Just got Radio Shack temperature/humidity sensors. Really like them, but the outside one doesn’t display 100% humidity when it’s raining, more like 90-97%?
A humidity of 100% at ground level would indicate a situation of extremely heavy fog or dew… where you can effectively see water emerging from the air if you wait long enough.
During rain, the humidity would not generally be 100% at ground level. It would be 100% up in the clouds, where the rain is actually precipitating out of air, and then falling down on you. (Tough once it’s been raining for a bit, the humidity would go up simply because there’s a lot of water around that the air at ground level can absorb.)
Does this make sense? Did I put my foot in my mouth? It seems pretty straightforward given what I know about weather.
Weather humdity is a relative measurement of how much water vapor is in the air compared to how much it can hold at full saturation at a given temperature and I think pressure. Water droplets such those that make clouds and raindrops do not count.
I’m not sure how an electronic detector works but the traditional method is wet bulb, dry bulb. Two thermometers are used, one with a wet pad over the bulb. Ambient air is blown over them, sometimes by swinging the thermometers on a string. When there is low humidity the wet bulb will become cooler. At 100% relative humidity the wet bulb will be at the same temperature as the dry bulb.
The weatherman on tv said, a while back, that on a cold, rainy day the humidity can be quite a bit less than 100%. As chrisk says, the rain comes from higher altitudes where the humidity can be much higher. When the rain falls to ground level where it’s too cold for it to evaporate, the humidity stays low.
Cool 
, huh?
Peace,
mangeorge
I would also imagine that the humidity can be 100% without any visible sign of precipitation or condensation, as long as the temperature remains perfectly constant and there are no nearby objects cooler than air temperature. If the air temperature drops at 100% humidity, that’s when you would get precipitation. If the air temperature rises, the relative humidity will drop because the warmer air can now take more moisture.
You’re describing a wet bulb-dry bulb psychrometer.
I doubt it uses a wet bulb-dry bulb psychrometer. It probably uses a capacitive RH sensor. (The capacitor’s dielectric constant is a function of RH.) While it’s not as accurate & stable as a psychrometer, it’s a heck of a lot cheaper to produce.
The Dopers are correct. Too many people think the air must be 100% saturated when it is raining. As said within this thread, those conditions exist at the cloud’s level. The air aound you can still be less than 100% saturated while it rains. - Jinx
That’s because the spaces between the raindrops are dry.
