Three things the atmosphere does to radiation passing through it:

• Scattering — photons are redirected unpredictably by particles or molecules.
• Absorption — photons are taken up by molecules (converted to heat or re-radiated).

• Transmission — photons pass through unchanged (this is what we want for imaging).

• Path length matters: low-sun / high-latitude imagery traverses more atmosphere and suffers more scattering + absorption.