The real atmosphere's energy balance includes not only radiation energy but also energy associated with water vapor evaporation and convection (see figure below). However, the atmosphere is still very close to total energy balance at each level.
First, let’s go through each set of arrows to see what is happening. The average solar irradiance at the top of the atmosphere is 342 W m-2, which we will represent as being 100% and then compare all other energy amounts to it.
- Leftmost two columns of yellow arrows: Of the solar irradiance coming into the atmosphere, most of the solar ultraviolet irradiance, about 3% of the total solar irradiance, mostly ultraviolet, is absorbed in the stratosphere and warms it, leaving 97% to make it to the troposphere. Another 17%, mostly at wavelengths just longer than solar visible wavelengths, is absorbed in the troposphere and another 30% is scattered back out to space by bright objects, such as clouds, non-absorbing aerosols, and snow, leaving 50% of the incoming solar irradiance to be absorbed at Earth’s surface.
- First column of red upward arrows: The Earth’s surface emits upwelling infrared irradiance that is 110% of the incoming solar irradiance. Of the 110% emitted by Earth’s surface upwards, only 12% is transmitted through the troposphere into the stratosphere and 10% of this 12% is subsequently transmitted through the stratosphere to space.
- Second column of red upward arrows: The troposphere radiates 89% down and 60% up, while 54% of this upward infrared escapes to space. Unlike our simple two-layer model in which we assumed that the troposphere emitted equally up and down, the real troposphere is more complex and the downward radiation exceeds the upward radiation because of the vertical distribution of temperature (with temperature decreasing with height through the troposphere), water vapor and carbon dioxide.
- Third column of red upward arrows: The stratosphere radiates 5% downward and 6% upward.
- Rightmost blue columns: Non-radiation vertical energy transport. Of the net 29% of irradiance absorbed at the Earth’s surface, 24% of it goes into latent heat. Latent heat quantifies the amount of irradiance necessary to evaporate liquid water at Earth’s surface to water vapor, which is transported upward by convection to form clouds, which releases this energy into the troposphere, warming it. The remaining 5% of net irradiance absorbed by the surface goes into sensible heat. Sensible heat is the conduction of energy between the warmer Earth’s surface and the cooler tropospheric air, thus warming the air and causing it to become less dense (higher virtual temperature) than its surrounding air, followed by convection, which moves warmer air upward.
At each level, the amount of energy going down must equal the amount of energy going up. Thus, at the top of the stratosphere, 342 W m-2 crosses into the stratosphere from space, and to balance this downward energy is 30% reflected solar irradiance upward to space and 70% upward emitted infrared radiation that makes it to space. At the top of the troposphere, there is 97% of 342 W m-2 solar irradiance and 5% infrared irradiance emitted by the stratosphere moving downwards and it is balanced against 30% upwelling reflected solar irradiance and 72% upwelling infrared irradiance. At Earth’s surface, the downward solar irradiance (50%) and downward emitted infrared irradiance from the troposphere (89%) balance the 110% upward emitted infrared irradiance from Earth’s surface, the 24% in latent heat and the 5% in sensible heat.
In reality, the Earth’s surface and atmosphere are not in simple radiative equilibrium, but are instead in radiative-convective equilibrium. Furthermore, the atmosphere is in radiative-convective equilibrium globally, but locally balance is far from achieved all over the Earth (see figure below). The absorbed solar irradiance is much greater near the equator than the poles because that is where the surface is most perpendicular to the incoming solar irradiance. The radiative and convective net upward energy transport is greatest at the equator as well (because Earth’s surface is warmer there than at the poles). Overall, there is significant net incoming radiation energy between 30oS and 30oN latitude and a net outgoing radiation energy poleward of 30o in both hemispheres.
This uneven distribution of incoming and outgoing radiation results in a flow of energy from the tropics to the poles (see figure below). It unleashes forces that cause a poleward motion of air. The poleward motion of warmer air, coupled with the Coriolis force that curves moving air to the right in the Northern Hemisphere and to the left in the Southern Hemisphere, causes the atmosphere’s basic wind structure, and thus its weather. We'll talk more about these forces and the resulting motion in the next few lessons when we discuss atmospheric motion (kinematics) and the forces (dynamics) that cause the motion that results in weather.
A series of slides shows you the vertical energy balance in the Earth system and can be found at this site that depicts the energy balance.
Quiz 7-1: Solving the Earth system's temperature problems.
- Find Practice Quiz 7-1 in Canvas. You may complete this practice quiz as many times as you want. It is not graded, but it allows you to check your level of preparedness before taking the graded quiz.
- When you feel you are ready, take Quiz 7-1. You will be allowed to take this quiz only once. Good luck!