The shallows layer in atmosphere is the troposphere.
Explanation:
Troposphere is also the immediate layer above the ground which extends up to 33000 feet above the sea level. Most of the living Habitat occurs in this layer of the atmosphere that is the troposphere.
Thus all the weather conditions that is winter summer and rainy occurs in this part of the atmosphere. Hence per square is the shallowest and the lowest part among all four layers.