VU Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the VU Observatory clear sky chart from 2005-11-05 to

2017-09-12
.

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
38%
15912
16%
46770
11%
15870
43%
15870
22%
31925

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
18%
900
11%
2506
4%
895
23%
895
14%
1911
January16th to endNight
21%
960
11%
2688
3%
960
27%
960
15%
2067
February1st to 15thNight
22%
743
11%
2154
3%
743
29%
743
17%
1553
February16th to endNight
24%
631
16%
1893
7%
631
33%
631
16%
1381
March1st to 15thNight
26%
720
13%
2160
8%
720
31%
720
17%
1569
March16th to endNight
30%
671
16%
2029
9%
671
38%
671
19%
1451
April1st to 15thNight
30%
539
14%
1797
10%
539
38%
539
20%
1224
April16th to endNight
33%
540
19%
1704
11%
540
42%
540
21%
1150
May1st to 15thNight
37%
540
15%
1620
9%
540
42%
540
21%
1128
May16th to endNight
51%
562
17%
1537
11%
561
56%
561
30%
998
June1st to 15thNight
55%
540
16%
1440
11%
540
58%
540
36%
891
June16th to endNight
54%
540
16%
1440
10%
540
58%
540
31%
935
July1st to 15thNight
65%
510
17%
1360
13%
510
67%
510
39%
834
July16th to endNight
71%
564
21%
1417
16%
531
72%
531
41%
835
August1st to 15thNight
74%
539
21%
1583
21%
536
75%
536
38%
903
August16th to endNight
67%
573
21%
1910
21%
573
71%
573
32%
1102
September1st to 15thNight
63%
563
27%
1800
26%
563
66%
563
34%
1028
September16th to endNight
59%
657
27%
1815
20%
657
64%
657
35%
1093
October1st to 15thNight
51%
660
26%
1851
20%
660
56%
660
32%
1155
October16th to endNight
29%
704
17%
2266
11%
704
37%
704
19%
1511
November1st to 15thNight
32%
700
21%
2276
14%
700
42%
700
22%
1445
November16th to endNight
26%
720
15%
2340
10%
720
34%
720
16%
1641
December1st to 15thNight
19%
876
10%
2496
3%
876
25%
876
14%
1902
December16th to endNight
16%
960
9%
2688
2%
960
22%
960
10%
2218

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
14%8%36%5%3445%7%35%
47897
Night
15%8%35%5%24446%37%
46938
Cloud CoverDay
13%8%36%5%3445%7%35%
47906
Night
15%8%35%5%3445%6%37%
46943
TransparencyDay
46%6%12%20%11%1
47583
Night
45%5%12%21%12%1
46770
SeeingDay
16%7%27%26%16%6%
5916
Night
18%8%28%29%13%1
15916

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%14%6%9%7%4435%6%20%
1432
16th to endDay
13%15%7%11%6%36%436%20%
1536
February1st to 15thDay
14%15%5%8%6%345%46%25%
1708
16th to endDay
14%11%6%9%6%345%45%28%
1580
March1st to 15thDay
19%11%47%5%245%5%7%27%
1790
16th to endDay
17%9%37%6%24448%30%
2205
April1st to 15thDay
19%9%35%434347%33%
2151
16th to endDay
19%8%245%2445%7%35%
2255
May1st to 15thDay
19%8%35%43446%7%32%
2331
16th to endDay
11%6%26%6%35%45%8%39%
2575
June1st to 15thDay
10%425%5%35%45%8%44%
2520
16th to endDay
9%4246%36%5%6%10%38%
2520
July1st to 15thDay
8%4146%35%46%10%44%
2520
16th to endDay
6%3245%35%6%7%9%42%
2655
August1st to 15thDay
7%425%5%25%5%6%9%45%
2364
16th to endDay
9%425%5%3446%8%44%
2292
September1st to 15thDay
12%425%435%5%6%7%44%
2068
16th to endDay
8%5%26%5%35%46%10%42%
1815
October1st to 15thDay
14%6%35%5%25%45%6%40%
1777
16th to endDay
19%8%35%6%4445%6%32%
1604
November1st to 15thDay
16%5%25%4245%46%41%
1582
16th to endDay
19%10%47%435%35%7%29%
1620
December1st to 15thDay
18%15%48%6%345%45%21%
1461
16th to endDay
21%16%6%8%5%2435%6%19%
1536
January1st to 15thNight
19%12%47%5%23446%27%
2506
16th to endNight
15%11%6%8%7%35%446%26%
2688
February1st to 15thNight
16%11%5%7%6%33346%32%
2152
16th to endNight
17%11%47%5%24345%31%
1893
March1st to 15thNight
17%10%46%5%34446%31%
2147
16th to endNight
16%8%36%425%5%5%7%33%
2029
April1st to 15thNight
17%8%36%5%35%5%45%34%
1797
16th to endNight
18%7%25%6%2433440%
1704
May1st to 15thNight
20%5%35%5%25%446%35%
1620
16th to endNight
10%436%5%35%45%7%44%
1535
June1st to 15thNight
9%425%32346%8%48%
1440
16th to endNight
8%5%146%347%5%9%42%
1440
July1st to 15thNight
6%3144245%5%8%53%
1440
16th to endNight
6%3134245%7%10%49%
1513
August1st to 15thNight
44234245%6%10%50%
1592
16th to endNight
10%323325%447%51%
1910
September1st to 15thNight
12%4133135%46%52%
1800
16th to endNight
8%424335%348%49%
1815
October1st to 15thNight
12%6%24423345%50%
1851
16th to endNight
20%7%25%32345%5%37%
2266
November1st to 15thNight
14%7%36%5%23446%41%
2276
16th to endNight
19%9%36%5%245%5%6%30%
2340
December1st to 15thNight
20%12%47%5%24447%25%
2496
16th to endNight
23%14%5%7%6%3444419%
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
17%14%6%8%8%4436%6%19%
1432
16th to endDay
13%14%7%10%6%45%5%35%21%
1536
February1st to 15thDay
14%15%5%9%6%34446%25%
1708
16th to endDay
14%11%6%9%6%3445%6%27%
1580
March1st to 15thDay
19%11%5%7%43445%6%26%
1800
16th to endDay
18%8%37%6%24447%31%
2204
April1st to 15thDay
19%9%35%434348%33%
2151
16th to endDay
18%8%3443445%7%35%
2255
May1st to 15thDay
19%8%35%425%45%7%32%
2331
16th to endDay
11%6%26%6%35%46%8%39%
2575
June1st to 15thDay
10%425%44445%8%44%
2520
16th to endDay
9%425%6%36%5%6%10%38%
2520
July1st to 15thDay
7%425%5%35%46%10%43%
2520
16th to endDay
6%3245%35%6%8%9%42%
2655
August1st to 15thDay
7%4245%346%7%8%45%
2364
16th to endDay
9%425%5%345%6%8%44%
2292
September1st to 15thDay
12%424445%46%7%43%
2068
16th to endDay
8%5%26%5%245%5%9%42%
1815
October1st to 15thDay
14%5%345%25%45%6%41%
1777
16th to endDay
19%7%26%6%445%5%6%32%
1604
November1st to 15thDay
16%6%35%4235%47%41%
1582
16th to endDay
18%10%5%6%5%3435%6%30%
1620
December1st to 15thDay
18%14%5%8%5%345%5%5%21%
1461
16th to endDay
21%16%6%8%5%2435%5%20%
1536
January1st to 15thNight
19%12%47%5%33446%27%
2506
16th to endNight
14%11%6%8%7%35%446%25%
2688
February1st to 15thNight
16%10%6%6%6%34436%32%
2145
16th to endNight
17%11%47%6%3435%5%31%
1893
March1st to 15thNight
18%10%5%6%5%34445%31%
2160
16th to endNight
16%7%36%425%5%6%7%32%
2029
April1st to 15thNight
17%7%35%5%35%5%5%5%34%
1797
16th to endNight
18%7%246%2433440%
1704
May1st to 15thNight
19%6%25%5%35%45%6%35%
1619
16th to endNight
10%426%5%34447%44%
1535
June1st to 15thNight
9%425%42346%8%48%
1440
16th to endNight
8%4245%35%6%5%9%42%
1440
July1st to 15thNight
6%3143245%5%8%53%
1440
16th to endNight
5%3143345%7%10%49%
1513
August1st to 15thNight
45%234245%6%11%50%
1592
16th to endNight
10%323325%45%7%51%
1910
September1st to 15thNight
12%4133225%47%52%
1800
16th to endNight
8%5%24335%348%49%
1815
October1st to 15thNight
12%6%24423345%50%
1851
16th to endNight
20%7%35%32345%5%37%
2266
November1st to 15thNight
14%6%36%423446%41%
2276
16th to endNight
19%9%36%5%245%5%6%31%
2340
December1st to 15thNight
20%11%47%6%24447%26%
2496
16th to endNight
23%14%5%7%7%35%34419%
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
63%412%13%5%
1433
16th to endDay
62%6%12%13%4
1536
February1st to 15thDay
58%5%12%15%6%1
1710
16th to endDay
55%6%12%19%41
1580
March1st to 15thDay
56%5%12%17%6%
1800
16th to endDay
53%5%12%21%5%
2205
April1st to 15thDay
49%410%21%13%
2151
16th to endDay
46%7%13%17%11%4
2255
May1st to 15thDay
50%6%11%22%7%
2331
16th to endDay
42%7%11%21%14%2
2579
June1st to 15thDay
36%7%10%23%19%1
2520
16th to endDay
38%9%13%18%17%1
2520
July1st to 15thDay
36%7%13%22%16%2
2371
16th to endDay
33%9%17%22%13%2
2488
August1st to 15thDay
32%8%16%24%16%2
2349
16th to endDay
36%7%12%22%16%4
2292
September1st to 15thDay
37%6%11%22%17%4
2068
16th to endDay
35%6%11%25%18%1
1815
October1st to 15thDay
41%5%11%24%15%1
1777
16th to endDay
50%6%14%19%8%1
1604
November1st to 15thDay
41%6%11%22%14%2
1582
16th to endDay
54%412%22%6%
1620
December1st to 15thDay
62%5%12%16%3
1461
16th to endDay
64%7%11%10%4
1536
January1st to 15thNight
57%5%12%16%7%
2506
16th to endNight
57%5%14%15%6%
2688
February1st to 15thNight
52%414%18%7%1
2154
16th to endNight
54%5%11%20%7%
1893
March1st to 15thNight
53%5%12%17%8%2
2160
16th to endNight
48%7%13%20%8%1
2029
April1st to 15thNight
48%5%10%19%14%1
1797
16th to endNight
47%48%19%15%4
1704
May1st to 15thNight
48%5%11%22%11%
1620
16th to endNight
37%7%13%21%16%2
1537
June1st to 15thNight
32%6%13%26%18%1
1440
16th to endNight
37%7%14%22%16%2
1440
July1st to 15thNight
28%8%14%25%20%3
1360
16th to endNight
27%6%15%27%17%4
1417
August1st to 15thNight
26%8%16%26%20%2
1583
16th to endNight
31%7%11%24%19%5%
1910
September1st to 15thNight
31%5%10%25%21%5%
1800
16th to endNight
33%48%27%22%3
1815
October1st to 15thNight
36%49%27%21%
1851
16th to endNight
46%5%11%24%10%1
2266
November1st to 15thNight
42%5%9%23%15%3
2276
16th to endNight
51%5%14%22%6%
2340
December1st to 15thNight
57%5%13%17%5%
2496
16th to endNight
67%410%12%4
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
25%13%33%18%8%
90
16th to endDay
20%16%34%23%31
96
February1st to 15thDay
20%18%45%15%
90
16th to endDay
20%9%38%17%12%1
81
March1st to 15thDay
32%18%27%16%4
90
16th to endDay
27%8%32%20%10%
273
April1st to 15thDay
26%8%31%21%10%1
270
16th to endDay
23%9%27%22%12%4
270
May1st to 15thDay
24%9%27%22%12%3
288
16th to endDay
16%7%31%24%15%4
475
June1st to 15thDay
11%428%31%18%4
495
16th to endDay
13%6%32%30%13%4
495
July1st to 15thDay
9%326%28%19%11%
475
16th to endDay
8%222%31%23%11%
515
August1st to 15thDay
9%215%29%25%17%
349
16th to endDay
15%215%26%26%13%
332
September1st to 15thDay
13%416%30%27%8%
299
16th to endDay
10%6%16%28%28%9%
254
October1st to 15thDay
11%6%24%31%19%5%
217
16th to endDay
25%9%42%20%2
96
November1st to 15thDay
10%10%25%28%21%4
90
16th to endDay
28%15%40%12%3
90
December1st to 15thDay
20%22%36%16%4
90
16th to endDay
27%28%39%32
96
January1st to 15thNight
24%13%37%19%4
900
16th to endNight
20%13%35%26%3
960
February1st to 15thNight
21%12%37%25%3
746
16th to endNight
23%12%34%24%5%
631
March1st to 15thNight
23%9%35%23%7%
720
16th to endNight
19%9%35%26%7%
671
April1st to 15thNight
20%9%36%26%6%
539
16th to endNight
21%7%33%27%9%1
540
May1st to 15thNight
22%429%30%11%
540
16th to endNight
12%324%39%19%
563
June1st to 15thNight
12%223%44%17%
540
16th to endNight
9%225%41%19%
540
July1st to 15thNight
8%118%44%24%2
510
16th to endNight
6%114%42%30%3
564
August1st to 15thNight
7%8%48%30%5%
539
16th to endNight
10%15%35%32%4
573
September1st to 15thNight
14%313%35%28%4
563
16th to endNight
10%5%20%32%27%3
657
October1st to 15thNight
16%6%22%34%18%1
660
16th to endNight
24%11%31%24%8%
704
November1st to 15thNight
19%11%33%22%11%1
700
16th to endNight
23%16%29%23%7%
720
December1st to 15thNight
27%14%31%21%4
876
16th to endNight
30%12%32%20%3
960
Page generated at 2017-09-13 07:39:12. Version ac04

Copyright 2017 A. Danko


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