How do simulate a cloudy day for a science fair project? I'm trying to help my 5th grade daughter with her science fair project.  She wants to test that water heats up more on a sunny day than a cloudy day.

To help her, I'm trying to figure out how to block a lot visible light but allow IR and UV through.

I know I could use smoke or dust, but can't figure out how to contain it for an experiment.
My suggestion would be to use shade cloth. You can get it in different densities and different colours. I know this is not the same as clouds, but get her to think about what the differences are and how that might affect the results. If I were a judge I would be very impressed to see a section like that on the poster.