This may be off, but it seems the Dew Point began to be reported as an indication of the likely lowest overnight temperature. The Dew Point differs from Rel. Humidity, but it is a function of RH. The reason - I assume - that it serves as an indicator of likely lowest overnight temp. is because of the latent heat stored in water vapor. As with water when it freezes, the temperature of water vapor in the air will not not drop below the “Dew Point” temperature until all of the water vapor is condensed into fog/mist droplets due to the relase of the latent heat. If the humidity level is stable (i.e. no large influx of air masses of dramatically different temperature or humidity), it is unlikely that there will be complete conversion of vapor to droplets during the overnight period. Hence, the lowest over night temp. likely to be reached would be the dew point. This is the same reason one waters outdoor plants before an expected light freeze, and why citrus orchards in Southern states spray water as a freeze protection - only the phase conversion is from liquid water to ice rather than water vapor to liquid water. Both processes release latent heat.
USA Today is not such a reliable source.
These are the same reporters who think that, since Guam is closer to California than to NYC, it must be a “westernmost” territory of the US. Of course, being on the other side of the international date line gives Guam the phrase “Where America starts her day,” hardly the phrase of a western location!
At 10 degrees F, in normal pressure, the air can hold no water; the “dew point” is the actual temperature. Dew point does not make much sense at temps below freezing. When moisture is introduced to that environment (10F) it quickly freezes (warming up the air a little as pointed out by FRB. Rapho) and floats off or falls to the ground. The air stays “dry.”
And the Staff Report in question is, What’s the “dew point” all about?
That is not true. Even on the coldest winter days, ice will sublimate if the air is dry enough. There is always an equilibrium vapor pressure associated with a given temperature. At -40 C (very cold), ice and water vapor will be in equilibrium at a vapor pressure of 0.1 Torr. If the air is too dry, ice will sublimate. If the air is too wet, frost will be deposited.
As for pressure, I assume you’re talking about ambient air pressure. But the pressure of the air has nothing to do with it. Vapor pressure is what determins how water changes phase, and vapor pressure is independent of the presence of any other gas.
Here’s a link that talks more about it.
http://www.madsci.org/posts/archives/may98/895551948.Ch.r.html
sford,
The cite does not appear to contradict my statement, nor does your comment.
You point out that ice goes directly to vapor (i.e., no water) when it is very cold. I agree. That’s why you can dry your clothes on a clothes line when the air is so cold that the water in the clothes freezes when hung on the line.
You also point out that, when the air is in a situation where the objects it surrounds are below freezing, frost collects on the objects, thus removing water from the air. I am not talking about water as a gas (vapor).
I will reassert that, when the air itself is below freezing and the pressure is “normal” (ambient, sea-level, whatever), the water freezes. The resulting ice sublimates eventually, again, no water.
When temperatures are below freezing (like your -40), what is the dew point? I claim that objects the same temperature as the air (i.e., between -40.5 and -39.5) will remove water from the air in the form of frost and will have no affect on water present in the air as an equalized vapor. Just for clarity, BTW, I am not measuring time in units of less than a few seconds - this is not the Olympics!
OK… but the air pressure is almost never “normal”. It changes, both over time and with altitude.
It is possible for water to remain liquid below the freezing point - it’s called “supercooled” - although not commonly seen on the ground. As soon as supercooled water hits a solid object it freezes.
Even sub-freezing air contains humidity, often quite high because the cold air can’t hold that much water. If the air temperature drops from that point you can get ice crystals sublimating out of the mix. Dew point is still important, it’s just that the precipitation is solid and not liquid. Dew point is the point at which the air reaches saturation and can hold no more water. Cool the air further and the water will come out, one way or another.
As an example, as you increase altitude the air typically gets colder and at some point it and the dew point match. This is usually where you see clouds. In fact, you can calculate pretty closely the difference between temperature and dewpoint if you know the altitude of the base of a cloud layer. In summer and at lower altitudes these clouds are water vapor. In winter and at high altitude clouds are actually microscopic ice crystals. Cirrus clouds are typically ice, even in summer, being very high and very cold.
And you don’t get frost when everything is the same temperature - you need a relatively cold object coming into contact with relatively warmer air (could be object -40, air -20, so both would be cold, just one is colder)
I’m on shaky ground here since I’m not very knowledgable on the subject. But I was under the impression that water vapor (the gas phase of water) is transparant. Clouds (that aren’t ice crystals) are liquid water droplets that are small enough to stay aloft.
Also, when Yuck talked about “normal” pressure, I suspect he meant “in the range of normal”. His points are meant to apply to situations commonly found on Earth, not in the lab and not in outer space.
You said, “At 10 degrees F, in normal pressure, the air can hold no water…” You went on to say, “The air stays ‘dry.’” I took those statements to mean “no humidity (water vapor).”
Air generally does not hold liquid water. Are you talking about clouds/fog? I don’t think so; the original discussion talked mostly about the difference between relative humidity and dew point. Humidity is water vapor. When the temperature is below freezing, there is still water vapor in the air (like Broomstick said). When people talk about “dry air”, they don’t mean that it contains no liquid (fog), they mean that it contains no water vapor (humidity).
Your claim was that it makes no sense to talk about “dew point” at temperatures below freezing. Is your quibble over the word “dew”? By taking the common meaning of the word, you’re right - no liquid water forms on surfaces that are at or below the dew point temperature. But if you generalize the concept, the dew point refers to the temperature at which water vapor changes state and is deposited on surfaces. If the air temp is 10 degrees F and the dew point is 5 degrees F, then frost will form on a surface that is 5 degrees or colder. Since there is no term “frost point”, I believe that “dew point” is used for below-freezing cases.
Actually, that isn’t what you asserted. You asserted that when moisture is introduced into below-freezing air, it freezes. True if that moisture is liquid water. But I’ve heard people talk about humidity as being “moisture in the air”. Sorry if I mis-interpreted you.
So although the term “dew point” might be considered a bit of a misnomer in cold temperatures, the principal still makes sense. The dew point is not necessarily the same as the actual temperature, as you claimed.
I distinguish, perhaps too much, between the liquid - water - and the gas - vapor.
Below freezing, we talk about the wind chill factor relating to the speed that the liquid evaporates from one’s skin and the resulting “chilling” (due to the fact that water requires energy to change states up, releases energy changing states down). The reason the calculation of wind chill factor does not distinguish relative humidty is because it only counts below freezing where relative humidity is virtually (my cop out!) zero.
Humidity, to me, has to do with “moisture,” not vapor, in the air. Again to me, moisture has to do only with the liquid, not the gas.
I agree that the amount of moisture dissolved in the air is affected by the vapor pressure, but assert that moisture cannot exist below freezing (adjusted appropriately for whatever pressure we are talking about - but I would like to stay a sea level).
Vapor pressure, of course, is important as it determines whether the water in your skin is going to evaporate or not - causing your skin to get colder. Thing is, above freezing, it is the moisture that makes you uncomfortable. Below freezing, water in the air is either ice or vapor, not moisture. Humidity is not present. “Exposed” water (the liquid) evaporates or freezes.
How much doo-doo could a Dew Point do if a Dew Point could do dew?
Well, someone had to say it.
Good clarification - I should have said “water vapor” and you are correct, for all practical purposes water vapor is transparent. Clouds and fog are, indeed, liquid droplets (or ice crystals) of minute size that are small enough to stay aloft.
I should note, however, that at higher temperatures and high humidity the water vapor can manifest as “visible moisture”, a haze too thin to be a cloud, but still visbile between oneself and the horizon. But that may be more picky than the present discussion requires.
Um… “wind chill” does not depend upon evaporation of anything, although evaporation certainly does make any temperature more chilly to the person experiencing the effect on their wet skin. In still air, the warmth of your skin heats the air directly adjacent and the temperature gradient is reduced, slowing (somewhat) heat loss because the temperature differences are not as extreme. With windchill, the adjacent air (which you have just warmed) is replaced by cooler air, which you must then heat up again. So the temperature gradient remains at its steepest and you lose heat faster than in still air. This effect would also speed evaporation or freezing as well, but those are effects of windchill and not causes of it.
Actually, while the absolute amount of water vapor in sub-freezing air is small, the relative humidty may, in fact, be quite high.
Also, windchill “counts” at considerably above freezing temperatures. In fact, it starts counting not very much below what is perceived to be comfortable room temperature. While you can not literally freeze to death at 50 degrees Farenheit a sufficient windchill factor can cause you to die of hypothermia at that temperature. So yes, it counts.
Well, OK, but in discussions of humidity please let everyone know you’re using your own definition. The official meterological definition of humidty involves the concept “water vapor”, or H2O in the gaseous state.
Yes, it can. I know this from personal experience.
On several occassions while flying my Cessna 150 with the standard carbeurated Continental engine on clear winter days (the high pressure of such conditions plus the cold air would mean a pressure at or even greater than standard sea level conditions), at a temperature of about 25-30 degrees I have experienced the phenomena of “cabeurator ice”. If you want the full technical explanation I’ll supply it in a later post, but to be brief, the air going through the airplane’s carbeurator undergoes a rapid drop in pressure, which can cause any moisture in the air to precipitate out in the form of ice, which then proceeds to clog your carbeurator and choke off the air going to the engine, stealing your RPM’s and eventually causing a decrease in altitude (at times all the way to a landing). Fortunately, this sort of clogging is not instantaneous and can be dealt with if caught early. The pilot’s handbook for this airplane (and all others with carbeurators) stress that even as low as 20 degrees Farenheit air can contain enough water vapor to cause carbeurator ice, even to the point of causing the engine to stop firing completely. This is not flying through clouds or fog - this is in clear air. It comes from water vapor in the air - in other words, from the humidity present in the air.
Apparently, I am not alone in my definitions:
http://www.dictionary.com/cgi-bin/dict.pl?term=humidity&db=*
and
http://www.graylab.ac.uk/cgi-bin/omd?humidity
both actually use the word “wetness” which, to me, applies only to the liquid:
http://www.dictionary.com/cgi-bin/dict.pl?term=wetness&db=*
and for “wet”
http://www.graylab.ac.uk/cgi-bin/omd?query=wet&action=Search+OMD
and for moisture:
http://www.dictionary.com/cgi-bin/dict.pl?term=moisture&db=*
“moist”
http://www.graylab.ac.uk/cgi-bin/omd?moist
I have lost the reference, but the weather service is changing the wind chill chart to reflect the fact that it doesn’t make much sense above freezing.
The following ref shows a wind chill chart on the new basis from Canada - notice that it ignores temps above freezing:
http://www.msc-smc.ec.gc.ca/windchill/education_documents/Wallet_card_e.pdf
If you find the right page, you will notice that a 100 mile an hour wind at -40 degrees blowing past a wooden board will make the board exactly -40 degrees - no less. Exposed skin will get a lower temperature until all moisture is gone, at which time it will be exactly -40. An exposed stiff that is thoroughly dried out will not reach any temperature lower than -40. It is the change of state of the water (up) that is cooling the skin to a temperature lower than -40.
http://www.usatoday.com/weather/winter/windchill/wind-chill-second-law.htm
touches on the issue.
As far as actual heat loss goes, everything warmer than -40 will eventually hit -40 and, with a stiff wind, will achieve that result faster. This is not the essential element of wind chill. Wind chill is confined to the effect on exposed skin. The skin cools at a speed that relates to the same wind speed and a much lower temperature. I do not dispute the reality that warming a greater volume of air with your body takes more heat than warming a smaller volume. I am saying that, it is the evaporization of the moisture (till it freezes) in your skin that makes a freezing wind effectively a lower temp.
Sorry for being long-winded. The point for me remains: Dew point is primarily for temps above freezing and wind chill is for temps below freezing. Both have to do primarily with moisture (recognizing that sublimating ice does get colder than the air around it do to the change of state - up).
You seem to miss the point why dew point is very usefull information when temperature is freezing (ie below 0 C).
FRB. Rapho in his response stated that but he talked only about night temperatures.
In extreme north (or south) there is time when sun don’t shine during days at all. The dew point is very usefull information because it tells you the lowest temperature that the is possible.
The reason is that when water vapor condenses either to liquid (water) or to solid (ice) it will release heat.
Thus when in Finland couple of years ago was measured last centurys lowest temperature in Finland (-52,3 C) the dew point was, surprise surprise, -52,4 C.
Yours: Topi
I cut Yuck’s series of definitions, although I did take the time to read them. I see nothing that excludes either my interpretation or his. I also feel compelled to point out that these definitions are from a general dictionary, geared towards the general public, and not from a professional meterological source which, being a technical profession, is likely to define such terms in a more specific and precise manner than “moist”, “wet”, and “damp”
The meterological definition that I recall being taught is that relative humidity is the percentage of water vapor in the air divided by the total amount of water vapor the air mass is capable of holding under current conditions. Actual liquid H2O is not humidity it is precipitation, although in the case of clouds and fog that precipitation is so minute and light that is can remain suspended within the air.
Uh… yeah, it DOES make a difference above freezing. You must not spend much time outdoors in the spring and fall, or else your region of the world is lacking in wind. Granted, the difference is not as great above freezing, but 40 degrees F with a 40 mph wind is noticeably different thatn 40 degrees F with no wind.
Simply choosing to not print something doesn’t mean the effect does not exist. Since a 60 kph wind at freezing results in a windchill factor of -9C (conditions quite common in Chicago, at certain times of the year) I find it hard to believe that at 1C a 60 kph wind would have no effect on my comfort level. In fact, I know from experience that it most certainly DOES make a difference.
Nowhere did I state that windchill would cool anything below the actual ambient temperature. What windchill does is accelerate the rate of cooling. You get to that -40 faster in windy conditions than in still air.
You are confusing the effects of evaporation with the effects of dry heat transfer. Evaporation, in and of itself, will cool an object. It can even cool it below the ambient temperature (after the moisture is gone the object’s temperature will rise to match the ambient). Windchill effects will speed evaporation over that occuring in still air but you’re still talking about two different phenomena.
Yes, it is.
Mainly because that’s what’s important to humans. But that doesn’t mean other things aren’t affected.
Well, you’re wrong.
Well, OK, that’s your point. But I still say dew point is a factor below freezing and wind chill exists above freezing.
Back in ninth grade science, forty years ago, we used a device called a sling psychrometer, which was two thermometers mounted on a stick, one having a little sock on the end. You wet the sock, and slung the stick over your head, and after a while, read the two thermometers. The relatively coolness of the “wet bulb” due to evaporation indicated the relative humidity, which you read off a chart.
It seem intuitively obvious that this wet bulb temp would be the same of as the dew point (the temp that water evaporates/condenses) but I have never seen the connection made.
Ok, you weather-gurus…what’s the skinny?
If you want to see the new American spin on the subject, <A HREF=“http://www.srh.noaa.gov/tulsa/windchill.html”>CLICK HERE</A>. Note that it acknowleges that windchill occurs at temperatures above freezing.
It also shows the equation used, along with a built in calculator. Note that if you plug in 0 as your temp, and as your windspeed, you actually get a somewhat warmer windchill temperature.
This seems bogus, but the small print has an escape clause. Just below the chart, it says that “calm” means at a brisk walk upto 4 mph, with no wind. And if you plug in a windspeed of 1 mph, or above, everything appears more normal.
However, this is not just a piece of fiction to defend the calculations. According to the Beaufort Scale, 1 mph and below is considered “calm,” and 3 mph or below is considered “light air”, whose direction can not even be shown by wind vanes.
(sford looks around for a weather guru. Seeing none, he decides to chime in.)
I don’t think they would necessarily be the same. But I’m not sure. Let me think this through.
As you sling the psychrometer, water evaporates from the wet bulb and cools. As it cools, it approaches the dew point. If it reached the dew point, then the water would be in equilibrium with the vapor in the air, and no further evaporation would take place. Hence the bulb would stop cooling.
So far so good. BUT, as you continue to sling it, the higher temperature of the ambient air would tend to re-warm the web bulb.
I believe that there is an equilibrium point, above the dew point but below the air temp, where water will continue to evaporate. But the heat removed from the bulb from the evaporation would equal the heat added to the bulb from the surrounding air.
But I have no idea where this equilibrium temp would lie. Perhaps half-way between the air temp and the dew point? Perhaps 90% of the way to the dew point? I’m not sure. If I had to guess, I would say it would be a lot closer to the dew point simply because I suspect evaporation is a more efficient cooler than ambient air is a warmer.
So I suspect your guess is pretty close.
Thank you normfromga! Very helpful.
FYI - you can’t include straight html tags in your posts. This message board uses something called “vB” codes. See http://boards.straightdope.com/sdmb/index.php?action=bbcode for info.
In my opinion, the “best” way to include a link is to let the message board software do it for you. Rather than seeing the link simply say “CLICK HERE”, I prefer a link that says the name of the web site. Just type the URL:
http://www.srh.noaa.gov/tulsa/windchill.html
http://www.srh.noaa.gov/tulsa/windchill.html
This is the correct address. I hope it hyperlinks better than my last try…8^)
I guess these things work better when you check out the rules at the top first, or at least to see how fast you were to help me fix my linkage.
I tend to agree with sford (sinse he tended to agree with me.)
I do not think the reheating was much of a factor. Once dry, the sock/wick probably applied pretty good insulation from the ambient air. Also, as I recall, there was some time limit as to how long that thing was “slung.” I don’t remember the exact time, but for my pudgy arms, it was too long.
(Probably OSHA has made them things illegal…carpal tunnel syndrom, you know.)
Still, I will drop by once in a while to see if a real guru DOES show up…(no offense)
Cheers,
Norm