Mississauga Centre RASC

65th Meeting

    Speakers’ Night

Day:                Friday June 9, 2006

Speaker:  John Gawrow

Observing our Atmosphere

John Gawrow, amateur astronomer, current member and former President of the Hamilton Centre RASC spoke about observing the phenomena of the sky. In the past, people were in touch with their environment but with modern life and cities, this is less the case. Amateur astronomers, however, automatically look up at the sky to ascertain the weather condition.  As amateur astronomers, we are lucky that we just have to step outside to observe. John encouraged broaden our outlook to beyond just the stars and to consider the atmosphere as well.

The atmosphere is divided into layers. The troposphere contains 75% of the mass and is 8 to 15 km thick (8 at the poles). The temperature drops 6 ½ degrees per km. It holds 1/10 of 1% of the Earth’s water, which sits just at the saturation point, from perfectly transparent to opaque or anywhere in between, either liquid or crystal (ice).  In the stratosphere, with the temperature inversion, it gets warmer with height. The mesophere extends to 80 km., above 99% of the atmosphere, and can contain noctilucent clouds. Finally, the aurora can be found in the uppermost thermosphere.  John showed images of various cloud types and emphasized that Earth’s atmosphere does more marvelous things than other planets. 

Rainbows are caused by sun shining through water droplets producing chromatic aberration with different colours coming to different foci. The blue band is dark due to scattering of blue light.  A secondary bow, outwards from the primary bow has its order of colours “reversed” because of an extra turn of the light around the droplet. The sky is much brighter beneath the primary bow and above the secondary one. Alexander’s dark band between the bows is due to more reflections in this area. Fainter bows can also be occasionally seen. A supernumary arc is an interference pattern caused by alternating bands showing that light is a wave.

The mis-shapen sun at sunset is due to refraction, with the sun actually below the horizon. The various colours of the sun are refracted in different ways – this allows one to see the green flash after the rest of the sun has disappeared.  The solar halo or parahelic circle is caused by ice crystals scattered randomly. The upper tangent arc or the solar halo looks like a pair of horns.  When vertically scattered, ice crystals produce sundogs or parahelia. John also described the circumzenithal arc, the sundog and moondog. The challenge for us is to notice these phenomena.

Charged particles of the solar wind enter the Earth’s atmosphere near the magnetic poles producing the aurora, the bottom part of which is 100 km high. High low density oxygen produces a brilliant red, lower down green; nitrogen glows blue or deep red. It is often bright enough to be seen from the city but can be confused with light pollution. Again, John encouraged us to look for this phenomenon.

Submitted by Chris Malicki, Secretary  Chris Malicki, Secretary                               back to Miss Centre. meeting reports page
Mississauga Centre RASC