How can we keep up this (seemingly) natural interest in our environment?
Model the behavior.
Right now you have most of the answers, which is great, but not so great when teens start to reject your ideas/authority figure status. So, make it a habit.
At first, just giving answers is fine, but it's even better to show kids, and it's not too early to start if the child is close to three. Experiments are fun, and simple things link baking soda and vinegar being able to blow up a balloon or be able to produce an invisible gas that can put out a candle are like a magic act to a kid! Just keep them short when very young and age appropriate. A prism or crystal to split light into rainbows is wonderful. Awesome things surround us!
Clouds are water vapor in the air. Heat water (not to boiling quite) and hold a big spoon over it that you just took out of the refrigerator. She can't yet see the water moving through the air, but there it is on the spoon. Just like in the clouds.
Say, "I don't know", and look things up together.
Propose questions of your own that might interest her. What will fall faster, a big marble or a little one? Why?
Rent books from the library on science and the environment.
My kids grew up in a two-physician home, so science and the scientific method was always a topic of conversation and a way to prove our 'beliefs'. I was also a naturalist by avocation and a molecular biologist by training, so I gave my kids diverse books that I thought they'd like. It is a big part of their lives still. (My kids know wildflowers, birds, trees, etc. My oldest was into survival on Nature alone, so just loved books like Hatchet and the Tom Brown survival guides. Cater to their preferences.)
Don't overwhelm them.
Do lots of other fun stuff, too. Balance it with other things they might like, like art, sculpting (who doesn't like to play with clay/Sculpy?), etc.
Have fun! It's not fun for them if it's not fun for you, and vs versa.
One of my favorite stories about teaching my kids: I was picking up after homeschool around noon, and I had intended to teach about the hemispheres but didn't get around to it. My oldest wandered in with a question he had about something unrelated (I think it was a TV show) and at the end of the conversation, I found myself with an orange and two rubber bands in my hands. "You know, 'Sam', if this orange was the earth..." "Mom!" interrupted my son, "I didn't come in here to learn anything!" I had to laugh and admit defeat.