The GPS signals will not be affected by clouds, rain, fog,
snow, etc., because of the combination of their passing
through water vapour, and the signal wavelength meaning
they "pass through" rain etc.
On the other hand, water will block/attenuate GPS signals.
For example, you can’t receive GPS signals underwater, although
having a GPS receiver antenna very close to the water surface
may allow some reception.
Most cloth that would be covering a GPS receiver’s antenna
will pass GPS signals(e.g. backpack, jacket, etc.), but cloth
with a metallic component will block/attenuate the signals.
Cloth that covers a GPS receiver’s antenna and gets wet
(e.g. from rain) can also block/attenuate the GPS signals.
In fact, you don’t even need the cloth – water on the case
that covers the GPS receiver antenna can block/attenuate
the GPS signals. That water could be drops, and/or a film
of water. I’ve seen that happen myself, when using my etrex
Venture in the rain – the signals were weak/blocked, but
a quick drying-off of the GPS case over the antenna brought
the reception back, which then degraded again as water
accumulated on the case. Similarly, GPS signal reception
is degraded in wet forest canopy conditions compared to
when the forest canopy is dry.