Wind is the movement of air caused by differences in atmospheric pressure. The wind near the ground typically decreases significantly after sunset. This daily pattern is a direct consequence of the sun’s influence on the atmosphere’s lowest layer, which undergoes a dramatic structural change between day and night. This shift involves examining how solar energy drives atmospheric mixing and how the lack of that energy allows friction to dominate.
Why Wind Speeds Increase During the Day
During daylight hours, the sun heats the Earth’s surface, which warms the air directly above it. This warming creates buoyant, less dense air that rises in a process known as convection. The rising air parcels generate vertical circulations and vigorous atmospheric mixing throughout the lowest part of the atmosphere.
This mixing action, termed thermal turbulence, is responsible for the stronger winds experienced at the surface during the day. Air higher up in the atmosphere moves faster because it is less affected by surface friction. The vertical turbulence transfers the momentum of this faster air downward toward the ground. Consequently, surface wind speeds increase and become gustier as peak heating occurs in the mid-afternoon.
The Role of Cooling and Friction at Night
As the sun sets, the ground rapidly cools by radiating heat into space. This cooling quickly lowers the temperature of the air immediately adjacent to the surface. Since cooler air is denser, this creates a layer of stable air near the ground that is reluctant to move vertically. This effectively suppresses the thermal turbulence that defined the daytime atmosphere.
This new, stable structure is known as the nocturnal boundary layer. Its formation effectively “decouples” the air near the surface from the faster air above it. Because vertical mixing is suppressed, the momentum from swift upper-level winds can no longer be transferred down to the ground. The air in this shallow, stable layer is then slowed dramatically by surface friction from obstacles like trees, buildings, and terrain features.
The result is a sharp reduction in surface wind speed, leading to the calm conditions often observed on clear nights. This stable layer is typically quite shallow, sometimes only a few dozen meters deep. It is responsible for the quietening of the wind near the surface. The cessation of turbulence allows for rapid surface cooling, which is why the coldest temperatures often occur just before dawn on calm, clear nights.
When Wind Does Not Die Down
The decrease in surface wind speed at night is a general rule, but it is not universal, especially when strong atmospheric forces are at play. When a large-scale weather system, such as a strong low-pressure area or a mid-latitude cyclone, produces a powerful pressure gradient, the wind remains strong regardless of the time of day. These systems generate winds that are too powerful for the nightly cooling cycle to suppress.
A more common nocturnal exception is the formation of the nocturnal low-level jet, where wind speeds actually increase a short distance above the surface. This jet is a concentrated band of fast-moving air that forms in the residual layer just above the stable, friction-dominated air near the ground. It is typically found between 100 and 300 meters above the surface and can feature wind speeds ranging from 25 to over 70 knots (46 to 130 kilometers per hour).
This acceleration occurs because the air layer is decoupled from surface friction, allowing the wind to accelerate due to an imbalance of forces. The core of this low-level jet often reaches its peak speed in the pre-dawn hours before the morning sun begins to warm the surface and reintroduce mixing. Local terrain features, such as mountain-valley systems, can also create persistent wind flows that override the standard daily cycle of wind speed reduction.