Night coldest right before sunrise?

  1. Of course night is colder than daytime in general but it seems to me that maybe half an hour before sunrise it seems to suddenly get much colder, like freezing cold. Maybe it's just my perception. Is it just that all the heat that has been absorbed during the day has dissipated by that time or is there something else going on there?
  2. jcsd
  3. russ_watters

    Staff: Mentor

    It isn't sudden, but the surface of the earth gets cooler for almost the entire night becuase the sun isn't out and it radiates.
  4. davenn

    davenn 3,884
    Science Advisor
    Gold Member
    2014 Award

    here's an interesting answer to your question, that I found on the www

    so there ya go :smile:

    berkeman likes this.
  5. I guess the suddenness is just my perception. Thx for the answers :smile:
  6. Well, that's interesting because I've said the exact same thing, verbatim, for 40 years. I have to wonder how many people are regularly up at sunrise and just sitting around watching the thermometer. I do regularly as I work from home and it is an extremely reliable phenomenon, particularly if there is no wind.

    The answers are all pretty obvious but none address our central observation. Namely, the slope of the curve changes dramatically just before sunrise. We understand about the gradual cooling, but why the acceleration in the drop- right before it goes up. If it was slow radiation away, wouldn't it take a while to recover? Yet that quick drop comes right before it starts to rise again. Suffice to say something definitely appears that way, since we've both noticed the exact same thing.

    Here's a theory. I wonder if the air pressure drops too. The sun will start to heat the atmosphere over an area before it actually rises enough to strike the ground. Maybe the increased molecular motion at higher altitudes leads to a small decrease in pressure at the ground and that leads to a faster cooling. But just for a short while because then the heating starts at the ground. Kind of far fetched, but at least it would explain what it feels like we're seeing, that the sunrise and heating actually causes the drop right before the actual sunrise at ground level.
  7. Maggnum likes this.
  8. Thanks for the link from 10 years ago - I thought it essentially answered the question.
    Maybe I missed it but the 'me' factor was not mentioned. At and following sunrise, the sun shines directly on my skin. I am directly receiving the IR from the sun, therefore I feel warm and don't really care that the air around me is still cooling. The direct affect of receiving IR from the sun is easily perceived - on a hot day (at a train station) with the sun beating down on your face and body, step into and out of the shade of a telegraph pole (or whatever shade you choose) - wow - what a difference!
  9. Dave's post is correct. It's a simple matter of the heat budget of the atmosphere. As long as heat (thermal energy) loss exceeds heat gain, temperatures will continue to drop. As soon as heat gain exceeds heat loss, temperatures start to rise.

    I do take exception with the "one hour after sunrise" statement. In tropical deserts, the nadir can be only minutes after sunrise; while at the Poles it can be a matter of days. A lot depends on cloud cover, humidity, vegetative cover, the movement of frontal systems, and the like. A nice, vague term like "shortly after sunrise" is probably more accurate.
  10. In some parts of the world (where I live being one), it is not the case that temperatures at ground level consistently fall during night, with a minimum before dawn.
    Changes of wind direction and the passage of frontal systems frequently can result in temperature rising during the night.
  11. jim mcnamara

    jim mcnamara 1,529
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    Gold Member

    An issue meteorologists local to Albuquerque TV have noted is that the that coldest temperature of the "night" is often after dawn. AFAIK nobody has given a satisfactory explanation - mostly a comment about radiative cooling changing as the upper atmosphere changes when then terminator moves off. Anybody know specifically?

    ABQ is at 5000 feet elevation with very low humidity, so radiative cooling is a big factor here - with larger diurnal temperature changes than a lot of lower, wetter places. Our temperature range today was 65F -> 96F, more than 30 degrees F.
  12. Not sure what you are asking. If you are on a flat area, the surface should warm and cool during the day/night cycle as in post 3. Even during the day there is radiation leaving the surface to space, or the upper atmosphere, which is at a lower temperature.

    With uneven ground, there is some interplay between the higher and lower surfaces, with movement of air and different radiation factors, since the surfaces will have an exchange of radiation with each other.
  13. jim mcnamara

    jim mcnamara 1,529
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    Not really asking - "wondering" more fits the bill. Is there a definitive answer to: why is the lowest temp here frequently - that means more than 50% of the time - well after sunrise - when this appears not to be the case in other areas. 'well after' means several minutes, circa 10-20 minutes.
  14. russ_watters

    Staff: Mentor

    Post #2 covers why the coldest temperature can happen after sunrise. I suspect that's a matter of sky clarity (clearer skies reflect less heat back to earth), since it isn't something I see here in Philly.
    Humidity lower down also plays a role in a place like Philly because humidity limits how cold it can get. When you start getting dew, it takes more energy to cool the air and fog (like clouds) inhibits the heat transfer.
    Last edited: Jun 30, 2015
  15. It certainly points up the fact that few temperature records are kept with short enough intervals to test the hypothesis. My experience was based on what the thermometer says, so there wouldn't be a "me factor".

    When I worked in Utrecht there was a sign on the wall that said, "Hier hebben we geen ruzie over dingen, gaan we berekenen het antwoord" (Here, we don't argue about things; we go compute the answer). It's disappointing that this could be debated for 10 years without anyone doing that.
  16. A friend just happened to randomly ask me essentially this same question this morning after coming back from an early morning run.

    [For simplification of course, I am ignoring all cases that involve the effects of wind/weather fronts, rain, cloud cover etc. and focusing on your "ideal" no wind, no cloud cover situation]

    The question itself can be a little misleading depends how it is phrased however. Compare the subtle differences in how people can essentially be asking the same question:

    "Why does it get colder when the sun begins to rise?" kind of sounds like the sun rising itself is making things/causing things to become colder and this is what needs to be explained, which is not the case.

    Whereas rephrasing the question as "Why does it still get colder when the sun begins to rise?" is a much better (and accurate) way of phrasing the question and indeed the actual observation.

    Now the other thread titled "Temperature begins to fall at dawn" can itself also be considered a poorly phrased "question" with the OP asking "I heard somewhere that temperature actually begins to fall at dawn and then goes up later on. Why?" The use of the word "begins" sounds like they are saying that up until sunrise, the temperature was steady, but when sunrise hit, the temperature began to drop and they want to know why. Well that may indeed have been an actual observation but I doubt it was one that was devoid of wind/weather front effects (convective), possibly also combined with cloud cover effects. However, from reading the OPs later posts I am sure this observation was not what they were asking about.

    Semantics aside, I think we can agree that everyone has been asking the same question/trying to understand the same phenomena (one that does not involve any wind/weather front or cloud cover effects) and I believe adequate explanations have been given.

    I will however propose an alternate way to explain/look at this situation. Radiant heat transfer is difficult to "see" so I attempt to construct an analogy to perhaps provide a better "visualization" for what is going on.

    Consider the universe like a very powerful vacuum cleaner sucking heat from the earth. Consider the sun like a very powerful hairdryer blowing heat towards the earth (Yes, ignore the fact that convective heat transfer is NOT primarily responsible for how the sun heats the earth and how the universe cools the earth). At night, the sun is obscured and it is just the universe continuously "sucking" heat away from earth, making the temperature continually drop.

    There are other "suns" in the sky at night however, we call them stars. They too are like our sun, a hairdryer blowing heat towards us but they are so far away their power is nothing compared to the power of the universe sucking heat away from the earth, or to the power of the sun! Now consider what happens when the sun begins to rise. It's like gradually putting more stars in the sky (or more hairdryers pointing towards the earth, or starting to reveal a very powerful heat blowing hairdryer). Would you think that it makes sense to assume that as soon as you catch a glimpse of the sun that the temperature on earth should immediately drop? Of course not. When you think of it like I have explained, it seems silly to expect that instantaneously the earth should start getting warmer. You need to reveal enough of the sun so that the power (rate) of heat being blown towards the earth just is enough to counteract the universe's opposite (rate) effect of trying to suck heat away from the earth. For earth, this apparently happens shortly after sunrise. This is essentially the turning point at which the rate of heat transferred to the earth exceeds the rate at which it is being sucked away by the universe.

    Conversely, understanding why temperatures are hottest not at peak sun but a few hours after can also be used by this analogy and the subsequent net direction of heat transfer at any given point in time.

    Curiously, a better exploration for what is going on here can be had by considering "what if" scenarios. For example, what if the sun "stopped moving" in the sky when the temperature in the morning just started to begin to increase? Would the temperature also stop increasing?

    This question itself makes it apparent that an understanding of "steady state" thermal systems (the concept of thermal equilibrium and time constants) is not only important to answer this new question, but to also answer the original question in more detail, which is in fact a much more complex/dynamic thermal situation to fully explain.

    Answering seemingly more simple "what if" scenarios can go a long way to building an understanding on how to answer or approach answering a more complex question.

    eg. How would temperatures on earth vary if the earth actually rotated faster/slower than it does?

    This great video is a perfect example:
    Last edited: Jul 7, 2015
  17. D H

    Staff: Mentor

    Humidity also plays a role higher up by increasing the effective temperature of the night sky, which in turn reduces the cooling rate at the surface. And of course if there are clouds, it's game over. Cooling is much diminished with a cloudy sky as opposed to a clear sky.

    Close but not quite. The upper atmosphere radiates heat to space, whose effective temperature is about 2.73 kelvin. The surface radiates head to the atmosphere. The only way to see the universe in the thermal IR band is to get above the atmosphere. The effective temperature of the night sky as viewed from the ground is much closer to 273 kelvin than 2.73 kelvin, even in the high desert.


    It's a simple matter of rates.

    Temperature rises if the net rate of thermal energy transfer from the surface is positive, falls if the energy transfer rate is negative. The amount of heat received from the Sun is small at sunrise, thanks to the obliqueness of the incoming solar radiation and to the very high air mass at sunrise. The amount of heat radiated skyward depends on the effective temperature of the sky, which can be rather low in desert areas such as where you live.

    The rising sun does three things:
    - The cross section to solar radiation increases with time, thanks to decreased obliqueness of the incoming solar radiation,
    - The amount of radiation increases with time, thanks to solar radiation having to penetrate less and less atmosphere (the "air mass"), and
    - The atmosphere itself warms up, thereby decreasing the net radiative transfer from the ground.
  18. I understand that things are more complex than my analogy (my analogy was really targeted at non-scientific readers), but I would still hold that it is fundamentally consistent with understanding/visualizing how this thermal system works from a macro level. The fact that you point out that the earths surface radiates heat to the atmosphere, which then radiates that heat to space does not break the basic analogous understanding of space being like a heat vacuum cleaner continuously "sucking" heat from the surface of the earth. Earth's surface heat eventually gets "sucked" away in to space, albeit being passed through to the atmosphere first.

    I was unaware of the actual temperatures you mention. Is it just coincidental that the two temperature you mention are exactly at a 1:100 ratio? Is it a coincidence?
  19. Well, many years ago my physics teacher taught that because the morning air is very moist, due the dropping temperature over the night reducing the moist carrying capacity of the air, but much of this moisture is in a transition zone - moving between liquid and vapor in the close to supersaturated air. Then, the first rays are sufficient to kick the air from the transition zone to vapor. However, this triggers a refrigeration effect because the radiation gain is insufficient to maintain the vapor state and air must draw in heat to maintain the vapor state. The theory that the morning air is the coldest because it has been continuously dropping overnight does not explain what is observed: that the temperature drops significantly when the first sun rays arrive and that it does not continue a gradual drop.
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