Describe the composition of the air
Air is a mixture of gases and aerosols that compose the atmosphere surrounding Earth. The primary gases of air include nitrogen (78%) and oxygen (21%). Trace gases and aerosols make up the remaining 1% of air. The trace gases include the noble gases argon, neon, helium, krypton and xenon, hydrogen and the greenhouse gases. The aerosols are solid or liquid particles having diameters in the region of 0.001 to 10 microns, and include dust, soot, sea salt crystals, spores, bacteria, viruses and a plethora of other microscopic particles, which may be natural or man-made.
Why are trace gases important?
The most important gases in the Earth’s atmosphere are the greenhouse gases. Most abundant in the troposphere, these gases include carbon dioxide, methane, nitrous oxide, water vapour and ozone, so called because they are involved in the Earth’s natural greenhouse effect. This keeps the planet warmer than it would be without an atmosphere. Apart from water, the most abundant greenhouse gas, by volume, is carbon dioxide. Carbon dioxide gases and other greenhouse gases help to keep the Earth 33 degrees Celsius warmer that it would otherwise be without an atmosphere.
How does air pressure change with altitude?
1. The gravitational attraction(*) between the earth and air molecules is greater for those molecules nearer to earth than those further away - they have more weight - dragging them closer together and increasing the pressure (force per unit area) between them. 2. Molecules further away from the earth have less weight (because gravitational attraction is less) but they are also 'standing' on the molecules below them, causing compression. Those lower down have to support more molecules above them and are further compressed (pressurized) in the process.
Structure of the atmosphere from base to top
The Atmosphere is divided into layers according to major changes in temperature. Gravity pushes the layers of air down on the earth's surface. This push is called air pressure. 99% of the total mass of the atmosphere is below 32 kilometers. Troposphere - 0 to 12 km - Contains 75% of the gases in the atmosphere. This is where you live and where weather occurs. As height increases, temperature decreases. The temperature drops about 6.5 degrees Celsius for every kilometer above the earth's surface. Tropopause - located at the top of the troposhere. The temperature remains fairly constant here. This layer separates the troposphere from the stratosphere. We find the jet stream here. These are very strong winds that blow eastward.
Stratosphere - 12 to 50 km - in the lower part of the stratosphere. The temperature remains fairly constant (-60 degrees Celsius). This layer contains the ozone layer. Ozone acts as a shield for in the earth's surface. It absorbs ultraviolet radiation from the sun. This causes a temperature increase in the upper part of the layer.
Mesophere - 50 to 80 km - in the lower part of the stratosphere. The temperature drops in this layer to about -100 degrees Celsius. This is the coldest region of the atmosphere. This layer protects the earth from meteoroids. They burn up in this area.
Thermosphere - 80 km and up - The air is very thin. Thermosphere means "heat sphere". The temperature is very high in this layer because ultraviolet radiation is turned into heat. Temperatures often reach 2000 degrees Celsius or more. This layer contains: Ionosphere - This is the lower part of the thermosphere. It extends from about 80 to 550 km. Gas particles absorb ultraviolet and X-ray radiation from the sun. The particles of gas become electrically charged (ions). Radio waves are bounced off the ions and reflect waves back to earth. This generally helps radio communication. However, solar flares can increase the number of ions and can interfere with the transmission of some radio waves. Exosphere - the upper part of the thermosphere. It extends from about 550 km for thousands of kilometers. Air is very thin here. This is the area where satellites orbit the earth.
Magnetosphere - the area around the earth that extends beyond the atmosphere. The earth's magnetic field operates here. It begins at about 1000 km. It is made up of positively charged protons and negatively charged electrons. This traps the particles that are given off by the sun. They are concentrated into belts or layers called the Van Allen radiation belts. The Van Allen belts trap deadly radiation. When large amounts are given off during a solar flare, the particles collide with each other causing the aurora borealis or the northern lights.
What characteristic defines the boundary between layers?
Between each layer of the atmosphere is a boundary. Above the troposphere is the tropopause; above the stratosphere is the stratopause; above the mesosphere is the mesopause; and above the thermosphere is the thermopause. At these "pauses," maximum change between the "spheres" occur. In addition to temperature, other criteria can be used to define different layers in the atmosphere. The ionosphere, for example, which occupies the same region of the atmosphere as the thermosphere, is defined by the presence of ions, a physico-chemical criterion. The region beyond the ionosphere is known as the exosphere. The ionosphere and the exosphere together make up the upper atmosphere (or thermosphere). The magnetosphere is the region above the Earth's surface in which charged particles are affected by the Earth's magnetic field.