BRAS Dark Sky Site
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the BRAS Dark Sky Site clear sky chart from 2005-11-06 to 2024-03-01.

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
55%
23753
22%
79444
18%
23647
60%
23647
27%
47843

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
35%
945
24%
4000
10%
926
44%
926
19%
2325
January11th to 20thNight
39%
948
29%
3922
12%
930
47%
930
23%
2114
January21st to endNight
35%
1040
17%
4446
10%
1023
43%
1023
18%
2819
February1st to 10thNight
37%
761
24%
2281
13%
761
45%
761
25%
1510
February11th to 20thNight
36%
743
20%
2219
11%
740
43%
740
24%
1452
February21st to endNight
29%
596
18%
1852
8%
596
35%
596
22%
1222
March1st to 10thNight
41%
542
25%
1986
18%
542
50%
542
25%
1164
March11th to 20thNight
40%
540
20%
1980
14%
540
48%
540
20%
1241
March21st to endNight
45%
594
25%
2153
18%
594
56%
594
30%
1177
April1st to 10thNight
44%
540
25%
1800
18%
540
53%
540
27%
1084
April11th to 20thNight
43%
533
24%
1775
18%
533
54%
533
26%
1070
April21st to endNight
54%
540
28%
1800
25%
540
64%
540
34%
995
May1st to 10thNight
57%
540
24%
1800
23%
540
65%
540
34%
1003
May11th to 20thNight
63%
531
20%
1770
22%
531
66%
531
32%
1023
May21st to endNight
73%
584
21%
1938
24%
582
75%
582
35%
1076
June1st to 10thNight
79%
540
17%
1628
19%
540
80%
540
32%
994
June11th to 20thNight
79%
540
18%
1620
17%
540
82%
540
35%
955
June21st to endNight
70%
540
14%
1620
15%
540
71%
540
26%
1073
July1st to 10thNight
79%
525
12%
1575
13%
525
80%
525
28%
1016
July11th to 20thNight
79%
522
14%
1566
16%
522
79%
522
28%
1004
July21st to endNight
78%
591
12%
1720
14%
558
79%
558
29%
1092
August1st to 10thNight
84%
537
13%
1778
16%
534
85%
534
37%
995
August11th to 20thNight
79%
534
14%
1776
18%
534
79%
534
25%
1115
August21st to endNight
74%
594
14%
1980
16%
594
74%
594
24%
1298
September1st to 10thNight
74%
540
20%
1800
23%
540
75%
540
28%
1118
September11th to 20thNight
69%
540
21%
1800
24%
540
69%
540
25%
1156
September21st to endNight
72%
679
27%
1995
25%
679
73%
679
34%
1186
October1st to 10thNight
71%
720
28%
2160
25%
720
72%
720
43%
1145
October11th to 20thNight
67%
720
33%
2160
27%
720
70%
720
40%
1167
October21st to endNight
61%
792
33%
2376
28%
792
65%
792
37%
1331
November1st to 10thNight
62%
740
31%
2220
27%
740
67%
740
39%
1222
November11th to 20thNight
53%
760
32%
2280
24%
760
61%
760
34%
1310
November21st to endNight
47%
760
29%
2280
21%
760
55%
760
30%
1405
December1st to 10thNight
45%
760
28%
2280
20%
760
53%
760
27%
1450
December11th to 20thNight
42%
805
23%
2739
14%
805
47%
805
21%
1774
December21st to endNight
35%
1037
12%
4369
9%
1026
39%
1026
16%
2762

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
11%6%36%6%46%5%6%8%38%
79094
Night
12%6%25%4345%6%8%46%
80342
TransparencyDay
42%8%11%17%16%6%
78824
Night
35%8%13%19%18%8%
80134
SeeingDay
13%5%16%24%25%17%
13872
Night
14%5%18%33%25%6%
23775

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%9%36%5%3445%7%37%
2847
11th to 20thDay
13%6%24423235%55%
2850
21st to endDay
23%9%46%434445%35%
3135
February1st to 10thDay
17%8%46%5%3445%6%37%
1894
11th to 20thDay
18%8%35%43446%6%40%
1848
21st to endDay
17%9%27%6%35%448%36%
1617
March1st to 10thDay
16%7%36%6%3445%7%39%
1980
11th to 20thDay
14%9%35%7%35%5%5%8%37%
1980
21st to endDay
10%6%27%6%345%5%8%44%
2200
April1st to 10thDay
10%7%45%5%46%6%5%8%42%
2160
11th to 20thDay
14%7%36%5%45%5%6%7%38%
2134
21st to endDay
9%435%5%36%6%6%9%43%
2160
May1st to 10thDay
9%5%36%6%46%5%6%7%42%
2160
11th to 20thDay
8%6%27%6%46%6%6%8%42%
2124
21st to endDay
6%5%47%8%6%7%7%8%9%33%
2337
June1st to 10thDay
7%5%36%8%5%8%7%8%10%33%
2326
11th to 20thDay
5%5%48%9%6%7%8%8%10%32%
2340
21st to endDay
6%6%49%10%6%8%9%8%10%24%
2340
July1st to 10thDay
8%7%48%10%6%8%8%8%9%24%
2340
11th to 20thDay
7%6%38%9%5%10%7%8%11%25%
2340
21st to endDay
8%7%47%9%6%8%8%7%9%27%
2492
August1st to 10thDay
5%5%38%8%5%8%9%9%11%30%
2148
11th to 20thDay
10%6%37%7%6%8%8%8%9%28%
2125
21st to endDay
11%6%37%8%5%7%6%7%8%31%
2376
September1st to 10thDay
9%5%37%7%6%6%6%7%10%35%
2160
11th to 20thDay
12%6%35%6%46%5%6%8%38%
2160
21st to endDay
10%5%26%6%35%46%7%47%
1941
October1st to 10thDay
7%425%4345%5%7%55%
1800
11th to 20thDay
9%425%43445%7%53%
1800
21st to endDay
11%4245%3445%6%53%
1980
November1st to 10thDay
8%5%344345%5%8%51%
1858
11th to 20thDay
10%6%3443445%5%53%
1900
21st to endDay
13%6%26%5%345%5%6%45%
1900
December1st to 10thDay
15%7%345%35%5%5%7%41%
1900
11th to 20thDay
15%10%37%5%45%46%7%36%
2307
21st to endDay
16%8%35%6%3446%6%39%
3135
January1st to 10thNight
17%7%34434446%45%
4108
11th to 20thNight
15%5%25%423345%52%
4120
21st to endNight
25%7%25%323347%39%
4532
February1st to 10thNight
15%8%36%5%35%45%6%42%
2281
11th to 20thNight
19%6%35%43335%5%44%
2218
21st to endNight
20%9%25%423446%40%
1852
March1st to 10thNight
14%7%36%43445%8%44%
1986
11th to 20thNight
14%7%45%5%35%5%5%8%40%
1980
21st to endNight
10%5%2442445%8%51%
2153
April1st to 10thNight
12%5%35%435%5%6%8%45%
1800
11th to 20thNight
12%6%35%5%25%45%6%47%
1775
21st to endNight
7%435%4245%6%8%53%
1800
May1st to 10thNight
9%42443446%9%51%
1800
11th to 20thNight
7%5%25%43447%8%50%
1769
21st to endNight
6%4245%346%7%10%49%
1938
June1st to 10thNight
5%424436%6%9%12%45%
1628
11th to 20thNight
3324446%7%9%13%46%
1620
21st to endNight
7%426%7%47%6%7%10%39%
1620
July1st to 10thNight
45%44437%8%9%11%42%
1620
11th to 20thNight
4425%5%35%8%9%14%41%
1620
21st to endNight
4335%6%47%7%8%14%39%
1821
August1st to 10thNight
33144246%7%13%51%
1790
11th to 20thNight
4325%7%5%5%7%10%14%38%
1776
21st to endNight
7%436%6%47%6%7%12%38%
1980
September1st to 10thNight
7%5%26%5%45%6%7%10%43%
1800
11th to 20thNight
9%7%26%5%46%5%7%8%40%
1800
21st to endNight
7%425%5%445%6%8%49%
1995
October1st to 10thNight
5%4235%245%47%59%
2160
11th to 20thNight
7%425%42345%6%58%
2160
21st to endNight
10%5%24334445%57%
2376
November1st to 10thNight
8%425%424346%58%
2220
11th to 20thNight
12%5%25%324445%55%
2280
21st to endNight
15%6%35%43345%7%47%
2280
December1st to 10thNight
15%7%36%5%23446%45%
2280
11th to 20thNight
14%9%35%43445%6%42%
2872
21st to endNight
17%10%36%5%24346%40%
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
45%6%7%13%22%8%
2848
11th to 20thDay
33%46%18%30%9%
2850
21st to endDay
52%45%12%19%7%
3135
February1st to 10thDay
48%6%8%13%17%9%
1894
11th to 20thDay
45%5%7%14%19%10%
1858
21st to endDay
47%5%8%16%16%8%
1617
March1st to 10thDay
44%5%9%18%16%7%
1980
11th to 20thDay
44%7%11%11%20%6%
1980
21st to endDay
38%7%9%20%21%6%
2200
April1st to 10thDay
39%7%11%16%18%8%
2160
11th to 20thDay
43%6%9%18%15%8%
2134
21st to endDay
35%8%10%19%23%4
2160
May1st to 10thDay
37%8%11%21%17%6%
2160
11th to 20thDay
38%8%10%24%18%2
2128
21st to endDay
42%9%15%21%12%1
2337
June1st to 10thDay
44%10%13%22%11%
2326
11th to 20thDay
42%11%16%19%11%1
2340
21st to endDay
49%13%17%15%5%
2340
July1st to 10thDay
53%12%17%14%4
2276
11th to 20thDay
48%14%18%15%4
2271
21st to endDay
48%12%19%15%6%1
2354
August1st to 10thDay
42%13%21%17%6%
2136
11th to 20thDay
47%13%18%15%41
2125
21st to endDay
48%10%17%14%8%2
2376
September1st to 10thDay
42%11%15%16%11%4
2160
11th to 20thDay
42%8%16%14%14%6%
2160
21st to endDay
36%7%11%21%17%8%
1941
October1st to 10thDay
28%6%10%21%21%13%
1800
11th to 20thDay
30%5%8%16%27%13%
1800
21st to endDay
31%5%7%15%26%15%
1980
November1st to 10thDay
32%49%19%26%10%
1858
11th to 20thDay
33%5%8%17%26%12%
1900
21st to endDay
39%6%7%17%22%9%
1900
December1st to 10thDay
40%7%10%19%16%8%
1900
11th to 20thDay
47%5%8%12%21%7%
2305
21st to endDay
43%7%10%14%24%3
3135
January1st to 10thNight
39%5%8%14%26%9%
4108
11th to 20thNight
33%5%6%18%26%11%
4120
21st to endNight
46%46%17%18%9%
4532
February1st to 10thNight
43%6%7%15%20%9%
2281
11th to 20thNight
43%5%6%16%18%12%
2219
21st to endNight
44%6%10%14%15%11%
1852
March1st to 10thNight
38%5%11%18%18%10%
1986
11th to 20thNight
41%7%12%14%17%7%
1980
21st to endNight
31%7%12%21%22%8%
2153
April1st to 10thNight
35%8%12%18%17%10%
1800
11th to 20thNight
37%6%10%19%17%10%
1775
21st to endNight
28%8%11%21%25%7%
1800
May1st to 10thNight
30%7%11%26%19%7%
1800
11th to 20thNight
30%9%14%23%21%3
1770
21st to endNight
28%8%19%28%16%2
1939
June1st to 10thNight
29%12%20%24%15%1
1628
11th to 20thNight
27%11%20%28%12%2
1620
21st to endNight
37%12%20%23%7%1
1620
July1st to 10thNight
31%15%28%20%6%
1575
11th to 20thNight
28%17%30%19%5%
1566
21st to endNight
29%16%29%19%7%
1720
August1st to 10thNight
21%14%33%24%7%1
1778
11th to 20thNight
32%15%28%20%5%1
1776
21st to endNight
36%14%22%16%8%3
1980
September1st to 10thNight
32%13%20%20%10%5%
1800
11th to 20thNight
37%11%17%17%13%5%
1800
21st to endNight
32%7%14%22%15%10%
1995
October1st to 10thNight
24%6%12%23%22%14%
2160
11th to 20thNight
26%6%9%19%25%15%
2160
21st to endNight
29%5%6%16%25%18%
2376
November1st to 10thNight
28%5%10%19%25%13%
2220
11th to 20thNight
32%5%8%16%25%15%
2280
21st to endNight
37%5%8%18%21%11%
2280
December1st to 10thNight
40%5%7%20%20%7%
2280
11th to 20thNight
42%6%7%13%19%13%
2873
21st to endNight
46%5%9%14%19%7%
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
21%13%35%20%9%3
270
11th to 20thDay
16%12%37%25%8%1
270
21st to endDay
27%9%36%24%5%
297
February1st to 10thDay
24%11%28%27%9%
270
11th to 20thDay
25%13%30%23%9%
270
21st to endDay
24%16%38%18%4
225
March1st to 10thDay
24%9%33%26%7%
240
11th to 20thDay
23%12%37%19%8%1
240
21st to endDay
17%8%36%24%13%1
286
April1st to 10thDay
18%7%28%31%14%2
420
11th to 20thDay
22%7%29%24%16%2
412
21st to endDay
11%5%29%30%21%4
420
May1st to 10thDay
11%5%22%34%23%4
423
11th to 20thDay
13%116%29%29%12%
447
21st to endDay
6%211%30%34%18%
485
June1st to 10thDay
10%28%29%33%18%
616
11th to 20thDay
7%211%25%37%18%
630
21st to endDay
10%37%23%30%27%
630
July1st to 10thDay
10%2419%34%32%
621
11th to 20thDay
7%2217%38%34%
618
21st to endDay
10%2319%35%32%
630
August1st to 10thDay
5%2212%32%47%
446
11th to 20thDay
8%219%31%49%
439
21st to endDay
12%3314%35%34%
495
September1st to 10thDay
10%3217%39%30%
450
11th to 20thDay
13%1419%38%25%
450
21st to endDay
11%35%21%37%23%
388
October1st to 10thDay
7%213%37%28%13%
270
11th to 20thDay
8%314%36%24%14%
270
21st to endDay
17%6%22%27%20%7%
297
November1st to 10thDay
14%120%31%28%6%
270
11th to 20thDay
16%6%24%32%17%4
270
21st to endDay
17%8%30%26%16%3
270
December1st to 10thDay
19%7%24%30%14%6%
270
11th to 20thDay
23%7%31%30%8%
270
21st to endDay
26%6%30%25%11%2
297
January1st to 10thNight
21%7%30%33%8%1
950
11th to 20thNight
18%8%29%35%10%1
950
21st to endNight
22%8%30%33%7%
1045
February1st to 10thNight
20%8%30%35%6%
761
11th to 20thNight
23%10%26%35%7%
743
21st to endNight
24%10%34%29%3
596
March1st to 10thNight
17%8%28%38%9%
542
11th to 20thNight
16%6%29%38%11%
540
21st to endNight
14%5%32%37%12%
594
April1st to 10thNight
14%7%30%37%11%1
540
11th to 20thNight
15%8%26%36%14%1
533
21st to endNight
8%427%44%15%1
540
May1st to 10thNight
11%222%46%17%1
540
11th to 20thNight
10%216%43%27%2
531
21st to endNight
9%18%39%36%7%
584
June1st to 10thNight
7%1439%41%8%
540
11th to 20thNight
416%33%45%10%
540
21st to endNight
9%16%24%44%16%
540
July1st to 10thNight
5%2121%52%20%
525
11th to 20thNight
42116%52%26%
522
21st to endNight
5%1218%54%20%
591
August1st to 10thNight
41117%51%25%
537
11th to 20thNight
5%114%53%27%
534
21st to endNight
8%1317%49%21%
594
September1st to 10thNight
9%2318%49%19%
540
11th to 20thNight
14%2421%48%11%
540
21st to endNight
10%2428%47%9%
679
October1st to 10thNight
8%110%35%39%6%
720
11th to 20thNight
10%313%41%31%3
720
21st to endNight
14%417%38%25%3
792
November1st to 10thNight
11%417%41%25%3
740
11th to 20thNight
15%5%21%39%19%1
760
21st to endNight
19%424%38%15%1
760
December1st to 10thNight
20%8%21%35%16%1
760
11th to 20thNight
21%5%24%37%12%
807
21st to endNight
26%7%25%33%9%
1045
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