Angel Light Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Angel Light Observatory clear sky chart from 2005-11-05 to 2018-01-12.

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
25%
15324
12%
48376
7%
15293
31%
15293
14%
35666

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
10%
765
6%
2662
4%
761
13%
761
5%
2220
January16th to endNight
9%
768
7%
2574
3%
768
15%
768
6%
2118
February1st to 15thNight
8%
704
7%
2287
2%
701
15%
701
6%
1933
February16th to endNight
11%
631
10%
1920
4%
631
19%
631
8%
1549
March1st to 15thNight
13%
720
9%
2085
3%
720
20%
720
10%
1638
March16th to endNight
20%
768
15%
2040
6%
768
27%
768
14%
1539
April1st to 15thNight
17%
718
11%
1796
3%
718
21%
718
15%
1403
April16th to endNight
25%
720
17%
1800
5%
720
31%
720
23%
1278
May1st to 15thNight
31%
384
17%
1464
12%
384
41%
384
16%
978
May16th to endNight
35%
374
15%
1497
14%
374
44%
374
19%
936
June1st to 15thNight
38%
360
12%
1440
11%
360
47%
360
16%
956
June16th to endNight
44%
360
12%
1440
14%
360
52%
360
22%
855
July1st to 15thNight
52%
340
13%
1360
17%
340
57%
340
23%
800
July16th to endNight
56%
376
15%
1418
18%
354
60%
354
24%
822
August1st to 15thNight
51%
515
14%
1586
10%
513
54%
513
28%
988
August16th to endNight
53%
744
18%
1890
10%
744
55%
744
34%
1225
September1st to 15thNight
42%
720
17%
1800
8%
720
45%
720
29%
1218
September16th to endNight
47%
717
23%
1955
17%
717
51%
717
30%
1267
October1st to 15thNight
32%
720
16%
2160
12%
720
38%
720
19%
1523
October16th to endNight
17%
768
11%
2364
5%
768
25%
768
11%
1809
November1st to 15thNight
18%
760
14%
2472
8%
760
26%
760
13%
1784
November16th to endNight
15%
780
12%
2724
6%
780
24%
780
11%
2030
December1st to 15thNight
11%
780
5%
2730
2%
780
15%
780
6%
2264
December16th to endNight
7%
832
4%
2912
1%
832
11%
832
4%
2533

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
17%11%47%6%3445%6%27%
48966
Night
15%12%5%8%6%34445%28%
48545
Cloud CoverDay
17%11%47%6%3445%6%27%
48975
Night
16%11%5%8%6%34445%29%
48551
TransparencyDay
56%5%10%17%8%
48653
Night
56%5%11%17%8%
48376
SeeingDay
24%14%28%18%10%3
5912
Night
23%17%28%21%8%
15325

By Month

Percentage Cloud Cover NE (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
13%16%7%14%8%46%5%45%14%
1520
16th to endDay
10%16%9%15%10%45%44313%
1647
February1st to 15thDay
13%17%9%12%11%45%44312%
1575
16th to endDay
16%15%7%12%11%4432416%
1551
March1st to 15thDay
17%16%6%11%7%3333420%
1864
16th to endDay
16%13%6%8%6%23346%28%
2183
April1st to 15thDay
19%14%6%6%5%23335%30%
2152
16th to endDay
16%14%25%5%23445%35%
2160
May1st to 15thDay
19%8%36%6%35%446%31%
2494
16th to endDay
16%7%36%5%35%5%5%8%31%
2618
June1st to 15thDay
24%7%35%5%3445%7%27%
2520
16th to endDay
17%7%26%7%345%5%9%29%
2520
July1st to 15thDay
13%6%26%6%46%6%6%9%31%
2520
16th to endDay
14%5%26%7%46%6%6%8%30%
2654
August1st to 15thDay
14%7%26%6%46%6%7%8%28%
2362
16th to endDay
14%6%35%6%45%6%6%7%32%
2300
September1st to 15thDay
19%6%36%43445%6%34%
2160
16th to endDay
15%6%25%434445%41%
2003
October1st to 15thDay
21%9%25%434446%33%
1800
16th to endDay
24%16%36%5%3434423%
1860
November1st to 15thDay
15%15%36%5%24446%29%
1716
16th to endDay
18%15%5%9%6%34346%21%
1563
December1st to 15thDay
15%22%9%11%7%2235%415%
1560
16th to endDay
17%20%11%12%8%3443310%
1664
January1st to 15thNight
15%15%6%14%10%5%6%34313%
2662
16th to endNight
12%14%8%13%10%5%6%5%45%13%
2574
February1st to 15thNight
11%17%8%16%11%45%33314%
2285
16th to endNight
13%14%6%13%9%36%44418%
1920
March1st to 15thNight
15%13%7%10%8%3433423%
2072
16th to endNight
16%13%6%8%424435%28%
2040
April1st to 15thNight
18%14%46%5%35%345%26%
1796
16th to endNight
17%11%25%423345%38%
1800
May1st to 15thNight
17%9%36%5%3435%6%35%
1464
16th to endNight
14%5%26%5%345%5%7%38%
1495
June1st to 15thNight
20%7%344345%5%6%33%
1440
16th to endNight
14%5%144345%6%9%40%
1440
July1st to 15thNight
7%425%5%36%5%6%9%42%
1440
16th to endNight
11%4145%245%6%7%45%
1514
August1st to 15thNight
10%5%24436%5%5%9%42%
1596
16th to endNight
11%5%25%4335%5%7%45%
1890
September1st to 15thNight
16%5%25%5%23447%41%
1800
16th to endNight
15%6%1442445%6%44%
1955
October1st to 15thNight
21%8%25%5%23446%34%
2160
16th to endNight
25%13%36%5%3443424%
2364
November1st to 15thNight
15%17%47%5%24345%29%
2472
16th to endNight
16%14%7%9%6%23345%24%
2724
December1st to 15thNight
18%17%8%11%6%35%43416%
2730
16th to endNight
18%19%9%14%7%3433311%
2912

Percentage Cloud Cover SE (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
13%14%7%14%8%46%44416%
1520
16th to endDay
10%16%9%15%10%46%34315%
1647
February1st to 15thDay
14%15%9%13%12%3444313%
1575
16th to endDay
16%14%7%12%11%35%33417%
1551
March1st to 15thDay
18%15%6%11%7%3324420%
1874
16th to endDay
17%13%6%7%6%23246%29%
2182
April1st to 15thDay
19%15%5%6%5%2333430%
2152
16th to endDay
16%13%25%5%33345%36%
2160
May1st to 15thDay
19%9%36%6%2445%6%31%
2494
16th to endDay
16%8%35%6%35%5%5%8%30%
2618
June1st to 15thDay
25%7%36%42445%7%28%
2520
16th to endDay
18%6%36%6%35%5%6%8%30%
2520
July1st to 15thDay
14%6%26%5%46%6%7%8%32%
2520
16th to endDay
14%6%36%6%36%6%6%8%31%
2654
August1st to 15thDay
16%6%26%7%36%7%6%8%28%
2362
16th to endDay
15%6%35%6%35%7%6%7%32%
2300
September1st to 15thDay
19%7%26%6%34446%34%
2160
16th to endDay
15%6%25%425%45%442%
2003
October1st to 15thDay
23%9%25%424445%33%
1800
16th to endDay
24%15%35%6%3344424%
1860
November1st to 15thDay
16%14%37%5%24445%30%
1716
16th to endDay
18%16%5%8%6%34336%22%
1563
December1st to 15thDay
17%21%8%12%6%2333416%
1560
16th to endDay
17%20%12%12%7%35%43311%
1664
January1st to 15thNight
14%14%7%13%9%5%7%44414%
2662
16th to endNight
12%14%9%13%9%5%5%445%15%
2574
February1st to 15thNight
11%16%8%15%11%45%33315%
2278
16th to endNight
13%13%6%13%10%35%445%18%
1920
March1st to 15thNight
16%13%7%9%8%35%33423%
2085
16th to endNight
15%13%7%8%5%23435%30%
2040
April1st to 15thNight
19%15%5%5%5%34445%27%
1796
16th to endNight
17%10%26%42424439%
1800
May1st to 15thNight
17%8%45%5%34336%36%
1464
16th to endNight
13%5%26%5%35%6%37%39%
1495
June1st to 15thNight
20%6%35%33446%7%33%
1440
16th to endNight
13%4245%335%6%9%40%
1440
July1st to 15thNight
8%5%25%5%35%6%5%9%42%
1440
16th to endNight
12%3235%3446%7%45%
1514
August1st to 15thNight
10%5%245%346%6%7%43%
1596
16th to endNight
11%5%15%5%3345%7%45%
1890
September1st to 15thNight
16%5%2443435%7%42%
1800
16th to endNight
15%5%14434446%44%
1955
October1st to 15thNight
21%8%35%5%23346%35%
2160
16th to endNight
25%12%36%5%34345%25%
2364
November1st to 15thNight
15%15%48%424445%30%
2472
16th to endNight
16%14%6%9%5%33345%25%
2724
December1st to 15thNight
18%17%8%11%7%3433416%
2730
16th to endNight
18%18%10%14%6%3443411%
2912

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
70%48%9%6%
1521
16th to endDay
72%5%9%8%4
1647
February1st to 15thDay
74%5%5%11%3
1577
16th to endDay
71%37%12%4
1551
March1st to 15thDay
65%411%12%5%
1874
16th to endDay
56%410%15%12%1
2183
April1st to 15thDay
57%39%20%8%2
2152
16th to endDay
50%48%20%15%1
2160
May1st to 15thDay
53%5%10%19%10%
2494
16th to endDay
49%8%13%18%10%
2622
June1st to 15thDay
53%7%10%19%9%
2520
16th to endDay
50%7%13%19%9%
2520
July1st to 15thDay
47%8%10%23%9%
2371
16th to endDay
47%8%14%20%7%
2487
August1st to 15thDay
50%7%13%20%8%
2348
16th to endDay
46%7%11%23%9%
2300
September1st to 15thDay
50%5%10%22%11%
2160
16th to endDay
44%49%24%15%1
2003
October1st to 15thDay
52%5%9%20%11%1
1800
16th to endDay
63%313%14%4
1860
November1st to 15thDay
54%6%12%16%8%1
1716
16th to endDay
63%5%11%12%5%
1563
December1st to 15thDay
72%410%8%3
1560
16th to endDay
78%48%6%2
1664
January1st to 15thNight
71%5%8%8%5%
2662
16th to endNight
70%48%11%4
2574
February1st to 15thNight
72%46%11%3
2287
16th to endNight
67%49%13%5%
1920
March1st to 15thNight
63%411%14%6%
2085
16th to endNight
56%49%17%10%1
2040
April1st to 15thNight
57%5%9%16%9%1
1796
16th to endNight
48%39%20%16%2
1800
May1st to 15thNight
49%5%10%19%13%1
1464
16th to endNight
43%6%14%22%11%1
1497
June1st to 15thNight
48%5%12%23%9%
1440
16th to endNight
38%6%15%24%13%
1440
July1st to 15thNight
36%8%12%27%13%1
1360
16th to endNight
36%6%15%24%13%2
1418
August1st to 15thNight
36%7%18%26%10%
1586
16th to endNight
36%5%15%28%13%
1890
September1st to 15thNight
41%5%13%25%13%1
1800
16th to endNight
39%411%27%15%1
1955
October1st to 15thNight
50%5%10%19%13%
2160
16th to endNight
60%413%14%6%
2364
November1st to 15thNight
55%411%16%9%2
2472
16th to endNight
60%313%16%6%
2724
December1st to 15thNight
71%410%10%2
2730
16th to endNight
76%5%8%8%2
2912

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
23%39%23%9%4
82
16th to endDay
30%30%33%32
83
February1st to 15thDay
31%44%22%2
90
16th to endDay
27%29%28%12%1
210
March1st to 15thDay
30%22%27%15%4
270
16th to endDay
27%25%27%14%5%
288
April1st to 15thDay
32%17%30%12%5%1
270
16th to endDay
26%14%29%17%9%3
270
May1st to 15thDay
32%13%27%13%10%3
288
16th to endDay
16%14%39%17%9%2
331
June1st to 15thDay
27%9%28%23%8%1
315
16th to endDay
22%10%36%18%10%1
315
July1st to 15thDay
18%6%30%26%13%4
305
16th to endDay
14%5%31%29%13%5%
326
August1st to 15thDay
16%7%26%24%17%7%
313
16th to endDay
13%8%25%25%13%12%
332
September1st to 15thDay
20%9%20%23%19%6%
315
16th to endDay
14%8%24%21%18%10%
314
October1st to 15thDay
25%9%26%18%16%3
315
16th to endDay
34%17%28%14%5%
321
November1st to 15thDay
29%13%29%17%9%
280
16th to endDay
29%31%32%7%
93
December1st to 15thDay
28%31%24%12%21
90
16th to endDay
37%32%29%1
96
January1st to 15thNight
24%29%31%12%1
765
16th to endNight
25%24%33%14%1
768
February1st to 15thNight
23%27%35%13%
704
16th to endNight
26%26%29%14%3
631
March1st to 15thNight
28%24%29%15%2
720
16th to endNight
23%25%26%18%5%
768
April1st to 15thNight
30%18%30%17%2
718
16th to endNight
24%17%29%19%9%
720
May1st to 15thNight
22%10%29%29%7%
384
16th to endNight
13%8%36%28%12%1
375
June1st to 15thNight
21%11%24%30%11%
360
16th to endNight
15%430%39%10%
360
July1st to 15thNight
10%225%42%17%1
340
16th to endNight
14%220%41%19%1
376
August1st to 15thNight
13%425%34%19%2
515
16th to endNight
13%421%32%24%2
744
September1st to 15thNight
19%6%24%31%15%2
720
16th to endNight
17%8%22%24%23%4
717
October1st to 15thNight
26%12%24%21%13%1
720
16th to endNight
33%15%28%18%3
768
November1st to 15thNight
24%17%34%15%7%
760
16th to endNight
26%22%28%15%41
780
December1st to 15thNight
30%26%26%13%2
780
16th to endNight
29%28%29%10%1
832
Page generated at 2018-01-12 22:01:33. Version ac04

Copyright 2018 A. Danko


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