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March 20, 2010

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The mystery of spring's arrival

WITH the days bound to get warmer sometime soon, the weather bureau will be telling us at some point that spring has arrived.

Not only that, it will tell us it arrived five days earlier.


You might wonder why the bureau was late in giving us the news. Or maybe it forgot to pay attention.

But no, it's just how the system works.

Shanghai Meteorological Bureau, the city's weather authority, declares the start of spring once the average temperature climbs above 10 degrees for five consecutive days. It considers the first of those five days the first day of spring.

Therefore, the beginning of spring is always five days before the announcement that spring has arrived.

In the same way, when the average five-day temperature climbs to 22 degrees, the bureau will declare the start of summer -- on the first day of that five-day run.

And when a five-day average goes below 22 degrees, autumn will be retroactively declared. When a five-day average drops under 10 degrees, the bureau will tell us we've been in winter.

Mother Nature

It's not the way many other nations do it. In the United States, for example, spring starts today (March 20). Spring always begins on the March Equinox whether the sun is shining or it's snowing.

And it's not the way Mother Nature does it. The trees and flowers are on their own schedules for blooming.

No, modern China has its own way -- derived from a 1930s theory by renowned climatologist Zhang Baokun.

In Zhang's theory, once the average temperature of a hou (a Chinese meteorology word for five days) is below 10 degrees Celsius, it is a sign of winter; above 22 is summer; and between 10 and 22 indicates spring and autumn.

The theory focuses on the average temperature of the hou, rather than the average temperature of each day of the consecutive five, said an expert from the China Meteorological Administration surnamed Li.

However, weather conditions and standards vary in different provinces and there is no national standard for defining the seasons. Different provinces have different understandings of Zhang's theory.

In the Shanghai bureau's system, the average temperature of any one of the consecutive five days needs to be above 10 before spring is declared.

"The declaration of seasons in this system can truly reflect people's real feel of the weather," said Shen Yu, senior engineer of the bureau.

"Usually, people can have a direct feel of the climate change between two seasons.

"The system reflects not only stable, but also sensible change of the climates."

But the Shanghai bureau, like many other weather bureaus across the globe, uses a different system when it tracks seasons' average temperatures for comparison purposes.

It groups seasons by months: March, April and May are spring; June, July and August are summer; September, October and November are autumn; and December, January and February are winter.

A few weeks ago, the bureau declared this winter to be a warm one; the data came from December to the end of February.

"In this way, we can compare the winter of every year more scientifically," said Chen Min, one of the chief service officers of the bureau.

Shen, for one, is not quite satisfied with this way of dividing seasons.

"It's somewhat absolutistic and doesn't count the real feel of human beings," Shen said.

Seasons divided

In the northern hemisphere, seasons can also be divided according to the change of the distance from the sun to the earth. In this way, spring starts on the spring equinox, around March 21, summer starts on the summer solstice about June 21, autumn starts on the autumnal equinox about September 23 and winter starts on the winter solstice about December 22.

"The astronomical theory was based on the earth's orbit around the sun, as the latter was the source of heat," Shen said. "The system was agreed by most countries in the northern hemisphere including China."

There are still a great many people in China, including many farmers, who follow the change of seasons by indications in the Chinese lunar calendar.

The calendar divides the year into 24 solar terms. One of these is Lichun, around February 4 -- the start of spring.

These are followed by other dates that mark the changes from winter:

Yushui, rains, around February 19;

Jingzhe, the awakening of insects, around March 5 (March 6 this year);

Chunfen, the spring equinox or the divide of spring, around March 20 (March 21 this year);

Qingming, clear and bright, fixed every April 5; and

Guyu, grain rains, around April 20.

Shen believes the lunar calender "is not that accurate."

"The lunar calender reflects the moon's orbit around the earth -- not the sun's," Shen said. "The lunar calender could be a reference but not a scientific standard."

However, more and more Chinese farmers have started to farm by following the weather forecast on television rather than the traditional lunar calender.

"Farmers in my village watch the TV weather forecast every day," said a farmer surnamed Gao who farms in the city's Chongming County. "It's more accurate."


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