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 2005-11-05 to 2017-09-19.

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
72%
15252
28%
46900
27%
15216
74%
15216
42%
24807

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 15thNight
64%
716
18%
2470
20%
716
66%
716
30%
1424
January16th to endNight
61%
768
24%
2496
26%
768
63%
768
28%
1530
February1st to 15thNight
59%
697
23%
2265
24%
697
63%
697
28%
1383
February16th to endNight
58%
632
25%
1895
25%
632
61%
632
30%
1186
March1st to 15thNight
57%
720
17%
2061
17%
720
58%
720
27%
1364
March16th to endNight
58%
768
20%
2055
16%
768
61%
768
31%
1347
April1st to 15thNight
60%
719
20%
1799
12%
719
62%
719
32%
1192
April16th to endNight
69%
720
25%
1800
13%
720
71%
720
48%
1039
May1st to 15thNight
74%
575
29%
1655
22%
575
76%
575
56%
788
May16th to endNight
79%
375
26%
1496
32%
374
80%
374
46%
618
June1st to 15thNight
77%
360
27%
1440
34%
360
81%
360
49%
571
June16th to endNight
89%
360
36%
1440
50%
360
90%
360
67%
473
July1st to 15thNight
95%
340
35%
1360
48%
340
96%
340
76%
413
July16th to endNight
93%
498
39%
1528
40%
466
93%
466
72%
578
August1st to 15thNight
91%
577
38%
1650
31%
574
91%
574
74%
685
August16th to endNight
93%
764
45%
1910
29%
764
93%
764
83%
845
September1st to 15thNight
93%
720
43%
1800
31%
720
93%
720
81%
812
September16th to endNight
87%
671
43%
1816
39%
671
89%
671
72%
818
October1st to 15thNight
76%
660
32%
1980
33%
660
79%
660
51%
972
October16th to endNight
77%
704
35%
2112
37%
704
80%
704
47%
1078
November1st to 15thNight
73%
700
28%
2250
31%
700
75%
700
42%
1158
November16th to endNight
65%
720
25%
2414
26%
720
69%
720
36%
1303
December1st to 15thNight
58%
720
20%
2520
21%
720
62%
720
26%
1522
December16th to endNight
50%
768
21%
2688
20%
768
55%
768
22%
1708

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%312212335%64%
47920
Night
11%312212335%61%
47087
TransparencyDay
24%38%19%24%20%
47602
Night
26%38%19%22%18%
46904
SeeingDay
10%5%18%32%26%6%
5877
Night
12%28%33%35%8%
15264

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 15thDay
18%5%13424336%46%
1467
16th to endDay
21%5%24324435%42%
1728
February1st to 15thDay
20%5%25%323335%44%
1594
16th to endDay
20%6%24313446%42%
1579
March1st to 15thDay
18%5%24425%45%5%40%
1896
16th to endDay
17%424424447%43%
2169
April1st to 15thDay
16%6%134145%46%44%
2149
16th to endDay
10%212212346%61%
2160
May1st to 15thDay
5%212212335%70%
2293
16th to endDay
6%312313345%64%
2615
June1st to 15thDay
5%212212235%69%
2520
16th to endDay
211112384%
2520
July1st to 15thDay
111112287%
2520
16th to endDay
1111111385%
2533
August1st to 15thDay
1111286%
2300
16th to endDay
112387%
2292
September1st to 15thDay
11122384%
2160
16th to endDay
31111122378%
1886
October1st to 15thDay
6%11213345%68%
1650
16th to endDay
8%31331233462%
1760
November1st to 15thDay
10%31222335%7%56%
1608
16th to endDay
14%414323336%51%
1545
December1st to 15thDay
19%6%24333425%42%
1440
16th to endDay
23%433223347%40%
1536
January1st to 15thNight
17%414323348%47%
2470
16th to endNight
18%423424345%46%
2496
February1st to 15thNight
19%413324347%44%
2257
16th to endNight
19%5%14312345%47%
1895
March1st to 15thNight
19%5%23323445%43%
2061
16th to endNight
18%413323337%47%
2055
April1st to 15thNight
13%4245%25%446%45%
1799
16th to endNight
9%212212346%61%
1800
May1st to 15thNight
5%21112345%70%
1655
16th to endNight
6%1221225%7%66%
1496
June1st to 15thNight
4112312235%72%
1440
16th to endNight
2111211384%
1440
July1st to 15thNight
11121287%
1440
16th to endNight
11112384%
1634
August1st to 15thNight
111222381%
1659
16th to endNight
1112386%
1910
September1st to 15thNight
11112385%
1800
16th to endNight
11111222479%
1816
October1st to 15thNight
7%212212236%67%
1980
16th to endNight
6%312223335%64%
2112
November1st to 15thNight
10%312213346%59%
2250
16th to endNight
13%412322246%55%
2414
December1st to 15thNight
20%423322345%46%
2520
16th to endNight
24%5%13313435%42%
2688

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 15thDay
39%312%20%17%7%
1467
16th to endDay
45%38%15%15%11%
1728
February1st to 15thDay
42%310%20%17%5%
1594
16th to endDay
43%311%26%11%3
1579
March1st to 15thDay
44%411%18%15%5%
1896
16th to endDay
41%5%14%19%16%2
2169
April1st to 15thDay
40%5%10%24%14%4
2149
16th to endDay
25%412%26%25%6%
2160
May1st to 15thDay
18%28%29%30%9%
2293
16th to endDay
22%411%28%24%8%
2619
June1st to 15thDay
19%310%23%22%21%
2520
16th to endDay
7%1314%36%36%
2520
July1st to 15thDay
7%118%26%54%
2371
16th to endDay
8%129%24%54%
2376
August1st to 15thDay
7%129%30%48%
2284
16th to endDay
6%1111%37%42%
2292
September1st to 15thDay
7%1312%34%40%
2160
16th to endDay
12%15%20%34%24%
1886
October1st to 15thDay
17%38%24%27%18%
1650
16th to endDay
26%37%23%24%15%
1760
November1st to 15thDay
27%314%26%20%6%
1608
16th to endDay
34%411%22%21%5%
1545
December1st to 15thDay
44%211%19%17%3
1440
16th to endDay
44%5%10%18%15%5%
1536
January1st to 15thNight
36%313%20%16%10%
2470
16th to endNight
39%48%15%17%15%
2496
February1st to 15thNight
40%39%21%17%8%
2268
16th to endNight
39%49%26%17%4
1895
March1st to 15thNight
43%410%19%16%5%
2061
16th to endNight
38%5%10%19%18%7%
2055
April1st to 15thNight
39%413%21%14%6%
1799
16th to endNight
24%414%23%24%8%
1800
May1st to 15thNight
16%311%25%28%14%
1656
16th to endNight
17%415%32%20%9%
1496
June1st to 15thNight
16%311%25%23%19%
1440
16th to endNight
9%1414%32%38%
1440
July1st to 15thNight
5%127%27%55%
1360
16th to endNight
8%129%23%53%
1528
August1st to 15thNight
9%1114%27%45%
1650
16th to endNight
5%1211%35%44%
1910
September1st to 15thNight
6%2411%36%38%
1800
16th to endNight
10%15%19%34%28%
1816
October1st to 15thNight
20%36%22%27%19%
1980
16th to endNight
22%45%22%26%17%
2112
November1st to 15thNight
26%213%26%20%11%
2250
16th to endNight
31%29%24%23%8%
2414
December1st to 15thNight
40%311%17%18%7%
2520
16th to endNight
44%410%15%17%8%
2688

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 15thDay
32%8%12%26%16%3
95
16th to endDay
26%316%25%21%7%
256
February1st to 15thDay
24%5%16%28%21%4
239
16th to endDay
25%7%21%28%15%2
238
March1st to 15thDay
21%11%25%17%19%4
270
16th to endDay
19%9%21%27%19%2
288
April1st to 15thDay
20%11%29%20%14%3
269
16th to endDay
14%13%23%25%18%4
270
May1st to 15thDay
6%7%29%30%19%6%
288
16th to endDay
9%6%19%26%28%9%
330
June1st to 15thDay
6%5%26%38%18%5%
315
16th to endDay
217%35%34%8%
315
July1st to 15thDay
8%43%40%7%
305
16th to endDay
14%42%32%10%
327
August1st to 15thDay
314%41%32%7%
315
16th to endDay
12%37%32%15%
332
September1st to 15thDay
2111%46%29%8%
315
16th to endDay
3219%32%33%8%
268
October1st to 15thDay
6%5%21%31%25%8%
255
16th to endDay
13%416%30%30%5%
272
November1st to 15thDay
11%114%21%35%15%
114
16th to endDay
12%5%15%40%25%
77
December1st to 15thDay
26%6%18%35%11%1
60
16th to endDay
17%47%32%29%7%
64
January1st to 15thNight
17%37%32%27%11%
720
16th to endNight
20%46%33%24%12%
768
February1st to 15thNight
21%29%32%27%6%
704
16th to endNight
21%311%35%21%6%
632
March1st to 15thNight
21%49%30%28%6%
720
16th to endNight
19%312%34%25%4
768
April1st to 15thNight
14%5%11%36%27%3
719
16th to endNight
10%311%35%30%8%
720
May1st to 15thNight
7%113%38%32%6%
576
16th to endNight
6%27%37%41%6%
375
June1st to 15thNight
5%10%38%42%3
360
16th to endNight
2341%47%5%
360
July1st to 15thNight
134%59%4
340
16th to endNight
225%62%9%
498
August1st to 15thNight
2235%54%5%
577
16th to endNight
1233%55%6%
764
September1st to 15thNight
2228%56%9%
720
16th to endNight
27%33%41%14%
671
October1st to 15thNight
8%18%34%35%11%
660
16th to endNight
8%9%31%35%14%
704
November1st to 15thNight
12%16%34%32%12%
700
16th to endNight
15%110%35%26%10%
720
December1st to 15thNight
22%28%33%24%7%
720
16th to endNight
27%311%30%21%5%
768
Page generated at 2017-09-20 01:49:52. Version ac04

Copyright 2017 A. Danko


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