The level of contamination is determined by factors not directly related to the environment. A third component, oxygen, is also necessary but is rarely the limiting factor except where water-logging occurs. It is worth remembering that conditions at ground level are usually more humid than atmospheric conditions. The cooler it is, the longer it takes. The level of moisture required is more difficult to measure because it is linked to temperature and rainfall.
Rainfall contributes to, and evaporation removes, moisture.
Take the example of H. contortus in a hot climate, such as Queensland, where the rate of evaporation is high. Here 100 mm of precipitation per month may be required to provide sufficient moisture. So high temperatures inhibit development because evaporation is high. Conditions in spring and autumn are best for development. A wet summer is favourable but a dry summer too hot and dry.