霰
自然系列的一部分 |
天氣 |
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溫帶季節 |
熱帶季節 |
寒帶季節 |
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风暴 |
降水 |
主題 |
氣象主题 |
形成
於某些大氣條件下,雪晶(snow crystal)與過冷水滴(supercooled water droplet)會一同形成,過冷水滴直徑約10微米,在溫度低於冰點、甚至攝氏零下40度時仍呈液態。當雪晶與過冷水滴接觸,過冷水滴會凝固於雪晶的表面,這個過程稱為撞凍(accretion),與形成霿凇(亦稱雺凇、霿凇)的機制類似。當撞凍持續到看不出原本雪晶外形時即稱為霰。[3]雖然霰又稱為軟雹,但與雹在形狀與結構有很大差異。雹的冰質地堅硬並呈現層狀構造,通常於雷暴時降下;霰質脆、碰觸時容易碎掉,於雨夾雪時代替雪花落下,常與冰珠同時出現。
- 霰粒
- 很多霰粒
- 霰積於地表
顯微鏡觀察
霰不易以光學顯微鏡觀察,因為凝固的過冷水滴接近儀器解析度的極限。若改以低溫掃描電子顯微鏡(LT-SEM)觀察,凝固的過冷水滴則清晰可見,其大小可達50微米。四種基本雪晶皆可形成霰,包括片狀、枝狀、柱狀及針狀冰晶。隨著撞凍持續,凝固的過冷水滴會漸漸覆蓋原本的雪晶,最後成為霰粒。[3]
- 過冷水滴凝固於柱狀雪晶的兩端
- 凝固的過冷水滴將雪晶完全包覆
霰與雪崩
霰常見於高緯度氣候,比雪來得緻密且顆粒較大,形似保麗龍的顆粒。由於高密度與低黏性,霰無法穩固的堆積在斜坡上,而累積厚度20至30公分的霰有很可能會崩落、形成硬層雪崩(slab avalanche)。依據氣溫及霰的特性,霰於降下後約一至二天變為較緊密及穩固。[4]
參考資料
- AVIATION ROUTING WEATHER REPORT, Present Weather Group 页面存档备份,存于. Department of Atmospheric Sciences, Texas A&M University.
- USA and International Code Change For Ice Pellets 页面存档备份,存于. Automated Surface Observing System, National Weather Service.
- "Rime and Graupel". Electron Microscopy Unit, Beltsville Agricultural Research Center, U.S. Department of Agriculture. Public domain. URL accessed 2006-07-23.
- Graupel 的存檔,存档日期2010-05-04., www.avalanche.org.
外部連結
辭典
- 3 results for:graupel. Dictionary.com, accessed September 12 2006.
- Graupel. Merriam-Webster Online Dictionary, accessed September 12 2006.
天氣詞彙
- Weather Glossary, G. The Weather Channel, accessed September 12 2006.
- All About Snow. National Snow and Ice Data Center (NSIDC), accessed September 12 2006.
- Terms used by meteorologists, forecasters, weather observers, and in weather forecasts 页面存档备份,存于. National Oceanic and Atmospheric Administration (NOAA), accessed September 12 2006.
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