When people hear “solar radiation” they associate it with negativity. However, solar radiation is something we all encounter on a daily basis and it aids in the formation of beautiful weather phenomena like rainbows and halos. Dr. Erika Navarro explains the different aspects of solar radiation below so you can stay informed!
What is solar radiation?
Solar radiation is the energy emitted from the sun. Radiation is emitted in the form of heat, and the hotter the object, the higher the emitted energy. This energy has a specific “fingerprint,” which we can map on what’s called the electromagnetic spectrum. This spectrum defines all the different types of light and energy that we observe both on the earth and in space. The sun is a very warm object (9,941 degrees F!), and so the energy emitted from the sun is high on the electromagnetic spectrum — the sun emits energy in the form of ultraviolet light, visible light, and infrared light.
Does solar radiation have an impact on clouds?
Yes! Clouds present in the atmosphere interact with the incoming sunlight. They can absorb it (which means they re-emit that energy back into the atmosphere and to space, or that they evaporate), or they can reflect it (which is why we can see them!). The types of particles in the cloud, as well as their size, all determine how it interacts with light.
How do different types of clouds react to solar radiation?
Thinner clouds (such as cirrus) are mostly transparent to solar radiation — they allow that energy to pass right through them and continue heading towards the ground, with very little being absorbed or being reflected back to space. Thicker clouds (such as cumulonimbus) have MANY more particles inside of the cloud, and so there is much more opportunity for the sunlight to be absorbed or be reflected. Often, these clouds are so thick that most of the light is scattered away and very little sunlight makes it out of the cloud, which is why they appear dark.
Please explain how solar radiation assists in the creation of the following phenomena:
Rainbows are formed by the interaction of solar radiation and a type of water droplet in the sky (usually a raindrop). When sunlight reaches and penetrates the surface of a raindrop, that light often changes its course slightly (“bending” or changing its angle relative to the ground) as it moves from the air into the water (a process called “refraction). Then, when that sunlight hits the back of the raindrop, it is reflected, and the beam of light turns around heads back into the direction in which it came. Then, when the beam of light hits the front of the raindrop again, it refracts (“bends”) again as the light moves from water back into the air. This process often causes the beam of light to “disperse,” which means that the different colors of light spread apart just enough that we can detect each color individually with our eyes.
Halos, Sundogs, and Sun Pillars
Halos, sundogs, and sun pillars are formed when light from the sun interacts with ice crystals present in the sky (often in very thin ice clouds). This sunlight “bends” (refracts) when it moves from the air into the ice particle, and can also reflect off the inside or outside of the crystal. The shape of the ice crystal is important, as well as how it is oriented and positioned in the sky. When ice crystals are spread randomly in the sky (no particular orientation) we often can see a circular halo around the sun as light refracts inside the ice crystals. When the ice crystals are vertically aligned (upright in the sky), they reflect light horizontally, and often “sundogs,” or bright spots to the left and right of the sun, are observed. When ice crystals are falling horizontally, or flat relative to the ground, the light reflecting off the surface of these particles can look like a sun beam, or what is called a sun pillar in the sky.
Sunsets are observed when the sun is very low in the sky. When this occurs, the light coming from the sun must travel a longer distance to reach your eye, which leaves more opportunity for the beam of light to interact with things in the sky (such as clouds or small particles). Often, this distance is long enough that most of the “blue” light is scattered away, leaving more of the “red” colored light behind. This is the light then usually reaches the ground (and our eyes). That is why sunsets are usually pink and orange, and why clouds usually make for a great sunset!
How do wavelength, frequency and energy vary across the electromagnetic spectrum, and how does this impact what we see in the sky?
Wavelength, frequency, and energy all vary across the electromagnetic spectrum. The electromagnetic spectrum is measured from high to low energy, with the highest energy waves (such as gamma rays and x-rays) having the shortest (lower) wavelength. Frequency acts in the opposite sense of wavelength, such that higher energy waves have a higher frequency and lower wavelength. Our eyes are a specific type of sensor designed to detect light in the visible part of the electromagnetic spectrum — that is, wavelengths of light from 0.39-0.76 micrometers. There are other wavelengths of light around us, however, we cannot see it with our eyes.
Walk us through the diurnal cycle of radiation.
The diurnal cycle of radiation is simply the daily cycle of sunlight that we receive from the sky. This cycle — increasing intensity of sunlight through the morning, decreasing in the afternoon, and no sunlight overnight — is all due to the daily rotation of the earth. As the earth rotates about its axis each day, the amount of sunlight reaching the earth changes. We receive less sunlight in the morning because the sun is at a lower angle in the sky, and that light has to travel a longer distance in order to reach the surface. However, from the morning until about noon, the surface of earth is rotating towards the sun, such that angle between the sun and the earth decreases, and the intensity of the light reaching the surfaces increases. At what is called “solar noon”, the sun is at the highest point in the sky and the sunlight reaching the ground is at the most intense for that day. Then, throughout the rest of the daylight hours the surface of the earth is rotating away from the sun and angle between the sun and earth increases again, and the intensity of sunlight reaching the earth decreases. This occurs until the earth rotates far enough that the sun completely drops off the horizon. At night, no solar radiation is received from the sun until the earth rotates around again so that the sun appears on the horizon.
What aspects of the atmosphere does the diurnal cycle impact?
Many different kinds! The diurnal cycle of radiation affects everything from clouds, to air temperature, even the surface of the ocean! Sunlight is the energy source for the planet, and so anything that interacts with it and receives that sunlight can exhibit a diurnal cycle.