A Foggy Morning Walk

Why can’t I fully grasp the dew point / relative humidity relationship??

Wiki: The dew point is the temperature to which a given parcel of humid air must be cooled, at constant barometric pressure, for water vapor to condense into liquid water. The condensed water is called dew when it forms on a solid surface. The dew point is a saturation temperature.  The dew point is associated with relative humidity. A high relative humidity indicates that the dew point is closer to the current air temperature. Relative humidity of 100% indicates the dew point is equal to the current temperature and the air is maximally saturated with water. When the dew point remains constant and temperature increases, relative humidity will decrease.

Ok, so I get this part…. because it was 40° F this morning, and the dew point was 39 and the relative humidity was 100%… the air was maximally saturated and therefore it was a foggy Morning Walk.

But why [again, according to Wiki], if the air was maximally saturated this morning, could the air be considered “a bit dry for some”, as any dew point below 49 indicates??  And what do the conditions need to be for rain or snow?  And why, if it’s 90° F and 75% relative humidity, is the dew point 81?  Check out this interesting calculator: http://www.dpcalc.org/

I guess I understand the weather like I understand everything else – visually.  Give me the facts, let me see what it’s doing outside and ok, I get it.  But ask me to predict… and I’ll turn on Channel 13 News and tell you what Paul Caiano has to say.


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