In this task, you will be presented with a passage and have to answer a question based on your understanding of the events inferred from the passage. Try to find the best answer that is most likely to fill in "_". Note that the URLs in the text have been replaced with [Link].
Q: Stunning footage has emerged showing wave-like clouds over the skies of Colorado. The Breckenridge Resort tweeted the video to highlight something known as Kelvin-Helmholtz waves, which can be found all over the solar system, including in Saturn's atmosphere. This beautiful pattern emerges in clouds when two different layers of air in the atmosphere are moving at different speeds. Kelvin-Helmholtz waves are everywhere. From Earth's clouds and ocean surfaces to the atmosphere of Jupiter, these waves have a striking pattern - a series of curled hills moving steadily in one direction. The shape of this pattern has a simple cause. A fast fluid, say wind, moving past a slower one, such as water, naturally creates this wave shape.Pattern created when when two layers of air move at different speedsThese Kelvin-Helmholtz waves are more common than first thoughtCan also be found in Saturn's atmopshere and Earth's magnetosphere 
 Question:The first paper compared Kelvin-_ waves to what was seen in the solar wind, the second team compared it to what was happening closer to Earth.
A: 
Helmholtz