Earth-Sun interaction

Temperature - a measure of the average kinetic energy of the molecules in that substance.  Therefore, when we talk about the temperature of the air, we're actually talking about how fast the air molecules are moving.

Temperature (K.E.) ~ mv2

Therefore as temperature increases the velocity of the air molecules increases.

Extremes of  recorded temperature
Highest temperature: +136°F (57.8°C) at El Azizia, Libya
Lowest temperature:  -126°F (-87.8°C) at Vostok Station, Antarctica

US highest temperature: +134°F (56.6°C) at Death Valley, California
US lowest temperature:     -80°F (62.2°C) at Prospect Creek, Alaska

IL highest temperature: +117°F (47.2°C) at East St. Louis (1954)
IL lowest temperature:     -36°F (-37.8°C) at Congerville (last January)

Temperature Scales
In this course you should (will) become familiar with three temperature scales and their units:

Fahrenheit (°F) -- German
Celsius (°C) -- Swedish
Absolute (K) -- "Scientific"

Fahrenheit Scale
Fahrenheit Scale (1714):
Ice melts at 32°F,
Water boils at 212°F.
N.B.: 180 degrees between melting and boiling point of pure water at sea level.

Celsius Scale
Celsius Scale (1742):
Ice melts at 0°C
Water boils at 100°C
N.B.: This scale was originally upside down.  It is one of several "Centigrade Scales."  100 degrees between melting and boiling point of pure water at sea level.

Thermodynamic (Kelvin) Scale
Kelvin or Absolute Scale (1800's):
No molecular motion at 0 K; use Celsiusí degree increment
Ice melts at 273 K
Water boils at 373 K

So how does the Earth's atmosphere get its heat?

First, a little about radiation.  It can be described by its wavelength and frequency.

Wavelength - the actual length, in meters, between any two crests (or troughs) in the wave, e.g., radio waves have a wavelength from ~100cm to 100s of meters; light has a wavelength of ~10-9 m.  Shorter wavelength radiation would be x-rays, UV rays, and light; longer wavelength radiation might be infrared, radar, TV, and radio.

Frequency - the number of wave crests that pass a particular point every second, measured in Hertz (Hz).

The relationship between wavelength and frequency is simply:

c = ln

c = speed of light (3 x 108 ms-1)
l = wavelength in meters, m
n = frequency in Hertz, Hz

Two laws describe how all things radiate:

1) Amount of energy they radiate (Stefan-Boltzman Law)
2) Maximum energy at which they radiate (Wein's Law)

Stefan-Boltzman Law:  E = sT4

E - amount of energy being radiated (Wm-2)
s - Stefan-Boltzman constant, 5.67 x 10-8 Wm-2K-4
T - temperature, K

Wein's Law: the wavelength of radiation that an object radiates in inversely proportionate to its temperature, i.e., hotter objects radiate at shorter wavelengths.

lmax = (2897 mm K) (T-1)

E.g., where does the Sun emit most of its radiation?
T = 6000 K
lmax = (2897 mm K) / (6000 K) = 0.48 mm (green light)

7 % Ultraviolet and shorter

X-rays, gamma rays, etc.; these are higher energy waves
44 % Visible light - what we can see with our eyes - we can see from approximately 0.4 to 0.7 mm
32 % Near infrared
11 % Far infrared
> 1 % longer wavelengths - microwaves, radio waves, etc.

Earth only receives ~ one two-billionth of the Sun's energy, but this amount accounts for 99.9 percent of the energy that the Earth uses to heat the planet (remaining from internal sources).

Heat is a form of energy transfer from one place to another due to differences in temperature.

Things donít have "heat" but they can be heated.

Energy
Energy is the ability or capacity to do work on some form of matter.

There are several forms of energy, including the following:

1. 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).
2. 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).
3. Internal energy is the total energy (potential and kinetic) stored in molecules.
4. 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.
5. Radiant energy is the energy that propagates through space or through material media in the form of electromagnetic radiation.
6. Latent heat is the heat energy required to change a substance from one state to another.

The First Law of Thermodynamics states that energy lost during one process must equal the energy gained during another.  Energy is neither created nor destroyed.

Methods of Energy Transfer

• Conduction - energy is transferred by the direct contact of molecules, not by the movement of the material, e.g., 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 a very good 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, e.g. 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, e.g., 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.

As the Earth orbits the Sun, there are three orbital factors that effect the amount of energy the Earth receives:

• Eccentricity - variation in the shape (circularness) of the orbit; duration of ~100,000 years.
• Obliquity (tilt) - angle the Earth's axis makes with the plane of its orbit; duration of ~41,000 years; ranges from 21.5° to 24.5° - now at 23.5°.
• Precession (wobble) - duration of ~26,000 years.
(Milankovitch cycle, pages 376 - 377 in book)

(Figure 14-6a, b, c, page 377 in Lutgens and Tarbuck's The Atmosphere, 2001) Eccentricity, Obliquity, and Precession

Since the Earth is tilted in it's orbit, not all the Earth receives the same amount of energy.

(Figure 2-2, page 29 in Lutgens and Tarbuck's The Atmosphere, 2001) Schematic showing reduced effectiveness of the Sun to heat as the angle at which the rays pass through the atmosphere decrease (hence they must pass through more of the atmosphere)

More energy is received at the equator than at the poles owing to the obliquity of the Earth's axis.

Solstices and Equinoxes
Since the Earth is tilted 23.5° on its axis, there is only one latitude on the Earth at any one time when the Sun's rays are striking at 90° (Sun is directly overhead).

As the Earth moves in its orbit, that latitude changes from a maximum of 23.5°S latitude to 23.5°N latitude.  Theses latitudes are given the special names of the Tropic of Capricorn (23.5°S) and Tropic of Cancer (23.5°N) [note typo in book, p.31 in figure]

(Figure 2-4, page 30 in Lutgens and Tarbuck's The Atmosphere, 2001) Schematic showing solstices and equinoxes.

Reflection, refraction, and albedo

Okay, so now we know that the Earth gets most of its energy from the Sun.  However, much of the energy that gets to Earth's upper atmosphere never makes it to the surface.

Radiation can be absorbed, reflected, scattered, refracted, or transmitted.

• Absorbed: energy is retained by a substance.
• Reflected: the surface returns a portion of the energy.
• Scattered: the electromagnetic waves are changed from propagating in one direction to all directions.
• Refracted: the electromagnetic waves are changed from propagating in one direction to another direction.
• Transmitted: energy passes through space or the media.