Royer Oaks Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Royer Oaks Observatory clear sky chart from 2005-11-05 to 2019-06-19.

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%
17851
30%
53946
27%
17818
75%
17818
45%
27888

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
61%
836
23%
2922
23%
836
66%
836
33%
1596
January16th to endNight
57%
896
25%
2906
23%
896
63%
896
31%
1682
February1st to 15thNight
58%
820
27%
2472
26%
820
63%
820
32%
1488
February16th to endNight
57%
736
22%
2207
23%
736
59%
736
29%
1392
March1st to 15thNight
62%
840
19%
2520
20%
840
64%
840
31%
1539
March16th to endNight
63%
896
21%
2326
16%
896
66%
896
38%
1429
April1st to 15thNight
61%
839
23%
2099
14%
839
63%
839
38%
1318
April16th to endNight
66%
832
26%
2080
14%
832
69%
832
51%
1148
May1st to 15thNight
70%
705
30%
1965
21%
705
71%
705
52%
1004
May16th to endNight
64%
594
27%
1910
23%
594
67%
594
42%
959
June1st to 15thNight
82%
420
35%
1680
40%
420
86%
420
55%
617
June16th to endNight
94%
398
39%
1592
49%
398
96%
398
81%
455
July1st to 15thNight
93%
370
36%
1480
45%
370
93%
370
66%
504
July16th to endNight
91%
512
38%
1641
38%
483
92%
483
63%
653
August1st to 15thNight
92%
765
39%
1910
26%
761
92%
761
83%
839
August16th to endNight
93%
828
44%
2070
28%
828
93%
828
82%
926
September1st to 15thNight
92%
780
45%
1950
34%
780
93%
780
79%
886
September16th to endNight
88%
776
44%
2145
40%
776
91%
776
76%
910
October1st to 15thNight
78%
780
35%
2229
34%
780
82%
780
56%
1079
October16th to endNight
81%
832
36%
2528
40%
832
83%
832
53%
1209
November1st to 15thNight
75%
820
32%
2665
35%
820
78%
820
45%
1322
November16th to endNight
69%
840
27%
2730
29%
840
71%
840
37%
1474
December1st to 15thNight
64%
840
26%
2783
25%
840
68%
840
32%
1588
December16th to endNight
57%
896
23%
3136
23%
896
62%
896
26%
1871

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
6%412212335%62%
54783
Night
7%412212345%63%
54127
TransparencyDay
23%49%20%26%15%
54463
Night
23%49%21%26%14%
53947
SeeingDay
10%10%30%34%11%3
8122
Night
9%212%34%34%7%
17862

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
12%6%1242445%8%47%
1675
16th to endDay
13%8%24324437%44%
2019
February1st to 15thDay
15%9%2331235%6%46%
2051
16th to endDay
15%12%35%423335%40%
1839
March1st to 15thDay
11%9%344245%5%5%42%
2100
16th to endDay
12%9%25%423436%45%
2599
April1st to 15thDay
12%8%35%323346%46%
2509
16th to endDay
8%5%2332335%7%53%
2485
May1st to 15thDay
6%313213347%61%
2643
16th to endDay
8%413222446%58%
2904
June1st to 15thDay
3112212336%71%
2940
16th to endDay
11112285%
2776
July1st to 15thDay
111121233476%
2730
16th to endDay
112212235%76%
2774
August1st to 15thDay
111112285%
2293
16th to endDay
11113483%
2484
September1st to 15thDay
1111122382%
2340
16th to endDay
111122382%
2145
October1st to 15thDay
5%312213345%67%
2059
16th to endDay
431332334465%
2048
November1st to 15thDay
5%5%23313346%60%
1854
16th to endDay
9%5%1232335%8%54%
1890
December1st to 15thDay
10%6%2332435%9%50%
1834
16th to endDay
13%9%24323349%44%
1792
January1st to 15thNight
12%41332336%7%51%
2922
16th to endNight
12%5%2332335%7%49%
2906
February1st to 15thNight
13%7%2331335%7%48%
2461
16th to endNight
15%9%24212346%45%
2207
March1st to 15thNight
10%7%2432445%6%47%
2520
16th to endNight
11%5%13234347%52%
2326
April1st to 15thNight
11%5%13223446%53%
2099
16th to endNight
6%312212346%65%
2080
May1st to 15thNight
4312213237%68%
1965
16th to endNight
8%31321323465%
1909
June1st to 15thNight
3211211126%75%
1680
16th to endNight
112289%
1592
July1st to 15thNight
11111122480%
1560
16th to endNight
11121212379%
1744
August1st to 15thNight
11112287%
1920
16th to endNight
1112386%
2070
September1st to 15thNight
11122285%
1950
16th to endNight
111122383%
2145
October1st to 15thNight
421221323472%
2229
16th to endNight
321231333470%
2528
November1st to 15thNight
6%313313246%63%
2665
16th to endNight
7%424323349%54%
2730
December1st to 15thNight
10%5%14324446%52%
2783
16th to endNight
13%6%2432335%7%47%
3136

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
34%6%10%22%19%6%
1675
16th to endDay
39%5%11%18%15%9%
2019
February1st to 15thDay
38%411%21%17%6%
2051
16th to endDay
46%5%10%22%14%1
1839
March1st to 15thDay
39%6%13%19%16%4
2100
16th to endDay
39%7%14%21%15%2
2599
April1st to 15thDay
37%6%12%24%14%3
2509
16th to endDay
28%6%12%29%20%2
2485
May1st to 15thDay
21%5%12%29%28%3
2643
16th to endDay
24%6%9%33%22%2
2907
June1st to 15thDay
14%49%22%33%15%
2940
16th to endDay
6%239%45%31%
2776
July1st to 15thDay
10%4411%26%42%
2581
16th to endDay
11%45%15%31%31%
2614
August1st to 15thDay
7%2310%37%39%
2279
16th to endDay
6%2311%45%30%
2484
September1st to 15thDay
7%25%13%37%33%
2340
16th to endDay
7%2423%39%22%
2145
October1st to 15thDay
18%48%21%29%17%
2059
16th to endDay
21%45%20%28%20%
2048
November1st to 15thDay
25%5%8%22%27%10%
1854
16th to endDay
30%410%19%25%9%
1890
December1st to 15thDay
30%6%12%18%21%10%
1834
16th to endDay
38%6%12%19%16%6%
1792
January1st to 15thNight
31%5%11%24%21%5%
2922
16th to endNight
32%6%12%18%18%11%
2906
February1st to 15thNight
36%48%23%21%5%
2472
16th to endNight
38%6%12%22%16%3
2207
March1st to 15thNight
34%7%14%21%18%4
2520
16th to endNight
30%7%16%27%15%2
2326
April1st to 15thNight
30%8%15%27%14%4
2099
16th to endNight
20%5%15%34%21%1
2080
May1st to 15thNight
17%5%11%32%29%3
1966
16th to endNight
22%5%10%30%28%2
1910
June1st to 15thNight
12%310%23%34%15%
1680
16th to endNight
31413%46%31%
1592
July1st to 15thNight
8%25%13%28%41%
1480
16th to endNight
11%35%15%27%36%
1641
August1st to 15thNight
5%1313%35%40%
1910
16th to endNight
5%2213%49%26%
2070
September1st to 15thNight
6%2413%36%36%
1950
16th to endNight
7%2423%38%24%
2145
October1st to 15thNight
17%36%24%30%18%
2229
16th to endNight
17%46%22%28%19%
2528
November1st to 15thNight
22%410%20%26%14%
2665
16th to endNight
28%5%10%18%26%10%
2730
December1st to 15thNight
32%5%10%20%21%9%
2783
16th to endNight
37%6%12%19%17%6%
3136

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
34%228%20%9%4
123
16th to endDay
18%5%30%29%12%2
353
February1st to 15thDay
22%7%30%24%11%3
386
16th to endDay
20%12%35%24%6%
342
March1st to 15thDay
18%14%34%21%8%1
390
16th to endDay
18%18%33%22%6%
416
April1st to 15thDay
16%20%30%23%6%2
389
16th to endDay
10%24%29%25%6%2
378
May1st to 15thDay
8%17%33%29%9%1
408
16th to endDay
15%18%28%28%6%3
458
June1st to 15thDay
210%30%37%14%4
435
16th to endDay
9%27%41%14%6%
388
July1st to 15thDay
2227%46%14%6%
365
16th to endDay
3116%59%11%7%
391
August1st to 15thDay
15%33%43%8%6%
359
16th to endDay
130%49%10%6%
396
September1st to 15thDay
26%31%46%8%3
375
16th to endDay
110%35%40%11%
375
October1st to 15thDay
413%38%35%7%
375
16th to endDay
7%7%24%39%19%
368
November1st to 15thDay
36%25%35%25%3
180
16th to endDay
17%9%18%30%21%2
180
December1st to 15thDay
10%8%28%39%11%1
164
16th to endDay
23%326%25%19%2
128
January1st to 15thNight
14%217%37%22%6%
840
16th to endNight
15%219%36%23%3
896
February1st to 15thNight
16%217%37%21%4
824
16th to endNight
19%315%36%20%3
736
March1st to 15thNight
13%415%39%22%5%
840
16th to endNight
13%315%37%26%2
896
April1st to 15thNight
13%416%36%25%3
839
16th to endNight
7%418%31%31%5%
832
May1st to 15thNight
6%217%38%33%1
706
16th to endNight
9%5%16%34%30%3
596
June1st to 15thNight
419%38%42%4
420
16th to endNight
339%49%7%
398
July1st to 15thNight
1114%70%10%
370
16th to endNight
1114%65%16%
512
August1st to 15thNight
1426%54%12%
765
16th to endNight
219%60%16%
828
September1st to 15thNight
1328%52%13%
780
16th to endNight
17%32%45%11%
776
October1st to 15thNight
411%39%35%7%
780
16th to endNight
48%33%40%11%
832
November1st to 15thNight
8%19%39%31%9%
820
16th to endNight
10%112%37%30%7%
840
December1st to 15thNight
12%214%40%24%5%
840
16th to endNight
16%318%40%17%3
896
Page generated at 2019-06-20 05:02:30. Version ac04

Copyright 2019 A. Danko


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