Green Flash

Other than green flashes, occasionally enough blue light is sufficient to be seen as a ‘blue flash’.

How do they occur? – The basics

Before we find out how they actually occur, there are a few things that we need to know about.

Atmospheric Inversion
Normally when an anomaly (deviation from the normal) occurs in the atmosphere (such as the green flash) where there is an increase in temperature, humidity or precipitation instead of a decrease (which is the normal), it is said that there is an inversion. Mostly, when something talks about atmospheric inversion, it is referring to temperature inversion.

 Normal atmospheric conditions are that in which the air near the surface of the Earth (within the lower atmosphere, called the troposphere) is warmer than the air above it. Usually, air temperature decreases with altitude, at a rate of approximately 1⁰C per 180m. These conditions occur mostly because the atmosphere is heated from the  Earth’s surface which, in turn, has been warmed by solar radiation.

There are several ways in which temperature inversion can occur, some of these include inversions occurring as a result of subsidence (sinking) or air in an anticyclone and kabatic winds. Temperature inversion can also occur when warmer, less dense air masses move over cooler, more dense air masses (effectively stopping an convection currents that may have been occurring in the cooler air mass) or over bodies of water. Also, they occur  when the radiation from the surface of the Earth exceeds the amount of radiation received from the sun. This occurs mostly during the night, winter or early morning (the coolest part of the day) when the angle of the sun is very low in the sky.

Inversion layers block the upward movement of air, effectively trapping pollution (such as smog) close to the surface of the Earth. By suppressing convection currents, violent thunderstorms can evolve if any moisture is present.

Refraction
Refraction is the change in direction of a ray of light as it passes obliquely from on medium into another (mediums must have different optical densities). In this case, refraction occurs because of the differences in the atmospheric layers (temperature and density) and allows for optical phenomena such as mirages, Fata Morgana (objects on the horizon appear elongated and elevated) and others to occur. Some well known visual distortions caused by the atmospheric refraction of light are as follows:

• Stars in the sky appear higher than they actually are (see Figure 1)                                                                                                        apparent position of star
• The setting sun is visible after it has                                                                                true position
  actually gone over the horizon and                                                                        
  its shape appears slightly flattened
• Stars appear to twinkle due to the
  varying refractions of light from the
  star as it passes through the atmosphere.

Figure 1 shows that as light travels from the star to the observer, it is bent accordingly (although quite exaggerated in the diagram) to the varying optical densities of the different layers of the atmosphere.  In the atmosphere, objects are refracted the most when they are lower in the sky (this occurs because the ‘angle of incidence on the atmosphere is at its greatest and light must travel through a wider atmosphere band’3). For example, at sunset or sunrise, the observer can still see the sun a few minutes after it has actually set.

 Speed of light & colours

Light travels at different speeds in different mediums, depending upon their optical density. When light enters mediums of different optical densities, the amount of refraction that will occur depends on the extent that the speed of light has changed. Figure 2 shows a table of different mediums with the speed of light in each and Figure 3 represents light travelling from air into a few of these mediums.

          air                                air                                           air

         water                                water                                     water

Light slows down in certain mediums that are more optically dense because the light is actually being absorbed and then re-radiated by the atoms which make up the medium. When different colours are travelling through these mediums, they react differently to the atoms, therefore travelling at different speeds and refracted through different angles (two colours shown in Figure 4)

How do they occur? – Putting it all together

As the sun sets below the horizon, refraction of the light occurs in the atmosphere to separate it into the visible spectrum of colours (just like the refraction of light in a prism). Since air is thinner at the top and denser lower in the atmosphere, sunlight rays curve slightly (in the same direction as the curve of the Earth), so higher frequency light (such as green and blue) curve more than the lower frequency light (such as red and orange). Therefore, the green/blue rays from the top of the setting sun remain visible after the other lower frequency light rays have disappeared beneath the curve of the Earth.

From atmospheric refraction, the white light                                                                                               
is separated into the colours of the visible
spectrum and because of the differing densities of the
atmosphere, the light curves toward the earth (this is why
it’s possible to see the sun even when the earth is in the way
of the observer). The blue/green rays with higher frequency
are curved more than the red/orange rays, which are curved
the least. Therefore, since the red/orange light is out of the
observers view, the blue/green rays are seen.

The green flash can be enhanced by atmospheric inversion. An inversion is a deviation from the normal atmospheric conditions, normally referring to a temperature inversion. Normally the air near the surface of the Earth is warmer than the air above it but under certain conditions, temperature inversion can occur where the air near the surface of the Earth is colder than the air above. Temperature inversions can occur when a warmer, less dense air mass moves over a cooler, more dense air mass (effectively stopping any convection currents that may have been occurring in the cooler air mass) or the radiation from the surface of the Earth exceeds the amount of radiation received from the sun (occurs mostly at night or during winter when the angle of the sun is very low in the sky.). The index of refraction of air is decreased as the temperature of air increases, hence green flashes are enhanced.

What are the best conditions for them to occur?

The green flash is rarely seen by the naked eye as the observer needs to know what they are looking for and certain specific conditions are required to occur.

As seen from the table on page 3, certain types of the green flash are observed best in different conditions, however they are most likely to be seen in clean air (more of the light from the sunset reaches the observer without it being scattered). You might expect that we will see a blue flash but the blue light is preferentially scattered (hence the sky is blue) out of our line of sight and the remaining light ends up looking green.

Conclusion

The ‘green flash’, though it has only been spotted several times, is a very rare occurrence but, most importantly, we’ve find out that it is real and does happen, although it is only seen by people that know what they’re looking for (or are very, very lucky). The ‘green flash’ occurs because of various physics concepts such as refraction, the speed of light and others, similar to other optical phenomena occurring in the atmosphere.

References

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