That's it. Pretty simple way to see a temperature trend.

Data







I used data from the CRU TS dataset . I simply averaged January 1911, January 1912, etc. to get a January normal, did this for each month, then subtracted it from each month/year combination to see if that month/year combination was hotter or colder than the average. The total range here is 1911 to 2016.

To read that, simply interpret 'dark' as 'colder' and 'orange' as 'hotter'. Looking at the Mexico City plot for example, you get a clear trend towards warmer temperatures in the last 50 years than in the 50 years before that. I ran this for a number of cities, and the results are below...