Tell me more about Dew Point...

[osideterry]

I just looked and my current temp is 72F, humidity is 13%, and the dew point is 23°. Is dew point a better indicator of moisture in the air than humidity?

[gsn]

Terry,

The dew point is the temperature air must be cooled, at constant barometric pressure, for water vapor to condense into water. The dew point is the saturation point,or the point which vapor becomes water.

When the dew point temperature falls below freezing it is the frost point.

Water is no longer formed at the dew point,it becomes ice crystals!

Edited November 19, 2008 by gsn

[freakypalmguy]

Hi Terry,

This may be more simple than what you are looking for, but the way I understand it, is if the temperature drops to the dew point it is saturated and can hold no more moisture, in turn creating fog, clouds, rain, etc. When I am looking at the current weather for an airport I am flying to, if I see the temp and dew point equal, I am expecting fog or low clouds. The dew point fluctuates and I believe it is a gauge of how much moisture is in the air. The higher the dew point, the more moisture in the air.

Ow! My brain hurts.

Matt

[freakypalmguy]

That was pretty good, Scott and I posted at the exact same time.

[gsn]

What do they say about great minds!

Only problem is ,I'm not sure I answered the question he asked?

I was always under the impression that the humidity revealed more about moisture content than the dew point. As the dew point flucuates according to rising and falling barometric pressure?

But I might have missed that class!

Bueler,Bueler???

Edited November 19, 2008 by gsn

[freakypalmguy]

You know, now that I look back at his question, neither did I. Let me see if I can get this right, or just confuse myself more

Humidity is how much moisture in the air, that's seems simple enough.

But I described dew point wrong when I said, "it is a gauge of how much moisture is in the air". Like we stated above, dew point is the temperature at which the air must cool to become saturated. The higher the dew point means there is more moisture in the air, but is not a direct measure of how much is present, it is still relating to the temperature of saturation.

Well, I think that's the best I can do. Terry will probably be forever confused.

Matt

[osideterry]

I basically understand it, but every searched source is obsessed with high moisture content, and oblivious to low moisture. Wikipedia flat stated that when the dew point temperature falls below freezing, frost occurs. That can't be true because my local dewpoint temp was 23° this afternoon.

Sites that discussed dewpoint where mostly explaining uncomfortable high humidity. I couldn't find one that dealt with uncomfortable low humidity.

[BS Man about Palms]

Let me throw in my Air Conditioning knowledge (or lack ) here. The dew point is the temperature at which the given "moisture/humidity" will condense to liquid from a vapor. In my business, I am more concerned with the SURFACE temperature. I. E. if Glass, wood, metal, skin, gets the surface temp to the dewpoint, the moisture in the air will condense on that surface as there is a transfer of heat from one to the other. Very helpful for me to remove the humidity/moisture from the air in a home, put it on the surface of my cool evaporator coil and drain it away. Thats the hot end.

Terry, from your end, its almost just a hypothetical statement when its that dry, because while yes, the surface would be below 32F so it would freeze, it would have to be that cold (the 23F you mentioned) for the moisture/frost to EVEN APPEAR on that surface. Not going to readily happen, catch my drift?

Kinda like a lesson I learned about Heat Pump systems a long time ago (Heat pumps are A/C systems that can run backwards (basically) to heat in the winter. I won't go into the heat transfer, etc, but suffice to say, the air and the coil outside are very cold. If they run long enough that way, the outdoor coil can freeze over with ice, and with no airflow through the coil, it stops heating. Well, kinda like with our palms, I found that the Heat pumps would freeze over at a much "warmer" temp on the coast than in the desert.......Yep, you got it, if theres NO moisture (like the desert area), theres NO moisture to make ICE or frost with!!! (in simple terms, I can have a coastal heat pump freeze over at 45F while it usually has to be below 35F inland or desert before I regularly see frost.)

[BS Man about Palms]

And now rereading your question and my answer, the most simple answer I can give is that IF you can attain the dewpoint temp, (in the air, wood, glass, metal, skin, etc.) you will SEE moisture begin to appear in a frozen or liquid state depending on if the point is above or below 32F.

[gsn]

Wikipedia flat stated that when the dew point temperature falls below freezing, frost occurs. That can't be true because my local dewpoint temp was 23° this afternoon.

Bill said this better than I am going too but,I beleive that your dew point at 23F is the temp that you would have to acheive before the vapor condensed and formed water,if you reached that temp ,naturally you would not have dew ,but frost,or ice as it is below 32F?? Any temp above 23F and no moisture would be condensed from the air,but below that temp (saturation point),frost or ice would form!

Did I say that right Bill?? My head did EXPLODE Matt!

Edited November 19, 2008 by gsn

[BS Man about Palms]

Wikipedia flat stated that when the dew point temperature falls below freezing, frost occurs. That can't be true because my local dewpoint temp was 23° this afternoon.

Bill said this better than I am going too but,I beleive that your dew point at 23F is the temp that you would have to acheive before the vapor condensed and formed water,if you reached that temp ,naturally you would not have dew ,but frost,or ice as it is below 32F?? Any temp above 23F and no moisture would be condensed from the air,but below that temp (saturation point),frost or ice would form!

Did I say that right Bill?? My head did EXPLODE Matt!

Works for me.

[osideterry]

So....

Warm air holds more water than cold air. The current amount of water in the air would saturate it at 23F.

I think I've got it.

[BS Man about Palms]

So....

Warm air holds more water than cold air. The current amount of water in the air would saturate it at 23F.

I think I've got it.

I'm not sure if thats right. Is there a specific answer you're looking for or just trying to learn more.

[osideterry]

I guess I just want to know what the numbers mean in relation to each other. Temp and humidity are obvious. Dewpoint seems to be a formula dependent on those two and barometric pressure.

Now resuming regular scheduled right brain activities already in progress.

[Daryl]

Dewpoint is the temperature that the existing air must be cooled down to for dew to form. Dewpoint is calculated from the wet-bulb air temperature. When the dry-bulb temperature and dewpoint are the same, you have 100% humidity. Personally, I think dewpoint temperature is a better indicator of how 'sweaty' it is than a relative humidity reading.

We are always observing 'relative humidity', which is based on the amount of moisture air can hold before it becomes saturated at a determined temperature. Warm air can hold a lot more moisture than cool air. So at 100F and 40% relative humidity, the air holds more moisture than 60F and 99% relative humidity. So the first example is much more 'sweaty' than the 2nd example even though it is only 40% humidity.

I know that if the dewpoint is 80F then I'm sweating my ****s off!

As for dewpoint and frost..I'd rather stay away from that!

Here is a map of Oz temps tonight...this shows temp, rh and dewpoint for all of our major cities and towns...the white blotches are thunderstorms.

Daryl

[osideterry]

What inspired my question was my experience Saturday. I never checked the dewpoint, but the air temperature was 96F, and the humidity was 12%. I was determined to finish my shadehouse, but struggled with feeling almost flu-like symptoms - scratchy eyes, sluggish, headache. High humidity would have stopped me from working. This just made it real unpleasant.

[spockvr6]

Personally, I think dewpoint temperature is a better indicator of how 'sweaty' it is than a relative humidity reading.

I agree Daryl!

[Mark_NoVA]

When you say 'humidity', you're just using a general common-sense term.

To actually measure 'humidity', there are a variety of different measurements. According to http://en.wikipedia.org/wiki/Humidity, there is:

Absolute humidity

Mixing ratio / humidity ratio

Relative humidity ('dew point' is related to this measurement)

Specific humidity

Weather forecasters usual mean 'relative humidity' when they mention some degree of humidity. However, as Daryl above explains, that's not a very good measurement of how 'humid' it feels to a human, so they often use dewpoint. Problem is, unless you have a common sense about how 'humid' a certain dewpoint feels, that's not all that useful.

Can't say I understand which of the other 3 measurements is better at indicating the amount of water in the air, and thus how 'humid' it feels, but it seems that the one of those measurements that does that best would be useful to hear for how 'humid' it is.

[Neofolis]

Here's my attempt at re-wording what everyone has already said. In answer to your original question, dewpoint is an indicator of how much moisture is in the air. Relative humidity is also an indicator of how much moisture is in the air, but only at the given temperature, therefore dewpoint is an absolute measurement of the moisture in the air, whereas RH is dependent on the moisture and the temperature. Although the dewpoint will vary with pressure changes, it is still a more clear indicator than RH, because pressure changes are usually small and have little effect on the dewpoint, however temperature changes can be large and will have a dramatic effect on RH.

Temp: 80°F

RH: 50%

DP: 60°F

Given the above example, if barometric pressure was unchanged the RH would reach 100% at 60°F, i.e. the dewpoint. Because warmer air can hold more moisture the RH will decrease as the temperature rises. Human comfort zone is often regarded as an RH of 50-60%, when the temperature is 68-70°F, which would usually equate to a dewpoint of 48-55°F. The further the dewpoint is above or below that comfort zone, the less pleasant it will be for humans, generally. Higher dewpoint usually equates to sweatiness and in the extremes running nose and eyes, light headedness and fainting, breathing difficulties. Lower dewpoints usually equate to dryness/thirst and in the extremes, sluggishness and aching. Higher dewpoints can also cause problems for arthritis sufferers, but high RH can also cause this.

In terms of frost, if you have a dewpoint of 23°F a frost will not form until a surface temperature falls to or below that point, because no moisture will condense on the surface until that temperature is reached. If you have a dewpoint above 32°F, then frost will form on any surface with a temperature at or below 32°F, because the moisture will condense on the surface and then freeze. Air temperature can not fall below the dewpoint, so if the dewpoint is 34°F, the air can not cool to below 34°F, because saturated air, i.e. with a relative humidity of 100% will have it's temperature maintained by it's moisture content. The moisture content can change though, allowing for changes in dewpoint, which will then allow for further temperature drops, if the dewpoint drops. Equally, if the dewpoint rises, the air temperature will be rising with it.

The only other thing I can think to add is that drier air conducts heat more readily, so it is more prone to large temperature swings, whereas air with a high moisture content will heat and cool more gradually giving more stable temperatures. This is most evident in some desert areas that have really high daytime temperatures and relatively much lower night time temperatures, but obviously there are other factors that can effect large temperature swings.

[BS Man about Palms]

Well stated Corey.

[osideterry]

Thanks for the clarity Cory. You also explained why we rarely have frost with our lowest temps here. Frost won't form on a palm at 27F if the dewpoint is 25F.

[gsn]

If I was smart, I would have posted what Corey said!

Excellent breakdown of the question,and answer Corey!

Edited November 21, 2008 by gsn