3. LIQUID AND VAPOUR
• Liquid evaporating from a surface has a cooling effect. And
different liquids have this effect to different degrees. For
example, rubbing alcohol has more of an evaporative cooling
effect than does water. Alcohol is what is called a volatile
liquid, meaning simply that it evaporates comparatively more
quickly than water. But regardless of the liquid, the principle
of evaporative cooling is the same. The idea is that in its liquid
state, the substance—whether water or alcohol—has a certain
heat content.
4. Heat
When a liquid evaporates, its molecules
convert from the liquid phase to the vapor
phase and escape from the surface. What
drives this process is heat. In order for the
molecule to leave the liquid surface and
escape as a vapor, it must take heat
energy with it. The heat that it takes with it
comes from the surface from which it
evaporated. Since the molecule is taking
heat with it as it’s leaving, this has a
cooling effect on the surface left behind
5. HUMAN PERSPIRATION
An example of evaporated
cooling is that of human
perspiration. We have pores
in our skin from which liquid
water internal to our skin is
escaping and converting to
water vapor in the air. As this
happens, there is a cooling
effect on our skin surface.
This is almost always
happening to one degree or
another. When we are
exposed to an environment
that is hotter than what is
comfortable for us, the
degree of perspiration or
evaporation increases. And it
follows that the cooling
effect increases
6. PLANT TRANSPIRATION
Plants do something similar. In plants the process is called transpiration.
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.
7. 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. This is because the wind is increasing the
evaporation rate of the liquid water from our skin surface and accelerating
the amount that's being converted to vapor.
WIND INCREASES EVAPORATION
