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Friday, 22nd November 2013

Describing the time

Weather is ever changing. Various weather conditions may occur within just one day. As such, the Observatory has to use a variety of terms to describe the temporal change in weather in the weather forecasts or warnings. Some might think that the most direct way is to specify the start and end time of weather event with a precision of down to one hour or even minute. Owing to uncertainties in evolution of weather and the regional difference in weather within Hong Kong, it is difficult, if at all possible, to adopt very precise terms to describe the timing of weather events in territory-wide weather forecasts. As such, when issuing weather forecast or warning, the Observatory always adopts common terms that the public are used to and easily understand, such as "small hours", "morning", "during the day" and "at night". However, these terms have to be used with care to avoid ambiguity. For instance, "afternoon" can be interpreted as either "midday to sunset" or "midday to midnight". Therefore, the context must also be considered when applying or interpreting these terms. If we say "sunny in the afternoon", there is no ambiguity in the meaning of "afternoon" as the whole phrase suggests that it refers to "midday to sunset".

There are indeed no universal definitions for most of these common terms for describing time. Sometimes we may even find different interpretations of them on the Internet. Attempting to assign rigid or quantitative definitions to these descriptions may not only cause unnecessary controversy or confusion, but also limit their application.

Notwithstanding the above, it is worthwhile to talk about some of the terms so that one can understand more clearly the way they are being used. For example, "small hours" generally refers to the period of time between midnight and sunrise. "Overnight" is the period after sunset and before dawn, spanning across midnight, while "at night" generally refers to the period between sunset and midnight. As mentioned above, we should always consider the context of sentence in which these terms for temporal description are used in order to avoid ambiguity.

Furthermore, "at first" and "later" are also frequently used in weather forecasts. "At first" is mainly used to describe the weather conditions or changes during the first half of the forecast period. For instance, if "a few rain patches at first" is given in the "Weather Forecast for Today" issued at 10 a.m., it means that there will be a few rain patches in about the first half period of the following fourteen hours. Similarly, "later" refers to the second half of the forecast period.

With advances in technology, numerical weather prediction models run by computers can forecast regional weather with precision down to an hour. In the future, the Observatory will launch this kind of automatic regional weather forecast to complement the weather forecast issued by forecasters, with a view to providing more comprehensive weather information to the public.

LS Lee and KL Lee

Last revision date: <30 Jan 2014>