Allen Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Allen Observatory clear sky chart from 2005-11-05 to 2018-02-22.

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
29%
16740
13%
48880
7%
16697
34%
16697
17%
35806

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%
975
4%
2716
1%
970
13%
970
5%
2426
January16th to endNight
13%
1040
5%
2912
2%
1040
16%
1040
8%
2512
February1st to 15thNight
14%
824
7%
2441
2%
823
19%
823
7%
2039
February16th to endNight
18%
659
9%
1977
4%
659
22%
659
10%
1595
March1st to 15thNight
25%
720
14%
2151
8%
720
31%
720
17%
1543
March16th to endNight
30%
704
18%
2048
10%
704
37%
704
20%
1407
April1st to 15thNight
24%
539
14%
1797
8%
539
33%
539
17%
1250
April16th to endNight
28%
540
20%
1647
9%
540
38%
540
23%
1087
May1st to 15thNight
31%
540
14%
1620
7%
540
39%
540
21%
1117
May16th to endNight
35%
562
16%
1503
6%
561
44%
561
29%
987
June1st to 15thNight
42%
540
13%
1440
7%
540
46%
540
31%
923
June16th to endNight
46%
540
14%
1440
6%
540
52%
540
34%
900
July1st to 15thNight
60%
510
16%
1360
12%
510
62%
510
41%
794
July16th to endNight
54%
564
16%
1417
9%
531
60%
531
38%
868
August1st to 15thNight
59%
539
22%
1554
16%
536
63%
536
40%
875
August16th to endNight
58%
573
21%
1831
18%
573
63%
573
31%
1072
September1st to 15thNight
52%
591
23%
1851
19%
591
57%
591
33%
1068
September16th to endNight
50%
717
23%
1980
16%
717
55%
717
34%
1226
October1st to 15thNight
37%
720
19%
2031
13%
720
43%
720
24%
1390
October16th to endNight
24%
768
11%
2496
7%
768
31%
768
14%
1845
November1st to 15thNight
22%
760
15%
2471
9%
760
31%
760
14%
1807
November16th to endNight
17%
800
7%
2555
4%
800
21%
800
9%
2088
December1st to 15thNight
10%
975
3%
2730
1%
975
13%
975
6%
2432
December16th to endNight
11%
1040
6%
2912
2%
1040
15%
1040
6%
2555

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
15%10%47%6%34446%32%
49383
Night
15%11%5%7%6%34446%31%
49048
Cloud CoverDay
15%10%47%6%3445%6%32%
49392
Night
15%10%47%6%34446%31%
49053
TransparencyDay
52%5%11%20%10%
49069
Night
53%5%11%18%9%1
48880
SeeingDay
19%15%30%20%9%3
6528
Night
20%13%29%26%9%1
16743

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
16%17%10%14%11%5%5%3239%
1552
16th to endDay
12%18%14%13%10%5%6%3338%
1664
February1st to 15thDay
11%16%10%13%14%5%5%23412%
1750
16th to endDay
12%17%8%11%11%35%43415%
1643
March1st to 15thDay
15%11%6%8%6%34446%28%
1798
16th to endDay
15%10%37%5%34446%32%
2187
April1st to 15thDay
21%13%44423335%31%
2151
16th to endDay
17%10%35%5%24446%35%
2313
May1st to 15thDay
18%7%25%5%2445%7%36%
2331
16th to endDay
11%5%244245%6%8%44%
2609
June1st to 15thDay
13%7%25%5%2446%7%40%
2520
16th to endDay
12%5%2443445%9%42%
2520
July1st to 15thDay
9%3244245%5%8%49%
2520
16th to endDay
9%424425%46%10%44%
2655
August1st to 15thDay
8%5%15%436%5%7%8%43%
2397
16th to endDay
14%6%2432335%8%44%
2369
September1st to 15thDay
15%415%5%35%346%45%
2097
16th to endDay
12%5%26%5%3446%7%40%
1980
October1st to 15thDay
19%9%25%5%24336%35%
1926
16th to endDay
24%10%47%6%33435%24%
1728
November1st to 15thDay
17%12%46%5%25%35%5%30%
1717
16th to endDay
20%21%7%9%5%2444416%
1732
December1st to 15thDay
18%19%13%15%8%2322211%
1560
16th to endDay
18%22%10%12%9%432338%
1664
January1st to 15thNight
19%17%7%13%12%4422213%
2716
16th to endNight
11%16%10%13%10%5%5%445%12%
2912
February1st to 15thNight
10%16%8%12%11%5%5%34317%
2439
16th to endNight
15%13%6%11%7%45%35%5%19%
1977
March1st to 15thNight
15%9%47%5%3445%7%32%
2138
16th to endNight
15%7%46%6%3435%7%35%
2048
April1st to 15thNight
20%9%35%43435%7%32%
1797
16th to endNight
18%6%25%423346%40%
1647
May1st to 15thNight
16%5%35%7%35%5%5%7%34%
1620
16th to endNight
10%5%35%5%25%45%9%43%
1501
June1st to 15thNight
11%5%24335%5%5%8%43%
1440
16th to endNight
9%4345%245%7%10%44%
1440
July1st to 15thNight
7%3133245%6%8%52%
1440
16th to endNight
7%422325%5%6%8%49%
1513
August1st to 15thNight
6%21442447%8%52%
1563
16th to endNight
11%5%24424446%49%
1831
September1st to 15thNight
12%5%13313246%52%
1851
16th to endNight
10%5%2443435%7%48%
1980
October1st to 15thNight
21%7%24424346%39%
2031
16th to endNight
24%9%36%5%34436%28%
2496
November1st to 15thNight
16%13%47%5%33445%29%
2471
16th to endNight
19%18%6%8%6%3444419%
2555
December1st to 15thNight
19%20%11%13%7%3433311%
2730
16th to endNight
18%21%8%10%8%3344411%
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
16%18%9%13%11%5%43339%
1552
16th to endDay
12%18%14%13%11%5%6%3238%
1664
February1st to 15thDay
11%16%10%12%15%45%33412%
1750
16th to endDay
12%17%9%12%10%45%43415%
1643
March1st to 15thDay
15%11%5%8%6%25%45%7%27%
1808
16th to endDay
17%9%47%5%3445%5%32%
2186
April1st to 15thDay
21%13%445%23345%31%
2151
16th to endDay
17%10%35%5%2345%5%35%
2313
May1st to 15thDay
18%6%35%5%2445%7%35%
2331
16th to endDay
12%5%244345%6%7%43%
2609
June1st to 15thDay
13%6%35%5%25%5%5%7%39%
2520
16th to endDay
12%5%25%6%3445%8%41%
2520
July1st to 15thDay
9%4244245%5%9%48%
2520
16th to endDay
10%4245%25%5%6%9%44%
2655
August1st to 15thDay
8%4245%35%5%7%8%43%
2397
16th to endDay
14%6%2442435%7%43%
2369
September1st to 15thDay
15%425%43435%6%44%
2097
16th to endDay
12%5%36%5%3446%7%40%
1980
October1st to 15thDay
19%8%36%5%24345%35%
1926
16th to endDay
25%10%48%6%23445%24%
1728
November1st to 15thDay
17%12%46%6%2445%6%29%
1717
16th to endDay
20%21%7%9%5%2343416%
1732
December1st to 15thDay
18%18%13%15%8%2413211%
1560
16th to endDay
19%21%10%12%9%433238%
1664
January1st to 15thNight
18%17%7%13%10%5%432213%
2716
16th to endNight
11%15%9%13%10%5%6%335%13%
2912
February1st to 15thNight
11%15%8%12%11%46%43418%
2432
16th to endNight
15%14%6%11%8%35%446%18%
1977
March1st to 15thNight
15%9%47%5%245%5%6%32%
2151
16th to endNight
15%8%36%6%4435%7%35%
2048
April1st to 15thNight
20%9%46%42435%6%32%
1797
16th to endNight
18%7%25%423346%40%
1647
May1st to 15thNight
15%5%25%7%5%5%5%5%7%33%
1619
16th to endNight
9%5%25%5%3445%9%42%
1501
June1st to 15thNight
11%6%245%25%46%8%43%
1440
16th to endNight
9%5%25%4235%7%10%43%
1440
July1st to 15thNight
7%3133245%6%8%53%
1440
16th to endNight
7%424335%5%6%8%49%
1513
August1st to 15thNight
6%3234245%6%9%51%
1563
16th to endNight
11%5%13434446%49%
1831
September1st to 15thNight
12%414323335%53%
1851
16th to endNight
11%425%43445%7%48%
1980
October1st to 15thNight
21%6%24423346%39%
2031
16th to endNight
23%9%46%5%34436%29%
2496
November1st to 15thNight
16%12%47%5%34445%30%
2471
16th to endNight
19%17%6%7%6%44435%19%
2555
December1st to 15thNight
18%19%10%13%7%3432311%
2730
16th to endNight
18%20%8%11%8%4434411%
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
81%46%6%1
1553
16th to endDay
82%45%5%2
1664
February1st to 15thDay
75%46%11%1
1752
16th to endDay
71%5%8%9%3
1643
March1st to 15thDay
56%5%13%16%6%1
1808
16th to endDay
51%5%13%22%6%1
2187
April1st to 15thDay
55%39%19%11%
2151
16th to endDay
49%311%17%14%2
2313
May1st to 15thDay
45%6%11%23%10%1
2331
16th to endDay
36%6%15%26%14%1
2613
June1st to 15thDay
41%5%12%24%15%
2520
16th to endDay
38%6%15%23%15%
2520
July1st to 15thDay
31%5%13%29%18%1
2371
16th to endDay
34%6%13%26%16%2
2488
August1st to 15thDay
33%7%15%30%11%1
2382
16th to endDay
38%412%26%15%1
2369
September1st to 15thDay
39%5%10%29%13%1
2097
16th to endDay
41%5%13%27%12%
1980
October1st to 15thDay
51%410%23%10%
1926
16th to endDay
61%5%14%13%4
1728
November1st to 15thDay
54%5%11%19%8%
1717
16th to endDay
71%411%9%2
1732
December1st to 15thDay
81%36%7%1
1560
16th to endDay
82%38%31
1664
January1st to 15thNight
79%35%7%2
2716
16th to endNight
73%411%7%2
2912
February1st to 15thNight
70%47%13%3
2441
16th to endNight
65%5%10%12%5%
1977
March1st to 15thNight
50%6%13%19%9%1
2151
16th to endNight
47%412%23%10%1
2048
April1st to 15thNight
51%39%19%14%
1797
16th to endNight
44%410%21%15%3
1647
May1st to 15thNight
47%5%11%21%12%1
1620
16th to endNight
38%5%10%26%14%2
1503
June1st to 15thNight
37%5%13%26%15%1
1440
16th to endNight
33%6%16%26%15%1
1440
July1st to 15thNight
26%7%12%30%20%3
1360
16th to endNight
29%7%17%27%15%2
1417
August1st to 15thNight
27%5%15%33%16%
1554
16th to endNight
33%5%12%26%19%2
1831
September1st to 15thNight
33%411%30%18%1
1851
16th to endNight
35%414%31%13%
1980
October1st to 15thNight
45%5%9%25%13%
2031
16th to endNight
55%6%14%18%5%
2496
November1st to 15thNight
54%6%11%17%9%1
2471
16th to endNight
67%5%13%9%2
2555
December1st to 15thNight
78%47%6%2
2730
16th to endNight
76%49%7%1
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
19%24%39%16%
135
16th to endDay
27%18%37%13%2
144
February1st to 15thDay
13%49%27%6%2
135
16th to endDay
24%35%28%11%11
100
March1st to 15thDay
33%24%28%9%31
98
16th to endDay
27%12%28%21%7%2
288
April1st to 15thDay
30%15%30%16%5%1
270
16th to endDay
25%14%33%12%12%2
270
May1st to 15thDay
24%18%28%18%5%3
288
16th to endDay
17%17%35%20%6%2
509
June1st to 15thDay
15%15%36%21%6%3
495
16th to endDay
14%14%35%23%8%3
495
July1st to 15thDay
10%11%32%27%13%5%
475
16th to endDay
9%9%34%27%12%6%
515
August1st to 15thDay
9%7%27%24%20%10%
370
16th to endDay
24%6%18%26%16%8%
332
September1st to 15thDay
13%6%24%25%20%10%
315
16th to endDay
12%11%21%29%20%4
314
October1st to 15thDay
23%11%27%24%10%2
287
16th to endDay
31%17%42%6%1
144
November1st to 15thDay
26%17%37%11%5%
135
16th to endDay
27%30%29%11%
135
December1st to 15thDay
34%36%19%8%2
135
16th to endDay
32%43%18%4
144
January1st to 15thNight
26%21%30%16%4
975
16th to endNight
21%16%36%21%4
1040
February1st to 15thNight
17%21%32%22%4
826
16th to endNight
24%16%30%22%4
659
March1st to 15thNight
20%18%31%21%7%1
720
16th to endNight
18%14%31%26%8%
704
April1st to 15thNight
25%14%32%21%5%
539
16th to endNight
23%10%34%21%8%1
540
May1st to 15thNight
18%11%33%27%7%1
540
16th to endNight
13%8%35%29%11%
563
June1st to 15thNight
12%9%30%33%13%
540
16th to endNight
10%7%30%36%14%
540
July1st to 15thNight
8%422%43%19%1
510
16th to endNight
9%326%37%21%2
564
August1st to 15thNight
7%222%44%20%2
539
16th to endNight
15%217%40%20%2
573
September1st to 15thNight
14%6%21%31%19%5%
591
16th to endNight
12%8%23%31%22%2
717
October1st to 15thNight
24%8%24%29%11%1
720
16th to endNight
30%13%26%22%6%
768
November1st to 15thNight
25%15%29%21%7%
760
16th to endNight
29%18%25%21%4
800
December1st to 15thNight
30%21%30%16%2
975
16th to endNight
27%18%31%19%2
1040
Page generated at 2018-02-23 11:21:35. Version ac04

Copyright 2018 A. Danko


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