> Most cities grow and develop over time and the parts of cities with more buildings and especially with more cement will radiate more heat. If an observation station was built somewhere less developed then city growth might artificially push up temperatures around the station. This effect might be strongest somewhere like Phoenix or LA.
The temperature record - at least, what's used for generating long-term trends - is homogenized to remove discontinuities (step-wise or linear) associated with this sort of drift or other significant changes. Regardless, this effect likely wouldn't matter for the stats the website illustrates.
> This one seems unlikely (well, I'm sure it happens but it's very likely to have already been corrected for) but aggregation standards might have changed over time. I could imagine in the 30s someone might check the thermometer every hour and record the current temperature in a logbook. Modern thermometers are almost certainly automated. If the "highest temperature" is measured by recording the temperature each minute and taking the max then you're going to get a higher number than the manual method.
When manual reading of the temperatures was the norm, you'd have a mercury and an alcohol thermometer to record the daily max and min, respectively. For a mercury thermometer, it would have a stopper that rises and records the maximum level the liquid reaches. A human needs to invert the thermometer to reset it. Same for an alcohol thermometer, albeit in reverse (records the minimum). So granularity of observation doesn't matter - the daily max, historically, is truly the maximum temperature taken since the last time a human reset the stopper.
> site selection might have changed: observation posts probably used to be buildings which needed to be large enough to fit equipment and the people to operate them. City real-estate is expensive and meteorology used to be a cost-center so these buildings were probably placed at the edge of town. I'm sure modern stations are much smaller, they don't need a full-time staff to operate them, so they can be placed closer to the (hotter) centers of cities. Alternatively: I'm sure many weather stations are now situated at airports, an option which was not available in the 30s, and airports are particularly pavement-heavy (hotter) parts of cities.
Also accounted for by homogenization. In most cases when siting changes, you consider that site a new station.