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Raining cats and dogs

   Written by: LEE Tsz-cheung and CHAN Ho-sun       June 2011

    "Raining cats and dogs" is an English idiom describing very heavy rain.  While the definitive origin of this phrase is unknown, there are several common versions about the source of this phrase widely circulated over the internet.  One more probable source is that, in the 17th century England, the drainage system was poor and the heavy rain would occasionally cause flooding, carrying with it drowned cats and dogs along with other debris.  The scene of dead cats and dogs floating on the flooded street after rainstorms is vividly depicted by this interesting phrase.

    While there is no record of "raining animals" in Hong Kong so far, showers and thunderstorms are very common in Hong Kong during the rainy season. Occasionally, there are heavy downpours which cause flooding and landslides.  On average, there are about 6.2 days with hourly rainfall over or equal to 30 mm at the Hong Kong Observatory Headquarters each year.  Most of them occur between April and September (see Figure 1).  The highest hourly rainfall at the Observatory Headquarters (up to May 2011) is 145.5 mm on 7 June 2008.  

    Figure 2 presents the records of the highest hourly rainfall at the Observatory Headquarters from 1885 to 2010.  It clearly shows that, the hourly rainfall record was broken three times since 1966 with the latest record in 2008 breaking the previous one by a wide margin of 30 mm.  Moreover, a study conducted by the Observatory reveals that the frequency of extreme heavy rain events has an increasing trend over the past 120 years or so.  In 1900, an hourly rainfall of 100 millimetres or above would occur on average once every 37 years (i.e. a return period of 37 years). By 2000, the return period had decreased to 19 years, meaning that such heavy rain has become twice as frequent.

    Such an increase in the extreme rainfall events in Hong Kong could be attributed to various factors including global climate change and local urbanization.  Climate change will likely affect the frequency of occurrence of extreme rainfall events in the long term.  It is because the tropospheric warming due to increased anthropogenic (human induced) greenhouse gases can lead to an increase in the water-holding capacity of the atmosphere.  The warming may also enhance the hydrological cycle and atmospheric instability.  A less stable atmosphere with more water vapour in the air will provide a more favourable condition for intense precipitation events.  For local urbanization, some studies suggest that heavier rain in urban areas may be partly attributed to the urban heat island effect that enhances the convective activities, the increased roughness over a city that slows down the storm movement and the increase in the concentration of suspended particulates from urban activities that helps the formation and development of rain-bearing clouds.  

    Figure 1   Average number of heavy rain days with hourly rainfall ≥ 30 mm in each month (1971-2000)
    Figure 2   Record hourly rainfall record at the Hong Kong Observatory Headquarters, 1884-1939 and 1947-2010.

      References :

    1. Wikipedia
    2. Observatory Blog "What is a normal year?"
    3. Min, S.K., X. Zhang, F.W. Zwiers and G.C. Hegerl, Human contribution to more-intense precipitation extremes, Nature 470, 378-381, 2011
    4. Allan, R. P. and B. J. Soden, Atmospheric warming and the amplification of precipitation extremes, Science 321, 1481-1484, 2008
    5. IPCC AR 4, WG1, Chapter 3, Section : Surface and Lower-Tropospheric Water Vapour, page 272-273, 2007
    6. Shepherd, J. M., H. Pierce, A. J. Negri, Rainfall modification by major urban areas: observations from spaceborne rain radar on the TRMM satellite, J. Appl. Meteor. 41, 689-701, 2002
    7. Cao, K., Z. Ge, M. Xue and Y. Song, Analysis of Urban Rain Island Effect in Shanghai and Its Changing Trend, Water Resources and Power 27 (5), page 31-33, 54, 2009
    8. Wong, M.C., H.Y. Mok and T.C. Lee, Observed Changes in Extreme Weather Indices in Hong Kong, Int. J. Climatol., October 2010, DOI: 10.1002/joc.2238, HKO Reprint No. 941

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Last revision date: <21 Dec 2012>