SEASONS, ENERGY BUDGET AND TEMPERATURE

SCATTERING AND REFLECTION OF SOLAR RADIATION

Scattering

-responsible for daylight and sky colors

-sun would appear as a fiery disk in an otherwise dark sky

-scattering means that the solar beam is turned in all directions is most effective in the shortest wavelengths

-when sun is overhead and atmosphere cloudless and dust free more than 50 percent of the blue is scattered while almost all of the red is transmitted

-when sun is near the horizon during sunrise and sunset the number

of air molecules traversed by the solar rays is greatest

and scattering is most effective then

-so we see red sunrises and sunsets-this also accounts for the weakness of the winter sun when it is lower in the sky

-very fine dust or smoke haze in the atmosphere leads to an

abnormal sky appearance when the particles scatter the solar beam

-sun may appear as a dull red ball over a polluted city

Rayleigh Scattering

-exact amount and direction of scatter depend on ratio of radius of

scattering particle to wavelength

-S prop to l/wavelength raised to 4th power

-for particles less than 1/10 wavelength of light amount of scatter is inversely proportional to fourth power of wavelength

-.i.e short waves are scattered better than long which accounts

for blue of sky

Mie Scattering

Reflection

-at sizes above about .7 microns, particles in the air such

as water droplets, ice crystals, salt, dust and combustion products reflect solar rays

-no discrimination as to wavelength

ALBEDO

-percent of incident solar radiation which is reflected

-dependent upon-angle of sun-higher when sun is low in the sky

-nature of surface

-rougher and wetter surfaces tend to have lower albedos

-light colored surfaces have higher albedos

-some variance with wavelength

-i.e. green plants reflect more in green that other wavelengths

-Examples

fresh snow 75-95

old snow 40-70

dry sand 30-60

wet sand 30

plants 10-20

concrete l7-27

blacktop 5-l0

woods 5-20

meadow 12-30

dark soil 7-10

water 3-10

RADIATION TYPES

Solar radiation generally has a wavelength of less than 3 microns and is termed shortwave radiation.

Because of Wien's Law, lower temperature objects like those on earth typically radiate at longer wavelengths (longer than 3 microns) and emit longwave radiation

RADIATION BUDGET

If we were to look at all of the radiant energy flows we would find that for every 100 units of solar radiation that enter the top of the atmosphere about 25 units are reflected back into space from cloud tops and dust and gases in the atmosphere (Rayleigh Scattering). About 20 units are absorbed by atmospheric components directly and rest pass on to the earth (about 55% of the original incoming radiation). Of these 55 units about 5 are reflected back into space and the rest are absorbed by the surface and stored as internal energy (heat).

Therefore for every 100 units of solar radiation that enter the top of the atmosphere about 30 are reflected back into space, 20 are absorbed by the atmosphere and 50 are absorbed at the surface

As the atmospheric gases and surface materials have absorbed radiation from the sun their internal energy levels are increased along with their temperatures. They in turn are going to radiate energy away. Atmospheric constituents radiate energy upward to space and downward to the surface. The surface radiates energy upward. Since most longwave radiation is not subject to reflection we needn't consider albedo.

What the atmosphere radiates directly to space is lost to the system but what is radiated downward is absorbed by the earth. When the earth radiates upward most of the radiation is absorbed by atmospheric gases (even with clear skies). This is the greenhouse effect. Some radiation does escape through bands where the atmospheric gases are ineffective absorbers.

If we work all of the totals for both short and longwave exchanges we find that

GREENHOUSE EFFECT

Because solar and terrestrial radiation peak at different portions of the spectrum they have different properties and interact differently with the atmosphere.

-atmospheric gases and aerosols absorb only about 19% of the incoming solar radiation while the atmosphere absorbs a proportionately greater amount of infrared radiation emitted by the surface of the earth.

-this is then remitted back to earth and keeps the temperature at the surface about 30 C higher than it would ordinarily be.

-this is done primarily by water vapor, carbon dioxide, methane and cfc's

-atmospheric windows are specific wavelengths which allow terrestrial longwave radiation to escape through clear skies thereby cooling the earth

-clouds also produce a greenhouse effect when they are present at night--but during the day they have a high albedo and prevent solar radiation from coming in

-overall they have a net cooling effect on the earth

-greenhouse effect is also the reason why mountaintops are usually colder because of decrease in concentration of greenhouse gases with altitude (also air is less dense)

DAILY CYCLE OF AIR TEMPERATURE

Generally there is a low point around sunrise with a high point near mid afternoon. The ground cools radiatively all night with negative energy balance (after sunrise ground begins to heat up).

Net radiation reaches a maximum at noon with the sun at meridian and distribution tends to be symmetrical.

Air temperature tends to rise past noon, for the radiation surplus, though beginning to diminish, is still positive.

-air temperature maximum generally occurs about 3 PM. After that air temperature tends to fall even though there is still a slight

surplus of net radiation

-if air temperature depended only upon the radiant energy absorbed at the ground the maximum air temperature would occur at sunset, but in the early afternoon mixing of the low air in turbulent motions increases in intensity and carries the heated air upward and replaces it with cooler air from aloft.

TEMPERATURE INVERSIONS

AIR TEMPERATURE CONTROLS

Radiational Controls

-time of day and day of the year

-cloud cover

-nature of surface cover

Air Mass Controls

-warm and cold air advection

CONTROLS OF TEMPERATURE

a. diff. heating of land and water

b. ocean currents

c. latitude

d. altitude

LAND WATER TEMPERATURE CONTRASTS

-thermal properties of water-great thermal inertia and high specific heat

-rather unusual because normally the boiling of substances increases as molecular weight increases (water has 18 atomic mass units). However, chemically related substances of greater molecular weight have boiling points 100 to 150 degrees celsius lower than water

-what is unusual is that water occurs in all three phases

WHY DOES WATER BEHAVE IN SUCH AN UNUSUAL MANNER?

-each water molecule consists of two hydrogen atoms bonded to one atom of oxygen

-this bonding involves the sharing of two electrons, one from the hydrogen atom and the other from the oxygen atom

-because the oxygen atom has a stronger attraction for the shared electrons than do the hydrogen atoms, it acquires a small negative charge and the hydrogen is left with a small positive charge

-because the hydrogen oxygen hydrogen atoms describe a 105 degree angle a charge separation develops on water molecules and they are dipolar

-opposite charges attract so that the dipolar water molecules link together in what is called hydrogen bonding and is rather strong and is in three directions

-this hydrogen bonding inhibits changes in the kinetic activity of individual water molecules so as heat is added to water the resulting increase in kinetic molecular activity and corresponding rise in temperature are rather small and water exhibits an unusually high specific heat so this means greater amounts of heat are required for water to reach its melting and boiling points

-the stabilizing effect of hydrogen bonding also means that an exceptionally great amount of heat is required to change the phase of water

-latent heat of vaporization is greater than fusion because in melting not all of the hydrogen bonds are broken but with vaporization all bonds must be broken

WIND CHILL

Humans are homeothermic which means that we regulate our internal or core temperature within 2 C of 37 C.

At ambient air temperatures of 20 to 25 C someone fully clothed and indoors will feel comfortable at rest

When exposed to ambient air temperatures above or below the range the body must initiate processes that maintain core temperatures

e.g. high temps you perspire

at low temps you shiver which is increased muscular activity which produces additional heat

blood vessel constriction and dilation at lower and higher temps respectively

CLOUD EFFECTS ON ENERGY BUDGET

Far more important than CO2

Two effects

1. at bottom, absorbs longwave from surface and at top it radiates heat to space

2. At top it reflects shortwave radiation

on the whole they tend to cool the earth by 10 to 15C as compared to an earth without clouds

Measurement of Air Temperature

-mercury in glass thermometer was perfected by Fahrenheit in the early 1700's but though it was used almost immediately to measure air temperature over 100 years elapsed before it was recognized that careful exposure was needed-reason is because the thermometer is sensitive to solar and terrestrial radiation from which it must be shielded

-two major distadvantages

1. Can't be used below ­40 C because of freezing

2. can't continuously record temperature

3. alcohol in glass is good for minimum temperatures

Stevenson Screen is a simple strong compromise solution

-white surface absorbs little sunlight

-wooden walls insulate interior from warming effects of residual solar absorption and from warming and cooling effects of terrestrial radiation

-louvered walls and floors allow for ventilation of interior by wind

-ventilation is least satisfactory, especially on days when wind is calm-sometimes a fan is used