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 2005-11-06 to 2024-03-08.

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
76%
24467
38%
79445
33%
24389
81%
24389
55%
35411

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 10thNight
71%
749
41%
3971
34%
736
77%
736
39%
1390
January11th to 20thNight
73%
748
48%
3882
37%
740
81%
740
46%
1242
January21st to endNight
62%
828
26%
4216
34%
814
72%
814
29%
2005
February1st to 10thNight
68%
760
39%
2280
35%
760
76%
760
52%
1095
February11th to 20thNight
65%
740
37%
2218
32%
740
75%
740
50%
1078
February21st to endNight
60%
628
33%
1884
30%
628
68%
628
47%
942
March1st to 10thNight
61%
750
37%
2086
28%
750
72%
750
53%
1047
March11th to 20thNight
72%
720
40%
1980
32%
720
80%
720
61%
958
March21st to endNight
72%
792
40%
2178
30%
792
80%
792
60%
1052
April1st to 10thNight
72%
720
38%
1980
26%
720
80%
720
65%
904
April11th to 20thNight
66%
682
40%
1902
25%
682
77%
682
70%
836
April21st to endNight
73%
540
50%
1620
37%
540
84%
540
76%
630
May1st to 10thNight
71%
540
44%
1620
31%
540
81%
540
71%
649
May11th to 20thNight
78%
531
44%
1594
36%
531
82%
531
76%
620
May21st to endNight
79%
583
43%
1746
33%
582
84%
582
81%
662
June1st to 10thNight
86%
540
39%
1620
33%
540
87%
540
73%
654
June11th to 20thNight
86%
540
41%
1620
33%
540
89%
540
84%
603
June21st to endNight
87%
540
35%
1620
31%
540
87%
540
67%
695
July1st to 10thNight
86%
525
29%
1575
24%
525
87%
525
55%
750
July11th to 20thNight
86%
522
27%
1566
22%
522
86%
522
45%
825
July21st to endNight
83%
591
29%
1674
25%
558
83%
558
39%
917
August1st to 10thNight
89%
537
34%
1602
30%
534
89%
534
60%
723
August11th to 20thNight
84%
576
36%
1644
29%
576
85%
576
53%
829
August21st to endNight
84%
792
35%
2065
27%
792
84%
792
57%
1064
September1st to 10thNight
83%
720
33%
1980
28%
720
84%
720
54%
1017
September11th to 20thNight
86%
720
43%
1980
38%
720
88%
720
69%
901
September21st to endNight
84%
720
43%
1980
35%
720
89%
720
69%
895
October1st to 10thNight
78%
720
44%
1980
35%
720
84%
720
68%
901
October11th to 20thNight
84%
720
45%
2131
40%
720
89%
720
75%
863
October21st to endNight
81%
792
46%
2376
40%
792
86%
792
72%
959
November1st to 10thNight
81%
740
43%
2220
40%
740
86%
740
62%
992
November11th to 20thNight
80%
760
41%
2451
41%
760
85%
760
62%
1024
November21st to endNight
68%
760
39%
2470
37%
760
76%
760
49%
1131
December1st to 10thNight
73%
760
39%
2470
39%
760
78%
760
51%
1133
December11th to 20thNight
68%
759
34%
2932
36%
758
74%
758
36%
1511
December21st to endNight
66%
822
25%
4332
36%
817
72%
817
30%
1914

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
3213323446%69%
79025
Night
3213323346%70%
80605
TransparencyDay
16%3411%25%40%
78752
Night
16%3412%25%39%
80400
SeeingDay
45%19%33%28%11%
14221
Night
4214%41%34%6%
24518

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 10thDay
3323223446%68%
2858
11th to 20thDay
332332345%5%67%
2860
21st to endDay
6%3234345%6%8%55%
3146
February1st to 10thDay
2314323446%68%
1900
11th to 20thDay
5%324333447%62%
1850
21st to endDay
3323323446%66%
1570
March1st to 10thDay
331343435%7%63%
2033
11th to 20thDay
3213323347%68%
1980
21st to endDay
221332333474%
2178
April1st to 10thDay
1111212346%76%
1980
11th to 20thDay
2112212225%79%
1999
21st to endDay
11111213485%
2340
May1st to 10thDay
121221223481%
2340
11th to 20thDay
11221223481%
2301
21st to endDay
11111112389%
2530
June1st to 10thDay
211112235%81%
2340
11th to 20thDay
111122389%
2340
21st to endDay
1211212235%78%
2340
July1st to 10thDay
3213425%45%7%66%
2340
11th to 20thDay
23145%46%6%7%10%51%
2340
21st to endDay
5%436%6%47%7%8%11%40%
2545
August1st to 10thDay
33244345%6%8%59%
2328
11th to 20thDay
2213435%6%8%9%57%
2252
21st to endDay
23134445%7%9%58%
2288
September1st to 10thDay
431333445%6%63%
1980
11th to 20thDay
3212212346%76%
1980
21st to endDay
121242234474%
1980
October1st to 10thDay
3112312336%75%
1980
11th to 20thDay
11121222484%
1828
21st to endDay
122123335%78%
1980
November1st to 10thDay
1211212248%76%
1859
11th to 20thDay
221322333474%
1725
21st to endDay
321432345%6%66%
1710
December1st to 10thDay
5%3135%24347%64%
1710
11th to 20thDay
4425%4245%7%7%56%
2169
21st to endDay
6%4134235%6%8%57%
3146
January1st to 10thNight
3314323445%68%
4097
11th to 20thNight
2212222335%74%
4110
21st to endNight
7%5%2332345%8%58%
4517
February1st to 10thNight
4413313446%66%
2280
11th to 20thNight
432332445%7%64%
2218
21st to endNight
4325%324445%63%
1884
March1st to 10thNight
431332446%6%64%
2086
11th to 20thNight
3213223337%71%
1980
21st to endNight
3113324346%70%
2178
April1st to 10thNight
2212222346%74%
1980
11th to 20thNight
221121222382%
1902
21st to endNight
21111223385%
1620
May1st to 10thNight
112112335%80%
1620
11th to 20thNight
11111133483%
1594
21st to endNight
21111112289%
1746
June1st to 10thNight
1111213126%81%
1620
11th to 20thNight
121122388%
1620
21st to endNight
221221323379%
1620
July1st to 10thNight
1222434449%65%
1620
11th to 20thNight
1214435%7%8%12%52%
1620
21st to endNight
2324435%7%9%16%44%
1773
August1st to 10thNight
11123235%5%12%65%
1611
11th to 20thNight
2213335%46%9%61%
1644
21st to endNight
221332445%8%65%
2065
September1st to 10thNight
421332446%7%65%
1980
11th to 20thNight
221221223479%
1980
21st to endNight
121222233478%
1980
October1st to 10thNight
2112312335%77%
1980
11th to 20thNight
111121223482%
2131
21st to endNight
122112235%81%
2376
November1st to 10thNight
2212323346%72%
2220
11th to 20thNight
2312212336%74%
2451
21st to endNight
331442335%6%65%
2470
December1st to 10thNight
5%313323447%65%
2470
11th to 20thNight
5%5%25%5%34446%57%
3041
21st to endNight
5%424425%5%5%5%59%
4521

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 10thDay
18%4411%31%32%
2859
11th to 20thDay
20%2313%36%26%
2860
21st to endDay
26%5%6%13%31%19%
3146
February1st to 10thDay
18%35%9%24%41%
1900
11th to 20thDay
23%3313%26%33%
1860
21st to endDay
19%2312%29%35%
1570
March1st to 10thDay
20%3210%28%37%
2033
11th to 20thDay
18%2310%25%42%
1980
21st to endDay
17%237%30%42%
2178
April1st to 10thDay
11%228%33%45%
1980
11th to 20thDay
11%116%25%55%
2002
21st to endDay
7%127%24%59%
2340
May1st to 10thDay
10%226%21%60%
2340
11th to 20thDay
9%126%20%60%
2304
21st to endDay
5%11422%67%
2530
June1st to 10thDay
8%11419%67%
2340
11th to 20thDay
411212%80%
2340
21st to endDay
11%235%13%65%
2340
July1st to 10thDay
17%5%6%10%20%42%
2276
11th to 20thDay
24%9%10%23%24%10%
2271
21st to endDay
32%11%11%24%15%7%
2404
August1st to 10thDay
22%6%8%16%20%28%
2313
11th to 20thDay
20%8%10%18%24%20%
2252
21st to endDay
21%7%9%19%23%21%
2288
September1st to 10thDay
22%5%7%13%26%28%
1980
11th to 20thDay
11%337%28%47%
1980
21st to endDay
14%338%23%49%
1980
October1st to 10thDay
12%3410%27%45%
1980
11th to 20thDay
7%136%24%58%
1828
21st to endDay
11%237%23%54%
1980
November1st to 10thDay
10%2210%32%44%
1859
11th to 20thDay
15%238%26%46%
1725
21st to endDay
19%3311%27%38%
1710
December1st to 10thDay
22%3313%25%34%
1710
11th to 20thDay
25%45%16%27%23%
2168
21st to endDay
22%6%5%20%27%20%
3146
January1st to 10thNight
18%349%35%32%
4097
11th to 20thNight
14%2217%39%26%
4110
21st to endNight
25%6%5%14%31%19%
4517
February1st to 10thNight
20%2412%20%42%
2280
11th to 20thNight
21%3314%25%35%
2219
21st to endNight
23%249%27%35%
1884
March1st to 10thNight
20%2211%27%37%
2086
11th to 20thNight
16%3210%23%47%
1980
21st to endNight
16%2310%31%37%
2178
April1st to 10thNight
13%228%30%45%
1980
11th to 20thNight
11%126%26%54%
1902
21st to endNight
7%127%24%58%
1620
May1st to 10thNight
9%229%23%56%
1620
11th to 20thNight
7%137%20%62%
1595
21st to endNight
6%115%20%68%
1746
June1st to 10thNight
9%11218%68%
1620
11th to 20thNight
5%11211%80%
1620
21st to endNight
11%2349%70%
1620
July1st to 10thNight
17%48%10%19%42%
1575
11th to 20thNight
20%8%12%25%20%14%
1566
21st to endNight
23%10%13%24%21%9%
1674
August1st to 10thNight
13%49%23%22%28%
1602
11th to 20thNight
18%5%9%24%24%19%
1644
21st to endNight
16%5%9%18%27%25%
2065
September1st to 10thNight
19%5%8%17%23%29%
1980
11th to 20thNight
12%237%27%49%
1980
21st to endNight
11%3310%25%49%
1980
October1st to 10thNight
11%3312%23%47%
1980
11th to 20thNight
9%128%24%56%
2131
21st to endNight
9%139%20%58%
2376
November1st to 10thNight
14%329%29%44%
2220
11th to 20thNight
14%228%27%47%
2451
21st to endNight
20%2212%25%38%
2470
December1st to 10thNight
20%2416%24%34%
2470
11th to 20thNight
29%35%11%26%26%
3041
21st to endNight
25%45%17%27%22%
4521

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 10thDay
5%6%24%34%26%4
270
11th to 20thDay
7%5%17%35%29%6%
270
21st to endDay
37%25%35%23%6%
297
February1st to 10thDay
5%7%30%34%20%4
460
11th to 20thDay
7%14%28%34%16%2
455
21st to endDay
5%12%35%34%12%4
382
March1st to 10thDay
411%29%37%16%3
449
11th to 20thDay
48%27%28%23%10%
420
21st to endDay
5%11%31%33%19%2
462
April1st to 10thDay
37%29%30%21%10%
420
11th to 20thDay
28%29%34%19%8%
412
21st to endDay
17%29%30%22%10%
420
May1st to 10thDay
17%26%34%21%10%
423
11th to 20thDay
25%23%39%21%9%
448
21st to endDay
15%25%39%25%6%
485
June1st to 10thDay
3221%39%27%8%
450
11th to 20thDay
1318%39%30%9%
450
21st to endDay
2113%42%30%12%
450
July1st to 10thDay
119%36%41%12%
446
11th to 20thDay
36%27%42%20%
444
21st to endDay
5%16%25%37%27%
487
August1st to 10thDay
4432%41%19%
447
11th to 20thDay
27%24%41%25%
437
21st to endDay
31421%43%27%
495
September1st to 10thDay
6%114%26%36%18%
450
11th to 20thDay
412%34%36%14%
450
21st to endDay
2419%31%25%20%
450
October1st to 10thDay
3318%34%30%13%
450
11th to 20thDay
2213%32%36%14%
298
21st to endDay
2314%28%39%14%
297
November1st to 10thDay
3417%30%36%10%
270
11th to 20thDay
4215%32%38%9%
270
21st to endDay
3424%34%26%9%
270
December1st to 10thDay
8%215%43%26%6%
270
11th to 20thDay
4324%43%22%4
270
21st to endDay
11%7%25%33%20%4
297
January1st to 10thNight
4215%49%26%4
760
11th to 20thNight
5%115%47%26%6%
760
21st to endNight
8%321%43%22%3
836
February1st to 10thNight
5%417%41%29%4
760
11th to 20thNight
6%321%47%20%3
744
21st to endNight
7%422%45%17%4
628
March1st to 10thNight
6%5%23%45%19%2
750
11th to 20thNight
4318%37%31%7%
720
21st to endNight
3417%44%27%4
792
April1st to 10thNight
2318%49%26%3
720
11th to 20thNight
3325%43%22%3
682
21st to endNight
3321%47%24%3
540
May1st to 10thNight
2321%43%30%1
540
11th to 20thNight
1217%48%30%3
531
21st to endNight
2216%48%30%2
584
June1st to 10thNight
119%54%33%2
540
11th to 20thNight
11%49%35%4
540
21st to endNight
216%40%48%3
540
July1st to 10thNight
17%36%50%6%
525
11th to 20thNight
25%30%55%9%
522
21st to endNight
315%29%52%10%
591
August1st to 10thNight
21230%56%8%
537
11th to 20thNight
21429%55%9%
576
21st to endNight
315%21%54%16%
792
September1st to 10thNight
5%5%29%49%10%
720
11th to 20thNight
317%31%48%10%
720
21st to endNight
1110%40%40%9%
720
October1st to 10thNight
3114%40%37%6%
720
11th to 20thNight
2112%42%36%8%
720
21st to endNight
2212%40%34%11%
792
November1st to 10thNight
3111%46%31%7%
740
11th to 20thNight
3112%42%36%7%
760
21st to endNight
5%218%41%28%6%
760
December1st to 10thNight
6%214%44%28%6%
760
11th to 20thNight
7%414%49%24%3
760
21st to endNight
7%316%46%23%5%
836
Page generated at 2024-03-05 05:24:01. Version ac07

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