Martin Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Martin Observatory clear sky chart from 2005-11-06 to 2024-01-13.

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
40%
24227
18%
74494
11%
24177
47%
24177
21%
49194

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
17%
759
17%
2459
6%
757
30%
757
13%
1666
January11th to 20thNight
20%
729
18%
2367
8%
729
33%
729
13%
1646
January21st to endNight
16%
792
19%
2574
5%
792
32%
792
14%
1685
February1st to 10thNight
21%
720
16%
2340
8%
720
32%
720
17%
1561
February11th to 20thNight
12%
701
14%
2199
4%
700
25%
700
11%
1536
February21st to endNight
16%
592
21%
1734
8%
592
30%
592
15%
1230
March1st to 10thNight
22%
720
20%
1980
9%
720
33%
720
22%
1293
March11th to 20thNight
26%
720
15%
1980
6%
720
34%
720
17%
1412
March21st to endNight
26%
792
15%
2178
7%
792
32%
792
21%
1499
April1st to 10thNight
29%
720
17%
1940
5%
720
38%
720
22%
1294
April11th to 20thNight
35%
711
21%
1775
9%
711
42%
711
28%
1164
April21st to endNight
36%
570
21%
1650
9%
570
45%
570
24%
1077
May1st to 10thNight
38%
540
15%
1620
7%
540
44%
540
22%
1079
May11th to 20thNight
43%
531
14%
1593
8%
531
47%
531
21%
1096
May21st to endNight
57%
582
13%
1745
10%
582
58%
582
26%
1164
June1st to 10thNight
63%
540
15%
1620
13%
540
65%
540
28%
1047
June11th to 20thNight
59%
540
13%
1620
11%
540
62%
540
28%
1028
June21st to endNight
64%
540
17%
1620
14%
540
68%
540
29%
1019
July1st to 10thNight
65%
525
13%
1575
13%
525
66%
525
25%
1066
July11th to 20thNight
74%
522
14%
1566
13%
522
75%
522
31%
962
July21st to endNight
71%
591
15%
1672
13%
558
72%
558
28%
1075
August1st to 10thNight
63%
537
15%
1600
12%
534
64%
534
25%
1088
August11th to 20thNight
65%
534
19%
1598
17%
534
66%
534
26%
1075
August21st to endNight
67%
720
18%
1908
14%
720
68%
720
30%
1275
September1st to 10thNight
63%
720
21%
1873
15%
720
65%
720
29%
1264
September11th to 20thNight
59%
720
22%
1980
19%
720
60%
720
29%
1293
September21st to endNight
47%
720
19%
1980
14%
720
52%
720
24%
1358
October1st to 10thNight
49%
720
20%
2123
18%
720
54%
720
27%
1359
October11th to 20thNight
41%
720
23%
2160
17%
720
51%
720
25%
1369
October21st to endNight
43%
792
27%
2376
21%
792
53%
792
28%
1451
November1st to 10thNight
44%
740
26%
2221
19%
740
53%
740
28%
1373
November11th to 20thNight
37%
760
24%
2468
16%
760
49%
760
24%
1466
November21st to endNight
36%
760
24%
2470
15%
760
47%
760
23%
1515
December1st to 10thNight
23%
760
17%
2470
8%
760
34%
760
14%
1703
December11th to 20thNight
26%
759
20%
2978
9%
759
40%
759
14%
1960
December21st to endNight
24%
828
11%
4482
8%
817
34%
817
8%
3046

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
15%7%36%6%45%5%6%8%36%
74932
Night
14%6%35%5%35%5%6%8%39%
74813
TransparencyDay
44%8%12%20%13%2
74654
Night
41%8%13%21%14%2
74610
SeeingDay
21%14%23%21%14%6%
14091
Night
18%13%22%28%16%2
24241

By Month

Percentage Cloud Cover SE (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
17%8%36%5%445%5%7%37%
1706
11th to 20thDay
18%10%37%6%45%5%5%5%32%
1638
21st to endDay
15%9%5%6%5%24446%38%
1782
February1st to 10thDay
13%9%49%6%5%5%45%6%34%
1620
11th to 20thDay
19%8%36%5%46%46%6%34%
1647
21st to endDay
20%9%37%6%33446%34%
1520
March1st to 10thDay
15%5%25%5%345%5%7%45%
1980
11th to 20thDay
18%9%37%6%45%447%34%
1980
21st to endDay
18%7%38%6%3435%7%35%
2178
April1st to 10thDay
11%6%36%5%3445%8%43%
2020
11th to 20thDay
14%7%36%5%445%5%6%41%
2134
21st to endDay
17%6%36%6%35%5%47%38%
2325
May1st to 10thDay
16%8%36%6%35%6%7%6%32%
2340
11th to 20thDay
18%8%37%6%46%5%6%8%29%
2301
21st to endDay
13%6%36%7%37%5%7%10%32%
2531
June1st to 10thDay
12%5%346%36%6%7%9%38%
2340
11th to 20thDay
9%6%36%7%46%7%7%10%36%
2340
21st to endDay
9%5%36%6%46%6%7%11%36%
2340
July1st to 10thDay
12%5%36%7%5%6%7%6%10%33%
2340
11th to 20thDay
9%436%7%47%7%8%11%34%
2340
21st to endDay
11%6%37%8%5%8%7%7%9%29%
2547
August1st to 10thDay
14%6%36%7%5%6%7%7%10%30%
2327
11th to 20thDay
14%6%36%6%6%7%8%7%10%28%
2303
21st to endDay
10%7%26%7%47%6%7%8%36%
2430
September1st to 10thDay
13%5%26%6%36%5%6%7%41%
2083
11th to 20thDay
15%6%36%6%35%6%5%8%38%
1980
21st to endDay
21%7%36%6%35%5%5%7%34%
1980
October1st to 10thDay
17%6%245%23346%47%
1836
11th to 20thDay
16%7%25%5%2445%7%44%
1800
21st to endDay
15%7%25%5%34446%45%
1980
November1st to 10thDay
12%6%25%42435%7%50%
1857
11th to 20thDay
12%8%345%35%5%5%8%44%
1710
21st to endDay
16%6%26%424347%45%
1710
December1st to 10thDay
17%11%47%6%35%5%6%6%32%
1710
11th to 20thDay
17%8%36%42435%7%41%
2153
21st to endDay
19%11%35%5%46%5%6%7%28%
3124
January1st to 10thNight
14%7%36%6%46%5%7%8%34%
2459
11th to 20thNight
18%9%47%6%445%5%7%32%
2367
21st to endNight
15%7%45%5%45%5%5%8%37%
2574
February1st to 10thNight
15%9%35%5%35%5%7%8%35%
2340
11th to 20thNight
19%8%36%5%46%5%5%6%33%
2197
21st to endNight
17%7%47%5%45%45%7%34%
1734
March1st to 10thNight
14%5%25%5%35%5%6%6%43%
1980
11th to 20thNight
18%6%36%6%35%6%7%8%33%
1980
21st to endNight
16%6%35%5%45%6%5%7%39%
2178
April1st to 10thNight
13%7%25%5%4446%9%40%
1940
11th to 20thNight
15%425%435%45%7%45%
1775
21st to endNight
15%6%35%5%34447%43%
1650
May1st to 10thNight
13%6%37%6%45%45%8%40%
1620
11th to 20thNight
16%7%37%6%35%5%48%36%
1592
21st to endNight
14%6%25%6%3447%10%38%
1744
June1st to 10thNight
9%5%35%5%35%6%8%11%41%
1620
11th to 20thNight
10%5%35%43445%11%45%
1620
21st to endNight
9%325%5%45%6%7%11%43%
1620
July1st to 10thNight
10%6%35%7%35%6%5%10%41%
1620
11th to 20thNight
7%5%24435%6%7%12%45%
1620
21st to endNight
6%425%7%37%6%8%12%40%
1771
August1st to 10thNight
13%6%36%6%345%6%10%38%
1611
11th to 20thNight
13%6%26%6%36%5%5%8%41%
1598
21st to endNight
10%426%5%46%6%7%10%40%
1908
September1st to 10thNight
11%6%25%5%45%46%7%45%
1873
11th to 20thNight
14%6%25%6%3345%8%44%
1980
21st to endNight
17%6%35%5%35%446%42%
1980
October1st to 10thNight
15%6%245%2336%6%46%
2123
11th to 20thNight
14%7%26%5%3435%6%45%
2160
21st to endNight
14%5%25%43445%7%46%
2376
November1st to 10thNight
14%7%34434445%49%
2221
11th to 20thNight
14%6%3442445%7%46%
2468
21st to endNight
14%5%345%345%5%7%45%
2470
December1st to 10thNight
19%9%46%5%3445%7%34%
2470
11th to 20thNight
14%7%48%5%35%5%5%8%36%
3042
21st to endNight
19%9%5%6%6%45%5%5%7%28%
4532

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
47%7%10%21%13%3
1707
11th to 20thDay
52%5%9%20%12%2
1638
21st to endDay
46%6%9%21%15%3
1782
February1st to 10thDay
51%6%10%19%13%2
1620
11th to 20thDay
50%6%11%19%12%2
1656
21st to endDay
51%5%10%16%14%4
1520
March1st to 10thDay
38%5%11%25%17%4
1980
11th to 20thDay
50%7%9%18%13%4
1980
21st to endDay
49%6%10%21%11%2
2178
April1st to 10thDay
38%7%11%23%16%5%
2020
11th to 20thDay
41%6%12%21%16%4
2134
21st to endDay
46%7%11%22%11%3
2325
May1st to 10thDay
48%9%13%17%11%2
2340
11th to 20thDay
50%9%13%19%6%2
2305
21st to endDay
45%12%15%20%8%
2531
June1st to 10thDay
40%11%15%21%13%
2340
11th to 20thDay
40%12%15%18%14%2
2340
21st to endDay
40%11%15%20%13%2
2340
July1st to 10thDay
44%12%13%18%10%3
2275
11th to 20thDay
39%14%17%16%14%1
2271
21st to endDay
46%12%15%18%9%1
2405
August1st to 10thDay
46%11%17%18%7%
2314
11th to 20thDay
46%13%15%18%8%
2303
21st to endDay
42%10%17%17%11%2
2430
September1st to 10thDay
40%9%13%19%15%3
2083
11th to 20thDay
43%8%10%22%15%1
1980
21st to endDay
49%6%11%17%15%1
1980
October1st to 10thDay
38%6%11%25%16%4
1836
11th to 20thDay
39%6%11%23%18%3
1800
21st to endDay
40%5%10%24%17%4
1980
November1st to 10thDay
34%5%10%23%23%5%
1857
11th to 20thDay
37%6%11%21%21%4
1710
21st to endDay
39%5%12%22%20%3
1710
December1st to 10thDay
50%8%11%16%13%3
1710
11th to 20thDay
43%411%19%20%3
2150
21st to endDay
51%7%13%19%9%1
3124
January1st to 10thNight
45%8%13%17%13%3
2459
11th to 20thNight
50%7%11%17%13%3
2367
21st to endNight
43%7%12%23%13%2
2574
February1st to 10thNight
44%7%15%19%13%1
2340
11th to 20thNight
49%7%10%18%12%4
2199
21st to endNight
48%7%11%20%10%4
1734
March1st to 10thNight
39%7%11%23%17%3
1980
11th to 20thNight
47%7%12%21%11%1
1980
21st to endNight
43%8%13%19%14%3
2178
April1st to 10thNight
41%6%15%20%15%3
1940
11th to 20thNight
38%5%13%20%20%5%
1775
21st to endNight
41%6%14%26%13%1
1650
May1st to 10thNight
42%8%15%21%12%2
1620
11th to 20thNight
44%9%17%21%8%1
1593
21st to endNight
40%9%21%24%7%
1745
June1st to 10thNight
34%12%18%23%13%1
1620
11th to 20thNight
35%10%19%23%13%1
1620
21st to endNight
32%13%17%24%13%2
1620
July1st to 10thNight
38%12%15%19%12%4
1575
11th to 20thNight
30%12%21%21%12%3
1566
21st to endNight
31%15%19%25%10%
1672
August1st to 10thNight
40%11%19%21%10%
1600
11th to 20thNight
39%10%15%23%11%1
1598
21st to endNight
35%11%17%24%10%2
1908
September1st to 10thNight
37%10%13%20%16%3
1873
11th to 20thNight
39%7%13%22%18%1
1980
21st to endNight
44%7%11%22%15%2
1980
October1st to 10thNight
37%7%12%23%17%3
2123
11th to 20thNight
40%5%9%24%18%3
2160
21st to endNight
36%6%10%25%17%5%
2376
November1st to 10thNight
37%6%9%20%23%5%
2221
11th to 20thNight
36%6%10%22%21%5%
2468
21st to endNight
38%5%10%25%17%4
2470
December1st to 10thNight
49%6%12%20%11%2
2470
11th to 20thNight
46%6%10%20%16%2
3044
21st to endNight
53%7%13%15%12%1
4532

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
36%28%20%11%4
270
11th to 20thDay
36%22%31%10%1
246
21st to endDay
37%25%25%10%3
264
February1st to 10thDay
30%28%30%10%2
240
11th to 20thDay
45%26%23%5%
240
21st to endDay
36%26%26%9%2
238
March1st to 10thDay
26%27%25%17%5%
420
11th to 20thDay
31%24%25%15%41
420
21st to endDay
26%22%29%17%5%1
462
April1st to 10thDay
24%25%24%17%10%
420
11th to 20thDay
24%19%26%18%11%3
412
21st to endDay
26%19%26%16%10%2
420
May1st to 10thDay
22%19%30%16%8%5%
423
11th to 20thDay
21%11%28%21%14%5%
447
21st to endDay
15%7%24%26%18%10%
485
June1st to 10thDay
13%8%27%25%18%8%
450
11th to 20thDay
11%10%25%26%20%9%
450
21st to endDay
10%10%29%27%16%8%
450
July1st to 10thDay
13%417%28%24%13%
445
11th to 20thDay
7%318%33%25%14%
444
21st to endDay
11%320%28%24%13%
488
August1st to 10thDay
18%7%18%21%24%13%
446
11th to 20thDay
12%5%16%27%25%15%
439
21st to endDay
11%316%29%22%19%
495
September1st to 10thDay
14%418%24%27%13%
450
11th to 20thDay
15%415%28%26%12%
450
21st to endDay
23%11%21%26%16%4
450
October1st to 10thDay
21%9%21%24%22%4
450
11th to 20thDay
17%17%21%26%17%2
450
21st to endDay
21%16%27%22%12%3
495
November1st to 10thDay
19%15%24%26%13%2
455
11th to 20thDay
28%20%22%22%7%1
270
21st to endDay
23%22%24%24%5%1
270
December1st to 10thDay
28%27%28%14%4
270
11th to 20thDay
27%24%25%16%7%
270
21st to endDay
32%18%27%14%9%
297
January1st to 10thNight
26%25%27%19%3
760
11th to 20thNight
26%21%28%22%3
729
21st to endNight
22%27%32%18%2
792
February1st to 10thNight
23%22%29%23%2
720
11th to 20thNight
30%25%29%16%
703
21st to endNight
25%23%33%18%2
592
March1st to 10thNight
21%19%34%22%3
720
11th to 20thNight
26%20%26%22%7%
720
21st to endNight
21%15%33%24%6%
792
April1st to 10thNight
22%20%25%27%6%
720
11th to 20thNight
19%17%26%26%12%
711
21st to endNight
18%15%25%33%9%1
570
May1st to 10thNight
16%13%27%29%14%1
540
11th to 20thNight
15%10%22%35%18%1
531
21st to endNight
15%215%31%32%4
584
June1st to 10thNight
9%216%44%26%3
540
11th to 20thNight
11%417%32%33%4
540
21st to endNight
9%315%41%29%4
540
July1st to 10thNight
12%28%38%34%6%
525
11th to 20thNight
6%28%34%44%5%
522
21st to endNight
6%29%39%39%5%
591
August1st to 10thNight
12%311%36%34%5%
537
11th to 20thNight
14%19%31%39%5%
534
21st to endNight
12%39%30%39%7%
720
September1st to 10thNight
13%311%31%36%6%
720
11th to 20thNight
14%413%31%33%5%
720
21st to endNight
21%6%19%31%23%1
720
October1st to 10thNight
18%10%17%28%25%3
720
11th to 20thNight
18%12%24%29%17%1
720
21st to endNight
19%12%23%33%13%1
792
November1st to 10thNight
18%10%24%33%14%1
740
11th to 20thNight
19%15%23%32%11%1
760
21st to endNight
19%16%25%30%10%1
760
December1st to 10thNight
26%20%27%22%5%
760
11th to 20thNight
19%18%31%26%6%
760
21st to endNight
26%17%27%27%3
836
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