Is it correct to say "When it started to rain, I was in the open air"?
This is fine, though being "in the open" implies being "in the open air". (This is Earth, not the vacuum of space, or under the water, so it would sound odd.)
Or should I say "I was not inside a building"?
Only if you specifically want to tell them that you weren't in a building.
This is because there are other places you could be when it started to rain besides in the open or in a building. For example, you could be:
- on the porch,
- under an overhang,
- in a car port,
- any other place that shelters you from the rain which aren't in doors.
In all those cases, you are outside, but definitely not "in the open".
14What the answers below don't say is that, while neither of these are the first thing a native speaker would think of, both of them are correct English and would be understood. – Hearth – 2019-05-13T14:30:59.750
2If you do want to use the word "open", which is fair enough, the usual phrasing there would be: ".. I was caught out in the open". "open air" is correct but it's not the usual idiomatic word choice. normally you'd simply say "outside" or "outdoors." – Fattie – 2019-05-13T23:50:29.237
2Do note that "When it started to rain, I was out on the streets!" would be perfectly normal and common in English. – Fattie – 2019-05-13T23:54:44.307