CRUST Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the CRUST 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
36%
25014
14%
74825
8%
24957
42%
24957
22%
51553

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
28%
950
12%
2837
5%
946
35%
946
16%
2134
January11th to 20thNight
24%
921
8%
2693
5%
921
27%
921
13%
2140
January21st to endNight
24%
990
11%
2745
4%
990
30%
990
13%
2181
February1st to 10thNight
32%
900
11%
2340
4%
900
37%
900
19%
1754
February11th to 20thNight
32%
877
16%
2274
7%
874
37%
874
21%
1682
February21st to endNight
33%
676
17%
1860
8%
676
40%
676
21%
1350
March1st to 10thNight
31%
642
11%
2084
8%
642
35%
642
16%
1526
March11th to 20thNight
34%
540
20%
1980
13%
540
46%
540
21%
1260
March21st to endNight
34%
594
15%
2178
13%
594
42%
594
17%
1458
April1st to 10thNight
34%
540
16%
1815
13%
540
42%
540
21%
1167
April11th to 20thNight
33%
533
17%
1599
14%
533
40%
533
22%
1100
April21st to endNight
36%
540
14%
1620
10%
540
42%
540
22%
1120
May1st to 10thNight
37%
540
15%
1596
8%
540
41%
540
24%
1067
May11th to 20thNight
40%
531
17%
1416
6%
531
44%
531
30%
925
May21st to endNight
41%
584
14%
1362
4%
582
45%
582
35%
907
June1st to 10thNight
48%
540
13%
1260
6%
540
52%
540
39%
819
June11th to 20thNight
46%
540
13%
1260
6%
540
51%
540
40%
826
June21st to endNight
52%
540
13%
1260
7%
540
55%
540
42%
810
July1st to 10thNight
65%
525
17%
1225
9%
525
67%
525
55%
698
July11th to 20thNight
56%
522
18%
1218
7%
522
59%
522
51%
715
July21st to endNight
65%
591
20%
1401
11%
558
65%
558
48%
801
August1st to 10thNight
66%
537
21%
1601
14%
534
69%
534
43%
862
August11th to 20thNight
57%
534
20%
1600
15%
534
63%
534
39%
897
August21st to endNight
53%
594
24%
1782
23%
594
60%
594
39%
1001
September1st to 10thNight
53%
540
21%
1754
20%
540
60%
540
38%
950
September11th to 20thNight
50%
540
22%
1980
19%
540
61%
540
33%
1035
September21st to endNight
43%
540
24%
1980
19%
540
53%
540
28%
1129
October1st to 10thNight
36%
674
18%
2114
13%
674
42%
674
26%
1333
October11th to 20thNight
31%
720
21%
2160
11%
720
39%
720
24%
1431
October21st to endNight
28%
957
14%
2541
4%
957
35%
957
20%
1847
November1st to 10thNight
30%
925
17%
2527
6%
925
38%
925
21%
1810
November11th to 20thNight
26%
950
12%
2660
6%
950
31%
950
16%
2045
November21st to endNight
25%
950
12%
2660
4%
950
32%
950
18%
1956
December1st to 10thNight
26%
950
8%
2818
4%
950
30%
950
14%
2185
December11th to 20thNight
19%
949
7%
3466
3%
949
23%
949
10%
2704
December21st to endNight
20%
1038
6%
5159
4%
1026
26%
1026
6%
3928

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
12%9%47%6%35%5%6%7%36%
74678
Night
11%9%48%6%45%5%5%7%36%
75099
TransparencyDay
47%5%15%24%9%1
74368
Night
48%5%14%23%10%1
74942
SeeingDay
15%9%30%29%14%3
14966
Night
15%7%32%33%11%1
25026

By Month

Percentage Cloud Cover NE (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
6%11%6%14%9%5%7%6%6%8%24%
1328
11th to 20thDay
8%13%6%13%10%5%6%8%8%5%18%
1351
21st to endDay
10%16%8%14%8%6%6%5%6%7%15%
1611
February1st to 10thDay
8%12%6%10%8%5%6%7%7%6%24%
1620
11th to 20thDay
7%12%410%10%5%9%5%8%8%23%
1574
21st to endDay
7%10%5%11%11%6%7%5%5%7%25%
1412
March1st to 10thDay
12%12%6%10%8%36%45%7%26%
1873
11th to 20thDay
11%11%6%9%6%35%5%6%8%31%
1980
21st to endDay
14%12%5%10%7%3446%8%28%
2178
April1st to 10thDay
13%12%5%8%7%45%5%5%6%31%
2139
11th to 20thDay
17%11%47%6%35%5%5%6%30%
2312
21st to endDay
19%9%47%6%3446%6%32%
2340
May1st to 10thDay
18%8%36%6%3445%7%37%
2362
11th to 20thDay
15%6%35%5%445%6%8%38%
2479
21st to endDay
16%6%36%5%45%5%5%8%38%
2911
June1st to 10thDay
14%6%25%5%345%6%9%41%
2700
11th to 20thDay
15%5%35%435%5%6%8%42%
2700
21st to endDay
10%5%36%6%36%6%6%9%40%
2700
July1st to 10thDay
7%314435%6%7%9%50%
2700
11th to 20thDay
9%4246%35%5%6%9%47%
2700
21st to endDay
6%323435%5%7%9%53%
2833
August1st to 10thDay
8%426%5%35%6%7%9%46%
2328
11th to 20thDay
11%6%25%4235%5%9%48%
2301
21st to endDay
10%5%245%24448%52%
2574
September1st to 10thDay
12%6%25%43436%7%49%
2201
11th to 20thDay
13%7%26%43445%7%45%
1980
21st to endDay
13%6%25%6%345%6%8%42%
1980
October1st to 10thDay
17%10%36%6%34346%39%
1828
11th to 20thDay
16%11%46%5%34447%37%
1800
21st to endDay
14%11%5%9%6%35%5%46%31%
1814
November1st to 10thDay
14%12%6%9%6%34445%32%
1549
11th to 20thDay
12%14%7%13%8%44445%26%
1520
21st to endDay
11%12%5%11%8%45%446%28%
1520
December1st to 10thDay
9%15%6%12%10%5%6%7%5%5%20%
1359
11th to 20thDay
10%13%6%12%8%5%6%5%6%6%23%
1679
21st to endDay
17%13%7%12%8%5%5%45%420%
2442
January1st to 10thNight
7%10%6%11%9%5%6%6%6%7%26%
2837
11th to 20thNight
9%13%8%12%10%5%6%6%6%5%21%
2693
21st to endNight
10%13%5%11%9%6%7%6%6%7%21%
2745
February1st to 10thNight
7%11%5%10%7%5%7%7%6%7%28%
2340
11th to 20thNight
7%10%5%8%8%47%6%6%8%30%
2273
21st to endNight
7%11%6%8%8%45%6%6%7%31%
1860
March1st to 10thNight
11%12%7%9%7%45%5%6%6%29%
2073
11th to 20thNight
10%8%48%6%36%5%46%41%
1979
21st to endNight
12%10%49%7%3445%8%34%
2178
April1st to 10thNight
12%9%46%6%346%6%7%39%
1815
11th to 20thNight
18%10%46%5%2446%6%35%
1599
21st to endNight
17%9%46%6%24447%36%
1620
May1st to 10thNight
15%9%36%6%24447%40%
1596
11th to 20thNight
14%6%26%425%5%6%7%43%
1416
21st to endNight
13%5%3442446%9%45%
1362
June1st to 10thNight
11%4244247%6%10%45%
1260
11th to 20thNight
9%425%5%35%5%5%9%47%
1260
21st to endNight
7%426%5%346%6%10%48%
1260
July1st to 10thNight
43132246%6%9%60%
1260
11th to 20thNight
43243245%6%10%58%
1260
21st to endNight
6%21442446%9%58%
1483
August1st to 10thNight
5%3145%3345%9%56%
1611
11th to 20thNight
9%325%43446%8%54%
1600
21st to endNight
9%42443335%6%56%
1782
September1st to 10thNight
9%423334448%56%
1754
11th to 20thNight
10%42342446%8%53%
1980
21st to endNight
14%5%344245%5%7%48%
1980
October1st to 10thNight
16%7%25%43335%8%41%
2114
11th to 20thNight
17%9%45%5%245%47%39%
2160
21st to endNight
17%11%36%5%35%5%5%6%34%
2541
November1st to 10thNight
11%11%6%10%6%445%5%6%33%
2527
11th to 20thNight
11%13%7%11%8%45%445%28%
2660
21st to endNight
11%11%48%7%5%6%5%5%6%32%
2660
December1st to 10thNight
9%13%6%11%10%47%6%6%6%22%
2818
11th to 20thNight
10%14%6%12%9%5%7%5%5%5%22%
3518
21st to endNight
18%13%6%10%9%45%5%45%21%
5225

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
59%6%14%16%5%
1329
11th to 20thDay
63%5%13%16%4
1351
21st to endDay
69%5%9%11%6%
1611
February1st to 10thDay
57%5%14%16%8%
1620
11th to 20thDay
57%5%12%17%8%1
1583
21st to endDay
59%5%12%18%6%
1412
March1st to 10thDay
60%5%14%17%5%
1875
11th to 20thDay
52%6%15%22%6%1
1980
21st to endDay
55%5%13%21%5%
2178
April1st to 10thDay
52%6%10%21%10%1
2139
11th to 20thDay
54%412%19%10%1
2312
21st to endDay
53%414%21%7%1
2340
May1st to 10thDay
48%414%23%11%1
2362
11th to 20thDay
43%6%12%28%11%1
2483
21st to endDay
45%5%13%25%11%1
2911
June1st to 10thDay
41%419%27%9%
2700
11th to 20thDay
40%5%17%27%10%
2700
21st to endDay
40%6%17%29%8%1
2700
July1st to 10thDay
28%6%17%35%12%1
2626
11th to 20thDay
33%6%19%27%13%1
2619
21st to endDay
27%6%17%36%14%1
2675
August1st to 10thDay
33%7%18%30%10%2
2314
11th to 20thDay
34%5%19%30%11%
2301
21st to endDay
33%414%35%14%1
2574
September1st to 10thDay
35%415%32%13%1
2201
11th to 20thDay
39%418%30%9%1
1980
21st to endDay
39%5%16%34%6%
1980
October1st to 10thDay
48%415%24%9%
1828
11th to 20thDay
49%412%26%9%1
1800
21st to endDay
55%415%19%6%
1814
November1st to 10thDay
56%414%18%8%1
1549
11th to 20thDay
62%5%12%15%6%
1520
21st to endDay
58%6%15%15%5%
1520
December1st to 10thDay
67%412%15%3
1359
11th to 20thDay
61%417%14%4
1680
21st to endDay
68%37%18%4
2442
January1st to 10thNight
56%6%12%20%6%
2837
11th to 20thNight
64%412%13%6%
2693
21st to endNight
62%6%10%15%6%
2745
February1st to 10thNight
54%5%14%18%8%
2340
11th to 20thNight
51%6%11%20%11%
2274
21st to endNight
51%5%12%22%8%1
1860
March1st to 10thNight
55%6%13%20%6%
2084
11th to 20thNight
45%413%26%11%
1980
21st to endNight
49%6%14%21%9%1
2178
April1st to 10thNight
44%5%12%25%13%2
1815
11th to 20thNight
49%311%21%13%3
1599
21st to endNight
50%411%24%9%2
1620
May1st to 10thNight
46%411%23%15%1
1596
11th to 20thNight
40%5%13%25%13%3
1417
21st to endNight
37%6%13%28%14%2
1362
June1st to 10thNight
34%5%18%31%10%2
1260
11th to 20thNight
34%5%16%27%15%3
1260
21st to endNight
30%7%19%33%10%1
1260
July1st to 10thNight
19%6%17%38%17%2
1225
11th to 20thNight
23%6%20%33%18%2
1218
21st to endNight
24%5%15%34%19%2
1401
August1st to 10thNight
26%415%37%15%2
1601
11th to 20thNight
29%5%19%32%13%1
1600
21st to endNight
30%414%33%18%1
1782
September1st to 10thNight
28%5%15%35%16%1
1754
11th to 20thNight
29%6%19%33%13%1
1980
21st to endNight
36%5%12%34%12%1
1980
October1st to 10thNight
42%415%27%11%1
2114
11th to 20thNight
46%312%29%10%1
2160
21st to endNight
51%5%14%23%7%
2541
November1st to 10thNight
51%412%21%9%2
2527
11th to 20thNight
59%414%15%6%1
2660
21st to endNight
53%6%18%17%6%
2660
December1st to 10thNight
62%417%14%4
2818
11th to 20thNight
64%414%14%3
3516
21st to endNight
66%410%15%5%
5225

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
12%8%39%28%11%3
270
11th to 20thDay
15%9%30%33%13%
252
21st to endDay
20%8%33%26%12%2
264
February1st to 10thDay
12%15%41%27%5%
240
11th to 20thDay
11%16%40%20%11%1
240
21st to endDay
9%6%44%35%7%
198
March1st to 10thDay
15%11%34%30%9%1
240
11th to 20thDay
16%10%32%25%14%4
240
21st to endDay
16%12%31%30%8%3
264
April1st to 10thDay
19%16%32%24%8%1
399
11th to 20thDay
24%14%27%25%8%1
590
21st to endDay
23%9%31%25%11%1
600
May1st to 10thDay
19%10%28%30%11%2
603
11th to 20thDay
18%9%29%27%13%4
625
21st to endDay
18%6%27%29%16%3
679
June1st to 10thDay
13%8%26%31%17%5%
630
11th to 20thDay
15%10%33%26%14%3
630
21st to endDay
12%7%29%26%19%6%
630
July1st to 10thDay
8%325%35%22%7%
621
11th to 20thDay
12%5%29%28%21%5%
618
21st to endDay
5%325%37%22%8%
684
August1st to 10thDay
7%424%34%25%5%
626
11th to 20thDay
13%423%33%18%9%
615
21st to endDay
12%7%25%31%20%4
693
September1st to 10thDay
12%8%29%32%17%2
491
11th to 20thDay
17%6%33%33%8%3
270
21st to endDay
13%10%38%23%13%3
270
October1st to 10thDay
18%9%27%30%13%3
270
11th to 20thDay
17%13%33%29%8%1
270
21st to endDay
21%14%36%20%8%2
297
November1st to 10thDay
20%10%28%30%11%1
270
11th to 20thDay
18%13%35%26%7%
270
21st to endDay
19%13%37%27%4
270
December1st to 10thDay
16%9%37%30%7%2
270
11th to 20thDay
16%12%38%30%4
270
21st to endDay
18%8%34%27%9%2
297
January1st to 10thNight
10%8%37%32%11%1
950
11th to 20thNight
16%8%35%32%8%
921
21st to endNight
15%9%36%30%9%
990
February1st to 10thNight
12%9%32%35%10%2
900
11th to 20thNight
13%8%37%29%10%2
879
21st to endNight
12%7%38%29%11%2
676
March1st to 10thNight
14%7%35%30%11%2
644
11th to 20thNight
12%7%39%33%8%1
540
21st to endNight
18%8%30%30%13%2
594
April1st to 10thNight
17%9%34%31%8%1
540
11th to 20thNight
23%8%30%29%9%1
533
21st to endNight
20%8%28%33%11%
540
May1st to 10thNight
20%6%29%34%10%1
540
11th to 20thNight
16%6%33%33%12%1
531
21st to endNight
15%5%34%33%13%1
584
June1st to 10thNight
12%5%29%38%15%1
540
11th to 20thNight
11%5%31%40%13%
540
21st to endNight
7%330%41%18%1
540
July1st to 10thNight
5%225%46%21%1
525
11th to 20thNight
6%330%43%18%1
522
21st to endNight
6%321%47%20%2
591
August1st to 10thNight
7%219%49%23%1
537
11th to 20thNight
10%423%40%21%2
534
21st to endNight
12%425%39%18%2
594
September1st to 10thNight
12%328%39%16%2
540
11th to 20thNight
12%430%39%14%
540
21st to endNight
15%7%31%34%12%
540
October1st to 10thNight
21%8%30%32%9%
674
11th to 20thNight
23%9%31%29%8%
720
21st to endNight
23%10%32%28%7%
957
November1st to 10thNight
16%8%35%32%9%
925
11th to 20thNight
17%9%36%29%8%1
950
21st to endNight
17%11%40%26%5%1
950
December1st to 10thNight
14%8%35%32%10%1
950
11th to 20thNight
17%10%36%29%8%1
950
21st to endNight
23%8%31%29%8%1
1045
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