VU Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the VU 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
36%
25410
15%
95682
11%
25210
42%
25210
17%
64128

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
18%
940
8%
4416
4%
926
26%
926
8%
3454
January11th to 20thNight
16%
950
7%
4412
3%
930
21%
930
6%
3630
January21st to endNight
19%
1041
6%
4849
3%
1023
25%
1023
7%
3904
February1st to 10thNight
19%
826
12%
2346
4%
826
25%
826
16%
1754
February11th to 20thNight
24%
740
17%
2219
7%
740
34%
740
18%
1543
February21st to endNight
18%
628
15%
1884
4%
628
28%
628
15%
1379
March1st to 10thNight
24%
758
17%
3688
7%
745
29%
745
11%
2354
March11th to 20thNight
27%
754
22%
3622
9%
740
37%
740
18%
1966
March21st to endNight
27%
678
9%
3497
9%
661
34%
661
10%
2493
April1st to 10thNight
26%
570
16%
1900
9%
570
37%
570
21%
1264
April11th to 20thNight
30%
563
19%
1875
12%
563
40%
563
20%
1254
April21st to endNight
30%
570
15%
1749
7%
570
38%
570
20%
1214
May1st to 10thNight
34%
566
18%
2779
8%
559
40%
559
14%
1775
May11th to 20thNight
41%
548
12%
2704
10%
541
47%
541
16%
1617
May21st to endNight
51%
605
14%
2706
10%
593
56%
593
19%
1572
June1st to 10thNight
56%
570
17%
1520
12%
570
61%
570
37%
928
June11th to 20thNight
50%
570
19%
1520
12%
570
55%
570
35%
956
June21st to endNight
52%
570
16%
1520
10%
570
56%
570
30%
1005
July1st to 10thNight
66%
543
16%
2363
14%
535
69%
535
24%
1402
July11th to 20thNight
62%
539
19%
2228
18%
533
66%
533
29%
1183
July21st to endNight
69%
605
16%
2475
17%
569
72%
569
25%
1422
August1st to 10thNight
73%
550
21%
2534
21%
544
75%
544
24%
1433
August11th to 20thNight
66%
550
17%
2617
20%
543
69%
543
23%
1427
August21st to endNight
66%
605
18%
3066
22%
605
71%
605
23%
1684
September1st to 10thNight
60%
570
26%
1900
25%
570
65%
570
32%
1084
September11th to 20thNight
63%
721
31%
2051
26%
721
68%
721
40%
1172
September21st to endNight
52%
760
24%
2090
17%
760
56%
760
32%
1324
October1st to 10thNight
51%
760
26%
2090
20%
760
56%
760
34%
1302
October11th to 20thNight
33%
760
19%
2306
10%
760
43%
760
23%
1490
October21st to endNight
28%
836
18%
2717
12%
836
37%
836
19%
1841
November1st to 10thNight
30%
743
22%
2413
14%
743
41%
743
23%
1513
November11th to 20thNight
24%
760
17%
2470
8%
760
36%
760
17%
1693
November21st to endNight
25%
760
17%
2470
10%
760
34%
760
17%
1731
December1st to 10thNight
18%
912
10%
2622
3%
912
25%
912
15%
1947
December11th to 20thNight
22%
950
11%
3219
5%
948
28%
948
13%
2405
December21st to endNight
13%
1039
5%
4845
3%
1026
18%
1026
5%
4013

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
13%9%38%5%4446%8%36%
101166
Night
15%10%48%5%4445%6%36%
97838
TransparencyDay
46%6%12%21%13%2
100957
Night
48%6%12%21%13%2
97719
SeeingDay
16%11%29%25%14%5%
15744
Night
18%10%30%30%12%1
25572

By Month

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 10thDay
15%15%11%12%6%5%445%6%17%
2527
11th to 20thDay
14%14%7%14%8%6%5%6%8%414%
2530
21st to endDay
22%24%6%10%434335%15%
2783
February1st to 10thDay
15%15%411%6%6%4346%24%
1833
11th to 20thDay
14%11%5%9%6%445%6%6%31%
1848
21st to endDay
16%14%5%10%45%5%36%5%27%
1570
March1st to 10thDay
18%15%5%8%44435%6%30%
3120
11th to 20thDay
11%8%47%5%45%46%8%38%
3283
21st to endDay
19%11%5%11%6%5%5%45%7%23%
4100
April1st to 10thDay
16%12%38%5%4446%7%32%
2280
11th to 20thDay
17%11%37%44335%7%36%
2254
21st to endDay
19%9%37%44445%6%34%
2429
May1st to 10thDay
18%7%27%45%5%46%9%34%
4082
11th to 20thDay
14%7%36%44446%8%41%
4131
21st to endDay
9%6%26%44446%9%45%
4824
June1st to 10thDay
10%5%26%45%5%46%8%44%
2660
11th to 20thDay
9%426%5%45%47%8%45%
2660
21st to endDay
10%5%27%6%45%5%6%10%39%
2660
July1st to 10thDay
12%6%27%5%5%5%5%7%10%35%
4440
11th to 20thDay
7%425%445%47%11%47%
4440
21st to endDay
8%6%37%5%5%5%5%7%8%42%
4813
August1st to 10thDay
7%5%26%5%4446%8%48%
4199
11th to 20thDay
9%5%26%445%46%8%48%
3853
21st to endDay
7%426%44446%9%49%
4180
September1st to 10thDay
10%5%26%45%446%8%46%
2280
11th to 20thDay
10%6%26%34447%8%47%
2109
21st to endDay
10%7%27%44446%9%41%
2090
October1st to 10thDay
12%6%36%44445%7%46%
2090
11th to 20thDay
18%7%37%5%4446%6%36%
1870
21st to endDay
20%9%38%5%5%5%36%7%29%
1881
November1st to 10thDay
12%8%36%5%345%6%8%42%
1672
11th to 20thDay
18%11%37%5%5%446%7%31%
1710
21st to endDay
18%12%39%5%4425%5%32%
1710
December1st to 10thDay
16%16%410%5%5%45%6%6%24%
1554
11th to 20thDay
13%16%7%11%6%5%5%35%6%24%
1918
21st to endDay
22%22%7%14%5%4334412%
2783
January1st to 10thNight
17%16%8%12%5%4434422%
4428
11th to 20thNight
15%19%7%14%5%45%5%6%415%
4440
21st to endNight
24%17%7%11%5%4334419%
4883
February1st to 10thNight
16%14%5%9%6%5%335%6%28%
2346
11th to 20thNight
15%11%47%6%5%445%7%34%
2218
21st to endNight
20%13%48%44435%431%
1884
March1st to 10thNight
17%11%47%44435%6%34%
3733
11th to 20thNight
11%6%26%5%4445%6%46%
3676
21st to endNight
19%10%5%9%5%45%45%6%28%
3548
April1st to 10thNight
14%9%37%5%45%45%7%37%
1900
11th to 20thNight
19%9%27%5%44345%39%
1875
21st to endNight
17%8%38%5%5%4445%37%
1749
May1st to 10thNight
16%8%37%5%5%435%9%35%
2813
11th to 20thNight
18%7%27%33346%7%40%
2761
21st to endNight
12%6%27%5%5%435%6%44%
2743
June1st to 10thNight
10%5%16%33437%10%48%
1520
11th to 20thNight
8%5%26%5%5%5%47%9%45%
1520
21st to endNight
10%5%26%5%5%5%5%6%8%41%
1520
July1st to 10thNight
10%426%5%45%47%9%44%
2530
11th to 20thNight
7%415%44445%7%55%
2530
21st to endNight
5%5%26%5%5%6%5%7%9%45%
2750
August1st to 10thNight
7%5%26%5%5%5%47%10%45%
2731
11th to 20thNight
8%5%25%44435%7%52%
2972
21st to endNight
9%435%44445%8%51%
3476
September1st to 10thNight
10%5%35%343446%52%
1900
11th to 20thNight
8%415%34436%8%53%
2051
21st to endNight
10%7%26%445%35%9%45%
2090
October1st to 10thNight
11%5%36%34436%6%51%
2090
11th to 20thNight
17%9%36%334346%43%
2306
21st to endNight
21%9%26%44345%6%36%
2717
November1st to 10thNight
12%9%37%43446%6%41%
2413
11th to 20thNight
15%10%39%445%45%6%34%
2470
21st to endNight
19%10%47%5%4446%6%32%
2470
December1st to 10thNight
16%14%48%5%4445%7%27%
2622
11th to 20thNight
16%14%410%6%5%5%45%6%25%
3279
21st to endNight
24%19%6%10%6%5%4345%15%
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
66%6%11%12%4
2528
11th to 20thDay
70%49%11%6%
2530
21st to endDay
72%5%8%12%3
2783
February1st to 10thDay
61%5%12%15%8%1
1833
11th to 20thDay
52%6%11%22%9%1
1858
21st to endDay
57%6%11%18%6%2
1570
March1st to 10thDay
55%411%19%9%1
3120
11th to 20thDay
43%6%12%28%10%1
3283
21st to endDay
60%9%14%14%3
4100
April1st to 10thDay
50%5%12%20%13%
2280
11th to 20thDay
47%6%10%22%13%3
2254
21st to endDay
48%7%13%18%12%2
2429
May1st to 10thDay
45%8%12%24%9%1
4120
11th to 20thDay
40%8%13%27%11%1
4135
21st to endDay
34%7%12%22%23%2
4837
June1st to 10thDay
36%8%12%25%18%3
2660
11th to 20thDay
34%8%11%24%20%3
2660
21st to endDay
39%9%14%19%16%2
2660
July1st to 10thDay
44%5%13%21%14%2
4371
11th to 20thDay
31%5%16%29%17%1
4365
21st to endDay
39%6%16%17%19%3
4662
August1st to 10thDay
34%5%15%28%15%1
4184
11th to 20thDay
34%6%12%31%16%1
3892
21st to endDay
32%5%14%26%20%4
4179
September1st to 10thDay
35%7%14%24%17%4
2280
11th to 20thDay
32%7%12%26%20%3
2109
21st to endDay
38%6%12%24%19%2
2090
October1st to 10thDay
38%6%12%24%18%2
2090
11th to 20thDay
47%5%11%22%14%1
1870
21st to endDay
53%6%12%19%9%1
1881
November1st to 10thDay
39%7%15%22%15%2
1672
11th to 20thDay
51%5%11%22%9%1
1710
21st to endDay
55%411%21%8%
1710
December1st to 10thDay
59%6%13%16%6%
1554
11th to 20thDay
60%412%19%4
1915
21st to endDay
76%5%9%7%2
2783
January1st to 10thNight
64%6%11%14%5%
4428
11th to 20thNight
71%5%11%10%4
4440
21st to endNight
69%410%13%5%
4883
February1st to 10thNight
57%412%17%8%2
2346
11th to 20thNight
49%5%13%21%11%1
2219
21st to endNight
55%5%10%18%10%2
1884
March1st to 10thNight
49%6%12%22%10%1
3733
11th to 20thNight
37%6%11%27%17%2
3676
21st to endNight
54%9%12%20%5%
3548
April1st to 10thNight
45%7%13%21%13%1
1900
11th to 20thNight
47%5%7%20%17%3
1875
21st to endNight
48%6%10%19%14%3
1749
May1st to 10thNight
47%7%11%22%12%2
2840
11th to 20thNight
41%8%14%26%11%
2762
21st to endNight
39%7%10%18%22%4
2751
June1st to 10thNight
31%7%14%26%20%2
1520
11th to 20thNight
34%6%12%23%22%2
1520
21st to endNight
39%8%15%22%14%3
1520
July1st to 10thNight
38%5%15%24%13%5%
2490
11th to 20thNight
29%413%24%27%2
2482
21st to endNight
34%6%17%19%19%5%
2662
August1st to 10thNight
34%6%15%29%15%1
2722
11th to 20thNight
31%5%13%28%21%1
2999
21st to endNight
33%5%11%29%19%3
3476
September1st to 10thNight
32%6%11%24%21%6%
1900
11th to 20thNight
28%6%8%28%27%3
2051
21st to endNight
37%6%10%26%18%3
2090
October1st to 10thNight
34%6%10%27%22%1
2090
11th to 20thNight
43%5%9%24%18%2
2306
21st to endNight
48%5%10%23%12%1
2717
November1st to 10thNight
41%5%11%23%17%2
2413
11th to 20thNight
49%6%12%22%10%1
2470
21st to endNight
52%5%12%23%8%
2470
December1st to 10thNight
55%6%14%18%7%1
2622
11th to 20thNight
57%5%14%18%6%1
3281
21st to endNight
72%5%10%10%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
23%14%33%24%6%1
270
11th to 20thDay
23%19%36%15%5%1
270
21st to endDay
24%18%33%20%5%
297
February1st to 10thDay
20%20%37%18%5%
270
11th to 20thDay
23%18%35%20%4
270
21st to endDay
24%19%40%14%4
225
March1st to 10thDay
23%21%36%17%3
270
11th to 20thDay
21%18%35%19%7%
341
21st to endDay
27%16%30%20%6%1
506
April1st to 10thDay
24%13%32%22%7%1
460
11th to 20thDay
21%17%32%16%11%2
452
21st to endDay
24%14%32%20%8%2
460
May1st to 10thDay
22%13%30%22%10%3
463
11th to 20thDay
21%8%30%23%15%3
511
21st to endDay
13%9%29%26%16%7%
734
June1st to 10thDay
12%8%28%29%19%5%
680
11th to 20thDay
11%6%32%29%19%4
680
21st to endDay
12%8%31%31%13%5%
680
July1st to 10thDay
9%5%21%31%20%13%
671
11th to 20thDay
10%5%24%30%21%10%
668
21st to endDay
9%324%34%22%8%
739
August1st to 10thDay
9%318%30%26%13%
543
11th to 20thDay
11%421%29%25%11%
479
21st to endDay
11%315%34%28%9%
539
September1st to 10thDay
11%6%20%33%23%7%
490
11th to 20thDay
10%416%28%27%14%
490
21st to endDay
14%10%24%24%19%9%
490
October1st to 10thDay
13%8%29%30%16%5%
490
11th to 20thDay
18%21%26%25%10%1
329
21st to endDay
29%14%37%16%41
330
November1st to 10thDay
15%12%38%23%9%3
270
11th to 20thDay
21%19%34%17%7%1
270
21st to endDay
27%11%39%20%3
270
December1st to 10thDay
18%20%37%18%6%
270
11th to 20thDay
19%19%40%19%41
270
21st to endDay
26%22%36%14%2
297
January1st to 10thNight
22%15%36%22%5%
950
11th to 20thNight
23%15%37%21%41
950
21st to endNight
22%14%34%25%4
1045
February1st to 10thNight
23%13%38%23%3
826
11th to 20thNight
21%14%35%25%4
743
21st to endNight
26%15%36%19%2
628
March1st to 10thNight
21%13%37%23%6%
760
11th to 20thNight
17%12%40%25%6%
760
21st to endNight
23%11%33%26%6%
683
April1st to 10thNight
17%13%39%24%6%1
570
11th to 20thNight
23%8%35%26%7%1
563
21st to endNight
20%11%33%28%8%1
570
May1st to 10thNight
20%7%34%29%9%1
570
11th to 20thNight
19%5%27%35%14%1
561
21st to endNight
12%423%40%20%2
617
June1st to 10thNight
10%422%45%18%
570
11th to 20thNight
11%328%40%17%1
570
21st to endNight
11%325%41%18%1
570
July1st to 10thNight
8%214%45%25%5%
555
11th to 20thNight
8%319%38%28%4
552
21st to endNight
5%216%46%30%1
624
August1st to 10thNight
7%29%49%30%4
567
11th to 20thNight
9%113%41%31%5%
564
21st to endNight
8%217%38%32%3
627
September1st to 10thNight
12%417%37%27%4
570
11th to 20thNight
9%319%37%29%4
721
21st to endNight
13%6%24%32%22%3
760
October1st to 10thNight
14%5%25%36%19%1
760
11th to 20thNight
22%13%28%27%9%
760
21st to endNight
25%11%31%25%7%
836
November1st to 10thNight
18%12%37%22%10%1
743
11th to 20thNight
20%18%33%24%6%
760
21st to endNight
23%14%34%23%6%
760
December1st to 10thNight
24%15%35%22%3
912
11th to 20thNight
23%12%35%25%4
950
21st to endNight
30%14%33%20%3
1045
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