Meteorology is the study of what happens in the atmosphere, the layer of gases that surrounds the Earth. 78% of the atmosphere is nitrogen (N₂), 21% is oxygen (O₂) and the remaining 1% consists of argon (Ar), carbon dioxide (CO₂), hydrogen (H₂), helium (He₂) and other assorted gases.
     Different parts of the atmosphere have different properties. The region farthest from the Earth is the thermosphere (formerly known as the ionosphere because the air is so thin the atoms are ions). Below the thermosphere is the mesosphere. The boundary between these two regions is called the mesopause, and it is found at an altitude of about 80km. The mesosphere is about 30km thick. The stratopause divides the mesosphere and the stratosphere. The stratosphere is a region of uniform, stable air. Because it is "above the weather," pilots prefer to fly in it. The stratosphere also contains the ozone layer. The next region is the troposphere. This is the region nearest the surface of the Earth. The boundary between the troposphere and the stratosphere is called the tropopause. The altitude of the tropopause is about 8km at the poles and about 18km at the Equator. Most of the water vapor in the atmosphere is found in the troposphere so this is where clouds form and where weather happens.
     Weather means the condition of the atmosphere at a given time and place. This includes temperature, pressure, humidity, cloudiness, precipitation, and wind. The energy that drives the weather comes from the sun. There is less sunlight near the north and south poles and more sunlight closer to the equator; so there is less heating of the atmosphere at northern and southern latitudes. The seasons also change because the Earth is tilted on its axis and due to the Earth's orbit around the sun.
     Pressure also causes the weather. Whenever there is a pressure difference, air will move from high pressure to low pressure.
     Because the Earth is spinning, Newton’s 1st Law of Motion (Inertia) is seen as the Coriolis force. The Coriolis “effect” means that the air moving in the northern hemisphere is pushed up to the north, around, and down in a clockwise circle; while the air in the sothern hemisphere is pushed down to the south, around, and up in a counterclockwise rotation. This rotation of the air and water currents is called the Coriolis Effect. However, storms, because of their low pressure, move the opposite direction. So in the Northern Hemisphere, tornadoes and hurricanes go counterclockwise, and clockwise in the Southern Hemisphere.
     The relative humidity is the actual amount of water vapor in the air. A relative humidity of 20% means that the air is very dry. A relative humidity of 80% means that the air is fairly moist. At a relative humidity of 100% the air is saturated (totally soaked). If any more water vapor is added to the air, it will condense and begin to fall as rain.
     Temperature also causes weather. The water vapor will condense if the temperature decreases, and rain will fall. If the temperature increases, water will evaporate or remain a gas, and will stay in the air.
     The atmosphere can be divided into four layers based on temperature variations. These temperature variations are caused by the absorption of solar energy through the atmosphere. The Earth's surface absorbs the most solar energy. Some of this energy is radiated back from the Earth as heat, which warms the troposphere. The average temperature in the troposphere decreases with altitude until the tropopause, the boundary between the troposphere and the stratosphere. The temperature begins to increase with altitude in the stratosphere. This warming is caused by a form of oxygen called ozone (O₃) which absorbs ultraviolet radiation from the sun. Ozone protects us from most of the sun's UV radiation, which can cause cancer, genetic mutations, and sunburn.
     The thinner mesosphere does not absorb solar radiation, so the temperature decreases with altitude. At the mesopause, the temperature begins to increase with altitude, and continues in the thermosphere because it is so close to the sun’s radiation.