The Tweet: “When you see a halo around the moon or sun, that means a storm is coming.”

Halos around the sun or moon are caused by light refracting through ice crystals in the Earth’s atmosphere. The crystals are found in cirrus clouds 20,000 feet above the Earth’s surface – the highest of the main cloud types. Cirrus only form in regions where the atmospheric temperature is below -20 degrees Celsius.

Light from the sun refracts twice as it passes through the ice crystals, once on entry and once on exit, bending the trajectory. To an observer, the bent light now forms a ring 22 degrees from the light source.

The phenomenon is not seasonal, but more likely to occur when the atmospheric temperature at high altitudes is lower. Cirrus are often an indication of the approach of a warm front approximately 24 to 36 hours away if they form robust and thick layers.  However, they can also be an indicator of fair weather. Irregular, patchy, and often dissolving cirrus clouds are often an indication of increasing high pressure and therefore dry and sunny weather.

Sun or moon halos are not always an indicator of approaching storms but will be seen most easily where the cirrus clouds form ahead of a warm front which brings with it wetter weather. Thunderstorms will only form if the warm air mass is unstable, and this will be indicated by the presence of low level stratiform clouds.