Notes: Mon 23 Sept

Why is the sky blue?

To begin with, the peak of the incoming energy spectrum from the sun is in the blue light wavelength. There's more of it coming in than any other wavelength.

All colors we see are wavelengths of light scattered back into our eyes. If the sky is blue that means something is taking the blue wavelength component of light and scattering it.

In this case, it is nitrogen that absorbs and scatters the blue light.

In general, this absorption and scattering depends on the medium that light travels through. Absorption is wavelength-dependent, meaning some particles, such as nitrogen, will only respond to certain wavelengths of light. As another example, ozone responds to a wavelength of 300 nanometers, which is UV radiation (while blue is 399.5 nm).

This wavelength dependence follows Rayleigh laws. General rule of thumb:

small particles (like our nitrogen or ozone) are wavelength dependent.
large particles (like water molecules or carbon dioxide) are wavelength independent, meaning they scatter and absorb all (or many) wavelengths.

Why are sunsets red?

Fact 1: Nitrogen scatters blue light.

Fact 2: At sunset, the sun is lower on the horizon. This means the sun's rays have more atmosphere to go through before they hit the ground (compared to when the sun is directly overhead).

Conclusion: If the sun's rays have to travel through more atmosphere during sunset, that means that there are more chances for the blue wavelengths to be scattered away by the nitrogen in the atmosphere. What's left over are the longer wavelengths: the beautiful reds, oranges, and yellows.

Furthermore, dust in the atmosphere enhances scattering of the blue light, causing "prettier" sunsets. Dust is always around but can be increased by volcanic eruptions and dust storms.

Why are clouds white?

Fact 1: The "color" white is actually a mix of all wavelengths of light.

Fact 2: Clouds are very reflective. The scatter light away and have a high albedo.

Conclusion: Clouds look white because they scatter all wavelengths of light, and all wavelengths of light put together is the "color" white.

Remember that they don't always look white...for instance, during a pretty sunset, the clouds take on the color of the sky. This is because they are highly reflective and will scatter the reds, oranges, and yellows that we just discussed.

Ozone Depletion

Introduction: We will address:
1. What is it?
2. Taking a systems approach to the problem, that is, looking at the problem in the hydro-, bio-, litho-, and atmospheres.
3. Geochemical cycles that ozone depleting materials are in.

[most of the information is in the handout. just a few points of the ozone depletion process will be re-stressed here]

Where does ozone form?
Mostly at low latitudes. The formation process requires sunlight and oxygen in the stratosphere.

Where is ozone depleted?
Mostly at high latitudes, especially the South Pole, not so much the North Pole. It is present at all latitudes, though, including midlatitudes.

Why is it worse over Antarctica and why is it worse in September?
Antarctica has a vortex over it during the cold winter months. In this cold vortex, clouds called "polar stratospheric clouds" (PSC) form. These clouds take the safe reservoirs of chlorine and precipitate out water and HNO₃. What's left is stable diatomic chlorine (Cl₂). When the sun interacts with this form of chlorine, it changes it into chlorine ions (Cl^-). This free chlorine goes on to attack ozone.

So it's bad over Antarctica because it's got the vortex and cold temperatures to start the ozone depletion process.

It's bad in September because that's when the sun comes back out in the Southern hemisphere. Remember when the sun interacts with the Cl₂ that the PSC's have been churning out all winter, the bad chlorine forms and enhances ozone depletion.

Recap of the timing

Winter:
PSC's take chlorine out of the safe reservoirs.

Spring:
the sun comes back after a winter of darkness and interacts with the chlorine to make the reactive (bad) chlorine. the ozone depletion cycle begins and continues until chlorine is no longer produced and the existing chlorine is tied up in safe reservoirs again. Well, chlorine production ceases when the PSC's dissipate in the spring/summer. To tie chlorine up in safe reservoirs, you also need a fresh influx of NO₂ and other molecules from air coming from low latitudes . Even though ozone is produced continuously at low latitudes, where sunshine is regular year-round, it takes time to replenish the ozone lost in the depletion process, which is why the hole does not disappear instantly.

Back to GS 201 homepage