Status and Trends of United States (US) Temperatures Since 1895 (June)
This is for June, compared to the same perods in each year. It is not year-to-date compared to other whole years, but rather for June compared to prior Junes since records began. June is shown because it is the month with most intense sunshine and the longest day.

The temperature trend for the period of record (1895 to present) is 0.11 degrees Fahrenheit per decade. 
(Source: NOAA's National Climate Data Center)



Status and Trends of United States (US) Temperatures Since 1895 (December)
This is for December, compared to the same perods in each year. It is not year-to-date compared to other whole years, but rather for December compared to prior Decembers since records began. December is shown because it is the month with least intense sunshine and the shortest day.

The temperature trend for the period of record (1895 to present) for December is 0.18 degrees Fahrenheit per decade. 
(Source: NOAA's National Climate Data Center)


Status and Trends of United States Temperatures Since
  1895

Analysis: The US temperature has retreated to near the 20th century average after a record or near record high for the prior month. Recent temperatures are not very different from the 1930s (time of the US western "dust bowl" years. If the advent of wide spread use of thermometers in the late 1800s had not coincided with the end of the last Little Ice, would this rise bbe held in a different context?

The projected temperature rise by IPCC is unrealistic, given that the USA and global temperatures have risen by only 1 deg F (.7 C) in 100 years or 150 years using the full instrumented data set during the height of industrial expansion. Even if all this rise is correct, and is attributable to human causes, it is a small amount in the natural variation of the Earth, and to suggest the rise would accelerate 5 fold (IPCC best estimate) in this century based on a gas that is 0.0004 of the atmosphere is hard to accept. Even after the release of the new data set and procedures by NOAA, which addressed some of the urban heat island issues and dropped the warming 44% (below IPCC 2007), significant other urban heat island issues still remain. There are also issues of calibration as measurement protocols have changed, issues about the design and placement of the temperature stations, and even the strongly held view by many skeptics that this is a natural rise as the Earth recovers from the Little Ice Age (circa 1500-1900) and even the last glaciation (11.7K years ago.)

If the city where you live has a higher temperature than its suburbs, you can imagine the impact of growth around the world on land-based temperatures. The USA has fixed many of these problems. This is likely why the global temperatures rise while those of the USA are somewhat more "normal", but the USA has also adjusted its data sets in ways that raise many questions and suspicions among scientists that are not part of the "consensus" group. Adjustment of at-sea and on-land and atmospheric data is legitimate and necessary as technologies evolve (e.g., a bucket of water on a sailing ship in the 1800s is warmer on a hot day than the water intake on a modern ship which is sampling cooler water 5-15 meters below the surface. The adjustment is difficult and original, unadjusted data must be preserved and kept available for possible future use. When results change, as when a series of warm or cold years disappear in the adjusted data, careful explanation and further review is necessary.

The table below summarises some of the differences in various weather elements in urban areas compared with rural locations (Source: British Met Office).

Sunshine duration 5 to 15% less
Annual mean temperature 0.5-1.0 °C higher

Winter maximum temperatures
1 to 2 °C higher
Occurrence of frosts 2 to 3 weeks fewer
Relative humidity in winter 2% lower

Relative humidity in summer
8 to 10% lower


Total precipitation
5 to 10% more
Number of rain days 10% more
Number of days with snow 14% fewer
Cloud cover 5 to 10% more
Occurrence of fog in winter 100% more

Amount of condensation nuclei
10 times more

 

The formation of a heat island is the result of the interaction of the following factors:

The precise nature of the heat island varies from urban area to urban area, and it depends on the presence of large areas of open space, rivers, the distribution of industries and the density and height of buildings. In general, the temperatures are highest in the central areas and gradually decline towards the suburbs. In some cities, a temperature cliff occurs on the edge of town. This can be clearly seen on the heat profile below for Chester.

Urban Heat Island at Chester, England

Urban heat island in Chester

 

The Urban Heat Island (UHI) describes the increased temperature of urban air compared to the rural surroundings. The term ‘heat island’ is used because warmer city air lies in a ‘sea’ of cooler rural air.

The figure below shows a stylised heat island profile for a city, showing temperatures rising from the rural fringe and peaking in the city centre. The profile also demonstrates how temperatures can vary across a city depending on the nature of the land cover, such that urban parks and lakes are cooler than adjacent areas covered by buildings.

Urban Heat Island Sketch

Sketch of an urban heat island

Source of chart and text: The Met Office

The higher urban temperatures are caused by the increased capacity of the urban land surface (eg. roads, buildings, pavements) to absorb and trap heat.

This results in towns and cities remaining noticeably hotter than the surrounding countryside, particularly at night on calm, clear summer nights. The UHI can add 5-6°C to the nighttime temperatures experienced. During the summer heatwave of 2003, differences of up to 9°C between city and rural temperatures were measured in London.


 

 




This page updated or reviewed in February 2018