With Kiwis waking to frosty conditions across the country, we turned to our meteorologist Chris Brandolini for some answers.

When we inhale, the warm air inside our lungs transports oxygen to our bodies and also produces water vapor. This water vapor, when exhaled into the cold air around us, can form that little cloud that some people see on a cold day.

But what causes that to happen? The answer has to do with temperature and humidity. The warmth of our breath, combined with the relative humidity in the air surrounding it, determines whether or not we can see our own breath. It’s impossible to say exactly what temperature it has to be for our breath to become visible as it evaporates, but it’s usually below 45 degrees Fahrenheit.

The warm air from our lungs loses its energy when it cools, and the water molecules begin to pack together more closely as they slow down. This process is called condensation, and it turns the water vapor into tiny droplets of liquid water and ice (solid water). These small drops appear to form a cloud, similar to fog.

The air can still be quite dry, though, even at temperatures below freezing. That can make it hard to see your own breath, or can cause problems for people with facial hair, like beards and mustaches, which can form a layer of ice that makes it difficult to speak. That’s why many people wear a beard muffler or a mouth cover, to prevent ice forming on their faces.