Blue sky thinking

Does the sky reflect light coming from the ocean which gives it its blue  colour?  This is a nice theory if you live close to the coast but let's say you are in Alice Springs -things get a bit dodgy.  The reason why the sky is blue is fairly complex, but the fundamentals are fairly simple to grasp.

Blue sky over SE New South Wales 3 Sept 2012

In terms of the earth's history, a complete explanation of the blueness of the sky has been only a recent occurrence.  A British chap by the name of Lord John William Strutt Rayleigh who was born in 1842 and died in 1919 was the first to explain why we see a blue sky and the process 'Rayleigh Scattering' was named after him.

Basically, when the sunlight arrives at the earth's atmosphere it is made up of different wavelengths which we see as different colours (for a visual effect go and find yourself a rainbow), but together look light white light.  

When this mixture interacts with particles in the atmosphere like molecules of nitrogen or oxygen some of the light is scattered, whilst the rest is able to pass through with little scattering.  In our atmosphere it is the shorter wavelengths or blues and violets which are preferentially scattered over the reds and yellows which go straight through the atmosphere.  When we look at the sky we see the blue light which is being scattered all over the place.  The sun looks yellow as the blue part of the spectrum has been removed from the white light which has travelled directly to our eyes (and is probably damaging them so try not to do it).

The sky is not all the same shade of blue, towards the horizon the blue fades towards white as the proportion of blue light to the other colours diminishes.  You can see this effect in the picture above.  One day I'll have a look at other colours in the sky but at the minute we will stay with the blue sky thinking.

So is the sky reflecting the blue ocean? No, in fact the ocean is more likely to be reflecting the sky (with some of its own Rayleigh scattering in the water to add to the effect).

The weather forecaster's 'Get out of jail free card'

Weather forecasters seem to have a difficult job at times so it might just pay to give them a bit of slack and allow them a bit of leeway every now and then.  However they do have their own 'Get out of jail free card' that can be whipped out at a moments notice for those days when it seems like it should have rained, indeed it may have looked like it rained and yet nobody actually felt the rain on the earth's surface.  This get out of jail free card comes in the form of VIRGA.

Virga over Mt Ainsle 29 August 2012

Virga is of course rainfall which evaporates before reaching the ground.  The word has its roots from a Latin word meaning twig or branch and the reason behind this definition becomes obvious when one observes the tendrils and extensions created from the base of clouds by the precipitation.  In Canberra we had an extravaganza of virga just last Wednesday.

More virga falling over Canberra 29 August 2012

The two key ingredients for virga are obviously rainfall and a layer of fairly dry air where the raindrops can evaporate into vapour.  Virga is responsible for those high hooked cirrus clouds we see with an approaching front, but can also be observed lower in the atmosphere.  On Wednesday the virga was generally falling from altostratus clouds which are what are referred to as mid-level clouds.

Virga from altostratus under cirrocumulus

Some of the more devastating features associated with virga are so-called dry thunderstorms with strong wind gusts (dry microbursts) and dry lightning.  It's always a worry as a source of ignition for bush fires, but we'll leave that till the summer. 

For now, virga can remain the forecaster's friend and also the get out clause when the forecast said rain yet nobody got wet. 

Rainbow Bright

A rainbow captured in Canberra last week

Rainbows are generally pretty fascinating things.  If you're lucky you may catch a glimpse of a second rainbow outside the main one.  As with a lot of things things, luck is relative because really there are actually infinite rainbows outside each other which would make for quite a spectacular display.  The only problem is that the light is so weak we can't see them.

Next time you see one, have a look at the light inside the rainbow, then compare it to the light outside.  It looks brighter inside, and its not just your imagination there is more light inside that rainbow highlighting its effects.

Also, each rainbow looks slightly different for each observer, so when you see one think of yourself being in a truly unique situation.

Mathematics is so great we can explain all of these things with equations, but here we will just marvel at what we can see.

Now for budding rainbow watchers what are the best conditions to capture rainbows?

1. Find somewhere where there is precipitation as it is the raindrops which are doing the work of refracting the light so we see the rainbows.
2. Showers are better than rain as there are breaks between the clouds which allow the sunlight to penetrate and produce the rainbow, so watch out for fluffy cumulus type clouds.
3. Don't look towards the sun as the optical magic occurs when the sun is behind you.
4. Lastly, go rainbow catching in the early morning or late evening, in the middle of the day the angles make the rainbow below the horizon  so you can't see it.  At night there's no sun so no rainbows (although moonbows are still possible).

Now the tips are there, so go and catch a rainbow, and try to get one with some banding inside the rainbow as that surely is the holy grail of rainbow watching!

Weathering

So, where does weather come from?

Weather - That's it right there!

Here, the weather generally comes from the west, occasionally from the east and is less frequent, but generally more interesting, when it arrives from the north or south. Here is Canberra, but it could be anywhere as weather is everywhere, but where does it come from?

'Weather' the word has its origins from 'we' meaning wind from around the 15th century, it is in other languages to like wetter in German.  Obviously these are not new ideas, indeed  meteorology was discussed by Aristotle around 340 BC, but weather itself has been around a lot longer than human beings.  In this blog we'll try to investigate the origins of weather with the help of my brain, Australia, my phone camera and the internet. Let's see how we get on......