Why does the temperture get colder just before dawn?

Question

Why does the temperture get colder just before dawn?

Answer 1

Part of the reason is that just at the point the sun is heating the surface it produces a slight increase in air pressure, it produces a small shock wave. This may have the effect of reducing the temperature locally by a degree or two.

Answer 2

I don't think that answer is correct.

To get a half degree C decrease in pressure by that abiabatic process, you need to reduce the pressure by about 6 hPa. And nothing like that happens just before dawn. The first part of the answer in the reference is correct.

"First, just before sunrise the ground has had the maximum time to cool.  So the air would have had the maximum time to cool also."

The second part is rubbish. I am staggered that it appears on a US Government web site. I wonder how long it's been there. I'm going to write a letter...

I think the question is a bit misleading. The air temperature is often lowest just before dawn. That is not the same as the air "getting colder" just before dawn.

Answer 3

The earth's surface is heated by the solar radiation during day time and the earth starts giving out this heat to space once the sun sets in the evening.This cooling of the earth continues throughout the night till the rise of the sun next morning.So the coldest time of the earth's surface occurs just before or at the time of sunrise.This is the reason why the lowest temperature is recorded just before or at the time of sunrise.

Answer 4

Ross' math is correct. The dry adiabatic cooling is one degree Celcius per 100 meters. Half a degree would then be 50 meters. At sea level, the pressure sinks by one hPa per 8 meters. So, 6 hPa would be roughly 50 meters. Correct!

I also agree with him that a fall of 6 hPa is very much and unlikely to happen at dawn.

Seafarers have this thumb rule: If the pressure drops 10 hPa in 8 hours, prepare yourself for a gale wind!

I fly my little aircaft across Europe and I am sometimes staying up to four hours in the air. Wherever I go,  I get QNH (pressure at sea level) to calibrate my altimeter and that ground control uses the same data as me to separate me from other traffic. During all these years, even flying four hours over a long distance, from high pressure to low; I have never registered a fall or rise of the pressure greater than say, 3 hPa.

I think the first part of website answer is correct; we all agree to that. But the second seems strange.

During high pressure condition, with clear sky, the heat from the earth's surface will radiate into space. The air cools down on the surface and the pressure slightly increases as cooler air is denser. When that happens near a coast, a offhsore breeze is generated: the air moves from land to sea where the surface temperature of the water remains pretty much the same

During day time, the cycle is inverted as air warms up over land, decreasing slightly the pressure and creating an onshore breeze, convection and nice cumulus clouds over land.

At the very moment of dawn, I don't see how the sun's rays can do anything to the air. Air is not warmed up by the sun but by the earth surface that is, itself, warmed up by the sun.

In a mountainous region, at dawn, you will experience this, for example: During the night, the cold air will sink down in the valleys. As the sun's first rays warm up the mountain tops, it creates an inversion: Warm air on top of cold air. If there is some humidity down in the valleys - as often where there are lakes and rivers, fog will form and stay down in the valleys until the sun is high enough to pierce through it, warm up the ground and dissolve the fog.