Sun Halos over Finland: Understanding Pauli Hänninen’s Photographs

Pauli Hänninen’s Finnish halo photographs show a sky busy with arcs and bright spots. A small group of people beneath the display gives its extraordinary scale a familiar reference.

Ice crystals redirect sunlight

Different crystal shapes, orientations and light paths can produce different halo features. The common 22-degree halo does not mean every crystal sends all incoming sunlight through exactly that angle.

Notice that several features can coexist

A circular ring, a horizontal arc and bright patches need not be the same optical form. The photographs invite comparison of their position relative to the Sun. The finished photographs alone do not establish what processing was used, and looking directly at the Sun is unnecessary for enjoying the recorded display.

Explore more: recognised cloud terminology; why ice can look blue.

The phone call that brought the family outside

In the original account, a friend called Pauli Hänninen and urged the family to look at the sky. Hänninen described fog, a temperature around minus ten degrees Celsius and coloured forms appearing as the clouds began to break. That eyewitness context is worth keeping: the photographs record a changing display that drew people outdoors, rather than a diagram assembled to illustrate just one optical effect.

A 22-degree ring and a circle around the sky

The World Meteorological Organization describes the common 22-degree halo as a ring centred on the Sun or Moon, with a radius of about 22 degrees. It can have a reddish inner edge and may be only partly visible. The number describes the ring’s angular radius in the observer’s sky, not the physical size of a cloud.

A parhelic circle is a different feature: it runs horizontally at the Sun’s angular height. Bright spots can appear along it. This distinction helps explain why a report of a display spanning the sky need not mean a 22-degree halo became enormous. Several optical features can be present together, and a wide photograph can capture more than the familiar ring around the Sun.

Compare the wide and vertical views

Compare the position of each bright patch with the horizon before trying to give it a name. The wider framing shows how features extend across the sky, while the vertical photographs leave more room above the landscape. A ring centred on the Sun and a line at the Sun’s height describe different arrangements, even when they appear together. The family beneath the display adds another useful reference, showing why this was an event worth stepping outside to photograph rather than simply an isolated optical detail.