Boulder Knolls Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Boulder Knolls Observatory clear sky chart from 2005-11-05 to 2017-09-22.

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
76%
16270
31%
47031
29%
16226
77%
16226
46%
25101

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 15thNight
71%
717
29%
2319
30%
716
74%
716
41%
1206
January16th to endNight
66%
768
33%
2304
32%
768
70%
768
38%
1295
February1st to 15thNight
71%
698
33%
2107
35%
698
74%
698
43%
1125
February16th to endNight
61%
632
26%
1895
30%
632
62%
632
30%
1182
March1st to 15thNight
71%
720
28%
2160
32%
720
73%
720
40%
1200
March16th to endNight
73%
768
27%
2034
24%
768
74%
768
42%
1179
April1st to 15thNight
66%
719
24%
1799
17%
719
67%
719
40%
1120
April16th to endNight
73%
720
29%
1800
20%
720
73%
720
48%
1053
May1st to 15thNight
71%
586
27%
1666
20%
586
71%
586
40%
978
May16th to endNight
67%
561
25%
1685
20%
561
68%
561
38%
978
June1st to 15thNight
75%
540
24%
1620
21%
540
75%
540
44%
890
June16th to endNight
90%
540
34%
1620
30%
540
90%
540
67%
702
July1st to 15thNight
87%
510
26%
1530
22%
510
87%
510
54%
745
July16th to endNight
85%
635
28%
1658
21%
596
85%
596
50%
873
August1st to 15thNight
85%
717
32%
1790
20%
713
85%
713
63%
912
August16th to endNight
90%
764
38%
1910
27%
764
90%
764
65%
972
September1st to 15thNight
86%
720
36%
1867
30%
720
86%
720
62%
938
September16th to endNight
87%
683
44%
1890
43%
683
88%
683
68%
849
October1st to 15thNight
80%
660
35%
1815
34%
660
81%
660
52%
952
October16th to endNight
75%
704
36%
2111
38%
704
77%
704
48%
1067
November1st to 15thNight
79%
700
34%
2275
38%
700
80%
700
49%
1078
November16th to endNight
77%
720
33%
2340
36%
720
79%
720
43%
1180
December1st to 15thNight
69%
720
32%
2340
35%
720
71%
720
38%
1253
December16th to endNight
64%
768
31%
2496
32%
768
67%
768
36%
1374

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
5%313212346%64%
47837
Night
6%423313346%60%
47238
TransparencyDay
21%36%16%26%25%
47539
Night
25%36%16%23%25%
47037
SeeingDay
7%6%25%41%15%3
5945
Night
7%16%32%39%11%
16282

By Month

Percentage Cloud Cover SW (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 15thDay
8%424322245%59%
1617
16th to endDay
10%5%13213346%55%
1920
February1st to 15thDay
8%324214345%58%
1751
16th to endDay
13%6%25%32335%7%46%
1579
March1st to 15thDay
6%31331335%9%57%
1800
16th to endDay
6%313323448%59%
2187
April1st to 15thDay
8%5%23323447%56%
2149
16th to endDay
6%311212236%66%
2160
May1st to 15thDay
6%424213336%62%
2283
16th to endDay
5%423212247%64%
2429
June1st to 15thDay
5%423212226%66%
2340
16th to endDay
111122481%
2340
July1st to 15thDay
1312323337%66%
2340
16th to endDay
321222345%7%65%
2393
August1st to 15thDay
1212211225%76%
2160
16th to endDay
112212345%76%
2292
September1st to 15thDay
1111212335%75%
2091
16th to endDay
111211123477%
1882
October1st to 15thDay
6%323312345%64%
1815
16th to endDay
6%312211145%69%
1757
November1st to 15thDay
5%212223345%66%
1584
16th to endDay
7%313212347%62%
1620
December1st to 15thDay
8%41332335%9%54%
1620
16th to endDay
13%323422246%54%
1728
January1st to 15thNight
7%413323346%59%
2319
16th to endNight
11%5%24322235%56%
2304
February1st to 15thNight
8%422313345%60%
2101
16th to endNight
14%6%3442334447%
1895
March1st to 15thNight
8%5%23224348%55%
2160
16th to endNight
7%5%1332345%8%54%
2034
April1st to 15thNight
10%7%24312446%52%
1799
16th to endNight
7%5%23213345%61%
1800
May1st to 15thNight
6%7%24313448%52%
1666
16th to endNight
7%7%45%211336%53%
1684
June1st to 15thNight
27%45%313449%53%
1620
16th to endNight
1112111347%73%
1620
July1st to 15thNight
14133134410%61%
1620
16th to endNight
221432447%9%58%
1772
August1st to 15thNight
2422213349%64%
1800
16th to endNight
213112346%71%
1910
September1st to 15thNight
2312222337%68%
1867
16th to endNight
22221113476%
1890
October1st to 15thNight
5%212312346%64%
1815
16th to endNight
7%413212336%63%
2111
November1st to 15thNight
3313313345%66%
2275
16th to endNight
6%313323346%61%
2340
December1st to 15thNight
9%5%13322447%55%
2340
16th to endNight
12%423212335%56%
2496

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 15thDay
27%25%12%24%25%
1618
16th to endDay
29%47%14%15%29%
1920
February1st to 15thDay
27%36%13%23%25%
1751
16th to endDay
36%47%14%23%12%
1579
March1st to 15thDay
25%37%20%22%21%
1800
16th to endDay
24%5%10%23%21%14%
2187
April1st to 15thDay
28%48%24%24%10%
2149
16th to endDay
20%38%24%25%17%
2160
May1st to 15thDay
24%36%22%25%17%
2283
16th to endDay
21%46%25%31%10%
2432
June1st to 15thDay
22%1418%31%21%
2340
16th to endDay
9%127%27%50%
2340
July1st to 15thDay
17%25%12%27%35%
2201
16th to endDay
16%49%23%28%16%
2244
August1st to 15thDay
13%1216%33%32%
2146
16th to endDay
10%3412%35%32%
2292
September1st to 15thDay
12%3311%36%33%
2091
16th to endDay
11%1311%35%36%
1882
October1st to 15thDay
22%35%17%25%25%
1815
16th to endDay
19%25%11%20%39%
1757
November1st to 15thDay
19%47%15%18%34%
1584
16th to endDay
22%36%13%19%34%
1620
December1st to 15thDay
28%310%14%16%26%
1620
16th to endDay
32%47%15%22%17%
1728
January1st to 15thNight
26%36%12%20%30%
2319
16th to endNight
31%27%10%15%31%
2304
February1st to 15thNight
26%36%15%19%28%
2112
16th to endNight
39%45%14%21%15%
1895
March1st to 15thNight
29%36%18%19%22%
2160
16th to endNight
28%410%19%22%15%
2034
April1st to 15thNight
32%48%22%19%12%
1799
16th to endNight
26%38%22%20%18%
1800
May1st to 15thNight
29%5%8%20%18%18%
1667
16th to endNight
32%38%24%19%12%
1685
June1st to 15thNight
28%18%18%26%16%
1620
16th to endNight
12%1210%30%42%
1620
July1st to 15thNight
18%3413%29%30%
1530
16th to endNight
21%49%23%27%13%
1658
August1st to 15thNight
16%1419%36%20%
1790
16th to endNight
14%2412%34%31%
1910
September1st to 15thNight
16%25%11%32%31%
1867
16th to endNight
13%1312%35%33%
1890
October1st to 15thNight
21%36%17%27%23%
1815
16th to endNight
24%2410%20%37%
2111
November1st to 15thNight
21%25%15%17%37%
2275
16th to endNight
24%37%15%16%33%
2340
December1st to 15thNight
28%47%15%19%24%
2340
16th to endNight
30%48%13%21%21%
2496

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 15thDay
29%317%37%12%
96
16th to endDay
15%10%29%33%9%1
288
February1st to 15thDay
8%10%30%37%12%1
266
16th to endDay
17%13%35%24%7%1
238
March1st to 15thDay
7%11%30%30%15%3
270
16th to endDay
6%12%32%32%13%2
288
April1st to 15thDay
10%14%36%26%8%3
269
16th to endDay
417%39%30%7%2
270
May1st to 15thDay
11%10%32%34%8%2
288
16th to endDay
10%9%30%36%12%1
330
June1st to 15thDay
5%329%42%16%2
315
16th to endDay
120%49%21%6%
315
July1st to 15thDay
412%51%25%5%
305
16th to endDay
46%54%25%7%
327
August1st to 15thDay
213%59%20%4
315
16th to endDay
119%57%22%7%
332
September1st to 15thDay
3123%54%13%3
315
16th to endDay
5%429%45%12%3
280
October1st to 15thDay
5%7%40%35%10%1
255
16th to endDay
10%232%34%17%3
217
November1st to 15thDay
36%13%27%37%11%
90
16th to endDay
6%8%22%37%22%2
90
December1st to 15thDay
7%118%36%31%4
90
16th to endDay
20%6%17%41%13%
96
January1st to 15thNight
8%111%43%26%9%
720
16th to endNight
13%19%39%28%7%
768
February1st to 15thNight
9%28%36%33%8%
704
16th to endNight
15%39%39%24%6%
632
March1st to 15thNight
10%38%40%32%5%
720
16th to endNight
8%27%37%34%8%
768
April1st to 15thNight
13%29%38%30%5%
719
16th to endNight
9%38%36%33%8%
720
May1st to 15thNight
10%19%34%36%7%
587
16th to endNight
9%29%33%40%4
563
June1st to 15thNight
48%40%40%4
540
16th to endNight
2232%57%5%
540
July1st to 15thNight
220%64%11%
510
16th to endNight
2115%61%18%
635
August1st to 15thNight
1118%60%16%
717
16th to endNight
111%60%26%
764
September1st to 15thNight
2120%57%17%
720
16th to endNight
3330%46%16%
683
October1st to 15thNight
5%15%38%37%10%
660
16th to endNight
11%5%29%36%17%
704
November1st to 15thNight
5%16%35%38%13%
700
16th to endNight
8%17%34%33%15%
720
December1st to 15thNight
12%19%38%28%9%
720
16th to endNight
15%210%38%24%7%
768
Page generated at 2017-09-23 03:43:43. Version ac04

Copyright 2017 A. Danko


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