Transformation of the atmosphere by ancient algae: The earliest living things on earth were probably primitive microscopic plants that lived in the oceans, such as algae and bacteria. Using a process called photosynthesis, algae make their own food supply (sugar) by using the energy from sunlight, carbon dioxide and nutrient elements dissolved in sea water, and give off free oxygen as a waste product. At some time about 1.8 billion years ago, the algae multiplied in great numbers in the oceans, giving off so much excess oxygen that it could not longer be contained in the oceans, and it escaped into the atmosphere. The geological evidence of this formation of free oxygen are the red beds, widespread deposits of reddish (rusted) iron minerals found in sediments eroded from rocks on land. Because free oxygen is chemically active, it converted the original gases in the atmosphere as follows:
 

Original Atmosphere	Chemical Changes	Present Atmosphere
methane (CH4)	CH4 + O2 = CO2 + H2O	carbon dioxide + water vapor
ammonia (NH3)	NH3 + O2 = N2 + H2O	nitrogen + water vapor
carbon dioxide (CO2)	CO2 --> CaCO3 (calcite) (biological processes; limestone formation)	CO2 is far less abundant today due to presence of limestones
water vapor (H20)	no change	no change

Formation of the Ozone Layer: One of the by-products of the excess oxygen in the newly transformed atmosphere was the formation of ozone (O₃), which is made up of a trio of oxygen atoms (O₃). In the upper atmosphere, ultraviolet rays in sunlight break apart some of the "normal" oxygen molecules (O₂) into chemically active, single oxygen atoms (O). These single oxygen atoms combine with O₂ to form ozone, O₃:
 

The ozone layer in the upper atmosphere acts as a shield which filters out much of the deadly ultraviolet rays from sunlight, making the Earth's surface safe for living things. Before the ozone layer was formed, the only safe environment for living things was the ocean, whose waters also act as a filter for ultraviolet rays. So, life on land could not have evolved until the ozone layer was formed.

Why the ozone layer is threatened today: For many decades, the use of halogens (which includes gaseous elements such as chlorine and fluorine) in compounds such as Freon, which are used as refrigerants and spray can propellants has caused these gases to drift into the upper atmosphere. Chlorine and fluorine are more chemically active than even single oxygen atoms, so these halogens cause the ozone (O₃) to decompose into oxygen. When the ozone layer is depleted, more ultraviolet rays from sunlight are able to penetrate to the surface, causing higher rates of skin cancer. Chemical companies plan to phase out the use of these ozone-depleting chemicals and substitute more environmentally-safe compounds.

In order to understand how the atmosphere and weather works, it may help to go over some basic physics and relate them to your everyday experiences and observations.

States of Matter: All matter is composed of atoms - commonly, these atoms are arranged into larger units called molecules. Chemical bonds between the atoms cause them to organize into different states of matter - solids, liquids, and gases. In solids, the bonds are fairly strong, and the atoms take on a definite shape. In liquids, the bonds are less strong, and the atoms take on an indefinite shape that can change and flow, but still remain together. In gases, the bonds between atoms are weak, and individual atoms or molecules are free to move about, such that gases occupy whatever space they are allowed to move into.

Kinetic Theory of Motion: All molecules are vibrating in constant motion. (Kinetic means "motion.") The common model used depicts molecules as tiny, microscopic spheres. Among the three common states of matter, molecules are vibrating fastest in gases, more slowly in liquids, and slowest in solids.

Heat & Temperature:Heat is the energy of motion in vibrating molecules. When we say something is "hot," we mean to say that the molecules are vibrating very fast. Temperature is a measure of the average amount of heat in a given object. All molecules vibrate, even in the extreme cold temperatures of outer space (below -450^oF.). Theoretically, the temperature at which molecules finally stop vibrating is called absolute zero, which is -459^oF.

How Heat Flows: Heat flows in just one way - "from a hotter place to a colder place." What this means is that molecules which are vibrating faster in one region (hotter) will transfer some of their vibrational energy to slower molecules (colder) until all the molecules are vibrating at about the same rate. In everyday experience, this explains why your hot meal gets "cold" if you leave it untouched for a while on the dining room table - the hot food simply transfers its heat to the surrounding air until everything is at room temperature.

Latent Heat and Changes of State: Whenever water changes state, from solid to liquid to gas, and vice versa, a large amount of heat energy (known as latent heat) is needed to break or make the bonds necessary to accomplish the change. This explains why water makes a good coolant.

Evaporation: To turn liquid water into water vapor (gas), you must add heat energy to cause the water molecules to vibrate sufficiently fast enough to become gas molecules; the heat may be absorbed from the surrounding air or any other object in contact with the water. This explains why you feel cooler when your sweat (mostly water) evaporates.

Melting of Ice: To change from a solid to a liquid, ice must absorb heat energy from the surrounding environment in order to break the chemical bonds which make up the solid, and thereby becomes liquid water. We know that is so, because when we put ice cubes into a warm drink, the drink gets cold (ice absorbs heat from the drink - in doing so, the ice melts).

Freezing of Water: When water freezes, heat energy must be removed from the water in order for the molecules to slow down sufficiently to assume their place in a solid form.

Pressure is a measure of the collective force exerted by vibrating molecules. In gases, pressure is the result of molecules which collide with each other. There are several ways to increase gas pressure in a localized system; all of these cause more collisions between gas molecules:

1. Increase the rate of vibration in the gas molecules (add heat).

2. Decrease the space (volume) of the system.

3. Add more gas molecules.

You can apply the example of an air balloon to the above. If you heat up the air inside a balloon, the air molecules will vibrate faster and collide more often - the pressure will increase, and the balloon will expand. If you squeeze the balloon, you will feel the pressure of the air increase as it occupies a smaller space. If you add more air to the balloon, the balloon will also expand due to greater air pressure.

So, the pressure of the surrounding air in the environment is called the atmospheric pressure. At higher altitudes, the atmospheric pressure is lower, because the gas molecules are spread further apart from each other and are likely to collide less often. At sea level, the amount of atmospheric pressure is standardized as 1 atmosphere, 1.013 bars, or 1,013 millibars.  Below sea level, the density of the atmosphere is greater, therefore, the atmospheric pressure is greater than 1 atmosphere. In everyday life, the variations of atmospheric pressure associated with weather changes are very slight, so a unit that is 1/1000 of a bar, called the millibar is used. Generally, bad weather is associated with low atmospheric pressure, while good weather is associated with high atmosheric pressure.  Atmospheric pressure for most weather conditions is generally within 50 millibars above or below 1,013 millibars. 

Why do your ears pop in airplanes and elevators?  When you move in a high speed elevator or take off in an airplane, your body attempts to adjust to lower atmospheric pressures that are present at higher altitudes. This causes your eardrums to "pop" outward because your internal air pressure inside your ears is relatively higher. If you took a high speed elevator down into a deep mine, your ears would also pop, but in this case, because you are moving down into an area of higher atmospheric pressure, your eardrums will "pop" inward.

Should you leave your windows open during a tornado? A tornado is an extremely violent storm that acts as a giant vacuum cleaner - it creates an extremely low atmospheric pressure in its path which causes houses to explode outward. Opening the windows was formerly thought to allow some of the internal "normal" air pressure to escape and prevent the house from exploding. This idea is now outdated - a tornado simply blows down houses, whether the windows are open or not.  Keeping the windows closed is advised, because doing so at least provides a barrier to deadly flying debris from a tornado. 

Humidity is a measure of the amount of moisture (water vapor) in the air. Measuring the absolute amount of humidity in the air is rather complicated (and may not be all that useful). However, it is important to note that relative humidity can easily be measured, using a hygrometer, or a sling psychrometer.

Saturation Point: Air has a certain capacity to hold water vapor, but this capacity depends on temperature; warm air holds more moisture than does cool air. Everyday experience tells us that during warm weather, the air in Chicago tends to feel "humid," "sticky" or "muggy," while during the winter months tend to feel "dry" (especially indoors). The maximum amount of water vapor that can be held in the air is called the saturation point. Any attempt to introduce more water vapor causes precipitation, the formation of liquid water. We can see this happening when we take a hot shower and close the bathroom door. The hot water from the shower introduces water vapor into a confined space; the saturation point of the bathroom air is reached fairly quickly - precipitation causes small droplets of liquid water ("fog") to form on cooler surfaces, such as the walls and mirror. Dew is precipitation of water vapor caused by the lowering of the saturation point during the previous night, when the air was cold; the temperature at which this occurs is known as the dew point.

Relative Humidity: Relative humidity is measured against the saturation point of the air at a given temperature. At the saturation point, the relative humidity is 100 percent. (During rainfall, snowfall, or fog, the relative humidity is also 100 percent.) Dew is seen on the grass in the morning following a cold night because the relative humidity reached 100% when the temperature dropped to a certain point.

In the bar chart below, a given amount of water vapor is represented by a bar; below this, the other bars represent the saturation points at three different temperatures. If you compare the sizes of each bar below, you can readily see that if the amount of water vapor does not change, the relative humidity percent goes down as temperature goes up:

Another analogy you can use is the size of a "jug" which represents the capacity to hold water vapor.  Increasing the temperature "increases the size of the jug."

Effect of Relative Humidity on Evaporation: When the relative humidity in the air is high, evaporation of water is slowed down, because the "air is already occupied" by water vapor. You feel sticky during humid summer weather because your sweat is unable to evaporate quickly. Only when the sweat evaporates does your skin feel cool. An electric fan does not cool the air - all it does is to speed up the evaporation of sweat from your skin; on a humid day, there is nowhere for the sweat to go. During the winter months, the cold air cannot hold much moisture, so the air feels dry. Indoors, to save heat, doors and windows are kept shut; this has the effect of drying out the air. Moisture from all kinds of places, from your wood furniture to the aquarium to your skin, will evaporate and make you feel uncomfortable unless you compensate by adding moisture to the air with a humidifier.