So yesterday I was comparing the weather in my fair city (Bakersfield) and that from which I just returned (Las Vegas). For about the last week, both places have had very similar weather- well over 100, averaging about 105 degrees. The only real difference in weather is that Bakersfield is down right tropical compared to Las Vegas, what with our humidity level of 13% and all.
So I took a look at the humidity in Vegas and according to Google weather, the reading was -1%.
Is that just an error thing? Does it have to do with the relative humidity? Or has the air just started turning to powder?
I suspect that online weather isn’t always 110% accurate, for I’m confident that we in Arvada currently have slightly more atmosphere than this 0.0 millibars would indicate.
IIRC, from when I lived in Phoenix, it’s never absolute zero in terms of humidity. Even on the driest day it’s about 3-10% humid. Like if you go to weather.com and search zip 85044, you’ll see 6% humidity.
I’d think Google Weather would have more error checking than that. Negative humidities don’t make literal sense. However, you could have an electronic humidity sensor that had an accuracy of a couple percent or so, and have a reasonably correct calibration built into an instrument around it, and get a negative value because the real humidity was a very small positive number and all the parts of the system were working normally including a bit of error.
Now, humidities above 100% ARE possible, and in some cases you can get several hundred percent relative humidity.
Water vapor doesn’t condense very readily unless it has something to condense on. Near the earth’s surface it can condense on the ground or vegetation, or on dust particles that are common near the ground. So at low altitudes, relative humidity rarely exceeds about 100-104%. Higher in the atmosphere, condensation nuclei can be few and far between and temperatures can drop faster than the water can condense. So at high altitude, relative humidity can temporarily reach several hundred percent. Such supersaturation isn’t really stable, but it can persist quite a while.
Relative humidity is the ratio of the water concentration in air to the equilibrium concentration you’d get at that temperature and total pressure if the air were in contact with a flat surface of pure water. Air equilibrated with a convex surface has a higher-th-100% RH, and air in contact with a concave surface, lower. In the case of convex surfaces, such as exists on the surface of droplets in clouds, this is called the “Kelvin effect” (though William Thomson, Lord Kelvin, Baron of Largs did so much that there are several things that might be called this).
Cloud chambers and condensation nuclei counters (instruments used in aerosol science (which studies particles, not the propellant packaging that stole the name from scientists)) create very high RH’s above 100, or do so with some substance other than water, such as butyl alcohol. Generally, the science and technology of water and its vapor in air is the same (except for the particular numbers) as that of other liquids and their vapors in air.
For some reason, relative humidity is a topic that confuses many, even technical people. FWIW I’m a physicist and I’ve worked a quarter century in thermodynamics, aerosol science, sorption, and related fields. I’ve even patented a dehumidifier and several vapor sorption devices, one of which most Dopers probably have a few of (though it’s small and probably unnoticed).
Say it ain’t so!