Observatory Park
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Observatory Park clear sky chart from 2005-11-06 to 2024-01-27.

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
28%
23319
13%
75097
8%
23264
34%
23264
15%
54950

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
7%
758
8%
2459
2%
757
13%
757
5%
2078
January11th to 20thNight
10%
760
6%
2470
3%
760
13%
760
5%
2096
January21st to endNight
10%
820
6%
2664
2%
820
16%
820
6%
2263
February1st to 10thNight
12%
720
6%
2340
2%
720
16%
720
6%
1943
February11th to 20thNight
11%
703
8%
2257
3%
700
17%
700
8%
1839
February21st to endNight
9%
592
11%
1776
1%
592
19%
592
9%
1423
March1st to 10thNight
16%
720
14%
2012
7%
720
23%
720
13%
1523
March11th to 20thNight
22%
720
16%
1980
8%
720
29%
720
16%
1458
March21st to endNight
22%
792
15%
2178
8%
792
29%
792
16%
1636
April1st to 10thNight
21%
720
12%
1922
4%
720
27%
720
17%
1429
April11th to 20thNight
23%
602
17%
1666
6%
602
31%
602
19%
1171
April21st to endNight
27%
540
16%
1620
6%
540
36%
540
20%
1129
May1st to 10thNight
33%
540
12%
1620
7%
540
36%
540
18%
1146
May11th to 20thNight
38%
531
16%
1592
11%
531
42%
531
21%
1076
May21st to endNight
50%
467
16%
1630
13%
467
56%
467
28%
964
June1st to 10thNight
41%
360
15%
1440
14%
360
51%
360
21%
863
June11th to 20thNight
46%
360
17%
1440
16%
360
56%
360
21%
860
June21st to endNight
49%
360
17%
1440
18%
360
57%
360
23%
842
July1st to 10thNight
61%
436
16%
1486
17%
436
65%
436
30%
853
July11th to 20thNight
56%
522
17%
1566
14%
522
60%
522
33%
933
July21st to endNight
58%
591
18%
1672
15%
558
61%
558
29%
1045
August1st to 10thNight
60%
537
19%
1600
15%
534
62%
534
32%
959
August11th to 20thNight
59%
534
20%
1598
14%
534
62%
534
29%
1010
August21st to endNight
54%
631
22%
1819
14%
631
57%
631
32%
1114
September1st to 10thNight
54%
720
20%
1854
12%
720
56%
720
30%
1226
September11th to 20thNight
56%
720
24%
1980
18%
720
59%
720
34%
1218
September21st to endNight
41%
720
19%
1980
14%
720
45%
720
24%
1379
October1st to 10thNight
38%
720
18%
2121
14%
720
44%
720
25%
1385
October11th to 20thNight
25%
720
14%
2160
8%
720
34%
720
15%
1528
October21st to endNight
17%
792
15%
2376
7%
792
27%
792
13%
1798
November1st to 10thNight
27%
740
16%
2388
11%
740
35%
740
18%
1628
November11th to 20thNight
16%
760
13%
2559
7%
760
24%
760
10%
1923
November21st to endNight
14%
760
10%
2660
5%
760
20%
760
8%
2092
December1st to 10thNight
10%
760
4%
2660
3%
760
13%
760
6%
2234
December11th to 20thNight
9%
760
6%
3275
2%
759
16%
759
5%
2666
December21st to endNight
8%
831
3%
4837
0%
817
14%
817
3%
4220

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%11%5%8%6%345%5%7%30%
74707
Night
16%12%5%8%6%35%45%6%30%
75328
TransparencyDay
53%7%11%18%10%1
74428
Night
55%6%11%17%10%1
75126
SeeingDay
22%19%29%19%9%3
13484
Night
23%17%27%23%10%1
23328

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
16%18%10%12%8%4444416%
1706
11th to 20thDay
16%20%10%15%7%33445%14%
1710
21st to endDay
15%19%11%17%9%3434310%
1836
February1st to 10thDay
17%19%8%13%10%4334417%
1620
11th to 20thDay
15%14%9%14%9%5%444417%
1590
21st to endDay
17%18%5%13%6%345%45%20%
1480
March1st to 10thDay
17%14%7%9%6%3346%6%26%
1948
11th to 20thDay
16%15%6%9%6%34446%29%
1980
21st to endDay
20%14%5%8%5%3435%5%29%
2178
April1st to 10thDay
15%13%6%7%5%2445%7%32%
2038
11th to 20thDay
19%12%46%5%35%446%32%
2258
21st to endDay
21%12%46%5%34345%35%
2340
May1st to 10thDay
17%10%37%6%45%6%46%31%
2340
11th to 20thDay
19%7%36%5%35%5%5%6%35%
2301
21st to endDay
11%5%36%6%46%6%6%8%39%
2642
June1st to 10thDay
16%6%35%5%36%6%6%8%36%
2520
11th to 20thDay
14%6%26%6%35%5%6%8%38%
2520
21st to endDay
13%435%5%35%6%6%10%40%
2520
July1st to 10thDay
10%5%35%6%45%6%6%9%44%
2424
11th to 20thDay
12%6%35%5%345%7%10%40%
2340
21st to endDay
11%5%25%5%45%5%7%10%40%
2547
August1st to 10thDay
8%6%36%6%45%7%7%9%41%
2327
11th to 20thDay
10%6%27%7%46%7%8%9%35%
2303
21st to endDay
12%6%26%6%35%6%7%8%39%
2518
September1st to 10thDay
16%7%36%425%47%7%39%
2102
11th to 20thDay
12%6%25%6%45%46%8%42%
1980
21st to endDay
19%7%36%5%2446%7%37%
1980
October1st to 10thDay
17%7%36%5%35%5%6%8%36%
1838
11th to 20thDay
22%9%36%7%35%45%7%30%
1800
21st to endDay
25%13%38%6%35%446%23%
1980
November1st to 10thDay
12%14%47%6%3445%6%34%
1686
11th to 20thDay
16%20%5%8%5%34447%24%
1619
21st to endDay
18%20%7%9%7%45%35%6%17%
1520
December1st to 10thDay
16%22%11%12%7%34435%13%
1520
11th to 20thDay
16%20%8%15%5%335%5%5%14%
1910
21st to endDay
23%24%8%12%6%344339%
2786
January1st to 10thNight
16%19%9%14%7%4434415%
2459
11th to 20thNight
18%18%10%14%7%3433415%
2470
21st to endNight
15%19%9%14%10%45%33415%
2664
February1st to 10thNight
19%15%6%13%9%35%44417%
2340
11th to 20thNight
17%14%7%13%8%46%445%19%
2255
21st to endNight
20%13%6%11%7%44445%22%
1776
March1st to 10thNight
14%13%6%10%7%35%5%5%5%28%
2012
11th to 20thNight
17%11%5%8%6%35%446%32%
1980
21st to endNight
17%13%5%9%6%3444430%
2178
April1st to 10thNight
16%13%5%7%5%3445%6%32%
1922
11th to 20thNight
17%11%36%5%26%5%6%6%34%
1666
21st to endNight
15%9%37%6%36%5%47%35%
1620
May1st to 10thNight
18%10%36%6%35%6%6%6%32%
1620
11th to 20thNight
13%9%46%6%35%46%8%35%
1592
21st to endNight
12%5%24445%47%10%44%
1629
June1st to 10thNight
10%7%37%6%35%5%6%7%42%
1440
11th to 20thNight
10%6%45%6%45%5%5%7%43%
1440
21st to endNight
10%5%36%5%35%5%6%10%43%
1440
July1st to 10thNight
7%4345%345%5%8%50%
1531
11th to 20thNight
9%425%435%5%6%11%46%
1620
21st to endNight
8%5%35%5%46%5%6%10%44%
1771
August1st to 10thNight
7%426%6%446%6%8%47%
1611
11th to 20thNight
9%6%35%6%45%5%5%8%44%
1598
21st to endNight
10%5%35%435%5%6%9%45%
1819
September1st to 10thNight
14%5%244245%6%8%44%
1854
11th to 20thNight
11%5%25%42445%6%51%
1980
21st to endNight
16%6%36%5%45%45%6%39%
1980
October1st to 10thNight
15%8%25%5%345%5%8%41%
2121
11th to 20thNight
18%8%37%5%35%46%6%32%
2160
21st to endNight
26%11%36%5%3445%5%27%
2376
November1st to 10thNight
15%11%36%6%35%45%6%35%
2388
11th to 20thNight
16%17%5%8%7%4445%6%24%
2559
21st to endNight
18%18%6%9%6%3445%6%20%
2660
December1st to 10thNight
19%20%10%9%6%35%43416%
2660
11th to 20thNight
16%20%8%13%6%34445%17%
3285
21st to endNight
22%22%9%11%7%44345%10%
4852

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
73%5%10%9%4
1707
11th to 20thDay
74%411%9%2
1710
21st to endDay
78%47%8%3
1836
February1st to 10thDay
73%5%8%10%4
1620
11th to 20thDay
71%5%8%11%6%
1599
21st to endDay
65%6%10%11%6%2
1480
March1st to 10thDay
58%5%13%18%5%
1948
11th to 20thDay
58%5%10%15%10%2
1980
21st to endDay
59%48%18%9%2
2178
April1st to 10thDay
52%5%12%17%13%1
2038
11th to 20thDay
53%5%10%17%13%2
2258
21st to endDay
53%5%10%20%10%3
2340
May1st to 10thDay
53%8%8%20%10%2
2340
11th to 20thDay
49%7%11%17%14%2
2305
21st to endDay
40%9%13%23%12%2
2642
June1st to 10thDay
44%7%11%25%13%1
2520
11th to 20thDay
42%9%13%21%13%2
2520
21st to endDay
39%7%11%25%16%1
2520
July1st to 10thDay
38%8%10%25%16%2
2359
11th to 20thDay
39%8%12%25%15%1
2271
21st to endDay
37%9%13%23%16%2
2405
August1st to 10thDay
36%10%14%26%14%
2314
11th to 20thDay
43%11%13%21%11%2
2303
21st to endDay
39%9%13%21%15%2
2518
September1st to 10thDay
42%8%13%20%13%3
2102
11th to 20thDay
39%8%12%24%15%1
1980
21st to endDay
47%6%11%23%13%1
1980
October1st to 10thDay
46%8%13%20%13%1
1838
11th to 20thDay
55%6%12%18%9%1
1800
21st to endDay
62%413%13%8%1
1980
November1st to 10thDay
50%6%13%18%12%1
1686
11th to 20thDay
61%5%11%13%7%3
1619
21st to endDay
68%5%10%12%5%
1520
December1st to 10thDay
75%5%8%10%3
1520
11th to 20thDay
72%6%9%10%2
1906
21st to endDay
80%48%7%1
2786
January1st to 10thNight
73%49%10%4
2459
11th to 20thNight
74%410%10%21
2470
21st to endNight
74%49%9%31
2664
February1st to 10thNight
71%5%10%11%3
2340
11th to 20thNight
68%5%8%13%5%
2257
21st to endNight
65%5%8%13%7%1
1776
March1st to 10thNight
56%6%11%19%8%
2012
11th to 20thNight
54%5%10%19%11%2
1980
21st to endNight
57%47%17%13%2
2178
April1st to 10thNight
54%6%10%17%11%2
1922
11th to 20thNight
49%6%11%19%13%3
1666
21st to endNight
48%6%11%20%12%3
1620
May1st to 10thNight
50%6%12%17%12%3
1620
11th to 20thNight
47%5%11%19%15%4
1593
21st to endNight
35%7%16%25%15%1
1630
June1st to 10thNight
40%7%11%25%15%2
1440
11th to 20thNight
39%7%12%21%18%3
1440
21st to endNight
36%8%13%24%18%1
1440
July1st to 10thNight
33%6%12%25%20%4
1486
11th to 20thNight
31%8%14%24%20%3
1566
21st to endNight
35%9%13%23%18%3
1672
August1st to 10thNight
32%8%13%30%16%1
1600
11th to 20thNight
37%7%15%27%13%1
1598
21st to endNight
34%7%13%25%17%3
1819
September1st to 10thNight
37%8%12%24%16%3
1854
11th to 20thNight
33%7%14%24%20%2
1980
21st to endNight
45%6%11%21%16%1
1980
October1st to 10thNight
41%7%13%20%18%1
2121
11th to 20thNight
51%6%12%20%11%1
2160
21st to endNight
58%6%11%16%9%1
2376
November1st to 10thNight
48%6%13%19%11%3
2388
11th to 20thNight
60%5%11%15%6%3
2559
21st to endNight
65%5%11%14%4
2660
December1st to 10thNight
73%5%10%8%4
2660
11th to 20thNight
70%5%12%11%2
3288
21st to endNight
77%6%10%6%1
4852

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
31%29%28%9%2
270
11th to 20thDay
32%34%25%7%2
270
21st to endDay
28%37%28%6%11
282
February1st to 10thDay
34%28%33%5%
240
11th to 20thDay
33%38%24%4
240
21st to endDay
37%34%25%4
198
March1st to 10thDay
29%28%29%12%3
388
11th to 20thDay
25%27%31%13%31
420
21st to endDay
35%22%27%12%4
462
April1st to 10thDay
27%28%26%14%5%1
420
11th to 20thDay
31%20%26%15%8%1
412
21st to endDay
29%23%30%13%42
420
May1st to 10thDay
26%23%30%16%32
423
11th to 20thDay
22%15%31%23%7%2
447
21st to endDay
14%10%32%27%13%4
485
June1st to 10thDay
17%13%37%21%11%2
450
11th to 20thDay
17%14%30%27%11%2
450
21st to endDay
14%12%36%27%8%2
450
July1st to 10thDay
10%8%25%29%21%7%
445
11th to 20thDay
14%11%28%27%15%5%
444
21st to endDay
10%8%34%28%14%6%
488
August1st to 10thDay
9%9%27%29%18%8%
446
11th to 20thDay
13%9%27%29%15%7%
439
21st to endDay
13%12%24%27%16%8%
495
September1st to 10thDay
17%9%25%25%19%5%
450
11th to 20thDay
12%8%25%28%20%9%
450
21st to endDay
20%14%28%21%12%5%
450
October1st to 10thDay
18%15%28%21%16%3
450
11th to 20thDay
25%23%33%15%41
450
21st to endDay
35%20%29%14%3
495
November1st to 10thDay
23%26%29%14%7%1
284
11th to 20thDay
35%24%24%12%4
227
21st to endDay
28%30%34%7%
180
December1st to 10thDay
26%37%24%8%31
180
11th to 20thDay
25%39%28%8%
180
21st to endDay
32%32%28%7%1
204
January1st to 10thNight
29%28%31%10%3
760
11th to 20thNight
33%25%27%13%2
760
21st to endNight
27%25%32%14%2
820
February1st to 10thNight
30%26%27%15%2
720
11th to 20thNight
29%25%31%14%2
703
21st to endNight
32%24%30%13%1
592
March1st to 10thNight
24%24%32%17%3
720
11th to 20thNight
26%23%27%18%5%1
720
21st to endNight
25%19%29%20%6%
792
April1st to 10thNight
26%18%29%22%5%
720
11th to 20thNight
24%16%34%18%7%1
602
21st to endNight
19%15%34%26%6%1
540
May1st to 10thNight
22%12%28%28%10%1
540
11th to 20thNight
17%10%28%34%9%2
531
21st to endNight
13%5%28%34%18%2
469
June1st to 10thNight
12%6%34%35%13%1
360
11th to 20thNight
12%7%29%37%15%1
360
21st to endNight
10%8%27%40%14%
360
July1st to 10thNight
7%420%40%23%6%
436
11th to 20thNight
10%6%24%34%24%3
522
21st to endNight
10%6%20%40%23%2
591
August1st to 10thNight
9%5%17%40%24%4
537
11th to 20thNight
10%317%38%28%3
534
21st to endNight
13%6%22%30%27%3
631
September1st to 10thNight
16%7%18%32%24%4
720
11th to 20thNight
14%5%19%35%23%5%
720
21st to endNight
19%11%24%26%16%4
720
October1st to 10thNight
21%9%28%23%17%2
720
11th to 20thNight
24%19%27%23%6%
720
21st to endNight
32%19%29%16%3
792
November1st to 10thNight
23%18%28%23%7%1
740
11th to 20thNight
28%23%27%15%5%1
760
21st to endNight
33%23%26%14%3
760
December1st to 10thNight
33%25%28%11%4
760
11th to 20thNight
30%28%28%13%1
760
21st to endNight
31%25%30%12%1
836
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