To experiment with this, you will need:

• a sink, garden hose, stream, or other source of flowing water

Start by turning on the water, and placing your hand into it. Unless you turned on the hot water tap, or your garden hose has been sitting in the sun, the water will probably feel cool. Now cup your hands together and let the water flow into them. Keep your hands there for a few seconds, and then move them slightly, so that the water flows over your hand instead of into your cupped palms. Do you notice a difference in the temperature?

Move your hands back and forth, from letting the water flow into your cupped hands to letting it flow across your hand instead. Why does it feel so much cooler flowing across your hand?

I noticed something very similar on a recent trip to Ginny Springs in Florida. Floating gently on giant inner tube, the water did not feel that cold, once I got over the initial shock. While I was moving at the same speed as the water, I was comfortable. When I grabbed a branch and stopped myself, the water felt much colder. Why?

You are probably thinking that the water in your hand feels warmer because your hand has warmed it. You are correct! If you put a thermometer into the water in your hand, it will probably read at the same temperature as the water flowing over your hand, but that is because you are measuring the temperature of the water in the center of the pool formed by your hands. Over by your skin, things are very different. There, heat energy is moving from your warmer skin to the cooler water. Your skin is cooled, the water is warmed, and the temperature difference between the two is decreased.

That difference in temperature plays a large role in what you feel. When we say that we "get used to" the temperature of a hot bath or a cold spring, quite a bit of the process involves decreasing the temperature difference between skin and the surrounding water. The less difference there is, the less heat or cold we tend to fell. Of course, that only works up to the point were the temperature threatens to cause damage. No matter how little the difference, boiling water is still going to feel hot.

As long as the water is not moving, that thin layer of warmer water stays next to your skin. On the other hand, if the water is flowing past you, there is a constant supply of fresh, cool water, so there is a much greater temperature difference. That is why the running water feels colder, and why the flowing spring felt colder when I was anchored.

That idea also plays a role in wet suits used for surfing, diving, and kayaking. The suit does not keep the water out. That is why it is called a wetsuit. If you can't keep the water away from your skin, the next best thing is to protect that layer of warmed water next to your skin. The wetsuit keeps it from being swept away as you swim, dramatically reducing the amount of heat that you lose. If your wetsuit fits loosely, your movements will cause that layer of warmed water to squeeze out, letting fresh, cold water flow back in. I can tell you from my experience kayaking, that is a very bad thing.

Have a wonder-filled week.