No significant trend has been found over the equatorial region. Figure 2 shows the total ozone data for four central U.S. stations. The values in the years since 1993 have not continued to decline in recent years as they had declined from 1979 to 1993. The implication of these trends and their changes needs to be examined in the context of changes in the ozone depleting gases and varying meteorological conditions. A map of Northern Hemisphere monthly mean total ozone amounts for March 1999 is shown in Figure 4 .
Lowest values appear in the tropical region.
The 1979 to 1986 base period is chosen because these values are indicative of the early data record. Most notable is the absence of large negative anomalies over the Arctic region. In fact, positive anomalies of up to 15 percent are shown over the Arctic, from Greenland to northern Europe. Negative anomalies are evident in a band at sub-tropical to high latitudes, peaking at up to 16 percent over the mid Pacific. Small positive anomalies appear over the tropical region.