View Full Version : the dawn dip

03-03-2008, 10:00 AM
My question has nothing to do with machining, but I pose it here anyway, hoping some of you guys may have noticed a strange natural phenomenon, and can offer an explanation.

It's the noticeable fact that the ambient air temperature, at least on cold winter mornings, actually drops when the sun rises above the horizon. When camping I have noticed this drop can be as much 5F˚, and this temperature dip can last for fifteen minutes or so. It's counter-intuitive. I noticed again this morning as I walked to the supermarket to get my morning newspaper.

Don't bother Googling "dawn dip" -- last time I tried that all I got was links to porn.

03-03-2008, 10:20 AM
On calm nights the air will stratify with warmer air near the surface. As soon as solar energy starts to be absorbed in the upper layers the atmosphere becomes much more turbulent, a phenomena easily observed when using a telescope in the early morning near sunrise. This causes the lower atmosphere to begin mixing allowing cooler air to reach the surface. The opposite is also seen in the development of winds that are caused by rising or especially falling air on large mountain sides called adiabatic winds, aka foehn winds, where cooler air falls to lower altitudes but is then warmed by compression of the atmospheric pressure.

Norman Atkinson
03-03-2008, 10:26 AM
Temperature Inversion?

Any physical geography types? You know, cold air coming down from hill tops.
If you live in the mountains, you see the clouds literally coming down on a still day.

Cairngorms and French Alps sort of things?



03-03-2008, 11:39 AM
Evan's description pretty well sums up what I have always learned, and yes at times it can be quite pronounced.

For another strange phenomena, lookup super cooled water. This happened to me on numerous occasions this winter. Quite often this winter as I worked in the shop I would put out several bottles of water, for a cool drink when I needed it later. When I returned to the water 4 or 5 hours later I would expect the plastic bottles of water to be completely frozen. But just about every time the water was liquid with no visible ice. As soon as you grabbed the bottle to pour out the water it would freeze in the neck, or if shaken, it would freeze the entire bottle solid.

The water was about 10-12 degrees below the freezing point of water at the time. Apparently under ideal conditions one can get water up to 40 degrees below freezing before ice will form.

03-03-2008, 12:03 PM
That phenomenom ("dawn dip") will be most pronounced on a dead calm morning (or nearly so).
The optimum condition for radiational cooling is NOT dead calm air, but a very slight breeze. It is not the air that's radiating heat into space, but the earth's surface. The thin layer of air in contact with earth then looses heat to the ground thru conduction, the ground in turn looses that heat thru radiation.
So in dead calm conditions only the lowest few centimeters of air can loose much heat, and the heat loss process (conduction) slows toward sunrise as the ground and lowest layers of air approach thermal equilibrium.

But with sunrise, and the resulting "stirring and mixing" it causes, the radiational heat loss can work its way up thru a thicker layer, for a brief period until offset by the heating of the ground by the solar insolation.

Almost all of the heat loss or gain within the air comes not from the sun, but from the earth. i.e. sun heats earth, earth heats air.

03-03-2008, 02:58 PM
Water doesn't behave in the way that other chemical compounds do. It is highly anomalous. For a good page on this see http://www.lsbu.ac.uk/water/anmlies.html


The air is only transparent in the visible spectrum. In the infrared it has much greater absorption to the point of being nearly opaque at some wavelengths. That is what constitutes the greenhouse effect. The night sky, which is space, on a clear night has an intrinsic temperature of about 3 kelvin, almost absolute zero. The atmosphere at night moderates that by radiating in the infrared so that the effective sky temperature on a crystal clear night is only about -70 or so. If it weren't for the infrared absorption and emission of the air this planet would be a solid block of ice.

03-03-2008, 03:57 PM
Not sure what you're meaning by "intrinsic" temperature. But the sensible temperature of the air, above the ground effects (ie. surface inversion) doesn't vary appreciably from day to night. Rawinsonde soundings launched from a given location in the middle of the day and at midnight, or any other time during the night will show essentially the same thermal structure (and moisture), aside from any differences due to advection (wind) at any particular level. Excluding of course the lowest couple of thousand feet.

The greenhouse effect is due to the water vapor content - not air. That's why it's called greenhouse. That's why it cools off a lot more on a clear night, and hardly at all under a heavy overcast.. the clouds/water vapor, when present, absorb the frequencies radiated by earth and re-radiate it back down.

I'm sure some frequencies are absorbed by pure, dry air, but in the main most of our atmospheric heat is first absorbed from the sun by earth which in turn heats the atmosphere.

As for IR, as I recall those ceiling IR heaters that became popular during the energy crunch in the 70's did NOT heat the air in the room, but rather the occupants and objects within. Of course I imagine the spectrum was selected with that in mind.

03-03-2008, 04:31 PM
Space acts as an infinite black body at a temperature of 2.7279 Kelvin. That is the intrinsic temperature of space which is the left over heat of the big bang.

The greenhouse effect is due to the water vapor content - not air.
Water vapor is one of the gases in what we call air, not just oxygen and nitrogen. And, as you say, the content doesn't vary appreciably from day to night. The water vapor is a major absorber of thermal radiation but much of that heat is nearly immediately kinetically transferred to the other constituent gases as well which are poor radiators since they are poor absorbers. Regardless of how you define "air" it is a major moderator of the night time temperature in the absence of cloud cover.

A cloud is not composed of water vapor but is mainly droplets of liquid water. It is a poor absorber and a cloud only absorbs about 2% of incident radiation. For that reason it keeps the night temperatures much higher when it is cloudy at night. The surface of the earth only absorbs about 65% of the incident radiation and the atmosphere about 16%. Water vapor accounts for about half of that 16% atmospheric absorbtion with other greenhouse gasses accounting for the rest. On a cloudy day the atmosphere actually absorbs more because the incident radiation is making two trips throught the atmosphere since the clouds reflect about 60 percent and transmit about 25 percent with the remainder absorbed by water vapor in the cloud or scattered diffusely.

The surface of the earth reradiates exactly as much as it absorbs. If it didn't we would fry.

03-03-2008, 04:51 PM
Two Langleys per day. ...give or take.

Forrest Addy
03-03-2008, 05:22 PM
I guse I can't contribute much. When I look into the night sky at the stars I think "Hey! That looks like a fishie! And there's a doggy!" Not in the asme class at all.

We all better listen to Evan. I think he's got the Injun sign on this kinda stuff.

03-03-2008, 05:48 PM
LOL Forest!!! I always knew it happened. Now I know why :)

Forest...your looking at the wrong part of the sky....I see blonds and redheads where I look :D