a-z

Mammatus

Rounded, smooth, globular ‘pouches’ clumped together and hanging underneath the base of a cumulonimbus cloud. They often form on the underside of an anvil and are accompanied by thunderstorms. The name translates to mamma and means ‘mammary cloud’.

Alpine glow

A beautiful illumination effect seen in mountainous regions around sunset, beginning when the sun is around 2º above the horizon. At its most spectacular, snow-covered mountains in the east assume vivid tones of yellow, pink and purple.
 

Air

The mixture of gasses that make up the Earth’s atmosphere. Excluding water vapour and dust, the main components of air are nitrogen, oxygen, argon, carbon dioxide, neon, helium, krypton, hydrogen, xenon and ozone.

Advection fog

A type of fog formed when warm, moist, stable air moves over a far cooler surface. It is most commonly found over cool sea areas in spring and summer, but does occasionally form over land in the winter, especially when the ground is frozen or covered by snow.

Aurora

Taking its name from the Latin for dawn, aurora is the phenomenon of visible light being emitted by the high atmosphere, caused when charged particles emitted by the sun are deflected by the Earth’s permanent magnetic field. In Britain, aurora usually appears as a grey-white glow on the northern horizon. The terms ‘aurora borealis’ (or northern lights) and ‘aurora australis’ (or southern lights) are applied to the occurrence of aurora in the northern and southern hemispheres respectively.