Fog Drip and a Solar Still

Where Does Summer Water Come From?

1On a June field trip, Tavita Fonoti's sixth-grade class visited a coastal redwood state park in northern California. The air under the giant trees was cool and gray, and drops of water fell from the branches even though it was not raining. Ranger Marisol Peña told the class that summers here are almost rainless, but fog rolls in from the ocean on many mornings.

2The ranger explained how the fog forms. Sunlight warms the surface of the Pacific Ocean far offshore, and water evaporates into the air above it. Winds push this moist air toward the coast, where it passes over a band of cold ocean water near shore. Fog is a cloud that forms close to the ground. When it drifts through the forest, its tiny droplets collect on redwood needles, join into bigger drops and fall to the forest floor.

3To measure this water, the park keeps two identical funnel gauges. One stands under the redwoods, and the other stands in an open meadow 200 m away with no trees above it. The ranger showed the class the totals from last summer.

4Back at school, Tavita and her partner Owen Marchetti built two solar stills to model the water cycle. They poured 500 mL of salt water into each of two identical dark bowls and set a small empty cup in the middle of each one. They covered each bowl tightly with clear plastic wrap and placed a pebble on the wrap right above the cup. They set Bowl A in a sunny spot on the school patio and Bowl B in the shade of the same patio. After 4 hours, they measured the water left in each bowl and the water in each cup.

5Finally, with their teacher's help, they placed 3 drops of water from Bowl A's cup and 3 drops of the salt water left in Bowl A on black paper and let them dry. The drops from the bowl left a white crust. The drops from the cup left nothing they could see.