From 9-9:10am on April 3, homes and businesses in Hon Gai, Ha Long City of Quang Ninh Province had to turn on their lights when the sky suddenly darkened. This phenomenon was also observed in other northern provinces. It was brought on by a large storm, with dense clouds covering the sun.

A sun-radiated halo appeared in some localities last year, such as Da Nang, Ba Ria - Vung Tau, Lao Cai, Da Lat and Binh Duong. A halo is an optical phenomenon produced by ice crystals creating colored or white arcs and spots in the sky. Many are seen near the sun or moon but others are elsewhere and even in the opposite part of the sky. In picture: Halo in Bac Kan last August.

Amazing clouds in the sky of Hanoi in August 2009, after heavy rains. Scientists said this phenomenon is not related to any major meteorological events, but the frequency of repetition of clouds [of this type] is very low, so this phenomenon is rare. They are Mammatus clouds.

In July 2012, iridescent clouds appeared in the sky of Hanoi and Vinh Phuc. This phenomenon was previously witnessed in Quy Nhon in June 2012, and is not too rare. Iridescent clouds are generated through diffraction.

Also in July 2012, Hanoians witnessed a double rainbow and colorful clouds. A rainbow is an optical and meteorological phenomenon that is caused by both reflection and refraction of light in water droplets in Earth's atmosphere, resulting in a spectrum of light appearing in the sky. It takes the form of a multicolored arc. Rainbows caused by sunlight always appear in the section of sky directly opposite the sun.

In a "primary rainbow", the arc displays red on its outer part and violet on the inner side. This rainbow is caused by light being refracted (bent) when entering droplets of water, then reflected inside on the back of the droplet and refracted again when leaving it.

In a double rainbow, a second arc is seen outside the primary arc, and has the order of its colors reversed, red facing toward the other one, in both rainbows. This second rainbow is caused by light reflecting twice inside water droplets.  

At 8am on September 1, 1989, many people saw these dragon-shaped clouds in Bac Ninh. Photo: Nguyen Duc Thin.