Weather Terms

Anticyclone / Area of high pressure	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 anti clockwise in the southern hemisphere. The weather under an area of high pressure is generally stable and fine.
Cloudy	The sky is covered with a total cloud amount of between six eighths and eight eighths.
Cold front	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 backing ( anti clockwise change in direction), accompanied by showers and thunderstorms.
Depression / Cyclone / Area of low pressure	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 clockwise in the southern hemisphere, and anti clockwise in the northern hemisphere.
Dew	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	Precipitation of water drops of very small size.
Fine	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	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	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	Hard pieces of ice falling from thick cumulonimbus cloud and accompanied by thunderstorms.
Isolated showers	The rain-bearing cloud amount is small and isolated, resulting in showers occurring only in small parts of the country at a time.
Monsoon	A monsoon is a seasonal wind flow due to the difference in surface pressure caused by the differential heating of seas and lands.
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	The whole sky is covered completely by a continuous, thick and opaque cloud layer.
Rain	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	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.
Scattered showers	The rain-bearing clouds do not stay together and are distributed spatially, resulting in showers occurring in parts of the country. It may be rain-free in other parts of the country.
Severe tropical storm	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.
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.
Squall	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	Showers accompanied by brief but sudden strong or gale force winds.
Sunny	Plenty of sunshine for most time of the day.
Sunny intervals	The sunshine is intermittent and the total sunshine duration is shorter than half of the forecast period.
Partly cloudy	Clouds covering half or less of the sky.
Thundery showers	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	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 lighting through the atmosphere. One can calculate how far away the lighting area is, by multiplying the time between sighting of lighting and hearing of the thunder with the ratio of 3 seconds to 1 000 m.
Tropical cyclone	A generic term for tropical depression, tropical storm, severe tropical storm and cyclone.
Tropical depression	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	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	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	A typhoon is the most intense tropical cyclone with maximum sustained wind speed reaching 118 km/h or above. It is equivalent to a hurricane. The only difference is that typhoon is used in the Pacific region while hurricane is used in the Atlantic and America.
Warm front	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.

Courtesy of the Hong Kong Observatory