IV · Look up
What can the Sun make today?
Every halo needs the Sun at the right height. Choose a place and a time, and see which ones are possible and where to look for them.
The Sun’s height and the halos it allows
Tracing sunlight through ice…
In the field
How to look
Halos sit in thin, high cloud: a milky veil the Sun still shines through. They come and go with it, so look while it is there.
- Hide the Sun
Stand so a roof edge, a lamppost or your hand covers it. Never look straight at it.
- Measure
Arm out, hand spread: thumb on the Sun, little finger on the ring. That span is about 22°.
- Look sideways
Sun dogs sit at the Sun’s height, just outside the ring, red on the side facing the Sun.
- Look up
When you see sun dogs and the Sun is below 32°, check overhead for the circumzenithal arc.
- Note it
Time, place, what you saw. A wide photo with something on the horizon tells us the scale.
At home
A halo in a glass of water
Fill a straight-sided glass almost to the top and stand it at the edge of a table in low sunlight. The light enters the flat water surface and leaves through the curved side, the same turn it takes through a plate of ice, and throws a small circumzenithal arc on the floor.
The trick has been known since at least 1920. Light it steeply from below, through the side, and the arc it throws on the wall behind is a circumhorizontal arc.
