Beware: Harvest Moon Rising

September 15, 2008

The harvest moon is coming, and with that, the first frost of the year. What does it mean for ag crops?
  • An early frost could have big impact
  • Soybeans, corn at risk
  • What the weatherman's saying now


It's that time of the year when the days get shorter, the nights longer, and the agriculture markets work themselves up into a tizzy over worries about the first frost of the season. The Harvest Moon is fast approaching, and along with it, perceptions that temperatures will soon plummet to below the freezing mark.

This fall, the timing of the first frost will likely be more critical than usual because wet conditions in the spring delayed planting and most crops are behind their normal development. Those thinking about dipping into soybean or corn markets will need to pay close attention, and one must wonder: Can the brilliance of the moon actually provide us with some clues as to where agriculture markets are heading?


Blame It On The Moon?

The Harvest Moon is the full moon that occurs closest to the Autumnal Equinox and usually takes place in late September or early October. This year, the Harvest Moon will be Sept. 15, just a little before the Autumnal Equinox on Sept. 22. Its name originates from how the unobstructed moon casts sufficient light to allow farmers to finish their harvest chores.

The Harvest Moon is often seen, at least anecdotally, as a potential signal of the first big frost. Part of the reason for its controversy is its unusual way of making an appearance. Throughout the year, the moon rises, on average, about 50 minutes later as each day passes. But during the Harvest Moon period, stretching from several days before the full moon night to several days after, this day-to-day timing of the moon rising is only around 30 minutes.

Heidi Stonehill, senior associate editor with the 200-year-old Old Farmer's Almanac, notes that the first frost often occurs during the Harvest Moon period in many places on the continent, from Alaska to New Mexico.

The first fall frost is usually a radiation frost, produced on clear nights with little or no wind. The clear skies allow heat from the ground to escape into the atmosphere, cooling surface temperatures. The other type of frost - advective - is rarely the first one of the season, and occurs when a cold front sweeps into an area.

Stonehill speculates the long-held fable that a full moon near the Autumnal Equinox would produce frost may originate with outdated beliefs it could clear away clouds to expose clear skies.

Research has been unable to prove a correlation between the full moon and the occurrence of frost.

The chicken-and-egg argument could even be made that the full moon doesn't cause the frost; rather, the clear skies associated with a fall frost simply make it easier to see the moon.


Global Warming Won't Diminish Risk Of Frost Anytime Soon

Even meteorologists often admit that predicting longer-term weather conditions can be a shaky science. One thing that is for certain, despite all the noise over global warming of late, a frost is still guaranteed sooner or later each fall. Farmers are just hoping it will hold off until after September.

Its timing is key, as frost is the main risk that can harm crops at their current late stage of development, by reducing quality and potentially yields. Any damage to crops in the United States would have serious consequences for global markets, given that the nation is the No. 1 exporter of corn and soybeans.

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