Virgil Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Virgil Observatory clear sky chart from 2005-11-05 to 2018-03-21.

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
28%
15627
12%
49240
7%
15595
33%
15595
14%
36497

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
13%
780
6%
2716
4%
776
17%
776
6%
2262
January16th to endNight
12%
832
7%
2766
3%
832
18%
832
7%
2285
February1st to 15thNight
10%
764
6%
2482
1%
761
16%
761
6%
2100
February16th to endNight
15%
683
9%
2087
5%
683
21%
683
9%
1673
March1st to 15thNight
15%
780
10%
2243
4%
780
21%
780
11%
1763
March16th to endNight
23%
792
15%
2117
7%
792
29%
792
16%
1599
April1st to 15thNight
19%
718
12%
1796
4%
718
23%
718
16%
1378
April16th to endNight
29%
720
17%
1800
6%
720
34%
720
24%
1259
May1st to 15thNight
36%
394
17%
1474
13%
394
45%
394
15%
1001
May16th to endNight
40%
374
16%
1497
16%
374
48%
374
19%
947
June1st to 15thNight
37%
360
12%
1440
12%
360
45%
360
16%
944
June16th to endNight
48%
360
12%
1440
14%
360
56%
360
23%
864
July1st to 15thNight
57%
340
12%
1360
17%
340
59%
340
23%
814
July16th to endNight
58%
376
14%
1418
16%
354
62%
354
24%
836
August1st to 15thNight
53%
539
13%
1609
10%
536
56%
536
27%
1022
August16th to endNight
58%
738
18%
1884
11%
738
59%
738
33%
1229
September1st to 15thNight
48%
720
16%
1800
9%
720
51%
720
28%
1244
September16th to endNight
49%
717
22%
1960
16%
717
53%
717
29%
1300
October1st to 15thNight
34%
720
15%
2160
12%
720
39%
720
19%
1535
October16th to endNight
19%
768
10%
2347
6%
768
26%
768
13%
1816
November1st to 15thNight
24%
760
14%
2472
9%
760
31%
760
15%
1792
November16th to endNight
20%
780
12%
2730
7%
780
27%
780
11%
2034
December1st to 15thNight
13%
780
5%
2730
2%
780
17%
780
6%
2255
December16th to endNight
10%
832
4%
2912
1%
832
14%
832
5%
2545

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%11%47%6%35%45%6%28%
49614
Night
16%11%48%6%35%446%28%
49411
Cloud CoverDay
16%10%47%6%35%45%6%27%
49624
Night
15%10%5%8%6%35%446%29%
49416
TransparencyDay
55%6%11%16%8%
49303
Night
54%6%12%16%8%
49240
SeeingDay
24%14%27%18%10%4
6135
Night
23%16%27%22%9%
15628

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
15%13%7%14%9%46%44413%
1552
16th to endDay
11%15%8%15%10%5%4345%15%
1807
February1st to 15thDay
16%14%9%15%10%35%34313%
1710
16th to endDay
18%14%6%10%11%35%33418%
1672
March1st to 15thDay
17%17%6%10%6%23435%20%
2036
16th to endDay
16%14%6%8%5%34345%27%
2234
April1st to 15thDay
19%13%5%7%424345%29%
2152
16th to endDay
16%12%25%5%24346%36%
2169
May1st to 15thDay
18%9%26%5%35%5%45%32%
2477
16th to endDay
14%8%35%6%35%45%8%33%
2618
June1st to 15thDay
22%7%25%435%5%5%6%30%
2520
16th to endDay
15%7%35%5%36%5%6%9%31%
2520
July1st to 15thDay
12%6%35%6%36%6%7%8%33%
2520
16th to endDay
11%6%35%7%37%6%6%8%32%
2651
August1st to 15thDay
16%5%25%6%46%6%6%9%30%
2340
16th to endDay
13%7%36%5%35%6%6%8%33%
2306
September1st to 15thDay
18%7%36%43445%7%34%
2160
16th to endDay
16%5%246%34346%41%
1998
October1st to 15thDay
20%9%35%6%24445%33%
1800
16th to endDay
22%15%35%6%34445%24%
1875
November1st to 15thDay
16%13%46%5%24445%30%
1713
16th to endDay
19%16%49%6%23345%23%
1560
December1st to 15thDay
16%20%7%11%6%33345%16%
1560
16th to endDay
19%18%11%13%7%3343411%
1664
January1st to 15thNight
17%13%5%13%9%47%5%4413%
2716
16th to endNight
14%15%6%12%11%5%5%44416%
2766
February1st to 15thNight
13%14%8%13%8%45%445%15%
2480
16th to endNight
15%12%6%11%8%45%445%19%
2087
March1st to 15thNight
17%12%7%9%7%34445%22%
2231
16th to endNight
15%12%5%8%5%24445%29%
2117
April1st to 15thNight
19%13%45%5%3435%6%29%
1796
16th to endNight
18%8%25%433345%40%
1800
May1st to 15thNight
17%7%37%5%345%46%33%
1474
16th to endNight
11%5%46%6%245%5%8%38%
1495
June1st to 15thNight
18%7%25%5%25%45%7%34%
1440
16th to endNight
12%5%25%435%5%6%8%41%
1440
July1st to 15thNight
7%5%25%5%35%5%6%10%42%
1440
16th to endNight
11%4244345%6%8%43%
1514
August1st to 15thNight
9%5%26%4345%6%8%42%
1620
16th to endNight
11%424425%448%45%
1884
September1st to 15thNight
14%6%25%5%25%45%6%42%
1800
16th to endNight
14%5%135%35%436%44%
1960
October1st to 15thNight
20%9%35%5%24245%35%
2160
16th to endNight
24%13%37%5%23345%24%
2347
November1st to 15thNight
15%13%47%5%35%345%31%
2472
16th to endNight
17%15%5%7%5%34446%26%
2730
December1st to 15thNight
20%16%8%10%7%3433416%
2730
16th to endNight
18%16%8%12%8%45%44411%
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%13%7%14%9%46%44314%
1552
16th to endDay
11%15%8%15%11%5%4445%15%
1807
February1st to 15thDay
16%13%9%15%9%35%34313%
1710
16th to endDay
18%14%5%11%10%4434418%
1672
March1st to 15thDay
17%17%6%10%6%33435%20%
2047
16th to endDay
16%14%6%8%5%34345%28%
2233
April1st to 15thDay
19%13%5%6%5%24345%30%
2152
16th to endDay
16%12%35%424446%36%
2169
May1st to 15thDay
18%9%35%6%35%446%32%
2477
16th to endDay
15%7%36%5%25%5%6%8%32%
2618
June1st to 15thDay
23%6%345%25%5%5%7%29%
2520
16th to endDay
15%7%35%5%46%5%6%9%30%
2520
July1st to 15thDay
11%6%36%6%36%6%6%8%33%
2520
16th to endDay
11%5%27%6%46%6%6%8%33%
2651
August1st to 15thDay
16%5%26%6%46%5%7%9%30%
2340
16th to endDay
13%6%35%6%45%5%5%9%33%
2306
September1st to 15thDay
18%7%36%5%2445%7%34%
2160
16th to endDay
16%5%246%25%436%41%
1998
October1st to 15thDay
20%9%35%6%34445%32%
1800
16th to endDay
22%15%35%6%35%445%24%
1875
November1st to 15thDay
15%14%46%5%2445%5%31%
1713
16th to endDay
19%15%5%8%6%24246%23%
1560
December1st to 15thDay
16%20%7%11%6%33345%16%
1560
16th to endDay
18%17%12%14%7%3433312%
1664
January1st to 15thNight
16%13%6%12%9%46%5%45%13%
2716
16th to endNight
14%14%6%12%10%5%6%44417%
2766
February1st to 15thNight
12%13%8%14%9%46%44416%
2473
16th to endNight
14%12%6%11%8%5%5%45%5%19%
2087
March1st to 15thNight
18%12%7%9%7%2435%5%22%
2243
16th to endNight
15%12%6%7%5%3435%5%29%
2117
April1st to 15thNight
19%12%46%5%3435%5%29%
1796
16th to endNight
17%8%35%433245%40%
1800
May1st to 15thNight
16%8%46%5%435%6%6%33%
1474
16th to endNight
11%6%36%6%345%6%8%38%
1495
June1st to 15thNight
18%7%35%42445%7%35%
1440
16th to endNight
12%4245%35%46%8%42%
1440
July1st to 15thNight
7%435%5%36%46%9%42%
1440
16th to endNight
11%3245%25%6%6%7%43%
1514
August1st to 15thNight
10%5%25%4345%6%8%42%
1620
16th to endNight
11%4245%25%438%46%
1884
September1st to 15thNight
14%5%24425%45%6%42%
1800
16th to endNight
14%5%245%35%436%44%
1960
October1st to 15thNight
20%8%46%5%23345%35%
2160
16th to endNight
23%12%47%6%23345%25%
2347
November1st to 15thNight
15%12%47%6%25%445%31%
2472
16th to endNight
17%14%6%7%5%24445%27%
2730
December1st to 15thNight
20%15%7%10%7%3434416%
2730
16th to endNight
18%16%8%12%8%45%5%4412%
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
71%5%8%9%4
1553
16th to endDay
71%5%11%8%3
1807
February1st to 15thDay
75%47%9%3
1712
16th to endDay
69%5%8%11%4
1672
March1st to 15thDay
65%6%11%11%5%
2047
16th to endDay
57%5%10%15%10%
2234
April1st to 15thDay
56%410%18%8%1
2152
16th to endDay
48%5%8%19%15%1
2169
May1st to 15thDay
52%6%11%20%9%1
2477
16th to endDay
47%8%13%16%11%
2622
June1st to 15thDay
51%7%12%19%9%
2520
16th to endDay
48%8%14%18%10%
2520
July1st to 15thDay
45%9%12%21%9%
2371
16th to endDay
46%9%16%19%8%
2484
August1st to 15thDay
47%10%14%19%7%
2327
16th to endDay
45%9%12%21%10%
2306
September1st to 15thDay
48%5%10%21%12%
2160
16th to endDay
44%5%9%23%15%1
1998
October1st to 15thDay
52%5%9%19%11%1
1800
16th to endDay
61%6%13%13%4
1875
November1st to 15thDay
53%6%14%15%8%2
1713
16th to endDay
62%5%12%13%5%
1560
December1st to 15thDay
69%6%11%8%3
1560
16th to endDay
77%5%9%6%1
1664
January1st to 15thNight
69%6%9%8%4
2716
16th to endNight
69%5%10%10%3
2766
February1st to 15thNight
69%7%8%9%4
2482
16th to endNight
63%6%11%12%5%
2087
March1st to 15thNight
61%6%11%14%5%
2243
16th to endNight
54%6%10%16%10%1
2117
April1st to 15thNight
55%5%10%16%9%1
1796
16th to endNight
45%410%20%16%2
1800
May1st to 15thNight
50%6%10%18%12%1
1474
16th to endNight
41%9%14%21%12%1
1497
June1st to 15thNight
46%7%13%22%9%
1440
16th to endNight
37%8%16%24%12%1
1440
July1st to 15thNight
35%10%13%25%14%1
1360
16th to endNight
36%7%16%22%13%2
1418
August1st to 15thNight
35%10%17%25%10%
1609
16th to endNight
36%6%15%26%13%1
1884
September1st to 15thNight
39%8%13%22%13%1
1800
16th to endNight
40%5%11%24%16%1
1960
October1st to 15thNight
50%5%10%18%13%
2160
16th to endNight
59%6%14%14%5%
2347
November1st to 15thNight
54%5%12%16%8%2
2472
16th to endNight
57%6%13%16%5%
2730
December1st to 15thNight
69%6%12%8%2
2730
16th to endNight
74%7%9%6%2
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
31%27%30%6%22
90
16th to endDay
24%32%34%5%3
129
February1st to 15thDay
31%36%30%2
135
16th to endDay
32%26%29%9%1
253
March1st to 15thDay
33%20%30%12%3
330
16th to endDay
26%22%28%14%7%
311
April1st to 15thDay
31%18%26%15%5%1
270
16th to endDay
27%13%26%18%9%4
270
May1st to 15thDay
30%14%26%14%10%4
288
16th to endDay
18%12%36%17%13%2
331
June1st to 15thDay
27%11%26%22%10%2
315
16th to endDay
20%13%33%21%9%2
315
July1st to 15thDay
15%7%30%28%12%5%
305
16th to endDay
14%6%25%32%15%4
326
August1st to 15thDay
16%5%29%20%18%8%
313
16th to endDay
13%6%25%26%13%15%
332
September1st to 15thDay
21%7%16%24%19%9%
315
16th to endDay
17%10%17%21%21%11%
314
October1st to 15thDay
25%8%25%19%15%5%
315
16th to endDay
33%15%28%16%7%
325
November1st to 15thDay
27%12%24%23%9%1
277
16th to endDay
26%26%34%11%1
90
December1st to 15thDay
28%23%28%12%5%1
90
16th to endDay
39%28%27%5%
96
January1st to 15thNight
25%28%29%14%3
780
16th to endNight
23%21%34%17%1
832
February1st to 15thNight
22%25%37%14%1
764
16th to endNight
25%22%32%16%2
683
March1st to 15thNight
28%22%30%17%2
780
16th to endNight
20%24%27%19%7%
792
April1st to 15thNight
31%18%27%19%2
718
16th to endNight
25%15%28%22%8%
720
May1st to 15thNight
23%11%24%30%9%
394
16th to endNight
12%7%32%32%14%1
375
June1st to 15thNight
21%10%25%30%11%
360
16th to endNight
15%425%40%13%
360
July1st to 15thNight
9%223%44%18%1
340
16th to endNight
14%218%44%18%2
376
August1st to 15thNight
13%224%33%22%2
539
16th to endNight
14%317%33%28%2
738
September1st to 15thNight
19%6%21%31%18%2
720
16th to endNight
16%8%21%24%23%5%
717
October1st to 15thNight
26%11%21%23%15%1
720
16th to endNight
31%15%27%20%4
768
November1st to 15thNight
23%15%31%21%8%
760
16th to endNight
26%21%26%18%5%1
780
December1st to 15thNight
30%24%26%15%2
780
16th to endNight
28%25%30%13%1
832
Page generated at 2018-03-22 11:34:24. Version ac04

Copyright 2018 A. Danko


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