Minor Snow: Rainbows hide

May 25th, 2012
by Jo Law

Bellambi (AWS) recorded 16.8mm for the first 25 days of May 2012 compared to a monthly average of 94.6mm from 1997 – 2011.  Several news headlines declared: ‘Rain just a trickle in driest May since 1859′, ‘Sydney is experiencing its driest start to May in more than 150 years’, and ‘Sydney’s driest start to May in 130 years’. These articles painted a picture of nervous weather statisticians attentively watching the rain gauge as clouds built up and precipitation moved north towards Sydney on the morning of May 21st. The online news article in the Sydney Morning Herald featured a rainbow against a mottled sky over central Sydney’s skyline. Perhaps the rainbows are not hiding afterall. After so many dry days, the continuous light rain on the night of May 24th seemed strange yet familiar.The news articles identified the continuous high pressure systems that had been present over much of Australia’s east coast as the main cause of the dry spell. At the same time, Sydney have been reportedly receiving much more sunshine (hours) per day than the average May. In Wollongong, the daylength is now around 10 hours, steadily decreasing as we head towards winter solstice. In Austinmer, the sun presently rises just before 7am and our close proximity to the escarpment means that we loose the sun around 3pm making daylight hours somewhat scarce.

At the formation of the earth-moon double-planet system about 4.5 billion years ago, the earth was rotating at a much faster rate – an earth-day was about 5 hours long.  At approximately 25% of earth’s diameter, 2% of earth’s volume, and 1.25% of earth’s mass,  the moon is substantial enough to slow down the earth’s rotation. At the same time, this tidal friction between the two planetary bodies is also slowing down the moon’s orbit. Geological records such as ancient tidal sediments show that 620 million years ago an earth-day is around 22 hours with 13 months corresonding to about 400 mean solar days in one year or one orbit around the sun.

2 Responses to “Minor Snow: Rainbows hide”

  1. 1 Lucas
    May 26th, 2012 at 18:13

    beautiful photo, jo.

    is this sunlight coming in through the window?

  2. 2 Jo Law
    May 26th, 2012 at 21:04

    Sunlight through the window refracted through a 4-sided acrylic ‘prism’….