On the heels of a very wet and warm atmospheric river we’ve brought in a cold and dry airmass that will give us lots of sunshine over the weekend, but will also result in some frigid temperatures. Winds will turn light out of the north, and overnight low temperatures will drop to the low teens in town, and likely to the single digits in the colder outlying valleys. Highs on Saturday and Sunday will struggle to reach the freezing mark before a slight warming trend raises the mercury to the upper 30s on Monday and the 40s through mid-week.

We’ve been talking about adiabatic cooling, the cooling of air as you go up in elevation due to pressure drops. But show quickly should air cool as you go up? Generally speaking, as you rise in the atmosphere the decrease in air pressure will cool the air at a rate of about five and a half degrees per thousand feet of elevation gain. So in a mixed atmosphere, the temperature around Lake Tahoe (6,500’) will be about 10 degrees cooler than here in Reno (4,500’). That rate (called the dry adiabatic rate) applies only when the air is not saturated with moisture.

If the air is saturated (such as you’d find in a cloud), that rate of cooling drops to about three degrees per thousand feet gain. Why the difference?

It’s because of the latent heat release from water when it condenses from vapor to liquid. Whenever water condenses, it needs to lose heat in order to make the change in phases. That heat released partially counteracts the cooling of the air due to the drop in pressure. So once you get in the clouds, it still gets colder the higher up you go… but not as fast as if the skies are clear.

Knowing this difference between dry and wet adiabatic lapse rates is critical in predicting the stability of the atmosphere, and therefore factors in when trying to forecast all kinds of weather.