All-Star Telescope Observing Site
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the All-Star Telescope Observing Site clear sky chart from 2005-11-06 to 2024-03-18.

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%
23659
14%
80991
9%
23588
46%
23588
21%
52689

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
33%
940
14%
4678
7%
926
40%
926
11%
3183
January11th to 20thNight
34%
943
20%
4612
9%
929
41%
929
13%
2983
January21st to endNight
32%
835
13%
4821
12%
824
44%
824
11%
2773
February1st to 10thNight
36%
760
14%
2660
11%
760
41%
760
15%
1846
February11th to 20thNight
34%
743
14%
2432
10%
740
42%
740
17%
1691
February21st to endNight
40%
628
17%
1930
14%
628
48%
628
22%
1307
March1st to 10thNight
39%
760
15%
2280
12%
760
44%
760
21%
1614
March11th to 20thNight
40%
752
18%
2101
10%
752
46%
752
25%
1443
March21st to endNight
36%
792
12%
2137
5%
792
41%
792
20%
1571
April1st to 10thNight
37%
668
14%
1748
5%
668
43%
668
22%
1270
April11th to 20thNight
33%
533
13%
1601
8%
533
37%
533
18%
1165
April21st to endNight
36%
540
11%
1620
8%
540
40%
540
20%
1135
May1st to 10thNight
37%
428
8%
1454
7%
428
41%
428
17%
991
May11th to 20thNight
50%
354
12%
1239
11%
354
53%
354
25%
748
May21st to endNight
45%
388
5%
1358
2%
388
47%
388
19%
912
June1st to 10thNight
45%
360
1%
1260
0%
360
45%
360
19%
856
June11th to 20thNight
38%
360
0%
1260
0%
360
38%
360
14%
909
June21st to endNight
53%
360
0%
1260
0%
360
53%
360
23%
780
July1st to 10thNight
58%
350
0%
1225
0%
350
58%
350
24%
763
July11th to 20thNight
56%
348
4%
1218
1%
348
56%
348
24%
761
July21st to endNight
67%
394
11%
1341
11%
372
69%
372
34%
706
August1st to 10thNight
58%
358
10%
1424
14%
356
60%
356
25%
817
August11th to 20thNight
62%
486
14%
1554
14%
486
65%
486
32%
899
August21st to endNight
58%
594
18%
1782
13%
594
61%
594
32%
1075
September1st to 10thNight
53%
630
19%
1714
11%
630
56%
630
35%
1059
September11th to 20thNight
50%
720
18%
1980
11%
720
54%
720
32%
1270
September21st to endNight
44%
720
21%
1980
12%
720
50%
720
34%
1230
October1st to 10thNight
43%
720
18%
2056
10%
720
50%
720
26%
1359
October11th to 20thNight
39%
720
16%
2308
10%
720
48%
720
26%
1464
October21st to endNight
35%
792
17%
2574
11%
792
43%
792
21%
1701
November1st to 10thNight
31%
853
17%
2518
9%
853
39%
853
21%
1757
November11th to 20thNight
31%
950
17%
2759
8%
950
39%
950
24%
1863
November21st to endNight
36%
950
14%
2850
7%
950
42%
950
23%
1903
December1st to 10thNight
37%
950
17%
2850
10%
950
43%
950
21%
1966
December11th to 20thNight
34%
949
15%
3467
9%
949
39%
949
17%
2313
December21st to endNight
34%
1031
12%
4940
8%
1026
41%
1026
16%
2606

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
10%9%47%6%45%6%6%8%35%
78082
Night
7%9%48%7%46%6%6%8%35%
81670
TransparencyDay
44%6%16%22%9%1
77787
Night
44%6%15%22%10%2
81545
SeeingDay
13%8%27%31%16%5%
14402
Night
11%6%30%35%15%2
23704

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
310%7%10%8%345%7%8%35%
2219
11th to 20thDay
310%49%8%36%7%7%8%34%
2206
21st to endDay
48%47%6%36%6%7%9%40%
2794
February1st to 10thDay
7%11%6%10%9%46%47%9%28%
1520
11th to 20thDay
6%11%5%9%9%5%7%7%7%7%27%
1635
21st to endDay
410%7%10%8%46%7%7%7%30%
1522
March1st to 10thDay
412%7%9%9%46%6%6%7%30%
1900
11th to 20thDay
6%10%5%8%6%46%6%6%9%35%
2024
21st to endDay
8%14%5%10%7%37%6%7%6%26%
2218
April1st to 10thDay
8%14%6%9%8%46%6%7%7%26%
2211
11th to 20thDay
13%14%6%8%6%35%5%5%7%27%
2310
21st to endDay
12%12%48%6%35%5%6%9%30%
2340
May1st to 10thDay
14%12%5%7%7%45%6%6%8%28%
2503
11th to 20thDay
10%7%36%7%46%5%6%8%37%
2654
21st to endDay
19%8%37%6%36%5%6%7%29%
2917
June1st to 10thDay
17%7%37%6%5%5%6%7%9%27%
2700
11th to 20thDay
20%9%47%7%5%6%6%6%7%23%
2700
21st to endDay
9%5%36%6%5%6%7%7%10%36%
2700
July1st to 10thDay
10%6%35%6%46%6%7%9%37%
2700
11th to 20thDay
13%5%24445%5%7%10%40%
2700
21st to endDay
8%424435%6%6%10%48%
2894
August1st to 10thDay
11%6%25%5%35%5%7%8%44%
2507
11th to 20thDay
11%414435%5%6%10%47%
2346
21st to endDay
11%4245%35%6%6%7%46%
2574
September1st to 10thDay
13%6%25%5%245%5%8%46%
2244
11th to 20thDay
15%8%345%3445%7%44%
1980
21st to endDay
9%8%26%5%346%6%8%42%
1980
October1st to 10thDay
13%9%36%436%5%5%6%39%
1904
11th to 20thDay
10%9%38%5%45%5%7%7%37%
1652
21st to endDay
6%10%49%8%45%5%6%8%34%
1782
November1st to 10thDay
9%15%6%8%6%25%46%7%31%
1541
11th to 20thDay
6%12%5%8%6%45%46%8%36%
1421
21st to endDay
5%12%6%7%7%445%7%9%35%
1330
December1st to 10thDay
6%10%410%8%5%7%6%6%6%32%
1330
11th to 20thDay
49%5%11%9%5%6%6%7%9%31%
1682
21st to endDay
38%48%7%36%5%6%9%41%
2442
January1st to 10thNight
312%6%11%9%5%6%6%7%8%27%
4726
11th to 20thNight
38%412%8%37%8%6%7%33%
4699
21st to endNight
310%5%8%7%46%6%7%9%35%
4885
February1st to 10thNight
6%10%5%12%9%5%6%5%6%6%30%
2660
11th to 20thNight
412%5%11%8%5%6%6%7%7%30%
2431
21st to endNight
310%7%9%7%5%6%6%6%8%34%
1930
March1st to 10thNight
410%5%9%8%5%7%6%6%9%30%
2280
11th to 20thNight
5%10%47%7%5%6%6%7%9%35%
2101
21st to endNight
7%13%5%11%7%45%6%6%9%27%
2137
April1st to 10thNight
7%12%5%6%8%46%5%7%7%30%
1748
11th to 20thNight
10%13%48%9%37%46%8%27%
1601
21st to endNight
12%10%48%7%45%5%8%7%30%
1620
May1st to 10thNight
14%11%37%7%445%6%6%32%
1454
11th to 20thNight
11%6%35%5%35%5%6%10%40%
1239
21st to endNight
19%6%35%5%46%6%7%8%31%
1358
June1st to 10thNight
18%6%36%5%35%6%7%8%32%
1260
11th to 20thNight
19%9%36%8%47%47%7%26%
1260
21st to endNight
9%6%37%6%36%5%8%7%40%
1260
July1st to 10thNight
10%5%35%5%36%8%7%10%39%
1260
11th to 20thNight
10%425%7%47%7%8%9%38%
1260
21st to endNight
6%4234245%6%11%52%
1420
August1st to 10thNight
9%6%25%7%345%7%8%43%
1432
11th to 20thNight
9%5%245%35%5%6%11%46%
1554
21st to endNight
9%6%245%35%6%5%9%46%
1782
September1st to 10thNight
10%7%245%345%5%9%46%
1714
11th to 20thNight
14%6%244345%6%8%43%
1980
21st to endNight
11%7%25%5%245%6%8%45%
1980
October1st to 10thNight
12%9%36%6%4445%7%41%
2056
11th to 20thNight
8%9%47%5%45%6%7%7%39%
2308
21st to endNight
7%10%5%8%7%45%6%6%8%34%
2574
November1st to 10thNight
8%13%5%8%6%36%5%5%7%34%
2518
11th to 20thNight
411%48%7%46%6%7%9%35%
2759
21st to endNight
49%5%8%7%47%6%6%9%35%
2850
December1st to 10thNight
5%10%5%10%7%46%5%6%7%35%
2850
11th to 20thNight
210%5%9%10%5%6%6%6%9%32%
3510
21st to endNight
39%38%6%35%5%6%8%44%
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
45%5%12%24%11%2
2219
11th to 20thDay
45%6%10%16%19%4
2227
21st to endDay
40%6%14%32%8%1
2794
February1st to 10thDay
51%6%14%17%9%2
1520
11th to 20thDay
52%6%14%17%9%2
1644
21st to endDay
50%7%13%19%10%1
1522
March1st to 10thDay
50%7%16%18%9%1
1900
11th to 20thDay
43%7%13%23%12%2
2025
21st to endDay
55%6%15%17%6%1
2218
April1st to 10thDay
52%7%16%18%7%
2211
11th to 20thDay
54%6%10%20%9%1
2310
21st to endDay
49%6%15%21%7%1
2340
May1st to 10thDay
52%6%16%19%7%
2503
11th to 20thDay
42%7%16%24%11%1
2658
21st to endDay
52%6%15%21%6%1
2918
June1st to 10thDay
50%7%17%19%6%1
2700
11th to 20thDay
56%7%14%16%5%1
2700
21st to endDay
38%8%21%25%6%1
2700
July1st to 10thDay
39%8%21%23%8%2
2626
11th to 20thDay
37%8%21%27%6%
2619
21st to endDay
29%7%23%30%11%1
2733
August1st to 10thDay
36%8%22%27%7%1
2490
11th to 20thDay
31%7%25%26%9%1
2346
21st to endDay
34%7%19%28%11%2
2574
September1st to 10thDay
36%5%20%25%11%3
2244
11th to 20thDay
40%5%14%28%12%1
1980
21st to endDay
38%6%17%24%12%3
1980
October1st to 10thDay
43%7%14%25%9%2
1904
11th to 20thDay
43%6%17%24%9%2
1652
21st to endDay
46%5%14%23%11%1
1782
November1st to 10thDay
52%6%12%19%9%2
1541
11th to 20thDay
46%5%16%22%10%1
1421
21st to endDay
44%6%17%21%9%2
1330
December1st to 10thDay
51%5%12%17%13%2
1330
11th to 20thDay
48%6%19%19%6%2
1684
21st to endDay
38%5%12%22%18%6%
2442
January1st to 10thNight
52%6%12%19%8%2
4726
11th to 20thNight
47%6%11%16%14%5%
4740
21st to endNight
44%5%15%24%11%1
4885
February1st to 10thNight
53%412%18%10%2
2660
11th to 20thNight
50%6%12%19%11%2
2432
21st to endNight
46%5%10%22%14%2
1930
March1st to 10thNight
47%8%15%22%7%1
2280
11th to 20thNight
43%8%14%21%13%1
2101
21st to endNight
53%5%15%19%7%1
2137
April1st to 10thNight
50%6%13%20%11%1
1748
11th to 20thNight
54%5%13%20%8%1
1601
21st to endNight
50%5%15%20%7%2
1620
May1st to 10thNight
49%6%15%20%9%1
1454
11th to 20thNight
39%6%17%25%13%
1239
21st to endNight
47%7%16%24%6%1
1358
June1st to 10thNight
47%7%17%20%9%
1260
11th to 20thNight
55%6%16%15%6%2
1260
21st to endNight
40%7%20%25%7%1
1260
July1st to 10thNight
37%10%20%24%8%1
1225
11th to 20thNight
38%10%22%22%8%1
1218
21st to endNight
26%9%20%32%12%1
1341
August1st to 10thNight
36%7%23%27%7%1
1424
11th to 20thNight
31%7%24%27%9%1
1554
21st to endNight
34%7%18%27%13%2
1782
September1st to 10thNight
35%5%17%26%14%4
1714
11th to 20thNight
37%6%16%25%11%4
1980
21st to endNight
35%5%17%26%13%4
1980
October1st to 10thNight
43%5%14%25%12%2
2056
11th to 20thNight
41%6%16%24%10%2
2308
21st to endNight
45%6%15%24%9%1
2574
November1st to 10thNight
48%7%12%21%11%2
2518
11th to 20thNight
44%6%17%20%12%1
2759
21st to endNight
43%7%17%20%9%3
2850
December1st to 10thNight
48%6%12%21%10%3
2850
11th to 20thNight
47%7%17%22%6%1
3507
21st to endNight
37%413%25%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%7%30%34%16%7%
180
11th to 20thDay
9%8%34%28%18%3
180
21st to endDay
9%8%30%30%20%4
198
February1st to 10thDay
12%9%34%27%11%7%
180
11th to 20thDay
11%6%34%33%13%3
256
21st to endDay
6%9%28%34%19%4
334
March1st to 10thDay
8%9%35%31%13%4
460
11th to 20thDay
7%8%33%34%11%6%
449
21st to endDay
15%9%34%25%11%7%
462
April1st to 10thDay
11%12%34%29%12%2
420
11th to 20thDay
17%10%27%27%16%2
412
21st to endDay
18%11%25%30%12%3
420
May1st to 10thDay
15%13%25%29%15%4
477
11th to 20thDay
15%6%25%32%19%4
625
21st to endDay
17%6%21%29%20%6%
679
June1st to 10thDay
17%9%27%32%12%4
630
11th to 20thDay
21%9%24%31%13%2
630
21st to endDay
10%5%24%33%22%6%
630
July1st to 10thDay
13%5%20%36%21%6%
621
11th to 20thDay
16%422%32%22%3
618
21st to endDay
7%423%37%21%8%
640
August1st to 10thDay
11%427%30%21%7%
447
11th to 20thDay
10%5%22%38%21%5%
437
21st to endDay
10%7%24%31%20%7%
495
September1st to 10thDay
15%8%30%28%15%4
450
11th to 20thDay
19%6%24%30%16%6%
450
21st to endDay
15%13%29%27%12%4
450
October1st to 10thDay
16%9%26%29%13%7%
374
11th to 20thDay
13%11%30%27%14%4
270
21st to endDay
14%12%35%27%10%2
297
November1st to 10thDay
19%7%30%30%10%4
270
11th to 20thDay
15%13%32%28%10%2
223
21st to endDay
9%12%31%23%17%7%
180
December1st to 10thDay
9%7%31%32%16%4
180
11th to 20thDay
8%10%39%28%11%3
180
21st to endDay
7%5%30%31%22%4
198
January1st to 10thNight
8%8%37%34%12%1
950
11th to 20thNight
9%7%38%35%11%1
950
21st to endNight
12%9%34%34%10%2
846
February1st to 10thNight
10%10%29%33%16%3
760
11th to 20thNight
8%6%35%36%14%1
744
21st to endNight
6%7%29%33%20%5%
628
March1st to 10thNight
8%5%33%37%14%3
760
11th to 20thNight
9%6%32%37%13%2
752
21st to endNight
13%5%29%35%14%4
792
April1st to 10thNight
11%8%29%34%15%3
668
11th to 20thNight
17%7%30%36%11%
533
21st to endNight
16%8%27%37%10%1
540
May1st to 10thNight
17%7%29%31%14%
428
11th to 20thNight
12%5%26%41%16%
354
21st to endNight
20%422%36%16%1
389
June1st to 10thNight
19%7%22%37%15%1
360
11th to 20thNight
20%428%34%14%1
360
21st to endNight
10%421%41%22%2
360
July1st to 10thNight
12%417%40%26%1
350
11th to 20thNight
12%223%43%19%1
348
21st to endNight
8%316%43%28%2
394
August1st to 10thNight
10%314%41%30%3
358
11th to 20thNight
10%417%43%25%1
486
21st to endNight
10%322%42%23%1
594
September1st to 10thNight
13%325%38%18%3
630
11th to 20thNight
17%421%36%20%2
720
21st to endNight
14%5%30%35%14%3
720
October1st to 10thNight
17%428%34%15%3
720
11th to 20thNight
11%433%36%13%3
720
21st to endNight
11%9%34%34%12%
792
November1st to 10thNight
15%7%35%29%11%2
853
11th to 20thNight
9%10%41%29%9%2
950
21st to endNight
7%7%39%35%10%2
950
December1st to 10thNight
11%8%32%33%13%3
950
11th to 20thNight
7%7%39%36%11%1
950
21st to endNight
11%5%34%36%12%2
1045
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