Just a simple evening sketch. Integrating an element into existing fog is always a fun compositing exercise.
The footage was taken on a hike a few years ago, during an uncharacteristically foggy day.
The sphere was modeled in blender and the foreground birds consist of stock elements, as well as existing birds from the plate that were luma keyed into place.
An important technique is the use of preview nodes, usually a gamma/gain up, gamma down, and oversaturation node. This allows you to evaluate whether the element is sitting into the shot in terms of shadows, highlights, and color gamut.
As a bonus, here's a grungy instagram version: