The Undulatus effect forms due to wind shear that occurs in the stable layer of the atmosphere. The disturbance in the atmospheric layer can be due to the sudden change in wind speed and direction caused by the approach of a frontal weather system or by simple atmospheric convection (uplift of warm air due to surface heating) mountain tops. The disturbance of the cloud layer causes the waves of air to move like waves on a pond. As the winds blow at different speeds through the cloud layer, the clouds pile up into curved ridges, bind together, and split into waves.
Undulatus Clouds
There are many potential explanations for sightings. We recommend eliminating the most common and mundane before jumping to less probable conclusions or you submit a report.
Resources
Traffic Control Trackers
Live flight tracking maps are available for monitoring airline, marine, and balloon traffic and trajectories in real-time or historically around the world.
ADS-B Exchange
FlightAware
Flightradar24
Marine Traffic
PlaneFinder
RadarBox
SondeHub Tracker (tracks both meteorological and amateur radio sondes, including radiosondes used by weather services and enthusiasts.)
SondeHub Amateur (tracks radiosondes launched by amateur radio operators and hobbyists.)
Space/NASA Launches
Planned launches occur regularly all over the globe. These tracks can help identify the potential missions or launches in your area.
SpaceLaunchSchedule.com
SpaceFlightNow.com
RocketLaunch.Live
Satellite Trackers
Satellite tracking can also be done in real-time with the aid of tracking maps. They are also helpful for tracking Starlink launches.
Heavens Above
N2YO
Satellitemap.space
Spot the Station (International Space Station)
Space Weather
There are a number of sites which track solar flares, magnetic storms, asteroids, and other events which are helpful for eliminating explanations related to astronomic phenomena.
Spaceweather.com
Spaceweather.gov
Eyes on Asteroids