If you take the averaging period down to 1 (box in the lower left), then the center line is the average temperature for each day. The top of the shaded region is the high for that day. If you zoom in on, say, 2008-05-15, you can see that the high was over 100 degrees that day.

In general, though, temperatures in SF rarely go above 80. When they do, it's newsworthy.

Ya I thought the same thing.

Using a 14-day moving average makes it clearer. A 100-day rolling period averages out nearly all the specifics from the data.

You can change the moving average - it's in the bottom left corner of the graph.

Periods of hot weather in SF typically last 3 days.

It's entirely feasible that it didn't go above 70 in SF for a year. SF is not LA.

The historical data presented in the graph shows more variance.