Why Observing Station
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Why Observing Station clear sky chart from 2005-11-05 to 2017-10-16.

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
77%
15903
38%
47253
33%
15864
82%
15864
59%
21601

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
71%
718
34%
2327
32%
716
79%
716
51%
1070
January16th to endNight
66%
768
38%
2493
35%
768
75%
768
46%
1214
February1st to 15thNight
73%
699
40%
2108
37%
699
81%
699
56%
968
February16th to endNight
65%
632
36%
1895
34%
632
72%
632
46%
964
March1st to 15thNight
68%
720
40%
1995
32%
720
77%
720
59%
959
March16th to endNight
75%
768
40%
2112
31%
768
82%
768
64%
996
April1st to 15thNight
67%
719
36%
1978
23%
719
76%
719
62%
926
April16th to endNight
72%
579
47%
1701
34%
579
82%
579
76%
673
May1st to 15thNight
72%
539
41%
1619
30%
539
80%
539
73%
639
May16th to endNight
79%
562
43%
1684
33%
561
85%
561
77%
656
June1st to 15thNight
86%
540
37%
1620
31%
540
87%
540
76%
639
June16th to endNight
90%
540
36%
1620
32%
540
90%
540
74%
656
July1st to 15thNight
85%
510
25%
1530
21%
510
86%
510
49%
769
July16th to endNight
87%
564
30%
1593
26%
531
87%
531
45%
831
August1st to 15thNight
87%
539
33%
1611
28%
536
87%
536
54%
766
August16th to endNight
85%
734
36%
1937
29%
734
85%
734
54%
1026
September1st to 15thNight
82%
720
32%
1980
27%
720
84%
720
53%
1028
September16th to endNight
87%
716
48%
1980
39%
716
91%
716
76%
848
October1st to 15thNight
78%
720
44%
2022
35%
720
84%
720
72%
871
October16th to endNight
85%
708
44%
2124
41%
708
88%
708
68%
888
November1st to 15thNight
83%
700
42%
2148
39%
700
87%
700
65%
917
November16th to endNight
75%
720
41%
2340
40%
720
80%
720
56%
1009
December1st to 15thNight
73%
720
39%
2340
40%
720
77%
720
51%
1075
December16th to endNight
69%
768
37%
2496
37%
768
75%
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
2212213346%70%
48157
Night
2212213346%70%
47453
TransparencyDay
16%2410%24%41%
47852
Night
15%2411%24%41%
47259
SeeingDay
4419%32%27%10%
5682
Night
3112%37%36%7%
15912

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
4113212346%68%
1611
16th to endDay
5%213313336%65%
1728
February1st to 15thDay
1224223236%68%
1751
16th to endDay
4313322447%62%
1579
March1st to 15thDay
3212323345%68%
1964
16th to endDay
2112312235%72%
2112
April1st to 15thDay
1111212335%74%
1970
16th to endDay
111112384%
2253
May1st to 15thDay
1111223480%
2331
16th to endDay
11111386%
2429
June1st to 15thDay
1111112384%
2340
16th to endDay
1111112481%
2340
July1st to 15thDay
321233445%8%59%
2340
16th to endDay
5%3246%37%6%7%10%42%
2467
August1st to 15thDay
33124345%6%9%54%
2340
16th to endDay
22134345%7%7%58%
2260
September1st to 15thDay
431323445%6%60%
1980
16th to endDay
21121122479%
1980
October1st to 15thDay
11111222480%
1938
16th to endDay
111112336%76%
1768
November1st to 15thDay
111212137%76%
1708
16th to endDay
322212235%73%
1620
December1st to 15thDay
432313336%66%
1620
16th to endDay
3313322447%63%
1728
January1st to 15thNight
3213312446%66%
2327
16th to endNight
5%313323335%63%
2493
February1st to 15thNight
2212213345%68%
2101
16th to endNight
4323324446%61%
1895
March1st to 15thNight
3212113245%70%
1995
16th to endNight
2112113236%73%
2112
April1st to 15thNight
221121223474%
1978
16th to endNight
111111187%
1701
May1st to 15thNight
1111335%80%
1619
16th to endNight
11111286%
1684
June1st to 15thNight
111211483%
1620
16th to endNight
1111112283%
1620
July1st to 15thNight
1112335%5%5%10%58%
1620
16th to endNight
1112325%7%10%15%48%
1701
August1st to 15thNight
111232345%10%62%
1620
16th to endNight
22332445%8%61%
1937
September1st to 15thNight
421231435%6%63%
1980
16th to endNight
11111122482%
1980
October1st to 15thNight
11121123480%
2022
16th to endNight
11111223478%
2124
November1st to 15thNight
122212235%75%
2148
16th to endNight
2213212245%70%
2340
December1st to 15thNight
5%312223436%66%
2340
16th to endNight
4314323346%62%
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%2312%26%36%
1612
16th to endDay
21%2313%25%32%
1728
February1st to 15thDay
18%2410%28%36%
1751
16th to endDay
21%2213%30%28%
1579
March1st to 15thDay
18%226%25%44%
1964
16th to endDay
15%137%28%43%
2112
April1st to 15thDay
13%117%32%43%
1970
16th to endDay
7%127%22%59%
2253
May1st to 15thDay
9%116%21%59%
2331
16th to endDay
6%115%22%62%
2432
June1st to 15thDay
7%1316%70%
2340
16th to endDay
9%12310%71%
2340
July1st to 15thDay
21%5%8%15%19%30%
2201
16th to endDay
32%9%9%25%18%4
2312
August1st to 15thDay
23%6%8%16%21%23%
2325
16th to endDay
21%6%8%17%25%20%
2260
September1st to 15thDay
22%5%6%12%27%25%
1980
16th to endDay
11%126%24%53%
1980
October1st to 15thDay
10%138%25%51%
1938
16th to endDay
11%137%22%53%
1768
November1st to 15thDay
10%129%32%44%
1708
16th to endDay
15%129%28%42%
1620
December1st to 15thDay
20%2313%28%31%
1620
16th to endDay
20%36%17%29%22%
1728
January1st to 15thNight
20%2411%24%36%
2327
16th to endNight
23%2312%26%31%
2493
February1st to 15thNight
17%1415%21%39%
2112
16th to endNight
22%3312%25%32%
1895
March1st to 15thNight
16%128%23%46%
1995
16th to endNight
14%128%31%41%
2112
April1st to 15thNight
15%127%33%40%
1978
16th to endNight
7%16%24%58%
1701
May1st to 15thNight
8%127%24%56%
1620
16th to endNight
7%115%21%63%
1685
June1st to 15thNight
8%217%70%
1620
16th to endNight
8%1238%75%
1620
July1st to 15thNight
19%5%11%15%17%30%
1530
16th to endNight
19%8%13%27%22%7%
1593
August1st to 15thNight
17%48%22%23%23%
1611
16th to endNight
18%5%8%18%27%21%
1937
September1st to 15thNight
19%47%17%23%27%
1980
16th to endNight
8%118%27%51%
1980
October1st to 15thNight
10%1111%24%50%
2022
16th to endNight
11%139%17%56%
2124
November1st to 15thNight
12%129%30%43%
2148
16th to endNight
17%1210%23%45%
2340
December1st to 15thNight
20%1413%27%32%
2340
16th to endNight
23%35%16%27%25%
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
15%218%42%20%1
90
16th to endDay
6%421%31%27%9%
96
February1st to 15thDay
38%28%36%22%1
266
16th to endDay
7%10%32%29%15%3
238
March1st to 15thDay
311%27%26%20%10%
270
16th to endDay
210%31%27%19%9%
288
April1st to 15thDay
48%33%25%19%8%
269
16th to endDay
211%33%31%14%7%
270
May1st to 15thDay
310%29%33%14%8%
288
16th to endDay
2425%35%26%5%
330
June1st to 15thDay
4124%39%23%6%
315
16th to endDay
119%45%31%10%
315
July1st to 15thDay
48%37%39%9%
305
16th to endDay
3429%38%23%
327
August1st to 15thDay
46%33%39%17%
315
16th to endDay
15%27%40%23%
332
September1st to 15thDay
7%114%29%33%13%
315
16th to endDay
5%216%37%24%13%
315
October1st to 15thDay
2417%33%31%9%
273
16th to endDay
5%27%18%47%20%
99
November1st to 15thDay
2225%31%27%11%
90
16th to endDay
7%424%38%21%3
90
December1st to 15thDay
3312%34%38%7%
90
16th to endDay
9%8%29%31%19%2
96
January1st to 15thNight
4115%43%28%6%
720
16th to endNight
7%217%41%25%5%
768
February1st to 15thNight
3213%41%32%4
704
16th to endNight
6%317%46%20%4
632
March1st to 15thNight
5%320%35%29%6%
720
16th to endNight
2216%37%33%7%
768
April1st to 15thNight
2321%42%26%2
719
16th to endNight
2321%42%26%4
579
May1st to 15thNight
1320%41%30%2
540
16th to endNight
2214%41%36%3
563
June1st to 15thNight
10%49%35%3
540
16th to endNight
1336%53%4
540
July1st to 15thNight
15%28%53%10%
510
16th to endNight
1325%55%12%
564
August1st to 15thNight
3226%57%10%
539
16th to endNight
3321%54%17%
734
September1st to 15thNight
45%24%48%16%
720
16th to endNight
17%34%45%10%
716
October1st to 15thNight
215%36%38%6%
720
16th to endNight
118%35%39%14%
708
November1st to 15thNight
2110%41%35%9%
700
16th to endNight
3212%39%33%8%
720
December1st to 15thNight
6%212%39%32%6%
720
16th to endNight
5%213%45%25%5%
768
Page generated at 2017-10-17 13:35:27. Version ac04

Copyright 2017 A. Danko


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