This page made possible by the sponsorship of: Frank M. Klicar.

Megamark Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Megamark Observatory clear sky chart from 2005-11-06 to 2024-11-05.

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
36%
25487
16%
96813
11%
25210
43%
25210
18%
63924

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%
943
10%
4402
3%
926
30%
926
10%
3230
January11th to 20thNight
19%
950
10%
4389
4%
930
25%
930
8%
3470
January21st to endNight
20%
1039
7%
4742
3%
1023
26%
1023
8%
3809
February1st to 10thNight
21%
912
12%
2432
3%
912
27%
912
17%
1803
February11th to 20thNight
25%
740
19%
2219
8%
740
34%
740
19%
1520
February21st to endNight
21%
628
17%
1884
5%
628
31%
628
18%
1326
March1st to 10thNight
23%
758
18%
3689
7%
745
29%
745
11%
2238
March11th to 20thNight
29%
727
21%
3653
10%
712
39%
712
16%
1970
March21st to endNight
24%
625
8%
3424
9%
605
33%
605
9%
2472
April1st to 10thNight
27%
570
17%
1900
10%
570
39%
570
21%
1247
April11th to 20thNight
29%
563
19%
1875
13%
563
39%
563
20%
1230
April21st to endNight
30%
570
14%
1810
7%
570
39%
570
20%
1241
May1st to 10thNight
32%
566
18%
2782
8%
559
39%
559
14%
1759
May11th to 20thNight
39%
550
13%
2557
8%
541
46%
541
17%
1482
May21st to endNight
46%
609
14%
2701
10%
593
51%
593
20%
1525
June1st to 10thNight
50%
570
17%
1520
11%
570
55%
570
36%
946
June11th to 20thNight
51%
570
20%
1520
12%
570
56%
570
37%
927
June21st to endNight
46%
570
16%
1520
9%
570
51%
570
29%
1016
July1st to 10thNight
63%
546
13%
2425
12%
535
66%
535
21%
1553
July11th to 20thNight
62%
540
18%
2220
15%
533
68%
533
30%
1157
July21st to endNight
67%
605
16%
2493
17%
569
71%
569
26%
1461
August1st to 10thNight
68%
553
21%
2403
18%
544
71%
544
26%
1327
August11th to 20thNight
68%
549
18%
2767
22%
543
72%
543
25%
1475
August21st to endNight
65%
607
18%
3077
23%
605
70%
605
25%
1589
September1st to 10thNight
61%
570
26%
1900
26%
570
66%
570
34%
1058
September11th to 20thNight
64%
665
33%
1995
29%
665
68%
665
40%
1103
September21st to endNight
50%
760
23%
2090
15%
760
55%
760
32%
1312
October1st to 10thNight
51%
760
23%
2203
19%
740
55%
740
32%
1350
October11th to 20thNight
35%
760
18%
2704
11%
740
45%
740
19%
1854
October21st to endNight
28%
836
14%
3317
11%
814
37%
814
15%
2328
November1st to 10thNight
31%
760
26%
2470
17%
760
44%
760
26%
1484
November11th to 20thNight
26%
760
18%
2470
9%
760
37%
760
19%
1638
November21st to endNight
24%
821
18%
2531
9%
821
34%
821
20%
1734
December1st to 10thNight
23%
950
13%
2660
5%
950
29%
950
18%
1946
December11th to 20thNight
24%
950
14%
3213
5%
948
32%
948
15%
2326
December21st to endNight
14%
1035
6%
4856
3%
1026
21%
1026
6%
4018

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
13%9%38%5%4446%7%37%
101306
Night
14%10%48%44435%6%38%
97818
TransparencyDay
44%7%12%23%14%2
104872
Night
44%6%11%22%15%2
102380
SeeingDay
16%11%29%25%14%5%
15803
Night
17%9%30%30%12%1
25634

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
14%12%6%13%6%5%5%345%27%
2527
11th to 20thDay
14%14%6%15%6%5%47%7%319%
2530
21st to endDay
18%24%6%13%5%4234417%
2886
February1st to 10thDay
17%14%49%6%6%5%45%5%26%
1746
11th to 20thDay
14%12%49%6%44446%34%
1848
21st to endDay
15%15%5%10%5%3345%6%30%
1570
March1st to 10thDay
15%15%38%5%43346%34%
3120
11th to 20thDay
10%7%37%5%5%5%46%7%40%
3273
21st to endDay
19%13%5%13%6%5%5%36%5%19%
4165
April1st to 10thDay
16%13%48%5%4436%6%31%
2280
11th to 20thDay
16%10%37%445%46%6%36%
2254
21st to endDay
19%8%48%5%4435%6%33%
2370
May1st to 10thDay
17%7%26%5%5%5%5%7%8%34%
4082
11th to 20thDay
15%7%26%44445%7%41%
4299
21st to endDay
11%6%25%5%45%5%7%9%42%
4824
June1st to 10thDay
10%6%25%35%5%46%7%45%
2660
11th to 20thDay
9%5%26%5%5%5%46%9%44%
2660
21st to endDay
11%6%26%6%5%5%6%8%8%37%
2660
July1st to 10thDay
10%8%37%6%6%6%5%8%9%33%
4440
11th to 20thDay
7%426%44447%11%47%
4440
21st to endDay
8%6%28%6%5%5%5%8%8%41%
4813
August1st to 10thDay
8%6%26%45%5%47%10%44%
4317
11th to 20thDay
9%5%26%445%48%10%42%
3729
21st to endDay
7%5%26%5%4446%11%47%
4180
September1st to 10thDay
11%6%26%445%35%7%48%
2280
11th to 20thDay
10%42434345%7%53%
2165
21st to endDay
9%7%26%44446%7%44%
2090
October1st to 10thDay
12%7%26%44335%7%47%
2031
11th to 20thDay
17%8%37%44445%7%37%
1900
21st to endDay
19%8%37%5%3435%6%36%
1881
November1st to 10thDay
13%9%36%34445%6%45%
1708
11th to 20thDay
16%11%48%445%25%7%33%
1710
21st to endDay
19%10%39%443345%35%
1647
December1st to 10thDay
15%12%39%6%5%4446%31%
1520
11th to 20thDay
13%14%5%10%5%5%5%446%29%
1918
21st to endDay
22%22%9%13%6%4322414%
2783
January1st to 10thNight
14%16%7%11%5%4434429%
4428
11th to 20thNight
14%19%7%12%6%445%6%419%
4440
21st to endNight
22%18%8%10%5%4424421%
4779
February1st to 10thNight
15%16%49%6%44346%32%
2432
11th to 20thNight
13%12%47%5%44346%37%
2218
21st to endNight
18%11%49%443345%35%
1884
March1st to 10thNight
18%10%36%434246%40%
3733
11th to 20thNight
10%7%27%5%4435%6%47%
3714
21st to endNight
20%11%5%10%45%435%5%27%
3466
April1st to 10thNight
16%10%37%34336%6%39%
1900
11th to 20thNight
20%7%36%43335%6%40%
1875
21st to endNight
17%9%37%5%45%36%7%35%
1810
May1st to 10thNight
16%7%38%5%45%46%7%35%
2813
11th to 20thNight
12%6%25%445%46%9%41%
2599
21st to endNight
10%7%36%44446%8%44%
2743
June1st to 10thNight
9%5%16%44436%7%50%
1520
11th to 20thNight
8%5%25%446%48%9%46%
1520
21st to endNight
10%7%26%5%5%5%46%8%42%
1520
July1st to 10thNight
12%6%38%45%6%45%8%38%
2530
11th to 20thNight
5%426%43437%8%54%
2530
21st to endNight
45%26%6%6%5%5%7%8%45%
2750
August1st to 10thNight
6%426%445%5%7%9%48%
2616
11th to 20thNight
7%426%44446%8%51%
3100
21st to endNight
7%5%25%44446%10%52%
3476
September1st to 10thNight
10%5%25%333347%54%
1900
11th to 20thNight
9%425%34426%8%55%
1995
21st to endNight
10%5%27%5%4446%7%46%
2090
October1st to 10thNight
10%5%26%34335%6%53%
2146
11th to 20thNight
17%7%25%334346%46%
2280
21st to endNight
20%9%36%434246%38%
2717
November1st to 10thNight
13%7%35%433346%49%
2470
11th to 20thNight
15%10%38%5%4435%7%37%
2470
21st to endNight
17%9%38%5%4335%6%37%
2531
December1st to 10thNight
17%11%410%5%4435%6%32%
2660
11th to 20thNight
15%14%49%5%44345%32%
3279
21st to endNight
23%18%7%11%5%5%4345%16%
4884

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
60%5%12%17%6%
2528
11th to 20thDay
64%5%10%12%8%1
2530
21st to endDay
71%48%12%5%
2886
February1st to 10thDay
59%6%12%13%9%2
1746
11th to 20thDay
52%410%23%9%2
1858
21st to endDay
55%6%10%20%7%2
1570
March1st to 10thDay
52%5%10%20%11%2
3120
11th to 20thDay
40%6%13%29%11%1
3273
21st to endDay
62%10%12%13%3
4165
April1st to 10thDay
52%7%10%20%11%
2280
11th to 20thDay
47%7%11%20%12%4
2254
21st to endDay
51%6%13%16%12%2
2370
May1st to 10thDay
44%8%12%23%12%
4120
11th to 20thDay
40%7%11%28%13%
4303
21st to endDay
35%8%13%22%18%5%
4837
June1st to 10thDay
35%8%12%26%17%2
2660
11th to 20thDay
35%7%11%24%20%3
2660
21st to endDay
41%9%13%21%13%3
2660
July1st to 10thDay
43%7%13%23%11%2
4371
11th to 20thDay
28%7%17%30%16%1
4365
21st to endDay
38%8%17%16%19%3
4662
August1st to 10thDay
33%8%17%28%12%1
4302
11th to 20thDay
32%8%12%32%15%1
3765
21st to endDay
31%6%13%29%18%2
4179
September1st to 10thDay
34%7%12%25%18%4
2280
11th to 20thDay
29%6%12%29%22%3
2165
21st to endDay
36%7%9%26%20%2
2090
October1st to 10thDay
26%48%29%32%2
3369
11th to 20thDay
31%5%12%26%25%1
3140
21st to endDay
41%5%9%27%14%3
3081
November1st to 10thDay
39%6%14%23%16%2
1708
11th to 20thDay
51%5%11%21%11%1
1710
21st to endDay
53%410%23%9%
1647
December1st to 10thDay
54%6%13%18%8%1
1520
11th to 20thDay
55%6%13%18%7%1
1915
21st to endDay
78%38%9%3
2783
January1st to 10thNight
59%5%10%17%9%
4428
11th to 20thNight
66%5%11%12%7%
4440
21st to endNight
67%5%9%13%5%
4779
February1st to 10thNight
56%411%17%11%2
2432
11th to 20thNight
49%410%22%14%1
2219
21st to endNight
51%5%11%19%12%3
1884
March1st to 10thNight
47%5%12%23%10%3
3733
11th to 20thNight
37%6%10%27%18%2
3714
21st to endNight
58%6%11%18%6%1
3466
April1st to 10thNight
44%6%12%23%12%2
1900
11th to 20thNight
46%5%9%21%16%4
1875
21st to endNight
47%7%11%20%12%3
1810
May1st to 10thNight
45%7%12%21%12%2
2840
11th to 20thNight
37%10%16%26%11%1
2600
21st to endNight
36%8%12%19%20%5%
2751
June1st to 10thNight
33%7%13%25%20%4
1520
11th to 20thNight
31%7%13%24%22%3
1520
21st to endNight
38%8%14%20%16%3
1520
July1st to 10thNight
42%7%15%19%13%4
2490
11th to 20thNight
26%7%14%30%21%2
2482
21st to endNight
32%9%17%18%21%4
2662
August1st to 10thNight
29%8%18%26%17%1
2607
11th to 20thNight
28%7%15%29%19%1
3130
21st to endNight
28%6%13%32%18%3
3476
September1st to 10thNight
30%6%10%26%24%3
1900
11th to 20thNight
27%5%9%30%27%3
1995
21st to endNight
35%6%12%27%17%3
2090
October1st to 10thNight
21%47%37%28%2
3575
11th to 20thNight
33%5%7%24%29%2
3779
21st to endNight
39%49%26%19%3
4467
November1st to 10thNight
37%5%9%26%19%4
2470
11th to 20thNight
47%413%22%13%2
2470
21st to endNight
49%412%23%12%
2531
December1st to 10thNight
53%5%11%21%8%1
2660
11th to 20thNight
54%5%13%18%9%1
3281
21st to endNight
70%6%9%11%4
4884

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
23%14%34%23%6%1
270
11th to 20thDay
23%18%37%17%41
270
21st to endDay
23%21%30%21%5%
297
February1st to 10thDay
23%21%37%16%3
270
11th to 20thDay
24%19%35%19%3
270
21st to endDay
24%21%38%13%21
225
March1st to 10thDay
26%22%36%16%1
270
11th to 20thDay
28%17%34%15%5%1
287
21st to endDay
28%15%30%18%6%2
506
April1st to 10thDay
23%17%31%23%6%1
460
11th to 20thDay
21%22%25%18%11%2
452
21st to endDay
22%17%31%22%7%1
460
May1st to 10thDay
23%12%30%24%9%2
463
11th to 20thDay
20%10%30%25%11%4
614
21st to endDay
15%10%31%22%16%6%
734
June1st to 10thDay
11%9%29%29%16%5%
680
11th to 20thDay
11%7%31%30%17%4
680
21st to endDay
13%8%33%29%13%4
680
July1st to 10thDay
8%422%30%22%14%
671
11th to 20thDay
12%5%24%31%17%11%
668
21st to endDay
6%429%33%21%6%
739
August1st to 10thDay
7%321%36%22%11%
629
11th to 20thDay
10%421%29%25%12%
479
21st to endDay
6%5%18%32%28%11%
539
September1st to 10thDay
12%5%22%32%22%6%
490
11th to 20thDay
10%318%30%28%11%
490
21st to endDay
13%12%24%26%19%6%
490
October1st to 10thDay
13%8%29%29%16%4
431
11th to 20thDay
17%24%32%17%9%1
300
21st to endDay
29%16%31%16%6%1
330
November1st to 10thDay
17%17%34%21%8%4
282
11th to 20thDay
26%18%33%16%6%1
270
21st to endDay
25%12%37%20%6%
270
December1st to 10thDay
19%20%38%14%9%
270
11th to 20thDay
20%16%41%18%5%1
270
21st to endDay
26%21%37%15%2
297
January1st to 10thNight
21%14%39%22%5%
950
11th to 20thNight
24%15%34%22%5%1
950
21st to endNight
21%12%36%28%3
1045
February1st to 10thNight
23%13%35%25%4
912
11th to 20thNight
21%13%36%26%5%
743
21st to endNight
23%14%38%21%4
628
March1st to 10thNight
21%14%35%23%6%
760
11th to 20thNight
16%13%36%27%7%1
732
21st to endNight
25%11%35%23%6%1
627
April1st to 10thNight
20%14%34%25%7%
570
11th to 20thNight
22%12%33%26%7%
563
21st to endNight
21%10%34%27%8%
570
May1st to 10thNight
18%8%37%28%9%1
570
11th to 20thNight
18%6%30%34%12%
561
21st to endNight
13%5%29%36%17%1
617
June1st to 10thNight
11%429%42%15%
570
11th to 20thNight
9%330%45%11%1
570
21st to endNight
10%432%37%16%1
570
July1st to 10thNight
8%317%43%26%3
555
11th to 20thNight
7%319%41%28%3
552
21st to endNight
5%218%47%26%2
624
August1st to 10thNight
6%214%47%29%4
567
11th to 20thNight
6%214%43%30%4
564
21st to endNight
7%217%41%30%2
627
September1st to 10thNight
12%218%38%27%3
570
11th to 20thNight
10%417%38%28%3
665
21st to endNight
12%7%24%34%21%2
760
October1st to 10thNight
12%6%27%35%19%2
760
11th to 20thNight
22%11%30%27%11%1
760
21st to endNight
25%12%30%25%9%
836
November1st to 10thNight
16%11%37%25%10%1
760
11th to 20thNight
19%15%34%26%6%
760
21st to endNight
20%12%37%24%6%
821
December1st to 10thNight
23%12%35%25%5%
950
11th to 20thNight
23%11%34%26%5%
950
21st to endNight
29%15%33%21%3
1045
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