Heat

Scritch-scritch-scritch-scritch.



Hiking along a county road, making four miles per hour. The scritch-scritch-scritch sound comes from my sneakers crunching along the fine gravel surface.

Besides sneakers and socks, I'm wearing hiking shorts and a tee shirt, a hiking vest and a ball cap. It's hot out -- 98 degrees -- and only occasionally do I feel a puff of light breeze.

The exercise feels good. Early in the hike my muscles were sore and joints stiff, but I hit a nice rhythm after a mile or so as pain eased and stiffness ebbed. Now, on the flat, I cruise along majestically, balanced on the comfortable side of exertion. My heart is thumping away at 110-120 beats per minute, and air flows smoothly in and out of my lungs like a metronome. Sweat flows freely from my pores and is nearly immediately evaporated by the dry heat.

Moon-tains!

Did I mention that it's hot?

On the down slopes I hit almost six miles per hour, legs stretching and feet pushing off against the scrunching gravel. I feel powerful and strong on the down slope.

On the up slope, not so much, My pace slows, my heart rate bounds up to 160-180 beats per minute, respirations increase and come with a good deal more exertion.

As I stride along in the still, hot air, the heat envelops me like a robe. The sensation -- and the science behind the sensation, is interesting.

The heat I feel is two different things. I feel radiant heat -- infrared radiation -- directly and indirectly from the sun. Where the sun's light touches me I feel its radiant warmth. And some of the warmth comes from sunlight reflected from the ground. Additionally I feel a slightly different wash of radiated heat from the graveled road and from the surrounding landscape. This warmth is re-radiated from surfaces which have been soaking in the sun's smile for hours.

There is also ambient heat, the heat of the air surrounding me. This is different than radiated heat. It's driven by the sun as well, but it's the heat of molecular motion.

The sun adds radiant energy to the air. Obeying the law of conservation of energy, radiant energy is transformed into kinetic energy, and the molecules of gas that make up the air begin to speed up. The faster the molecules move, the hotter it is.

That's really what we are measuring when we use a thermometer, the speed of the molecules. As those gas molecules bash into the thermometer, some of their kinetic energy is transferred to the thermometer, causing the molecules in the thermometer to speed up, too.

In the thermometer -- depending on the type of thermometer of course -- a column of liquid expands or the steel of a spring expands, and that expansion is measured against a background scale, and that tells us what the temperature is.

So as I stride along in the hot, nearly still air, I can easily tell the difference between the feel of the sun's direct, radiated heat and the ambient warmth of the air.

As the warm air envelops me it's both comforting and uncomfortable. I'm working hard and my body is making a lot of heat too. My internal thermostat is designed to maintain a temperature of 98.6 degrees, but it's hard to radiate heat away from my body when the air is at about the same temperature. So I sweat, and evaporative cooling helps a bit, but would be more efficient if there was more of a breeze. So that's the uncomfortable part.

On the other hand, there's something comforting in the way the warm air envelopes me and holds me close. It's almost a feeling of déjà vu, as if something deep and primal in the core of my being feels right at home.

I wonder about that, about the sense of déjà vu. Is it just an illusion, or does something in me really recognize the enveloping heat? Could it be a dim memory from my personal past, or something deeper, a genetic memory from the plains of Africa?

I'll never know for sure, but it's fun to think about.