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What is a dewpoint?


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I just did a search and got alot of good information defining a dewpoint...  

Unfortunately, it was info overload and just made my eyes swim and my head spin...  

Can anyone tell me what a dewpoint is without making my head do loopdeloops? :)

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(PiousPalms @ Jan. 02 2008,22:11)

QUOTE
I just did a search and got alot of good information defining a dewpoint...  

Unfortunately, it was info overload and just made my eyes swim and my head spin...  

Can anyone tell me what a dewpoint is without making my head do loopdeloops? :)

The temperature to which the air must be cooled for water vapor to condense, i.e. to make dew, or frost.

In my post I sometimes express "my" opinion. Warning, it may differ from "your" opinion. If so, please do not feel insulted, just state your own if you wish. Any data in this post is provided 'as is' and in no event shall I be liable for any damages, including, without limitation, damages resulting from accuracy or lack thereof, insult, or any other damages

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The important thing about the dew point is that it "indirectly effects the rate of temperature fall". A higher dew point (more water vapor in the atmosphere) slows the rate of fall. Whereas, lower dewpoints mean the temps fall faster.

From a History of the Florida Citrus Freezes

John Attaway

NW Hillsborough County, FL (Near Tampa)

10 miles east of the Gulf of Mexico

Border of Zone 9b/10a

Lakefront Microclimate

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In practicality and in context with our other winter weather posts, it is the theoretical low point of resistance for the overnight low temperature.  That is if you have a dew point of 30F, it is theoretically possible with all other conditions being perfect (no wind, no urban heat factor, etc.) for the temperature to reach that low overnight.  The more moisture in the air, the higher the dewpoint and therefore a higher "low".

Steve

USDA Zone 9a/b, AHS Heat Zone 9, Sunset Zone 28

49'/14m above sea level, 25mi/40km to Galveston Bay

Long-term average rainfall 47.84"/1215mm

Near-term (7yr) average rainfall 55.44"/1410mm

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(steve 9atx @ Jan. 02 2008,22:24)

QUOTE
In practicality and in context with our other winter weather posts, it is the theoretical low point of resistance for the overnight low temperature.  That is if you have a dew point of 30F, it is theoretically possible with all other conditions being perfect (no wind, no urban heat factor, etc.) for the temperature to reach that low overnight.  The more moisture in the air, the higher the dewpoint and therefore a higher "low".

Steve

The temperature can fall below the dewpoint, but to do so, water must condense out of the air, and in so doing, heat is released.  This process thus tends to slow the fall in temperature.

So, higher dewpoints are "good" in this sense.

But, they are "bad" in that on nights with calm air, higher dewpoints allow frost to form much more easily.

Larry 

Palm Harbor, FL 10a / Ft Myers, FL 10b

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Exactly, so with a dew point of 6F in Tampa we are probably in deep trouble.

NW Hillsborough County, FL (Near Tampa)

10 miles east of the Gulf of Mexico

Border of Zone 9b/10a

Lakefront Microclimate

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(tropical1 @ Jan. 02 2008,22:30)

QUOTE
Exactly, so with a dew point of 6F in Tampa we are probably in deep trouble.

Kyle-

Look at the bright side.  We probably dont have to worry about frost :D

Well...excuse me for seeing the glass half full!  :P

LOL

Larry 

Palm Harbor, FL 10a / Ft Myers, FL 10b

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Thank you for these explanations of the effects of dew point on temperature fall and formation of frost.

So - the low dewpoints tonight are preventing frost from forming, but are also responsible for allowing the temps to fall more.  Like taking the good with the bad.

St. Pete

Zone - a wacked-out place between 9b & 10

Elevation = 44' - not that it does any good

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Larry

I can honestly say that in my whole life of watching the weather in SE Texas (similar to your climate), I have never seen the low actually go below the dewpoint.  I don't think y'all have to seriously worry about temps in the single digits tonight.

Steve

USDA Zone 9a/b, AHS Heat Zone 9, Sunset Zone 28

49'/14m above sea level, 25mi/40km to Galveston Bay

Long-term average rainfall 47.84"/1215mm

Near-term (7yr) average rainfall 55.44"/1410mm

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ok no frost is a good thing.  :D

No I dont think we will hit anywhere near single digits, but if we elude a hard freeze it will be a miracle.

Snow flurries are forecast for Daytona Beach Area (see NWS Melbourne).

NW Hillsborough County, FL (Near Tampa)

10 miles east of the Gulf of Mexico

Border of Zone 9b/10a

Lakefront Microclimate

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(SunnyFl @ Jan. 02 2008,22:38)

QUOTE
So - the low dewpoints tonight are preventing frost from forming,

The gusty winds are also preventing frost.

Larry 

Palm Harbor, FL 10a / Ft Myers, FL 10b

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(steve 9atx @ Jan. 02 2008,22:38)

QUOTE
Larry

I can honestly say that in my whole life of watching the weather in SE Texas (similar to your climate), I have never seen the low actually go below the dewpoint.  I don't think y'all have to seriously worry about temps in the single digits tonight.

Steve

Yeah...no way we will fall below our dewpoints tonight.  

Ive watched the temp fall below the dewpoint several tiems here.  Its fun to watch your weather station as it happens as one can actually see physics in action!  As the RH nears 100%, the temperature flattens right out and holds steady for a good while before it drops again.

Of course, as this is happening all the nice skyward facing plant leaves are being drenched with water primed to cool off radiationally and create a big mess :P

Larry 

Palm Harbor, FL 10a / Ft Myers, FL 10b

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(tropical1 @ Jan. 02 2008,22:42)

QUOTE
ok no frost is a good thing.  :D

No I dont think we will hit anywhere near single digits, but if we elude a hard freeze it will be a miracle.

Snow flurries are forecast for Daytona Beach Area (see NWS Melbourne).

snow flurries???

I had to go see for myself - yep, the NWS site says Daytona could see flurries before 11am.   Yikes!

St. Pete

Zone - a wacked-out place between 9b & 10

Elevation = 44' - not that it does any good

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CURRENTLY-TODAY...LARGE MASS OF STRATOCU OVER THE COASTAL WATERS LED

TO A FLURRY OF SNOW FLURRIES ALONG THE NORTH HALF OF THE COAST THIS

MORNING.

MORE DETAILS ABOUT THE FLURRY REPORTS WILL BE CONTAINED IN A PUBLIC

INFORMATION STATEMENT THAT WILL BE RELEASED LATER THIS AFTERNOON.

ADDITIONALLY...MEDIA CAN UTILIZE THE MELBOURNE CHAT ROOM FOR MORE

INFORMATION.

Jon

Brooksville, FL 9a

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Funny, but here we are always concerned with the dewpoint from a heat perspective not a cold perspective.

The dewpoint here is currently 25C (77F) and it is very sultry!

Daryl

Gold Coast, Queensland Latitude 28S. Mild, Humid Subtropical climate. Rainfall - not consistent enough!

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My simplistic understanding is that, if you have temperature A and relative humidity B, then the dewpoint is the temperature at which the relative humidity would be 100%.  Therefore, frost or condensation will form on any surface that has a temperature at or below the dewpoint, regardless of the air temperature.

]

Corey Lucas-Divers

Dorset, UK

Ave Jul High 72F/22C (91F/33C Max)

Ave Jul Low 52F/11C (45F/7C Min)

Ave Jan High 46F/8C (59F/15C Max)

Ave Jan Low 34F/1C (21F/-6C Min)

Ave Rain 736mm pa

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William, not sure if your question was answered or you other info you were looking for, but several have touched on it.  But to try to put it simply:

The Dewpoint is the temperature that any surface must be (glass, wood, metal, skin, plants, etc.)   must be in order for the moisture (or humidity) in the air to condense on THAT surface.  As Larry noted, that change of state will release some heat/energy onto that same surface. (helping to "warm" it) in effect.  

I have to use dewpoints all the time as I repair air conditioners, and I use little examples that people recognise.

If you have your nice, Ice cold Ice tea in a glass and its not very humid, you will notice very little condensation on the outside and it will stay cool a while, if its a bit more humid, you'll see more condensation on the glass, and it will get "warmer" a little faster, if its "real" humid, you'll see ALOT of condensation, almost a pool around it, and your drink will warm up quite quick.    Now just so all that makes sense for humidty and its effect on dewpoints, in each example, the temp of the drink, and the outdoor temp started the same, only the humidity changed!!!.

If you are humid all the time, you may have a hard time visualizing that! :D

The same effect is happening at lower temperatures too, but as was said, the higher the dewpoint, the higher the humidity,

the lower the dewpoint, the lower the humidity. The out door temp can remain the same.  (keep in mind, its surface temperature that is the "dewpoint")

Zone 10a at best after 2007 AND 2013, on SW facing hill, 1 1/2 miles from coast in Oceanside, CA. 30-98 degrees, and 45-80deg. about 95% of the time.

"The great workman of nature is time."   ,  "Genius is nothing but a great aptitude for patience."

-George-Louis Leclerc de Buffon-

I do some experiments and learning in my garden with palms so you don't have to experience the pain! Look at my old threads to find various observations and tips!

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Another way to look at it is.  The dewpoint is a measurement of the actual amount of moisture in the air.  If the temperature is 30°C/86°F or 20°C/68°F, but in both cases the dewpoint is 15°C/59°F, then both would have the same amount of moisture in the air.  Because warmer air has the capacity to hold more moisture the relative humidity at 30°C/86°F would be about 40%, which is lower than the relative humidity at 20°C/68°F which would be about 73%.  Therefore, the air at the lower temperature is more saturated, but has the same amount of moisture.  When temperatures fall to the point at which relative humidity equals 100% (the dewpoint), the air is totally saturated, i.e. holding the maximum amount of moisture that it can hold at that temperature.  At that point the moisture will condense.  Effectively the water/moisture always wants to condense, but it cannot do so until it comes in contact with a surface at or below the dewpoint, so any such surface will attract the moisture.

If you have a sealed environment where the air is contained and you have a palm tree and something made of metal (a good heat conductor, which loses it's heat more readily) a drop in air temperature would cause the metal object to reach the due point before the palm tree.  Water would then condense on the metal object before condensing on the palm tree, because the metal object would release it's stored heat, faster than the palm tree and the air, reducing it's surface temperature to the dewpoint before the palm tree surface or the air temperature reached the dewpoint.  This would have multiple effects.  The energy released from the metal as it cooled would effectively slow the cooling of the air or even increase the air temperature.  The water condensing on the metal surface would release more energy, also reducing the cooling of the air.  The moisture having, condensed on the metal surface, would mean that there is less moisture remaining in the air, reducing the dewpoint.

]

Corey Lucas-Divers

Dorset, UK

Ave Jul High 72F/22C (91F/33C Max)

Ave Jul Low 52F/11C (45F/7C Min)

Ave Jan High 46F/8C (59F/15C Max)

Ave Jan Low 34F/1C (21F/-6C Min)

Ave Rain 736mm pa

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