1.1. Air temperatures
1. Hourly air temperatures
2. Hourly urban-rural temperature differences (Δt)
3. Typical UHI intensity cycles
4. UHI maps
- Hourly air temperatures
On an annual average, cities are warmer than rural areas and this can go up to several degrees. For a city like Strasbourg Eurometropole (France), the city center is 1 to 2 degrees warmer than the countryside. The temperature differences between the city and the countryside are especially strongest on sunny days, when intense solar radiation is stored in the urban fabric. The heat accumulated in town during the day is released and trapped into the urban canyons at night. This is the well-known nighttime urban heat island (UHI). Urban heat island is essentially a nocturnal phenomenon since the air temperature is the same between the city and the countryside during the day (Figure 3).
2. Hourly urban-rural temperature differences (Δt)
The urban heat island intensifies over time as heat from solar radiation is stored in the urban fabric (accumulation effect, Figure 4). This is clearly visible at the start of this winter period of 11 sunny days: the heat island is only +2 ° the first night (february 17-18), but it reaches +6.5 ° at the end (february 26-27, red circle) before the weather changes really (march 1). During daytime of the whole period, the heat island disappears systematically around midday and the town becomes a urban cool island (green circle). Urban cool islands appear preferably during seasons when the sun is low over the horizon and on cool days. The air over rural areas can warm faster during the morning than air enclosed in the shaded urban canyons.
3. Typical UHI intensity cycles
The intensity of the urban heat island depends mainly on the amount of solar radiation that the urban fabric has absorbed. It is therefore a phenomenon whose intensity varies according to daily weather conditions (insolation), but also according to the seasons (Figure 5a and 5b). The urban heat island is strongest during the summer period (JJA), when solar radiation is most intense, and decreases during the intermediate periods: spring (MAM) and autumn (SON). It reaches the minimum intensity in winter (DJF). The cool island is systematically present during the day, in autumn, spring and winter.
X axis: normalized time (0 = midnight; 0.25 = sunrise; 0.5 = solar noon; 0.75 = sunset).
Y axis: temperature difference between town and country (in ° C).
4. UHI maps
It is possible to obtain maps of heat island. Several techniques can be used (satellite images, observation networks, simulations). Nocturnal satellite infrared images can be exploited to map UHI. However, this technique suffers of several disadvantages. Among these: 1) emissivity of the surface elements must be known to obtain realistic surface temperatures; 2) surface temperatures are sometimes significantly different from air temperatures...
Maps of air temperatures can be obtained with temperatures measured by networks. For that it is necessary to use sophisticated geostatistical processing techniques which allow information of different kinds to be crossed (i.e. air temperatures and land use). Figure 6 is an example of heat island map obtained with such techniques. It shows that there is a brutal transition (cliff) between the countryside and the city. It shows also that the urban heat island is not homogeneous in the city: fresh zones (basins) appears in places where vegetation is dense (parks...).
