An urban heat island is a metropolitan area with an average temperature higher than that of the surrounding countryside. Each city has a typical heat island, the characteristics of which depend on, for example, the large-scale weather conditions and the urban building density, as well as the distribution of waters and green areas. The urban heat island phenomenon and the proximity of the coastline add further complexity to the mesoscale climate of Helsinki.
The data obtained from the Helsinki Testbed project reveal the mesoscale differences in climatic conditions in the capital. A number of Vaisala WXT510 weather transmitters have been placed in various places around the city. In addition to temperature and humidity, the devices measure wind speed and direction, barometric pressure and liquid precipitation. The goal has been to build a geographically comprehensive city-centre network of transmitters, which have been placed to specifically facilitate research on the urban heat island phenomenon.
The location of observation stations must be planned carefully, since the distance from the sea greatly influences the temperature of coastal cities such as Helsinki. Topography is another key factor. Both distance and topography have a notably larger impact on temperature than the heat island phenomenon, which is thus easily obscured by the first two. For the weather transmitters to truthfully portray the strength of the urban heat island, they must be topographically representative and lie at an equal distance from the coastline.
July of 2006 was hot, but coastal Helsinki was cooled by the cold sea. The average temperature in Harmaja was 1.9° C lower than in Kaisaniemi, and the temperature at sea did not exceed 25° C on any day of the month. The chilly sea was clearly felt on the shores of Kaivopuisto, which was the only downtown neighbourhood with an average July temperature lower than that in Kaisaniemi. Moreover, the number of days in which the temperature exceeded 25° C in Kaivopuisto was half that recorded in Kaisaniemi.
The city centre was warm: the Sörnäinen and Mannerheimintie transmitters logged Finland’s highest average July temperature that year: 19.9° C, exactly one degree warmer than Kaisaniemi. The difference to that measured in Kaisaniemi was exactly one degree Celsius. The sea has a cooling effect on the southernmost parts of the city centre. Bulevardi, for example, is not quite as warm as downtown areas farther north. An interesting observation is that the temperature difference logged by the Mannerheimintie and Kaisaniemi weather transmitters, located equally far from the sea, exactly matched the difference between the towns of Lahti and Jyväskylä. In terms of temperature, a distance of 190 km (between Lahti and Jyväskylä) translates to a few hundred metres in the centre of Helsinki!
However, Kaisaniemi is not the only cool haven in the hot city centre: Hietalahti, a green area of the same size, is another one. The microclimates of the two resemble one another, the only major difference arising in the winter, when inversions are more common. The lower-lying of the two, Kaisaniemi, is colder in the winter – and most likely the coldest park in downtown Helsinki.
The physical conditions of the ground have a large impact on the emergence of urban heat islands through short- and long-wave solar radiation. Urban buildings, and fabrics in general, usually have good thermal capacity. The ground and buildings absorb considerable amounts of heat from solar radiation on sunny days. As a result, cities are usually cooler than rural areas throughout the morning until noon. In the countryside, surface materials have lower thermal conductivity, which is why a larger share of the heat energy absorbed in the ground goes to heating the air layers closest to the ground. This can be clearly seen in Figure 2, which depicts the temperature change of a clear solar day in July. Although the Hietaniemi park is several degrees colder at night than the area around the Mannerheimintie weather transmitter, the temperature difference begins to level out early in the morning, and for a while before noon, the park is warmer. As illustrated in the figure, there is hardly any inversion in the built-up urban areas, where the night-time temperature is nearly the same at ground level as at the top of the Olympic Stadium tower, 72 metres from ground level.
Since the albedo (reflective capacity) of the ground as well as the wind speed are usually lower in urban areas, at some point of the day the city temperature rises to equal or exceed that of rural areas. The maximum temperature is reached later in the city than in the countryside. Rural areas cool quickly in the evening, whereas the heat absorbed in the stone material in cities reduces the net radiation of surfaces, and the temperature decreases slowly.
The warmth of the downtown area is also apparent in the data for February 2007, when the sea was iced over. The highest temperatures were measured in the very city centre, around Kaivopiha on Mannerheimintie. The difference in average temperature compared to Kaisaniemi equals 1.4° C, which exactly matches the difference between the towns of Lahti and Jyväskylä, as before. Even the Harmaja lighthouse island, far off the coast, is over one degree colder than the city centre.
In the dead of winter, built-up areas are warmer than large parks throughout the day. Solar radiation has a minor impact in the winter, giving a bigger role to other factors, such as the direct heating effect of traffic and other human activities, and the physical conditions of the ground. In the parks, the snow functions as a strong insulator, while the streets are characterised by a heat flux from the ground into the air. As the spring advances, solar radiation takes on a bigger role, since bare buildings and streets have a considerably lower albedo than snowy surfaces and can thus absorb heat energy (Figure 3). Moreover, there is no inversion at ground level on Mannerheimintie, where the temperature nearly equals that measured at the top of the Olympic Stadium tower.
The Helsinki urban heat island has made Kaisaniemi seem a warm weather observation station – which it is, on a larger scale. For example, it has one of Finland’s highest effective temperature sums in the growing season. On a smaller scale, however, Kaisaniemi stands out as a very cold place. The city streets, where people move, are considerably warmer.
The Testbed observations can also be used to estimate the intensity of the Helsinki heat island by comparing city-centre data to those of a topographically comparable station located outside the centre at an equal distance from the sea. The station in the Roihupelto green area of the Radiation and Nuclear Safety Authority Finland constitutes a station comparable to that of Sörnäinen. In geographical terms, it represents the temperature conditions of sparsely built suburban areas. No observations could be made in Roihupelto in the summer months because the station was out of service, but the average temperature in the winter was 1.1° C lower than that of Sörnäinen. Since the urban heat island phenomenon is not quite as strong in Sörnäinen as the city centre, and since Roihupelto is not fully unbuilt, it is safe to assume that the temperature difference was larger than the 1.1° C observed.
The heat island phenomenon can also be studied by setting up an observation station, which corresponds to the geographical conditions of the centre of Helsinki, outside the city. Such a station is located in Santahamina, which is of a similar form and at a similar distance from the coast as the city centre. The place of measurement was chosen to represent the average elevation of Santahamina to ensure that any temperature difference between it and the city centre was caused purely by the heat island phenomenon, not by the topography or distance to the sea. Based on the results, the average temperature in 2012 of sparsely built areas outside the city centre was approximately 0.65° C lower than that of the large parks downtown (Kaisaniemi) and 1.1° C lower than that of built-up areas in the centre (Central Railway Station). This closely corresponds to the results obtained in, for example, the Helsinki Testbed project.