This afternoon a unique visual and radar feature began migrating its way across the Chicagoland area - gravity waves.
What are gravity waves? A gravity wave is vertical wave in the atmosphere. In order for gravity waves to develop, there must be something a triggering mechanism to displace the parcel of air. A simple way to understand them is to envision a rock being tossed into a pond. In today's example, a round of showers and thundershowers moved through the area and displaced the stable parcel of air that was sitting in place over the Chicagoland area.
Gravity waves require stable air to be in place before the disturbance moves through. If the air was unstable, the air would continue to rise without creating a wave(s). When the parcel of air is displaced, it initially will rise but since the parcel is stable, it will then sink in an attempt to restore equilibrium. However, the momentum the initial displacement creates will cause the air to overshoot its equilibrium level in the vertical (both above and below). This is why you see the ripples. This rising and sinking motion will continue a certain distance away from the initial displacement until the air once again stabilizes and reaches equilibrium.
It's also important to note that the upward motion in a gravity wave is the most favorable region for the development of clouds. Thanks to our community of followers, several images have already been shared with us of roll-type clouds giving us a great visual cue of the gravity waves traversing the area. Conversely, the sinking region of the gravity wave is not suitable for cloud development which is why we often see rows of clouds.
In the radar GIF above I have the typical reflectivity and velocity products displayed in order to easily see the gravity waves traversing the region. At the beginning of the loop you can see what is left of the triggering mechanism (the region of showers and thundershowers) as it moves off into Indiana.
We've seen several reports of winds gusting over 35mph with these gravity waves as well. Just another interesting weather phenomenon to add to what has been a crazy week here in Illinois.