A: The nine umbrellas in the 9-day weather forecast are the icons representing the forecasts for the probability of significant rain (PSR), a concise design to provide users with reference on the upcoming rainfall trend. The icons are colour-filled according to the forecast probability on each day and there are five possible states corresponding to the five categories of probability annotated to the right of the icon in text format as follows: (For details, please refer to Note 3 of 9-day weather forecast)

A:In addition to the colour-filled umbrella icons mentioned above, the forecast categories of the PSR are also presented as words on the right of the icons, namely 'Low', 'Medium Low', 'Medium', 'Medium High' and 'High'. Text version of the PSR is easier to disseminate via text-only version of HKO website or weather bulletins that do not allow attachment of graphics. People with colour deficiency may face difficulty understanding the forecast that is presented as icon only.

A: This is to distinguish the probability of 'any rain' from that of 'significant rain', the latter being defined as 10 millimetres of rainfall or more accumulated generally over Hong Kong in a day. As the set threshold for 'significant rain', the number '10' is the fixed portion in the umbrella icon design that will not change with the forecast.

A: Heavy rain of various intensities often affect Hong Kong in summer. Providing the chance of rain or no rain regardless of the amount of rainfall may not be sufficient to reflect the weather impact most concerned by members of the public. After studying the characteristics of local rainfall records, the Hong Kong Observatory takes the average daily rainfall amount during Hong Kong's rain season (April to September), i.e. 10 millimetres, as the threshold to define 'significant rain'.

A: Hong Kong is located in the subtropics with complex terrain. Rainfall development is often rapid and volatile, changing vastly in tens of minutes or even shorter time scale. Besides, Hong Kong is small in spatial extent. Rain areas, especially heavy rain, are also relatively small in coverage. Their actual swath and impact to Hong Kong can therefore be highly uncertain. It is still a big challenge to provide quantitative rainfall forecast with precision in time and location even based on state-of-the-art science and technology. Hence accurately predicting how many millimetres of rain will fall over Hong Kong out to nine days ahead is technically a daunting task. If we issue quantitative precipitation forecast for Hong Kong too early but the anticipated rainband suddenly weakens or moves elsewhere, false alarms or confusing information may result. Over time the adverse effect of 'crying wolf' may surface, reducing public awareness on precipitation forecast in the long run. To conclude, presenting rainfall forecast in terms of probability is currently a more preferred approach that can highlight the inherent uncertainties while outlining the probable rainfall trend at the same time.

A: To make the most out of PSR, members of the public could make judgments and decisions according to their purpose of various activities and the risks that they can bear.

Example (1). Choosing to go Staycation on either Saturday or Sunday: Even if you may enjoy staying outdoors, raining shall not pose a safety threat in general if shelter is available readily. Cancelling the planned activites may not be needed even if the concerned PSR are relatively high provided that wet programme is in place. If rain is forecast for both days, it will naturally be smarter to choose the day with lower PSR.

Example (2). Planning to go camping or hiking during long vacation: It will be hard to find shelter quickly under rainy or unsettled weather while at the countryside. The safety risk associated with outdoor activities will be generally higher under such circumstance. If the PSR is 'Medium' or above, please prudently consider postponing or cancelling the activity according to your own conditions and the risk that you could bear.

Please note: Even if the PSR is not high, having more intense rain over isolated areas cannot be completely ruled out. Stay alert and pay attention to actual weather conditions when engaging in outdoor activities with relatively high risk. Stay tuned to the Observatory's latest weather forecasts and warnings for the changes in weather.