How to judge whether there is sunset glow in a day?

We have developed a WeChat applet that can automatically forecast the sunset: Lijing Weather.

In addition to the forecast of sunrise and sunset, there are also the forecast of clouds and rainbows, astronomy, stargazing, meteorology, satellites and so on.

Everyone must have been screened by the sunset in Beijing on May 22nd 17. Before sunset, at 6: 29 pm on May 22nd, 17, everyone who paid attention to my official WeChat account "Frost toad" should have received the sunset warning of the official WeChat account.

Although the problem continues, at sunset, the sky is full of burning clouds.

Today's sunset began to turn red at 19:30, burned all day after 8 minutes, and began to fade after 10 minutes. If you don't predict in advance, the shooting effect will be greatly reduced.

It is better to teach people to fish than to teach them to fish.

So, what about the sunset forecast?

At the beginning of the year, I sent a tweet "See when the sunset will appear according to satellite pictures". But this push lacks many details of specific methods. Based on the re-observation in recent months, I decided to update this sunset warning push.

Sunset principle:

Lengthy version principle (can be skipped):

We already know that sunlight is a mixture of red, orange, yellow, green, cyan, blue and purple. These colors of light have different wavelengths. Red light wave is the longest, followed by orange light wave, and purple light wave is the shortest. Air molecules and countless tiny dust and water droplets float in the air, which can disperse all kinds of rays from the sun. This is called scattering. The shorter the wavelength of light waves in the sun, the easier it is for light like purple and blue to be scattered; The longer the wavelength, the less likely it is to scatter light such as red and orange. In the morning or evening, the sunlight is oblique, and its distance through the air layer is relatively long, so the scattering is greatly weakened. Purple light is the weakest, and red light or orange light is the weakest. These weakened colored sunlight shines on the sky and clouds, forming bright and dazzling rosy clouds.