C.J. Observatory & Retreat
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the C.J. Observatory & Retreat clear sky chart from 2005-11-05 to 2017-09-22.

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
63%
15252
35%
46905
26%
15211
74%
15211
50%
22562

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%
716
34%
2327
28%
716
72%
716
48%
1100
January16th to endNight
57%
768
38%
2496
29%
768
72%
768
46%
1187
February1st to 15thNight
58%
700
36%
2247
26%
697
74%
697
55%
1000
February16th to endNight
46%
632
43%
1813
24%
632
66%
632
55%
862
March1st to 15thNight
50%
720
41%
1980
20%
720
68%
720
56%
951
March16th to endNight
55%
768
40%
2112
21%
768
70%
768
61%
976
April1st to 15thNight
45%
569
37%
1828
18%
569
68%
569
53%
801
April16th to endNight
42%
540
43%
1800
22%
540
72%
540
59%
704
May1st to 15thNight
50%
539
34%
1796
23%
539
68%
539
60%
727
May16th to endNight
55%
563
36%
1684
23%
561
68%
561
56%
767
June1st to 15thNight
66%
540
32%
1620
26%
540
72%
540
58%
737
June16th to endNight
75%
540
25%
1620
21%
540
78%
540
49%
811
July1st to 15thNight
77%
510
20%
1530
18%
510
78%
510
38%
863
July16th to endNight
75%
564
19%
1593
19%
531
76%
531
31%
979
August1st to 15thNight
77%
539
20%
1683
21%
536
77%
536
30%
1021
August16th to endNight
77%
573
25%
1910
28%
573
78%
573
32%
1096
September1st to 15thNight
78%
540
26%
1800
30%
540
78%
540
33%
1024
September16th to endNight
75%
659
37%
1902
29%
659
81%
659
54%
944
October1st to 15thNight
72%
660
38%
1980
28%
660
79%
660
58%
913
October16th to endNight
76%
704
43%
2112
31%
704
85%
704
68%
890
November1st to 15thNight
73%
700
44%
2100
32%
700
83%
700
66%
887
November16th to endNight
65%
720
43%
2160
32%
720
75%
720
54%
1026
December1st to 15thNight
63%
720
39%
2316
30%
720
75%
720
50%
1083
December16th to endNight
56%
768
36%
2496
29%
768
66%
768
45%
1213

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
4212323446%63%
47966
Night
3212323446%64%
47111
TransparencyDay
20%25%15%27%27%
47662
Night
20%25%16%28%26%
46916
SeeingDay
7%8%20%29%25%8%
5169
Night
4421%38%26%5%
15261

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%21221235%5%67%
1611
16th to endDay
6%322212346%63%
1728
February1st to 15thDay
323312236%71%
1606
16th to endDay
221131235%5%69%
1658
March1st to 15thDay
3213313335%68%
1980
16th to endDay
2111212335%73%
2112
April1st to 15thDay
213212146%72%
2120
16th to endDay
11111113481%
2160
May1st to 15thDay
211213345%75%
2151
16th to endDay
211212335%74%
2429
June1st to 15thDay
212213337%70%
2340
16th to endDay
221222446%7%62%
2340
July1st to 15thDay
6%3145%36%7%7%11%41%
2340
16th to endDay
6%5%26%7%48%8%7%10%32%
2467
August1st to 15thDay
6%2245%36%6%8%10%41%
2268
16th to endDay
5%3245%45%6%6%8%46%
2292
September1st to 15thDay
8%4245%25%5%6%8%46%
2160
16th to endDay
5%21221225%7%67%
1866
October1st to 15thDay
311321335%6%67%
1650
16th to endDay
111121232476%
1760
November1st to 15thDay
1212212236%74%
1759
16th to endDay
4313212225%69%
1800
December1st to 15thDay
5%211313336%66%
1641
16th to endDay
3422223347%63%
1728
January1st to 15thNight
5%312323336%64%
2327
16th to endNight
6%312312335%65%
2496
February1st to 15thNight
2112312437%69%
2246
16th to endNight
2212213345%70%
1813
March1st to 15thNight
3112212445%69%
1980
16th to endNight
221211223376%
2112
April1st to 15thNight
2212212246%71%
1828
16th to endNight
11221223478%
1800
May1st to 15thNight
212112335%75%
1796
16th to endNight
212312335%71%
1684
June1st to 15thNight
312214345%70%
1620
16th to endNight
22123245%7%8%57%
1620
July1st to 15thNight
32135%46%7%9%12%40%
1620
16th to endNight
4235%5%46%7%8%14%35%
1701
August1st to 15thNight
44245%46%7%8%12%38%
1692
16th to endNight
43246%46%8%8%10%40%
1910
September1st to 15thNight
5%424436%6%7%9%46%
1800
16th to endNight
4213323346%67%
1902
October1st to 15thNight
3112213336%70%
1980
16th to endNight
11111213478%
2112
November1st to 15thNight
112212235%77%
2100
16th to endNight
5%31221223469%
2160
December1st to 15thNight
5%21221235%6%65%
2316
16th to endNight
5%313313345%63%
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
19%15%16%35%21%
1612
16th to endDay
22%2215%36%21%
1728
February1st to 15thDay
17%1416%29%30%
1607
16th to endDay
16%2114%36%27%
1658
March1st to 15thDay
19%1212%32%32%
1980
16th to endDay
14%129%34%36%
2112
April1st to 15thDay
15%1110%37%33%
2120
16th to endDay
9%18%28%51%
2160
May1st to 15thDay
12%1211%29%42%
2151
16th to endDay
13%139%26%45%
2432
June1st to 15thDay
15%117%22%51%
2340
16th to endDay
19%3413%20%38%
2340
July1st to 15thDay
32%5%11%19%17%14%
2201
16th to endDay
40%6%17%22%10%1
2312
August1st to 15thDay
32%6%14%23%17%5%
2253
16th to endDay
32%5%12%20%17%10%
2292
September1st to 15thDay
31%49%21%21%11%
2160
16th to endDay
17%2315%37%23%
1866
October1st to 15thDay
17%26%14%26%33%
1650
16th to endDay
13%1216%32%34%
1760
November1st to 15thDay
14%1315%35%29%
1759
16th to endDay
19%1214%35%25%
1800
December1st to 15thDay
20%2421%33%18%
1641
16th to endDay
21%26%24%32%13%
1728
January1st to 15thNight
22%2416%35%19%
2327
16th to endNight
22%2218%36%18%
2496
February1st to 15thNight
16%2317%31%29%
2256
16th to endNight
17%1216%36%25%
1813
March1st to 15thNight
16%2316%29%31%
1980
16th to endNight
14%1212%33%35%
2112
April1st to 15thNight
16%1210%37%30%
1828
16th to endNight
11%1110%28%46%
1800
May1st to 15thNight
13%1210%26%45%
1797
16th to endNight
16%239%25%42%
1685
June1st to 15thNight
16%129%22%46%
1620
16th to endNight
20%36%14%16%37%
1620
July1st to 15thNight
29%5%16%21%15%11%
1530
16th to endNight
35%6%22%24%9%1
1593
August1st to 15thNight
34%7%15%22%14%5%
1683
16th to endNight
32%6%14%22%15%8%
1910
September1st to 15thNight
32%5%12%21%14%13%
1800
16th to endNight
19%3416%35%20%
1902
October1st to 15thNight
16%16%15%29%30%
1980
16th to endNight
11%1217%29%37%
2112
November1st to 15thNight
11%1117%36%30%
2100
16th to endNight
19%1214%32%29%
2160
December1st to 15thNight
20%25%22%31%18%
2316
16th to endNight
23%26%22%33%11%
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
16%6%27%33%11%4
90
16th to endDay
6%8%28%30%14%12%
96
February1st to 15thDay
5%218%31%35%6%
90
16th to endDay
6%26%26%22%15%3
159
March1st to 15thDay
8%15%30%26%15%4
270
16th to endDay
7%20%30%24%12%3
288
April1st to 15thDay
6%21%34%20%14%2
269
16th to endDay
6%20%40%22%9%
270
May1st to 15thDay
319%37%26%12%1
288
16th to endDay
310%34%35%13%2
330
June1st to 15thDay
45%24%40%19%6%
315
16th to endDay
3213%40%31%8%
315
July1st to 15thDay
7%434%38%14%
305
16th to endDay
8%1422%40%21%
327
August1st to 15thDay
7%1420%44%21%
315
16th to endDay
6%26%23%42%19%
332
September1st to 15thDay
12%16%34%37%7%
315
16th to endDay
4319%39%25%6%
243
October1st to 15thDay
13%116%21%30%17%
90
16th to endDay
4116%29%34%14%
96
November1st to 15thDay
10%426%32%15%11%
90
16th to endDay
13%12%25%28%17%2
90
December1st to 15thDay
45%17%35%30%6%
90
16th to endDay
13%12%22%36%12%2
96
January1st to 15thNight
6%323%38%21%6%
720
16th to endNight
8%423%39%18%4
768
February1st to 15thNight
3429%37%20%4
704
16th to endNight
47%38%37%11%1
632
March1st to 15thNight
48%33%34%16%2
720
16th to endNight
47%30%42%14%1
768
April1st to 15thNight
39%37%36%10%1
569
16th to endNight
111%41%33%10%1
540
May1st to 15thNight
36%36%43%8%1
540
16th to endNight
4431%42%16%
563
June1st to 15thNight
4122%49%19%1
540
16th to endNight
3212%48%30%1
540
July1st to 15thNight
5%434%48%5%
510
16th to endNight
5%326%52%11%
564
August1st to 15thNight
6%1223%51%14%
539
16th to endNight
4325%54%10%
573
September1st to 15thNight
6%1234%50%4
540
16th to endNight
5%11%39%34%7%
659
October1st to 15thNight
4117%45%25%5%
660
16th to endNight
2116%41%28%8%
704
November1st to 15thNight
2119%37%29%8%
700
16th to endNight
6%318%36%27%6%
720
December1st to 15thNight
6%322%41%21%5%
720
16th to endNight
6%5%25%41%17%4
768
Page generated at 2017-09-23 04:43:44. Version ac04

Copyright 2017 A. Danko


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