Moisture in the air: Water exists in three states; in the air, solid, liquid and invisible water vapor. Water vapor or moisture in the air is known as humidity.
After rain has fallen there are some standing puddles of water. When water is heated the molecules move apart. It heated enough, the molecules will break off and fly into space. We call this process evaporation. Seventy-eight percent of the earth is covered with water. Warm air over bodies of water evaporates some of the water into the air in the form of water vapor.
Other than lakes, streams, and oceans sending up steady streams of water vapor, green plants send up an amazing amount also.
Relative humidity is the amount of vapor the air is holding expressed as a percentage of the amount the air could hold at that particular temperature. Relative humidity goes up when air with a given amount of water vapor cools and drops when the air is warmed. Warm air can hold more water than cold air.
Warm moist air rises and will slowly cool. When it cools to the point where the relative humidity reaches 100%, clouds will form. The process of evaporation, condensation, and precipitation is called the water cycle.
Heat energy: In order for the air to be mixed, heat is needed. All of the changes that take place in the atmosphere are due to temperature changes in different parts of the atmosphere.
The sun is a glowing ball of gas that radiates 74 million watts from each square meter of ratio. This energy can illuminate 740,000 one hundred watts light bulbs. Most of the sun’s radiant energy misses the earth because it radiates in all directions. About 43% of the radiant energy reaches Earth’s surface and is changed to heat. The earth is warmed by absorbing, storing and recycling radiant energy.
The sun’s energy that is absorbed by the Earth is spread out in the atmosphere in three ways: (1) conduction: direct transfer of heat energy from one substance to another; (2) convection: transferring of heat energy in a fluid such as gas or a liquid, and (3) radiation: the transfer of heat energy through empty space.
Winds: Winds are caused by convecting currents of air. The hot air at or near the equator rises. The hot air then circles around the upper atmosphere, cools, and drops near the tropics. Sometimes the hot air drops as far north as the Arctic or as far south as the Antarctica. The hot air that falls at the tropics spread south and north forming surface winds. The winds that blow back toward the equator are called “trade winds”. Those winds that blow away from the equator are called “westerlies”.
The cold heavy air that sinks, causing regions of air pressure, are the source of major winds. The main winds of the world all blow from high pressure regions to low pressure regions.
At 50 degrees north and south the westerlies are the main winds. In the northern hemisphere the main winds are the south westerly and in the south they are north westerly; meaning they blow from the southwest or northwest. Winds get their names from the direction from which they blow. Trade winds are north easterly in the northern hemisphere and south easterly in the southern hemisphere.
Air masses of North America often move from west to east. This is also the direction of the prevailing westerlies wind. The great flow of air is caused by the rotation of the earth and planet wide patterns of heating and cooling. The turning of winds is known as the Coruolis effect, which is due to the rotation of the earth.
Air pressure: Air is an invisible gas and has weight. The weight of the atmosphere when pressed down on the earth causes force on each square inch of surface it touches. The face is referred to as atmospheric pressure of air pressure.
Air pressure changes with altitude and temperature. At sea level, per square inch, the atmospheric pressure is about 14.7 pounds. At higher elevations there are fewer air molecules spread out and as the air is cooled the molecules come closer together. The more molecules there are in the same space, the greater the atmospheric pressure. Warm air has less pressure than cold air.