Heat Transfer :

• Always from warmer to cooler objects

• conduction: the transfer of heat energy by molecular activity - molecule to molecule contact
• convection: the transfer of heat energy by the movement of a mass or substance from one place to another
• radiation: can be transfered through a vacuum

B. Forms of Atmospheric Radiant Energy:
 

                                           
                                       Wavelength
 

• Sun (shortwave): ultraviolet (.01-0.4), visible (0.4-0.7), near infrared (IR) (0.7-4.0)

• Earth-Atmosphere (longwave): thermal IR (4.0-100)  ....... units are in micrometers (1 micrometer equals one-millionth of a meter)

C. Basic Radiation Laws:

• all objects emit radiant energy
• hotter objects radiate more total energy per unit area than cooler objects
• the hotter the radiating body, the shorter the wavelength of maximum radiation
• objects that are good absorbers of radiation are good emitters as well
• the rate @ which solar energy falls on a surface located at the top of the Earth's atmosphere is a constant (solar constant)
• A "blackbody" is an object that is perfectly efficient at absorbing and radiating radiation (blackbodies do not exist in nature, but represent and ideal)

                                                        
 Intensity of Radiation Emitted by the Earth-Atmosphere System as a Function of Wavelength
 

D. Paths Taken by INSOLATION

• As insolation moves through the atmosphere three things can happen to it:

• reflection - occurs at the interface between two different media (e.g. air & cloud) when some of the radiation striking the interface is thrown back
• albedo - the ratio of reflected to incident radiation (an object w/albedo of 1 (or 100%) is a perfect reflector) - light colored objects have high albedos; dark objects have low albedos
• scattering - dispersal of radiation in all directions
•  absorption - radiation is converted to heat and emits that heat (radiation) according to #2 (above)

Direct Insolation - insolation that is transmitted directly through the atmosphere to the earth's surface

Diffuse Insolation - insolation that is scattered &/or reflected to the earth's surface

• the direct and diffuse insolation ("total solar")are either absorbed or reflected by the earth's surface

• (that portion which gets absorbed is converted to heat)

 Basic Principles:

• while the earth behaves like a blackbody - the atmosphere does not!!

 
• the gases that comprise the atmos. are "selective absorbers" - they absorb some wavelengths and are transparent to others.

• selective absorbers usually emit radiation at the same wavelength which they selectively absorb at.

Important Selective Absorbers:

• water vapor (H2O) **

• carbon dioxide (CO2) **

• nitrous oxide (N2O)

• methane (CH4)

• Ozone (O3)

• These gases are poor absorbers of visible (shortwave) radiation, but good absorbers of infrared (longwave) radiation

                                                    
Absorbtion of Radiation by Selected Components of the Atmosphere
 
 

The greenhouse effect thus works in the following manner:

• direct and diffuse shortwave radiation from the sun is absorbed at the earth's surface

• the earth radiates longwave energy into the atmosphere where some of it is absorbed by the various greenhouse gases

• these gases gain kinetic energy and collide w/neighboring air molecules which increases the average KE of the air, which results in an increase of air temperature.

• these same greenhouse gases also emit longwave radiation - some of which is transmitted to the earth's surface where it is absorbed and thus heats the ground.

• the earth then reradiates longwave energy upward, where, once again, it is absorbed by the greenhouse gases and warms the lower atmosphere

• thus, the greenhouse gases act as an "insulating layer" keeping some of the earth's radiant energy from escaping to space; and keeping the lower atmosphere considerably warmer than it would otherwise be (~59 F higher than without the GH gases)
                                                                                            
               Heating of the Atmosphere            Upward Trend in Atmospheric Carbon Dioxide Levels

• the Earth-atmosphere energy system is in a state of balance; i.e., 100 units of energy come into the system and 100 units leave.
• i.o.w.: the earth and atmosphere must return to space as much energy as they take in - otherwise the earth's average surface temperature would change

Budget: 100 Units of Insolation

30 units ...... reflected to space by clouds, the earth, and the atmosphere (thus, the albedo for the Earth as a whole (planetary albedo) is 30%

20 units ...... absorbed by clouds and the atmosphere

50 units ...... absorbed at the earth's surface

........................................................................

70 units .........radiated back to space by the Earth-Atmosphere system

• the earth maintains a delicate balance between incoming and outgoing energy, and essentially there is no yearly gain or loss of total energy, thus the average temp of the earth remains fairly constant from year to year.

• in contrast, the Earth's surface receives a surplus of energy, while the atmos. exhibits a deficit

Net Radiation and Energy Budget by Latitude
 

incoming - outgoing = net radiation

• have positive values of net radiation at lower latitudes

• negative values toward the poles

• poleward of ~ 36 degrees N & S lat, values of net radiation are negative - thus, a net loss in energy !!

• thus, we have an overall imbalance in radiant energy from the equator to poles!!

• this leads to atmospheric & oceanic circulations !!

                            
            Energy Budget by Latitude
 
 

G. Energy Balance at Earth's Surface
 

• "boundary layer climate" - the climate at or near the Earth's surface
• "microclimatology" - the study of this layer

• "net radiation" (NET R) - the net (resultant) flux of all the short and longwave fluxes at the Earths' surface:

• sw (down) varies by season, latitude, and cloud cover
• sw (up) is a function of albedo

• at night Net R is negative because SW (down) = 0, and the surface continues to lose L (up) from the surface
• Fig. 4-17: Net R values are positive ~ equatorward of 70 degrees N & S latitude, and negative poleward of 70 degrees N & S latitude
• Net R is expended as sensible heat, latent heat, ground heating, and conversion of heat energy to biochemical energy through photosynthesis
• sensible heat; heat that you can sense; it gets transferred from ground to air, and air to ground via conduction and convection (18% of NET R)
• latent heat of evaporation; heat energy stored in water vapor after water evaporates (dominant expenditure of NET R)
• ground heat; energy that flows into and out of the ground by conduction (is zero overall, because the energy stored in spring & summer is lost in fall & winter)
• Photosynthesis by plants utilizes 8% of NET R, converting it into biochemical energy

(1) desert site (Fig. 4-20a)

(2) midlatitude moist meadow site (Fig. 4-20c)

(3) urban site (Table 4-1):

• "urban heat island effect": urban microclimates are warmer on average than areas in the surrounding countryside
• urban surfaces are more conductive and have a higher heat storage capacity; and thus have higher temperatures: higher max and min temps
• urban areas have lower albedos, and thus produce higher NET R's - which is expended in producing higher values of sensible heat (H)
• urban rain water runs off quickly (doesn't soak into the ground); and hence isn't available to be evaporated: thus little transfer of heat by latent heat of evaporation (and latent heat of condenstion)
• irregular geometric shapes of cities traps insolation; this energy gets conducted to the surface which leads to higher temperatures
• human activities (anthropogenic activities) exacerbate the urban heat island effect (e.g., through the burning of fossil fuels)
• air pollution (gases & aersols) create a higher albedo, but increase L (down); also air pollution may lead to precipitation downwind from urban areas (more condensation nuclei)