Δp(972) and Δp(S) are pressure indices derived by Hong Kong Observatory to
forecast the arrival time and intensity of winter monsoon surges in Hong Kong between
October and March/April.
Δp(972) is the pressure difference between Chenzhou (WMO station 57972, 25.8N,
113.0E) and Hong Kong. Positive values indicate pressure over Chenzhou is higher than Hong
Kong. According to Lai(1989)1, a Δp(972) value of
7 hPa or above indicates a northerly surge is on the way to Hong Kong:
|Δp(972) (hPa)||Arrival time (hours)|
Look for duplicate signals (Δp(972) stays at 7 hPa) which may suggest a delay of the
surge. If Δp(972) increases by 2 hPa in 6 hours or Δp(972) is expected to reach
10 hPa, the surge may bring strong northerlies to Hong Kong.
Δp(S) is the pressure difference between Shanghai (WMO station 58362, 31.4N, 121.5E)
and Hong Kong. Positive values indicate pressure over Shanghai is higher than Hong Kong.
According to Chang(1989)2, a Δp(S) value of 8 hPa or
above may suggest the arrival of easterly surge to Hong Kong with strong easterlies over offshore
In late winter, the arrival of northerly surge may be affected by other factors, e.g. cold
air blocked by Nanling. Care should be taken when interpreting the indices, especially
when Δp(972) and Δp(S) are of comparable magnitudes.
1Lai, S.T. (1989), Short-range forecasting of northerly
surges, Hong Kong Observatory Technical Note 83.
2Chang, K.M. (1989), Prediction of the strength of
overnight easterly winds in Hong Kong in winter, Hong Kong Observatory Technical Note