2.2: 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. And so we need to understand what determines the kind 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 2.2.1). 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, as visualized in Figure 2.2.1, is the distance between two consecutive peaks of an Electromagnetic Wave. (CC BY 4.0; Alicia Mullens)

The Electromagnetic Spectrum 

The Electromagnetic Spectrum (Figure \(\PageIndex{1}\)) breaks down 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 \(\PageIndex{1}\): The Electromagnetic Spectrum. (CC BY 4.0; Wikimedia Commons). Alternative description of image 

Table 2.3.1 provides a more in-depth description of the different types of radiation, their wavelength ranges, and each type.

Table \(\PageIndex{1}\)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; used in medical treatments.
Hard X-ray	10⁻¹² – 10⁻¹¹	10²⁰ – 10¹⁹	Shorter wavelength X-rays with higher energy, used in 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 with more penetration depth.
Ultraviolet (UV-C/B/A)	10⁻⁹ – 4×10⁻⁷	10¹⁷ – 10¹⁵	Non-visible light just shorter than visible; UV-C is germicidal, UV-B causes sunburns, UV-A penetrates deeper into 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, 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, GPS.
Radio – VHF	1 – 10	10⁸ – 10⁷	Very High Frequency; used in FM radio, 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.

2. 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

Because radiation waves can become so short, we must introduce two new units into our toolbox: The nanometer (1nm = 0.000000001m) and the micrometer (1µm = 0.000001m). 1000nm = 1µm. 

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. 

Law: The Warmer and 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. 

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

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: 

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

Where \(λ_m\) is the wavelength (in micrometers) most emitted by an object, and T is the temperature in Kelvin. To convert wavelength 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.      

4. Using this equation for Wein’s Law, the wavelength most emitted by the Sun (T = 6000K) is approximately: 
   1. 48.3 µm   
   2. 483 µm   
   3. 0.483 µm
5. According to the Electromagnetic Spectrum, the radiation most emitted by the sun is in the form of: (to convert your answer from question 4 to nanometers [nm], multiply it by 1000)
   1. Infrared rays (741nm-1mm)    
   2. Visible Light (400-740nm)    
   3. Ultraviolet Rays (10nm-399nm). 
6. On the other hand, Earth, with a temperature of approximately 290K emits radiation primarily as __________. 
   1. Infrared Rays       
   2. Microwaves       
   3. Gamma Rays 
7. 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 things can happen to it. 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 have a significant impact on the amount of heat the Earth receives and, thus, its temperature.