The Weather of April 2012

     With the prevalence of the warm maritime airstream for most of the time except the early part of the month, April 2012 was warmer than usual.  The monthly mean temperature of 23.9 degrees was 1.3 degrees above the normal figure of 22.6 degrees. The daily mean temperature of 28.5 degrees at the Observatory on 30 April  tied with that on 26 April 1994 as the highest in April since record began.

     Affected by frequent passages of troughs of low pressure across the South China coast, there were also several heavy rain episodes in the month.  The total rainfall in the month was 294.9 millimetres, about 69 percent above normal, more than compensating for the rainfall deficit of the first three months.  The accumulated rainfall since 1 January was 388.6 millimetres, about 16 percent above the normal figure of 336.0 millimetres for the same period.  There was less sunshine, the total bright sunshine hours being 88.1 hours for the month, 13 percent below normal.

     Under the influence of the northeast monsoon, the weather in Hong Kong was mainly fine and relatively dry for the first four days in the month.  Affected by a trough of low pressure over the coastal areas of Guangdong, the weather turned cloudy with rain and squally thunderstorms on 5 April.  With a fresh to strong easterly airstream prevailing over the coast of southeastern China, it was slightly cooler with some rain for the ensuing three days.  

     Affected by a broad and rain-bearing cloud band, it was cloudy with rain and a few thunderstorms in the morning on 9 April.  The rain bands gradually moved away from Hong Kong, there were sunny intervals in the afternoon.  With the prevalence of a warm and humid maritime airstream, the weather became mainly fine and warm on the next three days with coastal fog in the morning.  A trough of low pressure brought a few showers and isolated thunderstorms to the territory on 13 April.  Under the influence of a warm and humid maritime airstream, it was warm with a few showers and fog patches on 14 and 15 April.  

     A trough of low pressure crossed the South China coast and brought some rain and squally thunderstorms to Hong Kong on 16 April.  Affected by a fresh to strong easterly airstream, it was windy and slightly cooler with rain on 17 April.  Meanwhile, another trough of low pressure formed over inland Guangdong on 18 April and brought some showers to Hong Kong.  With the trough of low pressure moving southwards in the ensuing two days, local weather deteriorated with heavy showers and squally thunderstorms.  More than 50 millimetres of rainfall were generally recorded over the territory on 20 April.  The trough of low pressure moved into the northern part of the South China Sea on 21 April and the cloud band covering southern China thinned out gradually.  Local weather improved with sunny intervals on that day. 

     With the establishment of a ridge of high pressure over southern China, it was sunny on 22 April.  Affected by an active southerly airstream, the weather was mainly cloudy with a few showers for the next two days.  Meanwhile, a trough of low pressure formed over southern China and edged across the Pearl River Estuary on 25 April, bringing locally heavy showers and squally thunderstorms to the territory that night.  While the trough moved into the northern part of the South China Sea, local weather improved gradually and there were sunny periods on 26 April.  

     Affected by a trough of low pressure lingering over the coast, the weather became unsettled with occasional heavy rain and squally thunderstorms for the following three days.  The rain was particularly heavy, necessitating the issuance of the first Red Rainstorm Warning this year on the morning of 29 April.  More than 70 millimetres of rainfall were recorded over parts of the New Territories while over 150 millimetres were recorded over Tuen Mun and Tsuen Wan.  With the rain easing off, there were sunny intervals in the afternoon and the temperature at the Observatory rose to 30.2 degrees, the highest of the month.  Under the influence of a warm southerly airstream, it remained very warm with sunny periods and a few showers on the last day of the month.  The daily mean temperature of 28.5 degrees at the Observatory on that day tied with that on 26 April 1994 as the highest record in April.

    One tropical cyclone occurred over the South China Sea and the western North Pacific in the month.

    Details of issuance and cancellation of various warnings/signals in the month are summarized in Tables 1.1 to 1.5.  Monthly meteorological figures and departures from normal for April are tabulated in Table 2.