Back up the originals! Rename the existing sky files in the env folder to original_desertbk.bak just in case you want to revert.
If you are tired of staring at the same generic, pixelated clouds over de_dust2 or the dull gray haze over de_inferno, it is time to explore the world of What Exactly is a Sky Pack? In technical terms, a "sky pack" is a collection of custom texture files (usually .tga or .bmp formats) that replace the default skyboxes in the game. A skybox is the 3D cube that surrounds the entire map, projecting the illusion of a distant horizon, sun, moon, stars, or clouds. cs 1.6 sky pack
Have a favorite sky pack? Let us know in the comments below! Back up the originals