Brooks Astronomical Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Brooks Astronomical Observatory clear sky chart from 2005-11-06 to 2024-01-14.

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
31%
24646
14%
75007
8%
24587
38%
24587
19%
52834

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
15%
950
8%
2648
3%
946
19%
946
9%
2148
January11th to 20thNight
16%
918
7%
2569
3%
918
20%
918
9%
2126
January21st to endNight
19%
982
8%
2764
3%
982
23%
982
12%
2215
February1st to 10thNight
17%
720
9%
2340
3%
720
23%
720
9%
1836
February11th to 20thNight
18%
700
13%
2100
5%
699
27%
699
12%
1604
February21st to endNight
15%
592
12%
1776
2%
592
24%
592
11%
1347
March1st to 10thNight
25%
720
16%
2160
8%
720
32%
720
18%
1494
March11th to 20thNight
27%
720
22%
2003
9%
720
37%
720
22%
1348
March21st to endNight
25%
779
16%
2165
7%
779
32%
779
19%
1550
April1st to 10thNight
24%
540
14%
1800
6%
540
34%
540
18%
1223
April11th to 20thNight
28%
533
20%
1674
10%
533
40%
533
20%
1114
April21st to endNight
32%
540
17%
1620
6%
540
41%
540
22%
1091
May1st to 10thNight
30%
540
14%
1620
6%
540
37%
540
21%
1122
May11th to 20thNight
36%
531
18%
1593
9%
531
46%
531
24%
1045
May21st to endNight
45%
583
15%
1578
9%
582
52%
582
32%
1007
June1st to 10thNight
44%
540
15%
1440
8%
540
50%
540
32%
915
June11th to 20thNight
46%
540
16%
1440
11%
540
51%
540
32%
910
June21st to endNight
49%
540
18%
1440
10%
540
55%
540
35%
895
July1st to 10thNight
63%
525
18%
1400
14%
525
66%
525
43%
808
July11th to 20thNight
58%
522
21%
1392
15%
522
64%
522
40%
821
July21st to endNight
57%
591
19%
1488
12%
558
61%
558
39%
885
August1st to 10thNight
59%
537
19%
1600
14%
534
64%
534
36%
938
August11th to 20thNight
60%
534
23%
1598
16%
534
67%
534
38%
908
August21st to endNight
54%
594
23%
1871
17%
594
60%
594
33%
1078
September1st to 10thNight
52%
580
21%
1840
17%
580
58%
580
32%
1071
September11th to 20thNight
54%
720
27%
1980
19%
720
59%
720
38%
1156
September21st to endNight
45%
720
21%
1980
15%
720
49%
720
28%
1291
October1st to 10thNight
40%
720
21%
1980
15%
720
47%
720
30%
1268
October11th to 20thNight
25%
720
14%
2206
7%
720
34%
720
18%
1563
October21st to endNight
21%
792
13%
2574
7%
792
29%
792
14%
1867
November1st to 10thNight
27%
740
21%
2406
12%
740
39%
740
21%
1548
November11th to 20thNight
20%
760
13%
2470
6%
760
29%
760
14%
1789
November21st to endNight
20%
760
11%
2470
4%
760
28%
760
13%
1830
December1st to 10thNight
16%
872
6%
2770
2%
872
20%
872
9%
2233
December11th to 20thNight
14%
950
8%
3392
2%
948
20%
948
8%
2662
December21st to endNight
11%
1041
4%
4860
1%
1026
17%
1026
4%
4128

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
13%9%47%6%45%5%6%7%34%
74505
Night
15%10%47%6%35%5%5%7%34%
75247
TransparencyDay
48%7%13%20%11%1
74207
Night
48%7%13%20%11%1
75065
SeeingDay
18%16%30%20%11%4
14507
Night
19%12%31%27%10%1
24654

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
14%15%6%10%9%46%5%5%5%20%
1517
11th to 20thDay
15%17%8%12%9%5%6%5%5%6%12%
1464
21st to endDay
14%16%9%12%10%47%5%45%15%
1589
February1st to 10thDay
12%13%5%11%10%5%6%6%5%7%20%
1620
11th to 20thDay
12%13%6%11%10%5%6%445%23%
1744
21st to endDay
13%13%6%10%10%5%6%5%46%22%
1480
March1st to 10thDay
12%10%48%7%45%46%7%30%
1800
11th to 20thDay
12%9%38%6%46%5%5%7%34%
1956
21st to endDay
17%9%37%7%45%446%33%
2206
April1st to 10thDay
17%13%47%6%3445%7%30%
2160
11th to 20thDay
16%10%46%5%35%5%5%7%35%
2235
21st to endDay
16%8%47%8%45%5%5%6%33%
2340
May1st to 10thDay
17%7%35%6%36%5%6%7%36%
2340
11th to 20thDay
16%6%37%5%35%45%7%39%
2301
21st to endDay
12%5%25%5%35%5%6%9%42%
2696
June1st to 10thDay
11%425%5%36%5%6%9%43%
2520
11th to 20thDay
11%5%35%5%45%5%5%9%43%
2520
21st to endDay
11%5%25%5%45%7%7%8%41%
2520
July1st to 10thDay
5%4245%36%6%7%9%49%
2520
11th to 20thDay
8%5%25%5%35%46%9%48%
2520
21st to endDay
8%425%5%45%5%7%10%44%
2739
August1st to 10thDay
9%5%26%5%35%6%8%8%42%
2327
11th to 20thDay
9%5%26%5%46%6%5%8%44%
2303
21st to endDay
11%5%25%5%35%5%7%8%44%
2483
September1st to 10thDay
10%5%36%7%5%5%5%6%8%41%
2102
11th to 20thDay
11%5%25%5%4446%9%46%
1980
21st to endDay
13%6%36%5%45%6%5%8%40%
1980
October1st to 10thDay
15%7%26%5%35%5%5%6%40%
1980
11th to 20thDay
20%10%47%6%35%6%5%7%28%
1751
21st to endDay
21%9%48%6%5%6%5%47%25%
1782
November1st to 10thDay
13%8%47%6%35%46%7%36%
1672
11th to 20thDay
13%14%5%8%7%5%5%5%6%6%25%
1710
21st to endDay
20%14%5%9%6%45%45%5%24%
1706
December1st to 10thDay
13%16%7%15%8%44435%19%
1406
11th to 20thDay
14%14%7%9%8%45%6%5%6%23%
1753
21st to endDay
26%18%6%10%7%445%4410%
2783
January1st to 10thNight
15%14%6%12%8%5%5%5%5%6%20%
2648
11th to 20thNight
16%16%7%10%6%45%5%5%6%19%
2569
21st to endNight
12%13%8%10%9%45%5%6%6%21%
2764
February1st to 10thNight
14%14%411%9%5%6%45%6%22%
2340
11th to 20thNight
13%13%5%9%8%45%5%46%27%
2100
21st to endNight
16%13%6%9%6%35%5%5%6%26%
1776
March1st to 10thNight
14%9%47%7%35%5%5%7%36%
2160
11th to 20thNight
13%7%36%6%45%5%5%6%41%
2003
21st to endNight
19%8%47%5%35%45%6%35%
2165
April1st to 10thNight
16%10%35%6%35%46%6%35%
1800
11th to 20thNight
18%8%26%5%3445%6%39%
1674
21st to endNight
17%9%345%335%5%7%39%
1620
May1st to 10thNight
13%8%37%7%35%5%5%8%36%
1620
11th to 20thNight
17%5%26%435%5%5%7%41%
1592
21st to endNight
12%4344345%6%8%47%
1577
June1st to 10thNight
10%6%35%5%35%5%5%9%44%
1440
11th to 20thNight
10%5%35%5%345%6%8%46%
1440
21st to endNight
8%425%5%35%5%5%9%49%
1440
July1st to 10thNight
5%4244245%5%10%55%
1440
11th to 20thNight
7%415%435%5%6%8%53%
1440
21st to endNight
7%4234346%7%7%54%
1576
August1st to 10thNight
7%325%5%35%5%7%9%50%
1611
11th to 20thNight
8%4234345%5%8%53%
1598
21st to endNight
10%5%345%2345%8%51%
1871
September1st to 10thNight
10%425%5%345%5%7%51%
1840
11th to 20thNight
10%42343345%7%54%
1980
21st to endNight
13%6%35%5%45%5%5%7%43%
1980
October1st to 10thNight
18%5%2442345%6%46%
1980
11th to 20thNight
19%10%37%5%25%445%34%
2206
21st to endNight
22%10%47%5%34447%30%
2574
November1st to 10thNight
13%9%36%5%35%45%7%41%
2406
11th to 20thNight
15%12%47%7%45%5%7%7%27%
2470
21st to endNight
18%12%48%6%45%5%5%7%26%
2470
December1st to 10thNight
15%15%7%11%6%5%6%5%5%5%20%
2770
11th to 20thNight
14%14%6%10%8%45%5%6%6%21%
3423
21st to endNight
28%20%6%8%6%3443415%
4884

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
64%8%12%11%4
1518
11th to 20thDay
73%8%8%8%2
1464
21st to endDay
71%7%8%10%5%
1589
February1st to 10thDay
63%7%11%14%4
1620
11th to 20thDay
63%6%10%13%8%1
1754
21st to endDay
62%6%10%14%6%1
1480
March1st to 10thDay
52%7%13%19%8%1
1800
11th to 20thDay
50%5%12%18%13%2
1956
21st to endDay
53%6%13%18%9%2
2206
April1st to 10thDay
55%5%11%18%10%
2160
11th to 20thDay
49%6%12%19%13%2
2235
21st to endDay
50%6%12%17%12%2
2340
May1st to 10thDay
45%8%13%21%11%2
2340
11th to 20thDay
44%7%11%22%14%2
2305
21st to endDay
39%7%15%25%14%1
2696
June1st to 10thDay
37%7%13%27%14%1
2520
11th to 20thDay
38%7%14%22%18%1
2520
21st to endDay
37%9%15%23%14%2
2520
July1st to 10thDay
28%9%14%28%17%3
2451
11th to 20thDay
33%8%12%27%18%2
2445
21st to endDay
33%9%15%24%17%3
2587
August1st to 10thDay
37%8%15%26%13%2
2314
11th to 20thDay
36%9%13%24%16%2
2303
21st to endDay
35%8%14%25%15%1
2483
September1st to 10thDay
39%8%16%23%12%1
2102
11th to 20thDay
35%7%15%29%13%1
1980
21st to endDay
41%8%14%25%12%
1980
October1st to 10thDay
44%7%10%24%14%2
1980
11th to 20thDay
55%7%14%18%6%
1751
21st to endDay
58%8%13%15%5%1
1782
November1st to 10thDay
45%7%14%22%11%
1672
11th to 20thDay
57%8%13%15%6%
1710
21st to endDay
61%6%15%13%5%
1706
December1st to 10thDay
68%7%10%12%2
1406
11th to 20thDay
61%8%14%14%3
1749
21st to endDay
74%8%8%7%3
2783
January1st to 10thNight
64%7%11%13%5%
2648
11th to 20thNight
65%8%11%10%6%1
2569
21st to endNight
62%7%12%12%5%1
2764
February1st to 10thNight
62%6%10%15%5%1
2340
11th to 20thNight
58%6%10%16%8%1
2101
21st to endNight
58%7%12%15%8%1
1776
March1st to 10thNight
48%6%13%22%10%1
2160
11th to 20thNight
42%7%12%21%15%3
2003
21st to endNight
51%5%11%21%11%2
2165
April1st to 10thNight
48%5%12%19%15%1
1800
11th to 20thNight
45%5%9%21%16%3
1674
21st to endNight
44%6%11%21%14%4
1620
May1st to 10thNight
45%8%12%20%13%3
1620
11th to 20thNight
42%7%11%20%18%3
1593
21st to endNight
36%6%12%27%15%3
1578
June1st to 10thNight
36%6%12%27%17%2
1440
11th to 20thNight
34%7%14%22%21%1
1440
21st to endNight
32%8%13%29%16%2
1440
July1st to 10thNight
24%9%12%31%21%5%
1400
11th to 20thNight
27%7%15%30%19%2
1392
21st to endNight
27%8%17%26%18%3
1488
August1st to 10thNight
27%10%16%31%15%1
1600
11th to 20thNight
28%7%15%31%17%2
1598
21st to endNight
32%6%12%29%20%2
1871
September1st to 10thNight
31%8%13%29%18%1
1840
11th to 20thNight
28%7%14%34%16%1
1980
21st to endNight
38%7%15%27%12%1
1980
October1st to 10thNight
37%6%12%27%16%2
1980
11th to 20thNight
50%6%13%21%9%1
2206
21st to endNight
54%6%13%18%9%
2574
November1st to 10thNight
42%6%14%24%14%1
2406
11th to 20thNight
52%8%16%17%7%1
2470
21st to endNight
56%7%16%16%5%1
2470
December1st to 10thNight
64%8%12%12%4
2770
11th to 20thNight
61%8%14%11%6%1
3425
21st to endNight
73%6%10%7%4
4884

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
24%19%37%14%41
270
11th to 20thDay
26%21%37%12%4
249
21st to endDay
26%20%36%15%3
264
February1st to 10thDay
26%30%31%12%1
240
11th to 20thDay
24%30%37%8%1
240
21st to endDay
36%32%27%6%
198
March1st to 10thDay
26%31%34%8%
240
11th to 20thDay
23%19%31%21%5%1
396
21st to endDay
28%17%30%18%6%1
462
April1st to 10thDay
24%20%32%17%6%1
420
11th to 20thDay
28%21%29%12%7%3
412
21st to endDay
21%19%31%20%8%2
420
May1st to 10thDay
24%20%29%16%8%3
423
11th to 20thDay
19%15%31%19%11%5%
447
21st to endDay
14%12%29%25%14%5%
650
June1st to 10thDay
11%12%37%26%10%4
630
11th to 20thDay
15%13%33%23%12%5%
630
21st to endDay
13%10%34%25%14%3
630
July1st to 10thDay
7%7%27%28%22%9%
621
11th to 20thDay
9%10%28%29%16%8%
618
21st to endDay
8%8%32%28%17%7%
680
August1st to 10thDay
9%7%26%24%21%14%
446
11th to 20thDay
10%5%21%31%21%12%
439
21st to endDay
14%8%23%28%20%7%
495
September1st to 10thDay
12%11%27%25%15%10%
450
11th to 20thDay
10%8%21%28%27%7%
450
21st to endDay
19%14%21%22%18%6%
450
October1st to 10thDay
21%15%25%21%14%4
450
11th to 20thDay
25%19%33%17%5%
401
21st to endDay
28%21%37%10%41
297
November1st to 10thDay
21%22%37%13%6%
270
11th to 20thDay
22%27%38%9%4
270
21st to endDay
25%28%32%12%3
268
December1st to 10thDay
29%28%27%9%6%1
180
11th to 20thDay
24%29%36%10%1
204
21st to endDay
30%27%32%10%11
297
January1st to 10thNight
22%20%34%18%5%1
950
11th to 20thNight
24%18%31%22%41
918
21st to endNight
18%14%37%24%7%
982
February1st to 10thNight
21%16%36%24%3
720
11th to 20thNight
21%18%33%23%4
703
21st to endNight
25%21%33%18%3
592
March1st to 10thNight
19%16%33%25%7%
720
11th to 20thNight
18%16%35%23%7%1
720
21st to endNight
23%15%32%24%5%1
779
April1st to 10thNight
21%15%35%23%5%1
540
11th to 20thNight
22%15%31%24%7%1
533
21st to endNight
21%11%33%27%7%1
540
May1st to 10thNight
18%9%36%29%7%1
540
11th to 20thNight
20%7%32%30%10%1
531
21st to endNight
14%5%30%32%17%2
584
June1st to 10thNight
11%7%30%37%13%1
540
11th to 20thNight
13%6%29%35%16%1
540
21st to endNight
9%429%39%18%1
540
July1st to 10thNight
7%420%42%24%4
525
11th to 20thNight
8%5%22%40%22%3
522
21st to endNight
8%229%40%19%1
591
August1st to 10thNight
9%322%43%20%3
537
11th to 20thNight
8%321%39%25%4
534
21st to endNight
12%6%22%34%25%2
594
September1st to 10thNight
11%5%25%35%20%4
580
11th to 20thNight
12%5%23%35%22%3
720
21st to endNight
15%9%26%28%19%4
720
October1st to 10thNight
21%8%27%28%15%1
720
11th to 20thNight
26%14%30%22%7%1
720
21st to endNight
28%14%33%20%5%
792
November1st to 10thNight
19%15%33%23%10%
740
11th to 20thNight
23%19%32%20%6%
760
21st to endNight
24%17%32%23%4
760
December1st to 10thNight
24%17%33%21%4
872
11th to 20thNight
24%18%37%18%3
950
21st to endNight
32%16%33%18%2
1045
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