Simple........... population growth.  The weather stations were originally on city outskirts.  Close enough to the city so that some guy could walk out twice a day and check the temp and rain level  Since 1960, global population has doubled. Now more than 90% of all weather stations are located within city limits and are therefore surrounded by concrete, roads and lighting which has steadily raised the temps near the weather stations.   Ninety percent of all global weather stations are now putting out corrupted and elevated temperature readings.  
Just search on UHI  effect. 

Following is a clear demonstration of this amplification. I think it is pretty indisputable evidence that the Urban Heat Island effect is quite substantial.

This is a very simple experiment. The intent of this experiment is to see how much the recorded temperature varies compared to the urban recordings. To accomplish this, a temperature recording device is mounted on a car which drives from a rural area into urban Oslo Norway and then back into a rural area, all the while recording the outside temperature. The total drive time is less than an hour.

Notice how much the temperature increases when the car enters urban Oslo.

The car made 2 runs and the paths taken are shown on the map.
This is JanuaryThis is August“”””””””””””””””””””
Hohenpeissenberg, Germany since 1791
Following is another temperature record which demonstrates that we are being deceived by politicians claiming that we are on the brink of a global climate catastrophe. Following is a temperature graph of data collected in Hohenpeissenberg, Germany since 1791. If this graph was viewed from 1880 to 2008, it would appear as though the warming was a substantial 2.5degC. This is the reason that climate liars always start their graphs at 1880. But if you look back to the period around 1830 vs 2008, as you can see, there is NO warming. FYI, there was a climate event known as the little ice age-LIA, which existed from about 1400 to 1880 during which the earth cooled by several degrees. As a result of this cooling, Vikings who had settled Greenland around 1000 AD had to abandon it in 1400 because they could not raise enough feed for their cattle.