Valley mist · Bamboo ValleyScience for kids

Where does valley fog come from?

In short: Fog is simply a cloud touching the ground. At night, moist air near the ground cools until it cannot hold all its water vapour, and tiny floating droplets form. Cold air collects in valleys, so they are often the foggiest places.

How it works

Air always contains invisible water vapour. Warm air can hold more of it than cold air. When air cools so much that it cannot hold it all, the extra vapour turns into tiny drops of water. This is called condensation, the same thing that puts droplets on the outside of a cold drink.

On clear, calm nights, the ground loses heat quickly to the sky, and the air touching it cools too. Cold air is heavier than warm air, so it slides down hillsides like water and pools on the valley floor. By morning the valley can look like a bowl full of milk.

When the sun comes up, the ground and air warm again and the droplets evaporate back into invisible vapour, so the fog lifts. Weather experts call it fog when you cannot see as far as 1 kilometre; thinner fog that you can see further through is called mist.

Where to see it in New Zealand

In the Waikato, foggy winter mornings are common. Low land along the Waikato River around Hamilton is often blanketed in thick fog, which sometimes delays flights. Drive through a river valley on a calm winter morning and you may see fog lying in the hollows while the hilltops are clear.

Try it together

Fill a glass with ice and water and wait a few minutes. Watch droplets appear on the outside: that is water vapour from the air condensing on the cold glass. On a cold morning, breathe on a chilly window and watch your own little fog form.

Kids also ask

What is the difference between fog and cloud?

There is no real difference. Both are tiny water droplets floating in the air. Clouds float high in the sky, while fog sits on the ground. Walking through fog is like walking through a cloud.

Why does fog usually appear in the morning?

Through the night there is no sunshine, so the ground and air keep cooling. Around dawn is often the coldest time, so that is when water vapour is most likely to condense into fog.

How can I stay safe in fog?

Fog makes it hard to see far. Look and listen extra carefully when crossing roads, and wear bright clothes so drivers can see you. Drivers slow down and turn on their lights in fog.

In the game, “Valley mist” is a nature card on Bamboo Valley.

Your child finds this card by reaching the right place or meeting the right weather on the island, and lights it up by answering the question on the back. Tap the button below and the game shows your child which island it is on and how to find it. Mochi Isle is a browser learning game for ages 5 to 18; the main island and the daily lesson are free.

Find “Valley mist” on the island ↗ All cards from Bamboo Valley