Check out the following site.  Some of it goes beyond the 2nd grade level, but I will be using it in the classroom.

http://eo.ucar.edu/webweather/index.html

Stratus Clouds

  The word stratus comes from the Latin word that means “to spread out.” Stratus clouds are horizontal, layered clouds that stretch out across the sky like a blanket. They are uniform gray in color and can cover most or all of the sky. Stratus clouds can look like a fog that doesn't reach the ground. Light mist or drizzle is sometimes associated with stratus clouds

 Fog

Nimbostratus clouds are low, dark, storm clouds that cover the sky like a blanket.

Cumulus Clouds 

The word cumulus comes from the Latin word for a heap or a pile. Cumulus clouds are puffy in appearance. They look like large cotton balls. Cumulus clouds usually form when warm, moist air is forced upward. As this air rises, it is cooled. If it is cooled below its dew - point temperature, condensation will occur.

Cumulonimbus Clouds

The clouds that produce heavy thunderstorms in summer are a form of cumulus clouds called cumulonimbus. Cumulonimbus clouds may extend upward for hundreds of meters.

Cirrus Clouds

Cirrus clouds are a third general type of cloud. The word cirrus comes from the Latin word for a tuft or curl of hair. Cirrus clouds are very wispy and feathery looking. They form only at high altitudes, about 7 km above the earth's surface. Cirrus clouds are composed of ice crystals and are so thin that sunlight can pass right through them.