Why does evaporative cooling happen

Plant roots "drink" water from the soil and transport it up through the stem to the leaves. Plant leaves have structures called stomata.

These are essentially pores that you can think of as comparable to the pores in our skin. One of the main functions of this process in plants is to transport water needed by plant tissues in other parts of the plant besides the roots. But this evaporative cooling effect also benefits the plant. This keeps the plant—which might very well be exposed to direct, intense sunlight—from overheating. And this also explains why, on a hot day, if we enter a forested area, we feel considerably cooler.

Part of that is due to the shade, but part is also due to the evaporative cooling effect from the trees through this process of transpiration. Wind increases the effect of evaporative cooling, and this is a familiar concept.

Anyone who's ever been swimming and has come out of the water into a calm environment, versus one that's windy, can attest to it feeling colder in the wind. The wind increases the evaporation rate of the liquid water from our skin surface and accelerates the amount that's being converted to vapor. Incidentally, this process also causes so-called wind chill. Even in colder conditions, when we're outside and our skin is exposed to the elements, a certain amount of perspiration occurs.

When it's windy, more evaporative cooling takes place from exposed skin. This explains the basics behind the so-called wind-chill factor. Donald Miller has a background in natural history, environmental work and conservation. When the water touches your skin, how does it feel? Blow softly over the skin area that you just covered with water. Does your skin feel any different when blowing on the water?

Can you sense a difference in temperature while blowing? How does it feel? Rinse your pipet with some rubbing alcohol and then suck up some of the alcohol with your pipet. Drop the same quantity of liquid on the back of your other hand and spread the liquid with your fingers. Does the alcohol feel different when it touches your skin? Again, blow over the area on your hand where you put the alcohol. What sensation do you feel? Does your hand feel warmer or cooler compared with water when blowing on the liquid?

Can you think of a reason why? Extra : Find out how fast rubbing alcohol and water evaporate. Put the same small amount of water and rubbing alcohol in two different cups and place them both in the sun.

Observe how long it takes for the liquids to completely evaporate. Depending on how warm it is, this might take some time. Which liquid vaporizes faster? You can even determine the evaporation rate by weighing the cups in the beginning and throughout your experiment to find out how much water is lost due to evaporation.

Build a Cooler. Make a Potato Shrink--with Saltwater. Get smart. Sign up for our email newsletter. Sign Up. Support science journalism. Knowledge awaits. See Subscription Options Already a subscriber? Create Account See Subscription Options. Continue reading with a Scientific American subscription. Subscribe Now You may cancel at any time. A ton of air-conditioning is the rejection of 12, BTUH. The figure below shows the relationship between water and air as they pass through a cooling tower.

The curve indicates the drop in water temperature point A to B and the rise in the air wet bulb temperature Point C to D in their respective passages through the cooling tower. This is clearly indicated in the psychrometric analysis of the air path in a cooling tower as indicated below. The true path is approximated by the dotted curved line from Point A to Point C.

In the analysis, air enters the tower at an unsaturated condition Point A. Before reaching the fill, it is saturated adiabatically as it travels to point B. Passing through the fill, it absorbs heat from the falling water, thereby increasing the total heat content of the air. Since the air is continually being washed with falling water, the process follows the saturation line to the final temperature of the air leaving the tower, Point C.

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