Indian summers around the world

Q: During September in Cyprus (an island in the eastern Mediterranean about 40 miles [64 km] south of Turkey), the temperature suddenly rises at least 3° to 4°C (5° to 7°F) for a week or two. Does the Earth getting closer to the Sun cause this "Indian summer?" Are there any strange air movements north and south? Viken, Nicosia, Cyprus

Indian summer in Maine [Corel]

A: No. The Earth getting close to the Sun doesn’t cause Indian summers. Instead, stalled high-pressure areas fuel the formation. But, yes. Winds do move from south to north in the Northern Hemisphere (and north to south in the Southern). The same phenomenon creates Indian summers in the US and elsewhere.

Take New England, for example. During the fall, high-pressure areas sweep out of the Northwest, move across the country, and sometimes stall, off the Atlantic Coast. This creates a high pressure over New England.

Higher pressure pushes air towards lower pressure, causing winds. Because of the Earth’s spin about her axis, the winds rotate counterclockwise about high-pressure zones in the Northern Hemisphere and, hence, about the New England high. The gyrating winds curve south, and fetch warm air north to New England. This creates an Indian summer there that lasts until the next strong low pressure and its cold front juts across New England. Sometimes many Indian summers occur in the fall and sometimes none at all.

"My take on all this is that Indian summers are part of the regular parade of weather systems that occur throughout the year," says David Schultz, meteorologist at the National Severe Storms Laboratory in Norman, Oklahoma. "If you get unusually warm weather late in the fall, people want to give it a name, but it’s really just part of the normal climatology."

Wondering about other parts of the world, I asked Bob McDavitt, a New Zealand meteorologist, if Indian summers occur in the Southern Hemisphere. "Yes," he said. High-pressure bands (called subtropical ridges) circle the globe normally near 30° N and 30° S. In the fall, however, they follow the Sun to somewhat higher latitudes — 35° to 45° — and cause the Indian summer phenomenon. So, it’s a warm delight that occurs worldwide. "We in the Southern Hemisphere get it in April or May, says McDavitt"

By the way, the name "Indian summer" has been around at least since 1778. According to the Glossary of Meteorology, the term probably refers to Indians using the warm days to increase winter stores.

Further Reading:

USA Today: High pressure system, graphic

National Weather Service, NOAA: Just what is Indian summer and did Indians really have anything to do with it by William R. Deedler, 1996

American Meteorology Society: The Glossary of Meteorology, 2000.

The Weather Doctor: The halcyon days of Indian summer by Keith Heidorn

(Answered Sep. 17, 2004)

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