Or, is this related to local geography (we live at the base of a mountain range so it could be that colder air “falls” off the mountain as the sun comes up)?
I’m not a meteorologist. But I suspect it’s for the same reason as why the weather keeps getting colder for some time after the winter solstice: both the planet and the atmosphere provide large enough heat sinks that there’s a delay before the change in sunlight causes warming.
But, the temperature (here) is realtively stable before sunrise – you would always expect the coldest part of the night ot be just prior to sunrise, all else being equal.
But, AS the sun rises, the temperature falls further! You would expect it to start to rise…
Air that has been cooling overnight may still be descending after sunrise. The sun also begins heating the earth and hotter air near the ground can rise and displace colder air that then descends.
Plus is it the dry bulb temperature or the wind chill? I haven’t noticed temps per se dropping as the sun rises as long as the air is still still, but I’ve definitely noticed it windier around sunrise, probably due to the energy from the sun stirring things up, which can make it feel colder.
And the angle of the sun’s very low; so there’s not as much heat coming through as there will be when the sun’s higher. So there’s not enough to counteract the other factors.
This is possible – i.e., a consequence of our location. We’ve noticed that the same plantings will reliably bloom a few weeks earlier just a few blocks from us where they are at a slightly higher location and that much further from the mountains. And, there are lots of microclimates undoubtedly influenced by their presence. E.g., we often suffer from microbursts in strong downdrafts while folks a block or two away won’t see any impact.
But, as I said, the temperature is stable prior to sunrise. It only sinks after the sun makes its appearance (so, the sun is causing the cooling).
Dry bulb.
Wind, here, doesn’t pick up until much later in the morning – 10ish and later. (wind chill isn’t a real factor, here; THI is, during certain seasons)
If there are any geography factors at play, you’re more likely to get a relevant response by being more specific.
Did you miss:
Not sure, but could there be an evaporative cooling effect? The air is likely to be near or at saturation, so a small input of energy from the sun will cause the dew/fog to evaporate and perhaps drop the temperature along with it. I don’t know if the physics of that works out, but it’s a thought.
Similarly, warming of the mountain and/or the air above by the sun can cause convective currents as others have mentioned already. Evaporation can be a factor there too, but only so much as it changes buoyancy and causes wind to bring in colder air from elsewhere. In this case “elsewhere” could be just a foot or two above ground, versus a thermometer that’s four or five feet above ground. That’d be an easy one to test.
Could there be low clouds that hang in place overnight? Those trap warmth below them. Could they burn off at sunrise and expose you to more radiational cooling before the sun angle is sufficient to start warming up the air?
Hmmm, that’s an interesting idea. But, I would tend to dismiss it as it is REALLY arid, here
(RH has been ~5-10%, recently)
No. Arid. We have delightfully DARK skies, at night. I’d never seen a meteor shower but now make time for them regularly. I feel sorry for folks who live in places with lots of cloud cover and light pollution. Seeing a “shooting star” is probably the only experience, now, that takes my breath away (literally a gasp of surprise)
Even first thing in the morning?
Our nighttime lows (typically times when RH “appears” to increase) are ~80F.
Today, it’s about 30% but the sun hasn’t finshed its yawn, yet…