In temperate climates, influenza displays distinct seasonality: infections mainly occur from November through March in the northern hemisphere, and from May to September in the southern hemisphere. The results of experiments in guinea pigs have revealed that aerosol transmission of influenza virus is most efficient in cold and dry conditions, and completely blocked at warm or humid conditions. Therefore it has been suggested that seasonality of influenza in temperate climates is regulated by temperature and humidity.

There have been additional studies in guinea pigs (see my post at http://bit.ly/Miloc for more information) but epidemiological studies in humans need to be done to determine if these conclusions are valid in the 'real world'.

In the original paper transmission of influenza was most effective at humidities of 20 to 35% and blocked at a humidity of 80%. Transmission occurred with greater frequency when guinea pigs were housed at 5 degrees C than at 20 degrees. It has been suggested that humidifying homes during the winter might impair spread of the virus; however once you leave the home, conditions of low humidity and low temperature would be encountered, so it's not clear that these parameters could be effectively manipulated to reduce spread. Hygrometers are available to measure humidity.