I waved my hand in front of my headlamp, and my leather mitt reflected bluish light. Otherwise, I couldn’t tell that the headlamp was still on. Light had gathered gradually and inexorably since we’d stood up from breakfast. Two solitary clouds shone scarlet and gold in the low sky above the eastern shore of Disappointment Lake. Our thermometer read twenty-two below zero when we woke in darkness, and the morning was about to get colder. On calm mornings, we registered a consistent 4 degree drop in temperature even as the sun rose. At VOBS, we call this phenomenon the “morning dip.” What might cause this counterintuitive cooling?
The morning dip is rooted in Earth’s heat budget, or the balance of energy received and given off by the planet each day. The ground absorbs heat from the sun throughout the day and radiates it back toward space. During the day, the sun sends enough heat to more than offset the ground’s heat loss, warming the lower atmosphere. At night, however, the ground radiates heat without any energy input. The ground and air lose heat throughout the night, until the sun rises again and begins adding back energy to the system. The longer the night, the colder the Earth’s surface becomes.
As the sun rises, the ground absorbs energy. The ground re-radiates that energy as heat into the air directly above it. The warming air rises and buoys the colder air above it. When the rising mass of cold air reaches our bodies (and thermometers), it feels as if the temperature has dropped everywhere. We shiver, throw on an extra hat, and run laps between the ice hole and the cook fire. The sun continues to warm the ground, and the increasingly warm lower air keeps pushing the cold air until it is out of our reach, raising the temperature again.
Only in vain does one hope for warmth from the just-risen winter sun in the frozen North. Though sunrise brings little warmth, it does kindle hope for the day’s discoveries. And save headlamp batteries.