Background Material
Most information is available in Chapter 1. Here is some additional
information on the standard atmosphere, and
some brief notes. You should know what Latitude and Longitudes are.
ATMOSPHERIC COMPOSITION
Air takes up space - it consists of billions of air molecules. Our atmosphere
has evolved over 100's of millions of years. Atmosphere is a gaseous envelope
- N2 - 78%, O2 21% trace gases - H2O, CO2, O3, CFC and Methane - Clouds
and Aerosols .
Gravity keeps the gases from going out to space. Gravity weakens as
you get farther away from surface, so the number of molecules decreases
with altitude.
DENSITY tells us how much matter is in a given space. Density
decreases with altitude.
PRESSURE - Even though we can't feel the constant bombardment
of air, the push of tiny molecules, we can detect rapid changes. Ears popping
- air collisions outside the ear drum lessen. Ear popping, occurs as the
molecules on the inside and outside equalize.
Pressure is force per unit area. The pressure at any level in the atmosphere
may be measured in terms of the total weight of the air above any point
- atmospheric pressure. Pressure always decreases with altitude.
Blow up a balloon - the number of molecules, and the speed, colliding
against the sides determines the air pressure inside. You have to do work
to blow up the balloon. Which brings us to the definition of energy, which
is the ability or capacity to do work on some form of matter. Work is done
on matter when matter is either pushed, pulled or lifted over some distance.
Energy must be conserved, but it can be converted between different forms.
Kinetic energy - energy associated with motion.
TEMPERATURE is a measure of the average speed of molecules. Vertical
distribution of temperature
Troposphere Stratosphere Mesosphere
10 km 50 km 85 km
Weather occurs in the troposphere and is the focus of this course. The
stratosphere is important particularly with respect to O3. There are interactions
between the stratosphere and troposphere even though an inversion exists.
Back to kinetic energy and the movement of molecules. Pressure, density
and temperature are related via the ideal gas law.
Hold pressure fixed - increase temperature, density decreases
decrease temperature, density increases
Cold air is more dense than warm air.
At the surface a parcel has the same temperature as air around
it. When we raise the parcel the air pressure is lower; molecules push
on "sides" to expand parcel - this is work; less energy (kinetic energy)
cooler temperature. Rising parcels of air always cool due to expansion.
Sinking or subsiding air always warms by compression.
ENERGY TRANSFER MECHANISMS IN THE ATMOSPHERE
Energy is everywhere. Energy is the ability or capacity to do work on some
form of matter. By doing work on something we give it energy, which it
can in turn use to do work on other things. Energy exists in many forms,
it cannot be created or destroyed, it merely changes form.
Kinetic energy - also depends on its mass.
Potential energy, represents the potential to do work. In atmospheric
science, gravitational potential energy.
Another example is radiant energy - the energy we get from the sun.
Heat is energy in the process of being transferred from one object
to another because of the temperature difference between them. After heat
is transferred it is stored as "internal energy" which is the total energy
stored in the molecules.
How is energy transferred in the atmosphere? Before we do this lets
discuss the concept of sensible heat, specific heat and latent heat.
Sensible heat - heat we feel and measure with a thermometer
Specific Heat - The amount of heat needed to raise the temperature
of one gram of a substance 1C. Water has a much higher capacity for storing
energy than other substances -e.g. sand or air. Water is therefore very
important in determining a regions climate/weather.
-
CONDUCTION - transfer of heat from molecule to molecule. warmer
colder. The greater the temperature difference the faster the transfer.
Some materials are better conductors of heat than others, metals are good
conductors, air is poor.
-
CONVECTION - transfer of heat by the mass movement of a fluid such
as air.
-
ADVECTION. - Meteorologists think of convection as vertically moving
air, horizontal winds can also move heat energy.
-
RADIANT ENERGY - travels in the form of waves that release energy
when they are absorbed by an object. These waves have electric and magnetic
properties - electromagnetic waves. Electromagnetic waves do not need molecules
to propagate them - speed of light 186,000 mi/sec. Wave characteristics
- horizontal distance between two successive crests is the wavelength.
-
LATENT HEAT - heat energy required to change a substance from one
state to another. We will discuss this method more later in the course.
Evaporation cools the environment. As molecules leave, faster ones leave
first, average speed decreases and so the temperature of the object cools.
Latent heat is responsible for keeping a cold drink with ice colder than
one without ice. As ice melts its temperature doesn't change, added heat
breaks down the rigid crystal pattern. When ice melts, heat is taken in;
when it freezes heat is liberated. Phase changing is important for transferring
energy in the atmosphere. For example, Evaporation over tropical waters,
the water vapor can move poleward and condense.
Visible radiation has a wavelength less than one millionth of a meter -
100th of the diameter of a human hair - micron. Eyes are sensitive to radiation
between 0.4 and 0.7 microns. (violet-red) The sun emits 44% of its
energy in this region. UV rays wavelength between about .1 and .4 microns.
All things no matter how small emit radiation. As the temperature is
increased, more total radiation is emitted. The hotter the temperature;
the greater the amount of energy at shorter wavelengths.
Planet reflects 30% of the sun's energy back to space, 50% makes its
way to the surface
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