4.2: Radiation

What is radiation?

Radiation is how the Sun heats the Earth. The Sun, being a large and hot object, radiates a massive amount of energy that makes an 8-minute, 92,000,000-mile journey to Earth. Meteorologists must have a keen understanding of radiation. You would be surprised how much radiation is surrounding you right now. As a society, we often equate radiation with nuclear disasters, gamma rays, Homer Simpson, and so on. However, the light that brightens your home at night, the Wi-Fi signal you receive on your phone, and the radio signal that brings your favorite radio station to you are all examples of radiation. However, they are all very different types of radiation. Therefore, we need to understand what determines the type of radiation a photon contains. There is one key property that defines this: The radiation’s wavelength (the Greek symbol λ, or lambda, is often used to denote wavelength). If you visualize a beam of radiation as a wave, the wavelength is the distance between successive peaks (as shown in Figure \(\PageIndex{1}\)). The wavelength determines two other key properties of radiation: Energy and Frequency. Both are inversely proportional to the wavelength. This means that as the wavelength of a beam of radiation increases, both its frequency (f) and its energy (E) decrease. Therefore, longer-wavelength radiation is low-energy and typically harmless.

Figure \(\PageIndex{1}\): The wavelength is the distance between two consecutive peaks of an Electromagnetic Wave. (CC BY 4.0; Alicia Mullens)

The Electromagnetic Spectrum

The Electromagnetic Spectrum (Figure \(\PageIndex{2}\)) shows the relationship between the wavelength, frequency, and energy of a category of radiation. AM Radio Waves have wavelengths of tens or even hundreds of meters, are extremely low-energy, and thus pass through things harmlessly. On the other hand, as the wavelength shortens, the type of radiation changes to FM Radio Waves, TV Waves, Microwaves, Infrared Waves, and so on, with each type becoming more energetic as the wavelength decreases.

Figure \(\PageIndex{2}\): The Electromagnetic Spectrum. (CC BY 4.0; Horst Frank via Wikimedia Commons). Alternative description of image

Table \(\PageIndex{1}\) provides a more detailed description of the types of radiation, their wavelength ranges, and the corresponding types.

Table \(\PageIndex{1}\) The Electromagnetic Spectrum explained

Region	Wavelength Range (m)	Frequency Range (Hz)	Explanation
Cosmic Radiation	≈ 10⁻¹⁵	≈ 10²³	Extremely high-energy radiation originating from space; the shortest wavelength and the highest frequency.
Gamma Radiation	10⁻¹⁴ – 10⁻¹²	10²² – 10²⁰	High-frequency radiation from radioactive decay and cosmic events; it is used in medical treatments.
Hard X-ray	10⁻¹² – 10⁻¹¹	10²⁰ – 10¹⁹	Shorter-wavelength, higher-energy X-rays are used for imaging and materials analysis.
Medium X-ray	10⁻¹¹ – 10⁻¹⁰	10¹⁹ – 10¹⁸	Intermediate energy X-rays used in diagnostics and research.
Soft X-ray	10⁻¹⁰ – 10⁻⁹	10¹⁸ – 10¹⁷	Lower-energy X-rays have greater penetration depth.
Ultraviolet (UV-C/B/A)	10⁻⁹ – 4×10⁻⁷	10¹⁷ – 10¹⁵	Non-visible UV light is just shorter than visible; UV-C is germicidal, UV-B causes sunburns, and UV-A penetrates deeper into the skin.
Visible Light	4×10⁻⁷ – 7.5×10⁻⁷	10¹⁵ – 10¹⁴	The portion of the spectrum visible to the human eye; used in vision and illumination.
Infrared Radiation	7.5×10⁻⁷ – 10⁻⁴	10¹⁴ – 10¹²	Emitted by warm objects; used in night-vision, heating, and communication.
Terahertz Radiation	10⁻⁴ – 10⁻³	10¹² – 10¹¹	Bridges microwave and infrared; used in spectroscopy and imaging.
Radar	10⁻³ – 10⁻²	10¹¹ – 10¹⁰	Used in detection systems for aircraft, ships, and weather.
Microwave (MW Oven)	10⁻² – 10⁻¹	10¹⁰ – 10⁹	Used for cooking, telecommunications, and radar.
Radio – UHF	10⁻¹ – 1	10⁹ – 10⁸	Ultra High Frequency; used in TV, cellphones, and GPS.
Radio – VHF	1 – 10	10⁸ – 10⁷	Very High Frequency; used in FM radio and aviation.
Radio – UKW	10 – 100	10⁷ – 10⁶	Ultra-shortwave radio; overlaps with VHF/shortwave.
Radio – Shortwave	100 – 1000	10⁶ – 10⁵	Used for long-distance radio transmissions.
Radio – Medium Wave	1 km	10⁵ – 10⁴	AM radio broadcast frequencies.
Radio – Longwave	10 km	10⁴ – 10³	Long-distance, low-frequency communication.
Low-Frequency (AC)	10³ – 10⁷	10³ – 10⁻³	Alternating current; used in power transmission.

3. Based on the visible light portion of the EM spectrum, _______ light has the shortest wavelength while _______ light has the longest wavelength.
   1. Red; Violet
   2. Yellow; Violet
   3. Violet; Red
   4. Blue; Green

4. Thus, recalling the relationship between wavelength and energy, the most energetic color is ______, while the least energetic color is ______.
   1. Red; Violet
   2. Yellow; Violet
   3. Violet; Red
   4. Blue; Green

Because radiation waves can become so short, we must introduce two new units into our toolbox: The nanometer (1 nm = 0.000000001 m) and the micrometer (1 µm = 0.000001 m). 1000 nm = 1 µm. There are a few key facts and laws of the electromagnetic spectrum that you need to know before we can thoroughly investigate how radiation affects our weather:

Fact and Law

Fact: Every object with a temperature above Absolute Zero (0 K) emits radiation! Yes, that includes everything around you, and you as well! We jokingly call this “Oprah’s Law” (Everybody emits radiation!), but it’s important to note that it’s a fact, not a law.

Stefan-Boltzmann Law: The warmer an object is, the more radiation the object emits. This is called the Stefan-Boltzmann law, and the equation for it is given as:

\[E= σT^4\]

Where E is the total amount of energy, T is the object’s temperature, in Kelvin, and σ is called the Stefan-Boltzmann Constant, and σ = 5.670373 x 10^-8 Wm^-2K^-4. Don’t worry about the equation; focus on the idea that Warmer Objects Emit More Radiation.

Wein’s Law: The warmer an object is, the shorter the wavelength it emits. This is called Wein’s Law (pronounced Vein’s law), and the equation for it is:

\(λ_m=\frac{2898 (μm K^{-1})}{T (K)} \)

Where \(λ_m\) is the wavelength (in micrometers) most emitted by an object, and T is the temperature in Kelvin. To convert a wavelength from micrometers to nanometers, multiply by 1000. Thus, not only is the amount of radiation emitted by an object dependent on the temperature of the object, but so is the type of radiation.

Let's answer the following questions based on these laws.

5. Based on the Stefan-Boltzmann Law, the Sun (T = 6000K) emits ____________ radiation than the Earth (T = 290 K).
   1. more
   2. less

6. Using the equation for Wien’s Law, the wavelength most emitted by the Sun (T = 6000K) is approximately:
   1. 48.3 µm
   2. 483 µm
   3. 0.483 µm
7. Converted to nanometers (multiply your answer to question 6 by 1000), the wavelength of radiation emitted by the Sun is approximately:
   1. 48,300 nm
   2. 483,000 nm
   3. 483 nm
8. According to the Electromagnetic Spectrum, the radiation most emitted by the Sun is in the form of:
   1. Infrared rays (741 nm-1 mm)
   2. Visible Light (400-740 nm)
   3. Ultraviolet Rays (10 nm-399 nm).
9. On the other hand, using the equation for Wein's Law, the Earth, with a temperature of approximately 290 K emits radiation primarily as __________.
   1. Infrared Rays
   2. Microwaves
   3. Gamma Rays
10. Thus, the _______ emits Shorter-Wave radiation (we’ll call this Shortwave Radiation from now on). In contrast, the _______ emits Longer-Wave radiation (we’ll call this Longwave Radiation from now on).
    1. Earth; Sun
    2. Sun; Earth

Now that we have a baseline understanding of the types of radiation emitted by the Earth and the Sun, we can discuss how they affect our atmosphere. Once Solar radiation, which extends beyond the wavelength calculated above, reaches the top of the atmosphere, several processes can occur. Radiation that enters the top of the atmosphere can either be:

• Reflected by clouds
• Reflected/Scattered by the Atmosphere
• Reflected by Earth’s Surface (Land and Ocean)
• Absorbed by the Atmosphere
• Absorbed by the Earth’s surface (Land and Ocean)

All of these factors significantly affect the amount of heat the Earth receives and, thus, its temperature.