Download Appendix I Summary of Different Winter Weather Phenomena in

Appendix I ­ Summary of Different Winter Weather Phenomena in Hong Kong Type
Appearance and Formation
Favourable Conditions in Hong Kong
Frost
Small ice crystals formed directly from deposition of In Hong Kong, frost usually occurs when water vapour in the ambient air onto cold surfaces there is no rain and the weather is dry. The at or below the freezing point of water without going state of sky may vary from clear to through the liquid state. overcast. Winds are usually from north to northeast with strength varying from light to There are different variants of frost, such as ground moderate. frost, advection frost, frost flowers, etc., depending on how it forms and its appearance. Geographically, frost usually occurs on: (i) low­lying grounds in the northern part of the New Territories, usually spatially localized; and (ii) high grounds, such as Tai Mo Shan and Lantau Peak, and may be widespread. For type (i), ground frost usually forms on very dry nights with clear sky and light to calm winds, conditions that favour enhanced nocturnal radiation cooling. Frost at Ta Kwu Ling on 29 December 2013. (Photo courtesy of Irwin Wong)
Ice/freezing rain
Transparent or translucent solid ice, formed when In most cases in the past, icing was reported
liquid water is exposed to ambient air with on very cold days on hill tops with zero or temperature at or below the freezing point of water. sub­zero temperatures. It may appear in the form of icicles or ice sheets in fish ponds, Moreover, under favourable atmospheric water tanks or buckets, as well as pools of conditions, melted snowflakes or rain may fall in water.
liquid form through a shallow sub­freezing layer near
the ground which is too thin to allow the super­
cooled raindrops (freezing rain) to refreeze before they come into contact with cold objects, forming icicles or ice film. Ice on Tai Mo Shan in 1970s
Rime Rough ice crystals with feather­like appearance, A rare phenomenon with only five days of Rime Rough ice crystals with feather­like appearance, formed when tiny, super­cooled water droplets in fog or cloud on mountains come into contact with objects at or below the freezing point of water. The rime “feather” can grow thick on the windward side of the objects.
A rare phenomenon with only five days of occurrence in Hong Kong since 1948, and in all cases found on hill tops at sub­zero temperatures, enshrouded in low stratus clouds with fog or drizzle. Rime on Tai Mo Shan on 16 December 2010
Sleet
No internationally agreed definition for sleet at the A rare phenomenon in Hong Kong with only
moment. Generally speaking, sleet refers to "mix of five days of occurrence and mainly on hill rain and snow (in Britain and Australia) or ice pellets tops.
(in America)". Sleet occurs in stratus clouds where snowflakes partially melt as they fall through a layer of relatively warm air (above freezing point) and then re­freeze into ice pellets in a colder layer close to the ground. Sleet (small ice pellets) on ground in US (http://en.wikipedia.org/wiki/Ice_pellets) Snow
Tiny ice crystals stuck together to become snowflakes with fluffy appearance, formed when tiny super­cooled cloud droplets freeze at temperatures well below the freezing point of water, usually only attainable high up in the atmosphere. For snowfall, usually the layer of air from ground to where snow is formed has to be at or below 0o C all the way, so that the snowflakes do not melt into rain before reaching the ground. Extremely rare phenomenon with only four reports since 1948 (one at Cape Collinson and three at Tai Mo Shan near its summit), all described as brief and slight snow and three of them with above freezing surface temperatures at the observation spot.
Snow in Washington, D.C. area on 17 March 2014 (Courtesy of K Y Kong)
Snow in Beijing on 12 November 2009 (Courtesy of T C Lee)