The Walden Effect: Farming, simple living, permaculture, and invention.

Estimating minimum temperatures with the dewpoint

Butternut storage

Just in case, we prepared for a potential early freeze by bringing in all of our storage vegetables off the porch. Some were ready to enter their final storage locations, while others need another couple of weeks curing on the rack. (And, since someone asked last week and I forgot to reply to their comment --- if you're curious about how to cure and store vegetables that don't require fancy preservation, you can read everything I know in Weekend Homesteader: October.)

Sun-bathing chicks

Next we covered up our pepper plants (the only summer crop that hasn't already reached quota in the preservation department) and settled in to wait for the Monday low.

DewpointOkay, I lied. I can't wait worth a darn, so I instead finally researched a topic I'd read a little bit about but never really delved into --- the dewpoint temperature. The idea is that gaseous water starts turning liquid at a certain temperature, and in the process a lot of heat is released. That release of heat makes it very difficult for moisture-laden air to drop below the dewpoint temperature. So unless a cold front is blowing in, you can guess the night's minimum temperature by using temperature and humidity at sunset to calculate the dewpoint. Here's a website that does the math for you.

Did my calculations work? Sunday night, it was 51 degrees with 76% humidity...and Monday at dawn it was 43 degrees outside. Exactly as estimated!

Of course, Tuesday was a little bit different. My weather guru had explained that our valley location means that the second day in a cold spell often attains lower temperatures than the official forecast. I can't recall his exact reasoning, but I think the deal is that the cold air from the previous day gathers in the mountains above us then flows down into our valley that second night. Sure enough, the second day of our cold spell saw temperatures a few degrees lower than our dewpoint estimate...but still above freezing. Phew!

Tired wheelbarrow

Okay, I say I'm relieved...but I'm actually just about ready for the gardening season to wind down. But it's worth a bit more pushing at this time of year to grow organic matter rather than weeds in fallow garden beds. So my tired wheelbarrow will get back to work for a few more weeks, and then we'll both enjoy our much deserved winter rest.



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About us: Anna Hess and Mark Hamilton spent over a decade living self-sufficiently in the mountains of Virginia before moving north to start over from scratch in the foothills of Ohio. They've experimented with permaculture, no-till gardening, trailersteading, home-based microbusinesses and much more, writing about their adventures in both blogs and books.



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