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ELI5 Yahaaa

An old rhyme, half right

Does a Red Sky Really Predict the Weather?

Red sky at night, shepherd's delight — and in some places it genuinely works.

1 Low sun shinesthrough a longslice of air2 Only red lightsurvives thetrip3 Weather usuallyarrives fromthat direction
The colour is physics. The prediction is geography.

  1. Step 1 of 5

    Low sunlight travels through far more air.

    Midday A short path throughthe airSunset A path many timeslonger

    At sunset the light comes in at a shallow angle and crosses a much longer path of atmosphere than it does at midday.

  2. Step 2 of 5

    The long path scatters the blue away.

    Air along the path Blue scatteredawayWhat is left arriving is red

    Air knocks short blue waves sideways. Over a long enough path almost all the blue has been scattered out, leaving red and orange to reach you.

  3. Step 3 of 5

    It only glows red if the air is clean and dry.

    Clear dry air Deep red — light getsthroughCloud and damp Dull grey — nothinggets through

    Thick cloud or heavy moisture between you and the Sun blocks the light entirely. A vivid red sky means the air in that direction is clear.

  4. Step 4 of 5

    In Britain, weather arrives from the west.

    1 Clear air tothe westtonight2 The winds carryit east3 A fine daytomorrow

    The prevailing winds bring tomorrow's air from the west. A red sunset says the air heading your way is clear, so tomorrow will probably be fine.

  5. Step 5 of 5

    Which is why the morning half also works.

    Red at night Clear air still to come— delightRed in the morning Clear air departing —warning

    A red sunrise means the clear air has already passed overhead and is going away east, with whatever is behind it about to arrive.

    The rhyme fails near the equator and in places where weather arrives from the east — it is local advice, not physics.

The short version

A red sky shows clean dry air in the Sun's direction, which predicts tomorrow only where weather reliably comes from the west.

Try it yourself

Note the sunset colour for a fortnight and the next day's weather. Count how often the rhyme actually gets it right.

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