Below is an executive summary, which you can read and then move on––and save yourself another 30 min on this topic. All that will follow the summary is just the justification for the summary, and the background needed to understand it.
Our perspective is always that of a mariner, looking for weather knowledge that will enhance the safety or efficiency of navigation.
Although the moisture content of the local air mass is a fundamentally crucial factor to the weather you will observe, local relative humidity (RH) values that you might measure or be told by someone else have no practical value to your weather work at all.
The numeric value is way too sensitive to immediate conditions (ie several feet, several seconds), in large part because the number is so sensitive to air temperature. Thus we do not need to include at this point any discussion of how to measure it.
Measured dew point could in principle be of some value in predicting the onset of fog in the rare case that this might aid your navigation and when isolated from any wireless communications. With communications to landbased resources you could likely obtain a better forecast for your location than you could make yourself. This measurement requires a good sling psychrometer using careful procedures (most packaged instructions are not adequate). You do this by measuring the dew point and air temperature over a period of time to see if they are converging, and then if nice linear lines are headed toward each other, you can project to the time they will meet and likely make fog.
On the other hand, we have such wonderful online resources these days, that we can actually look at this type of data from around the country and see that this does not always work. The two numbers could approach each other all late afternoon and evening and then just change directions before they meet. Fog often forms when the air cools at night on clear days.
In short, we are better off simply understanding this principle and not hope for any practical application of our own underway. The amount of effort and equipment to do it right cannot be justified by the success rate you might obtain.
Thus we should just give up on these two topics and devote whatever time and resources we have to barometric pressure, which can be measured very accurately with the right instruments, and the results are indeed extremely valuable to our navigation. The same can be said of properly calibrated wind instruments. Wind instruments on many vessels, like barometers, are more often not calibrated than calibrated. We see them change and assume they are right. When they are not calibrated, we do not get the most useful information from them, and thus eventually lose perspective on how important they could be.
Relative humidity is indeed important for good health at home, and crucial to protecting good cigars and musical instruments, and there are ways to measure that with a useful accuracy, which we will add valuable information later on below, along with the pictures and data from our last round of measurements, which are just wrapping up. (Notice the word is "last," not "latest.")
More to follow...