The USGS Water Science School
This picture shows a local thunderstorm moving across Yellowstone Lake, Montana, USA. Likely this picture was taken in the summer, as that is when more localized storms occur. Other rainstorms are more "frontal" in nature, with large expanses of featureless and uniform nimbostratus types of clouds bringing precipitation over a large area. But often you see a landscape similar to the one in this picture, with rain falling in a small area (and often moving right towards your picnic) from a relatively small cloud. The sheets of rain are easily seen. Often these storms are intense but brief.
Credit: Jim Peaco, National Park Service