Wind speed tends to drop at night due to a phenomenon called "nocturnal boundary layer." During the day, the Earth's surface is heated by the sun, which in turn warms the air directly above it. This warm air rises, creating vertical motion and contributing to the formation of convective currents. These convective currents enhance the mixing of air, resulting in higher wind speeds during the day.
However, at night, the Earth's surface cools down as it loses heat to the atmosphere. This cooling effect causes the air near the surface to cool as well. As a result, a stable layer of cool air forms close to the ground, known as the "nocturnal boundary layer." This stable layer acts as a barrier to the vertical mixing of air.
With reduced vertical mixing, the transfer of momentum from higher altitudes to the surface diminishes. Consequently, wind speeds near the surface decrease during the night as compared to daytime conditions. Additionally, at night, the atmosphere often becomes more calm and stable, leading to less turbulent conditions and further reducing wind speeds.
It's important to note that this is a general explanation, and the specific behavior of wind at night can vary depending on local geographic and meteorological conditions.