For the fourth time this last month, the morning temperature has been about 4 degrees colder in Burlington than the National Weather Service predicted. You'd think they'd learn, especially when frost is involved. Last night they said the low would be 34, and now at 5:30 it's 31. It won't be a surprise if it drops another degree in the next hour before it starts to warm.
One of the keys to deciding if there will be an unpredicted frost is to look at the dew point, which is the temperature at which the air will be so saturated with water that it will condense on colder surfaces. Warm air holds a lot more water than cold air, so the dew point is not a measure of the actual water content. The temperature can never go lower than the dewpoint, (but the dewpoint can drop as the temperature goes lower.) So the dew point is a good guide to how low the temperature can/will go. If 34 is predicted and the dew point is 32, there's a good chance you won't get frost. If 34 is predicted but the dew point is 17 (as it was earlier this week), then the temperature is likely to drop, and frost is a good possibility.
If you're caught by an unexpected frost, the best thing to do is go out as early as you can (before the sun hits the plants) and spray with water. This will warm the plants and the ground, raising the temperature around the plant.
The amount of moisture in the air affects plants in other ways. Really dry air dehydrates plants, and really humid air promotes some fungal diseases.
When the dew point reaches into the 50's and 60's then we're usually uncomfortable, especially if it's not very warm. (If it's 90, then the amount of water in the air relative to what it can hold will be lower, and it actually may be more comfortable.) But by the time the dew point is 70 people generally find it oppressive. And when it's hot with a high relative humidity our sweat can't evaporate off our bodies, and we lose our ability to cool down.
In the meantime, this next week looks to be great weather. We could use a little more rain, but in our sandy garden that's almost always true.