temperature glass with both hands.

Weird! The hand that held the cold glass will report that you are holding a hot glass. The hand that held the hot glass will tell you that you are holding a cold glass. But both hands are holding the same glass!

Your contradictory sensations are a further indication that the thermal sensors in your hands do not sense temperature directly. These sensors sense temperature change caused by heat flowing into or out of your hands. When two objects are at different temperatures and in contact, heat flows from the hotter one to the cooler one. That's one way to tell which one is hotter.

In the experiment you just tried, heat flows from the room-temperature glass into the hand that was cooled by holding the cold glass. Heat flows into the room-temperature glass from the hand that was warmed by holding the hot glass. The hot hand is losing heat, so it senses that the glass is cold. The cold hand is gaining heat, so it senses that the glass is hot.

You may have noticed that we are talking a lot about heat flowing and not much about what heat is. That's Paul's doing.

Pat has noticed that physicists get peculiar and jumpy when they are talking to non-physicists about heat. Say something is hot, and a physicist will nod grudgingly. But say anything that implies that a hot object contains a lot of heat, and a physicist will look pained.

Paul says that to use the word "heat" correctly, you must always be able to add the word "flow" immediately after it. To a physicist, heat is a form of energy that transfers from one body to another. (Pat has responded to Paul's insistence by suggesting that we simply add a character named Flo (short for Florence) to any discussion of heat. As in "Turn up the heat, Flo!" This suggestion makes Paul laugh and look vaguely distressed simultaneously, and that, for Pat, makes it worthwhile.)

Annoying as the physicist's pickiness about heat may be, Pat agrees that this pickiness is understandable when you start mucking about trying to understand heat and temperature. Back in the 18th century people spent a lot of energy (and expelled a lot of hot air) trying to clarify the difference between heat, thermal energy and temperature. They ended up concluding that heat is a form of energy that moves from one body to another. (That's the flow that Paul insists on.)

Thermal energy, on the other hand, is a form of energy contained within a body due to its temperature. And temperature is a measure of the energy per molecule contained in a body. A lot of energy can flow out of a coin at 0 degrees Celsius into another coin at –200 degrees Celsius. The energy available due to the temperature of the coin is the thermal energy contained in the coin.

Here's a comparison that may help you get a better understanding of heat, thermal energy, and temperature. Consider the wick of a burning candle and a thousand-ton iceberg floating off the coast of Antarctica. The burning wick is at a much higher temperature than the iceberg. But the thermal energy contained in the massive iceberg is much greater than the energy content of the tiny amount of hot gas of the candle flame. And as for heat (or if you prefer, heat flow), more heat flows from the warm atmosphere into the melting iceberg than flows out of the small candle flame into the atmosphere.

So let's take this discussion from the theoretical to the practical. As Paul stood on the edge of the crater on Mount Erebus with the wind whipping around him, what mattered was not the quantitative measure of the temperature. What mattered was whether his flesh was going to freeze, a condition known as frostbite.

On Mount Erebus, Noel Wanner, another Exploratorium staffer on the expedition, took off his face mask to shoot video. In just minutes Noel's nose turned an inhuman shade of white, indicating the first stage of frostbite. Noel quickly covered his nose and cupped his gloved hands around it to warm it up again.

How long it takes for exposed flesh to freeze depends on how rapidly heat flows out of the flesh compared to how rapidly heat was carried into the flesh from inside the body. Heat loss from the body depends on both the air temperature and the wind. If you've ever felt comfortable on a cold day—then shivered when you were hit by a blast of wind--you know the cooling effect a wind can have. When the wind wasn't blowing, Paul reports that -30^oC did not feel dangerously cold.

In an attempt to quantify the cooling effect of the wind, people use a table known as the wind chill index. You've probably heard weather forecasters talk about wind chill, saying something like, "The temperature is 5 below zero, but with the wind chill it will feel like 20 below, so bundle up." The idea is simple—the wind speeds up the loss of heat from your body, so the air temperature feels colder than it really is. When Noel removed his face mask on Mount Erebus, he inadvertently tested the predictions of the wind chill table.

Antarctic explorers Paul Siple and Charles Passel did the original work on wind chill in the winter of 1941. They exposed plastic cylinders of water to different temperatures and wind velocities and measured how long it took the water to freeze. From these measurements, Siple came up with the first wind chill tables.

In 2000, the weather experts of the US and Canada came together to revise Siple's wind chill table. They put volunteers into a wind tunnel that blew cold air onto their faces. Temperature sensors were attached to the faces of the volunteers and their core temperatures were measured by thermometers inserted into places where aliens who abduct humans are reputed to insert probes. This allowed researchers to quantify the heat flow out of human flesh and create a more accurate table. (You can find this table on the Web http://www.crh.noaa.gov/fsd/windchill.htm.)

Wind chill quantifies the cooling rate of human flesh. It is meaningless to inanimate objects like rocks and bicycles. If the temperature is -30^oF and the wind is blowing at 40 mph, the wind-chill is -70^oF, according to the new table. That means a human will lose heat as if it were a calm -70^oF day. But it doesn't mean that your bicycle will act as it would in –70^oF weather. The bicycle will cool off rapidly at first in the wind. But as the bicycle approaches -30^oF, its heat loss will slow down and stop and its final temperature will be -30^oF. Unheated objects come to the temperature of the environment. The wind chill just shortens the time it takes to reach the final temperature.

When you get right down to it, the temperature that you feel isn't necessarily the same at the temperature that a thermometer measures. That's true in cold weather—like the windy top of Mount Erebus. It's also true in hot weather—like the sticky heat of the Belizian jungle.

You've probably heard that old weather cliché (usually drawled by someone who's too hot to move): "It's not the heat; it's the humidity." Paul points out that what people really mean to say is that "It's not the temperature; it's the humidity." Because it really is the heat, Flo.

You see, sweating is your body's way of cooling down. When water evaporates, it absorbs thermal energy from its surroundings and carries it away. When sweat evaporates from your skin, heat flows into the evaporating water and your skin gets cooler. This works just fine—unless the water doesn't evaporate.

A measurement that gives you some information on how rapidly the water will evaporate is relative humidity, which tells you how much water vapor is in the air compared to how much the air could hold at a given temperature. Relative humidity is usually given as a percentage. If the relative humidity is 80 percent, then the air contains 80 percent of the water vapor it could hold at that temperature.

The higher the relative humidity, the slower your sweat is to evaporate—and that's where the problem comes in. On a hot day, you sweat. If the relative humidity is high, the sweat doesn't evaporate.

Pat can testify that this is not pleasant. In this situation, you don't cool off. You feel the sweat clinging to your skin and you wish you had a cold beer and you know that the research lab doesn't have any and you think that this is just plain uncivilized and . . . but we digress. Let's just say that hot humid air makes it difficult for a body to produce a heat flow and keep a core temperature below 105^oF.

The wind chill index is one attempt to establish the "apparent temperature," that is, the temperature you feel as opposed to the temperature measured by a thermometer. A similar measure named the heat index attempts to quantify discomfort at higher temperatures. (You can find the heat index at http://weather.noaa.gov/weather/hwave.html#Heat%20Index%20Chart.)

Paul and Pat both survived their extreme environments, emerging with no permanent damage. They find themselves with an increased appreciation for temperate climates—and an increased respect for temperature extremes.

When Paul describes the conditions in at the top of Mount Erebus, where he spent several nights, it's easy to understand this respect. Seven scientists, one mountaineer, one science teacher, and two writers were crammed into a one-room building to eat and work. At night, each person went out alone to a one-person tent to sleep.

Paul remembers thinking that this was a perfect set-up for a horror movie. As someone who has watched these movies, he knows the secret to survival is simple: "Don't go out alone--the monsters will get you." In particular, he thought about the movie, The Thing, in which a group of Antarctic researchers thaw out an extraterrestrial they find frozen in the ice—with consequences that prove to be very unpleasant to anyone caught out alone.

In Paul's Antarctica, the situation was just as intense and just as frightening. But the monster wasn't an extraterrestrial alien. The monster was the weather.

To learn more about Pat Murphy's science fiction writing, visit her web site at www.brazenhussies.net/murphy. For more on Paul Doherty's work and his latest adventures, visit www.exo.net/~pauld.