Earth & Sky

What is an ice fog?

I was outside early in the morning during the winter and noticed that although the sky was clear, everywhere in the air was something like tiny floating, shimmering crystals – not snowflakes. What was I seeing?

This description sounds like an ice fog. On very cold, clear nights, the ground radiates infrared energy out to the atmosphere and space. As the ground gets colder, the air in contact with the ground also cools. On a still night, when there’s enough moisture in the air, the air temperature may go down to the dewpoint, or “frostpoint” – the temperature at which condensation occurs.

These are the normal conditions required to create a regular fog, but you need a couple more things to make an “ice fog.” First, the temperature needs to be cold enough for the moisture in the air to form tiny ice crystals. You also need just enough air motion to stir the air gently, so that more air can be cooled. What you get is a shimmering fog.

At times, you may also see what are called “light pillars,” columns of light that extend upwards from streetlights and porchlights. These occur when light reflects off the tops and bottoms of the ice crystals as the crystals flutter down like slowly falling leaves. When sunlight is the source of light, they’re called sun pillars.

More resources on “climate-weather”…