Popular name for undulatus. Some forms are the result of shear instability (Kelvin–Helmholtz), and some result from gravity waves. Billow clouds are present when there is sufficient moisture present in the upward motion of the waves to make the wave structure visible by condensation of cloud droplets. Billows formed from gravity waves exhibit broad, nearly parallel, lines of cloud oriented normal to the wind direction, with cloud bases near an inversion surface. The distance between billows is on the order of 1000–2000 m.