Why does dew point vary during the day?

I’m looking at dew point data from a local weather sensor. I notice that the dew point is not constant (or very slowly moving) as I expected, but rather starts at a relatively high level in the morning, drops as much as ten degrees during the afternoon heat (Arizona desert here - and it sure as hell isn’t a dry heat this time of year), then return to around the original level overnight. It terms of scale, I am talking about dew points in the low 60s dropping to the middle to low 50s then back again, as temperatures rise from the low 80s to the mid 100s (~105F, not ~150F obviously)and back again.

Is this some sort of measurement error (sensors drift out of calibration at higher temperatures) or is there some meteorological explanation for this apparent daily cycle of absolute humidity?

From Wikipedia:

So what are you seeing as far as diurnal variation of barometric pressure? Does it correlate with the reported dewpoint?

I saw that in Wikipedia also. There is a correlated variation (pressure is lower in the afternoon), but the variation seems slight 5-7mb from high to low during the day. Maybe that’s it - I didn’t look at pressure data, I just assumed that pressure would be higher due to higher temperature. I’ll check the equations given in that Wikipedia page.

I’m a meteorology nerd, though I’m not an expert on desert climates, or the nuances on dewpoint.

I do, however, have a link that I can share, to the National Weather Service’s graphical forecast for Phoenix. It’s a dynamically-created graph, relative to the current time / forecast, so it won’t look the same if you come back to it tomorrow, but what I’m seeing is, for the next few days, at least, the dewpoint (the green line in the upper portion of the graph) is, indeed, peaking at dawn (or shortly after), and then dropping off during the day, with a low point reached around sunset.

I have no idea if that’s a regular occurrence or not, and the variation isn’t quite as great as what you’re seeing on that local weather sensor (it’s more like a variation of 5 degrees or so over the course of the day in the forecast), but it does suggest that it’s an actual meteorological phenomenon, rather than an equipment issue.

I played around with some equations I found on the UofA website. The effect of barometric pressure is very slight.

Dew point temperature is the temperature the air must be for the relative humidity to be 100% … thus it is a measure of the moisture in the air AND it’s independent of air temperature … air temperature can go up, and RH will go down if dew point stays the same … the NWS website usually has a link on the city page that will give you a cute lil’ graph of the past three days of temps, dew points and RH … the correlation between dew points and day/night cycles in Phoenix looks weak to me right now … but if you find in the URL of that graph page where it says “72”, change it to “336”, hit return, the new page shows a full two weeks past weather … here I see a very close correlation last week, y’all had some storm activity, and now this week the correlation breaks down …

I’d guess the wind direction changed … I’m not familiar enough of the local climate in Phoenix to say much more … but here in Western Oregon winds from the west increase dew points and winds from the east crash them … like clockworks … winds from the south and it’s already raining …

Humid air is lighter than dry air (it’s not instinctive). Molecular weight of water is 18 and air is around 29.

As the temperature rises during the day on land, it creates sufficient temperature currents (buoyancy) for the lighter (humid) air to rise up.

The evaporation of water into the air is more during day time too (higher temps) but the buoyancy / temperature currents offsets this.

This seems related to a phenomenon called “afternoon mixing” that I found in a paper discussing dew point dropping in the afternoon (in Kansas, but the idea is the same). As the ground heats up the air near the ground rises, causing turbulence which mixes the air up to a certain altitude into a homogeneous layer. This mixed layer has a lower absolute humidity than the air near the ground does overnight, so the dew point drops. I am guessing that when the sun goes down the mixing slows or stops and the absolute humidity at the surface increases due to evaporation/transpiration.

That ground heat induced turbulence can reach a pretty high altitude, as anyone who has enjoyed a late afternoon summertime landing at Sky Harbor can attest…