When talking to people, there seems to be some common belief that when it rains, for example, the relative humidity is 100%. Today, watching the weather channel while it’s snowing outside, the relative humidity dropped from 95% to 91%!
So, I wonder if relative humidity must be for a highly specific “regime” of air, for lack of a better term. For example, I’d accept that the air is saturated at the cloud level where the act of precipitation is occuring. But, that doesn’t mean the air at ground level must be 100% saturated, too, right?
Since clouds are the result of air saturation and condensation, the RH at cloud level is 100%, whether it is raining or not. The water droplets in the cloud (or ice crystals) will not fall unless other conditions exist. Although the RH throughout the atmosphere may approach 100%, it need not be 100% at ground level. If it were 100% at ground level but not above, then you would have fog, but no rain.
Relative Humidity, is the ratio of the current absolute humidity to the maximum possible absolute humidity in the air at the given temperature.
So say air can hold a maximum of 30% of humidity at a given temperature. But you measured air that has 20% humidity.
So, the reltative humidity is = 20/30 = 67%.
In practice relative humidity is usually measured by measuring the dry bulb temperature and wet bulb temperature. These readings are used with a psychrometric chart to find the relative humidity.
Sorry, I misunderstood the question. Yes relative humidity varies with altitude due to of course presence of moisture, pressure (not sure about pressure) and temperature. Higher temperature means the saturation humidity is higher, so even with the same amount of moisture, hotter air will have lower relative humidity. (Warm air rises up and expands and cools down)
To expand on this: Capacitive RH sensors are what are commonly used nowadays. They’re cheap and maintenance-free. Psychrometers, on the other hand, are more accurate and perform better at high temperature / high RH levels, but are more expensive and much less convenient.
A little background… Our lab calibrates RH chambers for our entire research institute. I needed a NIST-traceable standard, so I purchased a $2000 Vaisala RH sensor + readout. The sensor was a capacitive-type. It was a pretty nice unit, but it couldn’t hold its calibration tolerance when exposed to high temperature & high humidity (e.g. 160 °F at 95% RH). Frustrated, I later learned that a psychrometer is the only RH measurement system that performs well at the upper extremes. We then purchased a psychrometer system from Thunder Scientific at the tune of $8000. Expensive to be sure, but it’s the only thing that works.
There’s also something called a “chilled mirror” RH system. They’re O.K., but have a more limited range vs. the psychrometer system.
Interesting Crafter. My only knowlede in Relative humidity was a program, I had written (in fortran :D) long back while I was an undergrad. There is an implicit relation between, the wet bulb temp, the dry bulb temp, relative humadity, dew point etc. I don’t remember exactly what the relation was but given any two (by any two I mean not the redundant ones) the others could be figured using a chart or the program.
BTW - it may interest you (If you don’t know already) that the early meters used a horse tail hair to measure its expansion wrt to rel humidity.
A couple of years ago I wrote a data acquisition program (using TestPoint) that acquired temperatures from wet and dry PRT sensors. The equation for converting the temperatures to RH was very hairy – it even accounted for “elevation above sea level.” I’ll dig it up and post it.
All the relevant equations are contained in the CRC Handbook of Chemistry and Physics.
As RH is virtually useless in judging outside comfort levels (despite weathermen’s constantly updating us like they had a clue) and, as the dew point temperature is an absolute measure of comfort (I’m talking air temps above 50 in the main) in my copious spare time I devised a simple mnemonic to convert RH to dewpoint.
‘For each 10% RH less than 100% subtract the series 3, 3.5,4,4.5 and so on from the air temp to get the dew point temp.’ Sounds complicated, but in fact it is simple, eg.,
Find the dewpoint for an air temp of 90F, and RH of 60%.
You have four 10% increments to subtract:
The dew point temp is thus 90-(3)-(3.5)-(4)-(4.5)=75F.
(Interpolate as necessary, thus for 45% at 82F.
You have 5-1/2 10% increments, so Dewpoint=82 - 3 - 3.5 - 4 - 4.5 - 5 - 5.5/2 = 59.5, round to 59F, a reasonably comfortable humidity.)
This is accurate to within 1 degree!
A quick rule of thumb, dewpoint under 60 good, dewpoint over 70 bad. In between–depends on what you are doing.
Just remember the sequence:
"three three-and-a-half four four-and-a-half five…’ .
andy_fl said: “Relative Humidity, is the ratio of the current absolute humidity to the maximum possible absolute humidity in the air at the given temperature.”
Tain’t so. Relative Humidity is the ratio of the current absolute humidity to the equilibrium absolute humidity in air at the given temperature (and pressure) in contact with a flat surface of pure water.
The relative humidity of air that has come to equilibrium with water depends on the radius of curvature of the water surface. If flat (infinite radius), the RH will be 100%. If the water surface is concave as seen from the air side, the RH will be below 100%. This is the case when the water surface is inside a fine capillary, such as a pore in silica gel, which is what makes this type of dessicant work. If the water surface is convex, the RH will be above 100%. This is the case when there are fine water droplets suspended in air, as in a cloud, and in this situation air is said to be “supersaturated”. Supersaturation must exist - you’ve no doubt heard of it - cloud chambers depend on it for the detection of atomic particles. This phenomenon generally is called the Kelvin Effect.
Surely Napier, you are correct - but I way trying to be non-technical there and trying to put it in layman’s terms.
sx633 - you are right. But, those are rules of thumbs. There are better equations than that. Usually the vapor pressure is calculated using the Antoine’s coefficeints. And there are other mass transfer relations which relate a lot of psychrometric properties. I doubt it will be there in the CRC handbook. Its more like to be in a Chemical Engineering Unit Operations or Transport Phenomenon book.
Relative humidity is important in Chemical / Power plants because of cooling towers.
We quite often have rain when the relative humidity is relatively low. Maybe 70% or so. However, up where the rain originated it is 100% or the water wouldn’t have precipitated out. In fact it often rains up in the air where you can see it but the rain evaporates before it gets down where we are.