Download Onset of southwest monsoon end of the fog season and start of the

 Onset of southwest monsoon ­ end of the fog season and start of the rain season 1. Southwest monsoon 2. The end of the fog season 3. The start of the rain season Written by: TONG Hang­wai March 2011 1. Southwest monsoon The basic driving force of the southwest monsoon is the large­scale differential heating between
landmasses and oceans. Starting from April, the Sun gradually moves north and the amount of
sunshine will increase in the northern hemisphere. The Asian continent will be heated up faster
than its neighboring oceans. As air over the land becomes lighter and rises, a low pressure
center will eventually form over the continent. The relatively warm and moist air from the ocean
will then in general flow towards this low pressure centre, thus leading to the onset of the
southwest monsoon. Fig. 1 shows the monthly mean of prevailing wind direction at Waglan
Island between 1971­2000. It clearly indicates that the prevailing wind direction in Hong Kong
veers from easterlies to southwesterlies between May and June. Fig. 1. Prevailing wind direction at Waglan Island (1971­2000). Back to content 2. The end of the fog season The fog season in Hong Kong usually begins in February and ends in April. When warm moist
maritime air moves over the relatively cool sea surface near the coast, air temperature will
gradually fall to the dew point. Water vapour in the air will condense into very small water
droplets which lead to the formation of fog (this is commonly called advection fog). Since the air
is warmer than the sea water underneath, the atmosphere is relatively stable and helps confine
the water vapour to the lowest level of the atmosphere. As the season proceeds, the sea water
will also warm up gradually such that the atmosphere over the ocean will become more unstable.
Hence, the onset of the southwest monsoon often marks the end of the fog season. Climatology
shows that the Observatory rarely observes fog in May, on average less than 0.2 day in that
month (Fig. 2). Fig. 2. Climatology of number of days with fog being observed at the Observatory (1971­2000). Back to content Back to content 3. The start of the rain season The southwest monsoon will bring in warm moist air and the warming up of sea water will fuel the
development of convective activities. Coupling with other favorable meteorological conditions
like low level convergence or upper level divergence, the southwest monsoon could sometimes
lead to heavy rain. Climatology shows that the amount of rainfall increases significantly in May
(Fig. 3). This correlates well with the onset of the southwest monsoon. Fig. 3. Climatology of monthly mean rainfall (mm) recorded at the Observatory (1971­2000) Back to content References: 1. Meteorology Today: An Introduction to Weather, Climate and the Environment. Author: C. D. Ahrens. Brooks/Cole Pub Co. Back to content