Create a stunning flat design illustration of a city skyline shrouded in fog, capturing the essence of urban life amidst the mist. The image should feature simplified skyscrapers and buildings, rendered in a harmonious color palette that reflects the mood of the fog. The fog should be represented with soft, layered gradients that gently obscure the lower parts of the skyline, creating a sense of depth and mystery. Incorporate elements like clouds or birds in flight, designed in a minimalist style, to enhance the composition. The overall atmosphere should evoke a sense of wonder and exploration, inviting viewers to imagine the stories hidden within the foggy skyline. Keywords: "flat design," "foggy skyline," "urban illustration," "mysterious atmosphere."