This is the default for Flight Sim World, a sim from DovetailGames which Orbx is developing the ground for. The clouds are provided by a feature called TrueSky which uses a volumetric skybox to simulate the sky. The shading is IBL or Image Based Lighting which refracts light off of the ground and clouds.
Brad provided a link above, but be cautioned that it recently left early access and lacks some features and addons that are still being brought up to standard you see in the sky and plane models. You may be best to catch it on sale, as its price fluctuates quite a bit.