One swallow doesn't make a summer

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  • A butterfly sitting on a lavender plant in Chester Zoo's gardens. According to the British Trust for Ornithology, the numbers and time of year that butterflies emerge from hibernation is dependent on the weather> Image: Peter Byrne/PA Wire

    This image shows how the seasons are caused by the Earth's tilt. The Earth is tilted on its axis by 23.5 degrees. Credit: NOAA, National Weather Service Weather Forecast Office, Albuquerque.

    Revellers gathering at Stonehenge in Wiltshire after watching the sunrise during the summer solstice, which marks the longest day of the year and the first astronomical day of summer. Image: Chris Ison/PA Wire.

    A synoptic overview for Monday 1st June, showing an area of low pressure, which is expected to move in from the Atlantic Ocean over the UK, bringing cool, breezy and wet conditions for the first meteorological day of summer. Image: MeteoGroup.

  • One swallow doesn't make a summer
    28.05.2015 14:03

    According to the Greek philosopher Aristotle, ‘one swallow does not a summer make, nor one fine day’. This well-known proverb probably relates to how the instance of one event does not necessarily demonstrate a trend or a permanent change. However, the proverb does not give us a clue to how many swallows or fine days are required for a summer nor indeed what to do if no swallows are spotted. So, what does ‘make a summer’? When will this summer start?

     

    Perhaps confusingly, summer can be defined in different ways and the timing of summer can depend on where you live. One method for defining summer could be by using phenology, the study of how the seasons affect the life cycle events of plants and animals, such as swallows. Phenology is obviously a very fluid method to use, in that the timing of these events, such as animal migration, varies from one year to the next. The first swallows usually migrate to the UK from South Africa sometime in April or May. Indeed, the arrival of these birds is probably more indicative of late spring than summer.

    Another method of defining summer is by using the astronomical calendar. We know that the Earth takes approximately 365 days, or one year, to orbit the sun and that the tilt of the Earth determines the seasons. In the northern hemisphere, the point at which the overhead sun travels its farthest north of the equator is known as the summer solstice, this also marks the longest day. The summer solstice also marks the start of the astronomical summer which lasts until the autumnal equinox (when day and night are nearly the same length all over the world).

    Some countries, such as the USA, denote the summer solstice as the official first day of summer. However, the date of the summer solstice can vary slightly from June 20th to June 21st (this is because it actually takes the Earth 365.24 days to orbit the sun which effectively changes the timing of the solstices slightly from one year to the next). This year’s solstice is June 21st.

    Meteorologists and climatologists prefer to break down the seasons into groups of three month periods. The meteorological summer is defined as the months of June, July and August in the northern hemisphere (and December, January and February in the southern hemisphere). This method is convenient for meteorologists and climatologists because it provides a consistent way of recording long term averages, such as temperature and rainfall averages. The meteorological definition of summer also reflects when much of the northern hemisphere, including the UK, usually experiences its warmest months.

    So meteorologically, Monday 1st June will be the start of summer this year. This begs the question of whether we will experience summer-like weather from Monday. The answer is no, not on Monday at least.

    The start of next week will be dominated by an area of low pressure crossing the UK from west to east, bringing cool, wet and at times breezy conditions for many. Indeed, some high rainfall totals are possible then and localised flooding cannot be ruled out.

    However, conditions look like becoming much more summer-like later in the week. An area of high pressure should move closer to the UK then, bringing generally drier, warmer and less breezy conditions. Hopefully, then, the swallows won’t regret this year’s migration here.

    By: Paul Mott