Why are the lower layer of the atmosphere denser than the upper layers?

Answer 1

If by "sky", what you mean "atmosphere", the answer is relatively straightforward.

Let's look at the possible heat sources for the earth and atmosphere.

In the case of the earth, heat comes from two places: pressure and solar radiation. The mass of the earth causes a non-trivial compression of the inner parts - that is, gravity keeps the earth tightly compressed into a ball. Basic physics tells us that raising the pressure without reducing the volume on something will significantly increase the internal temperature of that thing. As the Earth is made of materials that generally will not compress, the gravitational forces keeping the Earth together also result in a very significant heating of the internal portions of the Earth.

As a secondary source, the Earth is heated by absorbed Solar Radiation. Earth, as a solid, absorbs radiation (in all forms) quite easily. Most of that absorbed radiation is converted into heat (that is, it raises the temperature of the earth). A portion (perhaps a majority) of that heat is then radiated back out into the atmosphere. However, the total amount of energy being absorbed by the Earth at any instant is much, much smaller than the heat caused by internal compression (i.e. absorbed solar radiation may raise the local earth temperature by up to 100 degrees F, but internal pressure can generate 3-5,000 degrees)

As for the atmosphere, it too has two major sources of heat: directly absorbed solar radiation, and "waste" heat radiated back from the Earth.

Compared to the solid Earth, the gaseous nature of the atmosphere does a very poor job of absorbing most solar radiation. In addition, much of the radiation that is absorb is not converted into heat - rather, it chemically alters the nature of the atmosphere (the Ozone layer is a prime example of the chemical alteration of the atmosphere by solar radiation). So, direct solar radiation only heats the atmosphere by a very tiny bit.

However, the Earth itself provides a significant source of heat, both from re-radiating solar radiation that the Earth has absorbed (mostly in the form of infra-red radiation, which doeseasily warm gases), and by directly heating gas by exposure to hot Earth (think of gases being expelled by volcanoes).

As an analogy, think of the Earth as an electric stove burner, and the atmosphere as the pot you put on it. The burner is very hot, driven by internal heating process (electric in the case of the burner, gravitational pressure in the case of the Earth). The pot on top only gets hot by "stealing" (absorbing) heat from the burner - it can't generate heat all by itself. Thus, while the Earth is mostly self-warming, the atmosphere can generally only heat itself by absorbing waste heat from the Earth.

Answer 2

entropy..the earth is heated from its core. The further you are from the core at the center of the earth the cooler it is.

Answer 3

The lower atmosphere is warmed from the surface upward by ground temperature. As the ground warms up, the heat rises and causes warmer temperatures.

Answer 4

due to the gravitational energy