On one of my very first deer hunts, I remember thinking that Dad didn’t dress me warm enough. We hadn’t even been in the field an hour and I was ready to call it a day.
“It’s always coldest at sunrise,” he whispered to me, seeing that I was shivering.
Knowing that temps wouldn’t continue to fall kept me there long enough to kill a doe. It was a memorable hunt, but more so because I think about that sentence every time I’m in a treestand at dawn.
Is it true, though? Or just a piece of inaccurate wisdom that my dad’s dad shared with him?
It turns out that on a normal day the lowest temperature is usually recorded about an hour after sunrise. This is because of solar and thermal radiation.
During the day, while the sun is high, the earth and atmosphere are being heated by solar energy. Once the sun disappears, so does that solar radiation. It’s at this point that the earth starts cooling. With every second that passes, the earth’s surface gets colder and that emission in turn cools down the atmosphere.
Once the sun rises, solar energy is once again directed towards earth. However, there’s a delay between the sun coming up and the sun’s energy arriving here. For that reason, the earth is still cooling even in the early part of the day while the sun is out.
In summary, once the sun disappears, so does the heat. The earth spends the entire night getting colder, which makes the atmosphere colder as well. The cycle doesn’t start over until the sun’s energy gets here, making the hour after sunrise the coldest part of the day.
This is most noticeable when nights are the longest in winter, which gives the earth more time to lose heat. However, the “coldest after dawn” effect isn’t as dramatic if it’s cloudy or foggy, which helps trap heat in the atmosphere. It’s also less noticeable if there’s snow on the ground, as this lets less energy escape from the surface.
Toss an extra layer on for those clear sky mornings, or use this as an excuse to hit snooze one more time.