Why is the sun­rise red be­fore a storm?

I am in New Zealand and Cy­clone Lusi is ap­proach­ing – the sun­rise is beau­ti­ful!

Guru Magazine - - Ask A Guru - Asked by @sineira via Twit­ter

To un­der­stand why the sun­rise you see be­fore a storm is red, you also need to un­der­stand why the sky is blue. When you look up on a cloud­less day (as it is to­day as I write this), the sky looks like a cool blue dome. This colour is caused by the way sun­light in­ter­acts with what is in our at­mos­phere – mostly ni­tro­gen and oxy­gen. Sun­light ap­pears white but is ac­tu­ally made up of a spec­trum of colours (all the colours of the rain­bow). As sun­light passes through the at­mos­phere, it strikes the gas mol­e­cules and other par­ti­cles in the air, ‘scat­ter­ing’ the light. It is the blue part of the scat­tered light that gives the sky its blue colour. How­ever, the thick­ness of the at­mos­phere changes its colour. When the sun is low in the sky, as is the case dur­ing a sun­set or sun­rise, the light passes through much more at­mos­phere be­fore it gets to our eyes. All the blue light has been scat­tered away by the time it reaches us, leav­ing only the red and or­ange light. The red­dish colours are made even more strik­ing when the light passes through a high pres­sure air zone – the high pres­sure trap­ping more dust and small par­ti­cles in the air, scat­ter­ing even more of the blue light. So, why a beau­ti­ful sun­rise be­fore a storm? The say­ing “Red sky at morn­ing, shepherd’s warn­ing” holds some truth in parts of the world where the pre­vail­ing winds are west to east (as they are in North­ern Europe, much of USA and New Zealand – see this in­ter­ac­tive map). Gen­er­ally speak­ing, weather pat­terns (such as storms or high pres­sure sys­tems) will ap­proach from the west in these ar­eas. There­fore, when the sun is ris­ing in New Zealand, the morn­ing light is pass­ing through the weather pat­tern that has just passed over­head. If this is a high pres­sure sys­tem, then the sun­rise will be red. And what fol­lows a strong high pres­sure weather sys­tem can of­ten be a storm.

An­swered by Dr Stu

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