What makes a sunset burn?

Let's talk about something close to our hearts: what makes a sky burn, anyway?

You might have heard that sunrises/sunsets are likely to burn when there is a "gap on the horizon", or when the sky is brighter toward the horizon. This is often correct, but why? And how come some skies blow up even though the horizon was solid grey just before sunset? And how come a big fog bank offshore doesn't block a burn?

When you see a colorful sunset, what you are looking at is sunlight hitting the clouds from below.

(Note that the sun has already set where you are -- that's why most good colors are afterburns, as the clouds are lit from the bottom.)

As you know, even on a clear sky day, during golden hour, the sunlight shifts toward yellow, then orange, then a deep red glow after sunset. We'll talk about why that happens another time, but for now, you can realize that clouds turn those same colors as they reflect yellow, then orange, then pink, then red sunlight to you as they are lit from below.

It follows, then, that, the two basic elements of having a colorful sunset are:

(1) having clouds where you are, and

(2) having an unobstructed path for the sun to light the clouds from the bottom.

For example: 

This sunset will not burn, because the light is unable to reach the clouds over your head from beneath. You'll see 100% cloud cover, but it will fade to grey. 

This sunset will burn, because the light has a clear path to reach the clouds over your head from beneath. You'll see 100% cloud cover, but it will burn. Now, imagine you're the viewer, and you're looking at the sunset to the left in the diagram. In this case, you'll see a "gap" of open sky on the horizon. In reality, this "gap" might be hundreds of miles of open sky, but you'll see the same little gap. 

On a satellite image, the above scenario might look something like this when it's setting up:

Don't worry, we'll talk more later about how to read a satellite image. For now, note that there are plenty of clouds overhead, but a large area of clear sky out to sea. As the afternoon progressed, the clouds moved a bit further to the right, and the open sky to the left created a visible gap on the horizon in an otherwise grey sky. This is how that one ended (look closely at the horizon and you will indeed see the open sky beyond the clouds). 

As the sun gets lower below the horizon and continues to light up the clouds from beneath, the color of the light on the clouds changes, just as the color would change if the sky were blue bird. Remember, it's the same light, but now you're seeing it reflect off the bottoms of the clouds. 

Assuming the light encounters no other obstructions (like blocking clouds), the main show is over when the sun is far enough below the horizon such that the light can no longer reach the bottoms of the clouds, because it can't make it around the curvature of the Earth. Otherwise, the show is over as soon as the light encounters other obstructions, like blocking clouds, tall mountains, or thick haze. Also note that there's no such thing as too many clouds, or clouds too thick over your head -- it's the path of the light to those clouds that matters.

This explains why:

* some burns are shorter than others (the light is obstructed for part of the possible burn time)

* some burns end early after they go yellow or orange (the light is obstructed for the later part of the burn time)

* some burns won't start until late but will be a dazzling pink or red (the light is obstructed for the earlier part of the burn time)

* an offshore fog bank does not block a burn, as long as the fog is not covering you. A few things to note here. First, if you put yourself in the place of the stick figure below, looking at the sunset, you'll see the big fog bank blocks your horizon. That means no sunstar at sunset. But the high clouds are so much higher than the fog that the fog hardly affects the burn at all. (In fact, it affects the burn in the same way that a small mountain would: it'll just end the burn a little early, because we've now introduced a small obstruction on the Earth's surface, which the sun won't care about until it is way below the horizon, already into the red stage of the burn.)

It also helps us explain a lot of other things, like crepuscular rays, why high clouds often disappear just before sunset, and many more -- we'll discuss those in the coming posts. 

Some clever folks may also be supposing the cloud mass I drew in the above diagrams was higher or lower than it is, and noticing that there must be some kind of relationship between how far away the light will come to light up the clouds, and the height of the clouds. There absolutely is, and we'll give you a hint: low clouds tend to burn less, and for shorter periods of time, and wispy high clouds are often able to burn for 45 minutes or even longer. We'll be exploring this in more detail later. 

Feel free to ask us questions or discuss in the groups!

- Jeff



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