Microclimatology of Pihlajamäki City Quarter

1.Location of Pihlajamäki

Pihlajamäki is located some 7.5 kilometres northeast of the Helsinki city centre. It is a suburban area, with both densely built apartment blocks and extensive unbuilt areas. The average distance to the Baltic Sea coastline is 10 kilometres (to the south), but the Vanhankaupunginlahti bay area is only 1.5 kilometres away.
Pihlajamäki has been built on a wide rocky ridge with an altitude ranging mainly from 20 to 40 metres above sea level. Bare rocks rise here and there to a height of 45 metres. The ridge is surrounded practically on all sides by a clay plain, which is 0-5 metres amsl (above mean sea level) south of Pihlajamäki and 10-15 metres amsl on the northern side.

2. Microclimate based on straight line measurements

Straight line temperature measurements were carried out in Pihlajamäki on 1 January 2003 during a strong ground inversion (Table 1). The line of measurement ran from one of Pihlajamäki’s highest plateaus (including the peak) 700 metres away to the valley of the Vantaa River. The altitude difference between these two points is 35 metres (Figure 1, elevation profile of the route). The route traversed Pihlajamäki along two rift valleys and covered a total of 1.7 kilometres (Figure 2).
On the highest plateau (altitude 40-45 metres amsl), the temperature was -20…-21 °C, and around -21 °C also in the lower plateaus (Pihlajisto, 35 metres amsl). In the rift valleys, the temperature was only 2-3 °C lower, that is, some –23 °C. Since the deepest rift valley descends southeast towards the Vanhankaupunginlahti bay, the cold air streaming down from the rocks apparently does not remain standing in the valley.
It is not until the Vantaa River valley that the temperature drops rapidly. In Savelanpuisto, the temperature was around –29 °C, or some 9 °C lower than on the high plateaus and some 5 °C lower than in the Pihlajamäki rift valleys. The valleys in Pihlajamäki are considerably narrower than the Vantaa River valley, weakening the effective outward radiation. The Vantaa River valley is approximately 2 kilometres wide, compared to the 200 metres of the Pihlajamäki rift valleys.
In the evening of 1 January 2003, the temperature in Savelanpuisto was some 1.5 °C lower than that at the Malmi airfield, which is located on a wide swampy clay plain. Although the Malmi airfield has a very cold microclimate, the Vantaa River valley is even colder around Savela.


Figure 1; Topography of straight line measurements. Pihlajamäki on the left, the valley of River Vantaa (Savelanpuisto) on the right. The distance is about 1,7 km.

Table 1. Temperature on 1.1.2003 at 18.30 in the area between Pihlajamäki and Savelanpuisto
temperature on 1.1.2003 at 18.30


Figure 2: Straight line measurements

3. Temperature conditions in Savelanpuisto during winter inversion

A temperature logger and protective barrier were placed in Savelanpuisto in the Vantaa River valley in early January 2003. The winter’s coldest days took place on 6-7 January 2003 during moderate inversion conditions, which were not very strong, since the Vantaa airport was, uncharacteristically, colder than the lower-lying Malmi airfield. Nevertheless, the temperature in Savelanpuisto was as much as 2-3 °C lower than at the Malmi airfield (Table 2). The lowest temperature measured in Savelanpuisto, -33.7 °C, was the lowest in the entire Metropolitan area. Not even Riipilä in Vantaa, in the Vantaa River valley, was as cold (minimum –33.4 °C).

lämpötila 6.1 -7.2.2003
Figure 3: Temperature in Pihlajamaki and in Savelanpuisto between 6.1.2003 at 18 and 7.1.2003 at 18.

On 6 January 2003 Pihlajamäki was still nearly as warm as Savelanpuisto. The sky was clear, but the north wind was moderate, preventing an inversion from developing. The wind died down in the evening, which led to a rapidly strengthening ground inversion and the Vantaa River valley filling with cold air. At night and in the early morning, the valley was 6 °C colder than Pihlajamäki. However, the difference was not as big as on 1 January 2003, when the difference was 9 °C.
In the course of the morning, the wind picked up, turned south and brought with it a snow front: the inversion quickly dissipated, and by the early afternoon Pihlajamäki was some 0.3 °C colder. During good mixing conditions, a temperature difference of 0.2–0.3 °C between Pihlajamäki and Savelanpuisto is exactly what the altitude difference (27 metres) leads one to expect.

Table 2: The Minimum Temperature 7.1.2003

Pihlajamäki -27,7
Savelanpuisto -33,7
Harmaja island -23,0
Isosaari island -24,5
Kaisaniemi park -27,0
Malmi airfield -31,0
Helsinki-Vantaa airport -31,3
Vantaa, Riipilä (valley) -33,4

4. Temperature differences in Pihlajamäki, the Malmi airfield and Kaisaniemi in 2002

The Pihlajamäki observation point is on Kiilletie, on one of the highest plateaus, in a park surrounded by apartment blocks, the closest of them 25-30 metres away. The observation point was equipped with a protective barrier and temperature logger (Hobo H8 ProTemp External Logger). The probe was calibrated at the Finnish Meteorological Institute.
The Malmi airfield is located in a cold, low-lying clay plain protected by hills, which is slightly swampy in many places. During an inversion, Malmi is always cold, while Pihlajamäki is relatively warm. The mean values of Malmi and Pihlajamäki are representative of the average conditions in suburbs.

Cold nights affect the mean temperature. In the clear months (August, September, October and December 2002), the average temperature of the Malmi airfield was 0.4-1.0 °C lower than that of Pihlajamäki (Table 3). November 2002 was quite cloudy, and the average temperature in Malmi was only 0.2 °C lower. On average, Pihlajamäki was 0.6 °C warmer than Malmi, a considerable difference, that is.

Table 3: Monthly Average Temperature

Month Kaisaniemi
Malmi Pihlajamäki
June 16,0 15,8 16,2
July 19,1 18,7 19,1
August 19,4 18,4 19,4
September 11,6 10,6 11,4
Oktober 1,5 0,5 1,0
November -1,9 -2,6 -2,4
December -7,1 -8,3 -7,5

At night-time, during clear and still weather, Malmi is considerably colder, with differences of 5-8 °C common. Table 4 shows the average monthly minimum temperature. The average difference in the clear months was 2-3 °C.

Table 4: Mean Daily Minimum Temperature
Month Kaisaniemi
June 12,4 10,5 11,9
July 15,7 13,4 15,3
August 15,0 11,2 14,6
September 7,9 5,4 7,4
October -1,0 -2,8 -1,5
November -4,0 -5,1 -5,0
December -9,2 -11,1 -9,7

At times of maximum temperatures (with the exception of mid-winter), the air near the surface is well mixed and the temperature decreases with altitude. Since Pihlajamäki is 20 metres higher, the temperature ought to be 0.2 °C lower than in Malmi. In the summer, however, the maximum temperatures are nearly the same. The Malmi airfield is open, while Pihlajamäki is protected by trees and building. This probably compensates for the cooling effect of altitude.

Table 5: Mean Daily Maximum Temperature

Month Kaisaniemi
Malmi Pihlajamäki
June 19,9 20,4 20,3
July 22,6 23,0 23,1
August 23,3 24,3 24,2
Seeptember 15,5 15,9 15,7
October 4,5 3,9 3,8
November 0,2 -0,5 -0,8
December -4,2 -5,2 -5,1