Enviro Test #2 - Earth's energy balance

Dominant components of the atmosphere: nitrogen and oxygen

4 billion years ago, carbon dioxide was 80% of the atmosphere, Nitrogen was about 10% and water was around 10%. Now it looks like the picture.

Albedo - values for different surfaces/global average

A portion of the incoming solar radiation is absorbed by the surface and a portion is also reflected away. The proportion of light reflected from a surface is the albedo. Albedo values range from 0 for no reflection to 1 for complete reflection of light striking the surface. Albedo can be expressed as a percentage (albedo multiplied by 100). For instance, grass has an albedo of about .23 (range:.15-.25). This means that of the incoming solar radiation that strikes the grass, 23% of it is reflected away. On the other hand, highly reflective surfaces like snow have an albedo upwards of .90, or 90% of sunlight is reflected away. The average albedo of Earth is approximately 0.31 (31%).

Direct/diffuse radiation

About 30% of the available solar radiation at the top of the atmosphere is reflected or scattered back to space by particulates and clouds before it reaches the ground. The gases of the atmosphere are relatively poor absorbers of solar radiation, absorbing only about 20% of what is available at the outer edge of the atmosphere. The remaining solar radiation makes its way to surface as direct and diffuse solar radiation. Direct solar radiation (S) is shortwave radiation able to penetrate through the atmosphere without having been affected by constituents of the atmosphere in any way. Diffuse radiation (D) is shortwave radiation that has been scattered by gases in the atmosphere. Scattering is a process whereby a beam of radiation is broken down into many weaker rays redirected in other directions. Together, direct and diffuse shortwave radiation accounts for the total incoming solar radiation or insolation (K↓). K↓ = S+D

Changes in atmospheric density, pressure and temperature with altitude

As altitude increases, air pressure and air density gets progressively lower because the lower you are the more air you have above you. The atmosphere has a mass of about five quintillion (5 × 1018 or 5,000,000,000,000,000,000) kg, three quarters of which is within about 11 km (6.8 mi; 36,000 ft) of the surface. (Earth's atmospheric pressure is 14.7 pounds per square inch or 1.04 kg per square cm).

Non-radiative heat transfer

Available net radiation is used to do work in the Earth system. The principal use of this energy is in the phase change of water (latent heat, LE), changing the temperature of the air (sensible heat, H), and subsurface (ground heat, G) or, Q* = H + LE + G Though there are other uses for net radiation like photosynthesis and the weathering of rocks, it is the three previously stated uses that are most important. LE, H, and G involve non-radiative transfers of heat. That is, conduction or convection/advection are responsible for the transfer of heat (thermal energy), not electromagnetic radiation. To illustrate the transfer of energy we'll use arrows either pointing away from or toward the surface of the Earth to indicate the direction of heat transfer. We will also use positive and negative signs to indicate that heat is being added to, or taken away from a body. Non-radiative fluxes directed away from a surface are positive. Thus positive values indicates a loss of heat from the surface while negative values indicate a gain.

Electromagnetic radiation

Electromagnetic radiation travels through space in the form of waves. Unlike heat transfer by convection or conduction, heat transfer by electromagnetic radiation can travel through empty space, requiring no intervening medium to transmit it. The quantity of energy carried in a wave is associated with the height or amplitude of the wave. Everything else being equal, the amount of energy carried in a wave is directly proportional to the amplitude of the wave. The type or "quality" of radiation depends on the wavelength, the distance between successive crests. The greater the distance between wave crests, the longer the wavelength. Q = σ T4 A where: Q = heat transfer per unit time (W) σ = 5.6703 10-8 (W/m2 K4) - The Stefan-Boltzmann Constant T = absolute temperature Kelvin (K) A = area of the emitting body (m2) λ = 2897 μm/T (μm = 1000 nm)

Subsurface heat (ground)

Ground Heat Transfer (G) The third major use of radiant energy is to warm the subsurface of the Earth. Heat is transferred from the surface downwards via conduction. Like in the case of sensible heat transfer, a temperature gradient must exist between the surface and the subsurface for heat transfer to occur. Heat is transferred downwards when the surface is warmer than the subsurface (positive ground heat flux). If the subsurface is warmer than the surface then heat is transferred upwards (negative ground heat flux).

Latent heat (Changes in energy state during evaporation/condensation)

Latent heat: is the energy released or absorbed when a substance changes from one state to another (evaporation, condensation and freezing of water).

Net Radiation Balance

Net Radiation Gathering all the radiation terms together we have net radiation: Q*= [(S+D) - (S+D)a] + L↓ - L↑ Net radiation can be positive, negative, or even zero. Net radiation is a positive value when there is more incoming radiation than outgoing radiation. This typically occurs during the day time when the sun is out and the air temperature is the warmest. At night, net radiation is usually a negative value as there is no incoming solar radiation and net longwave is dominated by the outgoing terrestrial longwave flux. Net radiation is zero when the incoming and outgoing components are in perfect balance, which doesn't occur too often. Q*: Net Radiation S: direct solar radiation D: diffuse radiation a: albedo L↓: atmospheric radiation (longwave) L↑: surface radiation (longwave)

Net shortwave radiation

Net shortwave radiation is the difference between incoming and outgoing shortwave radiation expressed as: K*= (S+D) - (S+D)a During the day, K* is a positive value as incoming always exceeds outgoing shortwave radiation. At night, K* is equal to zero as the Sun is below the horizon.

Influence of coastal environment on seasonal variations in temperature - importance of differences in specific heat of land and water

See graphics on powerpoint.

Seasonal variations in incoming shortwave (solar radiation)

See picture. See powerpoint - good graphic.

Geographic and seasonal changes in surface temperatures

See powerpoint. good graphics.

Sensible heat

Sensible heat is heat energy transferred between the surface and air when there is a difference in temperature between them. A change in temperature over distance is called a "temperature gradient". In this case, it is a vertical temperature gradient, i.e., between the surface and the air above. We feel the transfer of sensible heat as a rise or fall in the temperature of the air. Heat is initially transferred into the air by conduction as air molecules collide with those of the surface. As the air warms it circulates upwards via convection. Thus the transfer of sensible heat is accomplished in a two-step process. Because air is such a poor conductor of heat, it is convection that is the most efficient way of transferring sensible heat into the air.

Specific heat (relative values for water and air)

Specific heat is the amount of energy needed to raise the temperature of 1 gram of material by 1oC. Specific heat is different for every substance, a function of the molecular structure. Water has an unusually high specific heat. Continentality is the tendency of land to experience more thermal variation than water, due to the land's lower specific heat capacity.

Temperature/heat

Temperature is proportional to the average speed of the atoms or molecules in an object (matter)(average kinetic energy). Temperature will decrease as altitude increases. As pressure decreases temperature also decreases. For a constant volume: P1 / T1 = P2 / T2 P: pressure T: temperature

Kármán line

The Kármán line, or Karman line, lies at an altitude of 100 kilometres (62 mi) above the Earth's sea level, and commonly represents the boundary between the Earth's atmosphere and outer space.

Net longwave radiation

The difference between incoming and outgoing longwave radiation is net longwave radiation expressed as: L* = L↓ - L↑

Longwave radiation

The energy absorbed at the surface is radiated by the Earth as terrestrial longwave radiation (L↑). The amount of energy emitted is primarily dependent on the temperature of the surface. The hotter the surface the more radiant energy it will emit. The gases of the atmosphere are relatively good absorbers of longwave radiation and thus absorb the energy emitted by the Earth's surface. The absorbed radiation is emitted downward toward the surface as longwave atmospheric counter-radiation (L↓) keeping near surface temperatures warmer than they would be without this blanket of gases. This is known as the "greenhouse effect". The difference between incoming and outgoing longwave radiation is net longwave radiation expressed as: L* = L↓ - L↑

Maximum wavelength - function of temperature

The maximum wavelength at which a body emits radiation depends on its temperature. Wein's (pronounced "weens") Law states that the peak wavelength of radiation emission is inversely related to the temperature of the emitting body. That is, the hotter the body, the shorter the wavelength of peak emission. The figure below shows the wavelengths over which the sun and earth emit most of their radiation. The Sun being a much hotter body emits most of its radiation in the shortwave end and the Earth in the longwave end of the spectrum. *The hotter the object the more electromagnetic radiation it gives off.* *The hotter the object, the shorter the wavelength.* Solar radiaton = short wave radiation. Terrestrial radiation = long wave radiation.

Radiation balance

The radiation balance of the Earth system is an accounting of the incoming and outgoing components of radiation. These components are balanced (incoming = outgoing) over long time periods and over the Earth as whole. If they weren't the Earth would be continually cooling or warming. However, over a short period of time, radiant energy is unequally distributed over the Earth, resulting in periods of localized heating and cooling.

Shortwave

The radiation balance of the Earth system is depicted in the above figure. (shortwave radiation is colored yellow and longwave radiation is in red.) Shortwave radiation from the Sun penetrates through space to the outer edge of the atmosphere unimpeded by the vacuum of outer space. If one places a surface oriented perpendicular to an incoming beam of light, 1360 W m2 of solar radiation will be received. This value is known as the solar constant but actually varies by a small amount as the Earth-Sun distance changes through the year. When the solar constant is averaged over the entire Earth surface we get a value of 342 W m2. Once solar radiation begins to penetrate through the atmosphere this amount begins to decrease due to absorption and reflection.

Thermals (convective processes)

Thermal column (or thermal) is a column of rising air in the lower altitudes of the Earth's atmosphere. Thermals are created by the uneven heating of the Earth's surface from solar radiation, and an example of convection. The Sun warms the ground, which in turn warms the air directly above it. (1) The warmer air expands, becoming less dense than the surrounding air mass. (2) The mass of lighter air rises, and as it does, it cools due to its expansion at lower high-altitude pressures. It stops rising when it has cooled to the same temperature as the surrounding air. (3) Associated with a thermal is a downward flow surrounding the thermal column. The downward moving exterior is caused by colder air being displaced at the top of the thermal.

Greenhouse effect

Water vapor and carbon dioxide do not significantly absorb solar (short wave) radiation, however, they do readily absorb long wave (thermal) radiation. In this way, H2O and CO2 in the atmosphere acts much like the glass on a greenhouse. These gases allow the sunlight (solar radiation) to enter and warm the surface. However, they act to trap the outgoing heat, resulting in rising air temperatures. These gases are termed "greenhouse gases" and the absorption of outgoing longwave radiation by these gases is referred to as the Greenhouse Effect. The average surface temperature of the earth is around 15°C, some 33°C warmer than it would be without the greenhouse effect.

Conduction/Convection - Transfer from surface (land and water) to atmosphere

When energy is added to water it will change states or phase. The phase change of a liquid to a gas is called evaporation. If we could see down to the molecular level we would find water being comprised of cluster of water molecules (H2O). The clusters are bound together by bonding between the hydrogen atoms of water molecules. The heat added during evaporation breaks the bonds between the clusters creating individual molecules that escape the surface as a gas. The heat used in the phase change from a liquid to a gas is called the latent heat of vaporization. We say it is "latent" because it is being stored in the water molecules to later be released during the condensation process. We can't sense or feel latent heat as it does not raise the temperature of the water molecules. When evaporation is taking place we say there is a positive latent heat flux (transfer). A positive latent heat flux is illustrated with an arrow pointing up away from the surface of the earth. This indicates that the surface is losing energy to the air above. Evaporation is a cooling process for a surface because energy is removed from the water as molecules escape the surface. This causes the surface temperature to decrease. You've probably experienced this cooling when water or sweat evaporates from your skin. Condensation is the phase change from a gas to a liquid. During the phase change, the latent heat that was taken up during evaporation is released from the water molecule and passed into the surrounding air. During this process latent heat is converted to sensible heat causing an increase in the temperature of the air. When radiation is absorbed by the Earth it will raise the temperature of the surface. But if the surface is water, some of that energy is used in evaporation rather than heating the water. As a result, with equal inputs of energy to land and water surfaces, land will heat up more than water. This is one of the reasons why it is cooler near large bodies of water.

Geographic (latitude) variations in incoming shortwave (solar radiation)

When the sun shines directly over the equator, the equator receives the most intense solar radiation, and the poles receive little. The insolation averaged over March, 2003 from NASA's CERES satellite. In March, the insolation is at its greatest right at the equator and drops off to nearly zero at the poles. Note that this is the insolation before it interacts with the atmosphere, so it is a fairly simple pattern.