A set of special stamps on the theme "Weather Phenomena" was released with associated philatelic products by Hongkong Post.
Situated in the subtropical zone, Hong Kong has a humid climate with fog and drizzle in spring. Summer, also a typhoon season, is hot with occasional showers and thunderstorms. Autumn normally brings pleasant breezes, plenty of sunshine and comfortable temperatures. Winter becomes cloudier, with occasional cold fronts crossing the territory, bringing frost at times. In collaboration with the Hong Kong Observatory, Hongkong Post introduces seven common weather phenomena in Hong Kong under the theme of "Weather Phenomena" to help the public understand the changes in our weather.
$1.70 A rainbow is an optical phenomenon. Sunlight is composed of light of different wavelengths. When there are water droplets in the atmosphere, sunlight gets refracted and reflected in them and a rainbow is formed. The stamp illustrates a double rainbow in the sky over the urban area of Hong Kong. When sunlight passes through water droplets and gets reflected twice, two rainbows are developed. The primary rainbow, formed by one reflection, is brighter while the secondary rainbow, produced by two reflections, is fainter.
$2.20 Frost forms when water vapour freezes into ice crystals. When the surface temperature of an object falls below freezing point, frost appears. The stamp portrays frost formed on the surface of a plant in cold weather in Hong Kong. Frost is white and is usually found on the ground or exposed objects.
$2.90 A cloud is formed when water vapour rises through the atmosphere and cools. A cloud is a mass of water droplets or ice crystals formed by the cooling and condensation of the lifted water vapour aloft. The stamp shows lenticular clouds in the sky over an urban area of Hong Kong. As air rises on the lee side of a mountain, water vapour condenses into water droplets, which accumulate and develop into lenticular clouds, taking the shape of a pile of saucers or lenses.
$3.10 Lightning is an atmospheric electric discharge phenomenon. In an unstable and moist atmosphere, water droplets and ice pellets in clouds will become electrically charged in a convective motion. When the building up of charges reaches a certain threshold, discharges take place between clouds or between clouds and the earth's surface. The stamp depicts cloud-to-ground lightning in the night sky of Hong Kong. Cloud-to-ground lightning, a lightning discharge between a cumulonimbus cloud and the ground, poses a greater threat to people and facilities on the ground.
$3.70 Fog refers to tiny suspended water droplets near the ground. Under light wind, and stable and humid conditions, when the air near the ground cools down sufficiently, the water vapour in the air condenses into tiny water droplets and fog is formed. The stamp pictures Lantau Island embedded in mist at dawn. In springtime, it is often foggy in Hong Kong and neighbouring areas as they are affected by alternate cold and warm air.
$5 Rain is a natural phenomenon of precipitation. Water droplets in the clouds will come down as rain when they accumulate to a certain weight. The stamp features Chek Lap Kok Airport in the rain. When a moist and warm air parcel rises and condenses as cloud, a significant amount of latent heat will be released into the surrounding air, facilitating the uplift of the air. Further sucking up of humid air will promote the full development of a cumulonimbus and eventually bring torrential rain.
$5 Stamp Sheetlet A typhoon is a classification of tropical cyclone with wind speeds near its centre reaching hurricane force. The stamp sheetlet showcases the satellite image of Typhoon Vicente when it approached Hong Kong in July 2012 and the Hurricane Signal No. 10 was issued by the Hong Kong Observatory.
Hongkong Post also issues a $20 special stamp sheetlet, printed with lithography plus a lenticular effect, to show the trajectory of Typhoon Vicente.