How do you dew?

Last Sunday morning as I was getting dressed to sing in my church choir I noticed a peculiar phenomenon as I looked out the window. The sliding glass was open about 8 inches and the screen was adorned with small droplets of dew, but only where the window was open.

Dewy screen

The vertical black bar in picture is the frame of the sliding portion; to the left we are looking through the window glass and the right side is open to the outside air. The screen lives on the outside of the frame so all of it is in the foggy outside air – why is there no dew where the screen is outside the glass? My first thought was that the open side was exposed to the relatively warm and drier air in the house (it was pretty clearly about 100% humidity outside) and so should be less likely to form dew.

Also, the dew drops were more dense toward the bottom of the window and sparser near the top – here are two closeups that I cropped from another shot:

Closeups

The droplets are rather washed out by the backlighting near the top so the pictures may not be convincing, but take my word for it.

Here’s what I came up by way of explanation: The outside air was saturated with moisture but it was quite still. Under such conditions the humidity can actually exceed 100% (supersaturation) because water vapor doesn’t really like to condense unless there is something to condense ON. But where the colder and heavier outside air met the warmer inside air, it started flowing through the screen into the house. Any sort of disturbance can be enough to get supersaturated air to start condensing on whatever is handy. And that inflow of air meant that some inside air had to get out somehow.

So I’m picturing cool air flowing in through the lower part of the screen, producing lots of condensation, and warmer air escaping through the upper part, evaporating some of the water.

That’s my story and I’m sticking with it.

Technical Note

I was able to get the screen into sharp focus since my new camera allows manual focus. Better yet, it has a “Manual Focus Assist” feature that temporarily blows the image on the camera’s screen up to the maximum, right to the pixel level, so you can really see what you’re getting. Very cool.

Personal Note

I am on jury duty this week and next. I didn’t have to go in on Monday but I was called in today and sent in with a panel for jury selection in a criminal trial. I shared some misgivings I have from my last jury experience and was peremptorily dismissed by the prosecutor. I actually sort of wanted to get on that panel since it was expected to be a 1- or 2-day trial, meaning I would likely be off the hook for Thursday’s hike. As it is I will call in on Wednesday evening for my next assignment, which could be on Thursday, of course. But if they go in any sort of predictable rotation there’s a decent chance I might be free for the hike to Lake Ann.