Skywalker Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Skywalker Observatory clear sky chart from 2005-11-06 to 2024-01-12.

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
49%
22838
16%
74937
14%
22798
53%
22798
22%
49441

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
24%
759
8%
2646
5%
756
28%
756
9%
2148
January11th to 20thNight
29%
726
6%
2540
5%
726
32%
726
9%
2056
January21st to endNight
34%
792
8%
2772
7%
792
37%
792
11%
2173
February1st to 10thNight
36%
720
10%
2495
8%
720
41%
720
14%
1820
February11th to 20thNight
41%
702
13%
2203
9%
700
48%
700
19%
1553
February21st to endNight
47%
592
18%
1776
15%
592
54%
592
23%
1203
March1st to 10thNight
46%
720
14%
2160
12%
720
50%
720
21%
1509
March11th to 20thNight
43%
720
16%
2119
12%
720
49%
720
22%
1480
March21st to endNight
44%
792
13%
1980
6%
792
47%
792
22%
1450
April1st to 10thNight
48%
720
15%
1800
7%
720
51%
720
25%
1263
April11th to 20thNight
44%
672
19%
1740
10%
672
49%
672
27%
1174
April21st to endNight
50%
536
18%
1616
14%
536
54%
536
26%
1015
May1st to 10thNight
54%
360
21%
1440
24%
360
62%
360
26%
791
May11th to 20thNight
58%
354
18%
1416
28%
354
62%
354
26%
779
May21st to endNight
58%
389
12%
1552
18%
388
62%
388
23%
901
June1st to 10thNight
61%
360
11%
1292
14%
360
63%
360
27%
743
June11th to 20thNight
57%
360
12%
1260
10%
360
61%
360
26%
740
June21st to endNight
71%
360
17%
1277
16%
360
73%
360
40%
619
July1st to 10thNight
80%
350
16%
1400
20%
350
83%
350
48%
568
July11th to 20thNight
75%
348
22%
1392
33%
348
82%
348
54%
529
July21st to endNight
87%
394
31%
1488
48%
372
93%
372
68%
482
August1st to 10thNight
80%
358
30%
1424
41%
356
86%
356
53%
540
August11th to 20thNight
78%
418
34%
1486
36%
418
81%
418
52%
632
August21st to endNight
75%
769
33%
1957
21%
769
78%
769
55%
1028
September1st to 10thNight
74%
720
32%
1800
18%
720
77%
720
57%
950
September11th to 20thNight
68%
720
28%
1872
19%
720
71%
720
42%
1114
September21st to endNight
63%
720
27%
1980
21%
720
67%
720
41%
1139
October1st to 10thNight
62%
720
22%
2092
20%
720
65%
720
35%
1230
October11th to 20thNight
54%
720
22%
2241
19%
720
60%
720
29%
1360
October21st to endNight
50%
792
20%
2574
18%
792
56%
792
24%
1666
November1st to 10thNight
45%
740
11%
2405
10%
740
48%
740
16%
1733
November11th to 20thNight
37%
760
12%
2617
9%
760
42%
760
14%
1863
November21st to endNight
36%
830
8%
2730
6%
830
38%
830
14%
2045
December1st to 10thNight
34%
950
10%
2850
6%
950
37%
950
14%
2194
December11th to 20thNight
26%
949
4%
3507
3%
949
28%
949
7%
2973
December21st to endNight
26%
946
5%
5038
5%
936
30%
936
5%
3978

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
10%8%36%6%35%5%5%7%41%
74562
Night
12%10%47%6%35%5%5%7%37%
75156
TransparencyDay
43%412%21%15%4
74264
Night
46%411%20%14%4
74972
SeeingDay
15%13%26%28%15%3
13583
Night
17%7%16%36%22%3
22851

By Month

Percentage Cloud Cover NW (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
23%17%5%10%7%36%346%15%
1519
11th to 20thDay
19%13%7%12%9%46%46%6%14%
1448
21st to endDay
17%16%6%11%8%5%4447%18%
1584
February1st to 10thDay
15%12%410%7%5%6%5%6%7%24%
1462
11th to 20thDay
12%12%47%7%45%6%6%7%30%
1645
21st to endDay
13%12%47%6%45%6%6%7%29%
1480
March1st to 10thDay
14%11%46%5%46%6%6%8%30%
1800
11th to 20thDay
15%10%37%6%35%5%6%8%32%
1836
21st to endDay
13%10%5%9%7%45%5%7%7%27%
2376
April1st to 10thDay
12%9%49%8%5%6%6%6%8%28%
2160
11th to 20thDay
9%8%47%7%45%5%6%7%38%
2171
21st to endDay
8%8%37%6%46%5%6%8%37%
2342
May1st to 10thDay
5%6%37%6%45%5%6%8%43%
2520
11th to 20thDay
9%7%37%6%46%6%6%10%38%
2477
21st to endDay
9%6%47%6%5%6%6%7%8%35%
2723
June1st to 10thDay
14%7%36%5%4445%8%39%
2664
11th to 20thDay
11%5%37%7%46%6%6%7%38%
2700
21st to endDay
5%434435%46%7%56%
2678
July1st to 10thDay
331342446%7%64%
2520
11th to 20thDay
3213423346%69%
2520
21st to endDay
2112222335%78%
2741
August1st to 10thDay
3313423446%67%
2507
11th to 20thDay
421332335%8%68%
2413
21st to endDay
34145%35%5%47%59%
2395
September1st to 10thDay
3313324346%66%
2160
11th to 20thDay
5%5%25%5%35%5%6%7%52%
2088
21st to endDay
8%5%245%25%5%5%8%49%
1980
October1st to 10thDay
9%7%37%6%3446%7%47%
1867
11th to 20thDay
11%5%35%5%36%6%5%8%44%
1718
21st to endDay
18%9%35%5%35%5%6%6%36%
1782
November1st to 10thDay
19%10%47%7%346%47%28%
1674
11th to 20thDay
19%12%48%6%345%45%29%
1561
21st to endDay
16%17%6%8%8%45%5%5%6%20%
1428
December1st to 10thDay
14%15%7%10%9%5%6%45%6%20%
1330
11th to 20thDay
24%20%6%11%6%445%45%11%
1682
21st to endDay
14%20%9%9%7%46%5%5%6%14%
2611
January1st to 10thNight
23%17%6%9%6%3446%6%16%
2646
11th to 20thNight
19%16%5%12%7%45%446%17%
2540
21st to endNight
14%16%7%11%7%46%5%46%20%
2772
February1st to 10thNight
13%14%5%8%6%35%5%6%8%26%
2495
11th to 20thNight
11%11%5%7%6%46%5%6%7%32%
2201
21st to endNight
10%9%47%6%45%5%7%9%35%
1776
March1st to 10thNight
12%10%46%7%46%5%6%8%32%
2160
11th to 20thNight
13%9%46%7%45%6%48%34%
2119
21st to endNight
10%9%48%8%47%6%7%8%30%
1980
April1st to 10thNight
9%8%37%7%5%5%5%6%8%36%
1800
11th to 20thNight
9%8%38%8%45%45%6%41%
1740
21st to endNight
6%8%37%6%36%5%7%7%41%
1616
May1st to 10thNight
8%326%6%45%6%5%9%45%
1440
11th to 20thNight
7%5%26%6%35%6%6%10%44%
1416
21st to endNight
7%7%38%5%36%5%7%9%39%
1552
June1st to 10thNight
10%5%35%6%45%5%7%9%43%
1292
11th to 20thNight
7%5%45%5%45%6%8%8%43%
1260
21st to endNight
44135%34448%60%
1277
July1st to 10thNight
221333346%8%66%
1440
11th to 20thNight
221231335%6%71%
1440
21st to endNight
111222336%80%
1576
August1st to 10thNight
3312222346%73%
1432
11th to 20thNight
231332236%7%68%
1486
21st to endNight
32123245%5%7%66%
1957
September1st to 10thNight
321333345%8%65%
1800
11th to 20thNight
7%42443446%8%53%
1872
21st to endNight
8%4245%2446%8%53%
1980
October1st to 10thNight
10%5%25%44345%8%50%
2092
11th to 20thNight
11%6%36%5%345%5%8%46%
2241
21st to endNight
15%7%345%45%46%7%39%
2574
November1st to 10thNight
18%11%36%5%446%7%8%27%
2405
11th to 20thNight
17%11%46%6%45%6%6%6%28%
2617
21st to endNight
18%12%411%7%45%445%25%
2730
December1st to 10thNight
14%14%5%10%7%45%45%6%23%
2850
11th to 20thNight
21%19%9%11%7%4444413%
3510
21st to endNight
19%20%7%10%7%46%6%5%5%12%
5072

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
71%5%11%8%5%
1519
11th to 20thDay
72%5%9%10%21
1448
21st to endDay
67%5%10%13%5%
1584
February1st to 10thDay
59%5%16%11%7%1
1462
11th to 20thDay
52%5%16%18%8%
1654
21st to endDay
53%6%13%16%11%1
1480
March1st to 10thDay
53%6%16%17%7%1
1800
11th to 20thDay
51%5%14%21%9%
1836
21st to endDay
57%5%17%16%5%1
2376
April1st to 10thDay
54%6%12%18%8%1
2160
11th to 20thDay
46%5%14%20%12%2
2171
21st to endDay
45%5%12%25%11%3
2342
May1st to 10thDay
39%410%29%15%2
2520
11th to 20thDay
44%5%12%25%13%1
2480
21st to endDay
45%413%25%12%1
2724
June1st to 10thDay
45%49%22%16%4
2664
11th to 20thDay
44%410%24%14%3
2700
21st to endDay
28%311%28%23%6%
2678
July1st to 10thDay
21%38%26%27%15%
2451
11th to 20thDay
18%37%21%33%18%
2445
21st to endDay
12%2322%42%19%
2589
August1st to 10thDay
20%28%26%30%15%
2490
11th to 20thDay
17%39%27%36%9%
2413
21st to endDay
26%36%29%30%5%
2395
September1st to 10thDay
21%210%30%28%9%
2160
11th to 20thDay
31%412%25%24%5%
2088
21st to endDay
34%5%17%28%12%4
1980
October1st to 10thDay
38%314%31%13%1
1867
11th to 20thDay
38%417%29%11%1
1718
21st to endDay
47%5%15%23%9%1
1782
November1st to 10thDay
56%6%14%16%6%1
1674
11th to 20thDay
57%6%15%15%5%2
1561
21st to endDay
63%5%14%14%41
1428
December1st to 10thDay
65%412%13%6%
1330
11th to 20thDay
76%412%6%3
1684
21st to endDay
69%313%10%5%
2611
January1st to 10thNight
69%5%11%10%5%1
2646
11th to 20thNight
68%5%13%11%3
2540
21st to endNight
65%5%13%12%41
2772
February1st to 10thNight
55%7%16%13%8%2
2495
11th to 20thNight
52%5%16%18%9%1
2203
21st to endNight
46%5%15%21%13%1
1776
March1st to 10thNight
50%5%14%21%10%1
2160
11th to 20thNight
49%412%22%12%1
2120
21st to endNight
50%5%15%19%9%1
1980
April1st to 10thNight
47%412%24%12%1
1800
11th to 20thNight
45%39%23%17%2
1740
21st to endNight
41%310%25%17%3
1616
May1st to 10thNight
36%38%31%18%3
1440
11th to 20thNight
36%410%29%17%3
1416
21st to endNight
40%412%29%14%1
1552
June1st to 10thNight
37%49%26%19%5%
1292
11th to 20thNight
37%5%9%25%18%5%
1260
21st to endNight
25%48%30%26%8%
1277
July1st to 10thNight
18%36%23%32%19%
1400
11th to 20thNight
14%37%22%30%24%
1392
21st to endNight
8%2317%41%30%
1488
August1st to 10thNight
15%25%19%34%24%
1424
11th to 20thNight
17%35%25%38%12%
1486
21st to endNight
19%47%25%34%11%
1957
September1st to 10thNight
18%36%28%30%14%
1800
11th to 20thNight
30%49%24%26%9%
1872
21st to endNight
30%313%32%16%6%
1980
October1st to 10thNight
35%212%32%19%1
2092
11th to 20thNight
38%414%28%16%2
2241
21st to endNight
44%413%27%11%1
2574
November1st to 10thNight
53%7%17%16%7%1
2405
11th to 20thNight
54%5%16%15%7%3
2617
21st to endNight
60%414%14%7%
2730
December1st to 10thNight
60%5%13%17%4
2850
11th to 20thNight
74%5%11%8%3
3507
21st to endNight
71%312%9%5%
5072

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
32%8%21%28%10%2
180
11th to 20thDay
31%9%18%23%14%5%
162
21st to endDay
33%328%23%12%1
176
February1st to 10thDay
21%17%30%23%9%
182
11th to 20thDay
21%13%29%25%10%1
367
21st to endDay
17%16%31%24%11%1
346
March1st to 10thDay
21%20%29%19%9%2
420
11th to 20thDay
22%15%22%24%15%1
420
21st to endDay
20%21%27%21%9%1
462
April1st to 10thDay
21%15%31%22%10%1
420
11th to 20thDay
17%28%24%20%9%2
412
21st to endDay
15%18%28%28%9%3
420
May1st to 10thDay
12%15%31%27%13%2
423
11th to 20thDay
17%14%26%28%13%3
448
21st to endDay
13%13%26%26%16%6%
485
June1st to 10thDay
19%12%20%28%17%4
594
11th to 20thDay
13%16%26%28%14%3
630
21st to endDay
7%11%21%33%23%5%
608
July1st to 10thDay
5%11%32%37%14%2
446
11th to 20thDay
6%11%31%32%16%5%
444
21st to endDay
39%26%37%19%6%
487
August1st to 10thDay
6%8%19%39%21%7%
447
11th to 20thDay
5%5%22%35%25%7%
437
21st to endDay
6%11%27%28%21%7%
495
September1st to 10thDay
6%9%30%29%21%4
450
11th to 20thDay
8%12%26%28%21%4
450
21st to endDay
12%15%28%25%15%5%
450
October1st to 10thDay
13%12%29%27%16%3
450
11th to 20thDay
18%10%27%24%18%3
368
21st to endDay
22%10%27%27%10%3
297
November1st to 10thDay
23%11%24%29%13%
270
11th to 20thDay
28%15%18%21%16%3
199
21st to endDay
27%7%21%28%15%3
180
December1st to 10thDay
23%12%24%21%16%4
180
11th to 20thDay
28%13%24%26%7%2
180
21st to endDay
29%12%21%28%10%
198
January1st to 10thNight
31%8%20%27%12%1
760
11th to 20thNight
28%7%19%29%15%2
726
21st to endNight
24%5%18%37%15%2
792
February1st to 10thNight
22%7%23%33%13%2
720
11th to 20thNight
18%8%21%34%19%1
704
21st to endNight
17%7%18%37%18%3
592
March1st to 10thNight
18%9%19%34%18%3
720
11th to 20thNight
18%9%18%33%20%2
720
21st to endNight
17%8%19%38%16%3
792
April1st to 10thNight
15%7%18%35%23%3
720
11th to 20thNight
13%8%23%35%18%3
672
21st to endNight
10%10%20%37%21%2
536
May1st to 10thNight
9%8%20%34%27%3
360
11th to 20thNight
9%8%14%31%31%7%
354
21st to endNight
11%5%15%35%26%7%
389
June1st to 10thNight
11%6%13%41%26%3
360
11th to 20thNight
10%7%16%42%21%4
360
21st to endNight
6%5%11%40%35%3
360
July1st to 10thNight
3310%40%41%4
350
11th to 20thNight
3314%43%36%1
348
21st to endNight
128%39%44%6%
394
August1st to 10thNight
3310%31%45%8%
358
11th to 20thNight
25%10%33%40%10%
418
21st to endNight
3412%38%36%5%
769
September1st to 10thNight
4413%43%30%6%
720
11th to 20thNight
7%6%11%30%40%7%
720
21st to endNight
11%415%36%29%5%
720
October1st to 10thNight
13%5%11%39%30%2
720
11th to 20thNight
14%7%16%33%27%3
720
21st to endNight
19%7%14%40%16%4
792
November1st to 10thNight
26%8%12%41%12%
740
11th to 20thNight
23%9%19%33%15%2
760
21st to endNight
25%7%14%33%19%2
830
December1st to 10thNight
22%7%22%35%14%1
950
11th to 20thNight
29%7%18%34%11%1
950
21st to endNight
29%9%16%37%8%1
955
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