When God divided the heaven from the earth, he used a firmament, and it divided the water which were above, from the waters which are below.
We have air and water down here. And why not air and water up there, it all used to be the same substance. So what is in heaven is what is down here on earth, that is what a plain reading of scripture says.
Genesis 1
6 Then God said, “Let there be a firmament in the midst of the waters, and let it divide the waters from the waters.” 7 Thus God made the firmament, and divided the waters which were under the firmament from the waters which were above the firmament; and it was so. 8 And God called the firmament Heaven. So the evening and the morning were the second day.