The atmosphere absorbs and transmits infrared (IR) radiation in a selective way — some IR wavelengths pass through easily, while others are strongly absorbed by gases. This plays a crucial role in Earth’s energy balance and greenhouse effect.
1. Selective Absorption by Greenhouse Gases
- Water vapor (H₂O), carbon dioxide (CO₂), methane (CH₄), ozone (O₃), and nitrous oxide (N₂O) absorb IR radiation at specific wavelengths.
- Each gas has its own absorption spectrum — “fingerprint” wavelengths where it is most effective.
- Example:
- CO₂ absorbs strongly near 4.3 μm and 15 μm.
- H₂O absorbs broadly in several bands, especially <8 μm and >12 μm.
2. The Atmospheric Window
- There’s a range of IR wavelengths (~8–14 μm) where little absorption occurs.
- IR in this “window” can escape directly to space, which helps Earth lose heat.
- Clouds and increased greenhouse gases can partially close this window, trapping more heat.
3. Transmission vs. Absorption
- Absorption: Gases take in IR energy → molecules vibrate more → later re-radiate heat in all directions (including back toward Earth’s surface).
- Transmission: IR passes through without much interaction — common in the atmospheric window.
- Scattering: Minimal for IR (mainly affects visible and shorter wavelengths).
4. Why This Matters
- Climate: More greenhouse gases → more IR absorption → enhanced greenhouse effect → warming.
- Weather satellites: Use transparent IR bands to “see” Earth’s surface or opaque bands to study clouds and gases.
- Astronomy: Ground-based IR telescopes work best in the atmospheric window; outside it, space-based telescopes are needed.