So, the TV weather man commented today that the temperature can never drop below the dew point temperature - implying it is a floor. This makes no intuitive sense to me.
I can see where once the temp hits the dew point value, they would drop in tandem - but why would the amount of moisture in the air (I’m assuming that and barometric pressure sets the dew point) prevent the temperature from dropping?
wikisayth
It’s misleading, what he says. All he’s really saying is that air temperature will always be equal to or greater than dew point, but as you point out, the dew point is dynamic and can drop in tandem with the temperature. It’s not like a dew point of 33º at 6:00 in the evening means that the temperature will never go below 33º during the night.
I may have to send the question to the weather guy - he definitely implied that the since the dew point was 55 degrees (or some number), that it wouldn’t get any colder than that. It’s not the first time that I’ve heard that station say that - but perhaps they are just misspeaking.
I did Google the question before posting it here, and found one cite/site (that I can’t pull up again) that said something about the energy required to condense the water out is large - but I can’t imagine that that energy release would be enough to stabilize air temperature, would it?
No. If he used the exact phrasing that you reported, it was quite misleading.
The dew point is the temperature at which the current amount of water vapor in the air is the maximum amount that the air can hold. But that amount of water is not fixed, so the dew point is not a barrier. If the temperature drops below the current dew point, some of the water vapor condenses out (as dew, funnily enough!), lowering the dew point.
So it’s true that the temperature can never be below the dew point. But only in the tautologous sense that the air can never hold more than the maximum amount of water vapor that it can hold.
I think that these people who present the weather are chosen more for their presentation skills than their knowledge of temperature inclines and even their weather knowledge. The guy was just reading a script and probably didn’t understand what he was saying.
Not quite right. If the [air] temperature drops below the dew point, some of the water vapor condenses out as fog, not dew.
Dew (or frost, if cold enough) forms on the surfaces of objects whose temperature drops below the dew point, usually because of heat radiation into space on a clear night. This is why dew doesn’t form much on cars parked in carports or under trees. It gives the impression that the dew has “fallen” from the sky when in reality, it forms in place because the heat has “risen” toward the heavens.
While that’s true, it isn’t what the meteorologists mean when they say something like on the 6 o’clock weather report something like “well, the dew point out there is 42 degrees so we don’t expect the low tonight to go much below 42.” What thy mean is that, yes, the amount of energy released in the condensation of fog is enough to stabilize the temperature.
While the relative humidity can change rapidly, the dew point (which can be thought of as a measure of absolute humidity*) not so much. It can change slightly during the day as the sun warms any water causing it to evaporate and drop slightly during the night as dew (for fog/frost) forms, but unless either dryer or moister air masses move into the area (usually in the form of fronts), the dew point is not going to change much.
*Dew point is dependent on air pressure. The air pressure does fluctuate, but this has a minor effect on dew point since the fluctuation is small. In instances where considerably higher air pressures are encountered (in a hyperbaric chamber, for example), the design of the chamber just account for the fact that “dew” will form on just about all surfaces when the pressure is brought up to 5 times atmospheric. Of course, this is far outside what would typically be considered “typical fluctuation”.
Quoted for ironic value. While there are most likely some cases where this true, I doubt this is one of them. Quite the opposite, in fact. What we have described in the OP is a person whose factual knowledge is high (he was correct), but failed at presenting this information in a manner that was easily understood. Probably a combination of only having a few seconds to explain it and what I call the “if I understand it, everyone understands it” syndrome.
I’m seconding Excavating’s point.
I’ve seen this before and its a subtle point. I’m guessing he did not mean that the temp will never go below his stated dew point.
What he meant was for tonight, with those current conditions, the temperature will drop until it gets to the dew point and stabilize.
As water goes through a phase change it tends to stabilize the temp (this is why a freezing an ice/water mix is normally locked at 32F. But once the ice is all frozen, the temperature of the ice will start to drop again)
Same here… if conditions are cold enough, the temp will drop below the previously stated dew point. But if the conditions aren’t too severe, the temp will drop to the dew point and stabilize as the water goes through the phase change to liquid.
Like I said, its a subtle point and not always easy to describe when you have 10 seconds to describe how cold its going to get tonight.
I’m glad I came back to the thread to see Excavating’s post, since I was about to send a querying email to the local weather guy.
And I think, given the above posts, his statement was accurate enough for a newscast. I would question why he (and others on this station) have started mentioning it - in general, I think saying “the low tonight will be 45 degrees” is fine. Citing the dew point as the controlling factor (especially if there are cases of sudden drops where it won’t hold) strikes me as an attempt to wow the audience with how scientific their forecast is.
And it is actually possible for the temperature to be below the dew point, just like it’s possible to have liquid water below 0C or above 100C: It’s just not a very stable situation.
I do use this knowledge to my advantage, for example when I’m going camping. I have to admit that I didn’t realize it until I first heard a weather newscaster make the suggestion that temps wouldn’t go below the dew point. I regularly use the dew point to guide what I should prepare for or if I need to bring my plants in, etc. So hearing this phenomenon from the weather guy was useful for my learning
Unless I have it wrong, water vapor condensing can release a bunch of energy. Take a cumulus cloud, for instance. Normally, they are a couple to a few thousand feet thick with the temperature at the base of the cloud at the dew point. Thermals push the air above this level spreading the cloud vertically and horizontally. But sometimes they will be kind of narrow and very tall. As the water condenses it releases heat which continues to rise and more water condenses. Heat is released and the cycle continues. Eventually, you may end up with rain or thunderstorms. Conditions have to be correct for this to happen. Some clouds may look like cotton candy but there is a tremendous amount of energy hiding in there.
Yup, you’ve got it right. A given mass of water vapor contains more energy than the same mass of liquid water. Whereas evaporating water draws heat from its surroundings (this is why sweating works), condensing water dumps heat into its surroundings, slowing the rate at which the temperature drops.
Meteorologists speak of the lapse rate, the rate at which the temperature of a parcel of air decreases as it moves upward through the atmosphere. Note that there is a dry lapse rate, and a moist lapse rate: the dry rate is what you get when there’s no condensation, and it’s about 5.4 degrees F per thousand feet. The wet rate is what happens inside clouds, and it’s 2.7 degrees F per thousand feet; the ongoing condensation of water as that cloud rises is what limits the temperature drop, sustaining buoyant ascent.
In fact, it takes several times more energy to convert between liquid water and water vapor than it does to convert between liquid water just above freezing to liquid water just below boiling.
As a weather nerd (and one-time aspiring meteorologist), I understand the concept of dew point, and how it relates to how cool the air can get. But, while it’s a relatively easy concept for me (and for actual meteorologists and fellow weather nerds) to grasp, it is not an easy concept for the true layman. I’ve tried to explain dew point to friends, and how it relates to relative humidity – these are intelligent people, but it’s not an easy concept to convey to them.
Humidity is something that laymen understand conceptually (i.e., humid air can make it feel even warmer than the straight air temperature), but most laymen have no idea about the idea that warmer air can hold more water vapor, nor that the same air will have a lower relative humidity at noon than it did at 6 a.m., simply because the air temperature rose. And, people misuse / misunderstand / misinterpret the relative humidity stat all the time (i.e., “It was 98 degrees, with 100% humidity” – which is an extremely rare condition, outside of tropical rain forests).
Dewpoint is, for a lot of purposes, a better way to convey the idea of how much water vapor / humidity there is in the air – once you understand what it actually is and how it works. And, I agree – many professional meteorologists either aren’t good at providing that layman-level description, or aren’t given the time to do so.
Let’s quantify things.
Assume the dew point is 10 [sup]o[/sup]C (50 [sup]o[/sup]F). For air to cool to a lower temperature, water must condense out. If it cools to 5 [sup]o[/sup]C (41 [sup]o[/sup]F), 2.6 g of water will condense out of each cubic meter of air.
That 2.6 g of condensation releases 5.9 kilojoules of energy from the latent heat of vaporization.
That 5.9 kJ of energy is enough to raise the temperature of our 1 cubic meter of air by 4.8 [sup]o[/sup]C (8.6 [sup]o[/sup]F).
What that means is that the cooling that took place (e.g., heat radiated away) was actually enough to bring the temperature of dry air all the way down to 0.2 [sup]o[/sup]C (32.4 [sup]o[/sup]F).
So the stabilization effect was the difference between an overnight low of 5 [sup]o[/sup]C (41 [sup]o[/sup]F) and an overnight low of 0 [sup]o[/sup]C (32 [sup]o[/sup]F).
I’m not sure about that. I never heard “dew point” mentioned until a few years ago. Now I hear it all the time. I think this is meteorologists attempting to educate the public about better and more useful weather concepts. At one point, nobody knew what “temperature” meant either. Now we are all pretty comfortable with the term and know what various numerical temperatures mean without having to have them explained all the time. I’m sure that was a long transition, too, with plenty of people talking about how the high-faluting weatherman was just trying to make himself feel smart.
But clouds are not vapor, they’re aerosols. Tiny droplets of liquid water suspended in the air. Water vapor is invisible. Clouds are always at the dew point, that’s how they form. A mass of air is cooled to its dew point, and the invisible vapor condenses into aerosol droplets, causing a cloud to form. What I’m not clear on is what makes them eventually fall as rain and snow. Cold fronts? Updrafts?
When parts of the cloud get cold enough to freeze, you suddenly have solid surfaces within the cloud. It’s easier for water to condense out onto solid surfaces, so those ice particles tend to grow. Eventually, they grow enough that they can’t be held in the cloud any more. Usually, they melt again on the way down, and we get rain, but sometimes, they don’t, and we get hail.
I think that’s just because you weren’t paying attention. I remember hearing the TV weatherman telling us what the dewpoint was in the 1960’s.
I’m pretty sure it has been a long time since people didn’t know what temperature was.
My education did a poor job of teaching me weather concepts. As I remember, it was taught is science class, but none of it really stuck; it must have been in 7th or 8th grade. In high school and college, I learned much of the underlying concepts (latent heat of vaporization, buoyancy of air masses, etc…) and I have been able to better understand what the weatherman is saying, but I am fairly certain they have been saying it for a long time.