What type of cloud is dark, gray, and produces continuous rain or snow?

A dark, gray cloud that produces continuous rain or snow is a nimbostratus cloud.

Nimbostratus is a broad, nearly featureless layer cloud associated with widespread, steady precipitation. Unlike a towering thundercloud, it usually has little visual structure and is not normally linked with lightning or thunder. Rain or snow from it may continue for hours or even days when a large frontal system moves slowly.

These clouds commonly develop along warm fronts or occluded fronts, where warmer air is lifted gradually over cooler air. Altostratus can thicken and descend as precipitation becomes more persistent, eventually forming nimbostratus. The Latin-derived prefix “nimbo-” means rain-bearing.

A common mix-up is cumulonimbus, which can also produce heavy precipitation but is vertically developed and often brings intense showers, lightning, and thunder. Altostratus may produce light rain or snow, but continuous moderate precipitation is more characteristic of nimbostratus. After a warm or occluded front passes, stratus or stratocumulus may replace it.