Observatoire Le Valhalla
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Observatoire Le Valhalla clear sky chart from 2005-11-06 to 2024-01-10.

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%
22548
13%
74694
8%
22510
36%
22510
15%
53553

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
15%
760
8%
2647
3%
757
20%
757
7%
2136
January11th to 20thNight
17%
720
9%
2520
4%
720
24%
720
9%
1989
January21st to endNight
21%
792
12%
2772
7%
792
29%
792
12%
2028
February1st to 10thNight
15%
720
10%
2340
3%
720
24%
720
10%
1830
February11th to 20thNight
17%
702
12%
2207
4%
700
28%
700
13%
1631
February21st to endNight
21%
592
15%
1776
6%
592
31%
592
14%
1303
March1st to 10thNight
20%
720
14%
2160
8%
720
28%
720
14%
1635
March11th to 20thNight
21%
720
15%
2057
5%
720
30%
720
15%
1503
March21st to endNight
29%
792
20%
2008
9%
792
36%
792
22%
1457
April1st to 10thNight
24%
720
15%
1800
4%
720
29%
720
19%
1352
April11th to 20thNight
26%
711
15%
1775
4%
711
32%
711
21%
1303
April21st to endNight
26%
608
16%
1688
6%
608
33%
608
18%
1216
May1st to 10thNight
29%
378
16%
1458
11%
378
39%
378
17%
987
May11th to 20thNight
33%
354
19%
1416
15%
354
47%
354
18%
892
May21st to endNight
32%
389
13%
1551
10%
388
45%
388
19%
957
June1st to 10thNight
33%
360
9%
1440
8%
360
43%
360
15%
988
June11th to 20thNight
38%
360
12%
1440
14%
360
48%
360
19%
912
June21st to endNight
39%
360
10%
1440
13%
360
47%
360
17%
937
July1st to 10thNight
48%
350
14%
1400
19%
350
56%
350
24%
810
July11th to 20thNight
50%
348
15%
1392
19%
348
58%
348
23%
824
July21st to endNight
48%
394
14%
1486
16%
372
53%
372
22%
875
August1st to 10thNight
55%
358
16%
1422
17%
356
60%
356
23%
840
August11th to 20thNight
52%
552
17%
1616
11%
552
55%
552
28%
1041
August21st to endNight
44%
792
17%
1980
8%
792
48%
792
31%
1346
September1st to 10thNight
46%
720
15%
1800
7%
720
48%
720
30%
1232
September11th to 20thNight
49%
720
21%
1849
12%
720
53%
720
33%
1189
September21st to endNight
43%
720
21%
1980
14%
720
48%
720
27%
1314
October1st to 10thNight
32%
720
17%
2137
11%
720
41%
720
20%
1477
October11th to 20thNight
29%
720
15%
2167
9%
720
37%
720
17%
1575
October21st to endNight
22%
792
11%
2574
7%
792
29%
792
13%
1933
November1st to 10thNight
27%
740
15%
2407
8%
740
37%
740
15%
1687
November11th to 20thNight
23%
760
15%
2574
9%
760
33%
760
13%
1838
November21st to endNight
19%
760
10%
2660
5%
760
26%
760
9%
2020
December1st to 10thNight
15%
760
8%
2660
3%
760
21%
760
8%
2150
December11th to 20thNight
15%
760
11%
3249
4%
759
24%
759
7%
2512
December21st to endNight
10%
824
6%
4846
2%
817
18%
817
4%
3834

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%9%37%6%35%5%5%7%33%
74731
Night
18%10%47%6%3445%6%32%
74942
TransparencyDay
50%7%12%19%10%1
74436
Night
52%7%11%19%10%1
74771
SeeingDay
21%15%30%21%10%3
13429
Night
22%13%30%26%9%1
22561

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
17%18%6%10%8%44335%22%
1509
11th to 20thDay
20%16%6%8%6%345%5%425%
1440
21st to endDay
13%14%5%9%6%46%5%5%7%26%
1584
February1st to 10thDay
16%14%5%11%6%46%5%46%24%
1620
11th to 20thDay
14%13%49%7%45%45%6%27%
1640
21st to endDay
20%13%5%8%7%35%445%26%
1480
March1st to 10thDay
16%11%5%9%9%44445%29%
1797
11th to 20thDay
16%12%47%7%35%5%46%31%
1902
21st to endDay
21%9%35%6%3445%5%35%
2347
April1st to 10thDay
21%10%47%6%34446%33%
2160
11th to 20thDay
22%7%36%5%3445%6%35%
2134
21st to endDay
24%8%36%6%34446%31%
2272
May1st to 10thDay
20%7%36%5%45%5%5%6%35%
2518
11th to 20thDay
18%7%36%5%35%5%47%39%
2478
21st to endDay
18%6%26%5%35%45%7%37%
2725
June1st to 10thDay
22%7%36%5%35%5%5%8%30%
2520
11th to 20thDay
15%5%36%5%35%5%6%7%39%
2520
21st to endDay
20%6%36%5%46%5%6%8%31%
2520
July1st to 10thDay
12%5%36%5%35%5%6%8%42%
2520
11th to 20thDay
13%5%25%5%46%6%8%9%38%
2520
21st to endDay
13%6%25%5%36%5%7%10%39%
2744
August1st to 10thDay
14%5%25%6%45%5%5%9%39%
2506
11th to 20thDay
11%5%25%6%46%5%6%9%41%
2265
21st to endDay
14%5%25%5%35%6%6%8%41%
2376
September1st to 10thDay
13%5%25%4345%7%8%42%
2160
11th to 20thDay
13%5%34425%5%47%49%
2110
21st to endDay
17%6%25%445%45%7%41%
1980
October1st to 10thDay
20%7%36%5%345%5%5%36%
1820
11th to 20thDay
20%9%37%8%45%5%5%5%29%
1792
21st to endDay
25%10%37%5%35%5%5%6%27%
1782
November1st to 10thDay
17%8%47%5%45%5%5%7%34%
1671
11th to 20thDay
17%10%47%7%46%5%5%6%29%
1601
21st to endDay
21%11%47%6%5%5%7%5%6%23%
1520
December1st to 10thDay
24%13%48%7%35%45%6%18%
1520
11th to 20thDay
22%13%5%9%6%3445%6%24%
1906
21st to endDay
29%16%5%8%5%3433320%
2772
January1st to 10thNight
18%16%6%10%7%445%45%20%
2647
11th to 20thNight
19%16%6%9%7%3444424%
2520
21st to endNight
12%12%5%9%9%45%446%29%
2772
February1st to 10thNight
16%12%6%10%8%46%6%5%6%22%
2340
11th to 20thNight
15%11%49%7%45%5%6%5%28%
2205
21st to endNight
16%13%5%8%6%35%5%4431%
1776
March1st to 10thNight
17%12%5%8%7%35%5%5%5%29%
2150
11th to 20thNight
17%13%46%6%35%345%33%
2056
21st to endNight
18%9%47%5%34435%38%
2008
April1st to 10thNight
24%8%45%6%44446%32%
1800
11th to 20thNight
22%7%37%5%2445%5%36%
1775
21st to endNight
22%9%35%6%35%446%33%
1688
May1st to 10thNight
22%6%36%6%35%45%5%36%
1458
11th to 20thNight
18%7%36%5%3335%7%40%
1415
21st to endNight
15%5%35%5%35%45%8%42%
1550
June1st to 10thNight
20%7%25%6%35%5%7%8%31%
1440
11th to 20thNight
17%6%35%435%46%7%40%
1440
21st to endNight
16%7%26%5%35%5%6%8%36%
1440
July1st to 10thNight
11%325%5%35%6%6%8%45%
1440
11th to 20thNight
12%5%26%5%45%5%6%10%41%
1440
21st to endNight
12%6%25%5%35%6%6%8%43%
1574
August1st to 10thNight
11%5%25%5%345%6%9%45%
1432
11th to 20thNight
12%5%25%5%345%5%6%46%
1616
21st to endNight
13%5%345%35%5%6%8%43%
1980
September1st to 10thNight
11%6%36%6%46%45%7%43%
1800
11th to 20thNight
15%6%345%23347%48%
1849
21st to endNight
17%4246%345%6%6%43%
1980
October1st to 10thNight
20%6%26%6%345%5%6%37%
2137
11th to 20thNight
20%8%37%6%35%5%5%6%32%
2167
21st to endNight
23%9%47%5%45%5%45%29%
2574
November1st to 10thNight
19%9%37%5%345%5%7%33%
2407
11th to 20thNight
19%8%47%6%45%5%5%6%32%
2574
21st to endNight
22%11%5%8%6%4445%6%26%
2660
December1st to 10thNight
20%14%6%9%7%445%5%5%20%
2660
11th to 20thNight
23%15%5%8%6%3444425%
3288
21st to endNight
31%16%48%5%2333421%
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
66%6%8%15%5%
1510
11th to 20thDay
63%6%9%11%10%1
1440
21st to endDay
57%6%12%15%9%1
1584
February1st to 10thDay
61%5%9%18%5%2
1620
11th to 20thDay
58%6%12%16%8%1
1649
21st to endDay
62%49%14%9%1
1480
March1st to 10thDay
58%410%16%9%2
1800
11th to 20thDay
52%7%12%15%11%3
1902
21st to endDay
52%5%8%18%14%4
2347
April1st to 10thDay
54%49%19%12%2
2160
11th to 20thDay
48%6%9%20%15%2
2134
21st to endDay
54%6%8%15%13%4
2272
May1st to 10thDay
49%6%9%18%15%2
2518
11th to 20thDay
45%5%11%20%16%3
2482
21st to endDay
45%8%13%19%14%2
2725
June1st to 10thDay
51%8%10%18%12%1
2520
11th to 20thDay
42%8%12%22%13%3
2520
21st to endDay
50%10%13%17%10%1
2520
July1st to 10thDay
37%9%13%25%14%1
2450
11th to 20thDay
40%10%15%27%10%
2445
21st to endDay
40%10%16%25%9%
2591
August1st to 10thDay
42%9%16%24%9%
2492
11th to 20thDay
37%10%18%25%10%
2265
21st to endDay
39%8%15%25%13%1
2376
September1st to 10thDay
38%9%15%26%13%
2160
11th to 20thDay
35%7%13%31%14%1
2110
21st to endDay
42%6%15%27%9%1
1980
October1st to 10thDay
48%7%11%23%11%
1820
11th to 20thDay
57%6%12%17%7%
1792
21st to endDay
57%7%12%16%7%1
1782
November1st to 10thDay
48%8%15%20%8%1
1671
11th to 20thDay
54%9%12%16%9%
1601
21st to endDay
60%9%14%14%3
1520
December1st to 10thDay
66%8%11%11%3
1520
11th to 20thDay
62%7%14%13%4
1906
21st to endDay
69%7%8%12%5%
2772
January1st to 10thNight
66%6%10%12%5%
2647
11th to 20thNight
64%7%9%12%8%1
2520
21st to endNight
57%5%11%16%10%1
2772
February1st to 10thNight
62%7%10%16%5%1
2340
11th to 20thNight
56%6%12%19%7%1
2207
21st to endNight
56%410%17%13%
1776
March1st to 10thNight
57%6%10%16%9%2
2160
11th to 20thNight
54%5%11%17%11%3
2057
21st to endNight
50%47%21%14%4
2008
April1st to 10thNight
54%5%9%18%13%2
1800
11th to 20thNight
49%6%8%21%14%2
1775
21st to endNight
51%6%8%18%12%4
1688
May1st to 10thNight
49%6%7%17%16%4
1458
11th to 20thNight
45%5%9%19%18%4
1416
21st to endNight
41%8%13%19%17%2
1551
June1st to 10thNight
48%9%12%20%10%1
1440
11th to 20thNight
43%8%11%21%16%2
1440
21st to endNight
44%9%14%19%13%
1440
July1st to 10thNight
34%9%13%25%17%1
1400
11th to 20thNight
37%8%14%29%11%
1392
21st to endNight
37%9%17%25%12%
1486
August1st to 10thNight
35%10%19%25%11%
1422
11th to 20thNight
38%8%17%26%11%1
1616
21st to endNight
38%8%16%26%11%1
1980
September1st to 10thNight
40%7%12%25%15%1
1800
11th to 20thNight
36%6%13%33%11%1
1849
21st to endNight
40%7%12%28%13%
1980
October1st to 10thNight
47%7%11%23%12%1
2137
11th to 20thNight
51%7%12%21%8%
2167
21st to endNight
57%7%13%15%8%1
2574
November1st to 10thNight
51%6%14%20%8%1
2407
11th to 20thNight
53%6%13%18%9%1
2574
21st to endNight
59%7%13%15%5%
2660
December1st to 10thNight
66%7%11%13%3
2660
11th to 20thNight
62%6%9%17%5%
3288
21st to endNight
68%5%9%14%4
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
25%19%26%24%41
178
11th to 20thDay
28%21%33%12%6%1
160
21st to endDay
19%20%38%16%7%
176
February1st to 10thDay
33%29%28%10%
240
11th to 20thDay
26%19%41%13%2
307
21st to endDay
30%19%29%19%2
346
March1st to 10thDay
25%19%33%20%4
420
11th to 20thDay
21%23%33%17%41
420
21st to endDay
27%18%28%17%9%1
462
April1st to 10thDay
25%20%31%15%7%1
420
11th to 20thDay
25%20%26%18%9%2
412
21st to endDay
28%17%31%14%8%1
420
May1st to 10thDay
21%18%34%19%6%2
423
11th to 20thDay
19%16%33%21%9%2
447
21st to endDay
19%16%32%23%8%2
485
June1st to 10thDay
24%15%28%23%8%2
450
11th to 20thDay
16%12%33%24%13%1
450
21st to endDay
20%13%30%24%10%3
450
July1st to 10thDay
11%11%30%28%16%4
445
11th to 20thDay
15%9%31%30%14%3
444
21st to endDay
16%7%31%29%14%3
488
August1st to 10thDay
15%8%26%32%14%4
446
11th to 20thDay
12%7%26%31%15%8%
439
21st to endDay
14%10%28%23%17%7%
495
September1st to 10thDay
14%9%32%23%16%6%
450
11th to 20thDay
13%11%25%24%17%10%
450
21st to endDay
17%13%29%20%17%4
450
October1st to 10thDay
22%13%27%20%14%3
450
11th to 20thDay
21%21%27%20%9%1
446
21st to endDay
30%16%28%20%6%
396
November1st to 10thDay
27%18%25%22%7%1
365
11th to 20thDay
26%22%33%16%4
261
21st to endDay
26%18%33%18%6%
180
December1st to 10thDay
29%17%28%20%5%1
180
11th to 20thDay
30%20%33%14%21
180
21st to endDay
35%19%28%15%31
198
January1st to 10thNight
25%18%34%20%3
760
11th to 20thNight
28%17%30%21%41
720
21st to endNight
19%15%36%24%6%
792
February1st to 10thNight
22%16%36%23%3
720
11th to 20thNight
21%19%34%22%4
703
21st to endNight
23%16%32%26%3
592
March1st to 10thNight
24%17%33%23%4
720
11th to 20thNight
25%17%32%22%41
720
21st to endNight
23%13%30%26%7%
792
April1st to 10thNight
27%15%28%25%5%
720
11th to 20thNight
24%15%31%24%6%1
711
21st to endNight
26%12%30%23%9%
608
May1st to 10thNight
24%11%31%25%8%3
378
11th to 20thNight
21%9%32%28%9%1
354
21st to endNight
17%7%37%29%10%
389
June1st to 10thNight
22%9%29%30%9%
360
11th to 20thNight
17%9%29%33%11%1
360
21st to endNight
18%8%28%30%15%1
360
July1st to 10thNight
11%6%30%35%17%1
350
11th to 20thNight
12%5%26%41%17%
348
21st to endNight
13%428%38%15%2
394
August1st to 10thNight
14%421%43%18%1
358
11th to 20thNight
13%424%33%24%3
552
21st to endNight
13%7%28%32%18%2
792
September1st to 10thNight
13%6%28%36%15%1
720
11th to 20thNight
19%6%20%33%20%2
720
21st to endNight
18%9%26%27%18%2
720
October1st to 10thNight
21%11%28%27%12%
720
11th to 20thNight
23%13%29%26%9%
720
21st to endNight
28%14%30%25%4
792
November1st to 10thNight
24%16%27%25%7%
740
11th to 20thNight
23%20%29%20%8%
760
21st to endNight
29%17%28%20%7%
760
December1st to 10thNight
29%17%30%19%4
760
11th to 20thNight
29%20%29%17%41
760
21st to endNight
36%16%29%15%5%
836
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