My six year old daughter asked me why she had to
wear sunscreen in the winter given that the sun does not melt the
fragile snow then how could it hurt her skin. I promised her an answer.
I understand the electromagnetic spectrum. I am not sure which
wavelengths will melt ice though. If it were below 32 degrees F
inside a microwave would ice still melt in it for example? Can you
melt ice with gamma rays or x-rays. If microwaves can melt ice then
why not its spectrum neighbor TV or FM radio? I think I get how UV
rays harm skin but i can get past the difference between radiation
and heat or temperature and how they interact. I am stumped by a six
There are many factors that are important to truly clarifying the
situation for your daughter. I will focus only on the distinction
between heat (energy) and temperature.
Do not feel bad. The problem you are having is due in part to the
fact that heat and temperature in the English language are often
confused with each other or used interchangeably. We say something
is hot when we mean it has a high temperature, but we also say that
there is a heat index for weather (which is also temperature).
So let us first try to be more specific with our usage of heat and
temperature. Heat is energy whereas temperature is a measure of how
fast, on average, the particles of a substance are moving. While in
a lot of cases, as energy in the form of heat is injected into a
system, the temperature does go up (the particles move faster and
faster), this is not always the case. For example, take a pot of
boiling water, you can raise the temperature of the water by adding
heat (energy) into it. However, when the water is at maximum rolling
boil, no amount of additional heat will make the temperature go past
the boiling temperature (the water molecules do not move any
faster). So here we have a case where the addition of heat into a
system only serves to speed up the rate of evaporation (convert
liquid phase to gaseous phase) - it does not raise the temperature
of the system. Heat and temperature are two distinct concepts.
So what are the important factors in the melting of ice? Let us
assume that you are near standard conditions (at sea-level pressure,
etc.), in this case the ice will only melt if it is at a temperature
of 0 degC. That is (just like in the case of boiling water), adding
energy into ice will initially raise its temperature, and then when
the ice reaches 0 degC the additional energy no longer raises the
temperature, rather the additional energy causes the speeding up of
the melting of the ice.
So in answer to your daughter's point that the sun "does not melt
the fragile snow so how can it harm her
skin" we have to say that melting of ice (under normal conditions)
only happens if the ice is already at 0 degrees C, otherwise the
energy of the sunlight only serves to raise the ice's temperature.
This means that the sunlight (energy/heat) does have an effect on
the ice - the temperature goes up - it is just that it is something
that we do not observe with our eyes.
The effect of the sunlight energy on skin, on the other hand, is not
temperature dependent. That is, whether skin is below, at, or above
0 degrees C, absorption of the sunlight energy will be the same.
This means that it is mostly irrelevant (with regard to sunlight
effect on skin) whether it is hot (summer) or cold (winter), energy
will still cause damage.
Greg (Roberto Gregorius)
Different parts of the electromagnetic spectrum are responsible for heat
(and hence the melting of snow) and for damage to skin cells. Heat comes
primarily from the infrared part of the spectrum -- wavelengths longer
than about 700 nanometers. Ultraviolet radiation -- wavelengths shorter
than about 400 nanometers are responsible for skin damage. Now with regard
to microwaves melting ice (or stated more accurately, heating water) the
wavelength (or frequency) used in a microwave oven is chosen specifically
to match an absorption of the rotational spectrum of water. So the
microwaves selectively "giggle" the water molecules very intensely. For a
six year old, you can show an analogy with a guitar or other stringed
instrument. If you pluck one of the lower pitched strings at a fret that
makes it match the pitch of a higher pitched string you can actually touch
the latter string lightly and feel it vibrating in resonance (to use the
technical term) with the fretted string. Just as with the acoustic
analogy, the molecular and electronic motions that absorb radiation are
usually quite specific, so that materials can be transparent to some
wavelengths of radiation but not others.
This is an addendum to your direct answer to this question:
The part of the spectrum that we cannot perceive by light or by
heat can sometimes be seen by other animals.
One very important example is in the perception of a healthy mate.
Those beautiful feathers of a male parakeet do not look any different
to us if we put sunscreen on him. However if we did so, he would
not find a mate--because the colors in a part of the ultra violet spectrum
that we cannot see make all the different to his potential mate!
Jeannine M. Durdik
Professor of Biological Sciences
University of Arkansas
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Update: June 2012