Why is this in Weather 2?
Most likely that Duo is trying to teach you more applications of 大约.
I got this as an audio exercise with no audio. I had to guess what the sentence might be. Lucky for me I guessed rightly.
Is it just me? Or does the "approximately" sound kind of twisted in there?
I used "roughly" instead of "around". My dictionary says its ok.
Yes, "roughly", "around", and "about" all are ok here.
what about 大概?
Use 大约 for approximating numbers. They talk about it in the tips for this lesson. 大概 seems more for use with situations e.g. 他大概不会来 He probably won't come.
I thought 岁 was only for low number ages (children). Is it really used for age 40?
Perhaps saying she's a big kid? :-D