Sierra Skies Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Sierra Skies 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
69%
23462
29%
74714
27%
23419
72%
23419
41%
39670

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
52%
756
16%
2646
15%
756
56%
756
24%
1629
January11th to 20thNight
60%
728
22%
2403
22%
728
64%
728
29%
1446
January21st to endNight
56%
792
24%
2574
24%
792
61%
792
28%
1574
February1st to 10thNight
58%
720
26%
2340
26%
720
63%
720
31%
1368
February11th to 20thNight
57%
700
27%
2152
25%
700
63%
700
33%
1268
February21st to endNight
55%
592
25%
1776
25%
592
59%
592
32%
1075
March1st to 10thNight
49%
720
17%
2102
16%
720
52%
720
23%
1433
March11th to 20thNight
56%
720
23%
1980
20%
720
59%
720
30%
1277
March21st to endNight
53%
792
18%
2093
13%
792
57%
792
30%
1394
April1st to 10thNight
60%
720
23%
1800
13%
720
63%
720
34%
1166
April11th to 20thNight
61%
711
26%
1779
13%
711
65%
711
43%
1072
April21st to endNight
70%
720
27%
1800
14%
720
74%
720
52%
1001
May1st to 10thNight
66%
685
28%
1765
17%
685
69%
685
52%
953
May11th to 20thNight
78%
354
32%
1416
36%
354
81%
354
47%
573
May21st to endNight
76%
388
28%
1552
31%
388
79%
388
46%
642
June1st to 10thNight
72%
360
28%
1440
32%
360
77%
360
45%
604
June11th to 20thNight
80%
360
36%
1440
44%
360
86%
360
66%
478
June21st to endNight
85%
360
36%
1440
47%
360
89%
360
67%
474
July1st to 10thNight
94%
350
38%
1400
50%
350
95%
350
78%
418
July11th to 20thNight
92%
348
40%
1392
51%
348
93%
348
72%
440
July21st to endNight
92%
579
39%
1663
37%
547
92%
547
70%
684
August1st to 10thNight
87%
537
38%
1602
32%
534
87%
534
69%
668
August11th to 20thNight
91%
682
40%
1750
27%
682
91%
682
79%
780
August21st to endNight
92%
792
44%
1980
28%
792
92%
792
83%
880
September1st to 10thNight
91%
720
46%
1800
32%
720
92%
720
84%
794
September11th to 20thNight
83%
720
45%
1800
34%
720
86%
720
71%
868
September21st to endNight
83%
720
43%
2011
40%
720
88%
720
73%
876
October1st to 10thNight
77%
720
32%
2160
33%
720
81%
720
55%
1016
October11th to 20thNight
78%
720
39%
2160
42%
720
82%
720
56%
1009
October21st to endNight
77%
792
39%
2376
40%
792
81%
792
54%
1130
November1st to 10thNight
70%
740
29%
2364
31%
740
72%
740
40%
1253
November11th to 20thNight
68%
760
29%
2470
30%
760
72%
760
40%
1280
November21st to endNight
65%
760
28%
2587
29%
760
69%
760
36%
1369
December1st to 10thNight
58%
760
22%
2660
22%
760
62%
760
29%
1514
December11th to 20thNight
53%
760
20%
3224
23%
759
56%
759
18%
2192
December21st to endNight
47%
824
13%
4817
18%
817
54%
817
12%
3072

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
9%313323345%64%
74735
Night
11%423323346%60%
75006
TransparencyDay
24%38%19%24%21%
74450
Night
27%49%19%22%19%
74812
SeeingDay
9%5%17%30%30%9%
14047
Night
11%212%38%31%5%
23483

By Month

Percentage Cloud Cover SW (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%8%345%3445%7%35%
1519
11th to 20thDay
17%424433446%49%
1594
21st to endDay
18%6%35%3345%5%7%41%
1782
February1st to 10thDay
20%5%24324446%47%
1620
11th to 20thDay
17%6%24424446%49%
1694
21st to endDay
18%5%25%323345%49%
1480
March1st to 10thDay
19%7%36%5%35%5%6%5%38%
1854
11th to 20thDay
20%5%25%426%35%7%41%
1980
21st to endDay
16%6%245%245%47%46%
2263
April1st to 10thDay
15%6%134245%5%6%48%
2160
11th to 20thDay
12%413423346%56%
2129
21st to endDay
9%312313447%63%
2160
May1st to 10thDay
6%323322446%65%
2189
11th to 20thDay
5%223323445%68%
2477
21st to endDay
6%312323347%65%
2723
June1st to 10thDay
5%313222346%68%
2520
11th to 20thDay
111121223482%
2520
21st to endDay
211111112484%
2520
July1st to 10thDay
1111122387%
2520
11th to 20thDay
11111122387%
2520
21st to endDay
11221223483%
2554
August1st to 10thDay
211111122384%
2328
11th to 20thDay
1111122486%
2151
21st to endDay
11111112290%
2376
September1st to 10thDay
211112388%
2160
11th to 20thDay
3212212335%76%
2160
21st to endDay
421221122479%
1946
October1st to 10thDay
4212323345%72%
1800
11th to 20thDay
6%212223345%70%
1800
21st to endDay
7%323322445%65%
1980
November1st to 10thDay
12%41333435%8%55%
1712
11th to 20thDay
12%42443435%6%53%
1710
21st to endDay
13%31432345%7%55%
1592
December1st to 10thDay
16%6%244345%37%46%
1520
11th to 20thDay
30%7%35%433347%31%
1928
21st to endDay
15%7%35%5%46%6%7%9%34%
2794
January1st to 10thNight
21%6%25%433448%41%
2646
11th to 20thNight
14%6%2443445%6%47%
2403
21st to endNight
18%5%235%2445%7%45%
2574
February1st to 10thNight
18%5%24323345%52%
2340
11th to 20thNight
15%42432435%8%49%
2152
21st to endNight
17%5%23423445%51%
1776
March1st to 10thNight
21%8%34424445%41%
2102
11th to 20thNight
21%5%2333345%7%45%
1980
21st to endNight
18%424334446%47%
2093
April1st to 10thNight
14%5%245%35%446%49%
1800
11th to 20thNight
11%324423446%57%
1779
21st to endNight
7%322313447%65%
1800
May1st to 10thNight
7%413223345%66%
1765
11th to 20thNight
6%212213347%67%
1416
21st to endNight
6%11221335%7%68%
1552
June1st to 10thNight
5%223312346%69%
1440
11th to 20thNight
111221223482%
1440
21st to endNight
31111222383%
1440
July1st to 10thNight
111122389%
1440
11th to 20thNight
11211112386%
1440
21st to endNight
112212235%80%
1761
August1st to 10thNight
21221233479%
1611
11th to 20thNight
111111234482%
1750
21st to endNight
111111121288%
1980
September1st to 10thNight
11111112389%
1800
11th to 20thNight
421111223479%
1800
21st to endNight
211222335%79%
2011
October1st to 10thNight
6%21322323472%
2160
11th to 20thNight
6%212212446%71%
2160
21st to endNight
6%312223236%70%
2376
November1st to 10thNight
12%423313447%57%
2364
11th to 20thNight
11%41332435%7%57%
2470
21st to endNight
13%413323346%58%
2587
December1st to 10thNight
18%41442335%6%50%
2660
11th to 20thNight
28%7%34423446%34%
3276
21st to endNight
20%7%26%6%45%5%46%35%
4862

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
49%413%20%11%3
1519
11th to 20thDay
36%413%18%16%13%
1594
21st to endDay
44%410%17%19%6%
1782
February1st to 10thDay
40%410%21%17%9%
1620
11th to 20thDay
37%410%20%21%7%
1704
21st to endDay
40%39%23%18%6%
1480
March1st to 10thDay
47%5%11%17%16%3
1854
11th to 20thDay
42%49%24%16%4
1980
21st to endDay
39%7%14%21%16%4
2263
April1st to 10thDay
37%5%12%24%16%6%
2160
11th to 20thDay
30%5%11%26%21%7%
2132
21st to endDay
22%413%30%26%5%
2160
May1st to 10thDay
21%410%26%29%9%
2189
11th to 20thDay
20%39%29%28%11%
2480
21st to endDay
21%5%13%29%25%7%
2724
June1st to 10thDay
19%410%25%29%13%
2520
11th to 20thDay
8%28%21%33%27%
2520
21st to endDay
8%2313%36%38%
2520
July1st to 10thDay
6%129%26%56%
2451
11th to 20thDay
7%1311%27%51%
2445
21st to endDay
8%227%25%56%
2412
August1st to 10thDay
9%2310%29%47%
2312
11th to 20thDay
5%2110%34%48%
2151
21st to endDay
5%1110%35%47%
2376
September1st to 10thDay
5%228%29%54%
2160
11th to 20thDay
12%35%20%37%23%
2160
21st to endDay
12%2418%35%29%
1946
October1st to 10thDay
16%37%18%27%29%
1800
11th to 20thDay
18%46%21%23%28%
1800
21st to endDay
22%37%20%26%22%
1980
November1st to 10thDay
28%410%22%23%12%
1712
11th to 20thDay
32%411%24%23%7%
1710
21st to endDay
29%411%22%24%10%
1592
December1st to 10thDay
38%411%21%17%9%
1520
11th to 20thDay
54%410%15%12%5%
1928
21st to endDay
43%6%19%19%9%3
2794
January1st to 10thNight
43%314%22%11%6%
2646
11th to 20thNight
37%5%12%17%14%15%
2403
21st to endNight
39%411%15%20%10%
2574
February1st to 10thNight
37%37%22%20%10%
2340
11th to 20thNight
35%48%23%21%9%
2153
21st to endNight
36%48%25%19%7%
1776
March1st to 10thNight
46%5%9%18%16%5%
2102
11th to 20thNight
40%48%21%19%7%
1980
21st to endNight
39%6%10%20%17%7%
2093
April1st to 10thNight
38%412%20%20%6%
1800
11th to 20thNight
29%5%14%25%17%10%
1779
21st to endNight
21%5%12%32%24%6%
1800
May1st to 10thNight
20%5%12%22%29%12%
1765
11th to 20thNight
17%412%31%23%12%
1416
21st to endNight
17%5%16%32%24%6%
1552
June1st to 10thNight
19%411%27%27%11%
1440
11th to 20thNight
9%29%21%32%28%
1440
21st to endNight
9%1314%33%39%
1440
July1st to 10thNight
5%2211%28%53%
1400
11th to 20thNight
7%2311%22%55%
1392
21st to endNight
9%339%25%52%
1663
August1st to 10thNight
11%2214%27%45%
1602
11th to 20thNight
7%2311%31%46%
1750
21st to endNight
6%1210%31%50%
1980
September1st to 10thNight
5%228%30%53%
1800
11th to 20thNight
11%2418%41%23%
1800
21st to endNight
10%26%18%32%33%
2011
October1st to 10thNight
18%35%17%27%29%
2160
11th to 20thNight
16%45%19%26%30%
2160
21st to endNight
18%36%20%28%25%
2376
November1st to 10thNight
28%410%22%19%17%
2364
11th to 20thNight
28%310%26%23%10%
2470
21st to endNight
30%38%22%24%13%
2587
December1st to 10thNight
36%411%20%18%11%
2660
11th to 20thNight
51%49%16%15%6%
3276
21st to endNight
49%6%13%18%10%4
4862

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
37%7%14%22%16%4
180
11th to 20thDay
20%8%13%30%22%8%
306
21st to endDay
20%418%27%24%7%
374
February1st to 10thDay
21%9%14%22%26%8%
340
11th to 20thDay
18%10%23%23%21%6%
390
21st to endDay
18%8%25%27%19%3
346
March1st to 10thDay
21%12%24%26%14%3
420
11th to 20thDay
23%8%23%24%17%6%
420
21st to endDay
18%10%27%25%18%3
462
April1st to 10thDay
17%9%25%27%17%5%
420
11th to 20thDay
14%8%29%25%19%5%
412
21st to endDay
7%7%21%31%22%11%
420
May1st to 10thDay
5%10%28%28%23%7%
423
11th to 20thDay
7%5%24%28%26%9%
448
21st to endDay
6%6%20%26%29%13%
485
June1st to 10thDay
46%20%30%28%11%
450
11th to 20thDay
2421%35%32%6%
450
21st to endDay
215%31%44%8%
450
July1st to 10thDay
19%37%45%7%
446
11th to 20thDay
117%36%48%8%
444
21st to endDay
18%37%43%12%
487
August1st to 10thDay
3112%40%36%9%
447
11th to 20thDay
111%36%40%11%
437
21st to endDay
119%35%45%10%
495
September1st to 10thDay
228%39%39%10%
450
11th to 20thDay
5%314%35%33%9%
450
21st to endDay
3218%33%34%10%
450
October1st to 10thDay
25%20%30%30%12%
450
11th to 20thDay
6%412%27%37%13%
450
21st to endDay
9%6%14%28%29%13%
495
November1st to 10thDay
12%7%13%26%30%11%
308
11th to 20thDay
11%315%30%30%11%
270
21st to endDay
13%317%27%33%7%
214
December1st to 10thDay
18%9%15%26%23%9%
180
11th to 20thDay
18%7%18%26%21%11%
180
21st to endDay
18%11%19%25%22%5%
198
January1st to 10thNight
22%414%30%22%7%
760
11th to 20thNight
16%411%39%24%7%
728
21st to endNight
20%213%36%23%7%
792
February1st to 10thNight
20%314%37%23%4
720
11th to 20thNight
16%5%16%36%21%5%
704
21st to endNight
19%417%38%19%3
592
March1st to 10thNight
22%5%17%32%21%4
720
11th to 20thNight
21%314%33%26%4
720
21st to endNight
17%418%37%20%3
792
April1st to 10thNight
14%414%41%25%2
720
11th to 20thNight
10%5%19%36%24%6%
711
21st to endNight
7%315%41%30%5%
720
May1st to 10thNight
7%319%38%29%4
685
11th to 20thNight
6%210%44%35%3
354
21st to endNight
6%113%40%40%1
389
June1st to 10thNight
4214%40%37%3
360
11th to 20thNight
1112%49%35%1
360
21st to endNight
317%46%42%2
360
July1st to 10thNight
1347%47%1
350
11th to 20thNight
11349%45%1
348
21st to endNight
1238%54%6%
579
August1st to 10thNight
36%40%48%3
537
11th to 20thNight
15%45%45%5%
682
21st to endNight
21339%53%4
792
September1st to 10thNight
115%37%52%5%
720
11th to 20thNight
418%37%43%7%
720
21st to endNight
2112%37%39%10%
720
October1st to 10thNight
6%110%41%33%8%
720
11th to 20thNight
7%210%36%39%6%
720
21st to endNight
6%311%35%32%13%
792
November1st to 10thNight
13%310%34%32%8%
740
11th to 20thNight
11%212%41%28%7%
760
21st to endNight
14%213%39%25%7%
760
December1st to 10thNight
18%314%35%23%7%
760
11th to 20thNight
26%312%33%22%4
760
21st to endNight
23%418%34%17%4
836
Page generated at 2024-01-09 13:24:23. Version ac07

Copyright 2024 Attilla Danko


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