Averted Vision Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Averted Vision Observatory clear sky chart from 2005-11-06 to 2024-11-01.

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%
23699
12%
96067
8%
23513
38%
23513
14%
67145

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
21%
752
11%
4390
6%
737
30%
737
7%
3035
January11th to 20thNight
20%
757
10%
4406
4%
740
27%
740
6%
3281
January21st to endNight
20%
830
7%
4807
7%
814
29%
814
7%
3673
February1st to 10thNight
16%
760
10%
2470
3%
760
24%
760
11%
1890
February11th to 20thNight
15%
743
12%
2250
4%
740
27%
740
12%
1672
February21st to endNight
19%
628
13%
1884
6%
628
29%
628
14%
1371
March1st to 10thNight
20%
756
15%
3715
8%
744
31%
744
9%
2729
March11th to 20thNight
22%
755
13%
3662
7%
740
31%
740
9%
2519
March21st to endNight
23%
832
12%
3388
6%
814
32%
814
12%
2424
April1st to 10thNight
21%
760
15%
1900
4%
760
27%
760
20%
1407
April11th to 20thNight
25%
751
13%
1875
5%
751
29%
751
20%
1395
April21st to endNight
27%
656
15%
1796
5%
656
33%
656
19%
1286
May1st to 10thNight
27%
531
12%
2732
8%
527
32%
527
9%
1916
May11th to 20thNight
29%
370
10%
2472
12%
364
41%
364
9%
1723
May21st to endNight
45%
405
9%
2727
14%
399
50%
399
11%
1584
June1st to 10thNight
44%
380
14%
1520
15%
380
52%
380
18%
984
June11th to 20thNight
44%
380
12%
1520
15%
380
50%
380
20%
938
June21st to endNight
46%
380
12%
1520
14%
380
52%
380
19%
962
July1st to 10thNight
53%
364
12%
2382
16%
360
58%
360
12%
1468
July11th to 20thNight
53%
364
11%
2256
19%
358
59%
358
13%
1460
July21st to endNight
54%
410
11%
2419
17%
383
61%
383
13%
1547
August1st to 10thNight
56%
375
12%
2397
17%
365
62%
365
13%
1607
August11th to 20thNight
50%
694
11%
2899
9%
684
53%
684
16%
1954
August21st to endNight
53%
819
14%
3233
9%
814
56%
814
22%
1991
September1st to 10thNight
51%
760
18%
1900
9%
760
54%
760
32%
1284
September11th to 20thNight
51%
760
22%
1990
15%
760
54%
760
34%
1242
September21st to endNight
44%
760
21%
2090
15%
760
49%
760
27%
1396
October1st to 10thNight
35%
760
15%
2190
10%
760
41%
760
18%
1579
October11th to 20thNight
37%
760
14%
2285
10%
760
43%
760
21%
1554
October21st to endNight
27%
836
13%
2717
7%
836
35%
836
15%
1984
November1st to 10thNight
30%
743
17%
2414
11%
743
39%
743
19%
1627
November11th to 20thNight
28%
760
18%
2470
11%
760
37%
760
18%
1687
November21st to endNight
27%
760
13%
2628
6%
760
36%
760
16%
1828
December1st to 10thNight
20%
760
9%
2660
5%
760
26%
760
11%
2023
December11th to 20thNight
24%
759
10%
3253
5%
759
31%
759
9%
2382
December21st to endNight
17%
829
6%
4850
3%
817
25%
817
6%
3743

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
16%8%37%5%5%5%46%7%34%
101369
Night
17%9%38%5%4445%6%35%
97581
TransparencyDay
47%9%13%19%10%1
101125
Night
50%9%13%18%10%1
97445
SeeingDay
21%16%29%22%10%3
14781
Night
20%12%30%28%9%1
23793

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 10thDay
13%11%5%10%6%5%5%46%5%30%
2517
11th to 20thDay
16%13%6%11%7%5%5%45%5%23%
2520
21st to endDay
16%13%5%10%6%6%446%5%24%
2833
February1st to 10thDay
13%13%49%6%5%5%5%6%6%29%
1710
11th to 20thDay
12%13%5%11%6%6%5%45%5%29%
1816
21st to endDay
19%12%47%6%45%35%432%
1570
March1st to 10thDay
23%13%49%5%44335%28%
3116
11th to 20thDay
13%10%48%6%6%445%7%33%
3281
21st to endDay
18%9%38%5%44345%37%
4164
April1st to 10thDay
19%9%38%5%34345%36%
2280
11th to 20thDay
16%7%28%6%5%5%35%6%37%
2254
21st to endDay
20%8%37%5%5%5%45%5%33%
2382
May1st to 10thDay
23%9%37%44435%6%30%
4200
11th to 20thDay
16%8%38%5%5%5%46%7%33%
4398
21st to endDay
12%6%25%5%5%5%46%8%43%
4837
June1st to 10thDay
15%7%37%6%5%6%46%9%33%
2660
11th to 20thDay
17%5%26%45%6%5%6%9%35%
2660
21st to endDay
16%6%27%5%5%5%5%7%8%33%
2660
July1st to 10thDay
10%5%26%5%5%5%5%8%10%38%
4440
11th to 20thDay
9%6%26%5%5%6%6%10%11%35%
4440
21st to endDay
13%6%27%5%5%6%47%8%36%
4814
August1st to 10thDay
15%5%27%5%5%6%5%7%9%35%
4423
11th to 20thDay
14%6%37%5%5%6%5%7%10%33%
3764
21st to endDay
11%6%27%5%46%5%7%8%38%
4180
September1st to 10thDay
12%5%25%45%446%7%45%
2280
11th to 20thDay
14%5%2434435%7%49%
2190
21st to endDay
16%6%24445%46%7%41%
2090
October1st to 10thDay
19%8%38%6%45%45%5%34%
1986
11th to 20thDay
15%6%37%5%4445%7%40%
1895
21st to endDay
23%7%37%445%45%7%30%
1881
November1st to 10thDay
13%8%37%5%4435%7%41%
1671
11th to 20thDay
17%8%37%5%5%5%36%6%36%
1710
21st to endDay
18%9%36%6%5%5%36%7%33%
1549
December1st to 10thDay
20%13%5%9%5%5%435%5%26%
1520
11th to 20thDay
22%10%37%5%4335%5%32%
1906
21st to endDay
25%17%5%10%443245%22%
2772
January1st to 10thNight
16%11%49%5%44346%34%
4429
11th to 20thNight
13%11%5%13%7%6%6%35%6%25%
4440
21st to endNight
22%13%49%5%45%34427%
4820
February1st to 10thNight
13%12%49%7%6%5%46%7%27%
2470
11th to 20thNight
11%12%410%6%5%446%5%34%
2248
21st to endNight
17%9%39%5%5%45%6%5%32%
1884
March1st to 10thNight
25%10%38%44435%5%28%
3724
11th to 20thNight
15%12%37%5%5%5%45%6%34%
3676
21st to endNight
17%14%39%44424435%
3466
April1st to 10thNight
20%8%37%5%4435%5%36%
1900
11th to 20thNight
19%8%47%5%5%335%6%36%
1875
21st to endNight
21%8%27%44335%5%39%
1796
May1st to 10thNight
24%9%38%44335%7%31%
2756
11th to 20thNight
18%9%39%5%5%5%35%6%32%
2495
21st to endNight
13%5%36%46%5%46%7%40%
2738
June1st to 10thNight
18%5%15%5%5%6%5%7%7%37%
1520
11th to 20thNight
18%6%25%43445%8%40%
1520
21st to endNight
15%5%26%5%445%6%8%40%
1520
July1st to 10thNight
11%6%27%6%6%5%47%8%38%
2520
11th to 20thNight
15%6%26%5%45%46%8%40%
2520
21st to endNight
13%7%26%5%45%47%7%39%
2739
August1st to 10thNight
19%7%27%44435%8%36%
2506
11th to 20thNight
16%6%37%445%47%7%37%
3027
21st to endNight
11%5%25%445%5%8%10%41%
3476
September1st to 10thNight
13%6%25%44446%7%45%
1900
11th to 20thNight
13%5%25%34325%6%51%
1990
21st to endNight
15%5%25%5%45%45%6%44%
2090
October1st to 10thNight
20%6%39%6%4435%5%35%
2190
11th to 20thNight
16%6%27%6%45%45%6%40%
2285
21st to endNight
23%7%38%44445%6%32%
2717
November1st to 10thNight
13%9%37%5%4435%5%41%
2414
11th to 20thNight
19%7%37%44435%5%39%
2470
21st to endNight
16%8%37%5%5%4447%37%
2628
December1st to 10thNight
19%12%38%5%5%435%5%29%
2660
11th to 20thNight
23%9%37%5%5%5%45%6%29%
3288
21st to endNight
26%14%5%8%5%33345%23%
4884

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
51%12%14%18%5%
2518
11th to 20thDay
60%10%12%12%5%1
2520
21st to endDay
61%10%11%12%5%1
2833
February1st to 10thDay
57%9%12%15%6%1
1710
11th to 20thDay
58%7%11%15%9%1
1825
21st to endDay
55%7%11%14%11%1
1570
March1st to 10thDay
58%8%11%14%8%1
3119
11th to 20thDay
48%12%17%13%9%2
3281
21st to endDay
50%8%7%15%17%3
4164
April1st to 10thDay
51%7%8%20%11%3
2280
11th to 20thDay
47%7%11%19%14%2
2254
21st to endDay
51%8%8%16%12%3
2382
May1st to 10thDay
53%9%11%18%9%1
4214
11th to 20thDay
45%12%15%17%9%1
4402
21st to endDay
36%11%14%25%13%1
4837
June1st to 10thDay
46%11%15%17%10%1
2660
11th to 20thDay
44%9%13%19%13%1
2660
21st to endDay
48%9%14%19%10%
2660
July1st to 10thDay
38%9%15%24%14%1
4370
11th to 20thDay
37%10%18%26%9%
4365
21st to endDay
43%8%16%21%11%
4661
August1st to 10thDay
45%9%16%20%10%
4409
11th to 20thDay
43%10%17%25%5%
3801
21st to endDay
38%8%17%26%10%1
4180
September1st to 10thDay
37%8%13%29%13%1
2280
11th to 20thDay
35%7%12%26%18%1
2190
21st to endDay
40%8%14%24%13%1
2090
October1st to 10thDay
51%8%10%18%12%1
1986
11th to 20thDay
42%9%16%23%8%1
1895
21st to endDay
54%8%13%15%10%1
1881
November1st to 10thDay
41%8%16%23%11%1
1671
11th to 20thDay
48%8%13%16%13%1
1710
21st to endDay
51%8%17%17%6%1
1549
December1st to 10thDay
61%8%12%13%6%1
1520
11th to 20thDay
56%7%14%17%5%1
1906
21st to endDay
66%7%9%13%6%
2772
January1st to 10thNight
51%11%16%14%8%1
4429
11th to 20thNight
59%10%12%13%6%1
4440
21st to endNight
61%8%11%13%6%1
4820
February1st to 10thNight
58%8%12%15%6%1
2470
11th to 20thNight
53%9%10%17%9%1
2250
21st to endNight
52%8%13%18%8%2
1884
March1st to 10thNight
54%11%9%17%8%1
3734
11th to 20thNight
50%11%13%18%7%1
3677
21st to endNight
53%6%7%13%18%3
3466
April1st to 10thNight
51%6%12%19%11%1
1900
11th to 20thNight
49%8%9%20%12%1
1875
21st to endNight
48%7%10%18%14%4
1796
May1st to 10thNight
53%9%13%14%10%1
2762
11th to 20thNight
51%11%12%16%8%1
2496
21st to endNight
40%11%13%19%15%2
2739
June1st to 10thNight
44%8%15%21%11%1
1520
11th to 20thNight
40%9%16%22%12%1
1520
21st to endNight
41%10%14%22%12%1
1520
July1st to 10thNight
42%9%14%19%14%1
2480
11th to 20thNight
43%9%14%22%13%
2472
21st to endNight
42%8%12%27%10%1
2651
August1st to 10thNight
47%9%15%20%8%
2496
11th to 20thNight
44%10%18%19%9%
3056
21st to endNight
37%7%17%28%11%
3476
September1st to 10thNight
37%8%13%27%14%1
1900
11th to 20thNight
33%7%13%28%18%1
1990
21st to endNight
39%9%12%23%16%1
2090
October1st to 10thNight
50%7%10%20%11%1
2190
11th to 20thNight
44%8%15%22%10%1
2285
21st to endNight
53%8%13%17%9%1
2717
November1st to 10thNight
45%8%11%20%13%2
2414
11th to 20thNight
47%7%13%20%11%2
2470
21st to endNight
49%8%17%19%7%
2628
December1st to 10thNight
58%9%13%15%5%
2660
11th to 20thNight
56%9%14%13%8%1
3288
21st to endNight
64%7%11%15%3
4884

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
26%19%29%17%8%1
180
11th to 20thDay
29%27%29%12%21
180
21st to endDay
20%21%35%18%6%1
215
February1st to 10thDay
27%30%32%10%
270
11th to 20thDay
26%26%34%12%2
424
21st to endDay
28%19%34%16%3
382
March1st to 10thDay
26%17%34%20%3
460
11th to 20thDay
25%23%32%15%5%1
460
21st to endDay
30%21%26%14%8%1
506
April1st to 10thDay
30%20%26%16%6%1
460
11th to 20thDay
28%25%24%14%7%2
452
21st to endDay
27%18%31%16%6%2
460
May1st to 10thDay
31%16%28%14%8%2
463
11th to 20thDay
17%16%34%24%7%2
487
21st to endDay
15%14%31%27%11%3
529
June1st to 10thDay
16%17%31%23%10%2
490
11th to 20thDay
18%12%33%26%10%2
490
21st to endDay
18%15%32%24%9%2
490
July1st to 10thDay
13%9%29%30%16%3
485
11th to 20thDay
11%8%31%31%16%3
484
21st to endDay
14%7%36%30%12%2
532
August1st to 10thDay
11%9%27%30%17%5%
486
11th to 20thDay
15%6%28%26%19%5%
479
21st to endDay
11%11%23%33%15%7%
539
September1st to 10thDay
14%11%23%28%16%9%
490
11th to 20thDay
14%10%25%21%18%12%
490
21st to endDay
18%9%27%26%14%7%
490
October1st to 10thDay
24%13%25%21%12%6%
490
11th to 20thDay
17%18%29%23%10%2
487
21st to endDay
29%14%26%24%6%1
429
November1st to 10thDay
21%15%26%24%13%
365
11th to 20thDay
26%16%31%20%5%1
370
21st to endDay
25%21%32%19%4
209
December1st to 10thDay
24%23%31%17%41
180
11th to 20thDay
33%19%33%13%2
180
21st to endDay
26%24%30%17%3
198
January1st to 10thNight
21%16%35%26%3
760
11th to 20thNight
25%17%31%24%2
760
21st to endNight
18%15%38%26%3
836
February1st to 10thNight
20%16%40%22%2
760
11th to 20thNight
19%20%40%20%2
743
21st to endNight
23%20%33%20%3
628
March1st to 10thNight
23%16%37%22%3
760
11th to 20thNight
24%16%33%22%4
760
21st to endNight
22%17%33%22%7%
836
April1st to 10thNight
25%16%34%23%3
760
11th to 20thNight
23%18%31%21%6%
751
21st to endNight
25%16%28%25%6%
656
May1st to 10thNight
26%11%28%25%9%
531
11th to 20thNight
19%10%37%28%6%1
374
21st to endNight
13%11%26%37%12%1
411
June1st to 10thNight
20%5%26%37%12%1
380
11th to 20thNight
19%6%27%38%11%
380
21st to endNight
13%430%42%11%
380
July1st to 10thNight
12%5%22%44%16%1
370
11th to 20thNight
14%420%43%17%1
368
21st to endNight
13%424%44%16%1
416
August1st to 10thNight
12%222%46%17%1
378
11th to 20thNight
14%423%36%20%3
704
21st to endNight
14%6%19%39%21%1
836
September1st to 10thNight
15%423%37%18%3
760
11th to 20thNight
15%7%23%29%20%5%
760
21st to endNight
16%7%26%32%17%2
760
October1st to 10thNight
24%11%23%27%15%1
760
11th to 20thNight
18%11%27%34%10%1
760
21st to endNight
27%10%30%27%6%
836
November1st to 10thNight
17%13%33%29%8%
743
11th to 20thNight
21%14%32%24%7%1
760
21st to endNight
20%14%35%25%6%1
760
December1st to 10thNight
26%15%33%22%4
760
11th to 20thNight
26%14%31%24%4
760
21st to endNight
28%14%35%20%3
836
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