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

Pre-spring (PSP): second half of February to end of March;
Early spring (ESP): end of March to middle of April;
Full spring (FSP): middle of April to first half of May;
Pre-summer (PS): first half of May to beginning of June;
Early summer (ES): beginning of June to end of June;
High summer (HS): end of June to end of July;
Late summer (LS): end of July to end of August;
Early autumn (EA): end of August to second half of September;
Full autumn (FA): second half of September to end of October;
Pre-winter (PW): end of October to end of November.
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NATURE RESEARCH
The criteria of such division were pointed at and justified in the preceding discussion and
figures. However, to complete this part of work, it might be appropriate to emphasize some
conclusions which yet need to be studied in greater detail. They are as follows: (1) each season
may be distinguished by a definite pattern of solar radiation, distinctive state of heat and water
balance, biological turnover, phenological state of vegetation; (2) during each season these
variables show a particular combination on numerical values and a distinctive landscape image;
(3) every season has an effect on the state of landscape in the following seasons (theoretically –
throughout the year); (4) transitions between the seasons are both gradual and sharp, but in all
cases these may be clearly identified by several quantitative and qualitative features.
From this chapter it also follows that differences between seasons may be related to air
mass occurrence (Figure 6). Since air mass types and their frequency is a factor which has been
left beyond the scope in landscaper research, two following chapters are devoted to this item.
4. Air mass occurrence
4.1. Air mass types
Latvia’s atmosphere in the context of air masses is related to other parts of Europe and
the northern hemisphere in general, and experiences day-to-day, as well as seasonal, changes.
Following M.Geb’s (1981) classification, air masses were ranked according to their heat
content. In addition to the determined backward trajectory, the typical 850 hPa temperature and
pseudopotential temperature were considered (Figures 7, 8). At the same time the air masses
arriving in Latvia were described in relation to their source region and some typical weather
conditions [Draveniece 2003].
4.2. Seasonal changes of air masses
Based on the 850 hPa temperature and pseudopotential temperature, the monthly mean
values for a 11-year (1990-2000) period were found. These might describe the average or
theoretical air mass in Latvia.
From October to March, the average air mass was transformed maritime subpolar air xP.
However, the average October air mass could hardly be distinguished from mP air. Regarding
the heat content, the corresponding temperatures take up an intermediate position between the
xP and mP air, so the decisive role was attached to moisture content. This finding fits in with
the mapping of European air mass source regions in winter drawn by M.Geb (1981). The xP is a
relatively temperate air in winter months and usually produces a sheet-like or layered (stratus)
cloud cover the entire sky. It commonly occurs when warm, moist mP air moves over cold
ground surface during winter. Stratus clouds produce little precipitation, usually in the form of
drizzle. Occasionally, xP may produce partly cloudy or fair weather, depending on the degree of
transformation.