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Feb 26 2012

Update on season changes (Geek edition)

The two posts on changing the seasons (here and here) resulted in a lot of interesting information in the comments and it seems like there is quite a geographical variation in how the seasons are demarcated, with the US possibly being an outlier in using the solstices.

Reader ahcuah is a kindred soul and has kindly sent me the data he collected over a full year of the daytime high and low temperatures. He lives fairly close to Cleveland and so the data is similar to what I would have gotten. In general, the shape of the graph and the locations of peaks and the valleys should be the same over the entire northern hemisphere (I think) and inverted for the southern, so the pattern he gets is of far greater general utility than just for his location.

The recorded temperatures are only to the nearest whole number and this causes a problem for figuring out what the coldest or warmest days are since there are many days around those two points that have the same temperature. For example, the coldest daytime high stays at 34oF from January 14 to January 24. One could make a reasonable estimate that the coldest day is at the midpoint, which would be January 19th, but ahcuah knows that there is a better way to check this.

He did a Fourier analysis of the entire set of data and found that it converged pretty rapidly, with six terms in the series being sufficient. Using these Fourier components, he was able to recalculate the daily temperatures to greater precision and found that the coldest day is actually January 19, agreeing with the rough estimate. The coldest recorded nighttime low stays at 19oF from January 10 through February 4, with January 22/23 being the midpoint. The Fourier analysis says that the coldest night is January 22, again agreeing with the rough estimate.

While we can understand why the coldest days occur in mid-January, a month after the shortest amount of daylight hours given by the winter solstice (the reason being that it takes some time for the Earth to cool down), no obvious explanation comes to my mind as to why the nighttime low lags behind the coldest daytime high by three days.

Similarly, the hottest recorded daytime high stays at 84oF from July 16 through July 27 and the Fourier analysis pins the hottest day at July 19. The highest recorded nighttime low stays at 64oF from July 20 to July 28, with the Fourier analysis giving the peak on July 24, lagging by five days. The hottest days occur about a month after the longest day at the spring solstice, again due to the time lag for the Earth to heat up.

In addition to the interesting puzzle of why there is a three day lag between the daytime and nighttime peaks, there is also issue of why the nighttime peak in winter is broader (26 days) than the daytime one (11 days) but in summer is narrower (9 days vs. 12 days).

4 comments

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  1. 1
    mnb0

    Always willing to falsify a theory I have checked how your proposal works for The Netherlands, St. Petersburg (Russia), Kyiv, Volgograd (former Stalingrad) and Irkutsk. The idea was that it would not fit the Russian and Siberian continental climate, with its autumn rain in October and thaw in May or June. Still the months December, January and February are according to Wikipedia significantly colder than the others. Only in Irkutsk winter could be extended with an extra month. The Russian and Ukrainian cities can have low extremes in March, but on average the temperature is above 0 °C.

  2. 2
    Pierce R. Butler

    The coldest recorded nighttime low stays at 19oF …

    Which was also the coldest nighttime low here in north central Florida this immediately-past (thermally)/present (calendrically) winter.

    Even though we had only one night get that cold, it probably did more damage to our bananas and citrus trees than that ten-day snap did to Ohio’s.

  3. 3
    Heidi Nemeth

    Lake Erie has a lag effect on Cleveland weather. All bodies of water exhibit some lag effect because water has a higher heat capacity than rock, soil or air. Lake Erie just does it better than most bodies of water. We call it “The Lake Effect”.

    The Lake Effect impacts Cleveland’s weather more than just Lake effect snow. Lake Erie, being shallow, can cool all the way to the bottom to 39 degrees Fahrenheit (4 degrees Celsius), turn over, and freeze on top, some years freezing solid shore to shore. (Right now the the surface temperature of Lake Erie is colder than any of the other Great Lakes except Superior. [[http://coastwatch.glerl.noaa.gov/cwdata/lct/glsea.png]])

    Freezing raises the albedo of the lake, meaning more solar radiation is reflected off the lake, keeping the lake colder longer.

    But the bigger effect is the enthalpy of fusion – how much energy it takes to melt ice (or how much energy water must lose to freeze.) As I remember from a high school chemistry experiment, it takes 86 times more energy to “warm” an amount of water from 0 degrees C (solid) to 0 degrees C (liquid) than it does to raise the temperature from 1 degree C to 2 degree C.

    The enthalpy of fusion keeps Lake Erie unfrozen (and Cleveland warm in the late fall) long after the surrounding areas have succumbed to winter. That same enthalpy of fusion causes Cleveland to stay cold long into the spring. (Perhaps this lag effect is partly why Mano thinks we should change the dates of the starts of the seasons.)

    The latent heat of the phase change stabilizes the temperature around the freezing point for months. It increases the length of the winter lows because warm periods in winter are usually warm enough to bring the temperature up to the melting point – but not warm enough to melt all the ice on Lake Erie.

    Summertime peaks are not associated with any phase change, so they are more labile.

    As to why the nighttime lows last longer than the daytime lows in winter, and summer daytime highs last longer than summer nighttime highs: Isn’t this likely because in winter, night is longer than day, and the sun’s flux is low in the daytime, while summer days are longer than summer nights and the sun’s flux is high?

  4. 4
    Mano Singham

    You have really thought this through! Your very last point is interesting and I need to think about it some more to see if I truly understand if it produces that effect.

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