Anderson Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Anderson Observatory clear sky chart from 2005-11-06 to 2024-01-27.

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
36%
23906
16%
74589
11%
23851
42%
23851
19%
52440

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
14%
760
11%
2459
4%
757
23%
757
9%
1950
January11th to 20thNight
13%
760
9%
2470
4%
760
19%
760
8%
2028
January21st to endNight
17%
820
11%
2664
5%
820
23%
820
9%
2126
February1st to 10thNight
15%
720
9%
2340
4%
720
19%
720
10%
1871
February11th to 20thNight
19%
702
11%
2152
5%
700
25%
700
12%
1665
February21st to endNight
15%
592
12%
1776
3%
592
24%
592
11%
1385
March1st to 10thNight
22%
720
15%
2077
7%
720
29%
720
16%
1539
March11th to 20thNight
27%
720
19%
1980
9%
720
35%
720
22%
1370
March21st to endNight
27%
792
16%
2178
9%
792
33%
792
19%
1596
April1st to 10thNight
27%
695
14%
1955
6%
695
33%
695
21%
1386
April11th to 20thNight
31%
533
19%
1615
10%
533
39%
533
22%
1095
April21st to endNight
31%
540
17%
1620
6%
540
40%
540
22%
1127
May1st to 10thNight
34%
540
11%
1620
6%
540
38%
540
16%
1190
May11th to 20thNight
41%
531
16%
1593
11%
531
46%
531
22%
1069
May21st to endNight
58%
582
16%
1745
14%
582
60%
582
31%
1071
June1st to 10thNight
50%
540
14%
1620
11%
540
54%
540
29%
1015
June11th to 20thNight
56%
540
16%
1512
13%
540
61%
540
32%
951
June21st to endNight
56%
540
19%
1440
14%
540
61%
540
33%
918
July1st to 10thNight
65%
525
16%
1400
14%
525
67%
525
34%
886
July11th to 20thNight
65%
522
18%
1553
16%
522
68%
522
36%
914
July21st to endNight
63%
591
18%
1672
14%
558
66%
558
33%
1005
August1st to 10thNight
67%
537
20%
1600
18%
534
69%
534
34%
947
August11th to 20thNight
65%
534
22%
1598
19%
534
67%
534
33%
977
August21st to endNight
68%
594
24%
1782
20%
594
71%
594
36%
1032
September1st to 10thNight
60%
695
22%
1955
17%
695
63%
695
33%
1197
September11th to 20thNight
65%
720
29%
1980
23%
720
68%
720
39%
1160
September21st to endNight
51%
720
24%
1980
19%
720
55%
720
30%
1280
October1st to 10thNight
49%
720
22%
2047
18%
720
54%
720
30%
1281
October11th to 20thNight
33%
720
19%
2160
12%
720
41%
720
22%
1469
October21st to endNight
28%
792
19%
2434
13%
792
36%
792
18%
1725
November1st to 10thNight
36%
740
20%
2406
15%
740
44%
740
24%
1519
November11th to 20thNight
24%
760
19%
2470
9%
760
37%
760
17%
1711
November21st to endNight
24%
760
14%
2471
9%
760
31%
760
14%
1825
December1st to 10thNight
17%
760
10%
2660
6%
760
23%
760
10%
2112
December11th to 20thNight
18%
759
10%
3092
4%
758
25%
758
10%
2293
December21st to endNight
14%
830
5%
4513
4%
817
21%
817
4%
3755

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%10%47%6%45%5%5%7%32%
75180
Night
15%10%47%6%3445%7%34%
74845
TransparencyDay
52%6%11%19%11%1
74898
Night
51%5%11%19%12%2
74647
SeeingDay
18%10%27%23%14%6%
14071
Night
19%10%28%29%13%2
23916

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
16%16%7%13%9%5%4445%16%
1706
11th to 20thDay
17%19%7%14%7%45%44315%
1710
21st to endDay
15%17%8%13%11%45%435%15%
1836
February1st to 10thDay
17%17%6%9%9%45%43419%
1620
11th to 20thDay
14%12%8%13%9%45%5%5%5%20%
1693
21st to endDay
17%13%5%10%6%5%5%445%26%
1480
March1st to 10thDay
18%12%7%9%7%34446%25%
1882
11th to 20thDay
16%14%6%9%6%35%5%5%5%27%
1980
21st to endDay
21%13%6%9%6%34446%25%
2178
April1st to 10thDay
13%12%5%7%7%35%5%5%7%31%
2020
11th to 20thDay
17%10%36%6%35%45%7%33%
2292
21st to endDay
21%9%37%5%35%447%33%
2340
May1st to 10thDay
16%9%48%6%36%6%5%6%32%
2340
11th to 20thDay
19%7%37%6%45%46%8%31%
2301
21st to endDay
9%436%6%46%7%7%9%39%
2531
June1st to 10thDay
14%6%36%5%45%5%7%8%37%
2340
11th to 20thDay
13%6%36%6%46%6%6%9%37%
2439
21st to endDay
11%436%6%36%6%7%10%39%
2520
July1st to 10thDay
11%6%25%5%35%6%7%10%40%
2520
11th to 20thDay
10%5%25%5%46%5%7%9%43%
2350
21st to endDay
9%5%35%7%35%6%7%11%39%
2547
August1st to 10thDay
8%5%25%5%46%6%7%9%42%
2327
11th to 20thDay
10%6%35%6%46%7%8%10%36%
2303
21st to endDay
10%5%25%6%45%6%7%10%40%
2570
September1st to 10thDay
14%5%25%6%3445%7%44%
1985
11th to 20thDay
11%6%346%346%5%8%44%
1980
21st to endDay
17%6%35%5%3445%7%41%
1980
October1st to 10thDay
14%7%36%5%345%6%7%41%
1912
11th to 20thDay
16%8%36%5%36%6%5%7%36%
1800
21st to endDay
20%12%46%6%445%5%6%28%
1918
November1st to 10thDay
9%10%5%6%6%345%47%40%
1672
11th to 20thDay
17%14%5%6%5%25%446%32%
1710
21st to endDay
18%14%6%8%6%35%45%6%26%
1700
December1st to 10thDay
17%16%9%12%7%34445%18%
1520
11th to 20thDay
15%16%7%12%7%3445%7%20%
2054
21st to endDay
22%21%10%10%8%4433411%
3124
January1st to 10thNight
16%16%7%10%8%445%4421%
2459
11th to 20thNight
17%17%7%12%9%45%34419%
2470
21st to endNight
16%17%7%11%10%34445%20%
2664
February1st to 10thNight
20%13%7%10%7%35%445%21%
2340
11th to 20thNight
15%13%7%9%7%46%45%6%24%
2151
21st to endNight
19%13%6%9%7%35%445%25%
1776
March1st to 10thNight
15%13%5%9%7%44435%31%
2077
11th to 20thNight
15%10%46%5%36%46%7%36%
1980
21st to endNight
18%11%47%6%34446%32%
2178
April1st to 10thNight
14%12%46%5%35%45%7%34%
1955
11th to 20thNight
16%7%46%6%345%5%7%36%
1615
21st to endNight
16%8%36%7%35%5%5%6%36%
1620
May1st to 10thNight
18%8%47%7%46%5%5%7%30%
1620
11th to 20thNight
18%7%35%5%3445%8%38%
1592
21st to endNight
9%5%245%35%6%7%12%42%
1744
June1st to 10thNight
10%6%45%5%35%5%6%8%45%
1620
11th to 20thNight
8%5%26%5%35%6%7%10%43%
1512
21st to endNight
10%325%5%45%5%7%10%44%
1440
July1st to 10thNight
8%426%5%445%6%8%48%
1440
11th to 20thNight
8%4145%35%5%7%9%50%
1606
21st to endNight
7%425%5%35%5%7%8%47%
1771
August1st to 10thNight
6%425%6%35%5%7%9%47%
1611
11th to 20thNight
9%5%35%5%4446%8%47%
1598
21st to endNight
7%424435%5%6%9%51%
1782
September1st to 10thNight
11%6%345%34447%50%
1955
11th to 20thNight
10%42442345%7%54%
1980
21st to endNight
12%7%345%245%5%6%46%
1980
October1st to 10thNight
12%5%35%5%4445%7%46%
2047
11th to 20thNight
18%8%36%5%3445%6%38%
2160
21st to endNight
24%8%36%5%34345%34%
2434
November1st to 10thNight
14%8%36%6%3346%7%41%
2406
11th to 20thNight
15%12%5%8%5%3435%7%33%
2470
21st to endNight
18%13%6%7%6%345%46%28%
2471
December1st to 10thNight
17%17%7%10%6%34445%21%
2660
11th to 20thNight
17%15%7%9%6%3445%7%23%
3129
21st to endNight
22%19%8%10%7%35%45%5%13%
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
71%48%12%5%
1707
11th to 20thDay
75%411%8%3
1710
21st to endDay
75%46%10%41
1836
February1st to 10thDay
69%49%12%6%1
1620
11th to 20thDay
66%5%7%14%7%1
1703
21st to endDay
62%312%14%7%2
1480
March1st to 10thDay
60%5%10%18%7%
1882
11th to 20thDay
59%5%9%17%9%1
1980
21st to endDay
61%5%9%18%7%1
2178
April1st to 10thDay
53%5%12%18%11%1
2020
11th to 20thDay
51%5%11%17%13%2
2292
21st to endDay
53%411%20%10%2
2340
May1st to 10thDay
52%7%11%20%9%1
2340
11th to 20thDay
51%6%11%18%12%1
2305
21st to endDay
41%8%13%24%13%1
2531
June1st to 10thDay
44%6%11%25%13%1
2340
11th to 20thDay
44%7%13%22%14%1
2439
21st to endDay
39%8%14%21%15%3
2520
July1st to 10thDay
40%7%11%22%18%2
2451
11th to 20thDay
35%8%14%24%17%2
2280
21st to endDay
37%8%15%23%14%3
2405
August1st to 10thDay
38%8%14%25%14%1
2314
11th to 20thDay
40%9%13%24%11%2
2303
21st to endDay
38%7%14%20%17%4
2570
September1st to 10thDay
40%7%11%21%18%4
1985
11th to 20thDay
38%6%11%26%17%2
1980
21st to endDay
44%411%21%20%1
1980
October1st to 10thDay
43%6%11%22%17%1
1912
11th to 20thDay
47%5%11%21%14%2
1800
21st to endDay
57%313%16%9%1
1918
November1st to 10thDay
43%5%11%23%16%2
1672
11th to 20thDay
54%49%20%10%2
1710
21st to endDay
60%5%11%16%8%1
1700
December1st to 10thDay
69%48%14%5%
1520
11th to 20thDay
65%410%17%5%
2051
21st to endDay
79%46%7%3
3124
January1st to 10thNight
66%48%15%6%
2459
11th to 20thNight
71%39%12%41
2470
21st to endNight
68%47%15%6%1
2664
February1st to 10thNight
67%410%13%6%
2340
11th to 20thNight
62%5%9%15%8%1
2152
21st to endNight
64%48%15%8%2
1776
March1st to 10thNight
57%49%19%11%1
2077
11th to 20thNight
47%5%11%22%12%3
1980
21st to endNight
54%49%20%11%2
2178
April1st to 10thNight
50%5%12%17%15%2
1955
11th to 20thNight
46%5%11%17%17%3
1615
21st to endNight
48%5%10%21%14%2
1620
May1st to 10thNight
54%7%10%16%11%3
1620
11th to 20thNight
46%5%12%17%17%3
1593
21st to endNight
34%8%16%26%14%2
1745
June1st to 10thNight
39%5%11%25%17%3
1620
11th to 20thNight
35%7%15%23%17%2
1512
21st to endNight
35%7%13%25%15%4
1440
July1st to 10thNight
34%7%13%23%18%5%
1400
11th to 20thNight
31%6%15%26%20%2
1553
21st to endNight
32%8%15%25%16%4
1672
August1st to 10thNight
32%7%15%28%19%1
1600
11th to 20thNight
35%6%15%28%15%1
1598
21st to endNight
30%7%15%24%20%4
1782
September1st to 10thNight
35%5%13%26%16%5%
1955
11th to 20thNight
29%6%11%26%25%2
1980
21st to endNight
38%6%10%23%21%2
1980
October1st to 10thNight
38%5%11%25%20%2
2047
11th to 20thNight
48%48%23%15%1
2160
21st to endNight
53%49%21%11%2
2434
November1st to 10thNight
43%412%22%17%2
2406
11th to 20thNight
52%49%21%12%2
2470
21st to endNight
58%412%17%8%1
2471
December1st to 10thNight
66%5%8%15%6%
2660
11th to 20thNight
61%413%17%4
3131
21st to endNight
73%5%10%9%3
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
23%20%35%16%5%1
270
11th to 20thDay
22%23%41%9%5%
270
21st to endDay
26%19%33%18%41
282
February1st to 10thDay
33%18%35%13%1
240
11th to 20thDay
26%22%35%16%2
240
21st to endDay
24%23%41%11%1
198
March1st to 10thDay
23%16%38%19%5%
322
11th to 20thDay
24%14%36%20%5%1
420
21st to endDay
31%13%30%21%5%1
462
April1st to 10thDay
21%14%34%20%9%2
420
11th to 20thDay
22%17%28%17%12%5%
412
21st to endDay
25%11%32%20%8%3
420
May1st to 10thDay
24%11%30%24%8%3
423
11th to 20thDay
23%6%30%23%13%5%
447
21st to endDay
12%6%23%29%22%8%
485
June1st to 10thDay
17%5%24%34%17%4
450
11th to 20thDay
14%7%24%28%21%6%
549
21st to endDay
11%7%29%30%17%5%
630
July1st to 10thDay
12%5%17%28%23%15%
621
11th to 20thDay
11%421%28%24%13%
453
21st to endDay
10%319%28%26%13%
488
August1st to 10thDay
10%415%27%28%16%
446
11th to 20thDay
12%417%29%26%12%
439
21st to endDay
10%5%16%27%26%15%
495
September1st to 10thDay
15%220%26%24%14%
450
11th to 20thDay
11%412%24%33%16%
450
21st to endDay
18%9%20%25%18%10%
450
October1st to 10thDay
13%5%28%25%20%9%
450
11th to 20thDay
16%13%34%26%9%2
450
21st to endDay
27%16%31%21%41
433
November1st to 10thDay
14%12%37%22%11%3
270
11th to 20thDay
22%20%33%16%9%
270
21st to endDay
27%15%38%17%3
265
December1st to 10thDay
22%16%41%15%42
180
11th to 20thDay
19%19%41%18%3
224
21st to endDay
29%20%32%16%3
297
January1st to 10thNight
24%17%36%19%4
760
11th to 20thNight
27%18%34%18%31
760
21st to endNight
23%16%35%21%4
820
February1st to 10thNight
26%14%36%21%2
720
11th to 20thNight
24%14%35%23%3
703
21st to endNight
28%13%37%18%3
592
March1st to 10thNight
23%13%35%24%6%
720
11th to 20thNight
19%14%36%25%6%1
720
21st to endNight
24%11%32%26%7%1
792
April1st to 10thNight
20%13%34%27%5%1
695
11th to 20thNight
19%11%35%24%10%2
533
21st to endNight
21%8%36%27%8%1
540
May1st to 10thNight
21%7%32%29%10%1
540
11th to 20thNight
23%425%32%14%2
531
21st to endNight
10%322%39%23%3
584
June1st to 10thNight
11%5%24%42%16%2
540
11th to 20thNight
10%5%21%44%19%2
540
21st to endNight
11%321%43%20%1
540
July1st to 10thNight
9%213%41%31%4
525
11th to 20thNight
10%216%43%25%5%
522
21st to endNight
9%214%42%29%3
591
August1st to 10thNight
8%212%46%28%5%
537
11th to 20thNight
11%213%36%32%5%
534
21st to endNight
8%314%37%33%6%
594
September1st to 10thNight
13%316%34%30%5%
695
11th to 20thNight
12%214%33%33%6%
720
21st to endNight
16%6%21%33%21%4
720
October1st to 10thNight
14%5%24%34%19%3
720
11th to 20thNight
22%9%30%29%9%1
720
21st to endNight
27%11%30%26%6%
792
November1st to 10thNight
18%11%31%27%12%1
740
11th to 20thNight
22%16%32%22%7%1
760
21st to endNight
25%13%32%24%5%
760
December1st to 10thNight
25%16%33%21%4
760
11th to 20thNight
25%16%35%20%4
760
21st to endNight
26%17%34%20%3
836
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