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The Weather of March 2021

7 April 2021

With relatively less cold air outbreaks from the north, March 2021 continued to be exceptionally warm in Hong Kong. The monthly mean maximum temperature of 24.8 degrees, monthly mean temperature of 22.0 degrees and monthly mean minimum temperature of 20.2 degrees were 2.9 degrees, 2.5 degrees and 2.6 degrees above their corresponding normals (or 3.4 degrees, 2.9 degrees and 3.0 degrees above their corresponding 1981-2010 normals) and all of them were the highest of the correspondingly monthly mean values of March on record. The month was also much drier than usual with a total rainfall of only 3.5 millimetres, about 5 percent of the normal figure of 75.3 millimetres (or 4 percent of the 1981-2010 normal of 82.2 millimetres) and the fourth lowest on record for March. The accumulated rainfall recorded in the first three months of the year was 65.6 millimetres, a deficit of 55 percent compared to the normal of 147.4 millimetres (or 59 percent below the 1981-2010 normal of 161.3 millimetres) for the same period.

Under the influence of an easterly airstream, the weather of Hong Kong was mainly cloudy with sunny periods on the first day of March. A cold front moved across the coast of Guangdong on the morning of 2 March. Locally, apart from a few light rain patches in the morning and at night, it was dry with sunny periods during the day on that day. The associated northeast monsoon and a band of clouds brought a few rain patches and slightly cooler weather to Hong Kong on 3 – 4 March.

With the moderation of the northeast monsoon and setting in of a humid maritime airstream, the weather of Hong Kong remained cloudy with a few rain patches and coastal fog on 5 – 6 March. The visibility at Waglan Island once fell below 200 metres on 6 March. Meanwhile, a trough of low pressure over southern China moved southwards gradually. There were a few showers and isolated thunderstorms on 6 March. A few millimetres of rainfall were generally recorded over the territory on that afternoon, and rainfall even exceeded 20 millimetres over the northern part of the New Territories.

A replenishment of the northeast monsoon reached the coast of Guangdong and brought cooler weather to the region on 7 March. Locally, it was cloudy with a few rain patches and relatively low visibility in some areas. The weather was a mixture of sunny intervals and a few rain patches the next day. With the cloud bands thinning out gradually, the weather of Hong Kong turned generally fine on the afternoon of 9 March. Affected by a fresh to strong easterly airstream, the weather became cloudier with a few light rain patches on 10 March. With the moderation of the easterly airstream and thinning out of the clouds, generally fine weather returned on the afternoon of 11 March and remained so on the next day.

Local winds strengthened from the east again and the weather became cloudier on 13 - 14 March. With the setting in of a relatively dry easterly airstream, sunny and warm weather returned to Hong Kong in the next few days. Affected by a relatively humid easterly airstream, the weather turned mainly cloudy with a few rain patches on 18 March. With the clouds thinning out, the weather of Hong Kong became fine on 19 - 20 March. With plenty of sunshine and under light wind conditions, the temperature at the Observatory soared to a maximum of 29.7 degrees on the afternoon of 20 March, the highest of the month. It was also the hottest Vernal Equinox on record.

A cold front moved across the coast of Guangdong on the morning of 21 March and brought cooler weather to the region. Under the influence of the associated northeast monsoon, local weather turned mainly cloudy that day and on the next few days. It was dry and appreciably cooler on 22 March with temperature at the Observatory falling to a minimum of 15.8 degrees in the morning, the lowest of the month. With the clouds dissipating and the setting in of a ridge of high pressure, local weather became generally fine on 25 – 26 March. Affected by a warm southerly airstream and the anticyclone aloft over the northern part of the South China Sea, the weather of Hong Kong was generally fine with hot afternoons for the rest of the month. The daily mean temperature of 26.6 degrees on 30 March was the highest on record for March. Moreover, the daily minimum temperature of 25.3 degrees on 30 and 31 March were both the highest on record for March.

There was no tropical cyclone over the South China Sea and the western North Pacific in March 2021.

Details of issuance and cancellation of various warnings/signals in the month are summarized in Tables 1.1 to 1.3.  Monthly meteorological figures and departures from normal for March are tabulated in Table 2. 
 
Warnings and Signals issued in March 2021
Table 1.1   Strong Monsoon Signal
Beginning Time Ending Time
Day/Month HKT Day/Month HKT
3 / 3 0545 3 / 3 1615


Table 1.2   Thunderstorm Warning
Beginning Time Ending Time
Day/Month HKT Day/Month HKT
6 / 3 0815 6 / 3 0900
6 / 3 1205 6 / 3 1300
6 / 3 1710 6 / 3 1900


Table 1.3   Fire Danger Warnings
Colour Beginning Time Ending Time
Day/Month HKT Day/Month HKT
Yellow 14 / 3 0600 14 / 3 1800
Yellow 21 / 3 1000 21 / 3 1800
Yellow 28 / 3 1000 28 / 3 1800


Table 2   Figures and Departures from Normal - March 2021
Meteorological Element Figure of the Month Departure from Normal(1991-2020) Departure from Normal (1981-2010)
Mean Daily Maximum Air Temperature 24.8 degrees C 2.9 degrees above normal 3.4 degrees above normal
Mean Air Temperature 22.0 degrees C 2.5 degrees above normal 2.9 degrees above normal
Mean Daily Minimum Air Temperature 20.2 degrees C 2.6 degrees above normal 3.0 degrees above normal
Mean Dew Point Temperature 18.0 degrees C 1.9 degrees above normal 2.3 degrees above normal
Mean Relative Humidity 79 % 3 % below normal 3 % below normal
Mean Cloud Amount 69 % 8 % below normal 10 % below normal
Total Rainfall 3.5 mm 71.8 mm below normal 78.7 mm below normal
Number of hours of Reduced VisibilityΔ 25 hours 75.0 hours below normal§ 75.0 hours below normal§
Total Bright Sunshine Duration 153.4 hours 53.4 hours above normal 62.6 hours above normal
Mean Daily Global Solar Radiation 13.60 Megajoule / square metre 2.89 Megajoule above normal 3.64 Megajoule above normal
Total Evaporation 87.3 mm 14.1 mm above normal 16.8 mm above normal


  Remarks : All measurements were made at the Hong Kong Observatory except sunshine, solar radiation and evaporation which were recorded at King's Park Meteorological Station and visibility which was observed at the Hong Kong International Airport.

  Δ

The visibility readings at the Hong Kong International Airport are based on hourly observations by professional meteorological observers in 2004 and before, and average readings over the 10-minute period before the clock hour of the visibility meter near the middle of the south runway from 2005 onwards. The change of the data source in 2005 is an improvement of the visibility assessment using instrumented observations following the international trend.
Before 10 October 2007, the number of hours of reduced visibility at the Hong Kong International Airport in 2005 and thereafter displayed in this web page was based on hourly visibility observations by professional meteorological observers. Since 10 October 2007, the data have been revised using the average visibility readings over the 10-minute period before the clock hour, as recorded by the visibility meter near the middle of the south runway.


  §   Departure from mean value between 1997 and 2020

daily values of selected meteorological elements for HK for March 2021

The percentile map of mean temperature of March 2021

  Remarks : Extremely high: above 95th percentile
Above normal: between 75th and 95th percentile
Normal: between 25th and 75th percentile
Below normal: between 5th and 25th percentile
Extremely low: below 5th percentile
Percentile and 5-day running average values are
computed based on the data from 1991 to 2020


Extract of Meteorological Observations in Hong Kong for March 2021