Why At microwave frequencies the wavelength shrinks toward the size of raindrops, so water droplets absorb and scatter the signal instead of letting it pass. This 'rain fade' grows worse as frequency climbs, becoming very noticeable above roughly 10 GHz, which is why satellite TV dishes drop out in a heavy downpour. Snow, fog and hail cause similar loss, so any form of precipitation in the path can cut your microwave range.
Watch out Wind and barometric pressure by themselves do not absorb RF; pressure and temperature matter only indirectly, by shaping temperature inversions and ducts that can actually extend VHF/UHF range rather than shorten it.
Microwaves + water drops = rain fade. Think of satellite TV going out in a thunderstorm.