1/31/00
HEAT TRANSFER IN THE ATMOSPHERE
We began with the following definitions to describe the state of a gas:
- Density: the ratio of an object's mass to its volume.
Imagine a box of air (we call this an air parcel), with flexible sides
that do not let air in or out. If we make the box get smaller (compressing the air)
we reduce it's volume, and so density must increase. If the box gets
larger (air expanding) then density decreases. We call this an inverse relationship
between volume and density.
- Temperature: a measure of the average speed (or kinetic energy) molecules
in a substance. We say average speed because not all molecules within a gas, for example,
move at the same speed; some move faster, some move slower. The faster they move, on average,
the higher the temperature. In this class we measure temperature using three different scales:
- Fahrenheit scale: Used for reporting surface observations;
- Celsius scale: Used by the rest of the world for pretty much everything. In the U.S., upper air measurements are reported in Celsius.
- Kelvin scale: Based on the definition of temperature given above; 0 Kelvin means absolutely no molecular motion ("absolute zero"). 273 Kelvin = 0 Celsius = 32 Fahrenheit.
Heat: A form of energy that is transferred from a hotter object to a cooler object. Heat is usually measured in calories, which is defined as the amount of energy it takes to heat 1 gram of water from 14.5 C to 15.5 C.
- Conduction: transfer of heat from molecule to molecule within a substance. Conduction
works by direct contact, so it is very effiecient in solids, where molecules are closely packed
together. In a gas, molecules are spaced far apart, relatively speaking, and so conduction is
not very efficient. Only a shallow layer oif air directly in contact with a hot surface
will be heated through conduction.
- Convection: the transfer of heat by movement of a fluid (liquid or gas). (More on this below).
The relationship between the temperature of a gas and its density is as follows: As temperature increases, its density will decrease (assuming its pressure remains constant). When a mass of air becomes less dense then the air surrounding it, it will become "lighter" and it will rise (i.e., "hot air rises"). This is what drives convection. A shallow layer of air in direct contact with a hot surface will be heated by conduction. This heated air becomes less dense and begins to rise (convection).
As it rises, the air
cools, sharing its heat energy with its environment. Cooler air
will flow in from the sides, replacing the rising air, and also becomes warmed by conduction.
The process is repeated over and over. The net result is that heat is
carried away from the surface and deposited into the atmosphere above. Note that
convection would not occur without conduction.
2/2/00
LATENT HEAT
As mentioned in Chapter 1 of Ahrens, water vapor is a very important constituent in our atmosphere; water is the only substance that exists as a solid, liquid, and gas at pressures and temperatures normally found at or near the earth's surface. What makes this so important is that every time water changes phase - i.e. from liquid to gas - an exchange of energy is involved. This energy is described as latent heat.
Evaporation occurs when a liquid water molecule acquires enough kinetic energy to break loose from its neighboring molecules. When this happens, the average kinetic energy (or speed) of the remaining water molecules is slightly lower, and so the temperature of the liquid will decrease. This is why evaporation is a cooling process. The energy required to cause liquid molecules to become gas moelcules is called the latent heat of vaporization, which translates to about 600 calories of energy to evaporate 1 gram of liquid water.
Condensation occurs when a gas molecule becomes slow enough (i.e. loses energy or
has lower temperature) that it can become bound to other liquid molecules. When this happens,
the average kinetic energy of the remaining air molecules is slightly higher than before, so the
air's temperature has increased. In this way, condensation is a warming process.
When 1 gram of water vapor condenses, about 600 calories of energy will be released to the
environment.
2/4/00
HEAT TRANSFER IN THE ATMOSPHERE: LATENT AND SENSIBLE HEAT FLUX
The term flux refers to the flow or transfer of energy from
Point A to Point B in the atmosphere.
- Sensible heat flux: This refers to the transfer of heat that we can
feel (i.e. "sensible") due to the processes of conduction and convection.
Conduction warms the very thin layer of air closest to the surface; conduction transports
this heat away from the surface to the surrounding atmosphere. Together these processes
transfer heat upward, from the surface to the atmosphere.
- Latent heat flux: Through evaporation of liquid water at the surface of the earth, the surface is cooled (i.e. loses energy). Through condensation of water vapor in the atmopsphere, air is warmed (i.e. gains energy). Together this transport of latent heat acts to take energy away from the surface and transfer it to the atmosphere. In the real atmosphere, there can be a great distance between where evaporation and condensation take place.
The processes of sensible and latent heat flux both act to transport energy
upward, from the surface to the atmosphere.
RADIATION
Along with conduction, convection, and latent heat, radiation is another important way that heat energy is transferred through the atmosphere. The main energy source for the earth and it's atmosphere is the sun, which as we all know produces light and heat. Both visible light and sensible heat are forms of electromagnetic radiation.
E/M radiation, as it is sometimes called, propagates at the speed of light (about 186,000 miles per second or 300,000 kilometers per second). It is characterized by its wavelength, which is commonly measured in units of millionths of a meter, referred to as a micrometer or micron. The human eye can detect E/M radiation having wavelengths of 0.4 - 0.7 microns - that is what we call visible light. Heat that we can sense is a form of infrared radiation - having wavelengths of about 0.7 to about 10 microns. Ultraviolet radiation is also emitted by the sun at wavelengths of 0.1 to 0.4 microns