Everybody knows what to expect during each of the various seasons throughout the year. But why do we even have season? What causes us to freeze during one portion of the year but swelter during another? It actually has a lot more to do with astronomy. The fact that our planet tilts toward with the Sun has quite a profound effect on our weather and, in turn, how we live our lives.
Let's tackle the ultimate question: Why does the Earth's tilt towards the sun lead to seasons? The answer lies in some basic physics and astronomy (sounds a lot scarier than it actually is). Essentially, as the Earth makes its yearly revolution around the Sun, the Earth's 23.5° tilt causes different areas of the planet to either lean toward or away from the Sun at different times of the year. Thus, when one portion of the Earth leans towards the sun, it receives the most amount of solar radiation and experiences summer. Conversely, another part of the Earth during this same time leans away from the Sun, receives the least amount of solar radiation and thus experiences winter. The diagram below visually depicts this journey. For us in the Northern Hemisphere, we lean towards the sun from June to August and then away from it from December to February. The Southern Hemisphere experiences the exact opposite cycle. As the Northern Hemisphere transitions from leaning towards the sun to away from it or vise versa, we experience a "transition" season which we call spring and fall.
It's interesting to note that the tropics are not nearly as affected by the seasons as folks in the mid-latitudes. Areas along the equator such as the Caribbean are typically leaning towards the sun no matter the time of year. In addition, the Earth's tilt also explains why the poles experience nearly 24 hours of sunlight during the summer and then nearly 24 hour of darkness during the winter.
Above: Image of how we get seasons based on Earth's tilt and path around the Sun (Image courtest of NASA)
Another question you may be asking yourself is why don't we have the hottest temperatures on the first day of summer and the coldest temperatures on the first day of winter? This has to do with the ratio of water to land. Water takes a longer time to heat up than land (ever notice how the ocean is warmer in September than June?) Therefore, the fact that water makes up 71% of the Earth delays our warmest and coldest temperatures past the first day of the season.
Above: Philadelphia's warmest temperatures occur during July and August, after the summer solstice (Image courtesy of Wikipedia)