imported>Johnjbarton: /* "When the air is cooled below the dewpoint" */ Reply

2025-05-17T17:44:35Z

"When the air is cooled below the dewpoint": Reply

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== Dew Point record ==

How come no info for LOWEST Dew Point ever recorded ? [[Special:Contributions/70.190.65.59|70.190.65.59]] ([[User talk:70.190.65.59|talk]]) 22:39, 13 December 2024 (UTC)

:Because the lowest dew point is 0 Celsius (32 Farenheit) Any temp lower than this is the frost point. [[Special:Contributions/104.139.98.96|104.139.98.96]] ([[User talk:104.139.98.96|talk]]) 12:16, 14 March 2025 (UTC)
:The lowest dew points happen in interior Antarctica, which is the region with the lowest air temperature. On average it is near Dome A, or Ridge A, two locations that are considered the best place for telescopes on Earth as there is 24-hour darkness and extremely low huimdity.
:For other stations, there might be too much junk data to look through. Some weather stations can struggle with extremely low relative humidity, or they might not report a humidity below a certain temperature. The difference in moisture in the air at a dew point of -40 and -60C is tiny.
:If you live in the US and want to know the record for you area, there is a site for US stations. Search for "Iowa Mesonet Plotter" and pick graph #153. There is an option for "Lowest Dew Point Temperature". [[User:Crunch41|Crunch41]] ([[User talk:Crunch41|talk]]) 03:37, 24 March 2025 (UTC)

== Apparent implication that condensation is impossible ==

''"In normal conditions, the dew point temperature will not be greater than the air temperature"''—Surely this would imply that in normal conditions condensation is impossible, since ''at'' the dew point evaporation and condensation are already in balance and above it evaporation outweighs condensation.

So for condensation to form at all, the dew point needs to be above the current temperature, and the rate of condensation will depend on the difference. A condensation rate of zero isn't going to get you any condensation.

I suppose ''Under equilibrium conditions, the dew point is normally . . . '' would be true though.

Condensation will form on a cold surface— but surely that's because air at the surface is cooled below its dew point. [[User:Musiconeologist|Musiconeologist]] ([[User talk:Musiconeologist|talk]]) 10:04, 4 February 2025 (UTC)

:Sorry, your claims don't make sense to me. The phrase
:* {{tq|In normal conditions, the dew point temperature will not be greater than the air temperature}}
:makes no assertion about condensation. I take "normal" to be the same as "equilibrium".
:I do think the sentence would be clearer as
:* In normal conditions, the dew point temperature is less than or equal to the air temperature. As the air temperature drops to the dew point temperature, relative humidity approaches its limit of 100% and water condenses into a liquid.
:[[User:Johnjbarton|Johnjbarton]] ([[User talk:Johnjbarton|talk]]) 16:27, 4 February 2025 (UTC)
::@[[User:Johnjbarton|Johnjbarton]] My point really is that a casual reader has ''normal atmospheric conditions'' in mind—the weather, maybe, or their kitchen—and that equilibrium isn't normal in an everyday sense. They think they're being told that it takes exceptional circumstances for the air temperature to drop below the dew point.
::Trying to explain what my logic was:
::Maybe I've misunderstood and need to go and dig out a physics textbook from university, but my understanding is that the dew point is the boundary between condensation occurring faster than evaporation (assuming there's some liquid water present to evaporate) and vice versa. So ''at''' the dew point, as an immediate mathematical consequence, the rate of condensation-minus-evaporation is zero, and above the dew point it's negative.
::BUT condensation is an everyday phenomenon that we see all the time, meaning the rate is sometimes positive, meaning the original statement can't be strictly true without contradicting everyday experience. So either my logic or definition is wrong, or the original statement is wrong. I was making that mathematical argument.
::On the other hand it seems plausible that the drop below the dew point is usually tiny, and also that condensation onto a surface typically involves a layer of air that's cooler than the bulk of the surrounding air by virtue of being cooled by the surface, in which case the main volume of air ''can'' be at or above the dew point and still allow condensation onto the surface.
::And it's also possible that analysing the situation in terms of the rate of condensation-minus-evaporation is inaccurate somehow.
::I hope that's a bit clearer, anyway. As an explanation on this talk page of what I mean, of course, not as suggested content for the article. [[User:Musiconeologist|Musiconeologist]] ([[User talk:Musiconeologist|talk]]) 19:39, 4 February 2025 (UTC)
:::I altered the content based on a new source. [[User:Johnjbarton|Johnjbarton]] ([[User talk:Johnjbarton|talk]]) 19:10, 5 February 2025 (UTC)

== boiling point ==

Is it possible to define the difference with the boiling point? thanks [[Special:Contributions/151.29.146.245|151.29.146.245]] ([[User talk:151.29.146.245|talk]]) 13:55, 26 March 2025 (UTC)

In particular, I have sources stating that the dew point decreases with height by 2 C/km and other sources stating that for the
boiling point the decrease is 3.4 C/km. <small class="autosigned">— Preceding [[Wikipedia:Signatures|unsigned]] comment added by [[Special:Contributions/151.29.146.245|151.29.146.245]] ([[User talk:151.29.146.245#top|talk]]) 15:28, 26 March 2025 (UTC)</small> 

:Perhaps you will share those sources with us. [[User:Johnjbarton|Johnjbarton]] ([[User talk:Johnjbarton|talk]]) 16:21, 26 March 2025 (UTC)
::A dew-point gradient of -2 C/km in Strahler, Physical Geography (3rd edition, 1969), page 186 and in Strahler, The Earth Sciences (2nd edition, 1971), page 293
::The boiling-point gradient of -3.6 C/km in an old italian book that you cannot get in your country or in INTERNET ... but -3.4 C/km in Stull, Practical Meteorology, page 90 ... my grandson has downloaded it for me and I am unable to give you the URL (or whatever its name)
::thanks. pietro spizzo [[Special:Contributions/151.29.146.245|151.29.146.245]] ([[User talk:151.29.146.245|talk]]) 16:55, 26 March 2025 (UTC)
:::Using the Strahler ref I added as section on altitude.
:::The boiling point and dew point are not directly related. Boiling point is a liquid temperature; dew point is a gas temperature. These two are not related unless you have a closed system, like a pressure cooker.
:::To vaporize water we have to add energy just beyond the boiling point, the latent heat of vaporization. When water condenses this latent heat enters the air, not primarily the water. The asymmetric loss of this heat makes the two values differ. Unfortunately I could not find a source to spell this out. [[User:Johnjbarton|Johnjbarton]] ([[User talk:Johnjbarton|talk]]) 19:23, 26 March 2025 (UTC)
::::thanks very much. pietro spizzo [[Special:Contributions/151.29.146.245|151.29.146.245]] ([[User talk:151.29.146.245|talk]]) 20:20, 27 March 2025 (UTC)

== "When the air is cooled below the dewpoint" ==

Third sentence of this article is wrong, as the temperature is never lower than the dewpoint. Condensation happens when the dewpoint approaches the temperature. [[User:Jacobi Jackson|Jacobi Jackson]] ([[User talk:Jacobi Jackson|talk]]) 04:54, 17 May 2025 (UTC)

:Since you did not bother to say, I assume this sentence is:
:* {{tq|When the air is cooled below the dew point, its [[moisture]] capacity is reduced and airborne water vapor will [[Condensation|condense]] to form liquid water known as [[dew]].}}
:There is nothing terribly wrong with this sentence. The concept of "dew point" assumes equilibrium and the sentence is about a temperature change. I would make the same criticism of your claim that "the dewpoint approaches the temperature" since the dewpoint is not a thing that can change. It's an equilibrium property.
:Nevertheless we can do a bit better
:* {{tq|When the air at a temperature above the dewpoint is cooled, its [[moisture]] capacity is reduced and airborne water vapor will [[Condensation|condense]] to form liquid water known as [[dew]].}}
:Similarly I would rewrite your claim as
:* {{tq|Condensation happens when warmer air cools to the dewpoint.}}
:[[User:Johnjbarton|Johnjbarton]] ([[User talk:Johnjbarton|talk]]) 17:44, 17 May 2025 (UTC)

Talk:Dew point - Revision history

imported>Johnjbarton: /* "When the air is cooled below the dewpoint" */ Reply