Washington ? Grandma may have been right about keeping a teakettle warming on the stove in winter to moisten the air.

Studies of seasonal influenza have long found indications that flu spreads better in dry air.

Now, new research being published today in the Proceedings of the National Academy of Science, indicates that the key is the absolute humidity — which measures the amount of water present in the air, regardless of temperature — not the more commonly reported relative humidity.

Relative humidity varies depending on air temperature; absolute humidity doesn’t.

“The correlations were surprisingly strong. When absolute humidity is low, influenza virus survival is prolonged and transmission rates go up,” said Jeffrey Shaman, an Oregon State University atmospheric scientist who specializes in ties between climate and disease transmission

The finding “is very important for the scientific community and the medical community to know to develop better prediction models of influenza,” Shaman said in a telephone interview. It will offer the chance to better understand and forecast the spread of the disease.

For the public, he added, it offers a “more elegant explanation for why we see these seasonal spikes” in flu. And, he added, it shows that in some cases it may be worthwhile to add humidity to the air. Beware of overdoing it, though — too much humidity can lead to other problems, such as mold.

The correlation with flu and low humidity is important because in cold winter weather, when flu is most common, even a high relative humidity reading may indicate little actual moisture in the air, and the less moisture there is, the happier the flu virus seems to be.

In their new analysis, Shaman and a fellow re-searcher said using absolute humidity explains 50 percent of influenza transmission and 90 percent of virus survival.