Analysis: Hot in the city

As the northern hemisphere approaches summer, we explore land surface temperatures that are featured in ClimateBits: Urban Heat Islands.

Note that these examples are intended for curious people looking for hands-on Earth data exploration. Primary scientific research will require additional analyses through other methods. For the basics on how to use the NEO tool, see ‘Analysis tool in 10 easy steps’.

Urban Heat Islands are places on land where buildings, roads, and other impervious surfaces trap more heat than the surrounding rural area. During summer, an urban place like New York City can be 4°C (7°F) or more warmer than surrounding rural areas. Vegetation plays a cooling role through transpiration. Cities such as Minneapolis, Chicago and St. Louis — where most trees were cleared to make way for pavement and development — are urban heat islands surrounded by cooler forests.

Demonstrate seasonal changes

Load March, June and September, 2016 for land surface temperature [day]. These are found under the ‘Land’ category. Note the difference between ‘land surface temperature’ and ‘average land surface temperature’ data sets, the latter being climatology. We use the former in this example. These are MODIS/Terra observations collected since February, 2000 at daily, 8 day and monthly temporal resolution. Here we compare [day] temperatures.

The warmest land is colored yellow; coolest land is colored light blue. Hottest places are in the tropics and during summer in areas where the land is driest. Coldest places are covered in snow and ice. Black areas are missing data — over the ocean or due to cloud cover or lack of sunlight. The values along the white transect on the large map are plotted for March (red), June (green), September (blue).

The white line drawn from south of Lake Michigan east to New York City shows that the transect was about 10°C cooler in March compared to June and September in 2016. As the month of maximum sunlight, June would be expected to be warmest, yet September temperatures were not much cooler due to the thermal inertia of the land.

Compare day/night seasonal changes

Now load March, June and September, 2016 for land surface temperature [night]. Night temperatures are also coldest for places covered in snow and ice, but have important differences from daytime temperatures for warm areas.

The same line drawn from south of Lake Michigan east to New York City corresponds to the plot of nighttime temperatures for March (red), June (green), September (blue). September temperatures were again very close to those in June, especially for the urban areas at either end of the transect (near Chicago and New York City).

Compare urban and rural day/night temperatures

Looking at a weekly map from the end of June, we can compare day and night temperatures with a focus on urban versus rural New York.

Land surface temperature [day] in red and [night] in green for the week of June 26-July 4, 2016. Histograms show temperature distributions around urban New York City (left) compared to rural upstate New York (right).

The first thing to notice is the higher daytime temperatures around New York City (maximum 37°C) compared to upstate New York (maximum 28°C). Second, are the higher nighttime temperatures around New York City (most of values are much greater than 19°C) compared to upstate New York (most of the values are less than 19°C). Notice especially that there is more overlap between daytime and nighttime temperature distributions for New York City. This is the urban heat island effect.

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