Our coldest months are December, January, and February in North America. Someone recently asked me how it can be colder in winter even though Earth is closer to the Sun than it is in summer.
Although I was taught that Earth’s distance from the Sun averages out to be about 93 million miles, in reality this distance varies from 91.4 million miles in early January to 94.5 million miles in July—so Earth is indeed closer to the Sun during our winter.
But when you think about it, Earth’s distance from the Sun can not be seen to be the reason for the seasons, as when it is winter for me in Pennsylvania, it is summer for my friend in Australia. As we all know, seasons in the Southern Hemisphere are the opposite of those in the Northern Hemisphere.
Instead, the answer lies in the 23.5-degree tilt of Earth’s orbit as it revolves around the Sun (see below).
You can see how this works by using a flashlight. If you point the flashlight straight ahead, you will see a circle of light, with the brightest light in the center.
If you tilt the flashlight, its light is more diffuse and weaker away from where it is shining directly.
The weaker light in the case of the Sun and our solar system means that Earth is receiving less energy from the Sun, which is why the weather is colder and the daylight hours are shorter in winter.
Since the amount of sunlight is at its minimum at the winter solstice, which occurs around December 22, you might expect that day to be the coldest of the year, on average. But instead, the coldest time of year in the Northern Hemisphere occurs in February, nearly 2 months later.
The reason this happens is that while Earth receives energy from the Sun, it also emits energy into space. And Earth’s Northern Hemisphere emits more energy than it receives for nearly 2 months after the winter solstice, which means that temperatures continue to trend downward. Think about what happens when you turn off the burner on an electric stove—it does not cool down to room temperature instantly, but instead remains hot for a while, until it finally reaches room temperature.
In a similar manner, Earth still retains warmth from the previous summer, and it takes more time to fully cool down after sunlight reaches its seasonal minimum.
Because land and water have different specific heat capacities, they warm and cool at different rates. Thus oceans and other large bodies of water heat and cool more slowly than the land surrounding them. This is why a sea breeze from the ocean brings cooling in the summertime and why the Great Lakes are able to produce snow squalls in the winter.
So, the seasons are not produced because of changes in how far Earth is from the Sun, but instead because Earth is tilted in its orbit. And there is a lag between when incoming solar energy reaches its high and low points and when we record the warmest and coldest temperatures because Earth not only receives energy from the Sun but also emits energy into space.