Ohio is famous (or infamous in the winter time) for windy days throughout the late fall, winter, and spring months. Wind is created due to differences in temperature and pressure which usually occurs along a frontal boundary or a low/high pressure system. However, have you ever noticed that the wind is often stronger during the day compared to night? The mechanism for this phenomena might surprise you.

The boundary layer: The atmosphere’s circulatory system

Earth’s atmosphere is composed of the following gases: 78% nitrogen, 21% oxygen, 0.9% argon and 0.1% other gases. Alright, enough of the basic science lesson but the point is that Earth’s atmosphere is made up of elements and compounds in a gaseous state. Gases behave in a similar fashion to liquids in the sense that they are both fluids. There are vertical and horizontal motions that affect every day weather. One of the locations in the atmosphere with copious amounts of movement is called the boundary layer.

During the day, the sun heats the Earth’s surface which causes air to rise. The atmosphere responds by developing vertical circulations which are called convective boundary layer circulations. On calm days, these circulations often result in cumulus clouds like the ones show in the graphic below. On stormy days, these circulations can be the trigger for summertime thunderstorm activity.

Graphic explaining the planetary boundary layer. The image is courtesy of Carlye Calvin from the University Corporation for Atmospheric Research (UCAR) in Boulder, Colorado.

Why is the wind stronger in the afternoon?

Typically, wind increases with height in the troposphere. In other words, there are stronger winds above the surface of the Earth compared to at the surface. When the vertical circulations develop in the boundary layer, they act to transport the higher wind velocities to the surface. These higher wind velocities tend to come in waves and the wind speed can be variable from one point in time to another. Therefore, the reason for higher wind speeds in the afternoon is due to the transport of stronger winds to the surface via the boundary layer. A graphical representation of the transport of strong winds to the surface is shown below.

Graphic showing strong winds high in the atmosphere being transferred down to the surface. Photo courtesy of the National Weather Service.

In fact, these vertical columns of rotation are the reason for turbulence when flying in an aircraft. This is why commercial airplanes cruise at 35,000 feet to avoid turbulence due to ascending and descending air. On non-active weather nights, the winds calm down during the evening due to the loss of the daytime heating. The circulations collapse and winds become more calm during the overnight hours. Now you have a bachelor’s degree in boundary layer meteorology 101 and you know the best time to fly a kite!