The featured chart presents the climatology of change in air temperature between 6 PM and 6 AM the next day for Des Moines. The shaded boxes represent the frequency at the given temperature and week of the year combination. The lines are smoothed averages over the period of record and for this year. One would perhaps expect to see the most negative values during the cold season as nights are the longest and drier atmospheric conditions allow for more cooling. The chart shows the opposite and is likely due to a number of factors including more daytime heating by the sunshine during the warm season leading to a larger potential for cooling than during the cold season which is dominated by air masses. What do you think?

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