Observations

Unit: D.U. (Dobson Unit)

Year\Month	Jan	Feb	Mar	Apr	May	Jun	Jul	Aug	Sep	Oct	Nov	Dec	Mean
1991	_	255	268	284	299	288	279	275	261	265	249	247	**
1992	_	238	248	289	303	291	284	286	275	262	237	227	**
1993	218	236	241	270	283	275	269	279	267	265	235	233	256
1994	252	250	268	287	285	_	_	_	_	_	_	_	**
1995	230	248	272	271	287	277	278	267	271	265	241	236	262
1996	236	253	261	282	279	271	263	260	256	248	230	220	255
1997	227	220	255	275	279	283	274	266	269	262	251	246	259
1998	250	270	260	290	292	280	279	280	276	266	259	241	270
1999	232	243	253	273	287	274	274	272	272	266	254	239	262
2000	246	255	264	274	290	281	281	285	279	260	244	250	267
2001	_	_	_	288	282	276	266	266	260	247	221	205	**
2002	232	229	246	286	288	297	286	282	288	281	264	252	269
2003	246	259	273	285	294	298	283	266	270	264	239	229	267
2004	239	245	257	279	285	287	282	276	274	272	252	245	266
2005	249	240	269	274	280	275	272	264	264	258	233	231	259
2006	233	240	252	263	281	277	280	273	274	265	251	234	260
2007	234	257	248	294	282	274	276	281	275	264	262	247	266
2008	240	242	260	271	281	281	284	282	279	264	252	237	264
2009	243	239	263	292	290	284	279	279	275	265	249	253	268
2010	249	252	267	284	296	286	284	279	274	264	249	243	269
2011	245	256	263	295	284	284	278	278	278	268	259	242	269
2012	236	247	259	294
Mean	239	246	259	282	287	282	277	275	272	264	247	238	264
Max	252	270	273	295	303	298	286	286	288	281	264	253	270
Min	218	220	241	263	279	271	263	260	256	247	221	205	255

Notes :

"-" indicates no data; "**" indicates annual average is not calculated due to some monthly data missing.
Total ozone is represented with a special unit of measurement. If all the ozone in a column over an area was to be compressed and spread out evenly, it would form an ozone layer. The thickness of ozone layer measured under standard temperature and pressure with 0.001cm is defined to be 1 Dobson Unit (DU).