We have talked about all of weather and climate originating because of the Earth's tilt on its axis in relation to the sun allowing for an uneven heating of the Earth's surface, resulting in weather developing to try to re-balance that temperature. Because of the Earth's constant axis of rotation, the Sun's relatively constant emission of radiation, and the Atmosphere's relatively unchanging properties, Meteorologists have been able to nail down a pattern to the temperatures and weather we experience on a yearly basis. There are times when one part of the Earth is receiving heat from the sun more directly than others, and some missed completely. During the course of the year, this changes, and eventually, most of the Earth experiences similar amounts of incoming solar radiation, or "Insolation", as part of the Earth that was tilted away from the sun at one time in a year, is now tilted towards the sun in another.
    In the middle latitudes, our year ("year" being marked as one full revolution of the Earth around the Sun) is divided into four seasons, each denoting the stage that location is in its travel around the Sun and the amount of insolation it's receiving.
Of the four seasons, the two that are most important are Summer and Winter, even though perhaps most of the turbulent weather we experience happens in the Fall and Spring. Summer is the time of year when a certain region is experiencing the most amount of solar radiation in the yearly cycle. For example, June in the northern hemisphere, and December in the southern hemisphere. Conversely, winter is the time of year when a position on the Earth is receiving the least amount of insolation. This would be December in the northern hemisphere, and June in the southern. The type of weather and temperatures we would normally experience is often directly related to the amount of incoming solar radiation we receive from the Sun. Fall and Spring are seasons typically experienced in the middle latitudes, where the fluctuations in insolation is the greatest. Spring represents the changing insolation on the way to a leveling off in Summer, but a general warming trend. Likewise, Fall is the time of year when a location is leaving it's Summer warmth and heading towards Winter. Since Spring and Fall are periods of dramatic change in how much direct solar energy places in the middle latitudes receive, there are often wide temperature swings on the Earth's surface, and often large storms that form to compensate for it.
    There are some locations that don't experience that wide variation of insolation. They would be located near the equator and in the arctic. Though the Earth spins on its axis in relation to the Sun, points near the equator always manage to get a similar share of insolation, and thus stay relatively warm year round. Equatorial countries don't usually experience wide temperature swings, either hot or cold, during the year. Likewise arctic locations are often constantly on the outskirts of the Sun's heat, and therefore are on the cold side all year. Since the Earth's axis is a 23.5º tilt, the limits of direct sunlight on the surface of the Earth are bound by the same angle. For example, if you were on the Equator, you would experience the sun going directly overhead at noon twice, once while everyone in the mid latitudes mark the start of Fall and once at the start of Spring. If you were to trace the path of the sun's direct-most rays on the surface of the Earth during the course of the year, it would go up slowly north from the Equator until the summer solstice, or June 22, where it reaches a maximum latitude of ... you guessed it, 23.5ºN. The Winter is the same way, where December 22 marks the date when the sun shines directly overhead at 23.5ºS. These lines of latitude mark the "Tropics", between which the sun shines most direct for most of the time. There are other lines of latitude that mark important relationships between the Earth and the Sun. These are the "Arctic Circles". They are at 66.5ºN and 66.5ºS latitude. They show the line above which in the Winter, the sun never makes it to brighten the day...at all. Conversely in the summer, the sun never leaves, and it's known as the "Land of the Midnight Sun". The arctic is obviously known for its colder weather and minimal dosage of insolation.
    Though most of the Earth experiences four different seasons, others do not. Places near the equator and poles don't get the wide variation in incoming solar radiation, so their seasons are not driven by changes in temperature. Rather, there are other forces at work near their location that govern the seasons. The Earth itself is a radiating heat body, with the ability to influence the temperature of land and oceans, in addition to the Sun. Large oceans do not change their temperature rapidly, regardless of the amount of solar radiation upon it. There are currents, however, within the oceans that travel and transport water of different temperatures around to help even out the overall oceanic temperature. Wherever currents and oceanic temperatures collide, the result is usually some sort of turbulence, and weather. Likewise the winds do similar things to the Earth's atmosphere. Seeing as the Earth makes a slow revolution around the Sun, interaction at these convergence zones can linger for a long time, then this zone would be moved to a different part of the Earth, leaving the original area quite tranquil. This is why some locations go by the "Rainy Season" and the "Dry Season". If this oceanic and atmospheric interaction is dominant enough, it can actually assert itself more prominently than the sunshine, resulting in localized areas only considering two seasons instead of four: a rainy, or "Monsoon" season, and a dry season. Two season regions are easy to spot, because they have several month periods where they can receive very little rain, and then weeks on end that it would rain at least a little bit each day.

Seasonal Variations

    Season types are not restricted to just latitude and angle of the Earth to the Sun. Though that is the main ingredient, other things can factor in to a location's seasons. For example, agricultural interests would talk of a 'growing season' to refer to a time from late spring to early fall that is perfect for growing and harvesting their crops. Depending upon the location, even within a small area, those 'seasons' have different beginnings and endings. Sides of a mountain that face the sun receive much more sunshine than the areas the mountain is shading behind it, thus it warms up a bit earlier over there. Likewise, snow usually sticks around a bit longer on the north face of a mountain, because of the more limited amount of sunshine hitting it, thus whenever possible, ski resorts are constructed purposefully on the north face to maximize any extra cooling and snowpack. On a larger scale, locations inside a city will receive warmer temperatures on average than a rural town because of all the increased urban activity taking place. Stepping out a bit further, locations near the shoreline will typically see cooler temperatures than inland locations in the Summer, and warmer temperatures in the Winter because of their proximity to the ocean, and the ocean's more stable dispersion of heat across it's large surface. Other things that affect the seasons on a grand scale include temperature variations of specific large region of ocean, the effects of a volcanic eruption, and even the subtle cycles of solar heat output, the Sun's tiny revolution in space, and even the 11,000 year cycle of the Earth's axis rotation, creating a cone-like path above the Earth. Many of these large scale variations in the way we experience seasons on the Earth develop over many lifetimes, yet can be responsible for drastic changes in seasons and climates, such as Ice Ages and the  like.
    A final element of seasonal variation that can have a noticeable impact on a local scale, but lesser degrees over time and distance is the issue of human activity. In the mid-1800s, all the way into the 20th century, many of the steel mills and various signs of the Industrial Age were unable to transport the smoke from their factories into the upper atmosphere to be dispersed, and the result was days without visible sunshine, cooler temperatures, and lots of sick people. Over time and years, the pH of rainfall can even be affected, with the Ohio Valley receiving the rainfall with the lowest pH (thus is more acidic). Also, in what is commonly referred to as the "Greenhouse Effect", carbon-based chemicals released by factories, cars, and other human activity is working with the natural existing carbon in the atmosphere to add a little extra help in keeping some of that warm sunshine heating the Earth's surface. Again, many of these effects are most visible on a localized level, but the Earth's atmosphere has dealt with a lot in its long history. Trying to mitigate these human influencers does come with a price, as it is quite costly to do, and perhaps unfeasible for a developing nation, as every nation that has achieved 'First World' status has an Industrialized period in its history whereby nothing was spared, least of all human health and environment, to bring that country into the modern age.