Principle
The driving force for atmospheric motion is the sun and, in
particular, the uneven distribution of solar radiation across
the earth. It is the primary job of the atmosphere to redistribute
energy in order to achieve a balance from pole to equator.
We need to understand methods of heat exchange in the atmosphere
to appreciate how the thin atmosphere keeps us alive. |
Energy
- Energy is the ability or capacity to do work on some
form of matter.
There are several forms of energy, including
the following:
- Potential energy is the energy which a
body possesses as a consequence of its position
in a gravitational field (e.g., water behind a
dam).
- Kinetic energy is the energy which a body
possesses as a consequence of its motion (e.g.,
wind blowing across a wind generator). It is dependent
upon an object's mass and velocity (e.g., moving
water versus moving air).
- Internal energy is the total energy (potential
and kinetic) stored in molecules.
- Heat (or thermal) energy is kinetic energy
due to motion of atoms and molecules. It is energy
that is in the process of being transferred from
one object to another because of their temperature
difference.
- Radiant energy is the energy that propagates
through space or through material media in the
form of electromagnetic radiation.
- The First Law of Thermodynamics states that energy
lost during one process must equal the energy gained
during another.
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Figure 1 -
Evaporation and Condensation
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Latent
Heat
- Latent heat is the heat energy required to change a substance
from one state to another.
There are basically three states
of matter: solid, liquid, and gas. The difference
between them is how the molecules are arranged. Solids
have tightly-packed molecules, liquids are still
bound together but not strongly enough to keep them
from flowing, and gas molecules are free-flowing,
not bound to one another at all. Energy is required
to change from one state to another because bonds
must be loosened, broken, tightened, or made. Energy
must be given to the molecules if bonds are to be
loosened or broken and taken from the molecules if
they are to be tightened or made.
- Energy is required to change from solid to liquid, liquid
to gas (evaporation), or solid to gas (sublimation). Energy
will be released to change from liquid to solid (fusion),
gas to liquid (condensation), or gas to solid.
- Latent heat of evaporation is the energy used to change
liquid to vapor.
IMPORTANT: The temperature does
not change during this process, so heat added goes
directly into changing the state of the substance.
About 600 calories of energy are needed for every
gram of water at room temperature. This is why you
cool when you step out of the shower. Heat is taken
from your skin to evaporate the water on your body.
- Evaporation is a cooling process.
- Latent heat of condensation is energy released when water
vapor condenses to form liquid droplets.
An identical amount of calories
(about 600 cal/g) is released in this process as
was needed in the evaporation process. This is one
mechanism of how thunderstorms maintain their intensity.
As moist air is lifted and cooled, water vapor eventually
condenses, which then allows for huge amounts of
latent heat energy to be released, feeding the storm.
- Condensation is a warming process.
- Latent heat of fusion describes both changing from solid
to liquid and from liquid to solid.
From solid to liquid, about
80 calories per gram are needed. From liquid to solid,
about 80 cal/g are released.
- Latent heat of sublimation describes both changing from
solid to gas and gas to solid.
Sublimation is rare as compared
to the other changes of state. From solid to gas
600 + 80 = 680 calories per gram are needed. From
gas to solid, 680 cal/g are released.
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Figure 2 -
Conduction, Convection and Radiation
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Methods
of Heat Transfer
- Conduction- energy is transferred by the direct contact
of molecules, not by the movement of the material.
Example:
putting your hand on a stove burner. The amount of energy
transferred depends on how conductive the material is.
Metals are good conductors, so they are used to transfer
energy from the stove to the food in pots and pans. Air
is the best insulator, so good insulating products try
to trap air and not allow it to move.
- Convection- energy is transferred by the mass motion
of groups of molecules resulting in transport and mixing
of properties.
Example: holding your hand over a stove burner.
In meteorology, we speak of convection predominantly as
that caused by rising currents of warm air. We refer
to all other mass motions of air as advection.
- Radiation- energy is transferred by electromagnetic radiation.
Example:
heat felt when standing away from a large fire on a calm
night. Everything that has a temperature above absolute
zero radiates energy. Radiation is not "felt" until
it is absorbed by a substance. It does not require
a medium to transfer energy through as do conduction
and convection.
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Specific Heat
- Specific heat is the amount of heat needed to raise the
temperature of one gram of a substance one degree Celsius.
- The specific heat of water is very high compared to other
substances, so water can store energy longer than most
other substances.
For example, the Gulf of Mexico
remains warm during the night, when air and soil
temperatures decrease rapidly.
Why is the Southern Hemisphere
summer generally not warmer than the Northern Hemisphere
summer although Earth is closer to the sun during
the Southern Hemisphere summer? Because most of the
Southern Hemisphere is water, which regulates the
seasonal temperatures.
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