Last week we were introduced to the concept of heat. We saw that Heat is the total internal kinetic energy of the atoms and molecules that make up a substance. Since heat is a form of energy, it is measured in Joules.
Heat will not flow between two objects of the same temperature.
Heat is really energy in the process of being transferred from one object to another because of the temperature difference between them.
The transfer of heat is normally from a high temperature object to a lower temperature object. Heat transfer changes the internal energy of both systems involved according to the First Law of Thermodynamics.
Heat can be transferred by:
Conduction is the transfer of heat within a substance, molecule by molecule. If you put one end of a
metal rod over a fire, that end will absorb the energy from the flame (this is radiation transferring
energy). The molecules at this end of the rod will gain energy and begin to vibrate faster. As they do
their temperature increases and they begin to bump into the molecules next to them. The heat is being
transfered from the warm end to the cold end.
As you can see air does not conduct heat very well. This is the idea behind a styrofoam coolers, the
air pockets between the styrofoam beads do not conduct heat very well. On the other hand, metals do conduct
heat very well. This is why metal seems cold when you touch it. The metal molecules are conducting your
body heat away from your hand quickly.
Convection is heat transfer by the mass movement of a fluid in the vertical (up/down) direction. This type of heat transfer takes place in liquids and gases. This occurs naturally in our atmosphere.
Warm air is less dense than cold air, making cold air heavier than warm air. On a sunny day, the
surface of the Earth is heated by radiation from the Sun. The thin layer of molecules touching the
surface are heated by conduction. We know air is a poor conductor of heat, so this warm mass of air
near the surface can not immediately transfer its heat away from the surface by conduction. This warm air
mass is buoyant and wants to rise upward because it is less dense, the heavy cold air takes the place of
the warm bubble. This rising warm light air is called a thermal in meteorology.
In lecture you learned that the pressure of the air decrease with height. This is an
important fact for the life of these rising thermals. Recall, P/ρT = R,
from the ideal gas law. For a rising thermal, you can think of this thermal as a parcel or a
balloon, there will be less pressure exerted on its wall as it rises. Therefore, the parcel will
expand and the temperature within the parcel will decrease.
Advection is the transfer of heat in the horizontal (north/east/south/west) direction. In
meteorology, the wind transports heat by advection. This happens all the time on Earth, heat is transported
in many ways. For example, wind blowing over a body of water will pick up evaporated water
molecules and carry them elsewhere, when the air with these water molecules cools, the water will condense
and release latent heat. The heat is being transfered by the wind.
Radiation allows heat to be transfered through wave energy. These waves are called Electromagnetic Waves, because the energy travels in a combination of electric and magnetic waves. This energy is released when these waves are absorbed by an object. For example, energy traveling from the sun to your skin, you can feel your skin getting warmer as energy is absorbed.
The energy a wave carries is related to its wavelength (measured from crest to crest). Shorter wavelengths carry more energy than longer wavelengths. Wavelengths are measured in terms of meters:
All things with a temperature above absolute zero emit radiation. Everything, your body, your desk, your house, grass, snow, the atmosphere, the moon, they all emit a wide range of radiation. The source of this electromagnetic radiation are vibrating electrons that exist in every atom that makes an object.
Emitted radiation can be:
The temperature of an object can tell us something about the emitted radiation.
For the Earth, T~300 K:
Our eyes are sensitive to visible light, with wavelengths between 0.4 and 0.7 μm. This happens to be the range at which the Sun emits its peak wavelengths of radiation. In fact, 44% of the Sun's radiation is within the visible spectrum. Over time, our eyes have evolved to see this peak wavelength.
Kirchhoff's Law says that good absorbers of a particular wavelength are also good emitters of that
wavelength, and poor absorbers of a wavelength are also poor emitters at the same wavelength.
How does Earth's atmosphere deal with solar radiation?
Out of 100% how much is reflected to space? How much is absorbed by the atmosphere? by the
Describe what is going on and how each method of heat transfer works in this example:
Heat Transfer and the Atmosphere