Refraction and the 4/3 earth
The atmosphere's density decreases with height, which slightly bends radio waves downward. Radar engineers approximate this by pretending the earth has a radius 4/3 of its real value. With this trick, signals travel in straight lines on charts and ranges work out correctly.
Ducting
Strong temperature inversions over water can trap radio waves in a layer near the surface. Ships sometimes detect targets at 500 km — far beyond line-of-sight — because the duct acts like a waveguide. Air-defence systems both exploit and curse this effect.
Atmospheric absorption
Water vapour and oxygen absorb specific frequencies. The 22 GHz water peak and 60 GHz oxygen peak are nearly opaque. Mobile networks place 5G near 28 and 39 GHz precisely to avoid those peaks; military satellites use 60 GHz precisely to hide in them.
Rain attenuation
At X-band, heavy rain can cut range by half. At Ka-band it can cut range to a few kilometres. This is why automotive radars are dual-mode and why weather radars deliberately operate at lower frequency.