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thickness of the ozone layer decreased in the Northern hemisphere
in 1993 after a volcanic eruption in the Philippines (1991), and
again in 1997 and 1998 for other reasons. In 1998, there was an
improvement in the stratospheric ozone concentration, but to a
level that was still less than that of the 1980's.
An "ozone hole" occurs when the amount of ozone falls
below 33% of the normal level. A hazardous ozone hole (46% of the
normal level) has been recorded only once in Latvia, on 28 January
1992. The thickest ozone layer over Latvia was observed on 11
March, 1979.
The current ozone amount can not be considered to be sufficient,
and stabilization to a normal level is expected in the second half
of this century.
The ozone concentration in the atmosphere varies seasonally,
being highest in spring (March to May) and lowest in autumn (September
to November). Depletion of the ozone layer is most intensive in
spring and winter.
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