Is the Observatory's rainfall forecast so elusive? One article teaches you how to understand the umbrella diagram and the numbers are the same every day

Why is there an open umbrella every day on the nine-day weather forecast?

(The original text is taken from the Observatory's website, with the title, introduction and text, edited and revised by Hong Kong 01)

The nine umbrellas on the nine-day weather forecast are icons of significant rainfall probability predictions, providing users with a concise and easy-to-understand rainfall trend reference.

The icon will be colored according to the predicted probability. There are five possible states as follows, corresponding to the five probability levels displayed in text on the right side of the icon...

A picture series to see the rainfall probability forecast and the meaning of related patterns (click to enlarge) ▼▼▼

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Q: Why are some Chinese characters inserted to the right of the umbrella icon of significant rainfall probability?

A: The prediction level of the probability of significant rainfall is not only displayed by the colored umbrella icon above, but also expressed in the words "low", "medium-low", "medium", "medium-high" and "high", which are indicated on the right side of the icon .

Text-only web pages or weather manuscripts cannot accompany images, and text versions of predicted probabilities are easier to distribute.

For people who are color-impaired, just looking at the icons may also pose some difficulties.

Q: Why does the umbrella icon for significant rain probability display the word "10" every day?

What exactly does it stand for?

Answer: This is to distinguish the probability of "significant rain" from the probability of "there is rain".

The threshold for "significant rainfall" is set at a daily cumulative rainfall of 10 mm over a wide area of ​​Hong Kong. Therefore, the number "10" is included in the umbrella pattern, which is a fixed part of the icon design and will not change according to the forecast.

Q: Why is 10mm used as the threshold for "significant rainfall"?

A: Hong Kong often experiences heavy rains of varying degrees in summer. If only the probability of normal rain/no rain is provided without considering the amount of rainfall, it may not be enough to reflect the impact of the weather that people are most concerned about.

Taking into account the local rainfall characteristics in the past, the Observatory uses the average daily rainfall in Hong Kong's rainy season (April to September), which is 10 mm, as the threshold for "significant rainfall".

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How to use the Observatory App "Rainfall Forecast" ▼▼▼

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Q: Why provide rain probability forecasts?

Wouldn't it be more practical to just state how many mm of rain?

A: In Hong Kong, which is located in the subtropical region and has a complex terrain, rainfall tends to develop rapidly and with randomness, even in tens of minutes or even less time.

In addition, since Hong Kong is not a large area, and the impact of rainfall, especially heavy rain, is also relatively small, the uncertainty of the impact on Hong Kong is relatively high.

As far as current technology is concerned, it is a big challenge to accurately predict rainfall at a fixed time, at a fixed point, and quantitatively. Therefore, it is technically difficult to accurately predict how many millimeters of rainfall will occur in Hong Kong up to nine days in advance.

If a quantitative rainfall forecast is issued for Hong Kong too early, but the rainband suddenly weakens or moves to other places, it may cause false reports or confusion of information. Over time, the adverse effect of "the wolf is coming" may easily be generated, and the public's vigilance of the rainfall forecast may be reduced in the long run. sex.

All in all, expressing the rainfall forecast by probability can not only express the uncertainty of the forecast, but also give the possible rainfall change trend, which is a preferable method at present.

Q: How can citizens take advantage of significant rainfall probabilities?

A: With the use of significant rainfall probability, citizens can make judgments and decisions based on the purpose of different activities and the risks they can bear.

Example (1) Want to choose a day to go to Staycation on Saturday or Sunday: Although there is a chance to be in the open air, if you can go indoors at any time to avoid the rain, the rain will generally not cause much impact or pose a safety threat .

With sufficient rain arrangements, even if the probability of significant rainfall is not low or even high, it may not necessarily be necessary to cancel the activity.

If rain is forecast on both days, of course, the days with a lower probability of significant rainfall will be counted.

Example (2) Want to go camping or mountaineering during a long vacation: When you are in the suburbs, it is difficult to find a safe place to hide immediately in the event of rain or unstable weather. In this case, the safety risk of the activity is generally higher.

If the probability of significant rainfall during the holiday period is "Medium" or above, please carefully consider whether it is necessary to postpone or cancel the event according to your own situation and risk tolerance.

Please note: Even if the probability of significant rainfall is low, it cannot be completely ruled out that there will be strong rainfall in individual areas.

When engaging in high-risk outdoor activities, you still need to be vigilant at all times, pay attention to actual weather changes, and keep abreast of the latest warning information from the Observatory.

The Observatory predicts that it will rain for several days on the weekend and uses various information other than the weather forecast to avoid the "fallen chicken" typhoon Ray. Will it be windy in December?

Before and after the winter solstice, it may be cloudy and rainy in Hong Kong. In the past 60 years, there have been 7 Observatory science popularizations.

The Observatory pointed out that the probability of significant rainfall is "moderate" on Wednesday and Thursday. Outdoor activities should beware of heavy rain. Why does the temperature fluctuate?

Hong Kong is located in the "enchantment" of air masses from north to south

Original: Questions and Answers on Probability of Significant Rain Authors: Chong Sining and Wang Zhuoyao, Hong Kong Observatory