Night coldest right before sunrise?

[fourier jr]

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?

 

[russ_watters]

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.

 

[davenn]

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

http://www.weathernotebook.org/transcrip... old but erroneous proverb states that: it’s always darkest just before the dawn. A related belief contends it is always coldest at the same time of day. Hi, I’m Bryan Yeaton for The Weather Notebook.

It seems logical to think that air temperature bottoms out just before sunrise and then begins to warm with dawn’s early light. Observations and physical theory, however, show the coldest time of day generally occurs some time after sunrise.

To simplify why, let’s only consider a clear night, with no passing fronts.

All objects gain heat from outside sources and radiate it away at the same time. When more radiant heat is lost than gained, the object cools. When more heat is gained than lost, it warms. If they are balanced, the temperature remains constant. Okay… that’s pretty basic.

Between sunset and sunrise, the Earth’s surface gathers no solar energy but continues to radiate away its stored heat. During the night, the surface also loses radiant heat faster than it steals heat from other sources, and thus its temperature, and that of the air in contact with it, drops steadily.

At dawn, when the first light beams across the landscape, the incoming solar radiation is very weak. It does not yet have enough strength to counter all the heat escaping from the surface. As a result, the surface continues to lose heat for some time following sunrise, and the air temperature continues to fall.

At some point, the solar rays shine strongly enough to counter the heat loss. The gain-loss balance is shifted, and the air finally begins to warm up. As a rule of thumb: the coldest temperature is about an hour after sunrise.

so there you go

Dave

 

[fourier jr]

I guess the suddenness is just my perception. Thx for the answers

 

[DrJohnSmith]

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.

 

[johnbbahm]

I think this was addressed well in this very forum about 10 years ago.
https://www.physicsforums.com/showthread.php?t=7408

 

[Marcus_B]

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!

 

[klimatos]

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



so there you go

Dave

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.

 

[rootone]

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.

 

[jim mcnamara]

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.

 

[256bits]

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.

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.

 

[jim mcnamara]

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.

 

[russ_watters]

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.

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.
[edit]
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.

 

[DrJohnSmith]

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.

 

[FreeBodyDiag]

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:

 

[D H]

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.

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.

Consider the universe like a very powerful vacuum cleaner sucking heat from the earth.

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.

----

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.

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.

 

[FreeBodyDiag]

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.

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.

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 Earth's 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 into 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?

 

[martin80729236]

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.

 

[Mike Dunn]

I often play golf at dawn and I always seem to notice that as soon as the Sun breaks over the horizon the temperature drops for about 10 minutes is pretty basic heat rises. So when the sun peeks over the horizon and blankets the surface. The cold air compresses which drops the temperature and gradually the heat of the sunlight ivertakes the cold and begins to warm up. This is only when there's no wind or clouds. My theory. Very general. But makes sence.

 

[krater]

Living in some terrain, continental river valleys for example which I have spent most of my life living in will challenge every common-sense theory one might have about surface level atmosphere conditions. Things like humidity become centrally important at times, very interesting stuff to witness.

 

[Norman Hibberd]

In the early dawn, the ground is usually covered with dew. As the dawn approaches, evaporative cooling sets in and the temperature drops quite rapidly until the night's moisture has evaporated, then warming steps up as the day sets in.
I'm often outside and at work before dawn and although cloud over, humidity etc does have an effect, the above generalisation holds true.

 

[Baluncore]

As the dawn approaches, evaporative cooling sets in and the temperature drops quite rapidly until the night's moisture has evaporated,

That does seem illogical, because there is no energy available to evaporate dew until shortly after sunrise. The cold air has less water because it has dropped moisture as dew. Until the solar energy input, insolation, exceeds radiative losses, the condensation process cannot be reversed.

We can consider three phases occurring during the night.
During the first part of the night the air begins to cool and so quickly approaches the “dew point” temperature where the Relative Humidity is 100%. At that point the game changes.

During the second part of the night, the temperature of the air continues to fall, but it maintains 100% RH by progressively precipitating dew or fog. The rate of temperature fall is slowed because condensation of water releases energy that counters the fall in temperature. As water is progressively lost the rate of temperature fall increases. If the night is long enough this phase blends into part three, as the temperature falls below 0°C.

During the third part of the night, the cold air has progressively less moisture remaining so the temperature drops with ever increasing rapidity. Below –20° there is no water remaining to hold back the temperature fall.

The maximum rate of cooling will therefore be in a very short period just after sunrise. The coldest temperature will be later, at the instant insolation counters radiative cooling.

 

[my2cts]

Check yr.no for your local temperature prediction on hourly basis.
It is a 100% noncommercial site.

 

[wsnell]

While observing the weather at the NWS in Montgomery, AL I noticed some of the same things that the commenters have noticed. That is that the temperatures dropped abruptly 2 to 5 degrees Fahrenheit just before or around dawn. While most have pointed to the radiative effect and this did occur on clear nights only, it does not explain the abrupt drop described by the original questioner. What could explain the abrupt drop is that the observation sensors (or persons) were in a swale or local valley but not at the lowest portion. These valleys filled with cold air sinking from the surrounding terrain which was cooled by radiation and when the coldest air filled the area sufficiently, the much cooler air reached the sensor (or person) and the temperature dropped abruptly in as little as 10 minutes. Hence the (true) sensation that it was much cooler just before sunrise and that it had dropped much more quickly than during the rest of the night.

 

[wsnell]

While I was an observer with the NWS in Montgomery, AL I noticed the same thing as the original questioner. That is, that on clear nights the temperature at our sensor would drop abruptly anywhere from 2 to 5 degrees Fahrenheit just before or around dawn. We surmised that the reason why this was happening was that our sensor was located in a swale but not at the bottom. Thus when the terrain around the area cooled the colder air sank into the deepest part of the valley first, allowing the higher part to cool at the normal nightly rate. But then, as it filled, the colder air reached the sensor causing an abrupt drop in the temperature. This could be what he was also noticing.

 

[FactChecker]

I think that the link https://www.physicsforums.com/threads/temperature-begins-to-fall-at-dawn.7408/ given by @johnbbahm in post #6 may explain a lot. The latent heat of liquification and of evaporation is very significant and would pull the air temperature down dramatically as frost melts and evaporates.

 

[Computmensch]

Hi

I suddenly quickly got back to this question wishing for an answer for may be 15 years. Right now, sitting this morning in DK - rather hot night here, 24 degrees in living room, have a fan on living just under the roof in an apartment. However, sun just started coming up a little while ago. Its not hot anymore but cold. Have to shut the fan off.

Would have imagined a well-known answer was already available? Apparently not. This should be researched!

I am a programmer and software developer. I have done "late life" for years. Now older rarely do it. But this evening fell asleep at 18.00 and woke up 23.30. So early up )

Anyway, I have experienced and paid particular noticed to this phenomenon for over 15 years.

IT IS NOT GRADUAL. Its more like the heat is blown away when the sun comes up - i.e. cold after sunrise.

I can say that for sure. I may be experienced it over 1000 times over 15 years. I recall at one point I would even have my clothes ready if windows were open in the summer.

More interesting and similar things happen when the sun comes up.

I recall once in late july around Darlana (Salen) in Sweden about 600 km north of Gotenburg. I was in a cabin at the slope of a small mountain. Working for Novell R&D I had taken some work there. While my family was sleeping I was working nights. Then getting up after lunch ... etc. So I started working at may be 21.00-200 until about 2.00-3.00 (visible sunrise around here).

I.e. the sunrise would stop me. I would go for a walk. Looking down at the forest and the river hidden inside the forest, I would not only experience the heat blown away but also the steam coming up from the river over the top of the trees in forest some kilometers away. Looked "cool". Not simply fog ... but cooled smoke like the dense low hanging fog coming out of "cold fog machines"

Anyway, let's keep looking for an answers. No theories here satisfy my experiences or practical life. What happens.

The basic thing: Any heat whatever the temperature - i.e. even indian summers ... the heat is blown away. It will change from warm to cold.

I even tried it a few times sitting on a bench in indian summer (i.e. 20 degress celcius) and it got cold when the sun started to rise ... (did not measure the temperature but got back to getting stunned and amazed by the "cold sunrise" again ...)

 

[Evo]

Hi

I suddenly quickly got back to this question wishing for an answer for may be 15 years. Right now, sitting this morning in DK - rather hot night here, 24 degrees in living room, have a fan on living just under the roof in an apartment. However, sun just started coming up a little while ago. Its not hot anymore but cold. Have to shut the fan off.

Would have imagined a well-known answer was already available? Apparently not. This should be researched!

I am a programmer and software developer. I have done "late life" for years. Now older rarely do it. But this evening fell asleep at 18.00 and woke up 23.30. So early up )

Anyway, I have experienced and paid particular noticed to this phenomenon for over 15 years.

IT IS NOT GRADUAL. Its more like the heat is blown away when the sun comes up - i.e. cold after sunrise.

I can say that for sure. I may be experienced it over 1000 times over 15 years. I recall at one point I would even have my clothes ready if windows were open in the summer.

More interesting and similar things happen when the sun comes up.

I recall once in late july around Darlana (Salen) in Sweden about 600 km north of Gotenburg. I was in a cabin at the slope of a small mountain. Working for Novell R&D I had taken some work there. While my family was sleeping I was working nights. Then getting up after lunch ... etc. So I started working at may be 21.00-200 until about 2.00-3.00 (visible sunrise around here).

I.e. the sunrise would stop me. I would go for a walk. Looking down at the forest and the river hidden inside the forest, I would not only experience the heat blown away but also the steam coming up from the river over the top of the trees in forest some kilometers away. Looked "cool". Not simply fog ... but cooled smoke like the dense low hanging fog coming out of "cold fog machines"

Anyway, let's keep looking for an answers. No theories here satisfy my experiences or practical life. What happens.

The basic thing: Any heat whatever the temperature - i.e. even indian summers ... the heat is blown away. It will change from warm to cold.

I even tried it a few times sitting on a bench in indian summer (i.e. 20 degress celcius) and it got cold when the sun started to rise ... (did not measure the temperature but got back to getting stunned and amazed by the "cold sunrise" again ...)

Just before dawn, your area has been in the darkest for the longest, so is likely to be the coldest, being without sun for the longest, now things like clouds, a front moving in with high winds, these kind of things can change that. Use some common sense.

A nice explanation. Which was post #3 of this thread! Thread closed.

Night coldest right before sunrise?