In the answer to another question about tefilla, I read: "that it was against halacha to stand outside and daven out in the open under the Kippas HaShamayim [canopy of the heavens]."
This is new information to me and I would like to know its source and if possible its reasoning. Also, is this always true, or are there cases where it is permissible to pray the Shemona Esrei outdoors?
As you yourself sourced, the problem is defined in the Gemara:
SA OC 90:5: "One should not daven in an open place like a field, because when you're in a private place you have fear of the King and your heart is broken". (Source Brachot 34)
From what I recall learning, it's easier to feel one is in the presence of The King when one is indoors; palaces tend to be "indoors"/enclosed areas.
When in the open, one feels more liberated and free - not a good frame of mind to be in when one is supposed to in a private audience with the King of Kings.
As a result, when there is no [appropriate] building available, one should prefer treed-in areas over the open-space. (I.e. - it's not forbidden - it's not inappropriate to daven in open areas.)
