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 2017-09-10.

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
30%
16012
13%
46749
7%
15969
35%
15969
17%
34161

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
11%
900
4%
2506
1%
895
14%
895
5%
2234
January16th to endNight
14%
960
5%
2688
2%
960
16%
960
7%
2332
February1st to 15thNight
15%
759
6%
2250
2%
758
19%
758
7%
1885
February16th to endNight
18%
631
9%
1893
4%
631
22%
631
10%
1532
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
17%
1360
12%
510
62%
510
41%
794
July16th to endNight
55%
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
59%
573
21%
1831
18%
573
64%
573
31%
1072
September1st to 15thNight
50%
572
22%
1797
19%
572
56%
572
31%
1047
September16th to endNight
49%
657
20%
1815
13%
657
54%
657
32%
1140
October1st to 15thNight
39%
660
20%
1862
14%
660
45%
660
24%
1272
October16th to endNight
23%
704
11%
2288
6%
704
29%
704
14%
1699
November1st to 15thNight
22%
700
16%
2276
10%
700
32%
700
15%
1636
November16th to endNight
18%
738
7%
2358
4%
738
21%
738
8%
1959
December1st to 15thNight
10%
900
3%
2520
1%
900
12%
900
5%
2247
December16th to endNight
12%
960
6%
2688
2%
960
16%
960
6%
2355

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%3445%6%32%
47891
Night
15%11%5%7%6%34446%31%
46917
Cloud CoverDay
15%9%47%6%3445%6%32%
47900
Night
15%10%47%6%34446%31%
46922
TransparencyDay
51%5%11%20%10%
47577
Night
53%5%11%18%9%1
46749
SeeingDay
18%14%30%20%10%4
6002
Night
20%13%29%26%9%1
16015

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
17%18%10%14%9%5%5%33310%
1432
16th to endDay
13%18%15%13%10%6%5%3227%
1536
February1st to 15thDay
11%16%10%12%13%5%5%23413%
1612
16th to endDay
11%17%8%11%11%35%435%15%
1580
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%43%
2039
16th to endDay
13%5%26%6%3446%7%38%
1815
October1st to 15thDay
19%9%25%5%24336%35%
1766
16th to endDay
25%10%48%6%43435%23%
1584
November1st to 15thDay
16%11%46%5%25%35%6%31%
1582
16th to endDay
21%21%7%9%5%2433414%
1599
December1st to 15thDay
18%18%13%15%8%2322211%
1440
16th to endDay
19%22%10%12%8%332338%
1536
January1st to 15thNight
19%18%8%13%10%4322213%
2506
16th to endNight
11%16%10%13%11%6%5%33412%
2688
February1st to 15thNight
10%16%8%12%11%5%5%34317%
2248
16th to endNight
15%14%7%12%8%45%35%5%18%
1893
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
13%5%24313346%51%
1797
16th to endNight
11%5%2443445%7%47%
1815
October1st to 15thNight
19%7%24424346%38%
1862
16th to endNight
23%9%36%5%34436%28%
2288
November1st to 15thNight
16%13%47%5%2345%5%31%
2276
16th to endNight
20%18%6%9%6%3443417%
2358
December1st to 15thNight
19%20%11%12%7%3434310%
2520
16th to endNight
19%21%9%10%8%2344411%
2688

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%10%13%9%5%433310%
1432
16th to endDay
13%19%14%13%11%5%5%3227%
1536
February1st to 15thDay
12%16%10%12%13%45%34413%
1612
16th to endDay
11%17%9%12%10%45%43416%
1580
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%5%25%43435%7%42%
2039
16th to endDay
13%5%36%5%3446%7%38%
1815
October1st to 15thDay
19%8%36%5%24345%35%
1766
16th to endDay
25%9%48%6%34445%23%
1584
November1st to 15thDay
16%11%46%6%2445%6%31%
1582
16th to endDay
21%21%7%9%5%2343415%
1599
December1st to 15thDay
18%18%12%15%8%2412211%
1440
16th to endDay
19%22%9%12%8%433238%
1536
January1st to 15thNight
18%18%7%13%9%5%422213%
2506
16th to endNight
11%16%10%13%10%5%6%435%12%
2688
February1st to 15thNight
11%15%8%11%10%5%6%33417%
2241
16th to endNight
15%14%6%11%8%35%446%18%
1893
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
13%425%323345%52%
1797
16th to endNight
12%425%43445%7%47%
1815
October1st to 15thNight
19%6%245%23346%38%
1862
16th to endNight
23%8%46%5%34436%28%
2288
November1st to 15thNight
15%12%47%5%3445%5%31%
2276
16th to endNight
20%18%6%7%7%4443417%
2358
December1st to 15thNight
19%19%10%13%7%3432311%
2520
16th to endNight
19%21%8%11%7%3434411%
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
80%46%6%1
1433
16th to endDay
83%4442
1536
February1st to 15thDay
74%47%11%1
1614
16th to endDay
71%5%9%10%3
1580
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
40%5%11%28%12%1
2039
16th to endDay
43%414%25%11%
1815
October1st to 15thDay
51%410%23%10%
1766
16th to endDay
61%6%14%13%4
1584
November1st to 15thDay
52%5%12%20%8%
1582
16th to endDay
72%410%8%2
1599
December1st to 15thDay
81%36%7%1
1440
16th to endDay
81%38%31
1536
January1st to 15thNight
79%35%8%2
2506
16th to endNight
74%410%7%2
2688
February1st to 15thNight
70%47%12%3
2250
16th to endNight
66%5%11%12%5%
1893
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
34%412%30%17%1
1797
16th to endNight
36%414%31%11%
1815
October1st to 15thNight
45%5%9%26%13%
1862
16th to endNight
55%6%14%17%5%
2288
November1st to 15thNight
53%5%11%18%9%1
2276
16th to endNight
69%5%12%9%2
2358
December1st to 15thNight
78%48%6%2
2520
16th to endNight
76%410%7%1
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
20%26%35%17%
90
16th to endDay
34%13%34%13%31
96
February1st to 15thDay
12%45%31%6%4
90
16th to endDay
22%32%30%12%11
81
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
14%6%25%24%19%9%
294
16th to endDay
14%11%21%27%20%4
254
October1st to 15thDay
21%10%27%25%11%2
230
16th to endDay
27%20%45%6%
96
November1st to 15thDay
15%18%42%14%8%
90
16th to endDay
33%25%26%13%1
90
December1st to 15thDay
34%36%21%5%2
90
16th to endDay
37%44%14%3
96
January1st to 15thNight
26%21%29%17%4
900
16th to endNight
21%16%36%21%4
960
February1st to 15thNight
17%21%32%22%5%
761
16th to endNight
24%16%31%23%4
631
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
15%7%22%32%18%4
572
16th to endNight
13%9%22%31%21%2
657
October1st to 15thNight
23%8%24%31%11%1
660
16th to endNight
29%14%26%22%6%
704
November1st to 15thNight
24%15%30%21%7%1
700
16th to endNight
30%16%25%21%5%
738
December1st to 15thNight
30%21%29%15%2
900
16th to endNight
28%16%31%19%2
960
Page generated at 2017-09-11 19:35:30. Version ac04

Copyright 2017 A. Danko


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