The coldest moment of the day comes around sunrise, not in the middle of the night. On a clear autumn morning you can feel it: frost on the bike seat, a railing so cold it almost stings, breath hanging in the air. But the sun has been gone for hours. Shouldn't the coldest moment have come at two in the morning?
What happens to heat at night?
The ground is a heat store. During the day the sun fills it up, and stone, soil and grass hold on to that warmth. At night the ground sends its heat out towards the sky as invisible heat radiation. That radiation never stops, but in daytime the sun tops it up faster than it drains away.
Think of a bucket with a hole in the bottom. As long as more water runs out of the hole than comes in from the tap, the level drops. Turning the tap on a little does not stop it. The level stops falling only when the water coming in matches the water running out.
The night is not coldest when it is darkest. It is coldest when the sun finally comes back.
The first sunlight is weak and slanting, so the ground keeps losing more heat than it gains for a while after sunrise. The lowest point comes when the tap and the hole are equal. Clouds act like a blanket and send some heat back down, which is why frost almost always follows clear nights. The same cooling makes fog over lakes (see why is it foggy in the morning) and dew on the lawn. More autumn phenomena are in the autumn outdoor laboratory.
Why does this matter for children?
It is a chance to discover that nature does not always do what we "know" it does. Everyone knows the night is cold. Few have checked when it is coldest. Measuring it touches on weather investigations, temperature as a form of energy, and turning a table of numbers into an explanation.
Try it at home: find the coldest moment
Suitable for: 6–12 years. Choose a clear, calm evening.
- Hang an outdoor thermometer in the shade, about a metre above the ground and away from the wall.
- Read it at bedtime and note the temperature and time.
- Read it again just before sunrise.
- Read it half an hour after sunrise.
- Read it once more mid-morning, touching the bike seat each time.
- Plot the numbers as dots: time across, degrees up.
⚠️ An adult should be outside with children when it is dark.
Did the numbers keep falling after sunrise? Where is the lowest dot? The dew on the grass comes from the same cooling, and you can read why in why is grass wet in the morning.
What happens if you try the same thing on a cloudy night?
Questions to wonder about
Where does the heat that leaves the ground tonight go?
If the sun is highest at midday, why is it often warmest in the afternoon?
If the Earth sends heat into space every single night, why does it not just get colder and colder, year after year?
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