This is related to the assumptions that warmer air temperatures and higher humidity environments will slow the transmission of the virus. While this is the case for influenza, some studies show that COVID-19 could follow the same patterns, however more research is necessary to prove such claims. As of now, claims related to weather conditions and the transmission rates of COVID-19 relation remain non-conclusive. Regarding "washing off" the virus from surfaces by water/rain, the only effective methods to disinfect and clean the surfaces as recommended by the CDC are: alcohol (70%), diluted household bleach solutions, and EPA-registered household disinfectants. Be sure to check the labels of each product before use for proper guidance.
More information can be found in the sources below:
Drafted 20 March 2020