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View Full Version : Why does it get colder at sunrise?


TXJon
01-01-2008, 08:36 PM
Seems like I can be comfortable before sunrise, but right before shooting light the temperature drops 15*. It makes no sense, why does this happen?

Tmag
01-01-2008, 08:38 PM
Oh Johnny?

idratherbhuntin
01-01-2008, 08:45 PM
Sun heats the earth. Just before sunrise is the longest point the earth has been without sun (heat) since dusk the day before. Ie... it is the coldest part of a TYPICAL day that does not have a cold font coming in.

BTW... the shortest distance between two points is a straight line.

Lulhorse
01-01-2008, 08:45 PM
Thats like asking why the Denver Broncos suck. I dunno!

Rat
01-01-2008, 08:53 PM
idrathebehuntin is right, it has to do with emitted radiation. All day the Earth soaks up radiation and emits it, (that's why it is hottest around 4:00 pm not 12:00 Noon) just before sunrise is the longest period without a 'recharge' so the emitted radiation is low, so it gets colder.

Dew point is usually hit pretty close to this time as well, again, due to the cooling, and the rising dew causes temperatues to feel colder. The more humid the air the colder it feels. water is eight times as dense as air and even small spikes in humidity cause us to loose heat faster (feels colder).

Chasin Elk
01-01-2008, 08:56 PM
Thats like asking why the Denver Broncos suck. I dunno!

I do his initials are MS:D :D :D

Higee
01-01-2008, 08:56 PM
How soon do you get to your stand before sunrise?

How much are you exterting yourself getting to the stand. Do you think the sweat is causing you to get colder as your waiting for the sun to come up?

Lulhorse
01-01-2008, 08:57 PM
Ok then why do the Boncos suck?

Lulhorse
01-01-2008, 08:58 PM
Ok thanks so they should ***** can Shanahan!
Actually saw that on Tshirts they were peddling at the game Sunday

cantexduck
01-01-2008, 08:59 PM
Who cares about the Broncos?

Good question Jon. I have always wantted to know the answer to that one!!!

Chasin Elk
01-01-2008, 09:07 PM
Its been a long run for Mike. Hes a good coach, but its time for a change. A new coach tends to be very motivated to be succesful in his own way. With a new team as far as players now would be the time to make this change.

Buglemintoday
01-01-2008, 09:10 PM
I think I am going to get one of those buddy heaters for the blind for next season :D

TXJon
01-01-2008, 09:12 PM
I don't buy the "coldest part of the day" theory. Sure, it's the coldest part of the day, that doesn't explain a sudden temperature drop.

The dew point/humidity spike is interesting. Even on cold, dry mornings I get that sudden cold, just at first light. Would that mean that stuff is getting warmer and releasing moisture?

It's not cooling sweat. I go in lightly dressed, and bundle up on stand. I can be comfortable in the dark for 45 minutes or so, then it gets light, and ****!

Bobcat
01-01-2008, 09:13 PM
Wondered the same thing.
Considered the same things discussed regarding legth of time since the air has been heated.
I also wonder if the air over the horizon that is being heated causes the air in front of it to be pushed causing the slight breeze just before sunrise. The cooled air combined with a slight breeze increases the windchill?

Smart
01-01-2008, 09:17 PM
I don't buy the "coldest part of the day" theory. Sure, it's the coldest part of the day, that doesn't explain a sudden temperature drop.



Often the temperature does fall for a short time after sunrise, though it is only a degree or two at most. And even if the temperature doesn’t fall as the sun is rising, the air may actually feel colder to you. That is because we are used to it being colder when it’s dark and warmer when it’s light. So as the sky begins to brighten in the morning and the temperature remains nearly the same, we might perceive that it is colder than it actually is. Also, wind generally increases during the day. The air sometimes begins to stir just after sunrise, and an increase in wind would make us feel colder.

If there are no fronts or precipitation nearby, the daily temperature cycle is primarily controlled by the radiation budget. This is a comparison between the incoming radiation from the sun (sunlight) and the terrestrial radiation given off by the earth’s surface (felt as heat.) Think of the sunlight shining down on earth as the same as putting pennies into a jar. As long as you keep putting pennies in, the money adds up. In the same way, as long as the sun is shining down on earth, the amount of radiation adds up. Let’s say that at some point you decide to stop depositing pennies into the jar and begin to take them out. Even though you’re withdrawing pennies, you still may have a lot of money in the jar. Likewise, when the sun goes down, the incoming radiation from the sun stops, but there is still a lot of radiation that has been absorbed by the earth, so we still feel heat near the earth even after the sun goes down. At night, the “withdrawals” of terrestrial radiation continue, and the ground and the air near it cool. Earth’s surface is typically in radiation “debt” from a couple of hours before sunset to near sunrise. When the sun comes up and the “deposits” of incoming radiation from the sun equalize with the “withdrawals” of radiation from earth, we get the coolest temperature of the day. Sometimes in winter when the sun is low in the sky, the earth’s surface can remain in radiation debt longer, and the coldest temperature of the day can actually occur as much as an hour after sunrise. As the sun gets higher in the sky, earth’s surface is in radiation surplus (the deposits exceed withdrawals), so the ground and the air near it warm.

Shane
01-01-2008, 09:26 PM
I've often experienced a noticable drop in temp right at or just after sunrise. It seems to be more noticable on clear days. I've always assumed the air was turning over, like a lake will do when temps change. If the sunrise warms the air, it will rise. Cold air would be pulled in underneath, if that happens. Seems like the "line" of where the turnover happens is the "sunrise line" as it moves across.

Just speculatin'.....

Sika
01-01-2008, 09:38 PM
I always thought air felt coldest at sunrise because at sunrise, as the air warms, the relative humidity drops and it becomes dryer.

Since moist air holds more heat than dry air, and the air is getting dryer, the it suddenly gets colder.

Tx_timber_rattler
01-01-2008, 09:45 PM
the wind?

Flash
01-01-2008, 09:50 PM
Smart... where the heck did all that come from?:D

Sika
01-01-2008, 09:55 PM
The air sometimes begins to stir just after sunrise, and an increase in wind would make us feel colder.


I always feel colder when a slight breeze begins to build just after sunrise. When it comes to staying warm, wind is the enemy! :mad:

shaft
01-01-2008, 09:58 PM
does that mean that the darkest hour is just before dawn?

Johnny
01-01-2008, 10:04 PM
I agree with what everyone said.:D ;)

DavisHollow
01-01-2008, 10:05 PM
Ok, I have a digital thermometer, and this happened to me last week hunting. It was 28 just before the sun came up, a good 15-20 minutes after sunrise it had dropped to 25. Calm, zero wind, no clouds. Temp was stable at 28 long before sunrise.

There was frost on the ground, so likely dew point was also 28, when tends to keep the temp from dropping further on calm nights since as heat dissipates, sublimation keeps the temp stable until the moisture is out of the air.

I'm sure the answer is related to the moisture content in the air, just not sure how the moisture content decreases at sunrise.

12ring
01-01-2008, 10:09 PM
Dang,

I feel "smarter" just reading all this! :)

Lungbustr
01-01-2008, 10:18 PM
Ive been told that as the sun is rising the heat forces colder air from higher up in the atmosphere down. Dont know how true it is, but i do know it does get colder right as the suns coming up.

PEC
01-01-2008, 10:27 PM
Even on what seams to be crystal clear nights there are some atmospheric blanks that are burned off as the sun begins to rise. This allows for the most radiational cooling which for a short period allows for a sudden drop in temp before the sun begins to reheat things. Which as, Shane, mentioned is why it is more noticable on clear nights or mornings should I say.

topduarte
01-01-2008, 10:29 PM
Seems like I can be comfortable before sunrise, but right before shooting light the temperature drops 15*. It makes no sense, why does this happen?

I noticed the same thing New Eve's morning!!

Got there at 6:15 am and when the sun came up at 7:28 am it felt colder for about 25 minutes!!

DavisHollow
01-01-2008, 10:38 PM
Even on what seams to be crystal clear nights there are some atmospheric blanks that are burned off as the sun begins to rise. This allows for the most radiational cooling which for a short period allows for a sudden drop in temp before the sun begins to reheat things. Which as, Shane, mentioned is why it is more noticable on clear nights or mornings should I say.

I'll bite, what is an "atmospheric blank"?

PEC
01-01-2008, 10:39 PM
blankets....sorry

Cropduster
01-01-2008, 11:08 PM
It is related to the angle of the suns rays to the point on the earth that you are at any given time. At or just before dawn the distance from the sun through the atmosphere is much greater than at any other time during the day or night, therefore less heat is being contained in your particular location. Oh yes and the temperature cannot fall below the dew point.

Tx_timber_rattler
01-01-2008, 11:11 PM
Somethings stirring in the air... Mr Frost is till here. You need to warm his heart.

Green Arrow
01-01-2008, 11:17 PM
I cant tell you I havent hunted in the morning in 4 years.

PEC
01-02-2008, 12:33 AM
Oh yes and the temperature cannot fall below the dew point.

The temp falls below the dew point almost everyday. The water will not condensate until the temp fall below the dew point. When them temp and dew point are equal the relative humidity it @ 100%. The temp has to go below that point for the air to start to condensate to get rid of some of the water.

delriowil
01-02-2008, 03:33 AM
I understood what Smart was saying. It has to do with radiation heat. The same as when it gets colder on clear nights and not as cold on cloudy nights.

txairbear
01-02-2008, 06:51 AM
Smart I heard that pennies in the jar thing was related to marriage:)

Brute Killer
01-02-2008, 11:39 AM
.

The temp falls below the dew point almost everyday. The water will not condensate until the temp fall below the dew point. When them temp and dew point are equal the relative humidity it @ 100%. The temp has to go below that point for the air to start to condensate to get rid of some of the water.

Water vapor in air condenses when the dewpoint temperature and the air temperature are equal. This is what causes fog.

Water vapor also condenses on solid objects when the surface temperature of the object and the dewpoint temperature are equal or if the surface temp is below the dewpoint temp. This is why you cold beer sweats on a summer day. The dewpoint temp of the air is considerably higher, on average, during the summer. Warmer air can contain more moisture than cooler air (because the cooler air causes the moisure to condense).

I would have a difficult time believing that air temperature can ever drop below dewpoint temperature. Object surface temp, but not air temp (unless a controlled atmosphere and then only very briefly). We trend both air temp and dewpoint temp where I work, because both temperatures have a big influence on part of my job, and I have never seen the dewpoint temp above the air temp, on the trend.

As for the original question, yes the temp drops just before sunrise and I have winessed a drop of 5 degrees or more very many times. But I don't know why this is.

I don't buy the "length of time out of sun's radiation" explaination, because the tepm doesn't just reach it's lowest point at this time, it actually drops a few to several degrees in a short time.

coop2564
01-02-2008, 12:07 PM
Here's a weatherman's version.
http://www.komotv.com/weather/asksteve/4346931.html

SEATTLE - One reader noticed that sometimes the temperature seemed to drop a couple of degrees right after sunrise.
On clear and calm nights, the ground radiates its absorbed heat back into the atmosphere, allowing temperatures to cool at night. But since cold air is heavier that warm air, the cold air will pool right near the ground.

In this case, in the moments before sunrise, it might be 40 degrees at your head but 36 degrees at your toes. When the sun comes up, the ground starts warming.

As that warm air rises, will begin to mix the colder air near the surface with the warmer air above -- thus making it warmer at your foot but cooler at your head (perhaps now it's 38 at both your head and your foot).

That would only happen on nights without much wind and generally clear skies. Otherwise, wind will keep that layer mixed up and cloudy skies don't allow much heat to radiate back.

TXJon
01-02-2008, 12:17 PM
That's why my feet get cold first?;)

I didn't realize this would go so long. I figured somebody out there would just know the answer.:)

I went out this morning. It got colder right at first light. Maybe I'll just have to live with the mystery.

q2xlman
01-02-2008, 12:23 PM
Dang,

I feel "smarter" just reading all this! :)

Me too!

Shane
01-02-2008, 12:32 PM
I think the turnover of colder/warmer air masses happens on a little larger scale that head-to-toe height, because you can feel it in an elevated blind or tree stand. :)

DWG
01-02-2008, 12:51 PM
Water vapor in air condenses when the dewpoint temperature and the air temperature are equal. This is what causes fog.

Water vapor also condenses on solid objects when the surface temperature of the object and the dewpoint temperature are equal or if the surface temp is below the dewpoint temp. This is why you cold beer sweats on a summer day. The dewpoint temp of the air is considerably higher, on average, during the summer. Warmer air can contain more moisture than cooler air (because the cooler air causes the moisure to condense).

I would have a difficult time believing that air temperature can ever drop below dewpoint temperature. Object surface temp, but not air temp (unless a controlled atmosphere and then only very briefly). We trend both air temp and dewpoint temp where I work, because both temperatures have a big influence on part of my job, and I have never seen the dewpoint temp above the air temp, on the trend.

As for the original question, yes the temp drops just before sunrise and I have winessed a drop of 5 degrees or more very many times. But I don't know why this is.

I don't buy the "length of time out of sun's radiation" explaination, because the tepm doesn't just reach it's lowest point at this time, it actually drops a few to several degrees in a short time.


Air temp will drop below dewpoint.

15894

dwright8
01-02-2008, 12:57 PM
I agree with what everyone said.:D ;)
I think im going to join with this guy on my answer

Rat
01-02-2008, 01:16 PM
Dew Point is just another name for Saturation Point. Water has two saturation points, liquid (boiling) and vapor (condensing).

In liquid form water reaches is saturation point @ 212* (@ sea level) and turns into a vapor. Liguid water is heat saturated and turns into a vapor, this is the liquid saturation point, or boiling point.

Conversly when water vapor is present in the atmosphere it can reach a temperature where ther isn't enough heat to keep it a vapor. This is the vapr saturation point for the atmosphere (it can no longer hold water vapor due to heat loss) and the water vapor condenses, causing dew or hoarfrost if the temperature is below freezing.

dew point can be below air temp, but usually not for very long, because relative humidity and temperature are dependant on each other. When the atmosphere has shed the water vapor either the dew point falls or the air becoms warmer.

Many times in the woods I have ran across micro-climes, low spots where the air is cooler than the surrounding area. Sometimes these areas will have dew already forming when the rest of the area doesn't, so you can see how sometimes the dew point can be lower than the air temperature, to a degree anyway.

I deal with saturation points in my job as well, not water per se, but the science is the same.

Johnny
01-02-2008, 01:41 PM
DWG, the chart you provided does not prove that air temp will drop below the dew point. It shows the temp and dew point meeting at the saturation point between 7 & 8 in the morning but the temp never goes below the dew point reading, because it can't.

Air temp will NEVER drop below the dew point temp. Their are a few instances when super-saturation has occured but it is rare and very brief.

Now to the question of why it's colder at the first hint of light. Many weather stations record their air temps 4 ft. off the ground. Have y'all noticed that sometimes on clear, calm/cold nights when the temperature gets down to say around 34 to 36 degrees and their is frost on the ground? Frost is frozen moisture. So how can it be frozen when the thermometer says 35 degrees? That's because you are getting that 35 degree reading above the surface (4 to 6 feet) but the ground is at freezing or below. So when the sun starts to rise, the heat starts to stir up the cold at ground level and into the air which causes the thermometer to drop a degree or two at first light. The sudden cold that we are feeling at first light while sitting in a deer stand is the colder temperatures at ground level being stirred up into the air.

Howard
01-02-2008, 01:48 PM
without reading all the crap above:)

my guess is our dumb butt has been sitting there for 30 min. or more and we are no longer moving or worked up from hiking in. We are still, body temp has now dropped and it always feels colder. I doubt temp has even really changed outside, just our body.

Rat
01-02-2008, 01:52 PM
DWG, what location did you pull that report from?

Rat
01-02-2008, 02:25 PM
Air temp will NEVER drop below the dew point temp. Their are a few instances when super-saturation has occured but it is rare and very brief.

Supersaturation occures every day, that is what forms clouds, fog and contrails on airplane wings. Wether or not the air temp is lower than dew-point is hotly debated, but I don't see how it can't be lower, the moisture content is greater than 100%, dew point must be higher.

Ouch
01-02-2008, 02:33 PM
What was the question?

I'm cold.

Target-panic
01-02-2008, 02:37 PM
But whose on first???????????????????

Brute Killer
01-02-2008, 02:39 PM
DWG, that top graph does not show dewpoint temp below air temp, accoring to my eyes. :) The red line is always above the green.

Mike Murphey
01-02-2008, 02:43 PM
What...???!!!
How about the colder air in the upper atmosphere comes down due to the sun heating it up....the warmer air goes up making the cooler air to come down...(or something like that)...it could also be the equation of the train going 50 mph going West, is like a train goes SW at 40 mph on the third Thursday of an even numbered month or something like that in (I slept that day in college)....

DWG
01-02-2008, 02:45 PM
DWG, that top graph does not show dewpoint temp below air temp, accoring to my eyes. :) The red line is always above the green.


My bad I looked at it wrong. ( Rat) That graph came off wunderground.com

Tmag
01-02-2008, 02:50 PM
I'm going with Smart on this one. He's a brainiac!

Brute Killer
01-02-2008, 02:54 PM
http://ww2010.atmos.uiuc.edu/(Gh)/guides/maps/upa/dewp.rxml (http://ww2010.atmos.uiuc.edu/(Gh)/guides/maps/upa/dewp.rxml)
Dew Point Temperature is defined as the temperature at which air would have to cool (at constant pressure and constant water vapor content) in order to reach saturation. Dew points provide insight into the amount of moisture in the air. The higher the dew point temperature, the higher the moisture content for air at a given temperature.

When the dew point temperature and air temperature are equal, the air is said to be saturated. Dew point temperature is NEVER GREATER than the air temperature. Therefore, if the air cools, moisture must be removed from the air and this is accomplished through condensation. This process results in the formation of tiny water droplets that can lead to the development of fog, frost, clouds, or even precipitation.

Brute Killer
01-02-2008, 02:58 PM
http://farm2.static.flickr.com/1217/1149317580_c2e8991648.jpg
:D :D :D :D :D :D :D :D :D :D :D :D :D :D

refugioco
01-02-2008, 02:58 PM
I have noticed this phenoma for a while. I go out at 5 and get the paper cars are unfrosted, go to leave at 7 frozen. I studied it this past week in Uvalde the temperature would drop 4- 6 degrees as the sun came up. I dont know the answer but it does happen.

Brute Killer
01-02-2008, 03:05 PM
http://www.newton.dep.anl.gov/askasci/wea00/wea00042.htm (http://www.newton.dep.anl.gov/askasci/wea00/wea00042.htm)

This guy says temp drop is related to barometric pressure.

WCB
01-02-2008, 03:08 PM
I can't beleive some thinks 25 is a bunch colder than 28...:rolleyes:

Chad_E
01-02-2008, 03:20 PM
What's on Second?

DavisHollow
01-02-2008, 04:02 PM
So get two thermometers, one at your feet and one at your head (presumably you are not short), and see if there is an inverse correlation between the temp at your feet when the temp at your head drops just after sunrise.

I buy the argument that the sun, as it rises, burns off (dissipates) a magical radition barrier in the upper atmosphere (much like a thin cloud layer keeps the temps warmer at night) which allows for a brief moment of additional radiation cooling until the sun materially begins to heat the surface.

Sounds good anyway.

And guys, the temp does not fall below the dew point, by definition. The dew point is, by definition, the point at which air is saturated with water. Any further cooling would cause enough moisture to condense or sublimate out of the air, so the dew point tracks the temp, as colder air can hold less moisture. An interesting factoid is that as moisture sublimates or condenses out of the air, heat is released, this is what keeps the temp from droping further. The opposite of what happens when water evaporates, the temp drops (we sweat, it evaporates, we stay cool but stink).