The ATMSPHR file extension is associated with Terragen and used for atmosphere renderered graphic data.
How to open:
Currently there is no specific information available, about how to open files with this particular extension. However, you can usually open or import 3D graphics files to various 3D modeling tools or engineering programs. Each program has it's own set of supported formats, so it's always worthwhile to check it's developer websites for available options.For general information try to look at:Basic info how to open CAD and other 3D graphic files
How to convert:
Various 3D models or CAD files can be typically exported to some exchange format or converted between different formats. Once again, not all CAD/3D modeling programs may allow conversion of every format. But, there is always the option of using some of the dedicated 3D conversion utilities with a broad file format support.
For general information try to look at:Basic info how to convert CAD and other 3D graphic files