Earth Sun Relationships and Photoperiodism

Electromagnetic radiation

Radiant energy traveling at the speed of light and manifesting as waves in electric and magnetic fields.

Electromagnetic radiation as waves

Electromagnetic radiation can be conceptualized as energy traveling in wave form.

Wavelength

The distance representing one complete cycle of a wave, measured between two corresponding points such as consecutive crests or troughs.

Symbol for wavelength

The Greek letter lambda (λ).

Frequency

The number of cycles in a waveform that occur over a unit of time; it can be thought of like beats per minute.

Wavelength vs. frequency

Wavelength and frequency are inversely proportional: as wavelength increases, frequency decreases, and vice versa.

Long wavelength

A wavelength associated with lower frequency and lower energy.

Short wavelength

A wavelength associated with higher frequency and higher energy.

Wavelength and energy

Shorter wavelengths correspond to higher-energy radiation, while longer wavelengths correspond to lower-energy radiation.

Red light

Visible light with longer wavelengths and lower frequencies than green and blue light.

Green light

Visible light with intermediate wavelengths and frequencies compared with red and blue light.

Blue light

Visible light with shorter wavelengths and higher frequencies than red and green light.

Electromagnetic spectrum

The full range of all possible frequencies of electromagnetic radiation.

Visible light

A subset of wavelengths within the electromagnetic spectrum that can be detected by the human eye.

Visible light wavelength range

The lecture gives approximately 390–700 nanometers for the visible light band.

Solar radiation peak

The peak intensity of solar radiation occurs within the visible wavelengths.

Gamma rays

A portion of the electromagnetic spectrum characterized by extremely short wavelengths and very high energy.

X-rays

A portion of the electromagnetic spectrum with very short wavelengths and high energy.

Ultraviolet radiation

A portion of the electromagnetic spectrum with wavelengths shorter than visible light.

Infrared radiation

A portion of the electromagnetic spectrum with wavelengths longer than visible light and associated with thermal radiation.

Radar

A region of the electromagnetic spectrum shown among longer-wavelength forms of radiation.

FM

A radio-frequency region of the electromagnetic spectrum.

TV

A radio-frequency region of the electromagnetic spectrum.

Shortwave radiation

Radiation primarily associated with incoming solar energy; it has shorter wavelengths and greater intensity and peaks in visible wavelengths.

Longwave radiation

Radiation primarily associated with energy emitted by Earth; it has longer wavelengths and peaks in thermal wavelengths.

Solar radiation

Radiation emitted by the Sun that reaches Earth primarily as shortwave radiation.

Earth radiation

Radiation emitted by Earth primarily as longwave or infrared radiation.

Why is solar radiation shortwave?

The Sun is much hotter than Earth, so its emitted radiation has greater intensity and peaks at shorter wavelengths.

Why is Earth's radiation longwave?

Earth is much cooler than the Sun, so its emitted radiation has lower intensity and peaks at longer thermal wavelengths.

Planck function

A description of energy intensity as a function of wavelength; the lecture uses it to show that solar energy peaks in visible wavelengths.

Power spectrum graph

A graph in which wavelength is shown on the x-axis and radiation intensity is shown on the y-axis.

Radiation intensity

The amount or strength of radiation emitted at a particular wavelength.

Stefan-Boltzmann Law

A law stating that any object warmer than absolute zero emits radiation and that the intensity of emitted radiation is proportional to the fourth power of its absolute temperature.

Absolute zero

0 Kelvin, equivalent to −273.15°C; the theoretical lowest temperature used in the Stefan-Boltzmann relationship.

Stefan-Boltzmann temperature relationship

Because radiation intensity depends on the fourth power of absolute temperature, warmer objects emit much more intense radiation than cooler objects.

Wien's Law

The wavelength of maximum radiation is inversely proportional to an object's absolute temperature.

Wien's Law temperature relationship

The warmer an object is, the shorter the wavelength at which its maximum radiation occurs.

Stefan-Boltzmann vs. Wien's Law

Stefan-Boltzmann's Law describes the intensity of emitted radiation, while Wien's Law describes the wavelength at which maximum radiation occurs.

Sun vs. Earth radiation

The Sun emits more intense radiation with shorter wavelengths, while Earth emits less intense radiation with longer wavelengths.

Energy balance

Incoming and outgoing energy must balance over time for Earth's overall energy budget to remain in equilibrium.

Earth's radiation budget

The balance between shortwave radiation received from the Sun and infrared (longwave) radiation transferred back to space.

Earth's energy budget

The accounting of incoming solar radiation and the processes by which energy is reflected, absorbed, transferred, and emitted back toward space.

Insolation

Incoming solar radiation reaching Earth.

Incoming shortwave radiation

Solar radiation entering the Earth system, primarily peaking in visible wavelengths.

Outgoing longwave radiation

Thermal radiation emitted by Earth and ultimately transferred toward space.

Atmospheric interaction with solar radiation

Solar radiation can be reflected, scattered, transmitted, or absorbed as it passes through the atmosphere and interacts with Earth's surface.

Albedo

A measure of the reflection of insolation; it describes the percentage of incoming solar energy reflected from Earth back into space.

Albedo percentage

The percentage of incoming shortwave solar radiation that is reflected rather than absorbed.

High albedo

A surface or system that reflects a large percentage of incoming solar radiation.

Low albedo

A surface or system that reflects a small percentage of incoming solar radiation and therefore absorbs a larger fraction.

Factors affecting albedo

Albedo varies as a function of sun angle, surface color, and surface texture.

Surface color and albedo

Lighter surfaces generally reflect more incoming solar radiation and therefore have higher albedo than darker surfaces.

Surface texture and albedo

The texture of a surface can influence how much incoming solar radiation is reflected.

Sun angle and albedo

The angle at which solar radiation strikes a surface influences the amount of radiation reflected.

Why does albedo matter?

Albedo determines how much incoming solar energy is reflected back to space versus retained within the Earth system.

Reflection

The process in which incoming radiation is redirected away from a surface or system rather than absorbed.

Scattering

The diffusion of radiation in different directions as it interacts with particles or molecules.

Rayleigh scattering

A scattering process in which small particles and gas molecules reorient the direction of radiation; for solar energy it approximates a random process.

Rayleigh scattering and sunsets

Scattering of solar radiation contributes to the colors seen in beautiful sunsets.

Transmission

The proportion of radiation that passes through the atmosphere without being absorbed.

Atmospheric transmission

Some incoming radiation passes through the atmosphere and reaches Earth's surface.

Absorption

The process of retaining incident radiation and converting it into another form of energy.

Radiation absorption and sensible heat

Absorbed radiation can be converted into sensible heat, such as when blacktop roads heat up on a summer day.

Absorption bands

Specific wavelength ranges in which atmospheric gases absorb radiation.

Molecular composition and absorption

The wavelengths absorbed by the atmosphere depend on the molecular composition of the atmosphere.

Top-of-atmosphere radiation

Incoming radiation measured at the top of the atmosphere before it undergoes all of the interactions occurring lower in the atmosphere.

Sea-level radiation

Radiation observed closer to Earth's surface, where atmospheric absorption has altered the radiation spectrum.

Why does sea-level radiation look different from top-of-atmosphere radiation?

Atmospheric gases selectively absorb particular wavelengths, producing absorption bands and changing the radiation spectrum.

Shortwave radiation budget

The accounting of incoming solar radiation and how much is reflected or absorbed before becoming energy within the Earth system.

Incoming solar radiation

100% of the incoming solar radiation is used as the starting point for the shortwave radiation budget shown in the lecture.

Reflected shortwave radiation

30% of incoming solar radiation is reflected by clouds, atmospheric particulates, and Earth's surface in the lecture's budget.

Atmospheric absorption of shortwave radiation

22% of incoming solar radiation is absorbed by atmospheric water vapor, dust, and ozone in the lecture's budget.

Surface absorption of shortwave radiation

48% of incoming solar radiation is absorbed at Earth's surface in the lecture's budget.

Shortwave budget calculation

100% incoming = 30% reflected + 22% absorbed by the atmosphere + 48% absorbed at Earth's surface.

What happens to the 48% absorbed at Earth's surface?

It is transferred away from the surface through processes including evaporation, convection, and longwave radiation.

Longwave radiation budget

The accounting of how energy absorbed by Earth's surface is transferred back toward the atmosphere and space through longwave radiation and other heat-transfer processes.

Convection

Convective heat transfer involving the combined processes of conduction and advection.

Sensible heat flux

The transfer of heat from Earth's surface to the adjacent atmosphere through direct contact.

Surface heating and convection

The Earth's surface heats adjacent air, which contributes to sensible heat transfer through convection.

Convection in the energy budget

5% of incoming solar radiation is represented as energy transferred through convection in the lecture's longwave energy-budget diagram.

Evaporation

The phase change of water from liquid to gas.

Evaporative heat loss

The transfer of energy away from the surface through the evaporation of water.

Latent heat flux

Energy stored in the random motions of water vapor molecules during evaporation.

Latent energy

Heat energy stored during the phase change of water from liquid to vapor and later released when water vapor condenses.

Condensation

The conversion of water vapor from the gas state back to the liquid state, during which latent energy is released.

Evaporation in the energy budget

25% of incoming solar radiation is represented as energy transferred through evaporation in the lecture's energy-budget diagram.

Longwave radiation from Earth

Earth emits absorbed energy as thermal longwave radiation.

Longwave radiation in the energy budget

18% of incoming solar radiation is represented as outgoing longwave radiation in the lecture's budget.

Longwave energy-budget values

The lecture shows 48% absorbed at Earth's surface, 25% transferred through evaporation, 5% through convection, and 18% through longwave radiation.

Energy-budget accounting

30% is reflected, 22% is absorbed by the atmosphere, and 48% is absorbed at Earth's surface; the 48% is then represented by 25% evaporation, 5% convection, and 18% longwave radiation.

Incoming vs. outgoing radiation

Incoming radiation is primarily shortwave and peaks in visible wavelengths, while outgoing radiation is primarily longwave and peaks in thermal wavelengths.

Main incoming-radiation processes

Albedo, transmission, and absorption are the major processes shown for incoming solar radiation.

Main outgoing-energy processes

Longwave radiation, convection, evaporation, re-radiation, and the greenhouse effect are shown as processes associated with outgoing energy.

Re-radiation

The emission of radiation by Earth's surface and atmosphere after energy has been absorbed.

Greenhouse effect

The process by which atmospheric components affect the transfer of outgoing longwave radiation, causing Earth to retain more energy than it would without the atmosphere.

Why is Earth's actual average temperature warmer than −18°C?

The greenhouse effect provides additional warming that is missing from a simple calculation based only on solar radiation heating the surface.

Earth's theoretical temperature without the greenhouse effect

The lecture states that based on the amount of solar radiation heating Earth's surface, Earth's average temperature should be approximately 0°F or −18°C.

Earth's energy equilibrium

For Earth's energy budget to remain balanced over time, incoming energy must ultimately be balanced by outgoing energy.

Thermal wavelengths

Longer wavelengths associated with Earth's outgoing longwave radiation.

Visible wavelengths

Wavelengths in the electromagnetic spectrum where solar radiation has its peak intensity and that are visible to humans.