Correlation with low humidity and low rates of airflow
Schulman and Kilbourne (1962) performed one of the few controlled experiments of flu transmission, using mice as a model, and concluded (using only four data points) that airborne transmission was positively correlated with low humidity and low rates of airflow.
For more, see: Schulman, J.L. and Kilbourne, E.D. (1962) Airborne transmission of influenza virus infection in mice. Nature 195, 1129–1130.