Have you ever wondered why on some cloud free nights you can see forever - well as far as Andromeda (M31) our neighbouring galaxy and on some nights while there are no clouds you can see very little ?  One of the principal reasons is that the sky transparency varies from night to night depending how transparent the air is over your observing site. Transparency should not be confused with 'seeing' which is how steady the air is, poor seeing will cause the image of a star to move and planetary detail to be invisible.

This page was inspired by the article on 'Clear(er) Skies Ahead' in the August 2008 edition of Sky and Telescope. As usual this article while explaining the technicalities quite well is heavily biased to observers in the USA and Canada. The Internet resources referenced in the article only contain data on Sky Transparency in North America and do not cover Europe. This page is designed to redress this balance and provide some links to resources that cover the UK & Europe.

This page is a child of the Croydon Astronomical Society Weather Page that contains many more resources related to Weather for Astronomy.

Sky Transparency or Aerosol Optical Depth (AOD) is where Meteorology and Astronomy meet as a result terms from both sciences appear on this page. As a result there is the potential for confusion as both use the phrase Air Mass to mean different things !

In Astronomy Air Mass is the amount of air that you observe through. Assuming that you are observing from near Sea Level if you observe vertically upwards ( your Zenith ) you will be looking through one air mass of air. As you move further from your zenith you are looking diagonally through the atmosphere so you are looking through more air this increases the nearer the horizon you look until at the horizon with an altitude of zero degrees you are looking through around 40 times as much air or 40 air masses. The number of air masses is not infinite as the Earth and the Atmosphere is curved so when you look horizontally your line of site still ends up in space. There is also the effect of refraction but that just complicates matters and you should not normally be observing anywhere near the horizon.

Meteorologists however use the term Air Mass in a different way. They treat the air in the atmosphere as parcels known as Air Masses which have similar characteristics depending on where they originated and where they have traveled. For instance air from the South will have traveled over Europe and will tend to be warm, dry, contain dust and may well be polluted. If the air comes from the West at least for the UK the Air Mass will have traveled over the sea and will be cooler and contain more water. A more detailed explanation of Meteorological Air Masses can be found on the UK Meteorological Office Education page on air masses. Meteorologists also use the term visibility but this is used to define how far you can see in a horizontal direction, the air higher up may have different characteristics and this may not provide a reliable guide.

Light that travels through air is attenuated by two different mechanisms, the first is Raleigh Scattering from air molecules. This cannot be reduced except by going higher above sea level so you reduce the amount of air molecules between you and the light source. Raleigh scattering is what makes the sky blue and is varies as the 4th power of the wavelength of light so the blue light is scattered much more than the red light. The diagram on the left shows the amount of light scattered at different wavelengths together with an approximation of the colours ( these should be the rainbow colours but violet seems to be missing !). The loss is shown as a linear scale so needs to be converted to magnitudes for astronomical use. The second mechanism is Raleigh Scattering from pollution molecules, known as aerosols  in the atmosphere, normally man made Nitrogen Dioxide and Sulphur Dioxide, these typically result in the Milky White sky that is so familiar to European observers. The level of pollution depends very much on where a particular Air Mass has been in it's meteorological history.

The loss of transparency due to pollution is of interest to climatologists as the scattering of light results in some of the incoming solar radiation being dissipated in the atmosphere rather than on the surface. Atmospheric scientists measure the amount of light that is lost by a factor called the 'Aerosol Optical Depth' while astronomers use a more familiar unit of magnitudes per air mass. Luckily conversion is easy just multiply the AOD value by 1.086 to convert it to magnitudes per air mass.

Sky & Telescope have two useful pages that go into the subject in more detail, the first is a page on extinction & transparency and the second a more detailed page on AOD including sources of data for North America.

Higher levels of AOD will have more effect on low contrast objects such as nebulae but little effect on bright objects such as the Moon & planets. However it should be noted that the effect is very dependant on wavelength. Observations in the deep red such as the Hydrogen Alpha line at 658nm will be little effected but those in the Blue Green such as the Oxygen III line at 501nm and the Hydrogen Beta Line at 486nm will be much more heavily effected.

Sources of data

If there is no scattering due to pollution the AOD will be zero, the article in S&T describes values of AOD of 0.2 as mediocre and 0.4 as poor but values can get a lot higher than this. The site below shows values as high as 3 !

While there are a number of sites on the Internet  that provide values of AOD on a historic basis there appear to be none in Europe that provide current values either from ground stations or from satellite observations. However there is an European site that has a daily forecast of AOD which allows you to view the forecast in 3 hour time steps. While not as accurate as actual observations this is better than nothing and is not effected by cloud as are ground and satellite observations. The current forecast is available from the GEMS forecast page located here. Note that the forecast shows both the natural and man made aerosols, the two different constituents can be selected by using either the Natural or Anthropogenic options - you can then see just how good the seeing would be without man made pollution. 

Calculations

While the guide above gives some indication of the results of the reduction in transparency as a result of high AOD values it is possible to calculate more precise values that take into account not only the altitude of the star or planet you intend to observe but also the wavelength that you may wish to image at.

If you have Microsoft Excel or Open Office 'Calc' on your computer you can download a spreadsheet to carry out the calculations here. This is a 'tidied up' version of the original Sky & Telescope spreadsheet that can be found here, the calculations are the same but you can destroy the calculations in the original as they are not protected.

 
 
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