Electromagnetic Radiation and Earth's Energy Balance

Flashcards covering key concepts from the lecture on electromagnetic radiation, blackbody radiation, Earth's energy balance, seasonal variations, atmospheric processes, greenhouse effect, and related phenomena. Flashcards are in a question-and-answer format.

What is electromagnetic radiation?

The emission or transmission of energy in the form of electromagnetic waves or photons through space.

How fast does light travel?

Light travels at 300,000 km/s (186,000 mi/s).

How are wavelength and frequency defined for a wave?

Wavelength is the distance between successive wave crests or troughs. Frequency is the number of wave crests passing a point per second, measured in Hz.

What is a blackbody?

An object that absorbs all incident radiation and emits radiation according to its temperature, described by the Planck Function.

Why does Earth radiate in infrared and the Sun in visible light according to Wien's Law?

Because Earth's temperature (~255 K) is much lower than the Sun's (~6000 K), Earth's peak radiation is in the longwave infrared, while the Sun's peak is in shortwave visible light.

What is the Stefan-Boltzmann Law?

The total energy flux emitted by a blackbody is proportional to the 4th power of its absolute temperature: E = σT⁴.

What is the role of greenhouse gases in Earth's energy balance?

Greenhouse gases (CO₂, CH₄, H₂O, N₂O, O₃) absorb outgoing longwave infrared radiation and re-emit it, trapping heat in the atmosphere and keeping Earth's surface habitable.

How does solar altitude affect the path length of solar radiation through the atmosphere?

With increasing solar altitude (Sun higher in the sky), the path length through the atmosphere decreases, reducing scattering, reflection, and absorption, and allowing more radiation to reach the surface.

Why is the Northern Hemisphere colder in winter even though Earth is closer to the Sun?

Seasons are controlled by Earth’s axial tilt (23.5°). In winter, the Northern Hemisphere is tilted away from the Sun, resulting in a lower solar altitude and shorter days, leading to colder temperatures.

What is the significance of the Tropics, Equinox, and Solstices?

The Tropic of Cancer (23.5°N) has the Sun overhead at noon during the summer solstice (June 21). The Tropic of Capricorn (23.5°S) has the Sun overhead at noon during the winter solstice (Dec 21). During an Equinox (March 21, Sept 23), the Sun is overhead at the equator, resulting in equal day and night everywhere.

Distinguish between scattering, reflection, and absorption of solar radiation.

Scattering is when solar radiation is dispersed in all directions by molecules/particles (e.g., Rayleigh scattering makes the sky blue). Reflection is a special case of scattering where radiation bounces back at an interface (albedo). Absorption is when radiation is converted into heat energy by gases, particles, or surfaces.

How does cloud cover affect minimum temperature?

Clouds absorb outgoing infrared radiation and re-emit it back toward the surface, which keeps nights warmer, resulting in a higher minimum air temperature compared to clear skies.

What is the Greenhouse (Callendar) Effect and how has atmospheric CO₂ changed since the Industrial Revolution?

The Callendar effect proposes that human-emitted CO₂ enhances Earth’s greenhouse effect, causing global warming. Since the mid-1700s, atmospheric CO₂ has increased sharply due to fossil fuel burning, cement production, and land use changes.

Which gases are the main greenhouse gases?

The main greenhouse gases are Carbon dioxide (CO₂), methane (CH₄), water vapor (H₂O), nitrous oxide (N₂O), and ozone (O₃).

What instruments are used to measure different types of radiation?

A Pyranometer measures incoming solar shortwave radiation. A Spectroradiometer (often on satellites) measures scattered/reflected visible light and emitted IR radiation. An Infrared radiometer measures outgoing infrared radiation.

What are solar flares and sunspots?

Solar flares are sudden energy releases from the Sun’s surface, often near sunspots. Sunspots are darker, cooler areas on the Sun's surface with strong magnetic activity, occurring in cycles of approximately 11 years.

What is albedo?

Albedo is the fraction of incoming solar radiation reflected by a surface. Earth’s average albedo is approximately 0.3 (30%). High albedo surfaces include ice, snow, and clouds, while low albedo surfaces include oceans and forests.

What is the significance of stratospheric ozone?

Ozone (O₃) in the stratosphere absorbs harmful UV radiation from the Sun, protecting life on Earth. The Montreal Protocol (1987) was an international agreement to ban ozone-depleting substances like CFCs.

What is the atmospheric window?

The atmospheric window refers to certain wavelengths of infrared radiation (8–12 µm) that escape directly to space without significant absorption by greenhouse gases, which is crucial for cooling Earth.

What is the difference between greenhouse gases and aerosols?

Greenhouse gases warm the Earth by absorbing and re-emitting infrared radiation. Aerosols (like volcanic ash or industrial pollution) generally cool the Earth by reflecting sunlight back to space.