Temperature and precipitation don't greatly affect the propagation of TV signals. There is more effect on UHF than on VHF, but it's usually not a problem for either. However, if you are receiving a station that is a long distance away (> 25 miles) weather may have a significant effect.
The change is most likely due to something around your house, like the siding or a shed or maybe a hill or cliff reflecting the signals differently when they are cold and wet. This could create what is called multi-path, where the signal from the station is canceled by the signal that is reflected by some other surface.
Have you tried moving your antenna to a different location or orientation? Depending on the type, rotation may help, or you may need to move it a couple of meters. No guarantees.
The other possibility is that some of your antenna wiring or even your TV may be reacting with the change in temperature and humidity. Some components can be very sensitive to that if they are not assembled properly (meaning at the factory). Check all your antenna connectors and make sure they are tight and not corroded. See if the picture jumps or cuts out when you jiggle each one. Don't mess with the power connections - that can be dangerous, and it won't be what's affecting your reception.
Otherwise, see if you can get another antenna and try it. If it does the same thing, it's either the propagation to your house, or your TV. If it fixes it, be happy!
Be happy anyway.
John - The Eternal Pessimist.