XN000022-1983-04-01 — Page 3

FRIDAY, APRIL 1, 1983

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WETTEST MONTHS CAUSED BY FREQUENT

BUT MINOR SIBERIAN COLD SURGES

***

HONG KONG'S PERSISTENTLY RAINY WEATHER IN THE PAST FEW MONTHS WAS GENERALLY CAUSED BY THE MORE MODERATE COLD AIR SURGES COMING FROM SIBERIA, TOGETHER WITH THE PERSISTENCE OF THE UPPER AIR SOUTHWESTERLY FLOW OVER SOUTH CHINA, IT WAS EXPLAINED BY A SPOKESMAN FOR THE ROYAL OBSERVATORY TODAY.

IN THE WINTER, HE EXPLAINED, A HIGH PRESSURE AREA OVER SIBERIA CONTINUOUSLY SENDS COOLER AIR SOUTHWARDS, CAUSING A DROP OF TEMPERATURE IN THE AREA.

WHEN AN INTENSE COOL AIR MASS FROM THE NORTH ENCOUNTERS THE WARM AND MOIST AIR FROM THE SOUTH, THERE IS A SUDDEN BUT SHORT DURATION OF DOWNPOUR. AFTERWARDS THE WEATHER BECOMES FINE AND DRY DURING THE COLD SPELL, HE SAID.

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HOWEVER IN THE PAST FEW MONTHS THE COLD SURGES FROM THE NORTH WERE FREQUENT BUT MINOR, AND THE UPPER AIR DISTURBANCE WERE PERSISTENT AND SLOW MOVING, HE ADDED.

** HE ADDED. AS A RESULT OF THESE UNCOMMON WEATHER FEATURES, HONG KONG EXPERIENCED TWO RECORD-BREAKING MONTHS OF RAINFALL IN FEBRUARY AND MARCH.

LAST MARCH'S RAINFALL, TOTALLING OVER 420 MM, WAS THE HEAVIEST KNOWN SINCE RECORDS WERE KEPT IN 1884, MORE THAN 100 MM ABOVE THE PREVIOUS RECORD MARCH MONTH OF 291.2 MM IN 1905. THE FEBRUARY RAINFALL FIGURE OF 241 MM WAS ALSO THE WETTEST, AND JANUARY RAINFALL OF 76.3 MM WAS THREE TIMES THE AVERAGE.

THE SPOKESMAN POINTED OUT THAT AVERAGE MONTHLY RAINFALL FOR THESE THREE MONTHS DO NOT NORMALLY EXCEED 100 MM.

''AND IT IS ALSO UNCOMMON TO HAVE THE WETTEST TWO MONTHS CONSECUTIVELY," HE SAID.

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AFTER MARCH, HOWEVER, FEWER COLD SURGES WILL BE COMING DOWN SOUTH FROM SIBERIA AND THE CONTINUOUS RAINS ASSOCIATED WITH THE CONFRONTATION OF COOL AND WARM AIR MASSES WILL BECOME LESS FREQUENT, THE SPOKESMAN POINTED OUT.

HOWEVER, UNSTABLE WEATHER ASSOCIATED WITH SURFACE TROUGHS OF LOW PRESSURE NEAR THE SOUTH CHINA COAST AND IN THE NORTHERN PART OF THE SOUTH CHINA SEA IS COMMON IN MAY AND JUNE.

''THE TROUGHS MAY MOVE NORTH OR SOUTH OF THE COAST BRINGING HEAVY RAINS,'' HE SAID.

ON THE QUESTION OF FORECASTING, HE POINTED OUT THAT COLD SURGES WERE EASY TO PREDICT, BUT FORECASTING THEIR STRENGTH AND THEIR EXACT TIME OF ARRIVAL WAS DIFFICULT. THIS WAS BECAUSE OF THE GREAT DISTANCE BETWEEN THE HIGH PRESSURE AREA IN SIBERIA AND HONG KONG AND THE EXISTENCE OF A MOUNTANT BARRIER IN THE NORTH OF GUANGDONG PROVINCE.

THE COLD SURGES MAY WEAKEN OR LINGER OVER GUANGDONG PROVINCE BEFORE ARRIVING IN HONG KONG, OR MAY EVEN BE BLOCKED BY THE MOUNTAIN BARRIER, HE EXPLAINED.

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