It is definitely a blip. The reason it doesn't appear in all the zoom levels is that the four-hourly zoom level takes the average temperature of the hourly data

It is definitely a blip. The reason it doesn't appear in all the zoom levels is that the four-hourly zoom level takes the average temperature of the hourly data over the four-hour period. Once you go up to the daily zoom level we switch to showing the high and the low, and you can see the blip again. That historical high is due to that point. Actually, I'm just sitting down today to start the process of writing quality control filters to get rid of this sort of blip. So hopefully it will be gone soon! :)