# Maximum Air Temperature Decrease Over Time

Based on the maximum air temperature Time series plot plot below. What could be a possible reason(s) for Maximum Temperature to decrease over time, even though minimum and mean temperatures are increasing? And actually, I only this behavior in the some months not all of them which is also interesting.

Someone did tell me that Maximum temperature tends to show this behavior in a timeseries, but didn't tell me why.

The area is a coastal region in North America and the data was collected from the PRISM model.

• What is the source of the graph?
– Fred
Jul 21 at 17:22
• @Fred, R linear model regression. Jul 21 at 17:32
• Do you see this trend over larger datasets... 40 years in one specific month? One could say it has more data in it than 41 years as it is separate locations, but it's clear from your graph that values for the year have limited independence from each other? A regression slope on such a limited period will tend to be driven heavily by one or two outliers (as it looks like 1986, 2012, and even 2021 a bit were?) Jul 21 at 18:14
• @JeopardyTempest, I did mention that I got the data from the PRISM model. Jul 21 at 18:15
• I've never used R... but believe the linear regression statistics (R^2, P values) may be useful in analyzing the strength of the regression and important to someone's answer... looks like this site has input on how to get those things. Jul 21 at 18:23