Yesterday where I live was one of those rare days in spring that might prompt the comment, “What a perfect day!” The air was fresh, the sky was blue, the temperature rose to about 75 degrees Fahrenheit, and a gentle breeze was blowing. While we all know that weather conditions can change drastically and become harsh, seasons come when most people can enjoy being outside.

Whether we feel thankful for the weather, or just ignore it, it may be of interest to ponder some things that go into providing our weather conditions on Earth. The cast of characters that play a behind-the-scenes role in our weather each have the potential to wreak havoc with the livability of our climate. Seeing how they usually all conspire together for our benefit could be called the perfect day conspiracy.

The Investigation Begins

Our undercover investigation of this conspiracy first takes us a long way from home, on a dive into the very core of our Sun. Conditions there are far from idyllic — for humans, but they’re just right for a sustained thermonuclear fusion reaction that converts hydrogen into helium. At a temperature of 15 million degrees Celsius, and a crushing density of 14 times the density of lead, this solar furnace annihilates four million tons of matter each second to produce what we blithely refer to as “sunshine.” To further add to the exotic origin of our sunlight, about 3.8 percent of the energy from each fusion reaction in the solar core comes from matter-antimatter annihilations between positrons and normal electrons, in a scenario as sci-fi sounding as the iconic warp drive in Star Trek.

The total power output from the Sun’s nuclear furnace is a steady 4×10²⁶ Watts, radiating uniformly into all directions of space. Since the Earth revolves around the Sun in a nearly circular orbit, the heat and light intercepted by the Earth stay constant to within a few percent throughout the year. Due to the large distance from the Sun to the Earth, out of all the power emitted by the Sun, the entire Earth only intercepts the tiniest fraction — less than one part per billion. Yet that fraction (0.45×10^-9 out of all the Sun’s power) is just right to give us seasons when the temperature outdoors is just right.

The Marvel of Rain

But what about those rainy days? Today, just a couple of days after I started writing this article, the skies are grey with clouds and rain is predicted for the next 24 hours. In the Midwest, rainy days and thunderstorms roll through frequently enough that the farm country across several states can predictably grow crops during the summer months without resorting to artificial irrigation. The marvel of regular rainfall hits home if you’ve ever lived in a drier region. When I lived in Southern California, the entire summer often lacked any measurable precipitation. 

We all know that rain clouds consist of condensed water vapor that evaporated primarily from ocean water, although surface water on land also contributes. With 71 percent of our planet covered by oceans, and an average planetary temperature of about 59 degrees Fahrenheit (15 degrees Celsius), plenty of evaporation occurs. The resultant clouds need to hold their water, however, for a journey of hundreds to thousands of miles to fall as rain on the interior of continents. Prevailing winds, powered by the Sun’s energy and the Earth’s rotation, are major factors in bringing rain clouds to regions far removed from any ocean. The water cycle is far more complex than can be described in a few sentences, but it’s all part of giving us a perfect day.

Explaining a Blue Sky

“Why is the sky blue?” is a familiar question, but the answer to how this atmospheric color contributes to our perfect day conspiracy may not be well known. Sunlight is composed of all colors, so when the Sun shines on the atmosphere on a clear day, why does the sky predominantly appear blue? It has to do with sunlight being composed of oscillating electric and magnetic fields that interact with the molecules of air (primarily N₂ and O₂). The electrons in the air molecules are set into oscillating motion by the light and this causes them to reradiate at the same frequency. However, and here’s the blue-sky secret, high frequency light (blue-violet) is reradiated almost ten times more strongly than low frequency visible light (red). When we look at the sky in any direction away from the Sun, we’re only seeing scattered or reradiated light, which favors the high-frequency blue color. 

This pleasant phenomenon of blue-light scattering also depends on the relative size of the air molecules being much smaller than the wavelength of light. When the atmosphere contains clouds or fog composed of water droplets much larger than the wavelength of light, sunlight is scattered without preference for color, causing clouds and fog banks to look white or gray.

A Rosy Glow

At the end of a perfect day, when the Sun is setting, the beams of light we see slant through the atmosphere nearly parallel (tangent) to the Earth’s surface. The light therefore journeys further through the air, so most of the bluish colors are scattered away by the time it reaches our eyes, and guess what? This also provides us with the gorgeous reddish-orange sunset colors that highlight and provide a rosy glow on any clouds lingering near the horizon.

So much beauty from an obscure electromagnetic phenomenon that nobody understood until about 150 years ago! From nuclear fusion in the Sun to Earth’s orbital radius, to atmospheric conditions and the interaction of light with molecules, to the properties of water, and many more details that I had to leave out, it seems like a line-up of more than “the usual suspects” conspired together to bring us a perfect day.