Gravity Cools the Atmosphere

This statement may sound strange to those who don’t understand that heat is the kinetic energy, or motion, of atoms/molecules. Actions which increase kinetic energy of atoms cause an increase in heat. Actions which decrease kinetic energy of atoms reduce heat energy thus cooling atoms/molecules. I will use molecules instead of atoms because gas atoms exist as parts of molecules and at atmospheric temperatures atoms in molecules behave as a unit. The following is a simplified view of atmospheric heating and cooling involving the rising and falling of air molecules. Air currents can cause warm air and cold air to mix with heat transferring from warm air to cool air.

Inertia is the property of matter in which an object in motion will tend to stay in motion unless acted upon by some force. Gravity is a force which can increase or decrease motion. If you push a rock off a cliff, gravity will cause a downward motion with the velocity increasing as the object falls. If you throw a baseball up into the air, gravity will gradually decrease its upward motion until the baseball stops going up and gravity starts to pull it back down to the ground.

Physicists determined in the 19th Century that heat was the motion, or kinetic energy, of atoms. Individual molecules have their own kinetic energy which physicists call “heat”. There is a common misconception that heating causes molecules to vibrate. Heat causes motion in molecules but molecules seldom have freedom of movement. Molecules in solids are held in place in a matrix. Attempts to move result in vibration unless molecules become hot enough to break the bonds of the matrix, such as when ice melts. Gas molecules bounce off each other like ping pong balls in a bingo machine which in effect is vibration.

As the sun heats the earth’s surface, air molecules in thermal contact with the surface begin absorbing heat energy from the ground. Two substances in what physicists call “thermal contact” will attempt to become the same temperature. Although the process is far more complicated than what happens with billiard balls on a pool table, the behavior of billiard balls is one way of visualizing how energy is transferred from molecule to molecule.

As air molecules heat up they begin to rise from the earth’s surface because warm air is less dense, and thus lighter, than cool air. As air molecules bounce off each other the area they cover spreads out and there are fewer molecules per cubic meter. The upward movement allows cooler air to flow in under the warm air and begin heating. The process continues as long as some air is cooler than the ground.

The atmosphere also receives heat energy from the evaporation of water. The water vapor comes from bodies of water and the ground as well as the evaporation of water from plants and animals. For example, the human body cools itself by perspiring water to the outside of the skin where it evaporates and takes the heat energy into the atmosphere.

The heat energy held by water vapor involves more than just its temperature. Water vapor also holds what physicists call latent heat which includes the heat energy that must be absorbed for water to go from a solid to a liquid [heat of fusion] and from a liquid to a gas [heat of vaporization]. Other gases also possess latent heat , but they are gases at atmospheric temperature so they don’t go through a change of state that would involve this heat. Water is normally a liquid or solid at atmospheric temperature.

When matter rises from the earth’s surface it must turn part of its kinetic energy into potential energy to overcome the force of gravity. This process affects all matter regardless of whether it is as big as a rocket or as small as a water molecule. The conversion of kinetic energy into potential energy doesn’t cause a loss of energy, just a change in status from what might be called “active” energy to “inactive” energy. The is analogous to charging a battery.

Objects, including gas molecules, above ground have potential energy because that energy will become kinetic energy if they fall, If some of the kinetic energy (i.e., heat) of gas molecules didn’t change to potential energy gas molecules would gain energy from the movement upward which is impossible.

When gas molecules rise the conversion of kinetic energy into potential energy causes them to slow down and thus become “cooler”, The cooling process is slow because of the low mass of gas molecules, particularly water vapor which consists of an oxygen atom and two atoms of hydrogen which is the element with the lowest mass. High air pressure blocks this cooling by preventing warm air from rising.

When matter begins falling back to the ground, gravity converts its potential energy back into kinetic energy. As gravity increases the kinetic energy of solid objects when they fall, the velocity of the object increases. Gravity generally doesn’t increase the kinetic energy of individual gas molecules[cause heating] as they fall back to the ground. Instead, gravity increases the kinetic energy of the air mass, or wind. An exception is the Chinook winds that sometimes occur along the eastern slope of the Rocky Mountains.

In its gaseous state water molecules are lighter than the other molecules of the other atmospheric gases. Although water molecules can fall back to earth as gas molecules, they usually condense into liquid droplets or freeze into ice particles. If the ice particles are large enough, they can acquire enough kinetic energy as they fall to cause damage to solid objects on the ground. If water drops freeze on tree limbs or power lines when they near the ground their kinetic energy will temporarily become potential energy which can become kinetic energy if whatever they attach to falls.

The amount of potential energy held by water is determined by the distance it rises above sea level rather than just its distance from the ground. Generally water droplets will transfer their kinetic energy to whatever they hit such as human skin. If sufficient water hits on a slope, the kinetic energy of the flood water can be sufficient to move dirt or in rare cases buildings.


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