2.5.4: Radiation Changes with Time

The amount of radiation the Earth receives from the Sun varies over many time scales, from thousands of years, to one year, to daily time periods. These will be discussed in the following sections.

Changes in solar radiation on both daily and annual time scales can be explained by Earth’s orbital path around the Sun. These aspects will be discussed more thoroughly in a later chapter, but for now, let’s have a brief introduction. There are three primary factors to consider: eccentricity, obliquity, and precession. Eccentricity is the circularity of a planetary orbit. For example, a circle has zero eccentricity. Obliquity is the degree of tilt in the axis of rotation. Finally, precession is the wobble in the rotational axis of a planet that slowly traces out a cone. The three orbital parameters are shown in the image below.

Visual displaying three orbital parameters: eccentricity, obliquity, and precession (NASA Public Domain).

Currently the Earth has a 0.0167 orbital eccentricity, 23.44 degree tilt from vertical, and precesses on very long time scales. In fact, all of these factors change slightly over very long time scales but for now, let’s consider them to be constant.

Seasonal Changes

Seasons occur due to changes in solar radiation that come from the position of Earth with respect to the Sun. The diagram below shows the position of the Earth with respect to the sun during each of the four seasons.

Schematic diagram showing the reason Earth has seasons (CC BY-SA 4.0).

During summertime in each hemisphere, the hemisphere is facing toward the sun. For example, in June in the Northern Hemisphere summer, the sun shines more directly on the Northern Hemisphere than the Southern Hemisphere. While it seems like a small change, the sun angle and the amount of solar radiation absorbed (per area) varies significantly throughout the year. This is because in addition to changing the angle of the Sun, the position of the Earth also changes the length of day throughout the year. Seasons are due to the tilt of the Earth. If the Earth’s rotation was not tilted, it would not have seasonal changes.

Pro Tip: The cause of seasons on Earth is the topic of a common misperception. Earth’s seasons are caused by its tilted orbit. While it is true that Earth’s orbit is not spherical, and it is closer to the Sun during Northern Hemisphere winter, the distance from Earth to the Sun is not the cause of seasons.

The diagram below shows the summer season in the Southern Hemisphere where there is a higher density of incident rays due to the higher Sun angle. The Northern Hemisphere is experiencing wintertime. When the Sun’s rays are at an angle as they are in the Northern Hemisphere, the same amount of energy is spread over a larger area than they would be if the Sun’s rays were perpendicular to the surface. Again, the angle of the Sun’s rays and the length of day change because of the Earth’s tilt.

Distribution of solar rays on Earth, with the summer season receiving the majority of the solar radiation (Public Domain).

Daily Changes

Daily changes are also called “diurnal.” The Earth absorbs radiation from the Sun during the daytime. This is only true for one location, but accounts for the increase in temperature throughout the day from a point perspective. What we likely have not experienced directly is that the Earth emits infrared radiation all day and night. The lack of incoming solar radiation and the emission of infrared radiation is what accounts for the decrease in temperature at night. The combination of nonstop outgoing longwave radiational cooling from Earth’s radiative emissions and daytime solar shortwave radiational heating results in a diurnal cycle of net radiation and temperature, as seen below.

The amount of radiative energy (y-axis, in W m-2) as a function of time (x-axis, in hours). Yellow shows net warming and blue shows net cooling. The temperature is coolest at sunrise, warms during the day and reaches its maximum temperature in the late afternoon. (Image Created by Britt Seifert).

In this chapter we focused on the radiation: how we can define and describe it by wavelength and intensity; the temperatures it corresponds to as well as how it changes over time scales. As a reminder, these were our learning goals:

1. Define black body radiation and Planck’s Law
2. Apply Wien’s law to compute the maximum emission wavelength
3. Use Stefan-Boltzmann’s law to compute radiative emittance
4. Describe Earth’s surface radiation budget, including shortwave and longwave components
5. Define and differentiate obliquity, eccentricity, and precession
6. Describe the cause of seasons on Earth
7. Describe the diurnal cycle of radiative fluxes

In the following chapter, we’ll begin to see how this radiation does work, and what that means for the environment.

Chapter 2: Questions to Consider

1. Are the following statements about radiation true or false?

Query \(\PageIndex{1}\)

2. Describe the relationship between wavelength, energy emission, and the temperature of an object.
3. If the temperature of the Earth is 257 K, what is the total radiative flux emitted? What is the peak emission wavelength?
4. How would the seasons change if the Earth’s tilt were greater?

Selected Practice Question Answers:

Query \(\PageIndex{2}\)