Robert Brownlee Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Robert Brownlee Observatory clear sky chart from 2005-11-05 to 2017-09-21.

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
72%
15965
34%
46765
29%
15924
77%
15924
52%
22979

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
60%
716
29%
2327
26%
716
67%
716
41%
1183
January16th to endNight
58%
768
32%
2322
25%
768
67%
768
38%
1270
February1st to 15thNight
62%
698
33%
2107
30%
697
69%
697
43%
1115
February16th to endNight
57%
632
29%
1895
28%
632
62%
632
33%
1131
March1st to 15thNight
61%
720
29%
2160
28%
720
68%
720
42%
1154
March16th to endNight
68%
768
31%
2028
24%
768
73%
768
50%
1110
April1st to 15thNight
61%
719
29%
1799
17%
719
66%
719
46%
1047
April16th to endNight
67%
720
33%
1800
18%
720
71%
720
55%
972
May1st to 15thNight
71%
569
34%
1649
23%
569
74%
569
57%
791
May16th to endNight
72%
561
32%
1685
25%
561
75%
561
56%
795
June1st to 15thNight
83%
540
33%
1620
30%
540
84%
540
67%
689
June16th to endNight
94%
400
41%
1480
45%
400
95%
400
80%
467
July1st to 15thNight
93%
395
35%
1415
38%
395
93%
395
69%
505
July16th to endNight
85%
606
32%
1632
25%
570
86%
570
63%
749
August1st to 15thNight
92%
717
38%
1790
25%
713
93%
713
80%
804
August16th to endNight
93%
764
44%
1910
30%
764
93%
764
79%
863
September1st to 15thNight
90%
720
39%
1860
32%
720
91%
720
71%
869
September16th to endNight
89%
680
46%
1881
43%
680
91%
680
78%
788
October1st to 15thNight
75%
660
39%
1815
35%
660
79%
660
57%
902
October16th to endNight
76%
704
39%
2139
39%
704
80%
704
54%
996
November1st to 15thNight
74%
700
35%
2275
36%
700
79%
700
54%
1016
November16th to endNight
69%
720
36%
2340
38%
720
74%
720
46%
1147
December1st to 15thNight
61%
720
31%
2340
28%
720
68%
720
37%
1250
December16th to endNight
52%
768
32%
2496
27%
768
61%
768
34%
1366

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
3313212346%67%
48081
Night
3413212235%67%
46964
TransparencyDay
18%36%15%27%29%
47775
Night
20%25%15%26%29%
46771
SeeingDay
7%8%31%35%11%4
5908
Night
7%211%32%34%11%
15978

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
7%413312247%59%
1611
16th to endDay
7%5%2432335%5%55%
1901
February1st to 15thDay
45%33313446%58%
1751
16th to endDay
9%8%35%322346%50%
1579
March1st to 15thDay
6%41322346%7%59%
1800
16th to endDay
4324323448%58%
2191
April1st to 15thDay
6%414313246%59%
2149
16th to endDay
3212213236%68%
2160
May1st to 15thDay
2312213345%67%
2301
16th to endDay
223211237%71%
2429
June1st to 15thDay
112112136%75%
2340
16th to endDay
11111287%
2473
July1st to 15thDay
211223245%73%
2454
16th to endDay
2212214347%67%
2422
August1st to 15thDay
1121223480%
2160
16th to endDay
111112335%77%
2292
September1st to 15thDay
112211235%77%
2100
16th to endDay
11221123380%
1871
October1st to 15thDay
2314312235%69%
1815
16th to endDay
221321333472%
1730
November1st to 15thDay
3322312246%67%
1584
16th to endDay
5%423213336%62%
1620
December1st to 15thDay
5%5%1432346%9%52%
1620
16th to endDay
9%7%24313347%53%
1728
January1st to 15thNight
6%5%23323247%58%
2327
16th to endNight
8%6%23323345%56%
2322
February1st to 15thNight
47%25%212336%59%
2101
16th to endNight
7%8%3442345%6%49%
1895
March1st to 15thNight
46%23324246%58%
2160
16th to endNight
25%3432335%8%58%
2028
April1st to 15thNight
45%34323346%58%
1799
16th to endNight
2324323335%67%
1800
May1st to 15thNight
23332347%69%
1649
16th to endNight
1214212336%69%
1684
June1st to 15thNight
1222123478%
1620
16th to endNight
111388%
1480
July1st to 15thNight
11111232478%
1500
16th to endNight
2222212335%71%
1743
August1st to 15thNight
1111212383%
1800
16th to endNight
11111211284%
1910
September1st to 15thNight
11121122479%
1860
16th to endNight
111111385%
1881
October1st to 15thNight
2314212235%70%
1815
16th to endNight
131322223372%
2139
November1st to 15thNight
2313213245%69%
2275
16th to endNight
3414313346%61%
2340
December1st to 15thNight
7%6%23323347%54%
2340
16th to endNight
8%7%2431235%455%
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
24%36%15%27%23%
1612
16th to endDay
29%26%14%21%24%
1901
February1st to 15thDay
25%35%15%25%24%
1751
16th to endDay
35%35%16%26%12%
1579
March1st to 15thDay
23%37%21%23%20%
1800
16th to endDay
24%49%22%25%14%
2191
April1st to 15thDay
26%36%22%28%12%
2149
16th to endDay
19%38%21%28%19%
2160
May1st to 15thDay
19%27%19%29%21%
2301
16th to endDay
16%37%23%34%15%
2432
June1st to 15thDay
13%15%13%33%30%
2340
16th to endDay
6%135%23%59%
2473
July1st to 15thDay
11%35%9%23%47%
2310
16th to endDay
15%37%18%24%30%
2270
August1st to 15thDay
7%3211%27%47%
2146
16th to endDay
9%349%34%38%
2292
September1st to 15thDay
10%25%10%30%41%
2100
16th to endDay
9%1311%33%41%
1871
October1st to 15thDay
19%2414%28%29%
1815
16th to endDay
17%2411%25%39%
1730
November1st to 15thDay
18%26%14%29%28%
1584
16th to endDay
23%25%14%23%30%
1620
December1st to 15thDay
25%49%19%20%21%
1620
16th to endDay
30%48%18%21%16%
1728
January1st to 15thNight
26%26%13%25%25%
2327
16th to endNight
29%38%12%19%26%
2322
February1st to 15thNight
26%25%16%22%24%
2112
16th to endNight
34%45%16%23%15%
1895
March1st to 15thNight
27%36%18%23%21%
2160
16th to endNight
23%39%23%25%14%
2028
April1st to 15thNight
27%27%23%23%15%
1799
16th to endNight
21%26%22%29%18%
1800
May1st to 15thNight
17%27%23%28%19%
1650
16th to endNight
17%38%23%31%15%
1685
June1st to 15thNight
11%1416%33%32%
1620
16th to endNight
5%36%23%59%
1480
July1st to 15thNight
9%1310%22%52%
1415
16th to endNight
14%28%16%28%29%
1632
August1st to 15thNight
7%1212%33%43%
1790
16th to endNight
7%1210%35%42%
1910
September1st to 15thNight
10%249%32%40%
1860
16th to endNight
8%213%32%41%
1881
October1st to 15thNight
18%1315%33%26%
1815
16th to endNight
17%2210%25%41%
2139
November1st to 15thNight
17%2415%27%32%
2275
16th to endNight
23%2415%22%31%
2340
December1st to 15thNight
28%47%15%21%23%
2340
16th to endNight
30%38%15%24%18%
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
27%7%15%38%8%1
90
16th to endDay
16%10%33%28%9%2
269
February1st to 15thDay
9%12%34%28%12%2
266
16th to endDay
22%14%30%24%7%
238
March1st to 15thDay
11%14%32%27%11%2
270
16th to endDay
9%13%40%24%9%1
288
April1st to 15thDay
12%19%38%17%8%3
269
16th to endDay
8%24%36%22%5%2
270
May1st to 15thDay
11%14%35%28%6%3
288
16th to endDay
8%10%40%28%8%3
330
June1st to 15thDay
38%37%36%9%4
315
16th to endDay
1229%43%13%8%
315
July1st to 15thDay
1121%51%15%7%
305
16th to endDay
3116%53%14%9%
327
August1st to 15thDay
1232%44%9%9%
315
16th to endDay
223%50%13%10%
332
September1st to 15thDay
4136%40%13%2
315
16th to endDay
2433%43%14%2
276
October1st to 15thDay
5%11%38%34%9%
255
16th to endDay
9%8%29%30%21%
209
November1st to 15thDay
36%17%30%30%12%
90
16th to endDay
8%10%28%32%20%
90
December1st to 15thDay
5%5%31%35%20%2
90
16th to endDay
17%6%26%34%14%1
96
January1st to 15thNight
10%417%37%21%8%
720
16th to endNight
13%416%34%24%6%
768
February1st to 15thNight
9%416%37%24%7%
704
16th to endNight
15%5%16%34%21%6%
632
March1st to 15thNight
8%5%17%37%24%7%
720
16th to endNight
6%415%38%27%7%
768
April1st to 15thNight
9%6%17%38%22%6%
719
16th to endNight
6%5%17%35%26%8%
720
May1st to 15thNight
7%116%36%31%7%
570
16th to endNight
5%116%36%31%8%
563
June1st to 15thNight
2110%41%39%4
540
16th to endNight
433%53%8%
400
July1st to 15thNight
2122%60%13%
395
16th to endNight
21320%58%13%
606
August1st to 15thNight
1123%53%19%
717
16th to endNight
116%55%25%
764
September1st to 15thNight
2224%48%21%
720
16th to endNight
26%28%45%17%
680
October1st to 15thNight
7%29%34%35%10%
660
16th to endNight
6%210%30%33%16%
704
November1st to 15thNight
5%113%34%33%11%
700
16th to endNight
9%213%33%29%12%
720
December1st to 15thNight
14%214%34%27%7%
720
16th to endNight
14%7%19%32%20%5%
768
Page generated at 2017-09-22 04:35:33. Version ac04

Copyright 2017 A. Danko


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