VSO
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the VSO clear sky chart from 2005-11-05 to 2017-09-24.

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
37%
14907
15%
46910
10%
14871
43%
14871
20%
32239

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
15%
716
11%
2434
4%
716
23%
716
9%
1897
January16th to endNight
16%
768
12%
2496
5%
768
24%
768
11%
1906
February1st to 15thNight
18%
701
9%
2287
3%
701
26%
701
11%
1760
February16th to endNight
25%
631
13%
2001
6%
631
34%
631
14%
1472
March1st to 15thNight
26%
720
14%
1980
7%
720
32%
720
16%
1507
March16th to endNight
32%
768
21%
2112
11%
768
40%
768
24%
1457
April1st to 15thNight
31%
718
16%
1835
7%
718
36%
718
22%
1334
April16th to endNight
42%
549
21%
1629
10%
549
50%
549
27%
1054
May1st to 15thNight
44%
528
17%
1608
11%
528
50%
528
24%
1066
May16th to endNight
52%
374
17%
1497
17%
374
60%
374
24%
859
June1st to 15thNight
49%
360
13%
1440
14%
360
58%
360
21%
862
June16th to endNight
56%
360
14%
1369
17%
360
62%
360
25%
802
July1st to 15thNight
65%
340
17%
1360
24%
340
69%
340
34%
679
July16th to endNight
63%
535
16%
1566
15%
502
65%
502
33%
899
August1st to 15thNight
70%
539
18%
1609
15%
536
72%
536
37%
932
August16th to endNight
65%
615
23%
1761
17%
615
67%
615
36%
1044
September1st to 15thNight
56%
720
22%
1833
13%
720
60%
720
34%
1190
September16th to endNight
58%
693
23%
1922
17%
693
61%
693
33%
1187
October1st to 15thNight
43%
660
20%
1980
16%
660
49%
660
24%
1305
October16th to endNight
26%
704
15%
2112
8%
704
35%
704
17%
1554
November1st to 15thNight
36%
700
19%
2351
15%
700
44%
700
22%
1527
November16th to endNight
26%
720
12%
2520
8%
720
32%
720
13%
1805
December1st to 15thNight
17%
720
7%
2520
3%
720
22%
720
9%
1952
December16th to endNight
13%
768
7%
2688
2%
768
19%
768
8%
2189

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%8%37%6%35%5%5%7%32%
47958
Night
14%8%36%5%3445%7%35%
47092
Cloud CoverDay
14%8%37%6%35%5%5%7%32%
47968
Night
14%8%46%5%3445%7%35%
47096
TransparencyDay
49%5%12%20%10%1
47658
Night
48%5%11%21%11%1
46910
SeeingDay
16%7%24%26%16%8%
5382
Night
18%8%28%29%14%1
14915

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%12%5%9%7%46%5%46%20%
1501
16th to endDay
12%16%6%12%9%45%45%5%16%
1728
February1st to 15thDay
14%15%7%11%7%345%5%6%18%
1575
16th to endDay
17%12%6%10%6%35%45%5%22%
1470
March1st to 15thDay
15%11%5%9%8%35%336%25%
1969
16th to endDay
15%9%47%5%3435%6%32%
2112
April1st to 15thDay
19%9%36%6%35%446%30%
2113
16th to endDay
17%8%36%423346%38%
2340
May1st to 15thDay
14%7%37%6%345%5%7%34%
2342
16th to endDay
11%6%35%5%35%45%8%40%
2618
June1st to 15thDay
12%6%26%5%35%5%6%8%37%
2520
16th to endDay
11%425%5%36%5%6%10%37%
2536
July1st to 15thDay
7%425%5%35%6%7%10%41%
2520
16th to endDay
8%5%25%6%46%6%7%9%36%
2494
August1st to 15thDay
7%5%26%6%46%5%7%8%38%
2340
16th to endDay
10%6%25%5%35%5%6%8%39%
2429
September1st to 15thDay
12%5%36%7%345%5%8%37%
2124
16th to endDay
13%5%36%5%245%5%7%40%
1897
October1st to 15thDay
15%8%36%5%35%445%35%
1650
16th to endDay
23%10%47%6%44345%25%
1760
November1st to 15thDay
12%6%38%6%25%5%5%8%33%
1504
16th to endDay
17%12%47%6%35%5%6%7%22%
1440
December1st to 15thDay
15%14%5%10%7%45%446%20%
1440
16th to endDay
19%13%6%10%8%45%5%5%5%14%
1536
January1st to 15thNight
19%12%6%8%7%35%445%22%
2434
16th to endNight
13%12%7%10%8%45%446%22%
2496
February1st to 15thNight
14%12%7%8%6%35%5%5%6%24%
2285
16th to endNight
14%11%48%7%35%446%28%
2001
March1st to 15thNight
18%7%48%6%45%5%46%27%
1968
16th to endNight
14%9%37%423345%40%
2112
April1st to 15thNight
17%8%36%5%34346%35%
1835
16th to endNight
11%6%35%6%2445%6%42%
1629
May1st to 15thNight
15%6%25%6%34448%37%
1608
16th to endNight
11%41443445%7%46%
1495
June1st to 15thNight
10%5%25%5%45%5%5%8%40%
1440
16th to endNight
9%315%435%5%6%8%44%
1369
July1st to 15thNight
5%2234335%410%54%
1440
16th to endNight
7%42432445%8%50%
1673
August1st to 15thNight
6%31343448%10%47%
1620
16th to endNight
7%4245%34446%52%
1761
September1st to 15thNight
12%324434448%47%
1833
16th to endNight
11%416%4345%5%7%46%
1922
October1st to 15thNight
16%6%25%5%2435%6%41%
1980
16th to endNight
21%8%36%4345%5%6%29%
2112
November1st to 15thNight
14%5%25%5%26%35%8%38%
2351
16th to endNight
18%10%45%5%35%5%5%7%27%
2520
December1st to 15thNight
20%13%48%6%3445%7%22%
2520
16th to endNight
20%11%7%10%7%35%46%6%16%
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%13%5%9%6%36%5%47%20%
1501
16th to endDay
12%15%6%12%8%45%45%5%16%
1728
February1st to 15thDay
14%16%7%11%7%34446%19%
1575
16th to endDay
17%11%6%9%7%35%345%23%
1470
March1st to 15thDay
15%12%5%8%8%45%345%25%
1980
16th to endDay
15%10%47%5%3445%6%33%
2111
April1st to 15thDay
19%9%36%6%35%446%30%
2113
16th to endDay
18%7%36%423345%39%
2340
May1st to 15thDay
14%7%37%5%345%5%7%34%
2342
16th to endDay
11%6%34435%45%8%40%
2618
June1st to 15thDay
13%5%25%6%35%5%6%8%37%
2520
16th to endDay
12%425%5%35%6%6%10%37%
2536
July1st to 15thDay
8%425%5%35%6%7%9%41%
2520
16th to endDay
8%5%25%6%46%6%7%9%37%
2494
August1st to 15thDay
8%5%25%6%45%6%7%8%38%
2340
16th to endDay
11%6%25%5%35%5%6%8%40%
2429
September1st to 15thDay
13%5%26%6%3445%8%37%
2124
16th to endDay
13%5%36%5%2446%6%40%
1897
October1st to 15thDay
16%8%35%6%35%446%35%
1650
16th to endDay
23%10%37%6%34345%26%
1760
November1st to 15thDay
11%7%46%7%25%5%5%8%34%
1504
16th to endDay
17%12%47%6%36%5%5%7%23%
1440
December1st to 15thDay
16%14%410%7%35%45%6%21%
1440
16th to endDay
19%13%7%9%8%45%6%5%415%
1536
January1st to 15thNight
19%12%6%8%7%34445%22%
2434
16th to endNight
13%12%7%9%8%35%446%22%
2496
February1st to 15thNight
14%12%8%8%6%35%45%5%25%
2278
16th to endNight
14%11%5%8%7%34446%28%
2001
March1st to 15thNight
19%8%47%6%35%5%46%28%
1980
16th to endNight
14%9%46%423335%40%
2112
April1st to 15thNight
18%8%35%6%34336%35%
1835
16th to endNight
11%6%35%6%34447%42%
1629
May1st to 15thNight
16%5%25%5%3345%7%38%
1607
16th to endNight
12%42442436%7%46%
1495
June1st to 15thNight
11%5%246%36%5%7%8%40%
1440
16th to endNight
9%315%435%5%7%8%44%
1369
July1st to 15thNight
5%22343345%10%54%
1440
16th to endNight
7%5%2342445%8%50%
1673
August1st to 15thNight
6%3134245%8%10%47%
1620
16th to endNight
7%4245%3445%6%52%
1761
September1st to 15thNight
12%41442445%7%46%
1833
16th to endNight
11%425%4245%5%8%46%
1922
October1st to 15thNight
17%5%245%2435%6%41%
1980
16th to endNight
21%9%36%4345%5%6%29%
2112
November1st to 15thNight
14%6%25%435%35%8%38%
2351
16th to endNight
18%10%46%5%35%5%47%27%
2520
December1st to 15thNight
20%12%5%8%6%3445%6%22%
2520
16th to endNight
19%12%7%10%7%25%46%6%16%
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%5%10%13%6%
1502
16th to endDay
69%5%8%10%5%
1728
February1st to 15thDay
66%410%13%4
1577
16th to endDay
63%311%14%5%
1470
March1st to 15thDay
60%412%15%6%
1980
16th to endDay
51%410%20%11%1
2112
April1st to 15thDay
54%411%17%11%
2113
16th to endDay
47%49%21%14%2
2340
May1st to 15thDay
48%5%9%23%11%
2342
16th to endDay
41%6%13%22%14%1
2622
June1st to 15thDay
42%6%12%23%13%1
2520
16th to endDay
39%6%14%23%15%
2536
July1st to 15thDay
35%7%12%25%18%1
2371
16th to endDay
40%7%15%22%11%2
2338
August1st to 15thDay
39%8%15%21%13%1
2327
16th to endDay
40%6%12%26%13%1
2429
September1st to 15thDay
42%6%13%24%12%
2124
16th to endDay
41%6%13%28%10%
1897
October1st to 15thDay
49%412%20%10%1
1650
16th to endDay
60%5%13%14%5%
1760
November1st to 15thDay
46%5%17%20%8%
1504
16th to endDay
58%6%14%14%6%
1440
December1st to 15thDay
63%6%15%11%2
1440
16th to endDay
70%6%10%10%2
1536
January1st to 15thNight
63%410%12%7%
2434
16th to endNight
61%410%16%5%
2496
February1st to 15thNight
61%5%11%15%5%
2287
16th to endNight
57%411%18%8%
2001
March1st to 15thNight
57%410%20%6%
1980
16th to endNight
47%47%23%15%2
2112
April1st to 15thNight
49%5%9%20%13%
1835
16th to endNight
42%48%21%18%4
1629
May1st to 15thNight
44%410%25%12%2
1608
16th to endNight
37%5%12%24%16%3
1497
June1st to 15thNight
40%6%11%24%16%1
1440
16th to endNight
36%7%13%23%18%1
1369
July1st to 15thNight
26%5%11%26%25%3
1360
16th to endNight
31%6%15%25%18%3
1566
August1st to 15thNight
29%7%16%30%13%2
1609
16th to endNight
31%5%11%31%17%2
1761
September1st to 15thNight
36%5%11%28%15%2
1833
16th to endNight
36%412%30%15%
1922
October1st to 15thNight
43%5%10%25%15%
1980
16th to endNight
53%5%12%19%8%
2112
November1st to 15thNight
42%6%14%21%14%
2351
16th to endNight
55%5%15%17%6%
2520
December1st to 15thNight
62%6%13%13%4
2520
16th to endNight
67%5%11%11%3
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
16%23%35%12%9%1
71
16th to endDay
29%16%35%16%2
96
February1st to 15thDay
20%17%46%13%2
90
16th to endDay
20%17%35%23%2
81
March1st to 15thDay
21%9%32%26%9%
270
16th to endDay
24%6%29%27%10%1
288
April1st to 15thDay
21%12%28%24%9%2
270
16th to endDay
18%10%27%22%12%8%
270
May1st to 15thDay
18%6%23%25%17%8%
288
16th to endDay
13%6%28%25%16%9%
331
June1st to 15thDay
16%5%22%32%19%3
315
16th to endDay
14%422%31%19%7%
331
July1st to 15thDay
11%215%32%24%13%
305
16th to endDay
10%220%29%20%15%
326
August1st to 15thDay
9%215%29%26%16%
313
16th to endDay
11%216%24%24%20%
332
September1st to 15thDay
11%214%28%25%18%
315
16th to endDay
13%414%22%24%20%
287
October1st to 15thDay
17%9%21%21%21%7%
255
16th to endDay
26%8%30%26%8%
272
November1st to 15thDay
13%11%31%28%10%4
92
16th to endDay
20%20%30%16%10%3
60
December1st to 15thDay
25%25%23%18%8%
60
16th to endDay
20%23%45%10%
64
January1st to 15thNight
24%15%35%18%5%
720
16th to endNight
19%13%38%22%6%
768
February1st to 15thNight
21%15%36%22%4
704
16th to endNight
22%9%33%30%4
631
March1st to 15thNight
23%10%33%25%6%
720
16th to endNight
19%8%32%27%10%1
768
April1st to 15thNight
22%10%30%30%6%
718
16th to endNight
13%5%31%35%13%1
549
May1st to 15thNight
18%225%33%18%2
528
16th to endNight
15%224%33%20%2
375
June1st to 15thNight
15%224%40%16%1
360
16th to endNight
10%223%45%18%
360
July1st to 15thNight
421%46%24%2
340
16th to endNight
10%117%43%24%2
535
August1st to 15thNight
7%112%41%33%3
539
16th to endNight
9%15%35%34%3
615
September1st to 15thNight
15%217%32%26%5%
720
16th to endNight
13%317%27%30%7%
693
October1st to 15thNight
19%6%24%26%18%4
660
16th to endNight
27%7%32%27%4
704
November1st to 15thNight
18%7%32%25%14%2
700
16th to endNight
23%13%29%22%9%1
720
December1st to 15thNight
28%14%32%20%4
720
16th to endNight
25%13%36%19%4
768
Page generated at 2017-09-25 09:02:19. Version ac04

Copyright 2017 A. Danko


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