heat because it has many more vibrating atoms. Even though it’s at a lower temperature,
the bathtub has a greater total energy.
Heat is taken in or released when an object changes state, or changes from a gas to a liquid
or a liquid to a solid. This heat is calledlatent heat. When a substance changes state,
latent heat is released or absorbed. A substance that is changing its state of matter does not
change temperature. All of the energy that is released or absorbed goes toward changing
the material’s state.
For example, imagine a pot of boiling water on the stove: that water is at 100oC (212oF).
If a cook increases the temperature of the burner beneath the pot, more heat enters the
water. But still the water remains at its boiling temperature. The additional energy goes
into changing the water from liquid to gas. This allows the water to evaporate more rapidly.
When water changes from a liquid to a gas it takes in heat. Since evaporation takes in heat,
this is called evaporative cooling. Evaporative cooling is an inexpensive way to cool homes
in hot, dry areas.
Substances also differ in theirspecific heat, the amount of energy needed to raise the
temperature of one gram of the material by 1. 0 oC (1. 8 oF). Water has a very high specific
heat, which means it takes a lot of energy to change the temperature of water. Let’s compare
a puddle and asphalt, for example. If you are walking barefoot on a sunny day, which would
you rather walk across, the shallow puddle or an asphalt parking lot? Due to its high specific
heat, the water stays cooler than the asphalt, even though it receives the same amount of
solar radiation.
Energy From the Sun
Most of the energy that reaches the Earth’s surface comes from the Sun. The Sun emits
energy in a continuous stream of wavelengths (Figure15.11). These wavelengths include
visible light, infrared, ultraviolet radiation, and others. About 44% of solar radiation is in
the visible light wavelengths. When viewed together, all of the wavelengths of visible light
appear white. But a prism or water droplets, for example, can break the white light into
different wavelengths so that you can see separate colors (Figure15.12).
Only about 7% of solar radiation is in the ultraviolet (UV) wavelengths. Of the solar energy
that reaches the outer atmosphere, UV wavelengths have the greatest energy. There are
three types of UV energy: UVC has the shortest wavelengths and is the most energetic;
UVA is the longest wavelengths and is the least energetic; and UBV is in the middle of the
two. UV radiation will tan or burn human skin. The remaining solar radiation is the longest
wavelength, infrared. Most objects radiate infrared energy, which we feel as heat (Figure
15.13).
Some of the wavelengths of solar radiation traveling through the atmosphere may be lost
because they are absorbed by various gases (Figure15.14). Ozone, for example, completely