Trajectory Example

Under the combined effect of a weak northeast monsoon and Tropical Storm Sinlaku, a dry continental airstream affected southern China. The backward trajectory map showed that the air mass reaching Hong Kong originated from the north. The continental air mass brought relatively dry and hazy conditions with sunny periods to the territory on 17 Sep 2008. Visibilities at the Hong Kong Observatory lingered between 4000 and 8000 metres.

On 26 Aug 2008, a weak area of high pressure prevailed over Hong Kong and the adjacent sea areas. The backward trajectory map showed that the air mass reaching Hong Kong originated from the South China Sea. Locally, it was hot with light to moderate southwesterly winds. Visibilities at the Hong Kong Observatory ranged between 12 and 27 kilometres.

On 26 August 2007, the weather chart showed that pressure gradient was relatively slack over the coastal area of Guangdong and winds over the region were light. From the backward trajectory map, we can see that the air mass was moving slowly near Hong Kong during the period from 24 to 26 August 2007. In Hong Kong, under the influence of light northerly winds, visibilities at the Hong Kong Observatory dropped to about 5000 metres on the morning of 26 August 2007.

Under the influence of winter monsoon, the air masses affecting Hong Kong during winter are mainly originated from the north or northeast. From the backward trajectory map in January 2009 which shows typical paths of air masses affecting Hong Kong in winter, it can be seen that air masses reaching Hong Kong can even come from Siberia, bringing us dry and cold weather to Hong Kong.

Under the influence of southwest monsoon, the air masses affecting Hong Kong during summer are mainly originated from the south or southwest. The backward trajectory map in July 2008 shows typical paths of air masses affecting Hong Kong in summer, indicating that air masses are originated from hot and humid Indochina and South China Sea, bringing us wet and hot weather in summer.