Sunrise, Christmas Eve, 2007
Christmas Eve dawned clear back in 2007, unlike this morning. We are finally getting some rain.
In this shot, the low-lying fog in the hollows is the result of a temperature inversion. In the lower 40,000 feet or so of the Earth’s atmosphere, the part called the troposphere, the temperature normally decreases about 3.5 F for every 1000 feet increase in elevation. That change in temperature with altitude is called the temperature lapse rate. We often see the effect of the normal lapse rate when we drive up the mountain to our house. We are about 600 feet higher than the valley below us, and the temperature at the house is often a couple of degrees lower than down in the flatlands. That’s consistent with a lapse rate of 3.5 F per 1000 feet. But on a clear night, the lapse rate reverses close to the ground as the surface radiates heat towards space and the ground cools the air nearby. The change in lapse rate that occurs then is called a temperature inversion. In a temperature inversion, the air gets warmer as you go up. Since the cool air becomes denser, it wants to sink. The ground, of course, stops it. Up on the mountain, however, the cool air can slide down the mountainside to pool in the lower elevations. So at night, as we drive up the mountain, the temperature sometimes gets a good bit higher. We have seen temperature differences of 5 F between the bottom of the mountain and our house.
If the air near the surface cools enough, it drops below the dew point and fog forms. That’s what happened in this picture. As the sun rises, it will heat the surface, and the air near the ground will start to get warmer and less dense. The air will begin to stir, mixing into the air above it, until the inversion disappears, taking the fog with it, and restoring the normal lapse rate.
