About the placement of tefillah (prayer), Shulchan Aruch says something very interesting. He says that a person should not pray in an open area, such as a field, because if a person is in a more private area, that person has more concentration—or, as he actually puts it, you have more sense of the presence of Hashem when you're in an enclosed area. What does this halacha (Jewish law) mean? You're not allowed to pray in an open area? So what it means literally is that if you're randomly making a minyan (prayer quorum), or praying on your own, and you're debating, “Should I pray outdoors or indoors?” you should make sure to pray in a place that, ideally, has walls and a roof, but at least has a gated area enclosing you. If you're not praying just plainly in the open, according to Shulchan Aruch, it's not the best.
For this reason, he says, the mind does not have a sense of the presence of Hashem unless you're in an enclosed area. More importantly, it's an area; a sense of privacy allows you to have a sense of Hashem. A sense of openness does not. You're thinking to yourself of different instances when you saw people pray outside, and you're wondering how that's okay. So Mishnah Berurah adds one thing. He says that people who are travelers are allowed to pray wherever they are, but if you have the opportunity, even if you're a traveler and you're out there in a parking lot, make sure to go over to an area that has trees. That's considered somewhat tzanu'a, somewhat closed in. It has that sense of privacy. The theme is that praying with a sense of privacy from the outside world is better than praying just out in the open.
Secondly, there's a concern that people are going to walk through your tefillah and disrupt it. These are the two concerns that we have in place. Sometimes the latter concern doesn't exist and only the former one does. But ultimately, all in all, a person is better off praying in a place that's considered somewhat enclosed, at least by walls or even by trees, than just praying out there in the open. Have a great day, Kabbalah (Jewish mystical tradition).