Why do we see a circle around the sun? Halo's simple explanation

Why do we see a circle around the sun? Halo's simple explanation

Sometimes a halo can be seen in the sky around the Sun or the Moon. Why does it appear?

Halo only occurs with cirrostratus or cirrocumulus clouds. Let’s talk about the former.

Cirrostratus clouds

These occur when humid air manages to rise really high - to over 6 km. Here water drops turn into ice crystals. 

Why can humid air rise that high? For instance, due to a warm front - when warm air crawls over a wedge of cold one and “creeps” up along it. 

Clouds

 

Cirrostratus clouds look like a white semitransparent sheet. They can cover the whole sky but you can see the Sun or Moon through them.

Such a cloud consists of ice crystals. Different optical phenomena can occur - depending on the form of the crystal.

For example, if the cloud consists of hexagonal crystals, we can see a halo around the Sun.

When you look at the Sun you see rays directed straight at your eyes. The ice crystals focus the rays so that they’re once again directed at your eye. See the image below:

Sun rays

 

Thus, we see the Sun and the circle around it.

Important: don’t look directly at the Sun, it’s dangerous. 

Cirrostratus clouds and halo come a day or two before the warm front. The latter brings rains. So get your umbrellas ready when you see a halo. 

Important! All of the above concerns temperate latitudes. Tropical latitudes and the equator have different patterns. 

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