Mary, Mary, How does your dewpoint vary?

Today we are modifying the old nursery rhyme which inquires about Mary's garden  to consider typical fluctuations in dewpoint temperature.  Since the dewpoint depends on the total atmospheric pressure, and that changes from day-to-day, the dewpoint will change (for a given amount of moisture in the air) as the barometric pressure changes. We'll also look at how dewpoint might change with altitude.
First, a little review:  The dewpoint is the temperature at which a fixed sample of air would first start to have water condense out if it were cooled at constant pressure.  This psychrometric chart illustrates the process.

  The dewpoint depends on the atmospheric pressure, as well as the amount of water that is in the air (the humidity ratio).  Barometric pressure fluctuations differ a lot for different places.  but a range of about 29-31 inches of mercury (in Hg) would cover most of the fluctuations for most places.  This is around a base of 29.92 in Hg.  The barometric pressure can drop a lot more during a hurricane, and pressures in the range of 26-27 in Hg have been recorded. 

This figure shows the variation in dewpoint for a humidity ratio of 0.010 grams of water per gram of dry air over the range of 29-31 inHg.  There isn’t a large variation over that small fluctuation in barometric pressure.

This figure shows the variation in dewpoint as a function of humidity ratio for the three different barometric pressures that we have been discussing: 29, 29.92, and 31 inHg.  Again the barometric pressure doesn’t make a large difference.

Finally, we know that pressure decreases with altitude and typical change is represented by the standard atmosphere.  This figure shows the variation of dewpoint with elevation for four different values of the humidity ratio.