The Urban Heat Island Effect (UHI) is an important factor in the calculation of global temperature. Big cities are the source of the UHI effect. Mostly the warmers say that they take care of it by adjustments. This is like being between the rock and the hard place because adjustments by the global temperature keepers nearly always increase the present temperature and decrease older temperatures. I don’t know how much too trust. For an example of the UHI, lets look at the temperature records from Central Park in New York City, New York and temperature records from West Point, New York.
Until about 1885 the two temperature records were closely related. After 1885 they began to separate and now New York’s temperatures measures about 2C higher than those measured at West Point. Between 1870 and 1880, the New York City population passed 1 million people. The US Census Bureau estimates the City’s 2014 population at 8.5 million. The country-side temperatures may rise but at a much lesser amount that the temperature in the big cities
West Point is 56 miles north of New York City. It is on the Hudson River while NYC is on the Atlantic Ocean. These location factors may have some effect but not much based upon the temperature record in the above chart.
The cause of the UHI results from urbanization. Professor Humlum, Professor of Physical Geography at the Department of Geosciences in University of Oslo, describes UHI as follows in the posting “Climate Reason – The Little Ice Age Thermometers study of Climatic Variability from 1660-2009”:
“The precise amount of artificial warming through proximity of buildings, vehicles, tarmac etc will vary according to whether the station remains in a large open space or becomes crowded in by development. There is clearly a limit to the UHI effect – as urbanisation grows artificial warming is likely to be spread more widely but will not necessarily be more concentrated. However, a difference of several degrees between urban and adjacent rural areas is common and the effect is particularly marked on still clear nights when the difference can be substantially greater. This renders many modern temperature records rather suspect as the correction factor (if applied) often appears to be inadequate. The end result is that the recorded temperature may read rather higher than it should do when trying to compare like for like with older less affected records.
The warming caused locally by the UHI effect is unambiguous. Some urban areas will welcome the additional warming, but to others it will be a real issue. However it appears to be one that does not attract as much attention as the much less obvious impact of co2.”
I live outside of Philadelphia. The Philly TV weather person will often say, “it will be X degrees tonight in the City and probably one or two degrees cooler in the suburbs.” The UHI effect on display.