Have you ever been flying over a cloud deck and noticed a rainbow halo around the shadow of your airplane? Besides being pretty to look at, this ring-shaped rainbow – called a “pilot’s halo” or “glory of the pilot” because we’re usually the only ones who get to see it – should be a clue to you that the cloud holds liquid moisture.
So what, you ask? If the temperature is in the freezing range and you descend into that cloud, you can expect icing.
