The science of summer


Ah, the lazy, hazy days of summer. Time to kick back, relax, and ponder the big questions, the ones humans have asked themselves from time immemorial. Questions such as…

What causes sunburn and tanning?

Red skin is inflamed skin, and inflammation is the body’s effort to speed the healing of damaged tissue by rushing more blood to the area. Fifteen percent of the sun’s energy reaches us as invisible ultraviolet radiation, which is powerful enough to penetrate the surface of the skin and cause damage to the skin’s cells. The body tries to keep this from happening by increasing the production of melanin, a brown pigment that resists the penetration of ultraviolet radiation.

How do boats work?

The scientific principle behind all floating objects is Archimedes’ Principle, named after the famous Greek philosopher who thought of it in the bathtub and ran naked down the street yelling “Eureka!” An object floats if the amount of fluid it displaces matches its weight. The more heavily laden the boat, the deeper it sits, because it has to displace that much more water to equal its weight.

The first boats were hollowed-out logs called “canaoa” by the Caribbean Indians, from whom we get the word canoe. The first form of boat propulsion was the paddle, whose principle is simple: you push the paddle against the water to force the boat forward.

The principle of the sail is equally obvious: air blows the sail forward and takes the boat with it. How you can sail against the wind by zigzagging back and forth at an angle to it (tacking) isn’t quite as obvious. Tacking works because the force of the wind against the sail is matched by the resistance of the water to having the boat’s keel and rudder pushed sideways through it. The boat, trapped between these two forces, squirts forward like a watermelon seed squeezed between two fingers.


The propellers on motorboats are really rotating wings, curved so that, as they rotate, the water flows faster over one side than on the other. The water pressure is lower on the side where the water flows fastest; the higher water pressure on the other side drives the boat forward.

How is ice cream made?

Well, first of all, I assume you’re talking about homemade ice cream, the kind you make yourself in an old-fashioned ice cream maker that you have to crank by hand. Ice cream is frozen by putting a mixture of milk, sugar, flavoring and other ingredients (depending on the recipe) in a tub and rotating it in a bath of salt and ice while a paddle keeps the mixture stirring.

Salting the ice makes the resulting ice water colder. Ice water is in a state of equilibrium. Some water molecules from the ice break free, while other molecules in the liquid attach themselves to the chunks of ice. If more molecules break free than attach, the ice melts; but until all the ice has melted, the temperature of the ice water remains the same: 0 degrees.

To break free from the ice, a molecule requires energy. As salt dissolves in water it breaks into charged particles (called ions) that prevent molecules in the liquid from attaching to the ice, but don’t interfere with molecules breaking free from the ice. These molecules get the energy to break free from the surrounding water and ice, which makes the temperature of the mixture drop. Because the salt acts as an antifreeze, the ice continues to melt, making the ice-salt-water mixture colder and colder, until a new equilibrium is reached 10 degrees below freezing–a good temperature for making ice cream.

The sugar within the ice cream mixture also acts as an antifreeze. As the water in the mixture freezes, the concentration of sugar in the remaining liquid increases, which drops its freezing point lower and lower–eventually, lower than the temperature of the ice-salt mixture. As a result, the ice cream never freezes completely solid.

Constantly stirring the mixture insures that the ice crystals that do form don’t grow too big and the ice cream chills uniformly. It also whips air into the ice cream, which increases its volume and also helps keep it soft.

There’s nothing like cranking away on an old-fashioned hand-operated ice cream maker to cause you to work up a sweat. Which brings up the question…

Why do we sweat?

Sweat comes from millions of tiny glands in our skin (particularly numerous hairless regions such as the palms and soles, which is why we get sweaty hands when we get nervous–although just why we sweat when we’re nervous, nobody really knows). Sweat glands respond to excessive heat (and the threat of having to make a speech) by secreting a salty, watery fluid. As this fluid evaporates from the skin, it cools the blood rushing by just underneath the surface of the skin in the tiny blood vessels called capillaries. This cooled blood is then recirculated through the body, where it picks up more heat that it gets rid of again the next time it passes through the skin’s capillaries. If we didn’t sweat, we could easily overheat to a dangerous degree: in fact, one of the symptoms of heat stroke is a shutting down of the sweat glands.

Sweat sometimes seems to attract insects, which may lead you to ask…

How does insect repellant work?

Insect repellents come in a variety of formulas from many different manufacturers, but they almost all have one thing in common: N, N-diethyl-m-toluamide, better known as DEET.

DEET was first synthesized in the 1950s, and it soon became apparent that many insects hate it. DEET repels a variety of mosquitoes, chiggers, ticks, fleas and biting flies, but don’t spray it on and then going out to tend the beehives: no skin-applied repellent is effective against stinging insects.

The concentration of DEET in insect repellents varies. The U.S. Army uses a mixture of 75-percent DEET in ethanol, and some commercial products that are even stronger. Deep Woods Off!, for instance, is 95 percent DEET, and Muskol is 100 percent DEET. (Pure DEET is unpleasantly oily, however, and lower concentrations work just about as well under most circumstances.)

A few people have unpleasant reactions to DEET, so they turn to natural alternatives such as citronella-based repellents, which provide short-term protection against mosquitoes, but aren’t very effective against ticks. Other plant-based repellents have been made from cedarwood, eucalyptus and lemongrass, all of which smell pleasant to humans but apparently not to insects. Unfortunately, most of them are pretty ineffective against mosquitoes, at least compared to DEET; although they do seem to work pretty well against flies.

Of course, one can hardly deal with all the Big Questions in such a short column, so I leave the further search for enlightenment with you. I suggest something cold to drink and a cool place to recline. You might also try closing your eyes.

Comfy? Good. Enjoy your contemplation.

I know I always do.

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