MVAS Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the MVAS Observatory clear sky chart from 2005-11-06 to 2024-01-13.

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%
22842
11%
74751
8%
22798
32%
22798
13%
55974

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
10%
760
8%
2649
4%
757
15%
757
5%
2211
January11th to 20thNight
12%
732
6%
2560
3%
732
16%
732
5%
2176
January21st to endNight
13%
792
7%
2642
3%
792
17%
792
6%
2190
February1st to 10thNight
11%
720
7%
2340
2%
720
16%
720
6%
1977
February11th to 20thNight
11%
703
8%
2205
2%
700
18%
700
7%
1832
February21st to endNight
13%
592
8%
1776
4%
592
18%
592
9%
1476
March1st to 10thNight
17%
720
10%
2160
4%
720
23%
720
11%
1708
March11th to 20thNight
17%
720
12%
1991
5%
720
23%
720
12%
1556
March21st to endNight
24%
792
16%
2049
6%
792
31%
792
16%
1557
April1st to 10thNight
19%
720
11%
1800
4%
720
24%
720
16%
1405
April11th to 20thNight
26%
711
13%
1775
5%
711
29%
711
19%
1338
April21st to endNight
27%
710
14%
1790
6%
710
31%
710
19%
1334
May1st to 10thNight
30%
401
14%
1481
12%
401
35%
401
14%
1038
May11th to 20thNight
35%
354
18%
1416
15%
354
45%
354
17%
907
May21st to endNight
44%
388
14%
1551
15%
388
51%
388
19%
974
June1st to 10thNight
46%
360
14%
1440
17%
360
54%
360
18%
937
June11th to 20thNight
46%
360
13%
1440
17%
360
51%
360
18%
901
June21st to endNight
46%
360
13%
1440
16%
360
53%
360
20%
899
July1st to 10thNight
56%
350
14%
1400
21%
350
59%
350
21%
820
July11th to 20thNight
60%
348
16%
1392
22%
348
62%
348
24%
828
July21st to endNight
57%
394
16%
1486
19%
372
59%
372
23%
885
August1st to 10thNight
53%
451
16%
1515
14%
449
56%
449
27%
925
August11th to 20thNight
49%
610
13%
1674
9%
610
51%
610
24%
1175
August21st to endNight
52%
792
17%
1980
9%
792
54%
792
30%
1342
September1st to 10thNight
49%
720
15%
1800
8%
720
52%
720
29%
1236
September11th to 20thNight
50%
720
22%
1818
14%
720
54%
720
32%
1205
September21st to endNight
43%
720
20%
1998
15%
720
48%
720
25%
1364
October1st to 10thNight
35%
720
12%
2160
10%
720
40%
720
18%
1561
October11th to 20thNight
29%
720
12%
2160
8%
720
35%
720
16%
1576
October21st to endNight
18%
792
10%
2496
6%
792
24%
792
10%
1975
November1st to 10thNight
27%
740
15%
2407
11%
740
34%
740
15%
1739
November11th to 20thNight
20%
760
14%
2535
10%
760
27%
760
11%
1884
November21st to endNight
18%
760
9%
2660
5%
760
23%
760
9%
2099
December1st to 10thNight
12%
760
4%
2660
2%
760
15%
760
5%
2239
December11th to 20thNight
13%
760
6%
3269
2%
759
18%
759
5%
2691
December21st to endNight
10%
830
2%
4836
0%
817
13%
817
3%
4014

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
18%11%48%6%35%5%5%7%29%
74740
Night
18%12%5%8%6%3445%6%28%
74989
TransparencyDay
55%7%11%17%9%1
74445
Night
57%6%11%16%8%1
74818
SeeingDay
26%17%27%19%9%3
13580
Night
26%15%26%24%8%1
22849

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 10thDay
13%20%10%12%6%45%435%16%
1517
11th to 20thDay
17%19%6%14%9%3445%415%
1456
21st to endDay
12%17%7%12%10%5%7%5%45%16%
1712
February1st to 10thDay
17%17%7%15%10%45%5%4412%
1620
11th to 20thDay
13%16%9%14%9%45%445%17%
1641
21st to endDay
20%17%7%13%8%3434417%
1480
March1st to 10thDay
17%15%7%12%6%4444422%
1797
11th to 20thDay
16%17%7%9%6%3433427%
1966
21st to endDay
21%12%48%5%33436%31%
2306
April1st to 10thDay
18%14%5%8%5%2445%5%30%
2160
11th to 20thDay
18%12%36%5%3445%5%34%
2134
21st to endDay
23%13%36%444445%31%
2168
May1st to 10thDay
21%11%36%5%35%45%6%32%
2493
11th to 20thDay
19%8%35%5%3445%8%36%
2478
21st to endDay
15%6%36%6%25%5%7%9%36%
2725
June1st to 10thDay
19%8%37%6%35%6%6%8%31%
2520
11th to 20thDay
21%5%345%45%6%6%9%33%
2520
21st to endDay
18%7%36%7%46%47%8%32%
2520
July1st to 10thDay
13%6%36%5%46%6%7%8%36%
2520
11th to 20thDay
12%6%36%6%46%6%7%8%35%
2520
21st to endDay
14%6%36%7%36%6%6%8%34%
2744
August1st to 10thDay
14%6%25%5%46%6%7%9%37%
2409
11th to 20thDay
15%7%26%7%46%6%8%9%30%
2207
21st to endDay
16%6%35%6%35%5%6%8%37%
2376
September1st to 10thDay
16%6%245%35%6%6%8%38%
2160
11th to 20thDay
17%7%35%433347%43%
2141
21st to endDay
16%6%25%6%36%5%6%7%36%
1958
October1st to 10thDay
22%9%37%6%44347%32%
1800
11th to 20thDay
21%9%26%5%35%6%5%6%31%
1800
21st to endDay
28%15%36%4345%36%22%
1859
November1st to 10thDay
17%12%46%5%35%5%5%7%31%
1671
11th to 20thDay
16%17%5%9%7%3444427%
1644
21st to endDay
18%17%7%10%6%3445%6%21%
1520
December1st to 10thDay
20%21%9%12%6%34345%15%
1520
11th to 20thDay
21%17%10%10%8%33335%17%
1906
21st to endDay
26%20%9%8%6%3334413%
2772
January1st to 10thNight
16%19%7%12%8%45%445%15%
2649
11th to 20thNight
18%19%7%13%10%4344415%
2560
21st to endNight
12%16%7%14%10%6%6%5%4416%
2642
February1st to 10thNight
17%16%7%16%9%45%44415%
2340
11th to 20thNight
14%14%7%14%10%45%5%45%17%
2203
21st to endNight
17%14%8%14%9%44345%17%
1776
March1st to 10thNight
16%13%8%13%8%34445%23%
2150
11th to 20thNight
17%14%6%9%7%345%45%26%
1990
21st to endNight
20%13%47%5%345%45%30%
2049
April1st to 10thNight
21%14%5%6%434446%29%
1800
11th to 20thNight
21%10%37%5%345%45%32%
1775
21st to endNight
21%11%36%5%2446%6%33%
1790
May1st to 10thNight
23%10%36%6%34446%32%
1481
11th to 20thNight
14%8%45%6%35%5%6%7%38%
1415
21st to endNight
15%5%26%5%35%5%7%8%39%
1550
June1st to 10thNight
19%5%26%5%35%5%5%7%37%
1440
11th to 20thNight
20%6%24425%5%5%9%38%
1440
21st to endNight
15%5%35%5%345%6%8%39%
1440
July1st to 10thNight
10%5%25%6%36%6%6%8%42%
1440
11th to 20thNight
11%5%35%435%6%6%8%44%
1440
21st to endNight
14%5%25%5%3446%7%45%
1574
August1st to 10thNight
12%5%25%5%345%7%9%43%
1525
11th to 20thNight
13%7%37%7%445%5%8%36%
1674
21st to endNight
13%7%35%5%2446%7%43%
1980
September1st to 10thNight
15%6%245%346%6%7%42%
1800
11th to 20thNight
18%6%2442335%6%47%
1818
21st to endNight
18%7%25%435%45%7%40%
1998
October1st to 10thNight
23%7%36%5%45%45%5%33%
2160
11th to 20thNight
22%9%46%5%35%45%6%31%
2160
21st to endNight
29%12%46%5%44445%23%
2496
November1st to 10thNight
17%12%38%6%35%45%6%31%
2407
11th to 20thNight
19%14%48%5%35%45%6%27%
2535
21st to endNight
17%18%7%9%6%3445%6%22%
2660
December1st to 10thNight
21%21%8%10%7%35%44415%
2660
11th to 20thNight
21%16%8%12%6%34445%16%
3288
21st to endNight
27%18%10%8%6%34335%13%
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
69%5%10%11%5%
1518
11th to 20thDay
73%5%8%8%42
1456
21st to endDay
70%5%9%9%5%1
1712
February1st to 10thDay
75%6%7%9%3
1620
11th to 20thDay
70%6%7%12%4
1650
21st to endDay
73%39%9%6%
1480
March1st to 10thDay
65%5%11%13%5%1
1800
11th to 20thDay
61%49%14%11%2
1966
21st to endDay
57%49%14%14%2
2306
April1st to 10thDay
57%48%20%9%2
2160
11th to 20thDay
52%5%12%19%12%1
2134
21st to endDay
55%5%8%18%11%2
2168
May1st to 10thDay
55%5%9%17%11%2
2493
11th to 20thDay
47%6%11%23%12%2
2482
21st to endDay
45%8%16%21%10%1
2725
June1st to 10thDay
50%8%12%18%11%1
2520
11th to 20thDay
47%7%14%22%9%1
2520
21st to endDay
49%8%14%19%9%
2520
July1st to 10thDay
44%9%11%24%12%1
2450
11th to 20thDay
43%10%15%25%7%
2445
21st to endDay
46%8%14%21%10%
2591
August1st to 10thDay
42%11%14%22%10%1
2395
11th to 20thDay
47%11%14%21%7%
2207
21st to endDay
44%8%13%24%9%1
2376
September1st to 10thDay
43%8%13%26%10%1
2160
11th to 20thDay
42%5%11%25%16%1
2141
21st to endDay
47%7%10%23%12%1
1958
October1st to 10thDay
53%7%10%19%10%1
1800
11th to 20thDay
50%8%14%21%6%1
1800
21st to endDay
64%7%10%10%7%1
1859
November1st to 10thDay
52%6%12%18%11%1
1671
11th to 20thDay
60%6%11%15%7%2
1644
21st to endDay
63%7%15%11%4
1520
December1st to 10thDay
73%6%9%10%3
1520
11th to 20thDay
69%7%13%8%4
1906
21st to endDay
75%47%10%3
2772
January1st to 10thNight
71%6%8%10%5%
2649
11th to 20thNight
73%6%8%8%5%
2560
21st to endNight
71%5%9%10%41
2642
February1st to 10thNight
72%5%8%10%5%
2340
11th to 20thNight
70%5%9%11%5%
2205
21st to endNight
70%5%9%10%6%
1776
March1st to 10thNight
64%5%11%14%6%1
2160
11th to 20thNight
62%5%9%16%8%1
1991
21st to endNight
56%5%8%17%12%2
2049
April1st to 10thNight
57%6%10%17%8%1
1800
11th to 20thNight
52%6%11%18%13%1
1775
21st to endNight
52%6%10%17%12%3
1790
May1st to 10thNight
54%5%10%15%14%2
1481
11th to 20thNight
44%8%10%22%14%1
1416
21st to endNight
41%9%14%22%11%2
1551
June1st to 10thNight
46%7%11%22%13%1
1440
11th to 20thNight
41%10%15%22%12%1
1440
21st to endNight
41%8%13%27%11%1
1440
July1st to 10thNight
37%9%14%25%15%2
1400
11th to 20thNight
36%9%16%26%12%1
1392
21st to endNight
38%8%15%23%14%1
1486
August1st to 10thNight
35%10%16%29%10%1
1515
11th to 20thNight
44%9%16%22%10%
1674
21st to endNight
39%8%14%26%12%1
1980
September1st to 10thNight
40%8%14%26%11%1
1800
11th to 20thNight
38%6%13%26%17%1
1818
21st to endNight
43%7%11%26%12%1
1998
October1st to 10thNight
51%7%13%18%10%1
2160
11th to 20thNight
52%7%14%17%8%1
2160
21st to endNight
64%5%10%12%8%1
2496
November1st to 10thNight
54%6%11%16%11%2
2407
11th to 20thNight
58%6%10%17%7%2
2535
21st to endNight
63%6%13%13%4
2660
December1st to 10thNight
72%6%11%9%2
2660
11th to 20thNight
70%6%9%11%4
3288
21st to endNight
73%5%9%10%2
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
33%19%28%17%3
180
11th to 20thDay
42%26%23%7%11
164
21st to endDay
28%27%32%9%3
234
February1st to 10thDay
45%27%23%5%
240
11th to 20thDay
33%32%28%6%1
308
21st to endDay
40%22%25%11%1
346
March1st to 10thDay
31%24%27%16%2
420
11th to 20thDay
32%26%25%13%3
420
21st to endDay
37%21%23%14%5%1
462
April1st to 10thDay
34%25%23%13%5%
420
11th to 20thDay
33%21%22%13%8%3
412
21st to endDay
36%18%25%14%6%2
420
May1st to 10thDay
30%19%29%13%8%2
423
11th to 20thDay
25%14%31%20%9%2
447
21st to endDay
18%13%33%21%12%4
485
June1st to 10thDay
21%16%28%24%8%2
450
11th to 20thDay
22%13%28%24%12%2
450
21st to endDay
19%15%34%21%8%2
450
July1st to 10thDay
14%10%28%28%14%5%
445
11th to 20thDay
14%10%28%29%16%4
444
21st to endDay
14%10%36%26%11%3
488
August1st to 10thDay
14%12%22%28%17%7%
446
11th to 20thDay
18%8%23%29%16%6%
439
21st to endDay
15%12%27%26%14%6%
495
September1st to 10thDay
18%9%24%25%17%7%
450
11th to 20thDay
16%12%24%21%16%10%
450
21st to endDay
18%14%26%24%13%4
450
October1st to 10thDay
27%12%26%17%13%4
450
11th to 20thDay
26%20%26%18%9%1
450
21st to endDay
38%15%26%16%5%
435
November1st to 10thDay
30%19%21%20%10%
365
11th to 20thDay
31%25%24%16%3
304
21st to endDay
43%15%28%13%1
180
December1st to 10thDay
39%17%31%9%21
180
11th to 20thDay
37%23%31%9%
180
21st to endDay
39%23%29%9%
198
January1st to 10thNight
31%25%28%14%2
760
11th to 20thNight
32%22%28%15%31
732
21st to endNight
22%24%33%18%2
792
February1st to 10thNight
32%21%30%16%1
720
11th to 20thNight
27%25%32%13%2
703
21st to endNight
30%21%31%15%2
592
March1st to 10thNight
29%19%29%21%2
720
11th to 20thNight
30%20%30%18%2
720
21st to endNight
30%18%24%21%6%
792
April1st to 10thNight
28%18%30%21%3
720
11th to 20thNight
27%17%27%20%8%
711
21st to endNight
28%14%28%24%6%
710
May1st to 10thNight
28%9%25%30%7%
401
11th to 20thNight
20%8%32%32%8%
354
21st to endNight
17%6%27%38%12%1
389
June1st to 10thNight
18%7%23%40%11%1
360
11th to 20thNight
20%7%21%39%13%
360
21st to endNight
16%5%26%43%10%1
360
July1st to 10thNight
11%5%21%43%19%1
350
11th to 20thNight
12%319%47%17%1
348
21st to endNight
15%419%46%15%1
394
August1st to 10thNight
14%5%22%38%21%1
451
11th to 20thNight
16%6%19%34%23%3
610
21st to endNight
15%7%18%39%20%1
792
September1st to 10thNight
17%6%23%34%18%2
720
11th to 20thNight
19%7%19%31%21%3
720
21st to endNight
20%10%23%30%15%2
720
October1st to 10thNight
28%9%22%24%16%2
720
11th to 20thNight
26%14%26%28%6%
720
21st to endNight
36%15%26%21%3
792
November1st to 10thNight
25%15%28%25%7%
740
11th to 20thNight
29%20%27%17%6%1
760
21st to endNight
30%20%28%18%4
760
December1st to 10thNight
35%23%26%14%3
760
11th to 20thNight
34%20%28%15%3
760
21st to endNight
33%26%26%14%1
836
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