What adjective describes a calm and peaceful atmosphere?

The adjective that best describes a calm and peaceful atmosphere is "tranquil." This term specifically conveys a sense of quietness, peace, and an absence of disturbance, which aligns well with the qualities one might associate with a serene environment.

While "halcyon" also denotes a peaceful and idyllic time, it specifically refers to a period of calmness, often in nostalgia, and is usually used in a more literary context. Similarly, "serene" accurately captures a state of calmness but can imply a level of beauty or majesty in that quietness. "Restful" suggests a place or situation that provides comfort and relaxation, but it does not inherently describe the overall atmosphere of calm in the same way that "tranquil" does.

Thus, in the context of creating a vivid image of a calm and peaceful atmosphere, "tranquil" is the most appropriate choice as it directly embodies the essence of peacefulness and serenity.