Has summer gone?

  • Sea surface temperatures. Photo credit: NOAA

    Forecast surface pressure for Sunday night (15/16th September 2013)

    Tilt of the Earth and the seasons. Photo credit: NOAA

  • Has summer gone?
    12.09.2013 13:44

    Just two weeks into the autumn (meteorologically speaking) and we have all noticed (particularly in the south) a distinct shift in the weather. The change this year feels particularly marked with hot sunshine giving way to cooler, fresher conditions. Why the sudden change? Is this based on reality or perception? Why does Autumn often herald a change?

    Temperatures last Thursday (5th Sept) reached 30.2C at Manston in Kent. The following day the temperature reached just 19C in the same place. This significant shift in temperature was brought about by a cold front with cooler conditions being introduced from the Atlantic.

    However, the way we feel about the weather is often based on our perception of recent history. In the south-east of England the average maximum temperatures should reach around 19C or 20C at this time of year. However, given the recent spell of hot weather (and the summer in general) we are more acutely aware of the onset of Autumn. If the summer had been more akin to average (in terms of temperature) then the onset of Autumn would be hardly noticeable.

    In the Autumn we often see a marked increase in the influence of the Atlantic on British weather. This is often associated with cooler and generally wetter conditions. To understand why Autumn often ushers in wet and windy weather we have to look at the global distribution of heat.

    During the summer the temperature difference between the poles and the equator becomes less marked. These temperature differences affect the pressure distribution across the globe. This in turn generates the wind which effectively redistributes some of this heat. In the winter, as the northern hemisphere begins to tilt away from the sun, the temperature contrast between the poles and the equator increases. The potential then exists for stronger winds as the atmosphere effectively tries to redress this growing temperature/pressure imbalance.

    The jet stream forms high up in the atmosphere along the steepest boundary between the warm and cold air masses. As already noted, this transition is most marked during the winter. A stronger jet stream is able to develop deep areas of low pressure and steer them towards the UK.

    This weekend will see a deep area of low pressure approach to the north of Scotland bringing wet and windy weather. Gales or severe gales may affect northern parts of the UK. Temperatures are expected to be close to the seasonal average at first, possibly becoming noticeably cooler next week as north or north-westerly winds develop.

    Autumn, it seems for now at least, is well and truly with us.

    By: James Wilby