Lecture #6:  Energy, Heat, and Radiation
Tuesday, 31 January 2001

Text Reading for Lecture #6

Pages 26-35 in the text

The energy for all types of weather ultimately comes from the sun.
In fact, the general circulation of the atmosphere is governed by
the sun and the geometry of Earth's orbit and axial tilt.

Of critical importance to this class is that energy can be stored, moved
from one location to the next, and then released -- violently!  This is the
concept of latent heating, which we'll discuss below.

Energy - The ability or capacity to do work on some form of matter.

Stored Energy - The total amount of energy stored (called internal
energy) determines how much work can be done.  An example is carbo
loading before a race.  We'll see that the atmosphere stores energy
in the context of water vapor, and when the vapor condenses to a liquid,
the energy is manifest as warming, which is a key ingredient in
thunderstorm updrafts.

Energy also is stored in the atmosphere on a spring day -- as the ground
warms and a stable layer prevents air parcels from rising vertically until
temperatures become sufficiently high.

Kinetic Energy - Energy of motion, equal to 1/2 * mass * square of velocity.
Recall our discussion of dynamic pressure?  Note that kinetic energy is
proportional to mass;  thus, a mass of air blowing 40 mph has less of an
effect on a building than does an equivalent mass of water!!

Potential Energy - Equal to mass * acceleration due to gravity * height
above ground.  Think of a bowling ball dropped from the roof of a
tall building.  Originally at rest, the ball converts potential energy into
kinetic energy (motion) as it falls.

In thunderstorms, potential energy (warm, moist air near the ground and
cool, dry air aloft) iis converted to kinetic energy when rising air condenses
to form intense updrafts and then precipitation, which leads to downdrafts
that bring cool air to the surface.  Thus, the purpose of the thunderstorm
is to remove the instability that gave rise to it!  In other words, the thunderstorm
carries warm air aloft and brings cool air to the ground, and this overturning
reverses the originally unstable configuration of the atmosphere.



In order to quantify the instability present in the atmosphere and thus forecast
the potential for thunderstorms and severe weather, we must examine some
basic concepts of heat transfer in the atmosphere.  (Note that the energy
available for thunderstorms is called the Convective Available Potential
Energy, or CAPE.  We'll talk about it later.)

Heat - Note that heat and temperature are different animals.  Temperature is
a measure of the average speed of atoms and molecules, whereas heat is
energy in the process of being transferred from one object to another
owing to the temperature difference between them (the energy always flows
from high to low temperature).  Thus, it is VERY IMPORTANT that you don't
think of heat as a substance or quantity, but rather as a process (heating).

Conduction - A point-to-point (by contact) transfer of energy.   Molecules
that are energetic impart energy to those nearby that are less energetic,
thus increasing their temperature.

In the atmosphere, heating by conduction is primarily important at the ground,
where air warms by directly contacting the surface.  Sunlight has very little
warming effect directly on air molecules.

Convection - The physical transfer of molecules or fluid from one region
to another.  Convection happens naturally in the atmosphere, as shown below.
To meteorologists, the term convection often is synonomous with convective
cloud or thunderstorm, but in reality, convection means the transport of a
fluid in any direction.  To distinguish vertical motion (convection) from that
in the horizontal, meteorologists use the term "advection" to describe
horizontal fluid transport.

Radiation - Both a noun and a verb.   Radiation refers to radiant energy, which
can take many forms (x-rays, gamma rays, alpha particles), and to the process
by which energy is transferred without some intervening medium.  The shorter
the wavelength of the radiation, the more energy it contains. 

The amount of radiant energy from the sun reaching the surface of Earth
depends upon latitude, time of year, and cloud cover/atmospheric particulates.
The Oklahoma Mesonet measures incoming solar radiation on a continuous
basis, and this information can be valuable in forecasting high temperatures
as well as thunderstorms.

Radiation incident upon an object can be absorbed, scattered, reflected,
or refracted.  Radars emit electromagnetic radiation that, upon encountering
precipitation particles, is scattered and reflected back to the antenna to
indicate the location and intensity of the precipitation.  We'll talk more about
radar later.

Optical phenomena in the atmosphere owe their existence to the manner
in which radiant energy is scattered, absorbed, reflected, or refracted.  We'll
examine later several such phenomena including rainbows, sun dogs, and
haloes.   The red colors of the sunset shown below were produced by
scattering, and the sky is blue because air molecules preferentially
scatter light in with short (blue) wavelengths.  So, if you don't look directly
at the sun, your eye intercepts colors with blue wavelength.

The albedo is the per cent of radiation returned from a surface, i.e., the
amount not absorved.  This quantity is critical in determining daytime
high temperatures as well as global weather.  Table 2.3 in the book shows
that the albedo of fresh snow is about 75-95%, while that of a grassy
field is 10-30%.  On average, the albedeo of clouds is 60%.  The albedo
of Earth, averaged over the planet, is about 30%, as shown below.

FOR NEXT TIME:  Atmospheric Moisture (Read pages 105-122)
After that, we'll talk about condensation/clouds (Chapter 6) and then
thermodynamic dagrams and storm development (Chapter 7).