Robinson Observatory
Clear Sky Chart
History

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

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
56%
24610
20%
74448
15%
24555
61%
24555
28%
47418

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
40%
759
22%
2459
15%
757
50%
757
27%
1472
January11th to 20thNight
48%
724
25%
2352
19%
724
60%
724
30%
1371
January21st to endNight
36%
792
22%
2552
14%
792
48%
792
25%
1581
February1st to 10thNight
41%
720
22%
2160
14%
720
50%
720
29%
1325
February11th to 20thNight
43%
700
23%
2099
13%
700
54%
700
34%
1212
February21st to endNight
43%
592
29%
1646
16%
592
54%
592
38%
964
March1st to 10thNight
46%
720
31%
1980
18%
720
57%
720
45%
1083
March11th to 20thNight
51%
720
26%
1980
18%
720
58%
720
39%
1139
March21st to endNight
46%
792
28%
2178
16%
792
54%
792
38%
1272
April1st to 10thNight
47%
720
24%
1980
16%
720
54%
720
40%
1118
April11th to 20thNight
48%
711
22%
1888
15%
711
53%
711
34%
1175
April21st to endNight
55%
720
28%
1800
16%
720
60%
720
43%
1077
May1st to 10thNight
62%
644
27%
1724
19%
644
66%
644
45%
964
May11th to 20thNight
66%
531
17%
1592
13%
531
69%
531
32%
993
May21st to endNight
69%
582
20%
1744
16%
582
70%
582
29%
1122
June1st to 10thNight
63%
540
14%
1620
11%
540
64%
540
21%
1156
June11th to 20thNight
71%
540
13%
1620
11%
540
72%
540
22%
1122
June21st to endNight
69%
540
10%
1620
8%
540
69%
540
23%
1126
July1st to 10thNight
71%
525
6%
1575
5%
525
71%
525
18%
1194
July11th to 20thNight
74%
522
6%
1566
5%
522
74%
522
19%
1134
July21st to endNight
73%
591
9%
1672
9%
558
74%
558
20%
1208
August1st to 10thNight
78%
537
7%
1600
7%
534
79%
534
23%
1121
August11th to 20thNight
76%
592
7%
1656
6%
592
76%
592
23%
1180
August21st to endNight
62%
792
8%
2166
8%
792
62%
792
18%
1630
September1st to 10thNight
66%
720
9%
1980
8%
720
66%
720
18%
1486
September11th to 20thNight
59%
720
7%
1980
7%
720
59%
720
19%
1478
September21st to endNight
62%
720
16%
1980
15%
720
64%
720
22%
1409
October1st to 10thNight
58%
720
21%
2026
18%
720
62%
720
26%
1352
October11th to 20thNight
65%
720
28%
2160
28%
720
69%
720
34%
1287
October21st to endNight
62%
792
26%
2376
24%
792
68%
792
34%
1416
November1st to 10thNight
53%
740
25%
2221
20%
740
61%
740
31%
1350
November11th to 20thNight
52%
760
26%
2280
22%
760
59%
760
29%
1419
November21st to endNight
48%
760
25%
2341
19%
760
57%
760
28%
1456
December1st to 10thNight
51%
760
27%
2470
21%
760
62%
760
32%
1422
December11th to 20thNight
50%
760
23%
2950
20%
758
57%
758
23%
1765
December21st to endNight
47%
832
14%
4455
20%
817
54%
817
15%
2839

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
9%6%37%7%5%7%6%7%9%36%
74873
Night
8%5%36%6%35%5%6%9%44%
74825
TransparencyDay
42%9%14%16%13%5%
74596
Night
36%9%16%20%14%5%
74621
SeeingDay
9%6%20%26%22%17%
14487
Night
9%5%21%33%26%7%
24620

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%6%25%5%35%45%8%45%
1706
11th to 20thDay
6%5%25%5%345%6%8%50%
1620
21st to endDay
13%5%35%6%3446%8%43%
1800
February1st to 10thDay
13%5%36%6%35%5%6%7%42%
1800
11th to 20thDay
6%5%35%7%345%5%5%53%
1748
21st to endDay
9%5%35%6%46%6%5%7%44%
1608
March1st to 10thDay
4425%5%45%5%5%8%53%
1980
11th to 20thDay
9%425%5%345%5%10%48%
1980
21st to endDay
7%425%6%46%5%5%8%48%
2178
April1st to 10thDay
7%4246%36%5%7%8%47%
1980
11th to 20thDay
9%6%25%7%47%6%7%9%37%
2018
21st to endDay
5%426%436%6%7%8%49%
2160
May1st to 10thDay
6%324435%5%6%8%53%
2250
11th to 20thDay
9%5%26%6%5%7%6%7%9%38%
2301
21st to endDay
7%5%47%8%5%7%7%8%9%34%
2531
June1st to 10thDay
13%8%38%9%5%8%7%6%10%23%
2340
11th to 20thDay
6%5%37%9%7%10%8%8%10%27%
2340
21st to endDay
10%6%38%8%6%8%7%8%9%27%
2340
July1st to 10thDay
9%7%48%9%6%10%8%7%9%22%
2340
11th to 20thDay
8%6%48%9%7%9%9%7%9%24%
2340
21st to endDay
7%6%39%11%6%8%8%7%11%23%
2547
August1st to 10thDay
5%6%38%10%6%8%9%8%11%23%
2327
11th to 20thDay
5%6%39%10%6%9%8%9%10%25%
2226
21st to endDay
10%8%49%10%5%7%7%7%12%22%
2184
September1st to 10thDay
8%7%39%10%6%8%8%8%9%24%
1980
11th to 20thDay
11%7%47%9%5%8%8%9%10%22%
1980
21st to endDay
10%8%49%8%5%7%6%7%9%28%
1980
October1st to 10thDay
12%7%38%7%47%6%6%8%31%
1931
11th to 20thDay
8%5%26%6%45%5%7%8%43%
1800
21st to endDay
8%5%25%6%5%6%5%7%8%44%
1980
November1st to 10thDay
8%5%26%5%47%6%7%8%42%
1857
11th to 20thDay
10%6%36%6%47%5%6%7%41%
1900
21st to endDay
10%6%37%6%35%5%5%9%40%
1834
December1st to 10thDay
8%5%26%5%45%5%8%9%45%
1710
11th to 20thDay
11%7%36%6%36%45%8%40%
2153
21st to endDay
12%7%38%7%46%6%7%7%34%
3124
January1st to 10thNight
11%6%35%5%36%5%5%7%45%
2459
11th to 20thNight
9%435%445%47%8%47%
2352
21st to endNight
12%6%36%4345%5%6%44%
2552
February1st to 10thNight
11%6%36%4346%5%7%45%
2160
11th to 20thNight
9%5%245%3445%7%53%
2098
21st to endNight
8%4245%34448%53%
1646
March1st to 10thNight
5%3234345%5%7%60%
1980
11th to 20thNight
8%415%4335%5%9%53%
1980
21st to endNight
6%4245%345%6%9%53%
2178
April1st to 10thNight
7%41442445%10%55%
1980
11th to 20thNight
7%426%5%445%7%7%48%
1888
21st to endNight
6%425%4345%5%8%56%
1800
May1st to 10thNight
5%314425%447%61%
1724
11th to 20thNight
5%426%6%35%6%7%8%48%
1592
21st to endNight
7%426%6%46%6%7%9%42%
1744
June1st to 10thNight
10%8%37%7%35%7%9%9%33%
1620
11th to 20thNight
5%6%46%6%5%7%7%9%12%34%
1620
21st to endNight
7%6%47%6%46%6%9%12%33%
1620
July1st to 10thNight
7%7%37%6%5%8%7%8%12%30%
1620
11th to 20thNight
6%5%37%6%6%8%8%8%12%31%
1620
21st to endNight
5%6%37%7%5%8%8%8%11%32%
1771
August1st to 10thNight
45%26%7%5%7%7%9%13%35%
1611
11th to 20thNight
45%36%7%5%6%9%10%13%33%
1656
21st to endNight
10%7%37%7%5%6%7%7%10%31%
2166
September1st to 10thNight
7%5%37%8%5%7%7%9%10%32%
1980
11th to 20thNight
10%7%48%7%5%5%6%7%10%30%
1980
21st to endNight
9%7%38%6%46%6%8%9%34%
1980
October1st to 10thNight
8%6%26%6%45%6%6%9%41%
2026
11th to 20thNight
6%425%5%35%5%5%8%50%
2160
21st to endNight
8%425%5%35%46%8%50%
2376
November1st to 10thNight
9%5%25%5%35%6%6%7%47%
2221
11th to 20thNight
10%6%25%6%45%46%8%45%
2280
21st to endNight
9%5%35%6%45%5%5%9%44%
2341
December1st to 10thNight
7%425%6%35%5%6%8%49%
2470
11th to 20thNight
12%7%45%424446%47%
3042
21st to endNight
14%7%36%6%36%5%6%8%36%
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
37%5%10%20%20%8%
1707
11th to 20thDay
31%6%7%24%24%8%
1620
21st to endDay
40%49%18%19%10%
1800
February1st to 10thDay
41%7%10%14%20%9%
1800
11th to 20thDay
32%5%9%19%24%10%
1758
21st to endDay
38%6%10%14%24%8%
1608
March1st to 10thDay
29%6%11%23%23%8%
1980
11th to 20thDay
33%6%13%24%15%9%
1980
21st to endDay
33%5%12%21%21%8%
2178
April1st to 10thDay
35%7%12%22%17%8%
1980
11th to 20thDay
40%9%11%18%19%5%
2018
21st to endDay
31%6%12%20%22%9%
2160
May1st to 10thDay
28%7%11%21%25%9%
2250
11th to 20thDay
40%10%15%18%14%3
2305
21st to endDay
44%9%13%18%15%1
2531
June1st to 10thDay
54%11%16%14%4
2340
11th to 20thDay
48%13%20%17%2
2340
21st to endDay
51%13%20%13%31
2340
July1st to 10thDay
55%16%17%10%2
2275
11th to 20thDay
50%14%23%12%
2271
21st to endDay
51%16%20%12%2
2405
August1st to 10thDay
47%18%24%9%1
2314
11th to 20thDay
49%16%24%9%2
2226
21st to endDay
54%14%18%10%3
2184
September1st to 10thDay
52%15%18%10%5%
1980
11th to 20thDay
51%14%20%12%3
1980
21st to endDay
51%10%14%12%10%3
1980
October1st to 10thDay
49%9%12%12%12%6%
1931
11th to 20thDay
37%8%11%21%19%4
1800
21st to endDay
36%5%11%16%21%10%
1980
November1st to 10thDay
38%6%12%17%20%8%
1857
11th to 20thDay
42%6%8%19%19%6%
1900
21st to endDay
40%7%9%20%19%5%
1834
December1st to 10thDay
35%8%12%24%15%5%
1710
11th to 20thDay
41%7%11%14%17%9%
2150
21st to endDay
47%6%10%17%13%7%
3124
January1st to 10thNight
37%5%11%19%19%9%
2459
11th to 20thNight
33%6%9%23%20%8%
2352
21st to endNight
40%410%18%18%10%
2552
February1st to 10thNight
37%5%12%17%17%12%
2160
11th to 20thNight
31%6%12%21%21%10%
2099
21st to endNight
30%5%14%20%24%7%
1646
March1st to 10thNight
23%6%13%28%23%7%
1980
11th to 20thNight
28%5%17%26%15%9%
1980
21st to endNight
27%7%12%26%19%8%
2178
April1st to 10thNight
26%7%19%25%16%8%
1980
11th to 20thNight
33%7%12%24%19%6%
1888
21st to endNight
26%7%13%25%22%7%
1800
May1st to 10thNight
24%5%14%26%22%9%
1724
11th to 20thNight
31%11%20%23%13%2
1593
21st to endNight
36%10%14%22%17%1
1745
June1st to 10thNight
45%11%18%22%4
1620
11th to 20thNight
40%14%21%23%2
1620
21st to endNight
40%14%25%16%5%
1620
July1st to 10thNight
44%18%24%12%2
1575
11th to 20thNight
41%17%29%12%1
1566
21st to endNight
41%17%27%15%1
1672
August1st to 10thNight
35%18%34%12%1
1600
11th to 20thNight
37%17%31%13%2
1656
21st to endNight
46%15%23%14%2
2166
September1st to 10thNight
43%17%21%13%5%
1980
11th to 20thNight
47%13%21%16%3
1980
21st to endNight
44%11%18%16%9%2
1980
October1st to 10thNight
39%10%14%19%12%6%
2026
11th to 20thNight
32%7%10%26%20%4
2160
21st to endNight
32%7%11%19%22%10%
2376
November1st to 10thNight
34%7%11%19%19%9%
2221
11th to 20thNight
37%7%11%20%18%8%
2280
21st to endNight
37%7%10%22%19%6%
2341
December1st to 10thNight
32%7%16%27%13%6%
2470
11th to 20thNight
38%6%10%19%17%10%
3044
21st to endNight
45%8%12%18%13%5%
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
16%13%41%23%6%1
270
11th to 20thDay
5%12%42%27%10%4
240
21st to endDay
16%18%42%19%5%
264
February1st to 10thDay
11%16%38%21%12%2
240
11th to 20thDay
10%10%35%31%10%5%
240
21st to endDay
10%13%28%29%20%1
326
March1st to 10thDay
313%35%32%15%1
420
11th to 20thDay
11%10%30%28%18%3
420
21st to endDay
8%9%31%29%20%3
462
April1st to 10thDay
9%12%31%29%16%5%
420
11th to 20thDay
9%9%30%29%17%7%
412
21st to endDay
7%5%27%33%17%11%
420
May1st to 10thDay
6%6%22%32%23%11%
423
11th to 20thDay
9%318%31%24%16%
447
21st to endDay
7%311%24%32%23%
485
June1st to 10thDay
16%312%25%25%20%
450
11th to 20thDay
8%29%24%26%31%
450
21st to endDay
7%17%18%28%38%
450
July1st to 10thDay
7%2412%29%46%
445
11th to 20thDay
5%1316%31%43%
444
21st to endDay
8%1420%25%42%
488
August1st to 10thDay
317%18%36%35%
446
11th to 20thDay
45%14%29%48%
439
21st to endDay
11%18%20%27%33%
495
September1st to 10thDay
11%27%17%26%37%
450
11th to 20thDay
10%310%26%24%27%
450
21st to endDay
10%29%25%32%23%
450
October1st to 10thDay
13%422%31%22%8%
450
11th to 20thDay
7%7%20%29%30%8%
450
21st to endDay
8%5%25%34%23%6%
495
November1st to 10thDay
11%7%22%31%25%5%
455
11th to 20thDay
13%9%26%33%15%4
460
21st to endDay
11%9%27%35%18%1
394
December1st to 10thDay
6%14%37%29%13%3
270
11th to 20thDay
13%12%30%33%11%1
270
21st to endDay
9%13%30%35%10%3
297
January1st to 10thNight
12%11%32%32%12%1
760
11th to 20thNight
10%7%28%41%13%1
724
21st to endNight
15%11%34%32%8%
792
February1st to 10thNight
15%7%32%33%13%
720
11th to 20thNight
11%8%33%36%11%1
703
21st to endNight
9%8%36%38%9%
592
March1st to 10thNight
5%7%38%38%11%1
720
11th to 20thNight
9%8%27%40%15%1
720
21st to endNight
7%7%34%37%14%1
792
April1st to 10thNight
7%8%33%39%13%1
720
11th to 20thNight
8%7%31%38%15%1
711
21st to endNight
6%5%29%41%18%1
720
May1st to 10thNight
5%421%46%21%2
644
11th to 20thNight
8%115%42%31%4
531
21st to endNight
7%29%35%41%5%
584
June1st to 10thNight
12%210%31%39%7%
540
11th to 20thNight
6%26%25%40%21%
540
21st to endNight
9%26%20%47%18%
540
July1st to 10thNight
8%2317%43%26%
525
11th to 20thNight
5%2415%48%28%
522
21st to endNight
5%317%50%24%
591
August1st to 10thNight
5%319%49%24%
537
11th to 20thNight
41417%49%27%
592
21st to endNight
12%18%23%37%20%
792
September1st to 10thNight
9%26%23%39%22%
720
11th to 20thNight
12%310%26%38%12%
720
21st to endNight
12%26%31%38%11%
720
October1st to 10thNight
12%317%36%27%6%
720
11th to 20thNight
7%316%34%34%5%
720
21st to endNight
11%318%38%27%3
792
November1st to 10thNight
10%5%23%35%25%2
740
11th to 20thNight
11%7%24%38%19%3
760
21st to endNight
11%5%27%41%15%1
760
December1st to 10thNight
7%427%38%22%3
760
11th to 20thNight
13%5%27%36%18%1
760
21st to endNight
11%6%28%37%17%1
836
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