Atmosphere and Climate
Earth's Atmosphere
The atmosphere is composed of air, but that doesn't fully explain what it is. Air is not some distinct molecule, but rather a mixture of various gases (whose composition varies). The vast majority of gases in the atmosphere are nitrogen (78%) and oxygen (21%). These gases are extremely important to life on Earth. The remaining 1% is mainly argon (an inert noble gas), and then various other gases in very small amounts. One of the more important ones is carbon dioxide (0.04%), as it has a massive role in heating the atmosphere.
The atmosphere can be divided into distinct layers, primarily based on a temperature gradient.
The layer of the atmosphere where we live is known as the troposphere. Tropo- means "change" and this is the layer where the weather is constantly changing and gases are mixing (particularly when we start talking about air circulation and wind). This is the densest layer of the atmosphere, with most of the gas molecules being found here. As you increase in altitude in the troposphere, the temperature decreases. Think about a mountain - it's quite easy to imagine one where there is snow at the mountain peak, but not at the base of the mountain. It is because of this "environmental lapse rate" or drop in temperature as altitude increases.
The second layer of the atmosphere is the stratosphere. This is the layer where the most ozone is found, and therefore where the ozone layer is. The ozone layer absorbs UV rays, which protects the organisms on Earth from DNA damage. Due to the absorption of this UV radiation, the stratosphere is heated, resulting in temperatures increasing in this layer.
The third layer, or mesosphere, switches and once again has temperatures decreasing as altitude increases. The mesosphere is the least studied, as it is too high for weather balloons, but too low for satellites. This is where most meteors vaporize, and the top of this layer (in the mesopause) is where the coldest part of the atmosphere is found.
The fourth layer is the thermosphere, the hottest layer of the atmosphere. The temperature increases to such an extent because of the oxygen and nitrogen atoms absorbing short-wave, high-energy X-ray and UV radiation from the sun. This results in gas molecules moving extremely quickly (which is how we measure temperature), but because the density of the air is so low here, it wouldn't actually feel hot like what we are used to down in the troposphere. Aurora occur here, as charged molecules from space collide with the gas molecules of the troposphere, exciting them, and resulting in the release of the excess energy as light.
The final layer, the exosphere, is where the atmosphere merges with space.
Solar Insolation
Solar insolation measures the amount of solar radiation reaching a particular area. The angle of the sun's rays determines the intensity of this insolation. The angle with which solar radiation strikes Earth has a strong impact on the intensity of insolation and the temperature of an area. The reason that the equator is the hottest part of Earth is that it receives the most solar radiation (per unit area) due to arriving at a perpendicular angle.
Another factor that impacts the temperature is the albedo of a surface, which refers to how much light is reflected by it. Surfaces with high albedo reflect a lot of light, resulting in lower temperatures. Imagine it's the middle of summer and you're about to go outside - would you rather wear a white or black shirt? It's that exact concept. Areas with higher albedos, like polar regions that are very light in color due to all the snow, reflect more light, meaning they absorb less light and therefore absorb less heat. On the other hand, areas with a lower albedo reflect less and absorb more, meaning they absorb more heat and warm up faster.
Seasons
When you think about the changing of seasons, probably the biggest different you think of is in temperature. Winter is colder than summer. The reason for this is the amount of solar insolation varies due to the tilt of the Earth. As Earth revolves around the Sun, its orientation to it changes. Periods of time with greater insolation have longer days, which results in warmer weather with the seasons. The summer has the longest days (and therefore the greatest amount of solar insolation) and the winter has the shortest.
During a solstice, either the Northern or Southern Hemisphere is tilted toward the Sun at its maximum tilt. During an equinox, the Northern and Southern Hemispheres are equally facing the Sun.
Geography and Climate
Climate is affected by Earth's geography, as well. The location of geologic features such as mountains and bodies of water can have massive impacts on weather and overall climate.
One example of this is known as the rainshadow effect. As warm, moist air moves toward a mountain, the air begins to move up the mountain. As it does so, the air cools, causing the water vapor to condense into precipitation. This results in dry, cool air falling on the other side of the mountain. It is fairly common to see areas of lush, green vegetation on one side of a mountain, with dry, desert-like conditions on the other side.
Oceans can have a powerful effect on climate as well. In one way, they work to moisten the air and help regulate the air temperature. If you've ever been to a beach, you've probably felt the "sea breeze" - the cooling air coming from above the ocean. As water has a high specific heat and requires a lot of energy to change its temperature, coastal areas are often more temperate.
Ocean currents can have a dramatic effect on climates in other ways, as well. The movement of warm or cold water toward a landmass via ocean currents can greatly impact the local climate. As these currents change, so too will the climates, as is best demonstrated by El Niño and La Niña.