Cooke Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Cooke Observatory clear sky chart from 2005-11-06 to 2024-01-14.

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%
24363
20%
74538
15%
24310
62%
24310
28%
46911

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
38%
757
22%
2459
13%
757
50%
757
26%
1504
January11th to 20thNight
49%
735
25%
2386
20%
735
59%
735
29%
1391
January21st to endNight
34%
792
23%
2435
13%
792
46%
792
24%
1553
February1st to 10thNight
41%
720
22%
2160
16%
720
48%
720
29%
1351
February11th to 20thNight
44%
700
22%
2099
12%
700
54%
700
32%
1238
February21st to endNight
41%
592
25%
1744
14%
592
51%
592
31%
1069
March1st to 10thNight
43%
720
29%
1980
16%
720
53%
720
40%
1138
March11th to 20thNight
50%
720
27%
1980
18%
720
57%
720
34%
1199
March21st to endNight
47%
792
29%
2178
18%
792
56%
792
40%
1241
April1st to 10thNight
48%
720
24%
1980
15%
720
55%
720
38%
1163
April11th to 20thNight
46%
711
21%
1953
13%
711
53%
711
37%
1161
April21st to endNight
58%
664
29%
1776
19%
664
63%
664
43%
1025
May1st to 10thNight
71%
540
28%
1620
22%
540
74%
540
48%
816
May11th to 20thNight
67%
531
18%
1592
12%
531
72%
531
32%
978
May21st to endNight
73%
582
25%
1744
20%
582
74%
582
37%
1022
June1st to 10thNight
64%
540
14%
1620
12%
540
65%
540
25%
1101
June11th to 20thNight
76%
540
12%
1620
11%
540
77%
540
27%
1055
June21st to endNight
73%
540
9%
1620
8%
540
73%
540
22%
1152
July1st to 10thNight
76%
525
6%
1575
6%
525
76%
525
20%
1144
July11th to 20thNight
76%
522
3%
1566
4%
522
76%
522
18%
1166
July21st to endNight
74%
591
7%
1672
7%
558
75%
558
20%
1219
August1st to 10thNight
75%
537
5%
1600
6%
534
76%
534
23%
1110
August11th to 20thNight
77%
534
6%
1767
7%
534
77%
534
18%
1254
August21st to endNight
62%
754
10%
2140
10%
754
62%
754
18%
1580
September1st to 10thNight
68%
720
8%
1980
8%
720
68%
720
18%
1470
September11th to 20thNight
62%
720
9%
1980
9%
720
63%
720
19%
1458
September21st to endNight
62%
720
19%
1980
16%
720
64%
720
23%
1379
October1st to 10thNight
57%
720
21%
1980
17%
720
62%
720
28%
1314
October11th to 20thNight
66%
720
31%
2119
30%
720
72%
720
41%
1166
October21st to endNight
57%
792
27%
2376
24%
792
64%
792
34%
1396
November1st to 10thNight
53%
740
27%
2221
21%
740
61%
740
37%
1246
November11th to 20thNight
52%
760
29%
2280
24%
760
60%
760
32%
1365
November21st to endNight
50%
760
25%
2470
20%
760
59%
760
29%
1468
December1st to 10thNight
47%
760
27%
2470
19%
760
58%
760
30%
1449
December11th to 20thNight
47%
760
24%
2940
20%
758
54%
758
24%
1694
December21st to endNight
46%
832
15%
4476
20%
817
54%
817
14%
2876

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
8%5%26%6%46%6%7%9%41%
74862
Night
9%5%25%5%35%5%6%9%44%
74904
TransparencyDay
37%11%15%17%14%6%
74585
Night
34%10%16%19%15%5%
74700
SeeingDay
9%7%21%27%23%14%
14271
Night
9%5%20%32%25%8%
24375

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
11%426%5%35%5%5%10%45%
1706
11th to 20thDay
8%6%25%5%35%5%5%10%48%
1647
21st to endDay
11%5%35%5%35%5%5%6%45%
1917
February1st to 10thDay
14%5%25%5%345%5%8%43%
1800
11th to 20thDay
9%42442445%6%55%
1748
21st to endDay
8%6%26%5%3445%7%50%
1511
March1st to 10thDay
6%32432446%8%58%
1980
11th to 20thDay
8%4244345%5%8%52%
1980
21st to endDay
7%425%5%35%5%5%7%53%
2178
April1st to 10thDay
8%324426%5%6%9%52%
1980
11th to 20thDay
9%5%25%6%47%5%6%8%43%
1956
21st to endDay
5%214435%5%6%9%55%
2197
May1st to 10thDay
421342445%9%60%
2340
11th to 20thDay
7%426%6%46%5%7%9%44%
2301
21st to endDay
6%425%7%47%7%6%9%43%
2531
June1st to 10thDay
10%7%27%7%47%7%7%11%30%
2340
11th to 20thDay
6%435%7%5%8%9%10%13%31%
2340
21st to endDay
8%6%27%7%5%7%8%10%11%30%
2340
July1st to 10thDay
8%6%36%8%6%10%9%9%12%23%
2340
11th to 20thDay
7%6%38%9%6%9%8%9%12%23%
2340
21st to endDay
6%5%48%10%6%8%10%9%11%24%
2547
August1st to 10thDay
8%5%27%7%5%8%9%8%12%29%
2327
11th to 20thDay
7%5%27%8%6%8%10%11%13%22%
2129
21st to endDay
10%6%38%8%5%9%7%9%12%25%
2196
September1st to 10thDay
8%5%37%9%6%8%8%10%13%24%
1980
11th to 20thDay
9%6%37%8%5%8%7%8%9%28%
1980
21st to endDay
7%5%47%8%5%6%6%8%8%34%
1980
October1st to 10thDay
9%5%46%7%46%5%6%8%40%
1980
11th to 20thDay
6%425%435%46%8%53%
1841
21st to endDay
7%335%5%35%5%5%9%51%
1980
November1st to 10thDay
7%425%5%4445%7%51%
1857
11th to 20thDay
8%6%36%6%3445%7%48%
1896
21st to endDay
8%5%35%5%45%5%7%9%43%
1710
December1st to 10thDay
8%425%5%35%6%7%8%47%
1710
11th to 20thDay
12%6%25%5%245%5%8%46%
2153
21st to endDay
14%7%37%6%35%5%5%7%36%
3124
January1st to 10thNight
11%7%25%5%25%5%5%9%43%
2459
11th to 20thNight
10%435%4345%6%8%47%
2386
21st to endNight
14%6%36%5%345%5%7%42%
2435
February1st to 10thNight
13%7%25%43446%7%45%
2160
11th to 20thNight
9%5%25%5%35%5%6%6%50%
2098
21st to endNight
9%6%37%6%3445%7%47%
1744
March1st to 10thNight
7%42442446%5%57%
1980
11th to 20thNight
8%5%25%425%46%8%51%
1980
21st to endNight
7%42442435%8%57%
2178
April1st to 10thNight
7%4244335%7%10%51%
1980
11th to 20thNight
8%425%5%3445%9%51%
1953
21st to endNight
7%314434448%57%
1776
May1st to 10thNight
43133245%5%9%62%
1620
11th to 20thNight
5%5%35%435%6%7%10%48%
1592
21st to endNight
5%3145%35%5%6%9%53%
1744
June1st to 10thNight
10%6%26%5%45%7%8%9%37%
1620
11th to 20thNight
5%425%6%46%7%9%14%39%
1620
21st to endNight
7%5%35%7%5%7%7%8%13%34%
1620
July1st to 10thNight
6%435%6%5%7%7%9%15%33%
1620
11th to 20thNight
45%36%8%5%8%9%11%14%27%
1620
21st to endNight
45%36%7%5%8%8%10%13%31%
1771
August1st to 10thNight
6%425%6%36%7%11%14%35%
1611
11th to 20thNight
5%5%26%6%6%7%9%10%14%30%
1767
21st to endNight
11%6%37%8%46%6%7%11%31%
2140
September1st to 10thNight
6%5%37%8%48%8%10%12%31%
1980
11th to 20thNight
10%6%38%7%5%6%7%8%11%29%
1980
21st to endNight
8%6%27%7%47%6%7%10%37%
1980
October1st to 10thNight
8%5%27%6%46%5%7%9%42%
1980
11th to 20thNight
6%4136%2436%9%55%
2119
21st to endNight
8%5%25%5%3345%8%51%
2376
November1st to 10thNight
8%42442445%8%54%
2221
11th to 20thNight
9%5%25%5%35%5%5%7%50%
2280
21st to endNight
8%5%36%5%35%46%9%46%
2470
December1st to 10thNight
8%5%35%6%345%5%8%48%
2470
11th to 20thNight
12%7%245%3445%7%48%
3042
21st to endNight
18%8%36%5%35%5%6%8%35%
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
35%7%11%19%19%8%
1707
11th to 20thDay
32%6%8%23%22%9%
1647
21st to endDay
38%6%8%16%24%9%
1917
February1st to 10thDay
39%8%9%15%18%12%
1800
11th to 20thDay
30%6%9%18%25%12%
1758
21st to endDay
32%7%11%17%21%10%
1511
March1st to 10thDay
24%6%13%22%25%10%
1980
11th to 20thDay
31%6%12%24%17%10%
1980
21st to endDay
31%6%12%22%22%7%
2178
April1st to 10thDay
28%9%16%20%19%8%
1980
11th to 20thDay
37%9%11%16%22%7%
1956
21st to endDay
24%8%13%23%24%8%
2197
May1st to 10thDay
22%7%14%24%24%9%
2340
11th to 20thDay
34%10%17%20%14%3
2305
21st to endDay
35%11%15%20%16%3
2531
June1st to 10thDay
47%14%17%17%5%
2340
11th to 20thDay
40%17%21%18%4
2340
21st to endDay
44%17%21%14%4
2340
July1st to 10thDay
48%20%21%9%1
2275
11th to 20thDay
49%18%23%9%1
2271
21st to endDay
47%18%23%10%1
2405
August1st to 10thDay
43%18%27%11%1
2314
11th to 20thDay
46%22%22%10%1
2129
21st to endDay
48%19%18%13%2
2196
September1st to 10thDay
47%20%17%12%4
1980
11th to 20thDay
48%15%19%13%4
1980
21st to endDay
43%13%15%16%10%3
1980
October1st to 10thDay
41%9%12%16%12%9%
1980
11th to 20thDay
29%7%11%21%25%7%
1841
21st to endDay
30%7%11%19%21%12%
1980
November1st to 10thDay
32%7%10%21%18%12%
1857
11th to 20thDay
35%7%11%18%20%10%
1896
21st to endDay
36%7%9%20%21%7%
1710
December1st to 10thDay
31%9%14%24%17%6%
1710
11th to 20thDay
37%6%10%18%16%13%
2150
21st to endDay
44%9%9%19%14%5%
3124
January1st to 10thNight
37%7%12%17%18%9%
2459
11th to 20thNight
33%6%11%20%22%7%
2386
21st to endNight
41%6%9%15%20%9%
2435
February1st to 10thNight
37%6%11%16%19%11%
2160
11th to 20thNight
31%7%11%19%22%10%
2099
21st to endNight
35%6%13%18%21%7%
1744
March1st to 10thNight
26%6%12%23%23%9%
1980
11th to 20thNight
30%8%13%26%14%10%
1980
21st to endNight
26%6%11%29%20%7%
2178
April1st to 10thNight
27%8%18%23%19%6%
1980
11th to 20thNight
30%6%14%25%20%5%
1953
21st to endNight
26%6%14%25%23%6%
1776
May1st to 10thNight
19%7%15%28%22%9%
1620
11th to 20thNight
30%12%19%22%16%2
1593
21st to endNight
27%10%16%24%21%2
1745
June1st to 10thNight
38%13%23%21%6%
1620
11th to 20thNight
32%15%30%21%3
1620
21st to endNight
38%17%24%15%5%
1620
July1st to 10thNight
37%20%31%10%2
1575
11th to 20thNight
40%21%31%8%1
1566
21st to endNight
37%21%30%11%1
1672
August1st to 10thNight
34%19%34%11%1
1600
11th to 20thNight
36%21%30%12%1
1767
21st to endNight
44%17%21%16%2
2140
September1st to 10thNight
41%20%23%12%4
1980
11th to 20thNight
45%15%21%16%31
1980
21st to endNight
39%14%17%17%11%2
1980
October1st to 10thNight
36%11%13%19%13%8%
1980
11th to 20thNight
25%6%13%25%25%5%
2119
21st to endNight
31%7%10%18%23%10%
2376
November1st to 10thNight
28%7%12%22%20%11%
2221
11th to 20thNight
34%6%10%20%20%10%
2280
21st to endNight
35%6%10%23%18%8%
2470
December1st to 10thNight
33%7%11%27%16%5%
2470
11th to 20thNight
35%6%10%18%19%11%
3044
21st to endNight
44%9%11%18%14%4
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
11%13%49%19%6%1
270
11th to 20thDay
11%12%38%26%11%2
249
21st to endDay
15%16%47%19%3
264
February1st to 10thDay
14%15%36%24%10%1
240
11th to 20thDay
11%10%38%27%12%3
240
21st to endDay
7%14%37%28%14%
229
March1st to 10thDay
5%14%40%30%9%1
420
11th to 20thDay
12%13%29%28%15%2
420
21st to endDay
8%11%33%29%16%2
462
April1st to 10thDay
10%13%31%29%15%2
420
11th to 20thDay
11%14%26%28%17%4
412
21st to endDay
7%6%28%34%20%6%
420
May1st to 10thDay
46%27%34%20%9%
423
11th to 20thDay
7%322%31%25%12%
447
21st to endDay
7%311%28%32%19%
485
June1st to 10thDay
12%311%26%30%18%
450
11th to 20thDay
8%39%26%29%26%
450
21st to endDay
6%26%20%33%33%
450
July1st to 10thDay
5%1417%33%39%
445
11th to 20thDay
5%15%15%38%37%
444
21st to endDay
8%1425%30%32%
488
August1st to 10thDay
9%2417%33%35%
446
11th to 20thDay
8%15%15%33%39%
439
21st to endDay
10%26%20%34%28%
495
September1st to 10thDay
9%46%20%34%28%
450
11th to 20thDay
8%36%29%35%20%
450
21st to endDay
8%29%28%33%20%
450
October1st to 10thDay
10%6%22%33%20%8%
450
11th to 20thDay
6%6%19%33%28%8%
450
21st to endDay
7%5%21%39%23%4
495
November1st to 10thDay
9%5%25%37%19%5%
455
11th to 20thDay
11%9%27%35%17%2
456
21st to endDay
8%10%36%32%12%2
270
December1st to 10thDay
6%12%39%27%12%4
270
11th to 20thDay
14%9%36%28%13%1
270
21st to endDay
9%11%32%35%10%1
297
January1st to 10thNight
13%11%32%31%12%1
760
11th to 20thNight
13%10%23%40%14%
735
21st to endNight
16%11%34%31%8%1
792
February1st to 10thNight
16%9%30%30%15%1
720
11th to 20thNight
11%9%28%37%13%2
703
21st to endNight
11%7%34%39%9%
592
March1st to 10thNight
8%9%36%36%10%1
720
11th to 20thNight
9%10%26%39%16%1
720
21st to endNight
9%8%30%38%15%1
792
April1st to 10thNight
8%8%28%39%15%1
720
11th to 20thNight
9%8%31%38%14%
711
21st to endNight
7%426%40%22%
664
May1st to 10thNight
5%415%46%26%3
540
11th to 20thNight
7%315%41%30%5%
531
21st to endNight
7%110%35%37%9%
584
June1st to 10thNight
11%210%29%39%9%
540
11th to 20thNight
6%1422%43%22%
540
21st to endNight
8%15%17%44%25%
540
July1st to 10thNight
7%2314%44%30%
525
11th to 20thNight
5%1212%44%35%
522
21st to endNight
415%14%48%28%
591
August1st to 10thNight
8%1311%49%28%
537
11th to 20thNight
6%1212%45%34%
534
21st to endNight
12%35%22%36%21%
754
September1st to 10thNight
8%16%24%42%20%
720
11th to 20thNight
11%38%28%39%12%
720
21st to endNight
10%310%28%41%7%
720
October1st to 10thNight
11%319%39%25%4
720
11th to 20thNight
5%6%16%39%33%2
720
21st to endNight
10%322%39%23%3
792
November1st to 10thNight
9%428%36%20%3
740
11th to 20thNight
10%6%25%41%17%1
760
21st to endNight
8%6%27%43%15%1
760
December1st to 10thNight
10%6%29%33%21%1
760
11th to 20thNight
12%7%28%35%17%1
760
21st to endNight
12%8%27%35%17%1
836
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