TNAG-1174-FCO40-1476-Proposed-replacement-airport-for-Hong-Kong-at-Deep-Bay-or-Ch-1982 — Page 108

CONFIDENTIAL

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(b) Visibility and cloud base

Visual observations of visibility and cloud base were made by Royal

Observatory observers at Ping Shan from 0800 to 1700 daily during the 12-month

period April 1949 March 1950.

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According to Ref. 1, there were 16 hours with visibility less than

1000 yards*. Ref. 3 shows that there were 23 hours and 8 hours with visibility

less than 1000 m at Chek Lap Kok and Hong Kong International Airport respectively

between 0700 and 1800 over a two-year period. There is probably not much

difference between Deep Bay and Chek Lap Kok in terms of visibility since both

locations are sheltered by hills from sea fog in southeasterly airstreams during

spring. The characteristics. of sea water in the two basins are also similar.

Considering low cloud covering 4 oktas or more, the daylight observations

at Ping Shan showed that the cloud base was lower than 1500 feet in about 12% of

This appears to be a very high frequency of occurrence since,

the occasions.

The

according to Ref. 3, the percentages. of daylight observations (0700 - 1800) with

cloud amount. equal to or exceeding 5 oktas and with cloud base lower than 1500 feet

are 0.35% and 0.47% at Chek Lap Kok and the existing airport respectively.

large difference between Ping Shan and the other locations is partly due to the

different cloud amounts chosen in the two studies (4 oktas vs 5 oktas). However,

the differences between data from Ping Shan and those from the other locations

are so large that substantial spatial variation in cloud conditions in the

northwestern New Territories near Deep Bay is suspected.

*Yard and mile were the units used to measure visibility at that time.

CONFIDENTIAL