What did the weatherman mean by that?

[Crunchy_Frog]

I was watching the news last night, and when it was time for the weather, the weatherman mentioned “relative humidity”

This may seem like a stupid question, but what does that mean? Relative to what?

[Tretiak]

Relative humidity is the amount of water in the air “relative” to the amount it can hold at it’s current temperature and pressure.

[sailor]

Air can only hold so much water vapor before it starts condensing. The amount goes up with temperature. Water at 8oF can hold more water molecules than water at 60F. Relatve humiditu is how much water the air has compared to the maximum it could have at that same temperature.

I find this way of measuring humidity a bit misleading and find it more meaningful if they use the dew point which is how far you would have to cool the air for it to be saturated. In essence they are the same thing but expressed differently.

[sailor]

make that:
Air at 80F can hold more water molecules than air at 60F. Relative humidity is how much water the air has compared to the maximum it could have at that same temperature.

[Engineer_Don]

Dew point is interesting for some, but wet bulb temp is the most useful humidity measurement for me. Relative Humidity (%RH), dew point temp, and wet bulb temp are all different ways of describing how much water is in the air. %RH is the percent of moisture in the air compared to the maximum amount of moisture that air can hold at that temperature. It tells you how much cooling effect from evaporation you can expect, so if it is 90 deg F. and 100% RH, you can sweat all you want and you won’t cool down. At 90 deg F and 30% RH, you can be comfortable, unless you are from Florida in which case you will get nose bleeds. This is the infamous “Dry Heat”.

Dew point is the temp a surface has to be to get moisture condensing out of the air onto it. If the dew point is high compared to the regular (dry bulb) temp, then the air is fairly wet. If is it is low (near 32) then it is dry.

Wet bulb temp is sort of the opposite of Dew point. It looks at the ease of evaporation of liquid water into the air. If the wet bulb is the same as the air temp (dry bulb), then their is 100% relative humidity. If it is low, then the air is dry and lots of evaporation is taking place.

[BrothaTJ]

Warning: Weather-related hijack in progress.
What are “cooling degree days” and “heating degree days”? I’ve heard these terms discussed by weather gurus before but I have no idea what they are.
Also, what is the “National Temperature Index?” They mention this daily (or nightly I guess) on World News Now during the weather and seem to laugh it off as an unimportant statistic. What the heck does it mean?

[AWB]

AAAAAAAAAAAAUUUUUUUUUUUUUUUUGGGGGGGGGGGGGGHHHHHHHH!!!

Air does not “hold” water vapor! Here is an excerpt from the Bad Meteorology Page that explains what really goes on.

***Bad Meteorology: ** The reason clouds form when air cools is because cold air cannot hold as much water vapor as warm air. *

When moist air cools, a cloud can form. This much is true. The process is responsible for the cumulus cloud over Vancouver and the cap cloud over Rainier, shown to the right. Ascending air always cools. The cumulus cloud formed when air over the sun-warmed ground became buoyant and rose; the cap cloud, when the wind (coming from the right) blew against the sloping side of the mountain and was forced up.

But did the clouds form because the colder air had a lower holding capacity for water vapor than the warm air? If you believe a legion of teachers (from grade school to university), TV weather broadcasters, and endless textbook writers, this is the reason. They speak of the air being saturated and one even published an illustration of the air being wrung out like a sponge as the temperature dropped (sigh…). Unfortunately, it is not true. Sure, a cloud may form as the temperature drops, but not because some mystical holding capacity of the air has decreased.

To claim that a temperature-dependent holding capacity of the air caused the cloud to form in cold air is to get (approximately) the right answer for the wrong reason. It is like trying to reduce the fraction, 19 / 95, by imagining that you can cancel the 9s. The right answer ensues, but for the wrong reason. And, if the process was wrong, it is unlikely to work the next time you try it in a slightly different situation.

The air (mainly nitrogen and oxygen) no more has a holding capacity for water vapor, than, say, water vapor has for nitrogen. The atmosphere is a mixture of gases. While saturation (which involves bonds between different molecules) is a real phenomenon in liquids it does not describe the interaction of atmospheric constituents.

So, what is going on?

Water molecules are constantly coursing back and forth between phases (another word for the three states: vapor, liquid, and solid). If more molecules are leaving a liquid surface than arriving, there is a net evaporation; if more arrive than leave, a net condensation. It is these relative flows of molecules which determine whether a cloud forms or evaporates, not some imaginary holding capacity that nitrogen or oxygen have for water vapor.

The rate at which vapor molecules arrive at a surface of liquid (cloud drop) or solid (ice crystal) depends upon the vapor pressure.

The rate at which vapor molecules leave the surface depends upon the characteristics of the surface. The number escaping varies with:

[list]
[li]the phases involved — molecules can escape from liquid more readily than from the solid (ice);[/li][li]the shape of the boundary — molecules escape more readily from highly curved (small) drops or ice crystals (convex);[/li][li]the purity of the boundary — foreign substances dissolved in the liquid or ice diminish the number of water molecules which can escape;[/li][li]the temperature of the boundary — at higher temperatures the molecules have more energy and can more readily escape.[/li]
And therein lies the origin of the myth. The temperature of a cloud droplet or ice crystal will be (nearly) the same as that of the air, so people imagine that somehow the air was to blame. But, if the (other gases of the) air were removed, leaving everything else the same, condensation and evaporation would proceed as before (the air was irrelevant to the behavior). To assign the behavior of water to an invented holding capacity of the air is like assigning your life’s fortunes to an invented influence of the constellations (and as we all know, nobody does that anymore).

So, what do you tell your students?

What appears to be cloud-free air (virtually) always contains sub microscopic drops, but as evaporation exceeds condensation, the drops do not survive long after an initial chance clumping of molecules. As air is cooled, the evaporation rate decreases more rapidly than does the condensation rate with the result that there comes a temperature (the dew point temperature) where the evaporation is less than the condensation and a droplet can grow into a cloud drop.

Evaporation increases with temperature, not because the holding capacity of the air changes, but because the more energetic molecules can evaporate more readily (with, of course, the caveat that evaporation is also influenced by things other than temperature, as described above).

If that explanation is not simple enough for your students, just present the facts: when the temperature drops below the dew-point temperature, there is a net condensation and a cloud forms.

But don’t ever teach nonsense by claiming that the air has a temperature-dependent holding capacity for water vapor.

[Tretiak]

Re: Heating Degree Days (HDD) and Cooling Degree Days (CDD)

These each represent a deviation from some base temperature. For example, if you have a HDD base of 65 degrees and the average temperature is 63 degrees for the day then you have two HDD. If your CDD base is 70 degrees and your average temperature is 77 degrees then you have 7 CDD. As far as I know there is no universally accepted base for eaither HDD or CDD.

[Engineer_Don]

While the explanation from the Bad Meteorology page is correct and true, I still find it to be effective to think of air as having a water holding capacity, much like a sponge. Psychrometrics is the science that deals with the thermodynamic properties of a strange mixture of gases in a fairly small temperature and pressure range, rather than an infinite range that the Bad Weather page seems to stress. This is primarily due to the availability of these specific gases in this pressure/temperature range. They call these gases “moist air” (which is actually just a mixture of various gases and very small droplets of liquid water, but we will call it moist air for now because mixture-of-various-gases-and-very-small-droplets-of-liquid-water (MOVGAVSDOLW)is kind of burdensome.) People study the behaviour of MOVGAVSDOLW because there seems to be a lot of it and our weather generally involves it in some form or another.

Also, the statements made before the clarification are still essentially true. In MOVGAVSDOLW (air) where the molecules are moving, spinning, and vibrating slowly (i.e. is colder) more water molecules will arrive on the droplet’s surface (net condensation), and in MOVGAVSDOLW where the molecules are moving, spinning, and vibrating faster, more water molecules will leave the droplets surface.

Just thought I should clear that up.

[Crunchy_Frog]

Thanks everyone for responding. I’m gonna go home to take a couple aspirin and have a lie down now, as some of this was really way too deep for me to be reading right before quitting time.

[ren]

The point of these two numbers is to help establish how much you’ll need to use heaters/air conditioners over a typical year in a given location. They are used by utility companies to help predict power and fuel requirements.

The “base” is usually a range of temperature (or a single temperature) within which it is considered that you would need neither heating nor cooling to feel “comfortable” (that’s where the controversy over what the “base” should be arises).

Deviations below the bottom of the base range indicate that you would use heating to increase the temperature and deviations above, that you would use air conditioning to lower it. The actual number described previously is the heating or cooling degrees. Heating/cooling degree days are this number summed over the days measured, eg:

Theoretical base range:  65-75 degrees

Day    Avg Temp[sup]*[/sup]   HD      CD
Sun       58       0       7
Mon       66       0       0
Tue       60       0       5
Wed       72       0       0
Thu       78       3       0
Fri       83       8       0
Sat       80       5       0
                  --      --
             HDD: 16 CDD: 12

[sup]* I am assuming average temp, since we need a single number to compare against the base. I’m actually not sure which number is used to compare against the base.[/sup]

…or alternately, I’ve seen the term used to mean the number of days (for either heating or cooling) on which the temp fell outside the base range, which in this example would be 5.