It usually is much windier during the day than at night. Why is that?
Jill Matlik, Chicago
Dear Jill and Tom,
You are correct: Daytime winds tend to blow with more force than nighttime winds. It is the result of diurnal surface heating causing air in the lower atmosphere to mix or overturn. Sunlight warms the Earth’s surface unevenly and that imparts uneven warmth to the overlying air. For example, during the afternoon air over Lake Michigan tends to stay cooler than air inland. This causes a variation in air pressure: higher pressure in cooler air, lower pressure in warmer air. In turn, this generates wind as air is forced toward lower pressure. Another important factor is nighttime cooing, which sets up a layer of cool, calm, dense air near the ground. This effectively shunts organized winds a few hundred feet aloft.