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The weather

Started by thecfarm, August 04, 2008, 08:46:58 PM

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moonhill

Tom, when I was younger and just out of High School, I went on a road trip with 2 friends of mine.  Our journey went through the south, it was Winter.  We got a kick out of the way the highway department in the south was handling the ice build up on the roadway.  They had a fellow sitting in the back of a pick up truck shoveling rock salt on the major highway.  I guess they never see ice on the roads, so they didn't have the spreader ready.  I bet they have them now.  This was in the  Louisiana area.  At times there was 6" deep ruts on the Interstate, tractor trailer size ruts, the Ford Escort didn't quite fit.

My sister reports these -40° temps with winds at 30mph and we ask her the same thing.  Why?

Tim
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Samuel

Its going on the 3rd straight week of -30 C here in Alberta.  We has a heat wave of -16 on boxing day, but it quickly dipped back down to -32 that night once the sun dipped.  Looking at the extended forecast there does not seem to be any let up in these temperatures.  One good thing is that the roads are dry and bare, with the exception of bridge decks that can be quite slippery...

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Tom

Quotewe ask her the same thing.  Why?

......they must have jobs up there.  :D

beenthere

Quote from: stonebroke on December 30, 2008, 03:45:51 AM
I always thought the colder it got the lower the humiidty was. Something about the air just colder hold any more water.

I believe talking "Relative Humidity" here. For a given amount of moisture in the air, the colder it is, the higher the "RH" is, as the cold air can't hold as much moisture before it comes out as rain/snow/fog.

But, take the high moisture air and bring it inside and warm it up, then the RH will take a nose dive.

Can get lumber to increase in moisture content outside in the cold, but decrease in mc inside where it is heated (at least here in the north  :) )
south central Wisconsin
It may be that my sole purpose in life is simply to serve as a warning to others

SwampDonkey

It's the dew point that is lower in the cold, typically less than 20 % here in winter. In summer when it climbs above 55% is when it is real close and sticky, the RH could be near 100 % cold or hot. In the house it's around 15 % RH with wood heat, without a humidifier and around 60 % outside on fair weather days.
"No amount of belief makes something a fact." James Randi

1 Thessalonians 5:21

2020 Polaris Ranger 570 to forward firewood, Husqvarna 555 XT Pro, Stihl FS560 clearing saw and continuously thinning my ground, on the side. Grow them trees. (((o)))

Warbird

Quote from: beenthere on December 29, 2008, 11:51:20 PMThe first time, the temp on main street was -50° F.  ::) ::) ::)

So you should change your name to "beenhere"!  ;D

Quote from: stonebroke on December 30, 2008, 03:45:51 AM
I always thought the colder it got the lower the humiidty was. Something about the air just colder hold any more water.

Stonebroke

Usually that is the case.  We get waves of humid air and I believe it takes a bit for it to freeze out of the air.  It is really neat when it happens.  There is a white 'powder' (not snow) over everything and you can see the air crystallizing.  Once this happens the air is ultra-clear.  Things outside have a sharp sound and you feel like you can see and hear much farther than normal.

It is -39°F this morning in town.  -29°F outside my house (we are usually a bit warmer because we live on a hill, although sometimes, the cold air rolls right over our house on the way down into the valley).  The weatherman said this morning that the high for today is only -35°.  And he moved the -50° lows from mid-week to tonight.

Coldest ambient temp I've ever seen is -65°.  If we hit that, I may stay home and work from here.  Machines don't like to run much past -45°...

beenthere

SD
I thought dew point was a temperature....where does the % number figure in?

If dew point is close to actual temp, then RH is high.

south central Wisconsin
It may be that my sole purpose in life is simply to serve as a warning to others

SwampDonkey

beenthere, yes, its temperature, not %. But, it's not the actual air temperature its temperature air must be cooled at a constant pressure to form dew. The higher the dew point the higher the amount of water in the air. We get what Warbird describes here on the day before a storm. It will be a real cold morning. Typically before a big thaw in winter. Everything covered in ice crystals and the air even has ice crystals and not a cloud but a haze or very high serous clouds, not snow. Dew point this time of year is around 20 °, not %.  ::) Dew point of 55 or more is very muggy weather up here, the air temperature will be close to 85° in the day and 65-70° at night. Of course that can be a lot higher in the south, we are just not used to it up here so much. Our outside air is dryer here in winter, it's only when we get those brisk southerlies that our humidity climbs high. The Canadian arctic is a frozen desert basically, we get way more snow down here. Muskox would starve to death.
"No amount of belief makes something a fact." James Randi

1 Thessalonians 5:21

2020 Polaris Ranger 570 to forward firewood, Husqvarna 555 XT Pro, Stihl FS560 clearing saw and continuously thinning my ground, on the side. Grow them trees. (((o)))

beenthere

Quote from: SwampDonkey on December 30, 2008, 01:49:05 PM
..............The higher the dew point the higher the amount of water in the air. .......................

Be more correct to say "the closer the dew point temp is to the actual temp, the higher the amount of water in the air".

And that is given as the relative humidity in %.

Can have 20° dew point and 30° actual, and have a lower humidity than having a 20° dewpoint and 22° actual temp.

Just want to decrease the confusion, not add to it.  ;D ;D   I won't hassle you anymore  :) :) :)
south central Wisconsin
It may be that my sole purpose in life is simply to serve as a warning to others

Gary_C

I thought, actually I am certain that I understand humidity and dew point of mixtures of water vapor and air very well, at least I did untill you guys started explaining it here.  ::)

Now I am not sure.   ???

:D :D :D
Never take life seriously. Nobody gets out alive anyway.

Warbird

Quote from: SwampDonkey on December 30, 2008, 01:49:05 PMThe Canadian arctic is a frozen desert basically, we get way more snow down here. Muskox would starve to death.

Yup.  Fairbanks (and the whole Interior of Alaska) is often described as an "Arctic desert".  The building I work in is on the edge of a hill overlooking the city.  If I had my camera, I'd take a pic and show you what SD described with the haze.  All the smoke from the stacks goes straight up in the air.  It is very beautiful.


SwampDonkey

Yeah , I just checked it with the math for estimating vapor pressures ::), which is what is actually used to calculate RH. Those estimates are made with dew point and actual temps though, but it ain't as simple as diving the two. There is some constants and logarithms involved. ;)

But it was true as stated. Because your adjusting actual and not dew point, an old snake oil salesman trick. :D
"No amount of belief makes something a fact." James Randi

1 Thessalonians 5:21

2020 Polaris Ranger 570 to forward firewood, Husqvarna 555 XT Pro, Stihl FS560 clearing saw and continuously thinning my ground, on the side. Grow them trees. (((o)))

submarinesailor

Let me see if I can explain this humidity thing a little better.

If you have an RH of 75%, which means that the air is holding 75% of the total moisture it can hold at a given temperature. 

NOTE: I don't have a psychometric chart with me, so these examples are off.

If the air temperature is 80 degrees, at 100% RH it can hold 200 grams of moisture per cubic foot.   If they say that the RH is 50% at 80 degrees, than the air is holding about 100 grams of moisture per cubic foot.
If the air temperature is 40 degrees, at 100% RH, the air could hold about 80 grams of moisture per cubic foot – it is not a linier scale.  40 grams of moisture would be an RH of 50% at 40 degrees.

IF the RH is at 100% for a given temperature, it is at it's saturation point and cannot hold any more grams of moisture – it's also called "it's raining or snowing".

Dew Point is the point at which the air can no longer hold it's moisture at a given temperature.  Example:  If the air is at 80 degrees with an RH of 80% during the day time.  It is holding about 160 grams per cubic foot.  As the sun goes down, the temperature will start to fall, but the grams per cubic foot are still the same – 160 grams.  At some point, the temperature will fall to a point(the Dew Point) where the air can no longer hold this amount of moisture. At this temperature, say 40 degrees, the extra moisture all fall out of the air.  The dew was fallen. 

Why do they report the dew point on the news?  The greater the amount of moisture in the air, the more energy or more change it takes to change the air temperature.  So the higher the dew point, the higher the night time temperature COULD be.  Look that the deserts, the RH is almost always low and the have very large temperature changes from night to day and back - as much as 60 to 100 degrees.   In the deep south where the RH is ALWAYS high, the temperature swings are not as great – say 20 to 40 degrees night to day and back.

I hope I got it all correct because this is the way I was taught in all the different energy classes and air handling schools I have been to.  If I got any of this wrong or a little off, PLEASE let me know.

Bruce


SwampDonkey

I'm not sure of the exact figures for the amount of moisture either, I didn't do the math. But your explanation reads correct to me.  I think it's expressed as grams of water/kg of air, vapour pressures in kpa for metric.  As far as the dew point don't forget barometric pressure, it's not just falling temperature. If pressure changes,so isn't dew point temp. ;D
"No amount of belief makes something a fact." James Randi

1 Thessalonians 5:21

2020 Polaris Ranger 570 to forward firewood, Husqvarna 555 XT Pro, Stihl FS560 clearing saw and continuously thinning my ground, on the side. Grow them trees. (((o)))

Gary_C

Bruce

That sounds like a good explanation to me, even though as SD was somewhat trying to say, it's more complicated than the numbers you used.

Since the relative humidity of an air-water mixture is defined as the ratio of the partial pressure of water vapor in the mixture to the saturated vapor pressure of water at a prescribed temperature, just using the actual weight of water in a ratio is somewhat inaccurate, but close enough to get an idea of how relative humidity is determined.
Never take life seriously. Nobody gets out alive anyway.

SwampDonkey

My local weather I get out of Presque Isle, Maine. It's the most accurate for my area because I'm not on the coast. Ted Shapiro is the meteorologist there and he often gives night courses on the subject. Every night he has a multiple choice question and he takes calls from the public about the weather or anything related to meteorology. He's certainly a knowledgeable fellow, and I think because of his presentation, he's probably the best meteorologist I've seen on TV. Others I watch, they ramble on so much you wouldn't know what the weather forecast was when they are done, I doubt they even know. :D Never knew until tonight, he went to college at University of Wisconsin.
"No amount of belief makes something a fact." James Randi

1 Thessalonians 5:21

2020 Polaris Ranger 570 to forward firewood, Husqvarna 555 XT Pro, Stihl FS560 clearing saw and continuously thinning my ground, on the side. Grow them trees. (((o)))

pigman

 I think Bruce gave a good explanation on the relationships of temperature and the amount of moisture in the air at different relative humidities. I have a psychrometric chart in front of me and I was going to check to see how close Bruce's memory was on the values. Well, it seems my chart gives the values in pounds of moisture per pound of dry air at the different temperatures and relative humidities. I guess I went to school before Al Gore invented the metric system. ::)  ;D
Things turn out best for people who make the best of how things turn out.

Tom

I'm confused.  How come Al Gore needs a metric psycologist?

Warbird

So when the moisture freezes out of the air, leaving a very fine white powder over everything, looking like crystals in the air as it slowly drifts to the ground, that is really the dew point being exceeded for the temperature that day?  Am I understanding this correctly?

If so, that isn't what I usually call "snow fall".  Snow falls from clouds, not out of mid-air.  Suppose I could be calling it "dew fall".  ;) 

Gary_C

Quote from: Warbird on December 31, 2008, 12:42:19 AM
So when the moisture freezes out of the air, leaving a very fine white powder over everything, looking like crystals in the air as it slowly drifts to the ground, that is really the dew point being exceeded for the temperature that day?  Am I understanding this correctly?


I think you understand, but are not stating it correctly when you say "the dew point being exceeded for the temperature." The air temperature can exceed the dew point temperature any time, but cannot drop below the dew point temperature without the moisture condensing and then rapidly freezing at the low temperatures you are talking about.

Quote from: Tom on December 30, 2008, 10:46:08 PM
I'm confused.  How come Al Gore needs a metric psycologist?

He needs one to tell him that he does not have even one gram of sense in his brain.   ;D ;D

Never take life seriously. Nobody gets out alive anyway.

SwampDonkey

And another thing, RH doesn't measure amount of moisture in the air. You can have a dew point of 32° and a air temperature of 32° with RH =100% and there might only be 5 g of water vapor per kg of air and it could be raining. In the same token, the dew point could be 77° and the air 77° with RH=100 % and have 20 g of water vapor in 1 kg of air. Drop the dew point, keep air temp at 77 ° the same and , RH falls, say to 50 % and water vapour falls to 10 g of water to 1 kg of air. Drop air temperature, also means dew point is falling, so isn't water vapour. But dew point isnt always dropping because air temperature is, just using the example here when dew point and air were intially the same. Because you could have a dew point steady at 32° and the air temp is falling after sun down from say 60° down the 36°, causing relative humidity to rise. Nice morning fog in September. ;D

Warbird, what Gary said. ;)
"No amount of belief makes something a fact." James Randi

1 Thessalonians 5:21

2020 Polaris Ranger 570 to forward firewood, Husqvarna 555 XT Pro, Stihl FS560 clearing saw and continuously thinning my ground, on the side. Grow them trees. (((o)))

pigman

Quote from: Tom on December 30, 2008, 10:46:08 PM
I'm confused.  How come Al Gore needs a metric psycologist?
I am sorry I got you confused, I seem to do that to people. I couldn't sleep last night worring about your confussion, so I went  to the local Holiday Inn Express . I didn't rent a room, I just sat in the lobby. I think I can now help with the confusion.
I didn't mean that Al Gore needed a psycologist, well some would he say does, it is the weather that has gone psycho. All the gasses produced by cars, people and cows has caused the weather to go psycho. Al Gore invented the metric system so it would be easier to measure the changes in the weather. We all know that the metric system is easier to use because all we have to do is move the decimal point to the left, right, up , down or somewhere. This caused a problem when using ink pens because the paper would tare when moving the decimal point. So Al Gore invented the computer so it would be easier to move the decimal point. After we started moving the decimal point  so easy on the computer, Al invented the internet so we could have a faster way of showing other people where the  decimal point was now located. I sure hope I have helped with your confusion Tom.
Things turn out best for people who make the best of how things turn out.

stonebroke

Tom

I am with you. I am way confused now.

Stonebroke

Gary_C

Quote from: stonebroke on December 31, 2008, 09:37:27 AM
Tom

I am with you. I am way confused now.

Stonebroke

About what???    ;D
Never take life seriously. Nobody gets out alive anyway.

SwampDonkey

Betcha didn't know wet air (higher water vapor) is lighter than dry air. ;D  Nice snap'n cold dry day has the coldest spots in the river valleys, as cold air flows like water down them valleys and is heavier air.  ;)
"No amount of belief makes something a fact." James Randi

1 Thessalonians 5:21

2020 Polaris Ranger 570 to forward firewood, Husqvarna 555 XT Pro, Stihl FS560 clearing saw and continuously thinning my ground, on the side. Grow them trees. (((o)))

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