Baltic Sea Ice Climate Workshop 2005 
32 
was then 30 cm at Umea (Fig. 3). The Bay of Bothnia and Northern Quark were all ice-covered, 
but in the northern Sea of Bothnia there was a wide lead. 
Fig. 3. The ice situation in the Northern Quark on 31 March 2005 according to the Swedish 
Meteorological Institute (http://www.smhi.se/). 
At the site the ice was bare, and the thickness was 25 cm (Fig. 4). There was a thin snow-ice 
crust on top, and then 7 cm of clear congelation ice, and the lower part was porous congelation 
ice appearing opaque due to light scattering in the voids after the sample had been raised into 
the surface. The temperature of the inner ice was -0.2 - -0.4°C, approaching zero at the 
boundaries. 
Fig. 5 shows the downwelling irradiance at ice surface and at the depths of 10 and 15 cm 
beneath the surface. The data are recorded in quanta irradiance/m 2 s, which transforms into 
W/m 2 by division with 4.6 (Arst, 2003); consequently, the maximum daytime irradiance was 
about 500 W/m 2 (under clear sky conditions). The albedo was according to our measurements 
0.3. Reducing the top irradiance by the influence of the albedo, it is seen that the irradiance 
level dropped by the factor of 0.25 in the first 10 cm and by the factor of 0.40 in the next 5 cm. 
Therefore, the resulting light penetration depths were 35 cm for the clear congelation ice and 
10 cm in the porous ice (the penetration depth is here defined as the distance across which the 
irradiance level drops to 1/e ~ 37% from the start level). In comparison, the light penetration 
depth in Santala Bay, Gulf of Finland, has been measured as 20-25 cm (Lepparanta et al., 
2003).