An increase in cloud amount can cause a decrease in diurnal temperature range by reducing the solar incoming radiation during day time and trapping of long-wave radiation at night. The diurnal temperature range has decreased worldwide since the 1950s and the coincidental increases in total cloud cover are often cited as a likely cause for the observed decrease in diurnal temperature range.

In Hong Kong, cloud amount is reported in oktas hourly by trained observers at the Hong Kong Observatory Headquarters. The daily mean cloud amount is taken as the average value of the observed hourly cloud amount in a day. Based on these daily mean cloud amounts, annual mean values were calculated. Regression result indicates that the annual mean cloud amount has been increasing at a rate of 1.1% per decade in the period 1961 to 2011. One potential cause for the increase in cloud amount over Hong Kong could be the likely increase in the concentration of condensation nuclei in the air that favoured the formation of clouds, which is known to be associated with urbanization and human activities in the region.

Annual mean cloud amount recorded at the Hong Kong Observatory Headquarters (1961-2011)