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Scott

Low Temp/ Dew Point

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Scott

Hi

While checking local forecasts, I noticed the dew point is much lower than the forecasted low temp.

I guess I don't understand. if the forecast low is 54 / 35,

do I need to worry about possible frost?

Thanks,

Scott

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spockvr6

Scott-

More knowledgable folks than I will need to chime in here for definitive answers.  I will offer my thoughts as a non-expert (who happens to have a weather station).

All else equal, the greater the dewpoint depression (i.e. the amount the dewpoint is below the air temp), the faster the temp will fall after sunset.  

The closer the dewpoint is to the air temperature. the slower the temperature will fall.  I believe this is due to the fact that, as the air temp approaches, and tries to drop below, the dewpoint, water must fall out of the air as a result (obviously RH cant go above 100%).  As that process happens, heat is released.  This is a physically property.

As heat is released, the air temp will frequently remain constant.  This is a good thing as it means the low the next AM will likely be higher than it would have been otherwise!

But, the bad part about this is that, the more humid the air, the more easily frost can form.

As for your question re: frost at 54/35.....do you mean the forecast low is 35F and the forecast high is 54F?  Or, is 54F the forecasted low and 35F is the forecasted dewpoint?

The 35F dewpoint only means that if the air temp were to drop to 35F, the RH would be 100%.  It does not define when frost starts forming.

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Scott

(spockvr6 @ Oct. 16 2006,14:06)

QUOTE
I assum you mean 54F air temp and 35F dewpoint?

The 35F dewpoint only means that if the air temp were to drop to 35F, the RH would be 100%.

Thanks for the response! Yeah, that's right. and the projected humidity is 36.

This is alot more confusing than I thought. Is RH relative Humidity?

So if the dew point was 29 - even though the temp was projected higher, than frost is probable?

Sorry to be so thick headed about this. ???

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MattyB

A projected dew point of 29F dosen't necessarily mean that frost is probable, it just means that if the temp. happens to drop all the way to 29F, then dew (100% relative humidity) would start to form.  I'm pretty sure that 29F low w/ 29F dew point would yeild frost because it's so cold.  I guess you could have a projected dew point of 29F with a projected overnight low of 45F and obviously you wouldn't have frost that night because it just wasn't cold enough.  El Hoagie explained all this to me once.  As far as Relative Humidity goes, it's the amount of water that the air can hold at a given temp expressed in a percentage.  For example:  if it's 50 degrees F out and the Relative Humidity is 75% that means that the air is 75% saturated with water.  As the temperature rises, like it does during the day, say to 80 degrees F, the RH % will drop.  This dosen't mean there's any less water vapor in the air than earlier only that the warmer air can now hold more water before becoming completely saturated.  The RH is "relative" to the temperature at the time.  90F w/ 50% RH is a heck of a lot more humid than 70F w/ 50% RH.  Sort of a round a bout way to describe it but I hope that helps a little. ???

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happ

Here's a link:

http://ww2010.atmos.uiuc.edu/(Gh)/guides/m...sfcobs/dwp.rxml

High dew points mean the air is saturated beyond muggy.    :P   Tomorrow evening santa ana winds will kick in. California experiences below zero dew point temperatures/ below 10% humidity.  Warm arid winds that suck the air/vegetation dry.   :angry:

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Scott

Thanks guys for the feed back. I'm amazed at how fast all that hot sun has faded this year. And it seems really early for some of the cold night temps. I really wanted to make sure I'm reading things right.

One of the things I first noticed when I joined the forum on the old site was the attention to temperatures etc. And obviously for a good reason.

Another lesson in palm care I guess.

Thanks again!

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BS Man about Palms

All the stuff said is right on here.  I'd have to dig out one or two books or at least charts, but I have to deal with this thru my job repairing air conditioners.  The Dewpoint...or "apparatus" dewpoint, is the point of SURFACE temperature, that the moisture in the air, (latent heat, or humidity) will condense on ANY surface! Wood, metal, skin, paper, etc.

The Hoag of course was right on with the dispersal of heat as humidity condenses. Thats why I , to accurately check an air conditioners performance,  have to know the wet bulb temp and dry bulb temp. Simply put, its a method to measure the humidity load in the air.  when you wet a small sock and place it over the mercury bulb of a thermometer and swing it around over your head (I'm not kidding) the lower the temp reads when you finish, the dryer the air is because of the wetsock "evaporating" over the bulb lowers the temp.  If the temp does not lower much at all, its quite humid., (the air is fairly saturated with moisture and can't hold much more)

As for the "heat" idea, in simple terms, think how long you have to run the burner on your stove to boil water, all that is needed to have a "change of state" from a liquid to a gas, being held as steam, humidity, heat energy, its all about the same....

   NOW, heres why we hate humidity so much, your body sweats "hoping" that as it evaporates we will have that heat "removed " from our skins surface, YET if it is HUMID enough, your SKIN temperature can be cool enough that the humidity in the air, says"Great, look at this nice hairy place to condense on!" and puts that stored heat energy on you all the while you are getting NO help in the evaporation department!!!

I could go on, but I hear a lot of snoring...

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BobbyinNY
A projected dew point of 29F dosen't necessarily mean that frost is probable, it just means that if the temp. happens to drop all the way to 29F, then dew (100% relative humidity) would start to form.  I'm pretty sure that 29F low w/ 29F dew point would yeild frost because it's so cold.  I guess you could have a projected dew point of 29F with a projected overnight low of 45F and obviously you wouldn't have frost that night because it just wasn't cold enough.  El Hoagie explained all this to me once.  As far as Relative Humidity goes, it's the amount of water that the air can hold at a given temp expressed in a percentage.  For example:  if it's 50 degrees F out and the Relative Humidity is 75% that means that the air is 75% saturated with water.  As the temperature rises, like it does during the day, say to 80 degrees F, the RH % will drop.  This dosen't mean there's any less water vapor in the air than earlier only that the warmer air can now hold more water before becoming completely saturated.  The RH is "relative" to the temperature at the time.  90F w/ 50% RH is a heck of a lot more humid than 70F w/ 50% RH.  Sort of a round a bout way to describe it but I hope that helps a little.

I had a slight touch of frost in my yard 3 nights ago when the temp dropped to 39.7.... I don't know what the dewpoint was though..... I looked all around my deck & my yard and the only frost I saw was on the glass surface of an outdoor bar that I have on my deck - everything else was just wet.... I found this kind of odd.... but stuff is still growing like crazy in my yard... my wife was amazed to see the pepper plants still producing peppers. My Musa Basjoo is still pushing up its new spear..

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palmtreesforpleasure

Hi Bobby

have you moved your palms to the greenhouse yet?

hope you and Cari are well

regards

colin

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BobbyinNY
Hi Bobby

have you moved your palms to the greenhouse yet?

hope you and Cari are well

regards

colin

Hey there Colin......

how's everything?...... Yeah, MOST of them are in the greenhouse now (Adonidias, P. Canariensis), and I have (2) portable Greenhouses set up in my backyard - one for my P. Roebelleni in the ground, and the other one  for all my smaller stuff.  The only trees I still have outside are my Queen Palm, and my P. Elegans which will be taken care of by the end of next week. I will post some pics soon.  

Tell Denise we said Hello and hope to get to visit Oz soon.

Bobby :)

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PalmsZA

Hi all

Today is warm. Temp at 14:00 - 24.3c, Dew point - 21.1c and RH of 82%. Now if I am reading what you guys have written correctly, if the temp were to drop suddenly to 21.1c then moisture would appear on any available surface?

Thanks for asking the question Scott, I have also wondered about DP and RH.

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BS Man about Palms

Well, I left out a little bit, but it's in the link on an earlier post (which I read later)  Anyway, if the AIR reaches the dewpoint, the humidity will want to condense, thus mist, fog, etc.  BUT the surface temp of "whatever" will have to reach the dewpoint before condensation will occur on that surface.

Then we can have different surface temps on different materials and whether they are "raised' or not (cooll above and below) think of a icy bridge in the winter that freezes before the road, Bobby's glass table.

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BobbyinNY
Then we can have different surface temps on different materials and whether they are "raised' or not (cooll above and below) think of a icy bridge in the winter that freezes before the road, Bobby's glass table.

Bill,

That's very interesting..... Why does glass freeze before other surfaces? is it a colder material?

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BS Man about Palms

(MattyB @ Oct. 16 2006,15:25)

QUOTE
A projected dew point of 29F dosen't necessarily mean that frost is probable, it just means that if the temp. happens to drop all the way to 29F, then dew (100% relative humidity) would start to form.  I'm pretty sure that 29F low w/ 29F dew point would yeild frost because it's so cold.  I guess you could have a projected dew point of 29F with a projected overnight low of 45F and obviously you wouldn't have frost that night because it just wasn't cold enough.  El Hoagie explained all this to me once.  As far as Relative Humidity goes, it's the amount of water that the air can hold at a given temp expressed in a percentage.  For example:  if it's 50 degrees F out and the Relative Humidity is 75% that means that the air is 75% saturated with water.  As the temperature rises, like it does during the day, say to 80 degrees F, the RH % will drop.  This dosen't mean there's any less water vapor in the air than earlier only that the warmer air can now hold more water before becoming completely saturated.  The RH is "relative" to the temperature at the time.  90F w/ 50% RH is a heck of a lot more humid than 70F w/ 50% RH.  Sort of a round a bout way to describe it but I hope that helps a little. ???

IIRC part of this is backwards. Cool air can hold more moisture than warm air.  That is why we will mix "cool" outside air with indoor air to reduce the condensation formed on the windows inside a house during the winter.

(from a HVAC perspective)

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BS Man about Palms

(BobbyinNY @ Oct. 17 2006,06:46)

QUOTE
Then we can have different surface temps on different materials and whether they are "raised' or not (cooll above and below) think of a icy bridge in the winter that freezes before the road, Bobby's glass table.

Bill,

That's very interesting..... Why does glass freeze before other surfaces? is it a colder material?

Not sure, but I suspect its a density or surface thing.  More of an ability to retain or disipate heat.

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Neofolis

The glass thing is usually because it conducts heat well.  Materials that conduct heat well like glass and metals will change temperature quickly based on environmental changes, so they will both absorb heat and lose heat quickly.  Because their temperature will change more rapidly than the air temperature there are often times when they will be colder than the air temperature and frosts will form on them when other warmer surfaces are free of frost.

Thermal conductivity is based on chemical composition of materials, but it will also be affected by mass.  So 50mm/2" thick steel will take longer to change temperature than 6mm/¼" thick steel with the same surface area.  There are other issues like colour and reflectiveness of surface, etc.

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Steve

Matty B was correct when stating that warm air holds exponentially more moisture than cold air.  I deal with this often when product testing at work.  We often perform environmental conditioning to simulate extreme conditions.  

I don't work in heating and air, so I can't say for sure why the cool air mix reduces condensation on windows.  I would guess that it may be because it has a stabilizing effect so that the air immediately on either side of the glass doesn't have such extreme differences in temp and/or humidity.  The cool air might act as an insulator or buffer zone to keep the warmer more humid air slightly away from the window.  Is the cool air added onto or near the window surface?

Man About Palms, you may have a Psychrometric chart you use at work that shows the relationship between absolute humidity and RH at various temperatures using wet bulb and dry bulb readings.  Pick a high dry bulb temp from the chart, and look at how much higher the "Humidity Ratio" or absolute humidity is at a given RH than at a lower temp.  For instance, at 35C/95F w/ 25% RH, the Humidity Ratio in grams moisture per kg dry air is around 9.  At 25C/77F w/ 25% RH the Humidity Ratio is around 5.  The humidity ratio lines are the horizontal lines on the chart.  The chart also illustrates the J-curve exponential relationship between temp and how much moisture the air can hold.  My chart is based on a barometric pressure of 760mm Hg (std pressure at sea level).

Below is a website from the National Weather Service.  You can calculate relative humidity if you know the dewpoint, and temperature.  Or you can calculate the dewpoint if you know temp and RH.  It's pretty cool.  There's some other calculations you can make with it as well.

An example:  If you have a dewpoint of 60F and the temp is 80F, the RH is 50% (air is 50% saturated).  If the temp is 70F w/ a 60F dewpoint, the RH is 71%.  The barometric pressure is not significant for this calculation, but I usually enter 760 mm Hg.  That is standard pressure.  The pressure will affect the wet bulb temp if you are interested in that.

Calculating Relative Humidity from known dewpoint

http://www.srh.noaa.gov/elp/wxcalc/dewpoint.shtml

Link to main website

http://www.srh.noaa.gov/elp/wxcalc/wxcalc.shtml

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Steve

Be sure to check out the Heat Index and Wind Chill calculations on that website as well.  Those are interesting too.

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elHoagie

MattyB gave a pretty good explanation of dewpoints and relative humidity.  Here's the deal with frost.

IF an object has a temperature below freezing AND a temperature below the dewpoint of the atmosphere, then frost will form on that object.  Similarly, if an object has a temperature above freezing AND a temperature below the depoint of the atmosphere then dew will form on that object.  If an object has a temperature above the dewpoint of the atmosphere, no matter how low the dewpoint is, then the object will stay dry.

So, how can Bobby have frost when the temperature is 39.7?  Most objects will lose energy by radiating it into space, especially on clear nights.  This means that most objects will have a temperature that is lower than the atmosphere until just after sunrise.  Usually exposed cars, lawns, rooftops, etc. are the coldest objects in the morning, and the first to form frost.  Sometimes frost can even form when the ambient temperature is well above 5C (42F), sometimes there will be no frost when the ambient temperature is just above 0C (32F).  It just depends on the characteristics of the atmosphere that night.

BS - the reason you bring in cooler air to reduce condensation on the windows is because cooler air tends to be drier since it can't hold as much moisture (like MattyB said).  So, by bringing cooler air into the house you've reduced the amount of moisture in the air.

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BobbyinNY

Excellent explanation, Jack...... makes perfect sense....

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BobbyinNY

Excellent explanation, Jack...... makes perfect sense....

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BS Man about Palms

(elHoagie @ Oct. 17 2006,10:55)

QUOTE
BS - the reason you bring in cooler air to reduce condensation on the windows is because cooler air tends to be drier since it can't hold as much moisture (like MattyB said).  So, by bringing cooler air into the house you've reduced the amount of moisture in the air.

Thats it!  Thanks Jack!   (sorry - Matt)

I can't say I've attended a "school for higher learning" just 20+ years of fixing the dang things...   :)

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Scott

(PalmsZA @ Oct. 17 2006,09:21)

QUOTE
Thanks for asking the question Scott, I have also wondered about DP and RH.

What's funny, I thought I would be the only one wondering this, glad there were others. :)

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happ

(elHoagie @ Oct. 17 2006,13:55)

QUOTE
MattyB gave a pretty good explanation of dewpoints and relative humidity.  Here's the deal with frost.

IF an object has a temperature below freezing AND a temperature below the dewpoint of the atmosphere, then frost will form on that object.  Similarly, if an object has a temperature above freezing AND a temperature below the depoint of the atmosphere then dew will form on that object.  If an object has a temperature above the dewpoint of the atmosphere, no matter how low the dewpoint is, then the object will stay dry.

So, how can Bobby have frost when the temperature is 39.7?  Most objects will lose energy by radiating it into space, especially on clear nights.  This means that most objects will have a temperature that is lower than the atmosphere until just after sunrise.  Usually exposed cars, lawns, rooftops, etc. are the coldest objects in the morning, and the first to form frost.  Sometimes frost can even form when the ambient temperature is well above 5C (42F), sometimes there will be no frost when the ambient temperature is just above 0C (32F).  It just depends on the characteristics of the atmosphere that night.

BS - the reason you bring in cooler air to reduce condensation on the windows is because cooler air tends to be drier since it can't hold as much moisture (like MattyB said).  So, by bringing cooler air into the house you've reduced the amount of moisture in the air.

Hey elHoagie. How are things in Altadena?

Sometimes the lawn-level temperature is several degrees lower than the thermometer reading, especially in a low-lying shallow.

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Neofolis

As Jack said the introduction of cooler, drier air helps alleviate condensation on windows.  The main problem with windows is that the surface temperature of the glass inside is much lower than the air inside at night, because the lower outside air temperature is reducing the glass temperature.  The glass is then often well below the interior dewpoint.  In reality the heat is being drawn out through the windows, because it is the easiest route for it to mix with the colder outside air.  This results in the warmest air in the room being, maybe a couple of inches away from the glass.  This can be overcome with ventilators above the windows, because the heat is drawn to the ventilators as it is an easier route to release it's energy into the cooler air.  When you introduce colder air into the room, not only is it drier, but you are also reducing the temperature difference between the glass surface and the air near to it.  You are also creating an easier route for the warm air to release it's energy into the colder outside air.  Air temperature inside becomes drier, it is also cooler, there is less hot air built up near the windows and the surface temperature of the glass can actually increase slightly.

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elHoagie

(happ @ Oct. 18 2006,01:12)

QUOTE
Hey elHoagie. How are things in Altadena?

Cold!  I wish I had the same low temperatures you have... and I will once I move to one of those hilltops in a couple years.  I had temperatures down to about 10C earlier this month, but at least it was a balmy 12C this morning  :P .

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happ

(elHoagie @ Oct. 18 2006,13:04)

QUOTE

(happ @ Oct. 18 2006,01:12)

QUOTE
Hey elHoagie. How are things in Altadena?

Cold!  I wish I had the same low temperatures you have... and I will once I move to one of those hilltops in a couple years.  I had temperatures down to about 10C earlier this month, but at least it was a balmy 12C this morning  :P .

It was chilly in the lower foothills and a little windy Lo: 58f/14c  Hi: 84f/28c.

San Francisco : 76/57

LA : 84/56

San Diego : 72/57

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