Let's go back to our house and test its roof with rain:

It's really raining.
View this step in the browser: https://larsvers.github.io/learning-d3-mapping-8-3. Code example 08_03.html.
It's hard to see in a static image, but the blue dots are in fact moving down when viewed in the browser. They are also moving at different speeds, which makes the rain look a little more realistic.