Rime

Frost (霜)

Frost is the deposition of water vapor from saturated air on a cold surface. It usually forms when a solid surface is cooled by radiation cooling to a temperature below the dew point of the adjacent air and the freezing point of water, i.e. 0 degree Celsius. Since the source of moisture is the water vapour contained in the ambient air, the heaviest coatings of frost often occur when the air temperature is around 0 C but not even lower, as very cold air cannot hold much water vapour. There are many types of frost depending on time and water vapour available, such as radiation frost (hoar frost), advection frost and window frost etc.

Rime (霧淞)

Rime is another type of frost which forms in a different mechanism other than radiation cooling. Rime usually occurs in heavily saturated air and windy conditions. In contrast to the radiation frost where water vapours deposit slowly and directly into icy feathers, rime forms rapidly and goes through the liquid phase before freezing. Rime is denser and harder than radiation frost and has an icy solid appearance. Rime formation is common when fogs or low-hanging clouds with supercooled or near-freezing water droplets come in contact with surfaces at or below the freezing point of water (0 C). In such cases, rime will grow in the windward side of the surfaces and it could be accumulated up to tens of centimeters.
Types of rime:

• Hard rime - white ice that forms when water droplets freeze rapidly as they touch an exposed object, such as tree.
• Soft rime - similar to hard rime, but feathery and milky in appearance.

Local case: Rime at Tai Mo Shan on 16th Dec 2010

An intense winter monsoon reached the coastal areas of Guangdong in the evening of 15th Dec 2010. The weather in Hong Kong became cold with some rain in the overnight between 15th and 16th.  At TMS, winds were strong to gale northeasterlies (see Figure 1) under the influence of thiswinter monsoon. Both air and grass temperatures fell below zero after 0500H (see Figure 2 and 3). Besides, the persistent precipitation before dawn (see Figure 4) provided a large amount of moisture while the relative humidity at TMS stayed at 100% throughout the period. Meteorogram at CLK indicated the lowest cloud base was around 1000 ft only (see Figure 5), much lower than the elevation of TMS which is 3140 ft.
Wind barbs at TMS on 16 Dec 2010 Figure 1. Wind barbs at TMS on 16 Dec 2010
Air temperature at TMS Figure 2. Air temperature at TMS fell below zero after 0500H and lingered around -2C afterwards.

Grass temperature at TMS Figure 3. Grass temperature at TMS stayed at 0C after 0500H
Rainfall at TMS Figure 4. Continuous precipitation in the overnight period but gradually eased off after dawn
Meteorogram

Figure 5. Cloud base down to around 1000 ft at CLK

Weather conditions at TMS in the morning of 16th Dec 2010

On the whole, the weather conditions at TMS in the morning of 16th Dec were *Windy - strong to gale northeasterlies *Heavily saturated air - continuous precipitation before dawn and RH stayed at 100% *Fogs/low-hanging clouds (around 1000 ft height) with supercooled water droplets - subzero air temperature after 0500H.

• Freezing surfaces - grass temperature stayed at 0C after 0500H.Hence, the conditions which favoured rime formation were all satisfied at TMS and rime frost was reported at TMS by HKO staffs in the morning of 16th Dec. Figure 6 to 9 were some photos of rime captured in that morning.
  Fogs/low hanging clouds at TMS
  Figure 6. Fogs/low hanging clouds at TMS in the morning of 16th Dec.
  
  Rime growing out on the windward side in the leading edge of anemometer
  Figure 7. Rime growing out on the windward side in the leading edge of anemometer
  Rime with an icy solid appearance Figure 8. Rime with an icy solid appearance
  TMS-20101216-101.jpg Figure 9. Rime on grasses
  
  
• More photos#Tai Mo Shan (16 Dec 2010)*References
  AccuWeather - http://www.accuweather.com/blogs/weathermatrix/story/17927/photos-hoar-frost-vs-rime-ice.asp
  Wikipedia - http://en.wikipedia.org/wiki/Rime
  NOAA NWS - http://www.srh.weather.gov/srh/jetstream/append/glossary_r.htm