I. Introduction to Meteorology

4. Common phraseology in weather forecast

A lot of different terms are used in weather forecast vocabulary which varies due to the differences in the target audience, effective time, and specific uses. Currently the Central Weather Bureau issues 12 different weather reports.

A weather report includes two major parts: the sky condition and weather phenomenon which change all the time naturally. The sky condition is the amount of clouds in the sky and its variation. The sky is divided into 10 equal parts and the amount of clouds is calculated as the portion of the sky that is covered by clouds. For example, when a forecast indicates "cloudy", this means 5/10 to 8/10 of the sky is covered by clouds. Also note that all clouds are counted regardless of whether or not they are transparent.

By looking at the forecast phraseology table we can learn the differences and the principles to use these various terms in a weather forecast.

As for a weather phenomenon, it consists of weather events such as precipitation, fog, dust, thunder, lightning, rainbow, secondary rainbow, etc. From the following tables, we can have a better insight into their meanings.

Weather report information table by the Central Weather Bureau

The Central Weather Bureau routinely issues and updates short-range and medium-range weather forecasts for a range from 12~36 hours up to seven days including: today's, tonight's, tomorrow's, and tomorrow night's weather forecasts, 7-day weather forecast, 7-day tourism forecast, comfort index forecast, fishery weather forecast, and forecast for major cities around the world, etc. The CWB also provides specialized weather forecast service for contracted large business and timely issues severe weather advisories/typhoon warning. As for the long-range forecast, the CWB currently provides products include outlooks for one month and three months.

Common terms in weather forecast
State of Sky Division or Standards of Use
Clear
Partly Cloudy
Cloudy
When 0-4/10 of the sky is covered by clouds.
When 5/10-8/10 of the sky is covered by clouds.
When 9/10-10/10 of the sky is covered by clouds.
Clear
Partly Cloudy
Cloudy
When the occurrence spans over 3/4 of the effective time period of forecast, a single term will be used for the forecast.
Mostly Clear
Partly Clear
Mostly Cloudy
The term describes a situation which accounts for over 1/2 but not 3/4 of the effective time period of forecast.
Clear Becoming Cloudy
Cloudy Becoming Clear
Partly Cloudy Becoming Cloudy
Cloudy Becoming Partly Cloudy
Clear Becoming Partly Cloudy
Partly Cloudy Becoming Clear
When the amount of cloud changes significantly, the forecaster will use the word "becoming" as a representation of the change.
(The first two terms are more common)
Common terms of weather phenomena
Terms for Precipitation Forecast and its Variations Meanings
Precipitation Pattern Rain Intermittent or continuous precipitation from stratiform clouds falling in a uniform and mild pace.
Showers Brief precipitation usually from convective clouds. It is characterized by the sudden start and end of the precipitation as well as the fine weather before and after its occurrence.
Thundershowers Showers accompanied by thunder.
Duration Occasional With (or simply omission of modifier) When the accumulative duration of precipitation is under 1/4 of the effective forecast period, the precipitation is classified as occasional, for example, cloudy with occasional showers;
If the duration is longer than 1/4 of the effective forecast period, then a modifier "with" may be used or omitted, for example, cloudy with showers.
Time of Occurrence Afternoon,Early Morning,Night,Noon,Evening The time of the precipitation, for example: occasional afternoon thundershowers.