Air is made of gas molecules that can move freely. Like all matter, air molecules are attracted to the Earth by its gravity, which draws objects to the center of the planet. Gravity is strongest at lower altitude (the height above sea level) and air molecules are packed closest together at sea level. Air is also compressed by the weight of all the air above it. The weight of a column of air from the top of the atmosphere onto a person's shoulders at sea level is more than one ton. But people and animals are not crushed because billions of molecules inside our bodies are pushing outward to compensate. The force of the air weighing down over a unit of area is known as its atmospheric pressure, or air pressure.
Atmospheric pressure decreases with increasing altitude because there is less gravity and less air to weigh down from above. Air density, then, is greatest at the Earth's surface and decreases with altitude.
Each 3.7-mile (6-km) increase in altitude reduces the weight of the atmosphere above it by half so that at 18,000 feet (5,500 meters) above sea level, air pressure is only half of what it is at sea level. People feel changes in atmospheric pressure while on an airplane or driving through the mountains when their ears "pop" as they go up or down in altitude. This occurs because the air molecules inside their ears maintain the density of the previous altitude until they have had a chance to equilibrate with the air pressure at the new altitude.
The dense packing of air molecules near the Earth's surface restricts the ability of each molecule to move. At sea level, a molecule can travel an average distance of less than one millionth of a centimeter before it collides with another molecule. Each collision between molecules releases heat, so the air at sea level is relatively warm. At higher altitudes, where the molecules are not packed in so tightly, they are less likely to collide, so the air becomes cooler.
The transfer of heat is important for driving the motions of the atmosphere. Heat transfer occurs in two different ways. Conduction is the transfer of heat through a substance that has different temperatures in different parts. Because warm atoms and molecules move more vigorously than cold ones, the particles in the warmer region strike their neighbors, transferring heat until it is evenly distributed. Convection transfers heat by the movement of currents. Think of a room with a floor heater. As the air near the heater becomes warmer, the air's density decreases and it rises. The air near the ceiling is pushed sideways by the rising air. The sideways movement of air is called advection. Because the air pushed along the ceiling is now far from the heater, it is relatively cool. When it becomes denser than the air beneath it, it sinks. The air then moves by advection along the ground until it again is near the heater. The circuit described above is a convection cell. Warm air rises to make a low pressure zone, cool air sinks to make a high pressure zone, and air moves between the two.
Something else happens to air as it rises or sinks. Warm air can hold more moisture than cold air, so as warm air rises and cools, it is able to hold less moisture, which may result in precipitation. On
A radiator in a room warms air that rises to the ceiling, where it cools and then sinks, creating a convection cell.
the other hand, sinking air becomes warmer and therefore can hold more water, which may result in evaporation of water from the Earth's surface.
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Renewable energy is energy that is generated from sunlight, rain, tides, geothermal heat and wind. These sources are naturally and constantly replenished, which is why they are deemed as renewable. The usage of renewable energy sources is very important when considering the sustainability of the existing energy usage of the world. While there is currently an abundance of non-renewable energy sources, such as nuclear fuels, these energy sources are depleting. In addition to being a non-renewable supply, the non-renewable energy sources release emissions into the air, which has an adverse effect on the environment.