University of Colorado Alpine Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the University of Colorado Alpine Observatory clear sky chart from 2005-11-05 to 2017-09-18.

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
31%
15957
24%
46883
9%
15910
45%
15910
25%
26156

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
16%
900
23%
2506
5%
895
34%
895
18%
1373
January16th to endNight
12%
798
27%
2526
5%
798
37%
798
19%
1292
February1st to 15thNight
11%
701
20%
2267
3%
697
32%
697
13%
1237
February16th to endNight
10%
632
25%
1894
2%
632
34%
632
15%
1053
March1st to 15thNight
20%
720
23%
2097
6%
720
37%
720
21%
1149
March16th to endNight
20%
690
23%
2034
6%
690
37%
690
21%
1170
April1st to 15thNight
21%
539
20%
1798
6%
539
38%
539
19%
1000
April16th to endNight
26%
540
20%
1800
9%
540
42%
540
19%
1050
May1st to 15thNight
30%
540
17%
1590
6%
540
42%
540
20%
1036
May16th to endNight
43%
563
16%
1498
8%
561
50%
561
31%
921
June1st to 15thNight
48%
540
22%
1440
12%
540
56%
540
44%
772
June16th to endNight
64%
540
27%
1440
18%
540
70%
540
51%
743
July1st to 15thNight
68%
510
17%
1360
12%
510
71%
510
34%
844
July16th to endNight
72%
564
21%
1456
18%
531
74%
531
38%
866
August1st to 15thNight
68%
539
18%
1647
17%
536
70%
536
35%
925
August16th to endNight
67%
573
25%
1910
25%
573
71%
573
33%
1053
September1st to 15thNight
56%
540
26%
1800
21%
540
66%
540
33%
982
September16th to endNight
55%
512
32%
1801
23%
512
67%
512
37%
885
October1st to 15thNight
34%
660
28%
1980
11%
660
49%
660
28%
1076
October16th to endNight
25%
721
29%
2129
7%
721
43%
721
26%
1159
November1st to 15thNight
20%
875
30%
2275
6%
875
36%
875
26%
1266
November16th to endNight
16%
900
30%
2427
5%
900
37%
900
24%
1300
December1st to 15thNight
11%
900
19%
2520
1%
900
27%
900
15%
1423
December16th to endNight
12%
960
21%
2688
3%
960
28%
960
15%
1581

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
5%5%25%5%35%5%6%9%45%
47927
Night
342432445%9%55%
47059
TransparencyDay
34%9%14%21%15%4
47605
Night
24%8%17%26%17%4
46883
SeeingDay
10%25%32%18%9%2
5867
Night
8%30%25%20%12%2
15960

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
43443346%13%52%
1432
16th to endDay
43441445%11%54%
1708
February1st to 15thDay
436%5%25%5%6%12%48%
1593
16th to endDay
437%42446%13%47%
1580
March1st to 15thDay
15%26%5%245%6%8%48%
1862
16th to endDay
49%24435%5%7%13%39%
2189
April1st to 15thDay
49%24436%5%6%9%42%
2150
16th to endDay
10%11%35%5%37%5%5%7%34%
2160
May1st to 15thDay
11%10%36%6%35%46%7%34%
2359
16th to endDay
10%7%25%5%37%6%6%9%33%
2616
June1st to 15thDay
5%325%5%36%6%7%9%44%
2520
16th to endDay
23145%36%6%6%8%51%
2520
July1st to 15thDay
8%5%25%6%46%6%7%7%39%
2520
16th to endDay
7%425%6%46%6%7%8%37%
2611
August1st to 15thDay
8%5%26%7%46%6%6%8%37%
2304
16th to endDay
6%425%5%5%6%5%6%8%41%
2292
September1st to 15thDay
8%3135%3447%6%50%
2160
16th to endDay
5%414423347%57%
1882
October1st to 15thDay
5%414435%45%6%52%
1650
16th to endDay
45%23434349%54%
1730
November1st to 15thDay
131343446%11%54%
1583
16th to endDay
1316%32346%8%57%
1530
December1st to 15thDay
427%7%3435%11%48%
1440
16th to endDay
2325%425%47%12%49%
1536
January1st to 15thNight
2233245%6%10%58%
2506
16th to endNight
3243234410%60%
2526
February1st to 15thNight
43432336%12%55%
2257
16th to endNight
5%2442335%11%54%
1894
March1st to 15thNight
132342446%13%53%
2097
16th to endNight
45%333245%49%54%
2034
April1st to 15thNight
37%13335%5%5%8%53%
1798
16th to endNight
8%9%2432335%8%48%
1800
May1st to 15thNight
10%7%243245%7%8%41%
1589
16th to endNight
7%5%234245%6%8%49%
1497
June1st to 15thNight
331321347%9%58%
1440
16th to endNight
121221337%8%64%
1440
July1st to 15thNight
43145%26%7%6%10%47%
1440
16th to endNight
4314425%5%6%8%51%
1555
August1st to 15thNight
5%3244235%6%10%52%
1656
16th to endNight
5%314435%5%5%8%51%
1910
September1st to 15thNight
7%4145%23446%55%
1800
16th to endNight
4312313446%62%
1801
October1st to 15thNight
4413323448%58%
1980
16th to endNight
36%2332335%7%57%
2129
November1st to 15thNight
13124244411%60%
2275
16th to endNight
131432335%10%60%
2427
December1st to 15thNight
435%3234411%56%
2520
16th to endNight
325%43446%12%51%
2688

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
24%12%25%22%12%3
1433
16th to endDay
25%8%20%27%14%4
1708
February1st to 15thDay
30%11%22%24%8%2
1594
16th to endDay
30%9%16%26%14%2
1580
March1st to 15thDay
31%6%17%26%16%1
1862
16th to endDay
36%8%15%25%10%3
2189
April1st to 15thDay
38%7%14%22%13%4
2150
16th to endDay
47%9%12%20%8%1
2160
May1st to 15thDay
48%8%12%18%10%
2359
16th to endDay
45%10%13%19%9%2
2620
June1st to 15thDay
32%10%14%15%19%6%
2520
16th to endDay
27%8%10%20%24%8%
2520
July1st to 15thDay
40%10%10%15%13%8%
2371
16th to endDay
40%11%14%19%9%4
2447
August1st to 15thDay
42%11%12%14%13%6%
2289
16th to endDay
37%10%12%19%16%3
2292
September1st to 15thDay
32%8%10%20%19%8%
2160
16th to endDay
27%5%11%26%26%2
1882
October1st to 15thDay
31%7%11%22%23%2
1650
16th to endDay
27%5%12%27%21%4
1730
November1st to 15thDay
24%8%17%21%22%6%
1583
16th to endDay
22%5%18%30%18%5%
1530
December1st to 15thDay
32%9%20%27%9%1
1440
16th to endDay
28%12%18%24%13%2
1536
January1st to 15thNight
19%10%25%25%16%2
2506
16th to endNight
20%7%20%28%18%4
2526
February1st to 15thNight
23%9%22%29%12%1
2267
16th to endNight
25%6%19%27%17%4
1894
March1st to 15thNight
23%6%20%28%17%2
2097
16th to endNight
27%7%15%33%14%2
2034
April1st to 15thNight
28%8%15%28%14%4
1798
16th to endNight
33%6%15%27%15%1
1800
May1st to 15thNight
37%10%15%23%12%1
1590
16th to endNight
30%8%19%25%13%2
1498
June1st to 15thNight
20%7%18%24%21%8%
1440
16th to endNight
16%6%10%27%26%12%
1440
July1st to 15thNight
29%10%15%20%15%8%
1360
16th to endNight
26%9%17%24%13%7%
1456
August1st to 15thNight
27%9%19%20%16%7%
1647
16th to endNight
28%7%12%22%23%4
1910
September1st to 15thNight
28%6%12%20%21%9%
1800
16th to endNight
21%6%11%30%25%4
1801
October1st to 15thNight
24%6%12%27%23%5%
1980
16th to endNight
24%5%14%28%23%3
2129
November1st to 15thNight
20%7%15%25%23%7%
2275
16th to endNight
20%6%15%31%22%4
2427
December1st to 15thNight
22%9%25%26%13%2
2520
16th to endNight
26%9%22%26%12%2
2688

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
7%41%21%17%11%1
90
16th to endDay
6%50%26%15%2
96
February1st to 15thDay
14%61%16%6%1
90
16th to endDay
455%23%12%3
81
March1st to 15thDay
11%50%26%8%1
152
16th to endDay
14%46%32%5%1
288
April1st to 15thDay
13%40%32%10%2
269
16th to endDay
24%41%23%8%2
270
May1st to 15thDay
16%27%32%15%6%1
411
16th to endDay
15%22%29%18%11%2
517
June1st to 15thDay
6%19%32%25%13%2
495
16th to endDay
314%38%22%17%4
495
July1st to 15thDay
10%8%29%27%17%6%
475
16th to endDay
7%6%33%25%20%7%
470
August1st to 15thDay
13%10%41%24%7%1
315
16th to endDay
11%10%38%27%9%2
332
September1st to 15thDay
10%19%40%20%7%2
315
16th to endDay
416%42%22%11%3
154
October1st to 15thDay
6%32%27%18%11%3
90
16th to endDay
17%28%29%18%33
96
November1st to 15thDay
326%27%20%18%3
90
16th to endDay
11%50%31%7%
90
December1st to 15thDay
12%58%23%5%
90
16th to endDay
366%18%7%4
96
January1st to 15thNight
11%45%24%12%5%1
900
16th to endNight
10%46%28%10%4
798
February1st to 15thNight
10%49%24%11%3
704
16th to endNight
7%49%28%12%2
632
March1st to 15thNight
6%43%25%15%7%
720
16th to endNight
8%37%31%19%3
690
April1st to 15thNight
9%30%35%17%6%
539
16th to endNight
13%28%28%20%7%
540
May1st to 15thNight
14%16%33%21%12%1
540
16th to endNight
9%13%29%28%15%2
563
June1st to 15thNight
5%8%32%33%17%2
540
16th to endNight
26%24%37%26%2
540
July1st to 15thNight
5%314%36%34%5%
510
16th to endNight
4110%33%41%8%
564
August1st to 15thNight
5%213%44%30%4
539
16th to endNight
6%313%34%33%8%
573
September1st to 15thNight
9%5%22%33%23%5%
540
16th to endNight
7%11%21%32%21%5%
512
October1st to 15thNight
7%30%24%22%12%2
660
16th to endNight
8%34%29%15%8%3
721
November1st to 15thNight
7%41%30%14%6%
875
16th to endNight
6%45%30%14%3
900
December1st to 15thNight
11%47%27%10%2
900
16th to endNight
8%48%27%11%4
960
Page generated at 2017-09-19 05:32:22. Version ac04

Copyright 2017 A. Danko


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