How they form These sheets of gray and white blanket the sky—the quintessential cloudy day (the word stratus is actually Latin for “layer”). They form when a relatively stable air mass (read: not very different in temperature from the air around it) condenses over cooler air.
What to expect Prolonged rain or snow is likely. The lower, thicker, and darker the cloud, the sooner the precipitation.
Warning signs First, high, faint cirrostratus clouds create a haze across the sky and a halo around the sun, hinting at rain or snow within 24 hours. The cirrostratus might then lower and thicken into altostratus clouds, obscuring the sun’s exact location and suggesting continuous precipitation within 12 hours. If the altostratus clouds descend farther, they become dark nimbostratus clouds, which entirely obscure the sun and often produce steady precipitation that can last hours or days. What to do Gauge if you need to bust out miles to reach a campsite or cross slick terrain (say, a scree field) before the interminable drizzle.