This Demonstration looks at the cycle of temperatures through a calendar year in a certain location. The maximum and minimum daily temperatures are plotted, averaged over a number of years from the starting year to the most recent full year, 2008. Seasonal fluctuation is the most pertinent feature of the plot.
The cities in the list are the continental U.S. cities for which there are exchange-traded weather contracts. These cover a variety of temperate climates and show seasonal variations typical of the temperate regions in the northern hemisphere.
Including more dates, with the starting year farther in the past, shows the underlying cycles with less noise (year-to-year variation). But it is also worth noting that sampling over decades combines the annual cycles with longer term trends and adds a bias accordingly. For instance, a long-term cooling pattern will bias a displayed long-term average with warmer years from the past. Similarly a long-term warming pattern will bias the displayed average with cooler data from the distant past. So while longer-term calculations may be smoother by signal-to-noise it is worth being aware of this bias from multi-decade trends.