A Harsh Mistress

For millennia, people have insisted that heavenly bodies have strange effects on life here on Earth. Astrologers claim your personality is defined by the position of the stars in the sky at birth. Children believe that stars grant wishes and full moons produce werewolves. While adults scoff at werewolves, many believe that the full moon brings more childbirths than any other time of the month.

None of these beliefs are backed by science. However, researchers at Arizona State University have found that the moon can lay claim to some pretty freaky effects on Earth’s weather. ASU climatologists Randall Cerveny and Robert Balling have found that movements of the moon influence some of our most extreme weather, such as hurricanes and tornadoes.

Imagine that the Earth’s equator expands out into the heavens, like a belt being loosened to fit a wider waist. The moon moves slightly up and down in relation to this celestial equator, and astronomers measure its latitude in the sky in this way. This measurement is known as declination.

The ASU researchers are examining weather records for four weather phenomena—major hurricanes, severe tornadoes, large hail and damaging thunderstorms—from the 1950s to the present. They are finding that all four types of weather are more prevalent when the moon’s declination is south of the celestial equator.

Cerveny says that improved weather records have allowed this research to happen.

“There was a big spurt of research on lunar impact in the 60s and 70s,” he says. “They had some really great ideas in the past, but they didn’t always have the weather records to check it out. Now we have the weather records. I’m kind of like an archaeologist of climate research.”

Severe weather is usually associated with atmospheric instability. The researchers speculate that the moon’s declination might influence this instability. To find out, they are looking at five different indices of atmospheric instability used in weather forecasting. They are finding a strong relationship between lunar declination and atmospheric instability, which most likely causes the increase in severe storms.

The idea that the moon influences weather is hardly surprising, Cerveny says. Everyone knows that the moon affects ocean tides. In addition, earlier studies have linked moon phases to precipitation and global temperatures. In 1995, Cerveny and Balling published a study in the journal Science showing a link between moon phases and daily global temperatures.

“It is minutely warmer under a full moon than a new moon,” says Cerveny. “There is a difference of about .02° C. It’s small but it’s significant.”

He suggests two reasons why the moon might affect temperature. “First, when there is a full moon you are getting that extra bit of sunshine reflected back on Earth—that’s what moonlight is. Also, during the full moon the Earth is slightly closer to the sun than during a new moon.”—Diane Boudreau