Visualize histograms of term popularity by time. Use the Hour / Day / Month control to change the histogram granularity before searching.
To scale by the number of tweets available for each block of time, check "Normalize by total number of tweets". Each column will then represent the number of tweets matching the query in that time block, divided by the number of tweets in that time block.
The tweets used to obtain these lists are those which can be determined to be within a particular time zone, using the user's meta-information. See the full paper for details.