As shown in the image opposite, with low UV, high Relative Humidity and low temperatures airborne virus particles are more abundant (greater viral load), more people catch the virus and breathe out more virus particles, getting trapped in the lower atmosphere. Airborne viruses often attach themselves to small water molecules in the atmosphere and are breathed in. Less sunny, higher moisture and lower temperatures weather conditions prevent the virus particles reaching higher levels in the atmosphere and are not irradiated, they build up in the lower atmosphere, increasing viral load. The worst possible conditions for virus spread is sunless, foggy (high moisture content), windless (air cannot move viral load laterally), and low temperature (air cannot move viral load vertically).
Mountainous areas covered in low cloud and windless conditions, has the same effect. The Italian ski resort in the first outbreak was an ideal breeding ground, and hence extremely high infection rate. Another meteorological consideration of wind, downwind of large conurbations, with cloudy (little UV), high moisture, and low temperatures there is a risk of moving higher viral load. For example, with a westerly wind over London there would be a higher risk of infection over East London, Kent, and Essex.
The opposite occurs when UV levels are high, lowering the moisture levels (RH) and increasing temperature (raising the air and hence viral load, by convection), the airborne viruses are then irradiated and killed, hence less virus particles (virions). Virus particles in these conditions, due to drier air and increased thermal conditions can reach high into the atmosphere, up to a point where UVB and occasionally UVC can irradiate them. This is the crucial time for gaining herd immunity, less virions, less risk of infection, and because of the small number of virions the ensuing respiratory disease will be less potent.