見到D好奇怪既彩虹 20分!!!

What you saw is not a rainbow. This phenomenon is called "halo effect"(日暈現象). The circular coloured band around the sun is a "halo"(日暈),which is formed by reflection and refraction of sunlight by ice crystals in high altitude clouds.
You may refer to the following two web-pages (one in Chinese and one in English) for more information on this phenomenon.
http://en.wikipedia.org/wiki/Halo_(optical_phenomenon)

http://zhidao.baidu.com/question/52504518.html

冇錯
彩虹其實係光圈形的
平時只係彩虹圈太大
只見到一半

冇落雨都見到
唔出奇
近水源的地方(河,瀑布...)
仲有冷氣機滴水
甚至眼鏡在某角度睇白光
都會有機會出現虹彩