We know that the Moon plays a significant role in our lives on Earth, from lighting up the night sky to setting the times of the ocean's tides. But a new study suggests that our favourite cratered satellite also influences something else on our planet: the chances of rainfall.
Scientists from the University of Washington looked at 15 years' worth of data supplied by NASA and the Tropical Rainfall Measuring Mission satellite owned by Japan's space agency. They found that when the Moon is high in the sky, it creates 'bulges' in Earth's atmosphere that cause a slight change in precipitation levels. The higher air pressure created by each oscillation leads to an increase in temperature, and because warmer air can hold more moisture, that means less chance of rain.
"As far as I know, this is the first study to convincingly connect the tidal force of the Moon with rainfall," said one of the researchers, Tsubasa Kohyama. "When the Moon is overhead or underfoot, the air pressure is higher... it's like the container becomes larger at higher pressure."
However, "No one should carry an umbrella just because the Moon is rising," he adds. The variations in rainfall levels are so slight as to be almost imperceptible to most of us.
Where the researchers' findings are going to be practically useful is in climate change modelling and long-term weather forecasts, where these differences can add up. In the future, the researchers also want to investigate whether heavy downpours and other extreme weather types are influenced by the lunar cycle.
The new report builds on earlier research carried out by Kohyama and his colleague John Wallace, published in 2014. This paper looked at the way in which the phases of the Moon influenced air pressure down on Earth - a phenomenon scientists have hypothesised about since the mid-19th century. The latest study, published in Geophysical Research Letters, links those same variations with rainfall.
The change caused by the Moon is about 1 percent of the total variation in rainfall, according to the researchers, so you probably won't see weather forecasters adjusting their maps with lunar data anytime soon. Statistically speaking, the variation caused by lunar activity works out as 0.78 micrometres (1 micrometre is 0.001 millimetres) per hour. The findings are more likely to be used in computer models of how weather patterns might evolve in the future.