ASK THE METEOROLOGIST
“Why are winds stronger in the afternoon?”
Something that we commonly see is that winds will start out calm or light in the morning hours, and then as the day progresses, the winds will start getting stronger by the afternoon. Right before the sun goes down is when we typically see the winds at their peak. But why?
Well, when thinking about wind speed, it is important to remember friction. Friction is one of the reasons we have varying wind speeds. Obviously, the more friction, the slower the winds. Having this in mind, we see that the winds are slower at the surface than they are in the upper levels of the atmosphere. Faster currents of air move above our heads while we see the lighter winds at the surface. That must mean that the winds at the surface are encountering more friction. What causes the friction at the surface? Terrain, buildings, mountains, towers, etc. Now that we figured out where the different wind speeds occur, we can understand how they change.
During the day, if we have enough low pressure, daytime heating will cause air to rise. Low pressure promotes rising air and high pressure promotes sinking air. When air rises it cools and condenses to form clouds. When air sinks it warms away from the dewpoint, so no condensation takes place. That is why when you hear the meteorologists talk about low pressure, it’s usually a cloudy day, and clear when they say high pressure.
When we get low pressure, air begins to rise as pockets and when the winds aloft hit the pocket of air, it is forced in a different direction. The wind can be directed downward to the surface where it mixes with the light winds to cause stronger winds. The later in the day, the more thermals that will be rising; therefore, more of the faster moving winds are deflected to the surface. That is why the strongest of the winds are felt in the late afternoon.
When the sun goes down, thermals stop rising and thus the balance resets itself.