COLD MORNINGS!

[gsn]

BS in another thread made me think of this phenomenon! It seems just before sunrise is the coldest part of the night most times.

I was wondering if anyone knew why the coldest part of the night was about an hour to an hour and half just before sunrise. It seems the temp can drop a few more degrees in that time frame.On the surface one would think it would get warmer as the sun begins it morning ritual of rising.

What is the mechanism behind this drop in temp just before sunrise?

I realize other factors might come into play ,but in general this what I have noticed since I started watching, recording temps!

[elHoagie]

Scott,

All things being equal, the coldest time of day is usually about 15 minutes after sunrise.  At night energy is radiated out to space, so the air tends to get colder over the course of the night.  It's not until just after sunrise that there is more energy being received from the sun than being radiated to space, and the temperature starts to increase.

Jack

[Trópico]

This is what one radio host calls the "Sun suck" effect.  

[MattyB]

Next, Jack will be trying to sell you on the theory of "Dark Matter" sucking all the heat out of your yard.  Come on Jack, stop making things up.

[gsn]

(elHoagie @ Jan. 18 2008,12:50)

QUOTE

Scott,

All things being equal, the coldest time of day is usually about 15 minutes after sunrise.  At night energy is radiated out to space, so the air tends to get colder over the course of the night.  It's not until just after sunrise that there is more energy being received from the sun than being radiated to space, and the temperature starts to increase.

Jack

Jack,

I understand about enery being radiated out into space at night,and getting colder as the night progresses.

But your explantion of why it gets colder just before and after sunrise I don't understand,sorry.

I am talking about seeing the temps hold fairly steady, say from 4:00 to say 6:00 (for an example) then dropping considerably just before ,or after the sun rises? I would think it would be something other  than just more energy being radiated out, than being received, that happens all night long, somewhat evenly. The temps drop because more heat is lost (radiated out) over the course of the night,as you stated. That doesn't explain the  sudden drop at sunrise to me, but what do I know?

Is there some other law of nature, or physics at work here?

[elHoagie]

(gsn @ Jan. 18 2008,11:27)

QUOTE

Jack,

I understand about enery being radiated out into space at night,and getting colder as the night progresses.

But your explantion of why it gets colder just before and after sunrise I don't understand,sorry.

I am talking about seeing the temps hold fairly steady, say from 4:00 to say 6:00 (for an example) then dropping considerably just before ,or after the sun rises? I would think it would be something other  than just more energy being radiated out, than being received, that happens all night long, somewhat evenly. The temps drop because more heat is lost (radiated out) over the course of the night,as you stated. That doesn't explain the  sudden drop at sunrise to me, but what do I know?

Is there some other law of nature, or physics at work here?

Scott - The only things I can think of that would cause relatively constant temperatures overnight, then a quick drop near sunrise, are the wind stopping, clouds/fog clearing, or cold air advecting in. It's tough to imagine a microclimate where one of these would happen every night, but I'm definitely not an expert on the topic...

MattyB - Of course I'm an expert on "dark matter". I think it deposits old tires in the most evil parts of the universe

[BS Man about Palms]

Not sure if its where Jack was going, but I think I follow a bit. If you look at the fact that the temp is radiating out as the "night" progresses, the longer the "night" the cooler you can get. Follow?....Now, As the Sun rises, (or for Matty and I, the Son, ) the "light" may reach your house before the heat does! That would be the "15 min after" part.  Or at least it makes sense to me.  

[osideterry]

How about this non-scientific theory. The cold has momentum... you don't expect it to just throw the breaks on, then take off in reverse just because the sun came up. But I do like Bill's speed of light faster than speed of heat theory... expect aren't they "traveling together"?

[spockvr6]

Scott-

I have noticed the same effect.  I too am a bit stumped.

[_Keith]

I think this explains it pretty well.  

http://www.islandnet.com/~see/weather/elements/dawncold.htm

[BS Man about Palms]

Hey! I was right!

[_Keith]

Although I am not sure how much the actual effect would be, this statement would also tend to imply, that the more densely planted the garden, the warmer (more cold hardy) it will be.  

"All objects, including you and me, radiate heat away at a rate proportional to their temperature (actually the fourth power) and receive energy back from every object radiating in their view. When more radiant energy is gained than lost, the object generally warms. When more energy is lost than gained, the object cools. When there is a balance between gain and loss, the object can maintain a constant temperature."

[spockvr6]

(keiththibodeaux @ Jan. 19 2008,09:40)

QUOTE

I think this explains it pretty well.  

http://www.islandnet.com/~see/weather/elements/dawncold.htm

That still does not fully address Scott's question!

I think we can understand the lag between solar radiation coming in, etc.....

But.....

Why does the temperature many times stay flat or drop slowly until a few hours before sunrise, then drop faster a few hours before sunrise?

[gsn]

Yeah what Larry said.

I understand the principle of heat radiating out durning the night. I'm not talking about temps dropping fairly evenly through the entire night.That I fully understand.

But that still doesn't explain the the 45 minutes before sunrise dropping sometimes a full degree or two ,everything being equal! This is almost always the case at least here that right around sunrise ,even if the temps have held fairly stable for the few hours before that,they drop durning this period.That is the question I am asking,what laws of science cause this phenomenon?

There must be some mechanism,or physics at work here?

[BS Man about Palms]

(spockvr6 @ Jan. 19 2008,07:16)

QUOTE

(keiththibodeaux @ Jan. 19 2008,09:40)

QUOTE

I think this explains it pretty well.  

http://www.islandnet.com/~see/weather/elements/dawncold.htm

That still does not fully address Scott's question!

I think we can understand the lag between solar radiation coming in, etc.....

But.....

Why does the temperature many times stay flat or drop slowly until a few hours before sunrise, then drop faster a few hours before sunrise?

Larry, don't you do HVAC design?   "Heat flows to cool".  Just after sunrise, the Sky "ain't hot yet".

[sonoranfans]

(gsn @ Jan. 18 2008,14:27)

QUOTE

(elHoagie @ Jan. 18 2008,12:50)

QUOTE

Scott,

All things being equal, the coldest time of day is usually about 15 minutes after sunrise.  At night energy is radiated out to space, so the air tends to get colder over the course of the night.  It's not until just after sunrise that there is more energy being received from the sun than being radiated to space, and the temperature starts to increase.

Jack

Jack,

I understand about enery being radiated out into space at night,and getting colder as the night progresses.

But your explantion of why it gets colder just before and after sunrise I don't understand,sorry.

I am talking about seeing the temps hold fairly steady, say from 4:00 to say 6:00 (for an example) then dropping considerably just before ,or after the sun rises? I would think it would be something other  than just more energy being radiated out, than being received, that happens all night long, somewhat evenly. The temps drop because more heat is lost (radiated out) over the course of the night,as you stated. That doesn't explain the  sudden drop at sunrise to me, but what do I know?

Is there some other law of nature, or physics at work here?

scott if dew is forming say between 4-6 am the condensation process releases heat and could slow cooling, level off the temps.  After condensation stops, the downward trend in temp would continue.  In reality all the clouds, moisture content and air movement changes the simple assumptions of the model.  That is why it can be difficult to predict even the overnight low temps.  When clouds move through my area the temps jump up a bit during the night as they catch the radiated heat of the earth(water being the #1 greenhouse gas).

[gsn]

(sonoranfans @ Jan. 21 2008,18:20)

QUOTE

law of nature, or physics at work here?

scott if dew is forming say between 4-6 am the condensation process releases heat and could slow cooling, level off the temps.  After condensation stops, the downward trend in temp would continue.  In reality all the clouds, moisture content and air movement changes the simple assumptions of the model.  That is why it can be difficult to predict even the overnight low temps.  When clouds move through my area the temps jump up a bit during the night as they catch the radiated heat of the earth(water being the #1 greenhouse gas).

[spockvr6]

(BS, Man about Palms @ Jan. 19 2008,10:38)

QUOTE

Larry, don't you do HVAC design?   "Heat flows to cool".  Just after sunrise, the Sky "ain't hot yet".

Yes.....but the question Scott has relates to the unexpected change in rate of temperature drop before sunrise.

[ruskinPalms]

I think that Tom Blank nailed it. I bet it has to do with the whole heat of condensation thing. The formation of dew from water vapor is an exothermic process, meaning heat is released to the surrounding air. That is why temps remain realatively constant once the dew point is reached. Once one gets under the dewpoint and/or some of the moisture is taken out of the air, the temperature drops at an increased rate.

I looked at several of my weather station graphs. Once the dewpoint is reached, the temp and the dewpoint tend to vary together. A lot of times, this is when there is fog. Probably, this is a limitation of my weather station sensors, but I was not able to see any graphs where the temp was actually lower than the dew point. And there were very few times that the decsribed effect was observed through the last couple of weeks on my graphs, but there were definitely a few times on the graphs that right around 645 to 715 the rate of temp drop definitely increased!

Could there be variations in Barometric pressure that tend to occur at that time of day that result in the temp drop?

[BS Man about Palms]

(spockvr6 @ Jan. 22 2008,04:13)

QUOTE

(BS @ Man about Palms,Jan. 19 2008,10:38)

QUOTE

Larry, don't you do HVAC design?   "Heat flows to cool".  Just after sunrise, the Sky "ain't hot yet".

Yes.....but the question Scott has relates to the unexpected change in rate of temperature drop before sunrise.

I missed the "rate" part..... or at least I've decided its too early to be up when its that cold and my concern waned.....(plus I don't have them there computer temp stations that monitor constantly for me)  

[MattyB]

Just an observation:  Unless you're on a southeast facing hill with no trees or obstructions in front of you, you won't actually see the sunrise above the horizon.  So of course it's not going to warm you up yet.

[gsn]

Matty,

What is a HILL? :laugh:

The Atlantic Ocean is my only obstruction! ::

[sonoranfans]

(gsn @ Jan. 21 2008,19:20)

QUOTE

(sonoranfans @ Jan. 21 2008,18:20)

QUOTE

law of nature, or physics at work here?

scott if dew is forming say between 4-6 am the condensation process releases heat and could slow cooling, level off the temps.  After condensation stops, the downward trend in temp would continue.  In reality all the clouds, moisture content and air movement changes the simple assumptions of the model.  That is why it can be difficult to predict even the overnight low temps.  When clouds move through my area the temps jump up a bit during the night as they catch the radiated heat of the earth(water being the #1 greenhouse gas).

Sonorafans,

I have been waiting for you to post on this subject.

I thought we would get some info regarding spectral light rays hitting the atmosphere causing the temps to drop at  or around sunrise.Or some other scientific explantion regarding the sun and light rays!

Oh well looks like I will have to go with Tropico's "SUN SUCK" effect! :laugh:

Thanks all who chimed in!