As sunshine is synonymous with warmth, it stands to reason that its absence at night denotes cold. This much is true. But when I first took weather readings in my back garden as a young child, I soon noticed that this was only true up to a point. One thing became obvious. Midnight doesn’t often coincide with the lowest temperatures.
On clear, still nights, the sun-warmed ground leaks heat away from the Earth into space all night long. Temperatures near the ground continue to fall beyond midnight, right up until dawn. In fact, it turns out, later still.
…temperatures can ‘bottom out’ nearer breakfast time
The first few rays of sunshine may begin to hit the ground at daybreak, but it takes a while for that heat to warm the air immediately above it. That’s important because officially we take our air temperature readings at 1.25 metres (or around 4 feet) above the ground. It translates to a lag of up to an hour.
Curiously then, our temperatures can ‘bottom out’ nearer breakfast time. And given that dawn in mid-winter is around 8am or even later, perhaps “overnight lows” should more accurately be described as “mid-morning lows”.