A blanket that lets light in
How Does the Air Keep Earth Warm?
Some gases let sunlight through but absorb the heat radiating back, keeping warmth near the surface.
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Step 1 of 5
Sunlight passes through the air easily.
Most of the Sun's energy arrives as visible light, and the gases in our atmosphere barely absorb it, so it reaches the ground.
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Step 2 of 5
The warmed ground radiates heat back up.
Everything warm gives off heat as infrared light, so the surface constantly radiates energy back towards space.
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Step 3 of 5
But that outgoing heat is a different light.
Infrared has much longer waves than visible light, and certain gases absorb those waves strongly while ignoring sunlight entirely.
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Step 4 of 5
Those gases send much of it back down.
Carbon dioxide, methane and water vapour absorb the heat and re-radiate it in all directions, so a good share returns to the surface.
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Step 5 of 5
Without it Earth would be frozen.
This effect keeps the planet roughly 33 degrees warmer than it would otherwise be, which is the difference between habitable and frozen.
Venus shows the extreme version: its thick carbon dioxide blanket holds the surface at about 465°C, hot enough to melt lead.
The short version
The light coming in and the heat going out are different lengths of wave — carbon dioxide, methane and water vapour ignore visible sunlight and absorb infrared strongly, sending much of it back down. That is worth about 33 degrees.
Try it yourself
Compare a parked car in sunlight with the shade outside. Light gets in through the glass; heat struggles to get back out.