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Gilbert Rotary Centennial Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Gilbert Rotary Centennial Observatory clear sky chart from 2005-11-05 to 2017-08-12.

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
80%
15503
38%
46534
35%
15465
83%
15465
57%
21698

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
77%
718
33%
2327
35%
716
80%
716
48%
1113
January16th to endNight
72%
768
37%
2496
38%
768
78%
768
43%
1259
February1st to 15thNight
76%
699
37%
2182
37%
699
81%
699
55%
996
February16th to endNight
71%
632
38%
1877
38%
632
75%
632
47%
968
March1st to 15thNight
75%
720
41%
1980
36%
720
80%
720
62%
945
March16th to endNight
80%
768
41%
2112
33%
768
86%
768
65%
978
April1st to 15thNight
75%
706
34%
1965
26%
706
80%
706
59%
953
April16th to endNight
75%
540
46%
1707
40%
540
84%
540
73%
654
May1st to 15thNight
79%
539
40%
1619
34%
539
84%
539
70%
667
May16th to endNight
82%
561
41%
1684
33%
561
86%
561
73%
673
June1st to 15thNight
89%
540
35%
1620
32%
540
89%
540
72%
661
June16th to endNight
92%
540
36%
1620
31%
540
92%
540
75%
649
July1st to 15thNight
86%
510
26%
1530
21%
510
86%
510
51%
753
July16th to endNight
83%
564
28%
1593
24%
531
84%
531
42%
881
August1st to 15thNight
85%
527
30%
1575
27%
524
85%
524
46%
814
August16th to endNight
85%
583
32%
1713
31%
583
85%
583
45%
922
September1st to 15thNight
81%
660
35%
1815
31%
660
83%
660
55%
930
September16th to endNight
84%
656
42%
1815
34%
656
87%
656
66%
842
October1st to 15thNight
82%
660
42%
1895
36%
660
85%
660
64%
859
October16th to endNight
88%
704
45%
2112
42%
704
90%
704
69%
886
November1st to 15thNight
85%
700
42%
2121
41%
700
87%
700
66%
904
November16th to endNight
79%
720
41%
2340
41%
720
83%
720
56%
1019
December1st to 15thNight
76%
720
38%
2340
41%
720
79%
720
49%
1091
December16th to endNight
72%
768
37%
2496
39%
768
75%
768
43%
1281

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
3212213346%69%
47434
Night
3212213346%68%
46732
TransparencyDay
16%2311%25%40%
47129
Night
17%2311%23%42%
46538
SeeingDay
4211%27%35%18%
5307
Night
418%41%39%5%
15513

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
5%222222235%66%
1611
16th to endDay
7%213313236%64%
1728
February1st to 15thDay
3213312236%67%
1676
16th to endDay
6%223223237%62%
1595
March1st to 15thDay
4112323345%68%
1980
16th to endDay
311222324470%
2112
April1st to 15thDay
212213337%71%
1982
16th to endDay
1111112481%
2250
May1st to 15thDay
11111235%80%
2331
16th to endDay
21111122382%
2429
June1st to 15thDay
1111112483%
2340
16th to endDay
11111222382%
2340
July1st to 15thDay
21144245%6%7%59%
2340
16th to endDay
4314425%7%7%11%45%
2467
August1st to 15thDay
23133245%7%9%55%
2290
16th to endDay
22342447%9%57%
2135
September1st to 15thDay
3213323447%63%
1815
16th to endDay
2111222480%
1815
October1st to 15thDay
211212335%75%
1735
16th to endDay
11212235%79%
1760
November1st to 15thDay
112112235%77%
1735
16th to endDay
4212212225%72%
1620
December1st to 15thDay
6%33323336%63%
1620
16th to endDay
6%413313346%60%
1728
January1st to 15thNight
5%212313346%64%
2327
16th to endNight
8%31232333463%
2496
February1st to 15thNight
4312222336%68%
2173
16th to endNight
7%212323336%62%
1877
March1st to 15thNight
4212112346%70%
1980
16th to endNight
2112112235%73%
2112
April1st to 15thNight
3112212336%69%
1965
16th to endNight
111112383%
1707
May1st to 15thNight
11112246%77%
1619
16th to endNight
211111383%
1684
June1st to 15thNight
1111222481%
1620
16th to endNight
111123482%
1620
July1st to 15thNight
12134245%6%11%56%
1620
16th to endNight
3213435%6%9%12%46%
1701
August1st to 15thNight
22234245%8%9%54%
1584
16th to endNight
2145%35%6%6%9%53%
1713
September1st to 15thNight
221242346%8%61%
1815
16th to endNight
11121224474%
1815
October1st to 15thNight
2111212135%74%
1895
16th to endNight
111211213479%
2112
November1st to 15thNight
2111212235%76%
2121
16th to endNight
411231223373%
2340
December1st to 15thNight
7%212212346%65%
2340
16th to endNight
8%313323335%60%
2496

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
22%1414%29%27%
1612
16th to endDay
22%1216%25%30%
1728
February1st to 15thDay
19%1317%28%29%
1676
16th to endDay
22%2215%32%24%
1595
March1st to 15thDay
18%27%30%39%
1980
16th to endDay
18%118%33%36%
2112
April1st to 15thDay
14%117%35%39%
1982
16th to endDay
9%17%24%56%
2250
May1st to 15thDay
8%17%24%56%
2331
16th to endDay
9%16%22%60%
2432
June1st to 15thDay
8%1216%70%
2340
16th to endDay
9%1311%74%
2340
July1st to 15thDay
20%47%15%19%32%
2201
16th to endDay
27%6%13%24%19%9%
2312
August1st to 15thDay
21%49%18%17%28%
2275
16th to endDay
20%46%19%27%21%
2135
September1st to 15thDay
20%3414%25%31%
1815
16th to endDay
10%16%26%53%
1815
October1st to 15thDay
13%229%26%46%
1735
16th to endDay
10%138%27%49%
1760
November1st to 15thDay
11%111%33%41%
1735
16th to endDay
16%1110%27%42%
1620
December1st to 15thDay
24%1316%29%23%
1620
16th to endDay
24%2421%30%15%
1728
January1st to 15thNight
22%2411%25%33%
2327
16th to endNight
26%2112%27%29%
2496
February1st to 15thNight
18%1415%25%35%
2184
16th to endNight
24%213%27%30%
1877
March1st to 15thNight
16%118%25%46%
1980
16th to endNight
14%118%32%42%
2112
April1st to 15thNight
17%26%29%44%
1965
16th to endNight
8%115%21%62%
1707
May1st to 15thNight
10%16%25%55%
1620
16th to endNight
9%16%17%65%
1685
June1st to 15thNight
9%1312%72%
1620
16th to endNight
8%1147%76%
1620
July1st to 15thNight
19%410%16%15%33%
1530
16th to endNight
24%6%11%21%21%13%
1593
August1st to 15thNight
21%5%8%18%15%30%
1575
16th to endNight
24%47%15%24%23%
1713
September1st to 15thNight
20%27%18%21%29%
1815
16th to endNight
12%25%23%55%
1815
October1st to 15thNight
13%219%23%49%
1895
16th to endNight
11%128%19%57%
2112
November1st to 15thNight
12%118%27%48%
2121
16th to endNight
17%119%23%45%
2340
December1st to 15thNight
21%2214%28%30%
2340
16th to endNight
27%1316%29%21%
2496

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
24%15%33%30%5%
90
16th to endDay
6%5%8%31%29%19%
96
February1st to 15thDay
4114%32%41%6%
191
16th to endDay
7%421%36%22%6%
238
March1st to 15thDay
47%16%33%17%20%
270
16th to endDay
25%20%31%26%14%
288
April1st to 15thDay
45%21%31%26%10%
269
16th to endDay
28%27%32%21%7%
270
May1st to 15thDay
25%23%28%29%11%
288
16th to endDay
2216%32%35%11%
330
June1st to 15thDay
329%33%33%17%
315
16th to endDay
1324%44%25%
315
July1st to 15thDay
31219%48%24%
305
16th to endDay
3213%40%39%
327
August1st to 15thDay
2216%42%36%
300
16th to endDay
17%48%31%
268
September1st to 15thDay
6%17%21%45%18%
255
16th to endDay
5%110%32%36%13%
255
October1st to 15thDay
4116%26%37%14%
175
16th to endDay
3116%55%23%
96
November1st to 15thDay
2111%31%40%14%
90
16th to endDay
4110%37%41%5%
90
December1st to 15thDay
4225%53%14%
90
16th to endDay
14%116%32%30%5%
96
January1st to 15thNight
6%18%51%28%3
720
16th to endNight
9%18%50%26%2
768
February1st to 15thNight
5%110%49%29%3
704
16th to endNight
9%112%50%24%1
632
March1st to 15thNight
4213%48%26%4
720
16th to endNight
2210%46%33%4
768
April1st to 15thNight
3213%50%28%1
706
16th to endNight
2118%48%25%3
540
May1st to 15thNight
1214%47%31%3
540
16th to endNight
211%42%38%3
563
June1st to 15thNight
115%46%42%2
540
16th to endNight
1133%59%3
540
July1st to 15thNight
2326%59%7%
510
16th to endNight
41112%66%13%
564
August1st to 15thNight
2115%65%13%
527
16th to endNight
2119%63%12%
583
September1st to 15thNight
316%25%54%9%
660
16th to endNight
27%37%44%6%
656
October1st to 15thNight
38%45%38%2
660
16th to endNight
1443%42%6%
704
November1st to 15thNight
216%47%37%5%
700
16th to endNight
5%8%46%35%4
720
December1st to 15thNight
8%17%46%34%2
720
16th to endNight
8%18%50%27%3
768
Page generated at 2017-08-13 11:16:39. Version ac04

Copyright 2017 A. Danko


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