BoonHill Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the BoonHill Observatory clear sky chart from 2005-11-06 to 2024-01-13.

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
30%
24724
12%
74930
8%
24668
35%
24668
16%
56338

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%
950
4%
2648
2%
946
16%
946
6%
2345
January11th to 20thNight
13%
911
4%
2549
2%
911
16%
911
6%
2266
January21st to endNight
17%
990
3%
2772
2%
990
18%
990
6%
2473
February1st to 10thNight
16%
792
6%
2390
3%
792
19%
792
7%
2068
February11th to 20thNight
17%
700
9%
2099
3%
700
22%
700
8%
1729
February21st to endNight
16%
592
8%
1776
1%
592
22%
592
10%
1443
March1st to 10thNight
25%
720
14%
2160
8%
720
31%
720
17%
1576
March11th to 20thNight
24%
720
18%
2039
8%
720
32%
720
19%
1473
March21st to endNight
27%
723
16%
2109
10%
723
33%
723
18%
1523
April1st to 10thNight
22%
540
12%
1800
7%
540
30%
540
15%
1304
April11th to 20thNight
27%
533
18%
1700
10%
533
36%
533
19%
1179
April21st to endNight
33%
540
17%
1620
8%
540
41%
540
21%
1120
May1st to 10thNight
30%
540
14%
1620
8%
540
37%
540
21%
1121
May11th to 20thNight
34%
531
17%
1528
9%
531
41%
531
23%
1051
May21st to endNight
44%
583
15%
1552
8%
582
51%
582
32%
999
June1st to 10thNight
42%
540
15%
1440
9%
540
47%
540
30%
945
June11th to 20thNight
45%
540
15%
1440
9%
540
49%
540
30%
935
June21st to endNight
50%
540
15%
1440
8%
540
55%
540
30%
939
July1st to 10thNight
62%
525
17%
1400
12%
525
65%
525
44%
805
July11th to 20thNight
57%
522
19%
1392
13%
522
62%
522
39%
835
July21st to endNight
57%
591
18%
1487
13%
558
59%
558
36%
931
August1st to 10thNight
60%
537
19%
1527
15%
534
62%
534
33%
924
August11th to 20thNight
61%
534
23%
1598
19%
534
66%
534
36%
925
August21st to endNight
53%
594
20%
1908
17%
594
57%
594
28%
1174
September1st to 10thNight
55%
543
20%
1803
18%
543
59%
543
31%
1048
September11th to 20thNight
52%
720
24%
1980
19%
720
56%
720
33%
1237
September21st to endNight
43%
720
18%
1980
14%
720
46%
720
25%
1361
October1st to 10thNight
39%
720
19%
1980
13%
720
43%
720
24%
1352
October11th to 20thNight
28%
720
12%
2240
8%
720
33%
720
15%
1678
October21st to endNight
23%
792
11%
2574
7%
792
30%
792
12%
2008
November1st to 10thNight
25%
740
15%
2406
10%
740
33%
740
15%
1750
November11th to 20thNight
14%
760
9%
2470
5%
760
19%
760
8%
2017
November21st to endNight
17%
781
7%
2491
3%
781
21%
781
9%
2061
December1st to 10thNight
14%
950
3%
2785
1%
950
15%
950
5%
2463
December11th to 20thNight
13%
950
3%
3356
1%
949
15%
949
5%
2961
December21st to endNight
9%
1040
3%
4871
1%
1026
12%
1026
3%
4319

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
14%11%5%9%7%45%45%7%30%
74603
Night
14%12%5%10%7%45%45%6%29%
75124
TransparencyDay
54%6%11%18%10%1
74304
Night
57%6%11%17%9%1
74944
SeeingDay
20%14%30%22%10%3
14623
Night
20%11%31%28%9%1
24733

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
12%17%9%17%11%6%5%33312%
1517
11th to 20thDay
15%16%11%18%13%5%33447%
1456
21st to endDay
12%19%13%15%12%5%6%5%328%
1584
February1st to 10thDay
9%17%8%13%15%7%7%5%35%12%
1569
11th to 20thDay
10%14%8%15%14%7%5%44315%
1748
21st to endDay
11%17%12%13%8%45%44418%
1480
March1st to 10thDay
12%12%6%11%9%45%45%6%25%
1800
11th to 20thDay
10%12%6%10%8%445%46%30%
1920
21st to endDay
17%13%49%7%3436%6%28%
2262
April1st to 10thDay
16%16%7%7%5%3435%6%26%
2160
11th to 20thDay
20%13%47%5%2445%5%31%
2208
21st to endDay
15%11%48%7%445%5%7%30%
2340
May1st to 10thDay
15%10%46%5%4445%7%36%
2340
11th to 20thDay
16%8%36%6%4445%6%37%
2363
21st to endDay
11%6%35%5%35%6%7%7%41%
2725
June1st to 10thDay
12%7%36%6%35%5%6%7%42%
2520
11th to 20thDay
13%7%26%5%3447%9%40%
2520
21st to endDay
13%6%35%6%45%5%7%8%38%
2520
July1st to 10thDay
7%425%435%6%8%11%46%
2520
11th to 20thDay
10%6%25%5%35%5%6%9%43%
2520
21st to endDay
9%5%26%5%36%5%7%9%43%
2743
August1st to 10thDay
10%6%25%6%46%6%8%9%39%
2399
11th to 20thDay
11%6%36%6%45%5%6%8%41%
2303
21st to endDay
13%6%26%5%35%5%5%9%41%
2446
September1st to 10thDay
15%5%35%6%46%5%5%7%39%
2138
11th to 20thDay
13%6%25%5%345%5%7%45%
1980
21st to endDay
14%8%36%6%35%5%5%8%35%
1980
October1st to 10thDay
17%10%36%5%34447%37%
1980
11th to 20thDay
22%16%49%7%35%446%20%
1717
21st to endDay
22%16%49%7%3445%6%18%
1782
November1st to 10thDay
14%16%6%7%6%345%6%7%28%
1672
11th to 20thDay
12%19%9%13%8%44346%17%
1710
21st to endDay
17%21%11%13%6%33325%16%
1687
December1st to 10thDay
14%20%13%18%8%3433411%
1385
11th to 20thDay
14%19%10%15%11%44435%10%
1826
21st to endDay
18%23%10%16%8%443238%
2783
January1st to 10thNight
14%15%9%16%11%6%5%43313%
2648
11th to 20thNight
13%18%9%16%11%4444412%
2549
21st to endNight
10%17%9%15%12%7%7%45%410%
2772
February1st to 10thNight
11%14%7%14%15%7%8%5%3414%
2390
11th to 20thNight
12%13%7%15%11%5%6%5%5%5%17%
2098
21st to endNight
14%15%6%13%10%45%445%20%
1776
March1st to 10thNight
12%12%6%8%7%5%45%5%5%32%
2160
11th to 20thNight
11%13%5%9%7%35%45%6%33%
2039
21st to endNight
16%10%48%6%35%446%33%
2109
April1st to 10thNight
16%12%6%7%6%36%5%6%5%30%
1800
11th to 20thNight
20%11%36%425%447%34%
1700
21st to endNight
16%8%46%6%3445%5%38%
1620
May1st to 10thNight
14%10%35%8%45%5%5%7%35%
1620
11th to 20thNight
15%9%37%5%445%5%8%36%
1527
21st to endNight
13%5%25%5%4436%8%45%
1551
June1st to 10thNight
14%6%25%5%245%6%8%43%
1440
11th to 20thNight
10%5%37%6%246%6%10%42%
1440
21st to endNight
10%6%35%435%5%5%8%45%
1440
July1st to 10thNight
7%3134245%5%9%54%
1440
11th to 20thNight
7%425%435%5%6%9%51%
1440
21st to endNight
7%6%26%5%25%5%6%8%50%
1575
August1st to 10thNight
8%335%5%35%47%10%46%
1536
11th to 20thNight
8%415%5%3446%8%52%
1598
21st to endNight
11%8%25%5%345%5%6%47%
1908
September1st to 10thNight
12%6%25%5%2335%7%49%
1803
11th to 20thNight
13%5%25%43446%6%49%
1980
21st to endNight
14%8%37%5%35%45%7%38%
1980
October1st to 10thNight
16%9%46%43435%6%41%
1980
11th to 20thNight
20%14%47%5%345%5%5%28%
2240
21st to endNight
23%14%47%6%34445%25%
2574
November1st to 10thNight
11%16%6%8%7%34347%30%
2406
11th to 20thNight
14%17%7%13%8%45%445%19%
2470
21st to endNight
16%17%8%12%6%345%45%18%
2491
December1st to 10thNight
14%18%11%17%9%45%43412%
2785
11th to 20thNight
12%20%9%18%9%45%43312%
3355
21st to endNight
20%25%10%14%7%3333310%
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
79%47%7%3
1518
11th to 20thDay
83%445%2
1456
21st to endDay
83%45%5%2
1584
February1st to 10thDay
76%5%7%10%2
1569
11th to 20thDay
74%5%7%9%4
1758
21st to endDay
70%49%13%41
1480
March1st to 10thDay
60%6%11%15%8%1
1800
11th to 20thDay
53%7%11%17%9%3
1920
21st to endDay
57%5%10%18%8%1
2262
April1st to 10thDay
59%5%10%17%10%
2160
11th to 20thDay
54%5%9%16%15%2
2208
21st to endDay
53%7%11%13%14%2
2340
May1st to 10thDay
48%5%10%24%12%2
2340
11th to 20thDay
46%5%12%20%15%2
2367
21st to endDay
39%8%13%24%13%2
2725
June1st to 10thDay
40%7%11%25%16%2
2520
11th to 20thDay
40%6%12%21%19%1
2520
21st to endDay
41%7%13%23%15%1
2520
July1st to 10thDay
30%7%13%29%18%2
2451
11th to 20thDay
36%8%11%27%17%2
2445
21st to endDay
35%8%13%24%18%2
2591
August1st to 10thDay
38%9%13%27%12%1
2384
11th to 20thDay
39%7%13%26%13%2
2303
21st to endDay
39%7%14%22%16%2
2446
September1st to 10thDay
44%6%11%23%14%1
2138
11th to 20thDay
37%8%14%29%12%1
1980
21st to endDay
46%6%14%24%10%
1980
October1st to 10thDay
48%6%10%24%11%1
1980
11th to 20thDay
65%6%10%14%4
1717
21st to endDay
67%6%11%11%4
1782
November1st to 10thDay
55%7%11%18%9%
1672
11th to 20thDay
69%5%11%11%4
1710
21st to endDay
73%412%8%3
1687
December1st to 10thDay
80%5%6%7%2
1385
11th to 20thDay
78%5%9%6%1
1823
21st to endDay
83%46%5%2
2783
January1st to 10thNight
77%5%8%8%2
2648
11th to 20thNight
78%5%8%7%3
2549
21st to endNight
78%5%9%6%2
2772
February1st to 10thNight
75%5%6%11%3
2390
11th to 20thNight
69%5%9%12%5%
2099
21st to endNight
66%5%11%13%41
1776
March1st to 10thNight
53%7%12%19%9%
2160
11th to 20thNight
51%6%11%19%12%2
2039
21st to endNight
53%5%10%21%10%1
2109
April1st to 10thNight
54%6%9%18%12%1
1800
11th to 20thNight
51%410%19%14%2
1700
21st to endNight
48%5%10%20%14%3
1620
May1st to 10thNight
47%6%12%21%11%3
1620
11th to 20thNight
47%6%10%20%15%3
1528
21st to endNight
37%6%13%27%16%2
1552
June1st to 10thNight
38%7%11%26%17%1
1440
11th to 20thNight
38%7%13%23%18%2
1440
21st to endNight
36%8%11%27%15%2
1440
July1st to 10thNight
26%6%15%30%20%3
1400
11th to 20thNight
28%8%15%27%20%1
1392
21st to endNight
32%8%15%27%17%1
1487
August1st to 10thNight
32%8%18%30%11%2
1527
11th to 20thNight
30%6%15%32%16%1
1598
21st to endNight
38%6%13%25%17%2
1908
September1st to 10thNight
34%6%14%26%18%2
1803
11th to 20thNight
34%7%14%32%12%1
1980
21st to endNight
44%6%13%26%9%1
1980
October1st to 10thNight
44%6%10%26%13%1
1980
11th to 20thNight
57%6%13%19%5%1
2240
21st to endNight
62%5%11%15%6%
2574
November1st to 10thNight
54%6%12%18%8%1
2406
11th to 20thNight
68%5%11%10%5%1
2470
21st to endNight
69%6%13%10%21
2491
December1st to 10thNight
78%5%7%8%2
2785
11th to 20thNight
77%6%9%6%2
3357
21st to endNight
80%6%8%42
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
19%21%33%20%6%
270
11th to 20thDay
28%16%39%14%2
246
21st to endDay
24%19%32%20%3
264
February1st to 10thDay
24%27%34%13%2
240
11th to 20thDay
22%27%38%13%1
240
21st to endDay
28%33%34%41
198
March1st to 10thDay
25%28%35%11%2
240
11th to 20thDay
24%14%31%23%7%1
360
21st to endDay
30%16%26%22%6%
462
April1st to 10thDay
28%15%36%16%41
420
11th to 20thDay
32%16%28%14%7%3
412
21st to endDay
25%14%28%24%7%2
420
May1st to 10thDay
25%21%28%18%7%2
423
11th to 20thDay
21%14%30%22%9%4
509
21st to endDay
15%13%31%27%12%3
679
June1st to 10thDay
13%13%36%24%11%2
630
11th to 20thDay
18%13%33%23%11%3
630
21st to endDay
16%10%33%27%10%3
630
July1st to 10thDay
9%7%29%33%17%5%
621
11th to 20thDay
12%9%28%32%14%5%
618
21st to endDay
10%7%33%30%13%6%
684
August1st to 10thDay
12%6%25%29%20%8%
518
11th to 20thDay
14%5%22%28%19%12%
439
21st to endDay
16%7%24%29%18%5%
495
September1st to 10thDay
19%6%24%27%17%6%
450
11th to 20thDay
11%7%25%32%17%8%
450
21st to endDay
22%11%23%24%15%6%
450
October1st to 10thDay
25%12%26%22%13%2
450
11th to 20thDay
34%16%27%20%4
339
21st to endDay
33%22%28%12%41
297
November1st to 10thDay
29%14%35%14%8%
270
11th to 20thDay
25%26%35%11%3
270
21st to endDay
28%23%35%10%4
270
December1st to 10thDay
29%21%29%16%22
205
11th to 20thDay
26%24%35%12%21
227
21st to endDay
30%25%31%12%11
297
January1st to 10thNight
22%15%35%21%6%1
950
11th to 20thNight
23%16%34%21%6%
911
21st to endNight
18%12%38%27%5%1
990
February1st to 10thNight
20%14%37%25%3
792
11th to 20thNight
20%16%35%24%5%
703
21st to endNight
23%18%36%19%4
592
March1st to 10thNight
17%14%36%24%8%1
720
11th to 20thNight
19%13%38%24%6%
720
21st to endNight
21%12%33%27%6%1
723
April1st to 10thNight
23%12%37%22%5%1
540
11th to 20thNight
25%13%32%22%7%
533
21st to endNight
19%11%32%30%7%1
540
May1st to 10thNight
19%9%35%28%8%1
540
11th to 20thNight
19%9%33%30%9%1
531
21st to endNight
15%6%29%35%13%1
584
June1st to 10thNight
15%6%31%37%10%1
540
11th to 20thNight
13%8%27%36%15%1
540
21st to endNight
14%5%25%41%14%1
540
July1st to 10thNight
11%322%41%22%2
525
11th to 20thNight
8%426%40%21%2
522
21st to endNight
11%421%41%21%1
591
August1st to 10thNight
10%220%46%20%1
537
11th to 20thNight
10%419%39%24%5%
534
21st to endNight
15%5%22%38%20%1
594
September1st to 10thNight
16%222%37%18%5%
543
11th to 20thNight
14%424%35%20%3
720
21st to endNight
18%10%23%31%15%3
720
October1st to 10thNight
21%9%26%31%12%1
720
11th to 20thNight
28%12%27%26%6%1
720
21st to endNight
34%11%27%21%6%
792
November1st to 10thNight
22%13%33%25%7%1
740
11th to 20thNight
25%14%38%19%4
760
21st to endNight
25%14%35%22%4
781
December1st to 10thNight
24%14%36%22%4
950
11th to 20thNight
24%16%37%17%41
950
21st to endNight
28%15%35%20%2
1045
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