You can see seasonal trends in a lot of the data - especially
the temperature data.  For example, to see some strong seasonal
trends, click on locations in the US in the following
map.

Then, in the time series plot it shows you, scroll down to 
the second plot - this is a box plot, with the data grouped
by month.  You can see definite seasonal trends in warm, 
versus cool months.

If you select a location in the extreme southern end of the map,
the seasonal temperature trend will, of course, be the reverse
of the seasonal trend in the northern hemisphere. (ie, in the 
southern hemisphere, January is the warm month :)