“What causes the temperature to drop as much as 5 degrees at sunrise? I start my morning walks before the ‘crack of dawn’ noting the temperature at that time and when I finish (approx. 45 minutes later), it has dropped as much as 5 degrees. I live in the south end of the Carson Valley in Gardnerville and I see this temp variation on a regular basis.”
Many wonder if they are just imagining that post-sunrise cool down, but it does occur often… especially on a clear calm morning. Here’s how it works. Because of radiational cooling, on clear nights the ground gets much colder than the air just a few feet above it. Since thermometers are placed about five feet above the ground, it will show a warmer temperature than the air touching the ground. Once the sun comes up, the sunlight excites the cold air in the first foot or so above the ground (which can be 10 or more degrees colder), which causes it to move around and mix into the next several feet of air. That “mixing upward” drops the temperature of the air at thermometer level.