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.
There’s an old story that if you see a pilot’s halo it means you have a guardian angel flying with you. I don’t know about that, but if your airplane isn’t approved for flight into known icing and the temperature is ripe for icing conditions, you should consider steering clear of that cloud layer.
You can ask ATC for pireps of icing conditions and check with Flight Watch for current and forecast conditions, but that’s still no guarantee you won’t pick up ice in the clouds if the temperature is in the range that’s conducive to icing.
If you need to descend through a cloud layer and suspect it may contain ice, delay your descent as long as possible. If you do pick up ice, it’s important to exit those conditions as quickly as possible, either by climbing back on top, descending below the freezing level or cloud layer, or diverting. With ice on your airplane, you’ll want to keep your speed up and consider leaving the flaps in the up position to avoid problems associated with a tail stall.
Better yet, if the temperatures are below freezing, enjoy the view of the pilot’s halo on top and descend only after you reach VFR conditions.