The Science Of Sunrises

Why does the sun rise in the east? For some people this may be the “duh” question of the article but perhaps not. I think people clearly know that the sun rises in the east but may not actually think about why. It is simply related to the rotation of the Earth. The Earth is spinning on its axis, and it spins toward the east. At the equator, the earth is spinning at a rate of roughly 1037 mph. Because, the Earth’s equatorial bulging, it is really more of an oblate spheriod than a sphere. This also affects the rotation rate, which decreases by the cosine of latitude. For example, if you live at a latitude of 45 degrees, the cosine of that value is .707. If you multiply .707 x 1037 mph, someone at a latitude of 45 degrees is spinning at 733 mph.

Changes in Earth's spin axis.

Why does a sunrise sky have its orange-reddish color? To answer this question, we have to define “scattering.” Small particles, droplets or molecules in our atmosphere can cause light to change directions. This is called scattering. There are different types of scattering including Mie, Rayleigh, Tyndall, Brillouin, and Raman. The two most common to meteorological processes are Rayleigh (scattering particle is small relative to the wavelength of light) and Mie (scattering spherical particle is about the same size as the wavelenth) scattering.

When the sun is low on the horizon during sunrises and sunsets, the sunlight travels through more of the atmosphere. Shorter wavelength colors (blues and violets) get scattered out. This leaves more of the longer wavelength colors like yellow, orange, and red. This is why sunrises often take on such colors. When the sun is higher in the sky, smaller particles Rayleigh scatter much of the blue wavelength, which is why the sky appears blue. The human eye is more sensitive to blue than violet otherwise our characteristic blue sky might be violet.

Sun at low horizon. Source: S. Corfidi and NOAA.

Doesn’t dust or pollution brighten the colors? The notion that dust and air pollution brighten sunrises and sunsets is a myth. In fact, they have the opposite effect. As Dr. Stephen Corfidi writes in an excellent essay on this topic,

Typical pollution droplets such as those found in urban smog or summertime haze are on the order of .5 to 1 um in diameter. Particles this large are not good Rayleigh scatterers as they are comparable in size to the wavelength of visible light (Mie scatterers if spherical). If the particles are of uniform size, they might impart a reddish or bluish cast to the sky, or result in an odd-colored sun or moon (it is this effect that accounts for the infrequent observation of “blue suns” or “blue moons” near erupting volcanoes). Because pollution aerosols normally exist in a wide range of sizes, however, the overall scattering they produce is not strongly wavelength-dependent. As a result, hazy daytime skies, instead of being bright blue, appear grayish or even white. Similarly, the vibrant oranges and reds of “clean” sunsets give way to pale yellows and pinks when dust and haze fill the air.

Corfidi also points out that pollutants also enhance light modification at low sun angles. This causes a reduction in the total light reaching our eyes and the overall brilliance of the sunrise.

Do clouds hurt sunrise or sunset views? According to Corfidi’s essay, some clouds are preferable for the best views.

Clouds catch the last red-orange rays of the setting sun and the first light of the dawn like a theatre screen, and reflect this light to the ground.

The best sunrises and sunsets seem to be associated with middle to high clouds rather than lower level clouds. Higher clouds receive sunlight that has not been altered by attenuation at lower levels. If you have a satellite dish for TV service as I do, attenuation is what happens when you lose the signal during rain.