VSOR Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the VSOR Observatory clear sky chart from 2005-11-05 to 2018-01-12.

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
27%
15366
18%
48317
8%
15334
39%
15334
21%
31346

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
17%
765
19%
2662
6%
761
33%
761
17%
1684
January16th to endNight
15%
768
19%
2688
3%
768
34%
768
15%
1708
February1st to 15thNight
18%
704
17%
2188
5%
701
32%
701
18%
1463
February16th to endNight
18%
631
23%
1893
7%
631
38%
631
20%
1241
March1st to 15thNight
17%
720
18%
2154
6%
720
32%
720
18%
1488
March16th to endNight
21%
768
23%
2036
7%
768
34%
768
22%
1391
April1st to 15thNight
22%
718
18%
1796
5%
718
30%
718
22%
1276
April16th to endNight
23%
588
18%
1668
5%
588
32%
588
20%
1138
May1st to 15thNight
23%
540
15%
1620
7%
540
31%
540
18%
1117
May16th to endNight
27%
422
11%
1544
7%
421
34%
421
17%
1008
June1st to 15thNight
28%
360
12%
1440
8%
360
40%
360
16%
964
June16th to endNight
37%
360
11%
1440
11%
360
46%
360
22%
866
July1st to 15thNight
43%
340
12%
1360
14%
340
49%
340
23%
799
July16th to endNight
43%
376
15%
1418
14%
354
51%
354
21%
877
August1st to 15thNight
44%
512
16%
1583
11%
510
48%
510
28%
975
August16th to endNight
54%
717
21%
1863
12%
717
57%
717
35%
1181
September1st to 15thNight
40%
720
20%
1866
8%
720
47%
720
32%
1206
September16th to endNight
44%
717
26%
1980
17%
717
52%
717
34%
1215
October1st to 15thNight
35%
720
23%
2004
11%
720
45%
720
26%
1297
October16th to endNight
26%
768
18%
2496
7%
768
40%
768
21%
1605
November1st to 15thNight
30%
760
20%
2472
10%
760
42%
760
21%
1585
November16th to endNight
25%
780
22%
2535
9%
780
41%
780
20%
1592
December1st to 15thNight
21%
780
17%
2699
5%
780
36%
780
17%
1746
December16th to endNight
16%
832
19%
2912
5%
832
33%
832
16%
1924

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
17%5%2442445%7%40%
49027
Night
17%6%25%424346%41%
48487
Cloud CoverDay
17%6%2442445%7%40%
49037
Night
16%6%25%424446%41%
48492
TransparencyDay
43%5%11%22%15%2
48715
Night
43%411%22%15%2
48317
SeeingDay
20%14%32%21%9%2
6245
Night
20%12%33%24%7%
15366

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
16%8%24424446%42%
1520
16th to endDay
15%5%25%42345%6%42%
1536
February1st to 15thDay
17%7%25%435%46%7%35%
1669
16th to endDay
14%7%35%43445%5%41%
1580
March1st to 15thDay
21%6%25%42445%7%36%
1796
16th to endDay
18%6%35%413445%40%
2187
April1st to 15thDay
19%6%35%434446%37%
2152
16th to endDay
18%5%235%24447%40%
2292
May1st to 15thDay
25%7%25%424346%32%
2332
16th to endDay
17%5%245%34348%39%
2579
June1st to 15thDay
26%423425%45%8%32%
2520
16th to endDay
15%5%25%5%35%5%6%8%35%
2520
July1st to 15thDay
12%5%245%36%5%6%9%37%
2520
16th to endDay
13%4245%36%5%6%7%39%
2654
August1st to 15thDay
12%5%245%35%5%6%8%38%
2355
16th to endDay
11%424435%5%5%8%44%
2336
September1st to 15thDay
14%5%14333446%47%
2094
16th to endDay
16%5%23323345%51%
1980
October1st to 15thDay
19%5%135%23246%43%
1953
16th to endDay
17%5%24423346%43%
1728
November1st to 15thDay
21%6%2432433443%
1716
16th to endDay
17%5%24333436%44%
1755
December1st to 15thDay
18%6%25%424445%40%
1589
16th to endDay
23%7%24425%345%36%
1664
January1st to 15thNight
16%8%25%424335%42%
2662
16th to endNight
16%9%35%5%24345%41%
2688
February1st to 15thNight
14%8%36%5%3445%6%37%
2187
16th to endNight
16%8%34323335%42%
1893
March1st to 15thNight
18%7%245%35%35%6%36%
2141
16th to endNight
16%7%35%42333444%
2036
April1st to 15thNight
16%8%345%25%45%5%37%
1796
16th to endNight
22%5%245%24335%40%
1668
May1st to 15thNight
22%6%245%24346%36%
1620
16th to endNight
20%6%245%2446%8%35%
1542
June1st to 15thNight
23%5%35%324447%34%
1440
16th to endNight
16%325%4345%5%9%40%
1440
July1st to 15thNight
11%5%25%5%35%46%9%40%
1440
16th to endNight
12%5%25%7%35%5%5%8%38%
1514
August1st to 15thNight
9%4245%36%5%6%8%44%
1593
16th to endNight
12%414425%5%5%8%45%
1863
September1st to 15thNight
14%4245%24447%46%
1866
16th to endNight
15%41431235%5%51%
1980
October1st to 15thNight
18%5%23423345%45%
2004
16th to endNight
19%5%25%424347%40%
2496
November1st to 15thNight
17%6%25%423345%44%
2472
16th to endNight
17%6%24423446%43%
2535
December1st to 15thNight
19%6%35%5%14346%38%
2699
16th to endNight
18%8%36%423346%38%
2912

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
15%8%2432345%5%43%
1520
16th to endDay
15%5%35%423447%42%
1536
February1st to 15thDay
18%7%25%4345%5%7%36%
1669
16th to endDay
14%7%36%42445%6%41%
1580
March1st to 15thDay
21%6%25%4235%5%7%35%
1806
16th to endDay
18%5%36%423445%41%
2187
April1st to 15thDay
18%7%25%5%34346%37%
2152
16th to endDay
17%5%235%2445%7%40%
2292
May1st to 15thDay
24%8%25%5%14345%33%
2332
16th to endDay
17%5%25%5%2435%8%39%
2579
June1st to 15thDay
25%5%2442445%7%32%
2520
16th to endDay
15%5%25%5%45%5%6%8%35%
2520
July1st to 15thDay
12%4245%46%5%6%9%37%
2520
16th to endDay
11%5%25%5%36%6%6%8%38%
2654
August1st to 15thDay
12%5%25%5%35%5%7%8%38%
2355
16th to endDay
11%4244345%5%8%43%
2336
September1st to 15thDay
14%425%423447%47%
2094
16th to endDay
16%424323345%50%
1980
October1st to 15thDay
20%424424336%43%
1953
16th to endDay
16%6%24433336%44%
1728
November1st to 15thDay
21%6%2332433443%
1716
16th to endDay
16%5%25%32435%6%43%
1755
December1st to 15thDay
18%6%35%5%34345%39%
1589
16th to endDay
22%9%14425%335%36%
1664
January1st to 15thNight
15%8%25%424335%42%
2662
16th to endNight
16%9%35%423345%40%
2688
February1st to 15thNight
15%8%35%5%34447%37%
2180
16th to endNight
16%8%34324346%42%
1893
March1st to 15thNight
18%7%245%34446%36%
2154
16th to endNight
15%8%345%2334443%
2035
April1st to 15thNight
17%8%25%5%345%5%6%37%
1796
16th to endNight
21%6%245%23435%40%
1668
May1st to 15thNight
22%7%24424347%35%
1620
16th to endNight
19%6%245%1445%8%35%
1542
June1st to 15thNight
22%6%35%334447%34%
1440
16th to endNight
15%3245%245%5%8%41%
1440
July1st to 15thNight
11%5%25%6%245%7%9%40%
1440
16th to endNight
12%435%6%36%5%5%8%37%
1514
August1st to 15thNight
9%425%5%35%6%6%7%43%
1593
16th to endNight
11%4144246%5%8%45%
1863
September1st to 15thNight
13%423425%45%6%45%
1866
16th to endNight
14%5%23313345%51%
1980
October1st to 15thNight
18%424333435%46%
2004
16th to endNight
18%5%25%334346%41%
2496
November1st to 15thNight
17%5%25%424345%43%
2472
16th to endNight
17%5%35%323345%44%
2535
December1st to 15thNight
19%7%35%424346%39%
2699
16th to endNight
18%8%35%423446%37%
2912

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
42%5%10%24%13%3
1521
16th to endDay
41%411%27%14%1
1536
February1st to 15thDay
46%5%12%20%13%1
1672
16th to endDay
43%5%11%21%15%2
1580
March1st to 15thDay
48%313%20%11%3
1806
16th to endDay
44%49%21%14%5%
2187
April1st to 15thDay
47%49%19%16%3
2152
16th to endDay
42%5%8%21%17%4
2292
May1st to 15thDay
53%47%17%14%2
2332
16th to endDay
43%5%13%22%13%2
2583
June1st to 15thDay
50%5%12%20%9%1
2520
16th to endDay
43%7%12%19%15%1
2520
July1st to 15thDay
41%7%11%21%15%2
2371
16th to endDay
40%6%15%21%12%3
2487
August1st to 15thDay
40%7%13%22%13%1
2341
16th to endDay
37%6%10%25%18%1
2336
September1st to 15thDay
36%5%10%23%20%2
2094
16th to endDay
37%37%24%24%2
1980
October1st to 15thDay
43%38%25%15%3
1953
16th to endDay
41%411%23%16%2
1728
November1st to 15thDay
45%311%23%14%1
1716
16th to endDay
41%5%11%24%15%1
1755
December1st to 15thDay
46%314%15%18%
1589
16th to endDay
50%412%19%11%1
1664
January1st to 15thNight
45%312%22%14%1
2662
16th to endNight
45%311%20%15%2
2688
February1st to 15thNight
46%413%21%12%2
2188
16th to endNight
44%39%22%16%2
1893
March1st to 15thNight
48%49%19%14%3
2154
16th to endNight
45%37%18%19%5%
2036
April1st to 15thNight
46%5%9%19%17%2
1796
16th to endNight
47%48%19%16%4
1668
May1st to 15thNight
49%410%17%14%3
1620
16th to endNight
45%6%16%19%9%2
1544
June1st to 15thNight
49%5%13%19%10%1
1440
16th to endNight
40%6%13%23%15%1
1440
July1st to 15thNight
38%6%14%26%14%
1360
16th to endNight
42%6%13%26%9%2
1418
August1st to 15thNight
36%7%15%23%15%2
1583
16th to endNight
35%5%12%27%19%
1863
September1st to 15thNight
37%5%11%24%19%1
1866
16th to endNight
34%47%23%27%2
1980
October1st to 15thNight
41%410%24%16%3
2004
16th to endNight
43%5%12%24%14%
2496
November1st to 15thNight
43%411%21%15%3
2472
16th to endNight
42%312%23%15%2
2535
December1st to 15thNight
46%314%21%12%
2699
16th to endNight
47%411%21%13%1
2912

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
17%26%32%15%7%
82
16th to endDay
21%26%39%12%
64
February1st to 15thDay
26%16%36%16%3
227
16th to endDay
15%20%37%20%5%
239
March1st to 15thDay
23%24%31%15%31
270
16th to endDay
21%20%35%11%10%1
288
April1st to 15thDay
25%17%30%18%7%1
270
16th to endDay
21%17%34%18%5%2
270
May1st to 15thDay
34%16%24%16%6%2
288
16th to endDay
20%13%36%22%6%
331
June1st to 15thDay
25%12%34%19%6%
315
16th to endDay
15%13%38%24%5%2
315
July1st to 15thDay
18%6%35%28%9%
305
16th to endDay
14%5%32%32%11%3
326
August1st to 15thDay
13%8%30%30%14%2
313
16th to endDay
13%6%28%27%19%4
332
September1st to 15thDay
17%7%30%25%14%4
315
16th to endDay
22%6%25%19%14%11%
314
October1st to 15thDay
23%9%23%22%14%5%
313
16th to endDay
19%15%32%22%9%
288
November1st to 15thDay
23%12%33%17%10%2
280
16th to endDay
20%20%30%18%9%1
285
December1st to 15thDay
22%26%33%12%5%
119
16th to endDay
23%29%33%13%
96
January1st to 15thNight
21%18%38%18%3
765
16th to endNight
23%17%37%18%3
768
February1st to 15thNight
19%17%39%22%1
704
16th to endNight
22%18%37%18%3
631
March1st to 15thNight
23%16%38%17%3
720
16th to endNight
21%15%36%19%6%
768
April1st to 15thNight
21%18%34%21%4
718
16th to endNight
24%14%34%21%5%
588
May1st to 15thNight
24%13%33%22%5%
540
16th to endNight
22%9%37%26%4
422
June1st to 15thNight
23%7%34%27%6%
360
16th to endNight
15%438%35%6%
360
July1st to 15thNight
12%233%40%10%
340
16th to endNight
12%232%39%13%
376
August1st to 15thNight
10%332%41%11%
512
16th to endNight
15%222%40%17%1
717
September1st to 15thNight
15%5%32%31%13%
720
16th to endNight
18%5%25%26%21%3
717
October1st to 15thNight
22%8%27%26%13%2
720
16th to endNight
22%14%32%21%8%
768
November1st to 15thNight
20%13%30%24%10%1
760
16th to endNight
20%19%30%21%8%
780
December1st to 15thNight
23%15%34%20%4
780
16th to endNight
24%17%37%17%2
832
Page generated at 2018-01-12 22:01:38. Version ac04

Copyright 2018 A. Danko


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