Clarke Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Clarke Observatory clear sky chart from 2005-11-06 to 2024-01-17.

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
35%
23429
15%
74931
10%
23380
41%
23380
18%
53360

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
13%
757
10%
2459
3%
757
20%
757
9%
1978
January11th to 20thNight
15%
748
8%
2430
4%
748
20%
748
8%
1994
January21st to endNight
17%
792
9%
2574
5%
792
22%
792
9%
2089
February1st to 10thNight
17%
720
9%
2340
4%
720
23%
720
9%
1885
February11th to 20thNight
16%
702
11%
2239
5%
700
23%
700
10%
1759
February21st to endNight
14%
592
12%
1776
3%
592
24%
592
11%
1405
March1st to 10thNight
22%
720
16%
2006
8%
720
30%
720
18%
1485
March11th to 20thNight
28%
720
18%
1980
10%
720
34%
720
20%
1416
March21st to endNight
26%
792
16%
2178
9%
792
31%
792
18%
1603
April1st to 10thNight
28%
720
14%
1928
6%
720
33%
720
20%
1386
April11th to 20thNight
31%
615
17%
1679
8%
615
37%
615
22%
1192
April21st to endNight
30%
540
18%
1620
7%
540
37%
540
22%
1105
May1st to 10thNight
39%
540
12%
1620
7%
540
42%
540
18%
1174
May11th to 20thNight
44%
531
16%
1593
11%
531
49%
531
22%
1095
May21st to endNight
59%
528
16%
1691
14%
528
63%
528
30%
1038
June1st to 10thNight
51%
360
16%
1440
19%
360
56%
360
23%
867
June11th to 20thNight
55%
360
18%
1440
21%
360
62%
360
23%
846
June21st to endNight
55%
360
17%
1440
20%
360
62%
360
24%
833
July1st to 10thNight
65%
511
14%
1561
14%
511
67%
511
30%
976
July11th to 20thNight
64%
522
17%
1566
16%
522
66%
522
35%
905
July21st to endNight
62%
591
15%
1672
14%
558
63%
558
28%
1081
August1st to 10thNight
66%
537
19%
1600
16%
534
67%
534
32%
963
August11th to 20thNight
65%
534
20%
1598
17%
534
67%
534
31%
983
August21st to endNight
63%
638
22%
1826
15%
638
65%
638
33%
1122
September1st to 10thNight
56%
720
23%
1864
15%
720
59%
720
34%
1202
September11th to 20thNight
65%
720
27%
1980
21%
720
68%
720
37%
1185
September21st to endNight
49%
720
22%
1980
17%
720
53%
720
29%
1286
October1st to 10thNight
49%
720
19%
2115
17%
720
52%
720
28%
1338
October11th to 20thNight
33%
720
16%
2160
11%
720
40%
720
19%
1515
October21st to endNight
29%
792
18%
2376
13%
792
37%
792
17%
1714
November1st to 10thNight
37%
740
21%
2370
16%
740
46%
740
24%
1512
November11th to 20thNight
24%
760
16%
2513
10%
760
33%
760
16%
1789
November21st to endNight
23%
760
13%
2660
8%
760
30%
760
12%
1974
December1st to 10thNight
18%
760
7%
2660
4%
760
22%
760
8%
2137
December11th to 20thNight
18%
759
8%
3258
5%
759
24%
759
7%
2533
December21st to endNight
15%
828
5%
4739
4%
817
21%
817
4%
3995

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
16%10%48%6%35%5%5%7%31%
74623
Night
16%11%5%7%6%35%45%7%33%
75180
TransparencyDay
52%6%11%19%10%1
74344
Night
51%6%11%19%11%2
74978
SeeingDay
20%13%28%21%13%5%
13524
Night
20%10%28%28%12%2
23444

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
17%16%8%12%9%44445%17%
1706
11th to 20thDay
17%19%9%13%7%34345%15%
1674
21st to endDay
16%21%8%12%8%4445%5%15%
1782
February1st to 10thDay
19%16%7%11%9%3434420%
1620
11th to 20thDay
17%12%8%12%8%445%5%5%20%
1605
21st to endDay
18%15%5%10%7%34446%24%
1480
March1st to 10thDay
18%10%6%9%7%43446%29%
1952
11th to 20thDay
17%14%49%6%35%445%29%
1980
21st to endDay
21%11%5%8%6%3445%5%27%
2178
April1st to 10thDay
15%11%5%6%7%35%45%7%32%
2030
11th to 20thDay
18%11%47%5%35%5%5%6%33%
2244
21st to endDay
20%10%46%6%3445%6%32%
2340
May1st to 10thDay
18%10%46%6%47%6%5%6%29%
2340
11th to 20thDay
18%8%47%6%45%6%6%7%30%
2301
21st to endDay
10%5%36%6%46%6%7%10%38%
2580
June1st to 10thDay
14%7%36%6%35%6%6%9%35%
2520
11th to 20thDay
14%7%36%6%46%5%7%8%36%
2520
21st to endDay
12%6%25%6%36%6%7%11%38%
2520
July1st to 10thDay
11%5%37%6%45%5%6%9%39%
2349
11th to 20thDay
11%6%36%6%46%5%6%9%39%
2340
21st to endDay
12%6%26%6%46%6%7%9%36%
2547
August1st to 10thDay
11%6%25%7%46%7%7%9%35%
2327
11th to 20thDay
11%6%36%7%47%6%8%9%33%
2303
21st to endDay
11%5%26%7%36%6%6%8%39%
2511
September1st to 10thDay
15%6%35%6%345%5%7%43%
2092
11th to 20thDay
11%6%36%5%45%5%5%8%42%
1980
21st to endDay
18%7%36%4445%47%38%
1980
October1st to 10thDay
16%7%26%6%35%45%8%37%
1843
11th to 20thDay
17%10%36%6%45%45%6%33%
1800
21st to endDay
22%11%47%5%43446%28%
1980
November1st to 10thDay
10%9%48%6%35%446%40%
1704
11th to 20thDay
15%15%5%7%5%3445%6%30%
1663
21st to endDay
16%15%8%8%6%46%5%5%5%23%
1520
December1st to 10thDay
19%17%11%10%7%35%44417%
1520
11th to 20thDay
18%16%8%13%6%3445%6%17%
1910
21st to endDay
25%23%8%10%7%33335%11%
2882
January1st to 10thNight
18%15%7%11%8%45%44420%
2459
11th to 20thNight
18%17%8%12%6%45%44418%
2430
21st to endNight
17%17%7%11%8%45%43420%
2574
February1st to 10thNight
21%14%6%10%7%45%346%20%
2340
11th to 20thNight
18%13%5%11%7%36%5%5%5%22%
2237
21st to endNight
20%13%6%9%8%35%445%24%
1776
March1st to 10thNight
16%12%6%7%7%35%445%32%
2006
11th to 20thNight
18%10%46%5%245%5%6%34%
1980
21st to endNight
17%11%5%8%7%3445%433%
2178
April1st to 10thNight
16%11%47%5%4445%7%34%
1928
11th to 20thNight
15%8%47%6%45%45%7%35%
1679
21st to endNight
16%9%46%5%25%5%5%7%36%
1620
May1st to 10thNight
18%8%36%7%46%5%5%6%32%
1620
11th to 20thNight
14%8%36%5%45%46%8%36%
1592
21st to endNight
8%425%5%46%5%6%11%43%
1690
June1st to 10thNight
9%5%36%7%35%45%8%44%
1440
11th to 20thNight
9%6%36%445%5%6%9%43%
1440
21st to endNight
10%5%35%435%6%7%8%44%
1440
July1st to 10thNight
7%5%46%5%45%6%6%8%46%
1606
11th to 20thNight
8%4245%345%7%10%48%
1620
21st to endNight
10%5%36%5%45%6%6%8%41%
1771
August1st to 10thNight
8%5%26%5%445%7%10%46%
1611
11th to 20thNight
9%7%26%43447%9%46%
1598
21st to endNight
10%425%5%35%5%7%8%47%
1826
September1st to 10thNight
12%6%345%3445%7%48%
1864
11th to 20thNight
11%42443445%8%51%
1980
21st to endNight
15%7%25%5%3445%8%42%
1980
October1st to 10thNight
14%5%35%5%3346%8%43%
2115
11th to 20thNight
17%8%47%7%35%45%6%35%
2160
21st to endNight
23%10%35%5%345%46%32%
2376
November1st to 10thNight
15%8%25%5%3445%8%41%
2370
11th to 20thNight
16%11%5%8%6%25%5%47%31%
2513
21st to endNight
18%14%5%7%5%35%45%7%26%
2660
December1st to 10thNight
21%15%7%10%6%45%435%20%
2660
11th to 20thNight
18%18%7%10%7%34445%22%
3285
21st to endNight
24%20%8%9%6%34445%13%
4756

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
70%5%9%11%5%
1707
11th to 20thDay
73%311%10%3
1674
21st to endDay
73%48%10%31
1782
February1st to 10thDay
70%48%13%5%1
1620
11th to 20thDay
66%6%8%14%7%
1614
21st to endDay
62%411%15%7%2
1480
March1st to 10thDay
57%5%12%20%6%
1952
11th to 20thDay
57%5%10%17%9%2
1980
21st to endDay
60%48%16%11%1
2178
April1st to 10thDay
51%6%12%19%12%1
2030
11th to 20thDay
51%7%11%16%13%2
2244
21st to endDay
53%6%11%19%9%2
2340
May1st to 10thDay
54%7%12%16%10%1
2340
11th to 20thDay
52%8%11%15%12%2
2305
21st to endDay
40%10%15%23%11%2
2580
June1st to 10thDay
45%7%14%22%12%1
2520
11th to 20thDay
44%8%13%21%12%1
2520
21st to endDay
39%9%13%24%13%1
2520
July1st to 10thDay
41%8%12%24%13%2
2284
11th to 20thDay
41%7%13%25%13%1
2271
21st to endDay
41%8%15%22%13%1
2405
August1st to 10thDay
42%8%15%25%9%
2314
11th to 20thDay
43%9%15%22%9%1
2303
21st to endDay
40%9%13%22%12%4
2511
September1st to 10thDay
41%6%13%20%17%3
2092
11th to 20thDay
38%6%15%24%15%1
1980
21st to endDay
45%6%11%23%14%1
1980
October1st to 10thDay
44%7%11%22%15%1
1843
11th to 20thDay
51%6%11%21%10%1
1800
21st to endDay
56%5%12%16%9%1
1980
November1st to 10thDay
44%5%10%23%15%2
1704
11th to 20thDay
55%6%9%19%8%3
1663
21st to endDay
62%5%10%16%6%1
1520
December1st to 10thDay
71%5%9%11%4
1520
11th to 20thDay
68%5%8%15%4
1906
21st to endDay
78%5%9%7%2
2882
January1st to 10thNight
67%49%13%5%
2459
11th to 20thNight
70%410%12%31
2430
21st to endNight
69%49%14%41
2574
February1st to 10thNight
67%5%9%14%5%
2340
11th to 20thNight
64%5%9%15%7%1
2239
21st to endNight
64%5%8%14%8%2
1776
March1st to 10thNight
55%49%20%11%1
2006
11th to 20thNight
50%5%12%20%10%2
1980
21st to endNight
54%5%8%18%13%2
2178
April1st to 10thNight
50%6%11%19%13%2
1928
11th to 20thNight
48%6%12%17%15%3
1679
21st to endNight
46%6%12%18%16%2
1620
May1st to 10thNight
50%8%10%17%12%3
1620
11th to 20thNight
47%6%11%18%16%3
1593
21st to endNight
35%8%17%25%14%2
1691
June1st to 10thNight
38%8%11%25%16%2
1440
11th to 20thNight
37%7%15%23%16%2
1440
21st to endNight
35%9%14%23%18%1
1440
July1st to 10thNight
35%9%13%23%17%3
1561
11th to 20thNight
29%8%17%25%18%2
1566
21st to endNight
38%9%16%21%14%3
1672
August1st to 10thNight
32%9%15%28%16%1
1600
11th to 20thNight
35%7%16%26%14%2
1598
21st to endNight
33%8%13%25%17%4
1826
September1st to 10thNight
35%7%12%23%18%5%
1864
11th to 20thNight
31%7%13%27%21%2
1980
21st to endNight
40%5%13%23%18%1
1980
October1st to 10thNight
38%7%13%23%18%1
2115
11th to 20thNight
49%7%10%22%12%1
2160
21st to endNight
52%6%10%20%10%2
2376
November1st to 10thNight
42%5%11%23%15%3
2370
11th to 20thNight
53%6%10%19%10%3
2513
21st to endNight
58%6%12%18%6%1
2660
December1st to 10thNight
67%5%10%12%5%
2660
11th to 20thNight
65%6%11%16%3
3288
21st to endNight
74%6%10%9%2
4756

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
29%21%34%10%41
270
11th to 20thDay
29%24%37%7%3
258
21st to endDay
27%27%34%8%21
264
February1st to 10thDay
34%20%36%9%
240
11th to 20thDay
29%30%31%10%
240
21st to endDay
27%31%33%9%
198
March1st to 10thDay
23%20%35%17%5%
392
11th to 20thDay
29%15%33%16%6%1
420
21st to endDay
32%16%28%18%5%1
462
April1st to 10thDay
23%20%30%17%9%2
420
11th to 20thDay
26%14%29%16%10%5%
412
21st to endDay
25%15%32%18%8%2
420
May1st to 10thDay
26%15%28%18%9%3
423
11th to 20thDay
19%11%34%20%11%4
447
21st to endDay
12%7%25%28%21%7%
485
June1st to 10thDay
15%6%28%31%16%4
450
11th to 20thDay
17%9%23%28%18%6%
450
21st to endDay
14%8%27%28%17%6%
450
July1st to 10thDay
13%416%27%25%15%
445
11th to 20thDay
10%5%23%28%23%10%
444
21st to endDay
10%427%26%22%11%
488
August1st to 10thDay
14%419%28%24%12%
446
11th to 20thDay
15%5%21%25%22%12%
439
21st to endDay
10%6%19%28%25%12%
495
September1st to 10thDay
15%6%20%24%23%11%
450
11th to 20thDay
10%417%29%27%13%
450
21st to endDay
21%12%22%23%14%8%
450
October1st to 10thDay
14%10%26%25%18%7%
450
11th to 20thDay
21%14%36%21%8%1
450
21st to endDay
31%15%28%19%6%
495
November1st to 10thDay
17%23%31%17%11%2
302
11th to 20thDay
25%23%29%14%8%2
248
21st to endDay
20%21%43%14%2
180
December1st to 10thDay
27%26%31%9%6%2
180
11th to 20thDay
25%21%39%13%2
180
21st to endDay
31%24%30%13%3
231
January1st to 10thNight
25%20%35%17%3
760
11th to 20thNight
27%17%34%18%4
748
21st to endNight
25%18%32%22%4
792
February1st to 10thNight
32%13%32%21%3
720
11th to 20thNight
26%16%38%19%2
703
21st to endNight
29%15%37%17%3
592
March1st to 10thNight
23%14%36%24%4
720
11th to 20thNight
23%14%31%25%6%1
720
21st to endNight
23%12%33%25%6%1
792
April1st to 10thNight
21%13%33%26%7%
720
11th to 20thNight
20%11%34%24%9%1
615
21st to endNight
19%9%36%28%8%
540
May1st to 10thNight
21%6%27%34%11%1
540
11th to 20thNight
19%426%37%12%2
531
21st to endNight
10%420%40%23%3
530
June1st to 10thNight
9%328%43%17%1
360
11th to 20thNight
11%6%22%43%18%1
360
21st to endNight
10%424%39%23%1
360
July1st to 10thNight
9%313%38%31%6%
511
11th to 20thNight
10%317%39%26%5%
522
21st to endNight
10%414%41%28%2
591
August1st to 10thNight
9%212%43%31%3
537
11th to 20thNight
12%213%39%29%5%
534
21st to endNight
11%217%35%30%5%
638
September1st to 10thNight
15%318%31%28%5%
720
11th to 20thNight
12%312%36%31%5%
720
21st to endNight
17%5%24%29%20%4
720
October1st to 10thNight
16%6%25%31%18%4
720
11th to 20thNight
22%9%31%27%10%1
720
21st to endNight
28%10%29%27%6%
792
November1st to 10thNight
20%10%29%28%12%1
740
11th to 20thNight
23%14%33%22%7%1
760
21st to endNight
26%16%31%21%6%1
760
December1st to 10thNight
30%15%32%19%4
760
11th to 20thNight
25%16%34%21%4
760
21st to endNight
26%16%35%20%2
836
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