iw2fvo, As long as the atmospheric temperature is above the dew point one, moisture will stay in a gaseous form. But there is more to it in the world of thermodynamics. First, water can be found in a gaseous, liquid or solid form. When it is still gaseous in an environment where it should be liquid, it is then called vapour, and when it is still liquid where it should be solid, it is called under-cooled. But why is that? Well, first we must differentiate between absolute humidity and relative humidity. Water molecules tend to mix with air molecules all the time; even ice evaporates as gas. This is called sublimation. Absolute humidity is the volume of water that is contained in a volume of air. The higher the temperature, the greater the volume of water it can contain. For example, at the average temperature at sea level and average pressure, of 15 C, a cubic meter of air can't contain more than 13 grams of water. When that happens, the air is saturated with moisture and the relative humidity is 100 percent. When the temperature goes down to 0 C (freezing point) the air can sustain only 5 gr/m3. The absolute humidity is then 5 gr/m3 but the relative one is still 100 percent because at saturation point, a.k.a. dew point temperature. What happens when the temperature sinks below dew point? Well, the parcel of air has to get rid of some of its moisture by condensing it into tiny water droplets. But that can only happen by giving away energy in form of heat and the tiny molecules have virtually no mass to dissipate that heat energy. They have to touch anything like a dust, smoke particle, pollen or the bonnet of your car, to condense. Likewise, it must give heat energy to go from liquid to solid and that's why you can get tiny drops of water, in the clouds, that are under-cooled all the way down to -40 C! The type of sensor you are using does not have the capability of measuring the Dew Point below 0C. therefor the calculation you are using is not valid below 0C.....

Dave Purola,
N8NTA