| Description |
Interpretation |
| Anticyclone / Area of high pressure |
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|
When the atmospheric pressure over a region is higher than its surrounding, it is called an anticyclone or an area of high pressure. The air mass surrounding an anticyclone will move clockwise in the northern hemisphere. The weather under an area of high pressure is generally stable and fine. |
| Bright |
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The sky is covered by a large amount of thin cloud with sunshine occasionally. |
| Cloudy |
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|
The sky is covered with a total cloud amount of between six eighths and eight eighths. |
| Cold front |
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The boundary line between an advancing cold air mass and a warm air mass. During the passage of a cold front, the local weather changes as follows : pressure increasing, temperature falling, wind veering (clockwise change in direction), accompanied by showers and thunderstorms. Generally, the cold fronts in southern China is not strong and may not bring so remarkable weather changes as mentioned above. |
Depression / Cyclone
/ Area of low pressure |
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|
When the atmospheric pressure over a region is lower than its surrounding, the system is called a depression or cyclone. The area of a depression has no definite size and its diameter can range from 100 km to 2 000 km. The weather under an area of low pressure is generally unstable. The air mass surrounding a depression will move counterclockwise in the northern hemisphere, and clockwise in the southern hemisphere. |
| Dew |
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|
Radiation cooling at night causes the air temperature to drop. When the air temperature falls below the dew-point, which is the temperature that the air becomes saturated with moisture, some water vapour condenses from saturated air near the ground to form dew. Dew usually appears in the early morning and condenses on grass or other ground objects. |
| Drizzle |
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Precipitation of water drops of very small size. |
| Fine |
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The sky is covered by a total cloud amount of less than six eighths. However, it can still be described as fine even though the total cloud amount is greater than six eighths if the cloud layer is thin enough to let plenty of sunshine to penetrate. |
Fog, Foggy, Mist,
Misty, Haze, Hazy |
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|
Fog is the suspension of small particles formed by the condensation of water vapour in the air near the ground surface, reducing the visibility to less than 1 000 m; it is called mist if the reduced visibility is equal to or above 1 000 m; it is called haze if the visibility is reduced due to small dust or smoke particles. |
| Frost |
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|
Frost will occur in very cold conditions. When the temperature near the ground falls to the ice-point or below, frost will usually form. There are two kinds of frost : ground frost and hoar frost. Ground frost is the frost formed from the condensation of water vapour in the air when the temperature of the ground surface falls below ice-point. Hoar frost is the ice crystals sublimated directly from water vapour near the ground and is usually deposited on the rim of leaves and cable masts. |
| Hail |
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|
Hard pieces of ice falling from thick cumulonimbus cloud and accompanied by thunderstorms. |
| Isolated showers |
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The rain-bearing cloud amount is small and isolated, resulting in showers occurring only in small parts of the territory at a time. |
| Monsoon |
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|
A monsoon is a seasonal wind flow due to the difference in surface pressure caused by the differential heating of seas and lands. The northeast monsoon generally prevails over southern China in autumn and winter while the southwest monsoon dominates in summer. |
| Intense winter monsoon |
|
|
In winter (normally December to February), the monsoon that brings cold weather with strong winds is called the intense winter monsoon. |
| Extreme cold surge |
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The intense winter monsoon that causes drastic temperature drop and very cold weather with gale force winds is called an extreme cold surge. |
| Occasional showers |
|
|
There is a large amount of convective clouds in the sky. Showers occur in most parts of the territory occasionally but the rainy periods may be different for different places. |
| Overcast |
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|
The whole sky is covered completely by a continuous, thick and opaque cloud layer. |
| Rain |
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|
Precipitation of water drops from deep and thick cloud layers. It is more persistent than showers but the rainfall amount is less. |
| Ridge of high pressure |
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|
A ridge of high pressure is an extension of an area of high pressure. The atmospheric pressure over such a region is higher than its two adjacent sides, figuratively similar to the ridge of a high mountain. The weather under a ridge of high pressure is generally stable and fine. |
| Seas |
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|
Winds blow over the sea surface and generate water waves called seas or sea waves. Sea waves can be described according to their wave heights as below:
| Description of seas or sea waves
| Height of seas or sea waves
|
| Calm |
0 - 0.1 m |
| Smooth |
0.1 - 0.5 m |
| Slight |
0.5 - 1.25 m |
| Moderate |
1.25 - 2.5 m |
| Rough |
2.5 - 4 m |
| Very rough |
4 - 6 m |
| High |
6 - 9 m |
| Very high |
9 - 14 m |
| Phenomenal |
Over 14 m |
|
| Scattered showers |
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|
The rain-bearing clouds do not stay together and are distributed spatially, resulting in showers occurring in parts of the territory. It may be rain-free in other parts of the territory. |
| Severe tropical storm |
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|
If a tropical storm strengthens to the extent that its maximum sustained wind speed ranges between 88 to 117 km/h, it will be called a severe tropical storm. |
| Severe typhoon |
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A severe typhoon is more intense than a typhoon and its maximum substained wind speed ranges between 150 to 184 km/h. |
| Showers |
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|
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. |
| Squall |
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A very strong wind that arises suddenly and lasts for at least one minute with a longer duration than gust. It is usually accompanied by thunderstorms. Besides the sudden changes in wind speed and direction, other meteorological elements such as temperature also change abruptly. Although the affected area is comparatively small, the destruction caused is not less than that of a tropical cyclone in some cases. |
| Squally showers |
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Showers accompanied by brief but sudden strong or gale force winds. |
| Sunny |
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Plenty of sunshine for most time of the day. |
| Sunny intervals |
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The sunshine is intermittent and the total sunshine duration is shorter than half of the forecast period. |
| Sunny periods |
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The sunshine is continuous and the total sunshine duration is longer than half of the forecast period. |
| Super typhoon |
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A super typhoon is the most intense tropical cyclone with a maximum sustained wind speed reaching 185 km/h or above. |
| Swells |
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|
Winds blow over the ocean surface and generate water waves called seas or sea waves. Propagating outward across the oceans, they are referred to as swells when they reach a distance far away from their originating area. Swells generated by the winds of a tropical cyclone travel at a speed much higher than the speed of movement of the tropical cyclone itself. As such, even though windy and rainy weather associated with a distant tropical cyclone is yet to affect Hong Kong, swells generated by the tropical cyclone may already have reached the coastal areas. When swells enter shallow waters, their wave heights would increase drastically, posing threats to people near the shoreline or operating over near-shore waters. |
| Thundery showers |
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|
Precipitation of water drops from cumulonimbus clouds. It is characterized by its sudden start and end with rapid changes in the intensity of rain, accompanied by thunders. |
| Thunderstorm |
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Regional severe weather coming from cumulonimbus clouds. Usually, a thunderstorm is accompanied by lightning and a rumbling sound (thunder), strong gust and occasional heavy rain. Under suitable weather conditions there will also be hails. The rumbling sound is caused by the sudden expansion and contraction of the air, as a result of heating and cooling of the air brought about by the passage of lightning through the atmosphere. One can calculate how far away the lightning area is, by multiplying the time between sighting of lightning and hearing of the thunder with the ratio of 3 seconds to 1 000 m. |
| Tropical cyclone |
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A generic term for tropical depression, tropical storm, severe tropical storm, typhoon, severe typhoon and super typhoon. |
| Tropical depression |
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A tropical depression is a cyclone formed over the tropical region and its maximum sustained wind speed is less than 63 km/h. |
| Tropical storm |
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A tropical storm is more intense than a tropical depression and its maximum sustained wind speed ranges between 63 to 87 km/h. |
| Trough of low pressure |
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A trough of low pressure is an extension of an area of low pressure. It is called a trough because the atmospheric pressure over the region is lower than its two adjacent sides, in contrast to a ridge of high pressure. |
| Typhoon |
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A tropical cyclone with maximum sustained wind speed between 118 and 149 km/h. |
| Upper Level Disturbance |
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A disturbance in the upper atmospheric flow pattern which usually makes the air aloft more unstable and conducive to clouds and precipitation. |
| Warm front |
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The boundary line between an advancing warm air mass and a cold air mass. The warm air mass climbs over the cold air mass, resulting in widespread rain areas ahead of the warm front. |
| Probability of Significant Rain |
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"Probability of Significant Rain" means the probability of a daily rainfall of 10 millimetres (i.e. around the mean daily rainfall in rainy season) or above generally over Hong Kong.
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