Some changes finally appearing in our forecast, although at this time any precipitation will be pretty sparse, and that won’t come until after the weekend. A dry cold front will drop down from the north, kicking up some stiff north breezes Saturday and dropping our high temperatures to the low 50s. After a recovery to the upper 50s on Sunday, another cold front will come through Monday, dropping our highs further into the 40s with a slight chance of a few rain or snow showers. Temperatures then rebound to the upper 50s by Wednesday.

Dew Point grass

We have been talking about the dewpoint. The dewpoint can give you a baseline for how low your temperatures can go. That’s because from a practical point of view, the temperature will not fall below the dewpoint. It doesn’t necessarily mean if the dewpoint is 25 degrees in the afternoon that the overnight low can’t fall below that number, but in order to do so the air has to lose some of its moisture. It usually does that by condensing some of the water vapor out of the air as the temperature meets the dewpoint. But that will slow down your cooling due to a thing called latent heat.

latent heat graph

Whenever water changes its state from vapor to liquid, it gives off heat in order to do so. This release of “latent heat” slows the cooling of the air. This is why dry air cools so much faster than “wet” air.

latent heat 2

Latent heat is a very important factor in the physical chemistry of our air. The amount of water vapor in the air not only is a factor in producing rain directly, but it changes the thermal makeup of the atmosphere as well. Computer models used in forecasting have to take latent heat into consideration in order to be at all accurate.

Hmmm…that gives me a great excuse for the next forecast that busts. “Latent Heat Error.” I like the sound of that.