eogrāfiski raksti folia geographica xii - Ģeogrāfijas un Zemes zinātņu ...

17
NATURE RESEARCH
Figure 1. The approximate boundary of the ecotone between the European boreal and nemoral zones (dashed line).
Isopleths of annual temperature ranges in Fahrenheit degrees, with Celsius equivalents in parenthesis after
R. C. Scott [1992].
2. Data and methods
To describe the division of the year into seasons and to trace the diachronous nature of
and links between the seasons, a comparative study of net radiation, air temperature,
atmospheric precipitation, air mass types and the state of vegetation was carried out, taking into
consideration the diurnal rhythms and interannual variations. While the annual course of
different air mass types is usually left beyond the scope in such kind of studies here it is
followed up. For this purpose, the observation data for a 11-year period (1990-2000) were
extracted at the Latvian Hydrometeorological Agency, and long-term means were considered,
too (Table 1).
Net radiation is defined as the difference between the gain through incoming short-wave
solar radiation and loss through reflected short-wave and emitted long-wave or terrestrial
radiation. In Latvia, actinometric observations, including net radiation measurements, are
carried out only in one station, yet the observation station site has been relocated three times.
For a long time (1948-1972) the station was located in Riga (56º58´N, 24º02´) and the longterm
means of net radiation used in this study have been calculated after the measurements
made in this site. Since 1991, after relocation from Zoseni (57º09´N, 25º54E´, Vidzeme upland),
the station is located in Zilani (56º31´N, 25º´55 E, on the Daugava). Unfortunately, a
considerable number of data gathered during the 11-year period of the study were rejected due
to low-quality and even missing measurements. As result, the authors used for this study the
data for 6 years (1986-1988, Zoseni, 1994-1996 Zilani).
Air masses were determined for the 11-year period using twice-a-day (00 UTC and
12 UTC) upper air observation data for Riga and also Liepaja (till 1992). M.Geb’s [1971]
method, which is based on climatological identification of air masses at the same time checking
them with respect to their typical heat content (pseudopotential temperature θ850) at 850 hPa
level, was applied. The method implies the analysis of frontal zones and weather systems at that
level. A distinction is made between maritime (m), transformed maritime (x) and continental (c)
air masses, considering that transformation of maritime air into x air requires at least 24 hours.
Transformation of an air mass is caused by several factors, such as surface temperature,
moisture and topography. The type of air mass trajectory, whether cyclonic or anticyclonic, also
has a bearing on its transformation.