Fifth Meridian Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Fifth Meridian Observatory clear sky chart from 2005-11-06 to 2024-01-15.

This page is a summary of past forecasts. Even though it's based only on forecasts, some people use these pages as clue to a site's climate. But climate data based on actual observation should be preferred (if you can find it!).

Cloud, Transparency and Seeing forecasts are summarized individually. But because good observing conditions require a combination of minimum cloud cover, transparency and seeing, the individual forecast summaries can be difficult to interpret. The observing conditions section counts the combinations of conditions needed for common types of astronomical observation.

Observing Conditions

The following tables count the occurrence of the combinations of forecast and darkness conditions needed for different observations. Conditions are rated by by what percentage of the hours the observation would likely be attempted are usable.
Observation TypeMinimum conditions for hour to be usableOut of hours
Clear of CloudsTransparencySeeingSky Brightness
Planetary:50%(no limit)Average(no limit)Nighttime Hours
Deep Sky:70%Average(no limit)5.9Nighttime Hours
CCD Imaging:70%Below AveragePoor(no limit)Nighttime Hours

Percentages are of nighttime hours per month, or per year, when a particular observing conditions was met. Deep Sky percentages never exceed 50% because the moon is up half the time. Deep sky percentages are much lower for charts at high latitudes because of fewer dark hours in the summer.

Overall

PlanetaryData
Pts
Deep Sky
Max 60%
Data
Pts
Both Planetary and Deep Sky
Max 60%
Data
Pts
Either Planetary or Deep SkyData
Pts
CCD ImagingData
Pts
Night
35%
23457
16%
74694
9%
23416
44%
23416
24%
46457

By Month

MonthDays of MonthPlanetaryData
Pts
Deep Sky
Max 60%
Data
Pts
Both Planetary and Deep Sky
Max 60%
Data
Pts
Either Planetary or Deep SkyData
Pts
CCD ImagingData
Pts
January1st to 10thNight
27%
950
15%
2836
6%
946
35%
946
19%
1927
January11th to 20thNight
28%
883
19%
2727
9%
883
37%
883
22%
1765
January21st to endNight
27%
792
22%
2772
10%
792
44%
792
21%
1684
February1st to 10thNight
30%
720
15%
2514
10%
720
40%
720
17%
1628
February11th to 20thNight
28%
704
15%
2273
8%
700
38%
700
18%
1539
February21st to endNight
32%
592
18%
1801
10%
592
44%
592
18%
1250
March1st to 10thNight
28%
720
16%
2160
7%
720
37%
720
20%
1475
March11th to 20thNight
30%
720
21%
2003
8%
720
41%
720
24%
1328
March21st to endNight
31%
792
13%
2149
4%
792
37%
792
22%
1510
April1st to 10thNight
30%
679
16%
1759
4%
679
37%
679
22%
1227
April11th to 20thNight
29%
533
15%
1601
8%
533
35%
533
19%
1128
April21st to endNight
30%
540
13%
1620
8%
540
36%
540
20%
1088
May1st to 10thNight
38%
478
10%
1545
7%
478
43%
478
19%
1016
May11th to 20thNight
45%
354
15%
1239
16%
354
50%
354
26%
738
May21st to endNight
40%
388
7%
1358
5%
388
43%
388
19%
912
June1st to 10thNight
37%
360
4%
1260
1%
360
39%
360
22%
789
June11th to 20thNight
35%
360
3%
1260
0%
360
36%
360
19%
834
June21st to endNight
55%
360
3%
1260
0%
360
55%
360
30%
693
July1st to 10thNight
60%
350
5%
1225
1%
350
62%
350
32%
665
July11th to 20thNight
53%
348
10%
1218
7%
348
57%
348
34%
644
July21st to endNight
62%
394
18%
1391
19%
372
71%
372
42%
624
August1st to 10thNight
60%
358
17%
1424
21%
356
69%
356
29%
733
August11th to 20thNight
60%
521
20%
1589
17%
521
64%
521
41%
853
August21st to endNight
50%
594
25%
1782
17%
594
56%
594
40%
957
September1st to 10thNight
49%
641
21%
1739
11%
641
54%
641
39%
1022
September11th to 20thNight
48%
720
21%
1980
12%
720
54%
720
37%
1192
September21st to endNight
41%
720
23%
1980
11%
720
49%
720
34%
1185
October1st to 10thNight
44%
720
21%
2051
12%
720
53%
720
29%
1300
October11th to 20thNight
36%
720
21%
2286
10%
720
49%
720
28%
1350
October21st to endNight
30%
792
20%
2574
9%
792
43%
792
23%
1608
November1st to 10thNight
26%
820
18%
2485
7%
820
37%
820
20%
1659
November11th to 20thNight
24%
950
22%
2702
7%
950
38%
950
26%
1695
November21st to endNight
27%
950
17%
2850
6%
950
38%
950
25%
1768
December1st to 10thNight
32%
950
18%
2850
9%
950
40%
950
23%
1865
December11th to 20thNight
27%
950
15%
3475
7%
949
37%
949
19%
2194
December21st to endNight
29%
1034
12%
4956
7%
1026
38%
1026
15%
2612

Individual Forecasts

The length of each colored bar in the "occurrence of each Forecast" column is proportional to the number of times the chart forecast that color. The percentages are of the total number of hours per month, or per year, with a particular forecast. The colors as the same as those used in clear sky charts.

Note: Transparency probabilities are only counted for those hours that have 30% or less cloud cover. Seeing probabilities are only counted for those hours that have 80% or less cloud cover. If that's too tricky, try Observing Conditions instead.

Overall

Occurrence of each ForecastData
Points
Cloud CoverDay
9%8%36%6%35%5%6%8%39%
74631
Night
7%8%47%6%35%5%6%8%41%
75153
TransparencyDay
41%6%16%23%12%2
74318
Night
39%6%15%24%13%3
74984
SeeingDay
14%15%32%26%11%3
14185
Night
11%11%35%30%12%2
23469

By Month

Percentage Cloud Cover NW (2006-)
Color Meanings:
Overcast 90% covered 80% covered 70% covered 60% covered 50% covered 40% covered 30% covered 20% covered 10% covered Clear
MonthDays in MonthOccurrence of each ForecastData
Points
January1st to 10thDay
38%5%8%8%45%46%9%40%
1329
11th to 20thDay
28%36%6%46%5%8%10%39%
1345
21st to endDay
311%5%9%8%45%6%6%8%36%
1584
February1st to 10thDay
39%6%10%8%46%6%7%10%33%
1445
11th to 20thDay
310%5%8%9%46%7%8%9%30%
1575
21st to endDay
212%5%10%9%45%6%6%9%32%
1453
March1st to 10thDay
210%5%8%8%46%5%6%10%37%
1800
11th to 20thDay
49%5%7%6%46%5%6%9%40%
1956
21st to endDay
7%12%5%10%8%5%5%5%6%8%29%
2204
April1st to 10thDay
9%12%6%9%7%45%5%7%8%29%
2200
11th to 20thDay
11%14%5%7%6%36%46%7%32%
2310
21st to endDay
12%11%46%6%35%5%6%8%34%
2340
May1st to 10thDay
14%10%48%6%45%6%6%7%30%
2414
11th to 20thDay
15%7%26%6%35%5%6%8%35%
2654
21st to endDay
21%7%35%6%46%5%5%8%29%
2917
June1st to 10thDay
17%8%36%5%46%7%7%8%29%
2700
11th to 20thDay
22%7%37%6%35%6%6%7%28%
2700
21st to endDay
9%5%25%5%36%5%7%10%43%
2700
July1st to 10thDay
9%5%25%5%36%6%7%8%43%
2700
11th to 20thDay
9%414435%6%6%9%49%
2700
21st to endDay
5%324435%5%5%8%57%
2840
August1st to 10thDay
9%5%244346%6%9%48%
2507
11th to 20thDay
10%4133345%7%9%51%
2310
21st to endDay
9%5%244345%6%8%51%
2574
September1st to 10thDay
11%5%25%43345%7%51%
2219
11th to 20thDay
12%7%244345%5%8%47%
1980
21st to endDay
9%8%2443445%8%49%
1980
October1st to 10thDay
11%9%36%6%335%6%7%42%
1908
11th to 20thDay
7%8%47%4345%7%9%40%
1673
21st to endDay
6%11%46%6%35%6%7%9%38%
1782
November1st to 10thDay
7%13%48%7%45%5%5%7%37%
1575
11th to 20thDay
5%9%46%5%36%46%9%43%
1473
21st to endDay
410%5%8%6%345%7%10%39%
1330
December1st to 10thDay
39%6%9%6%45%6%5%8%38%
1330
11th to 20thDay
310%6%11%7%46%6%6%7%35%
1682
21st to endDay
38%46%5%445%6%8%47%
2442
January1st to 10thNight
310%5%9%8%46%6%7%9%34%
2836
11th to 20thNight
46%49%7%47%5%6%9%38%
2727
21st to endNight
39%47%6%45%6%7%9%39%
2772
February1st to 10thNight
39%6%8%6%47%6%6%9%36%
2514
11th to 20thNight
412%6%9%6%46%5%6%8%34%
2272
21st to endNight
48%6%10%10%46%45%6%36%
1801
March1st to 10thNight
310%5%10%8%46%6%6%9%34%
2160
11th to 20thNight
48%5%7%6%46%6%7%10%39%
2003
21st to endNight
5%12%5%10%6%25%5%7%9%34%
2149
April1st to 10thNight
7%12%6%8%5%345%6%8%36%
1759
11th to 20thNight
10%11%5%9%6%35%5%5%6%34%
1601
21st to endNight
10%10%38%6%36%46%7%37%
1620
May1st to 10thNight
13%10%45%5%36%6%5%8%35%
1545
11th to 20thNight
11%7%36%5%4346%9%43%
1239
21st to endNight
21%7%35%5%345%6%8%35%
1358
June1st to 10thNight
14%7%35%6%3447%8%39%
1260
11th to 20thNight
20%6%35%5%45%5%5%8%34%
1260
21st to endNight
7%424435%6%6%10%48%
1260
July1st to 10thNight
8%324445%46%9%51%
1260
11th to 20thNight
8%423325%46%8%53%
1260
21st to endNight
6%22333336%8%61%
1473
August1st to 10thNight
8%4246%3446%8%51%
1432
11th to 20thNight
8%42432346%10%54%
1589
21st to endNight
9%41332346%7%56%
1782
September1st to 10thNight
12%41442347%7%52%
1739
11th to 20thNight
14%7%13323448%53%
1980
21st to endNight
10%6%2433445%9%49%
1980
October1st to 10thNight
12%8%25%5%25%45%7%45%
2051
11th to 20thNight
8%7%25%5%345%7%9%45%
2286
21st to endNight
6%10%46%5%35%47%9%40%
2574
November1st to 10thNight
8%11%46%6%36%47%8%37%
2485
11th to 20thNight
39%36%6%345%7%9%43%
2702
21st to endNight
39%5%8%5%35%5%6%9%42%
2850
December1st to 10thNight
48%48%6%47%6%6%9%37%
2850
11th to 20thNight
18%49%9%36%5%7%10%36%
3510
21st to endNight
310%37%6%35%5%6%7%45%
5214

Transparency (when cloud cover <= 30%)
Color Meanings:
Too cloudy to forecast Poor Below Average Average Above average Transparent
MonthDays in MonthOccurrence of each ForecastData
Points
January1st to 10thDay
41%6%15%25%9%4
1329
11th to 20thDay
38%7%15%26%10%4
1345
21st to endDay
43%8%14%20%14%2
1584
February1st to 10thDay
45%8%15%21%9%3
1445
11th to 20thDay
46%8%15%21%9%2
1584
21st to endDay
48%5%14%18%14%1
1453
March1st to 10thDay
42%9%15%22%11%1
1800
11th to 20thDay
40%6%14%24%14%2
1957
21st to endDay
52%7%16%18%7%1
2204
April1st to 10thDay
50%7%16%19%8%1
2200
11th to 20thDay
51%5%13%20%10%2
2310
21st to endDay
47%5%16%21%10%1
2340
May1st to 10thDay
51%6%14%21%7%
2414
11th to 20thDay
45%5%15%24%10%1
2658
21st to endDay
51%7%16%18%7%1
2918
June1st to 10thDay
49%8%16%17%9%1
2700
11th to 20thDay
53%6%14%17%8%1
2700
21st to endDay
33%8%20%24%12%3
2700
July1st to 10thDay
34%6%18%26%12%3
2626
11th to 20thDay
31%6%17%27%17%3
2619
21st to endDay
24%6%15%31%19%5%
2682
August1st to 10thDay
30%7%20%28%13%3
2490
11th to 20thDay
28%6%21%30%11%3
2310
21st to endDay
29%6%15%30%17%4
2574
September1st to 10thDay
33%413%26%18%5%
2219
11th to 20thDay
35%6%14%27%15%3
1980
21st to endDay
35%6%13%29%14%3
1980
October1st to 10thDay
40%7%16%24%11%2
1908
11th to 20thDay
39%5%17%24%12%3
1673
21st to endDay
41%7%14%24%11%3
1782
November1st to 10thDay
48%5%13%21%12%1
1575
11th to 20thDay
38%6%19%24%12%1
1473
21st to endDay
39%6%18%23%10%3
1330
December1st to 10thDay
42%7%14%20%12%4
1330
11th to 20thDay
46%6%18%20%8%1
1684
21st to endDay
33%5%12%25%19%6%
2442
January1st to 10thNight
45%7%14%20%11%3
2836
11th to 20thNight
41%6%16%22%10%3
2727
21st to endNight
38%7%14%23%15%2
2772
February1st to 10thNight
43%7%13%22%12%3
2514
11th to 20thNight
47%6%13%21%11%1
2273
21st to endNight
49%6%9%22%12%3
1801
March1st to 10thNight
45%7%15%23%9%1
2160
11th to 20thNight
38%9%14%25%13%1
2003
21st to endNight
46%7%17%20%9%1
2149
April1st to 10thNight
45%7%13%23%12%1
1759
11th to 20thNight
49%6%12%23%8%1
1601
21st to endNight
45%6%14%24%9%2
1620
May1st to 10thNight
46%6%15%24%10%
1545
11th to 20thNight
39%415%32%8%2
1239
21st to endNight
47%6%15%22%8%1
1358
June1st to 10thNight
43%5%16%24%11%
1260
11th to 20thNight
47%6%14%23%9%2
1260
21st to endNight
30%7%20%26%14%2
1260
July1st to 10thNight
30%6%21%27%14%2
1225
11th to 20thNight
28%6%17%30%15%4
1218
21st to endNight
20%5%18%31%21%5%
1391
August1st to 10thNight
30%6%16%30%14%4
1424
11th to 20thNight
25%6%18%33%14%3
1589
21st to endNight
26%5%13%31%21%4
1782
September1st to 10thNight
30%416%29%16%4
1739
11th to 20thNight
32%5%14%29%16%5%
1980
21st to endNight
33%5%17%26%15%4
1980
October1st to 10thNight
38%6%13%26%16%2
2051
11th to 20thNight
34%6%16%27%14%3
2286
21st to endNight
40%6%15%26%12%1
2574
November1st to 10thNight
44%7%14%22%12%2
2485
11th to 20thNight
36%7%17%23%17%1
2702
21st to endNight
38%5%18%22%12%4
2850
December1st to 10thNight
42%7%15%21%11%4
2850
11th to 20thNight
41%7%21%22%7%1
3507
21st to endNight
36%5%13%26%14%6%
5214

Seeing (when cloud cover <= 80%)
Color Meanings:
Too cloudy to forecast Bad 1/5 Poor 2/5 Average 3/5 Good 4/5 Excellent 5/5
MonthDays in MonthOccurrence of each ForecastData
Points
January1st to 10thDay
5%12%39%24%13%7%
180
11th to 20thDay
10%14%33%28%11%4
168
21st to endDay
10%14%40%25%7%3
176
February1st to 10thDay
13%23%41%18%22
163
11th to 20thDay
7%21%38%25%8%
240
21st to endDay
11%15%37%27%9%
319
March1st to 10thDay
8%26%39%17%7%3
420
11th to 20thDay
9%22%35%23%8%3
420
21st to endDay
13%24%34%19%9%2
462
April1st to 10thDay
15%25%33%21%5%
420
11th to 20thDay
21%15%33%20%10%1
412
21st to endDay
20%22%27%21%7%2
420
May1st to 10thDay
19%13%28%27%8%4
436
11th to 20thDay
20%11%22%28%15%4
625
21st to endDay
20%10%23%29%15%3
679
June1st to 10thDay
17%14%32%23%12%3
630
11th to 20thDay
21%14%29%23%9%2
630
21st to endDay
11%11%30%29%15%4
630
July1st to 10thDay
10%9%29%33%16%3
621
11th to 20thDay
10%11%31%33%14%3
618
21st to endDay
6%12%34%28%16%3
586
August1st to 10thDay
10%11%32%33%11%3
447
11th to 20thDay
8%10%32%33%14%4
437
21st to endDay
9%13%36%23%14%5%
495
September1st to 10thDay
13%16%31%28%9%3
450
11th to 20thDay
16%13%28%26%13%3
450
21st to endDay
19%20%28%24%8%2
450
October1st to 10thDay
15%14%28%29%12%3
378
11th to 20thDay
14%15%35%19%13%4
270
21st to endDay
18%16%35%22%6%2
297
November1st to 10thDay
19%17%34%20%7%2
270
11th to 20thDay
14%18%38%20%9%1
248
21st to endDay
8%18%33%23%8%8%
180
December1st to 10thDay
7%17%38%26%10%2
180
11th to 20thDay
8%19%39%22%9%3
180
21st to endDay
9%11%32%29%14%5%
198
January1st to 10thNight
9%17%39%25%9%1
950
11th to 20thNight
9%12%44%27%8%1
883
21st to endNight
7%16%41%28%8%1
792
February1st to 10thNight
8%18%36%26%9%2
720
11th to 20thNight
10%12%39%29%9%1
704
21st to endNight
8%10%36%28%15%2
592
March1st to 10thNight
9%11%41%25%12%2
720
11th to 20thNight
9%15%39%26%9%2
720
21st to endNight
10%11%36%30%9%3
792
April1st to 10thNight
13%14%34%29%10%1
679
11th to 20thNight
17%9%34%31%9%
533
21st to endNight
14%13%32%30%10%
540
May1st to 10thNight
18%11%27%30%14%
478
11th to 20thNight
15%8%25%30%22%1
354
21st to endNight
23%6%23%32%15%1
389
June1st to 10thNight
18%8%29%29%14%2
360
11th to 20thNight
22%9%29%30%11%
360
21st to endNight
10%6%23%38%22%1
360
July1st to 10thNight
10%420%43%23%
350
11th to 20thNight
9%427%41%17%2
348
21st to endNight
6%5%22%43%22%2
394
August1st to 10thNight
10%415%43%25%2
358
11th to 20thNight
9%322%44%20%2
521
21st to endNight
10%6%28%39%16%1
594
September1st to 10thNight
12%5%27%35%17%3
641
11th to 20thNight
18%5%25%33%17%3
720
21st to endNight
15%9%31%29%12%3
720
October1st to 10thNight
16%7%28%33%14%3
720
11th to 20thNight
12%9%36%28%13%2
720
21st to endNight
12%11%41%28%7%1
792
November1st to 10thNight
14%14%40%25%7%
820
11th to 20thNight
12%18%43%23%31
950
21st to endNight
9%15%41%25%8%2
950
December1st to 10thNight
8%13%40%27%9%2
950
11th to 20thNight
6%15%42%30%7%1
950
21st to endNight
11%13%37%28%10%2
1045
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