The type of cloud that creates a flat, gray sky is called stratus.

Here's why:

* Stratus clouds are low-lying, sheet-like clouds that cover the sky like a blanket.

* They are made up of tiny water droplets, which means they are not thick enough to block out all the sunlight, but they do diffuse it, creating a dull, gray appearance.

Other types of clouds, like cumulus or cirrus, tend to have more distinct shapes and are less likely to produce a flat, gray sky.