The Autumn Equinox

  • A diagram showing the overhead sun at the equator on Wednesday 23rd September 2015. Photo credit: Przemyslaw Idzkiewicz.

    A visible satellite image of Europe on the morning of 22nd September 2016 illustrating the near vertical division between night and day across western Europe.

    A forecast chart of jet-stream strength across the North Atlantic next week as a deepening low pressure system tracks towards the UK. The bright pink shading illustrates very strong wind-speeds at upper levels in the atmosphere.

  • The Autumn Equinox
    22.09.2016 13:17

    Equinox is a word derived from the Latin aequinoctium, aequus (equal) and nox (genitive noctis) (night). An equinox is an astronomical event that occurs when the plane of the earth’s equator passes through the centre of the sun. The autumn equinox this year occurs on September 22nd at 14:21 UTC. On an equinox, the length of day and night are approximately equal in their duration over the entire planet. However, the length of both day and night are not exactly equal due to the angular size of the sun and atmospheric refraction.



    This interesting split in our 24 hour day is linked to the reason the Earth has seasons at all.  It is the passage of the sun which determines seasonal changes around the world, and is responsible for the timing of hurricane seasons, monsoonal rains, alongside more subtle changes such as the leaves of deciduous trees changing colour and eventually falling. The planet spins on a 23.5 degree tilted axis with respect to its orbital plane. This means that as the earth travels along its ~365 day orbit, the northern and southern hemisphere tilt closer to or farther from the sun.


    As the UK heads deeper into Autumn, the northern hemisphere will tilt farther and farther away from the sun, receiving the suns rays at an increasingly acute angle. This is what creates the increasingly long shadows and enables the atmosphere and land areas to cool down. By late December the sun will reach its lowest point, marking the December solstice. This years winter solstice will occur on December 21st at 10:44 UTC.


    Whilst the coming equinox signifies a change towards cooler and darker days for the UK and Ireland, the sun’s southward passage means warmer and brighter days to come for those in the Southern Hemisphere.


    As the days shorten in the northern hemisphere the horizontal temperature gradient between the Pole and Equator will increase.  This will lead to the strengthening of the North Atlantic jet-stream which will enable the development of more intensive low pressure systems at mid-latitudes which will, at times, threaten the UK. An example of the impact on the UK of the increasing horizontal temperature gradient is expected next week. The strengthening North Atlantic Jet-stream will interact with the lingering energy from ex-Tropical Storm Karl producing a number of intensive low pressure systems in the vicinity of the British Isles. Forecasters are currently focussed on the potential for stormy conditions next Tuesday (27th September) with a low pressure system expected to race north-east across north-western parts of the UK. There remains uncertainty on the exact track and intensity of the system but many northern and western areas are likely to experience wet and windy conditions throughout the week.