In atmospheric optics, an aureole is a bright disc that envelops its light source. Commonly they may also interchangably be called coronae, but some sources specify that the corona is only the vibrant edges of an aureole. I use this convention for this writeup.

Note that an aureole is not a ring around the sun or moon—which would be properly called a halo—but a complete disc. More than just appearance, aereole and halos differ in their causes: the halo is caused only by ice crystals, while aureole can occur from many types of particles.

Aureolae around the brilliant sun are understandably difficult to detect. They are often seen around the much dimmer moon. The disc commonly varies in size between 2 and 10° around the source. The size of the disc is inversely proportional to the size of its constituent particles.

Aureolae are caused by the same refraction that causes rainbows, but aureole particles are between 0.002 and 0.1 mm (orders of magnitude smaller than rainbow-causing drops) and most commonly suspended in a very thin flat horizontal layer in the atmosphere. Their coronae display an unsaturated spectrum, with blue towards the source and red away from the source. Coronae are sharpest and produce the most saturated colors when the particles are all of the same size and the layer very thin. Conversely, thick layers of varied sized particles will produce diffuse and colorless effects.

As thin clouds pass before the source, an aureole and its corona may sleepily shift and change accordingly.

Water is the most common atmospheric particulate to form this effect, but there are other sources:

  • Bishop's Ring: Named for the first scientist to accurately describe them, Sereno Bishop, these faint and diffuse aureolae are formed by extremely small aerosols (0.002 mm, an order of magnitude smaller than common aureole particles) spread in the atmosphere. They are not known to display vibrant coronae. They are larger than most other types, spreading out to 28° around the source. Krakatoa's eruption caused the first recorded Bishop's Ring in 1883. It is believed that volcanic eruption is the only source of mass aerosol dispersal to cause this effect.
  • Pollen:
    • Pine: Common. Because of the shape of the pine pollen, these aureolae can form bright bulges, called "knots", in cardinal points at the edge of the amber-colored aureole. April-June season.
    • Spruce: Similarly shaped pollen to pine, these aureolae may also display knots in their April and June season.
    • Alder: February-March season.
    • Hazel: February-March season, rare.
    • Birch: March-April season.
    • Walnut: April-June season.
    • Barley: Too heavy to float in the wind for long, barley aureolae are seen only from barley fields in June and July.
    • Rape: Also too heavy to float in the wind, yellowish rape aureolae are seen only from rape fields in April through August, and most commonly in dry, warm weather when the plants are all blooming simultaneously.

You can easily create your own aureole effect by exhaling on a cold window pane at night and observng a distant light source through the condensation.