All the Water in the World by George Ella Lyon, Katherine Tillotson
Water Dance by Thomas Locker
Make a shaving cream cloud in a glass of water and add blue dye. See http://www.hookedonscience.org as a reference.
Rain by Manya Stojic
Make it rain in a jar: http://www.weatherwizkids.com/experiments-make-rain.htm.
Use cardboard paper towel or toilet paper rolls to make rain sticks. First have the students decorate the tubes with paint or markers. Then help them tape a circle of construction paper to one end and put dry corn, rice, beans or a combination of all three inside. Cover the other end with a circle too and your preschoolers will have a fun musical instrument that sounds like rain.