Rankin Science Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Rankin Science Observatory clear sky chart from 2005-11-06 to 2024-01-15.

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
39%
24137
19%
74468
11%
24085
48%
24085
21%
47574

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
20%
758
20%
2459
6%
757
36%
757
16%
1486
January11th to 20thNight
22%
739
20%
2399
8%
739
35%
739
15%
1558
January21st to endNight
19%
792
23%
2574
7%
792
38%
792
18%
1536
February1st to 10thNight
25%
720
19%
2338
9%
720
37%
720
19%
1436
February11th to 20thNight
14%
701
17%
2099
4%
700
27%
700
14%
1360
February21st to endNight
17%
592
21%
1776
6%
592
31%
592
17%
1201
March1st to 10thNight
23%
720
23%
1985
8%
720
36%
720
22%
1252
March11th to 20thNight
23%
720
16%
1980
6%
720
33%
720
18%
1353
March21st to endNight
25%
792
15%
2178
7%
792
32%
792
19%
1487
April1st to 10thNight
29%
720
19%
1980
7%
720
39%
720
24%
1231
April11th to 20thNight
33%
681
22%
1776
8%
681
43%
681
26%
1136
April21st to endNight
41%
540
23%
1620
12%
540
51%
540
25%
1013
May1st to 10thNight
36%
540
13%
1620
6%
540
43%
540
22%
1069
May11th to 20thNight
41%
531
13%
1593
7%
531
45%
531
20%
1100
May21st to endNight
53%
582
13%
1745
10%
582
54%
582
22%
1181
June1st to 10thNight
58%
540
13%
1620
11%
540
60%
540
24%
1103
June11th to 20thNight
59%
540
13%
1620
8%
540
62%
540
28%
1032
June21st to endNight
60%
540
15%
1620
11%
540
63%
540
27%
1040
July1st to 10thNight
64%
525
11%
1575
9%
525
65%
525
24%
1056
July11th to 20thNight
68%
522
11%
1566
9%
522
68%
522
25%
1048
July21st to endNight
67%
591
13%
1672
10%
558
69%
558
25%
1122
August1st to 10thNight
61%
537
12%
1600
10%
534
62%
534
22%
1130
August11th to 20thNight
59%
534
17%
1598
14%
534
61%
534
21%
1150
August21st to endNight
63%
679
17%
1884
14%
679
65%
679
27%
1278
September1st to 10thNight
57%
720
19%
1980
14%
720
59%
720
26%
1329
September11th to 20thNight
54%
720
20%
1980
16%
720
56%
720
25%
1360
September21st to endNight
47%
720
20%
1980
14%
720
52%
720
23%
1352
October1st to 10thNight
45%
720
23%
2060
18%
720
52%
720
28%
1310
October11th to 20thNight
39%
720
26%
2160
18%
720
52%
720
26%
1307
October21st to endNight
39%
792
29%
2376
19%
792
53%
792
29%
1379
November1st to 10thNight
41%
740
27%
2260
17%
740
54%
740
27%
1329
November11th to 20thNight
38%
760
26%
2470
17%
760
53%
760
27%
1381
November21st to endNight
35%
760
26%
2470
14%
760
49%
760
26%
1412
December1st to 10thNight
25%
760
21%
2470
9%
760
39%
760
16%
1613
December11th to 20thNight
26%
759
23%
2933
11%
758
42%
758
17%
1701
December21st to endNight
23%
830
12%
4452
8%
817
35%
817
9%
2743

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%7%36%6%45%5%6%8%36%
74931
Night
14%6%25%5%35%5%6%9%40%
74863
TransparencyDay
43%8%13%20%14%3
74654
Night
39%8%14%21%15%4
74659
SeeingDay
22%15%23%21%13%6%
14008
Night
20%13%21%26%17%3
24150

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
13%9%36%5%3345%10%40%
1706
11th to 20thDay
15%8%46%6%35%5%47%38%
1656
21st to endDay
13%7%35%43445%6%46%
1782
February1st to 10thDay
12%9%36%6%35%6%6%7%37%
1620
11th to 20thDay
16%9%37%5%34447%37%
1748
21st to endDay
18%6%35%6%3445%7%37%
1480
March1st to 10thDay
12%6%25%5%345%5%7%46%
1973
11th to 20thDay
17%7%46%6%36%5%5%7%34%
1980
21st to endDay
18%8%36%6%35%46%7%35%
2178
April1st to 10thDay
11%5%35%6%35%5%7%8%42%
1980
11th to 20thDay
18%6%35%43446%8%40%
2146
21st to endDay
15%7%35%6%45%5%5%7%39%
2340
May1st to 10thDay
14%7%35%6%45%5%7%9%36%
2340
11th to 20thDay
18%8%47%6%45%5%5%8%30%
2301
21st to endDay
15%7%36%5%46%6%7%10%31%
2531
June1st to 10thDay
14%7%35%7%5%7%6%6%10%31%
2340
11th to 20thDay
10%6%47%6%46%7%9%8%32%
2340
21st to endDay
10%6%36%7%5%7%7%8%10%31%
2340
July1st to 10thDay
11%7%38%8%5%7%7%8%10%26%
2340
11th to 20thDay
11%8%47%9%5%7%7%8%9%25%
2340
21st to endDay
12%7%39%7%5%8%7%8%8%25%
2547
August1st to 10thDay
14%8%37%8%6%7%7%8%10%23%
2327
11th to 20thDay
13%7%47%9%6%7%8%7%7%23%
2303
21st to endDay
14%7%36%7%46%6%7%9%30%
2451
September1st to 10thDay
15%6%37%6%46%5%7%8%34%
1980
11th to 20thDay
18%7%36%6%36%6%5%8%32%
1980
21st to endDay
20%5%35%6%345%6%6%35%
1980
October1st to 10thDay
18%6%245%34446%45%
1900
11th to 20thDay
14%6%25%42345%7%48%
1800
21st to endDay
16%5%35%423446%49%
1980
November1st to 10thDay
12%5%24433436%54%
1815
11th to 20thDay
12%425%43445%7%50%
1710
21st to endDay
15%5%34423448%47%
1710
December1st to 10thDay
15%9%38%7%45%45%7%33%
1710
11th to 20thDay
13%5%244235%6%9%46%
2153
21st to endDay
18%6%35%5%35%5%5%7%38%
3124
January1st to 10thNight
12%6%35%6%345%6%10%40%
2459
11th to 20thNight
14%7%27%6%35%5%6%8%36%
2399
21st to endNight
12%7%35%5%3446%7%45%
2574
February1st to 10thNight
13%6%35%5%346%7%8%40%
2338
11th to 20thNight
16%9%25%435%45%9%37%
2098
21st to endNight
14%8%36%6%45%5%6%9%36%
1776
March1st to 10thNight
12%5%25%6%445%6%7%45%
1985
11th to 20thNight
15%7%25%6%45%6%6%7%37%
1980
21st to endNight
14%8%26%5%345%5%9%38%
2178
April1st to 10thNight
11%6%26%5%245%7%9%43%
1980
11th to 20thNight
15%5%3442446%8%44%
1776
21st to endNight
14%5%25%425%45%8%45%
1620
May1st to 10thNight
14%6%35%5%25%5%8%8%38%
1620
11th to 20thNight
15%7%35%6%35%46%9%36%
1592
21st to endNight
14%7%36%5%35%5%8%10%36%
1744
June1st to 10thNight
11%5%36%436%6%8%12%36%
1620
11th to 20thNight
9%5%245%35%5%8%11%43%
1620
21st to endNight
8%425%6%45%6%9%12%38%
1620
July1st to 10thNight
10%6%25%6%45%6%8%11%38%
1620
11th to 20thNight
9%435%5%46%6%8%13%37%
1620
21st to endNight
6%436%7%48%7%8%12%36%
1771
August1st to 10thNight
14%6%25%6%46%5%6%12%35%
1611
11th to 20thNight
13%7%37%5%45%7%6%9%35%
1598
21st to endNight
12%7%25%5%35%6%7%10%38%
1884
September1st to 10thNight
12%8%35%6%345%6%8%41%
1980
11th to 20thNight
18%7%25%5%2445%7%39%
1980
21st to endNight
19%7%25%435%445%42%
1980
October1st to 10thNight
17%6%3432445%7%45%
2060
11th to 20thNight
15%6%2442445%6%48%
2160
21st to endNight
13%6%24323337%52%
2376
November1st to 10thNight
13%8%2432335%8%50%
2260
11th to 20thNight
14%42442445%7%50%
2470
21st to endNight
13%6%244245%6%8%46%
2470
December1st to 10thNight
15%8%37%5%35%45%8%36%
2470
11th to 20thNight
14%6%36%6%345%5%7%41%
3042
21st to endNight
20%8%36%5%35%5%6%8%32%
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
39%5%14%23%15%5%
1707
11th to 20thDay
45%5%12%19%15%4
1656
21st to endDay
37%412%27%17%4
1782
February1st to 10thDay
42%6%15%20%13%3
1620
11th to 20thDay
45%5%14%18%15%3
1758
21st to endDay
45%6%12%18%14%6%
1480
March1st to 10thDay
36%5%12%23%19%6%
1973
11th to 20thDay
47%6%11%18%14%4
1980
21st to endDay
48%6%13%20%11%2
2178
April1st to 10thDay
35%6%13%24%17%4
1980
11th to 20thDay
40%6%12%18%18%5%
2146
21st to endDay
42%7%13%23%12%3
2340
May1st to 10thDay
42%10%15%20%11%2
2340
11th to 20thDay
50%9%14%19%7%2
2305
21st to endDay
45%10%15%23%7%1
2531
June1st to 10thDay
45%11%15%19%8%1
2340
11th to 20thDay
43%11%16%17%12%2
2340
21st to endDay
42%12%14%17%13%1
2340
July1st to 10thDay
47%14%15%15%9%1
2275
11th to 20thDay
48%14%16%11%10%1
2271
21st to endDay
48%14%16%16%6%1
2405
August1st to 10thDay
51%12%15%16%6%
2314
11th to 20thDay
52%13%13%14%6%1
2303
21st to endDay
45%11%16%19%7%3
2451
September1st to 10thDay
44%8%14%18%13%3
1980
11th to 20thDay
46%7%12%19%14%2
1980
21st to endDay
46%8%12%17%14%3
1980
October1st to 10thDay
40%5%10%20%20%5%
1900
11th to 20thDay
34%5%11%24%19%6%
1800
21st to endDay
34%49%26%19%7%
1980
November1st to 10thDay
31%411%21%27%7%
1815
11th to 20thDay
33%5%10%22%23%7%
1710
21st to endDay
36%39%24%23%4
1710
December1st to 10thDay
48%6%10%18%14%4
1710
11th to 20thDay
32%5%13%22%20%7%
2150
21st to endDay
43%6%9%25%15%2
3124
January1st to 10thNight
37%5%16%21%15%5%
2459
11th to 20thNight
43%7%13%19%14%4
2399
21st to endNight
37%5%13%25%16%4
2574
February1st to 10thNight
38%6%15%23%15%3
2338
11th to 20thNight
43%415%20%12%6%
2099
21st to endNight
42%8%13%19%13%5%
1776
March1st to 10thNight
35%6%13%20%19%6%
1985
11th to 20thNight
43%6%14%23%12%2
1980
21st to endNight
42%8%14%21%13%3
2178
April1st to 10thNight
33%8%15%24%16%4
1980
11th to 20thNight
36%5%13%21%17%7%
1776
21st to endNight
37%7%15%25%15%2
1620
May1st to 10thNight
40%9%16%25%9%1
1620
11th to 20thNight
43%11%16%21%9%1
1593
21st to endNight
41%11%18%21%8%1
1745
June1st to 10thNight
38%13%18%20%10%1
1620
11th to 20thNight
33%12%22%22%10%1
1620
21st to endNight
33%14%20%20%10%3
1620
July1st to 10thNight
37%14%18%18%11%2
1575
11th to 20thNight
36%14%23%17%8%2
1566
21st to endNight
35%15%22%20%7%1
1672
August1st to 10thNight
41%13%18%17%10%
1600
11th to 20thNight
43%13%15%18%9%1
1598
21st to endNight
38%11%18%22%9%2
1884
September1st to 10thNight
40%9%14%20%14%3
1980
11th to 20thNight
43%8%12%20%16%1
1980
21st to endNight
43%8%10%20%16%3
1980
October1st to 10thNight
38%6%12%21%21%3
2060
11th to 20thNight
35%5%10%25%19%5%
2160
21st to endNight
32%49%24%22%8%
2376
November1st to 10thNight
33%6%10%21%23%7%
2260
11th to 20thNight
32%410%22%22%9%
2470
21st to endNight
34%411%26%21%5%
2470
December1st to 10thNight
43%7%12%21%13%4
2470
11th to 20thNight
38%412%21%20%5%
3044
21st to endNight
47%6%14%20%12%1
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
37%26%27%8%1
270
11th to 20thDay
34%22%33%10%1
252
21st to endDay
34%31%26%7%1
264
February1st to 10thDay
30%28%31%9%2
240
11th to 20thDay
48%28%21%4
240
21st to endDay
40%27%28%5%1
198
March1st to 10thDay
28%24%30%14%4
413
11th to 20thDay
32%27%25%13%31
420
21st to endDay
29%26%27%13%5%
462
April1st to 10thDay
25%23%27%18%6%1
420
11th to 20thDay
24%21%22%20%11%2
412
21st to endDay
24%19%28%15%10%4
420
May1st to 10thDay
22%20%32%13%8%6%
423
11th to 20thDay
22%12%24%24%12%6%
447
21st to endDay
20%10%18%24%17%11%
485
June1st to 10thDay
17%7%26%27%15%8%
450
11th to 20thDay
11%8%22%29%21%9%
450
21st to endDay
12%9%29%27%14%9%
450
July1st to 10thDay
11%417%28%26%12%
445
11th to 20thDay
9%7%14%33%24%12%
444
21st to endDay
12%414%34%23%13%
488
August1st to 10thDay
17%5%21%24%17%15%
446
11th to 20thDay
12%6%15%27%26%14%
439
21st to endDay
17%412%28%22%17%
495
September1st to 10thDay
14%314%29%27%12%
450
11th to 20thDay
20%6%14%25%27%8%
450
21st to endDay
22%11%20%25%17%5%
450
October1st to 10thDay
26%10%18%23%18%5%
450
11th to 20thDay
19%18%24%22%15%2
450
21st to endDay
21%17%26%24%9%3
495
November1st to 10thDay
22%13%30%22%11%3
413
11th to 20thDay
23%23%28%21%41
270
21st to endDay
23%19%33%19%5%1
270
December1st to 10thDay
30%27%28%13%1
270
11th to 20thDay
27%18%38%13%31
270
21st to endDay
30%23%33%10%41
297
January1st to 10thNight
28%21%27%20%4
760
11th to 20thNight
27%19%28%21%5%
739
21st to endNight
24%21%32%20%3
792
February1st to 10thNight
23%19%30%25%3
720
11th to 20thNight
30%24%28%17%1
703
21st to endNight
26%21%32%19%2
592
March1st to 10thNight
20%21%32%23%4
720
11th to 20thNight
27%18%27%20%8%1
720
21st to endNight
24%17%28%24%7%
792
April1st to 10thNight
22%18%26%26%7%
720
11th to 20thNight
22%17%23%25%12%1
681
21st to endNight
17%14%20%34%13%1
540
May1st to 10thNight
19%12%25%29%14%1
540
11th to 20thNight
17%9%21%31%19%3
531
21st to endNight
18%6%13%27%28%8%
584
June1st to 10thNight
13%316%33%29%5%
540
11th to 20thNight
11%418%26%34%7%
540
21st to endNight
7%5%19%32%34%4
540
July1st to 10thNight
12%39%31%40%6%
525
11th to 20thNight
9%48%28%43%7%
522
21st to endNight
8%211%29%42%9%
591
August1st to 10thNight
15%310%27%37%7%
537
11th to 20thNight
16%39%27%37%8%
534
21st to endNight
14%310%27%39%9%
679
September1st to 10thNight
15%5%10%28%34%8%
720
11th to 20thNight
20%5%10%25%34%5%
720
21st to endNight
23%7%14%27%27%3
720
October1st to 10thNight
19%11%18%22%24%6%
720
11th to 20thNight
20%14%22%28%14%3
720
21st to endNight
21%15%20%29%12%2
792
November1st to 10thNight
20%12%21%30%14%3
740
11th to 20thNight
24%12%21%33%9%2
760
21st to endNight
21%15%24%28%11%1
760
December1st to 10thNight
28%17%24%24%7%
760
11th to 20thNight
22%18%26%26%6%1
760
21st to endNight
30%16%24%25%5%
836
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