This morning weather.com showed the relative humidity for my area at 6%. The temperature was 80 degrees.
Is it possible to have relative humidity even lower? Where and when would this occur?
An extreamly hot day. Maybe over 115 degrees.
Or an extreamly cold day in the artic or anartic. Any moisture in the air will freeze.
I looked at the National Weather Service Relative Humidity Tables- (Download yours here!) and the lowest RH on there is 1%. I personally slung an RH of 2% while I was working for the Forest Service (Dry bulb 107F, wet bulb 59F) in Sedona, AZ.
Wrong. Relative Humidity is the ratio of the actual moisture content of air to the maximum content for air at that temperature. While extremely hot air can theoretically hold a great deal of moisture, so that a hot dry day will produce a very low RH, freezing cold air has a very low capacity for moisture, so that while the absolute humidity is near zero, the RH is often close to 100%.
At first look you are right.
But air at a very low ttemperatrure can hold moisture but the moisture would freeze become solid and drop out.
Well hang on - if a water droplet is tiny enough to float in the air in liquid form, why would it suddenly plunge Earthwards in solid form?
Zero air is compressed air sold in the same type of cylinders as liquid oxygen or acetylene, and has less than .1 ppm hydrocarbons (and, I’m assuming, H[sub]2[/sub]O).
Why would you assume this? Water is not a hydrocarbon.
Perhaps because it doesn’t doesn’t even make the list of ingredients?
>Wrong. Relative Humidity is the ratio of the actual moisture content of air to the maximum content for air at that temperature.
Wrong. Relative humidity is the ratio of the actual moisture content of air to the content for air at that temperature such that it is in equilibrium with a flat surface of water also at that temperature. In clouds the relative humidity is generally well above 100%, which is supersaturated. Because of the Kelvin effect, the curvature of the surface of water in an equilibrated system dictates the humidity of the air part.
>But air at a very low ttemperatrure can hold moisture but the moisture would freeze become solid and drop out.
Also wrong. At temperatures above freezing an air and water system will equilibrate with some vapor pressure for the gasseous water part of the system. You know that at temperatures that allow water to be liquid, some part of the water in a system will be in a gas phase. The same situation exists below freezing. An air and water system will equilibrate with some vapor pressure for the gasseious water part of the system. Water moving from the solid to the gas phase is said to be sublimating, not evaporating, but it’s still going back and forth between a gas and nongas phase.
The vapor pressure of water in an equilibrating system does get much smaller as the temperature goes down, but it doesn’t disappear or even suddenly change at freezing.
This. It’s called zero air because it’s got zero “stuff” in it. It’s used in applications where you need extremely dry air.
Water is soluable in air and visa versa. It is a “solution”, that is it disolved. Water and ice would be a mixture. The Ice may be small enough to float or be carried by wind currents, but the ice is not desolved.
As a follow up, what location has the lowest relative humidity that is reported on a regular basis? As of right now, weather.com shows zip code 85014 having 9% relative humidity. The temperature is 80 degrees F and the dewpoint is 17 degrees F.
Yes, that definition is what I meant. I wanted to keep it simple, and since we’re talking about the low humidity realm, supersaturation is a bit off-topic. For the benefit of others, supersaturation occurs in the immediate vicinity of water droplets due to the high surface area, but water droplets are not themselves “humidity” – they’re liquid water.
As Napier ably pointed out, water vapor exists in equilibrium with ice. The absolute humidity of air at sub-zero temperatures is very low, but that’s exactly why its RH is quite likely to be near 100%.