what is weather?

Weather is a term that encompasses phenomena in the atmosphere of a planet. The term is normally taken to mean the activity of these phenomena over short periods of time, usually no more than a few days (see also Weather report). Average atmospheric conditions over significantly longer periods are known as climate. Usage of the two terms often overlaps and the physical concepts underlying them are closely related.
Basic mechanism

When used without qualification, "weather" is understood to be the weather of Earth. At large scales, weather results from temperature differences around the globe, which arise mainly because areas closer to the tropics receive more energy per unit area from the Sun (see also Sunrise and Sunset) than regions nearer to the Earth's poles. On local scales, temperature differences can occur because different surfaces (such as oceans, forests, or ice sheets) have differing physical characteristics such as reflectivity, roughness, or moisture content.
Surface temperature differences in turn cause pressure differences. A hot surface heats the air above it and the air expands, lowering the air pressure. The resulting horizontal pressure gradient accelerates the air from high to low pressure, creating wind. The simple systems thus formed can then display emergent behaviour to produce more complex systems and thus other weather phenomena. Large scale examples include the Hadley cell and other forms of atmospheric circulation. An smaller scale example would be coastal breezes.
Because the Earth's axis is tilted relative to its orbital plane, sunlight is incident at different angles at different times of the year. In June the Northern Hemisphere is tilted towards the sun, so at any given Northern Hemisphere latitude sunlight falls more directly on that spot than in December (see Effect of sun angle on climate). This effect causes seasons. Over thousands to hundreds of thousands of years, changes in Earth's orbital parameters affect the amount and distribution of solar energy received by the Earth and influence long-term climate (see Milankovitch cycles).
Terrestrial weather

On Earth, regularly occurring weather phenomena include such things as wind, cloud, rain, snow, fog and dust storms. Less common events include natural disasters such as tornadoes, hurricanes and ice storms. Almost all familiar weather phenomena occur in the troposphere (the lower part of the atmosphere). Weather does occur in the stratosphere and does affect weather lower down in the troposphere, but the exact mechanisms are poorly understood [1].
The Earth's atmosphere is a chaotic system, so small changes to one part can have large effects elsewhere. This makes it very difficult to accurately predict weather changes more than a few days in advance, though weather forecasters are continually working to extend this limit through the scientific study of weather, Meteorology.
Chaos theory says that the slightest variation in the motion of the air will grow with time. This idea is sometimes called the butterfly effect, from the idea that the motions caused by the flapping wings of a butterfly eventually could produce marked changes in the state of the atmosphere. Because of this sensitivity to small changes it will never be possible to forecast perfectly, although there still is potential for vast improvement.

Hong Kong's climate is sub-tropical, tending towards temperate for nearly half the year. During November and December there are pleasant breezes, plenty of sunshine and comfortable temperatures. Many people regard these as the best months of the year. January and February are more cloudy, with occasional cold fronts followed by dry northerly winds. It is not uncommon for temperatures to drop below 10 C in urban areas. The lowest temperature recorded at the Observatory is 0 C, although sub-zero temperatures and frost occur at times on high ground and in the New Territories.

March and April can also be very pleasant although there are occasional spells of high humidity. Fog and drizzle can be particularly troublesome on high ground which is exposed to the southeast, and air traffic and ferry services are occasionally disrupted because of reduced visibility.

May to August are hot and humid with occasional showers and thunderstorms, particularly during the mornings. Afternoon temperatures often exceed 31 C whereas at night, temperatures generally remain around 26 C with high humidity. There is usually a fine dry spell in July which may possibly last for one to two weeks, or for even longer in some years.

July to September are the months during which Hong Kong is most likely to be affected by tropical cyclones, although gales are not unusual at any time between May and November. On average, about 31 tropical cyclones form in the western North Pacific or China Seas every year, and about half of them reach typhoon strength (maximum winds of 118 kilometres per hour or more).

When a tropical cyclone is about 700 to 1000 kilometres southeast of Hong Kong, the weather is usually fine and exceptionally hot, but isolated thunderstorms sometimes occur in the evenings. If the centre comes closer to Hong Kong, winds will increase and rain can become heavy and widespread. Heavy rain from tropical cyclones may last for a few days and subsequent landslips and flooding sometimes cause considerably more damage than the winds.

The mean annual rainfall ranges from around 1300 millimetres at Waglan Island to more than 3000 millimetres in the vicinity of Tai Mo Shan. About 80 percent of the rain falls between May and September. The wettest month is August, when rain occurs about four days out of seven and the average monthly rainfall at the Observatory is 391.4 millimetres. The driest month is January, when the monthly average is only 23.4 millimetres and rain falls only about six days a month.

Severe weather phenomena that can affect Hong Kong include tropical cyclones, strong winter monsoon winds, and thunderstorms with associated squalls that are most frequent from April to September. Waterspouts and hailstorms occur infrequently, while snow and tornadoes are rare.