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 2005-11-05 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
25%
15390
13%
49240
7%
15358
32%
15358
14%
35428

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
7%
2716
3%
776
18%
776
7%
2188
January16th to endNight
15%
832
9%
2912
4%
832
23%
832
9%
2209
February1st to 15thNight
13%
764
9%
2482
3%
761
21%
761
8%
1947
February16th to endNight
18%
683
13%
2064
5%
683
27%
683
11%
1550
March1st to 15thNight
15%
780
12%
2330
6%
780
22%
780
11%
1798
March16th to endNight
26%
792
21%
2064
9%
792
34%
792
19%
1436
April1st to 15thNight
21%
718
16%
1796
5%
718
27%
718
17%
1337
April16th to endNight
21%
645
16%
1725
3%
645
30%
645
16%
1229
May1st to 15thNight
26%
372
16%
1452
11%
372
38%
372
13%
980
May16th to endNight
26%
375
12%
1497
7%
374
41%
374
14%
974
June1st to 15thNight
26%
360
8%
1440
6%
360
36%
360
11%
1014
June16th to endNight
34%
360
9%
1440
8%
360
42%
360
14%
925
July1st to 15thNight
44%
340
14%
1360
18%
340
52%
340
22%
786
July16th to endNight
42%
376
12%
1418
13%
354
51%
354
22%
824
August1st to 15thNight
49%
386
14%
1457
12%
384
56%
384
20%
894
August16th to endNight
44%
750
19%
1896
9%
750
49%
750
31%
1234
September1st to 15thNight
43%
720
14%
1800
7%
720
45%
720
27%
1249
September16th to endNight
45%
717
22%
1953
14%
717
50%
717
30%
1246
October1st to 15thNight
30%
720
17%
2145
10%
720
39%
720
20%
1461
October16th to endNight
21%
768
11%
2442
7%
768
28%
768
13%
1826
November1st to 15thNight
24%
760
15%
2479
9%
760
33%
760
13%
1777
November16th to endNight
17%
780
11%
2730
5%
780
25%
780
9%
2041
December1st to 15thNight
12%
780
8%
2730
2%
780
19%
780
7%
2177
December16th to endNight
8%
832
10%
2912
2%
832
17%
832
6%
2326

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%7%36%5%35%45%6%33%
49626
Night
18%9%36%5%34445%32%
49409
Cloud CoverDay
17%7%36%5%3445%7%33%
49635
Night
18%9%46%5%34446%32%
49415
TransparencyDay
50%7%12%19%9%1
49313
Night
52%6%11%18%9%1
49240
SeeingDay
20%20%27%19%9%2
6093
Night
20%24%24%21%8%
15390

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
21%14%5%9%6%3434421%
1552
16th to endDay
14%16%5%7%6%35%5%46%24%
1664
February1st to 15thDay
16%12%411%6%46%5%46%22%
1710
16th to endDay
19%11%47%6%35%445%26%
1694
March1st to 15thDay
18%14%48%8%3433425%
1950
16th to endDay
18%7%35%5%2345%5%38%
2288
April1st to 15thDay
23%8%36%5%33345%33%
2152
16th to endDay
21%6%25%5%34446%35%
2235
May1st to 15thDay
20%6%36%5%35%445%33%
2511
16th to endDay
20%5%25%435%45%7%35%
2618
June1st to 15thDay
23%6%35%5%34447%31%
2520
16th to endDay
19%425%5%36%5%6%8%32%
2520
July1st to 15thDay
12%435%5%36%5%5%8%39%
2520
16th to endDay
12%5%25%436%5%7%9%36%
2654
August1st to 15thDay
13%426%6%45%5%5%9%36%
2492
16th to endDay
11%3144346%5%9%45%
2294
September1st to 15thDay
16%5%2442445%7%42%
2160
16th to endDay
14%425%324346%49%
2007
October1st to 15thDay
20%6%26%5%245%5%5%35%
1813
16th to endDay
22%9%36%6%35%5%45%26%
1782
November1st to 15thDay
16%8%45%5%36%5%5%6%32%
1706
16th to endDay
21%9%46%6%4345%5%27%
1560
December1st to 15thDay
24%12%38%6%35%45%7%19%
1560
16th to endDay
26%14%48%5%2443421%
1664
January1st to 15thNight
19%14%6%9%7%345%4420%
2716
16th to endNight
14%14%6%8%8%44435%25%
2912
February1st to 15thNight
14%13%5%10%8%46%5%5%5%22%
2480
16th to endNight
17%11%5%7%5%35%5%5%428%
2064
March1st to 15thNight
20%12%5%7%6%3434427%
2317
16th to endNight
16%10%35%42333442%
2064
April1st to 15thNight
25%7%35%5%23335%33%
1796
16th to endNight
21%6%24434345%37%
1725
May1st to 15thNight
22%6%35%6%24345%36%
1452
16th to endNight
18%5%35%5%2445%6%37%
1495
June1st to 15thNight
23%7%245%25%45%6%31%
1440
16th to endNight
15%5%25%426%5%6%8%36%
1440
July1st to 15thNight
10%315%436%5%7%9%42%
1440
16th to endNight
11%5%14435%5%6%7%44%
1514
August1st to 15thNight
9%6%26%5%245%7%8%41%
1467
16th to endNight
11%4235%34447%48%
1896
September1st to 15thNight
15%5%25%5%34446%41%
1800
16th to endNight
16%42342335%6%48%
1953
October1st to 15thNight
19%6%26%42445%6%38%
2145
16th to endNight
23%8%46%5%34445%29%
2442
November1st to 15thNight
21%7%36%434445%33%
2479
16th to endNight
22%9%48%5%34445%27%
2730
December1st to 15thNight
21%15%5%7%6%3435%422%
2730
16th to endNight
24%16%47%6%2433422%
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
22%13%6%8%5%4444421%
1552
16th to endDay
14%13%5%8%6%35%45%6%25%
1664
February1st to 15thDay
15%12%5%10%6%36%5%46%23%
1710
16th to endDay
19%11%6%6%7%35%35%427%
1694
March1st to 15thDay
17%13%5%8%7%35%33426%
1960
16th to endDay
18%6%36%434345%39%
2287
April1st to 15thDay
22%8%35%5%34445%32%
2152
16th to endDay
19%6%35%5%24446%36%
2235
May1st to 15thDay
19%6%37%5%34446%33%
2511
16th to endDay
18%6%26%5%35%447%35%
2618
June1st to 15thDay
22%6%35%43445%7%31%
2520
16th to endDay
18%425%5%36%5%6%9%31%
2520
July1st to 15thDay
12%426%5%25%5%6%8%39%
2520
16th to endDay
12%425%5%36%48%10%36%
2654
August1st to 15thDay
12%5%26%7%35%5%5%8%35%
2492
16th to endDay
10%3144445%6%8%45%
2294
September1st to 15thDay
15%5%25%42345%7%42%
2160
16th to endDay
13%5%24323347%47%
2007
October1st to 15thDay
19%7%35%5%35%46%5%35%
1813
16th to endDay
22%10%47%43445%6%26%
1782
November1st to 15thDay
16%8%45%5%35%5%5%6%32%
1706
16th to endDay
21%9%37%5%35%45%7%27%
1560
December1st to 15thDay
24%12%37%5%45%5%5%5%19%
1560
16th to endDay
25%14%49%5%23345%21%
1664
January1st to 15thNight
19%15%6%9%7%3445%419%
2716
16th to endNight
13%15%6%8%7%45%445%25%
2912
February1st to 15thNight
14%12%5%11%7%46%5%45%21%
2473
16th to endNight
17%11%5%8%5%35%44428%
2064
March1st to 15thNight
20%12%47%6%35%345%26%
2330
16th to endNight
16%10%45%42334441%
2064
April1st to 15thNight
25%7%35%5%2325%5%32%
1796
16th to endNight
21%7%25%424436%36%
1725
May1st to 15thNight
21%8%45%5%23336%35%
1452
16th to endNight
18%435%5%34448%36%
1495
June1st to 15thNight
23%7%245%26%45%6%30%
1440
16th to endNight
15%5%25%5%25%6%5%9%36%
1440
July1st to 15thNight
10%3236%35%5%7%10%42%
1440
16th to endNight
10%5%25%435%6%7%7%42%
1514
August1st to 15thNight
10%426%5%345%6%9%40%
1467
16th to endNight
12%4244245%5%8%46%
1896
September1st to 15thNight
15%5%26%5%24346%41%
1800
16th to endNight
16%423433346%47%
1953
October1st to 15thNight
18%7%26%424446%37%
2145
16th to endNight
23%8%37%6%23445%28%
2442
November1st to 15thNight
20%8%36%5%44446%33%
2479
16th to endNight
22%10%48%5%24445%26%
2730
December1st to 15thNight
21%15%5%8%6%3444422%
2730
16th to endNight
24%15%5%8%6%3333421%
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
66%6%7%13%5%
1553
16th to endDay
59%7%12%13%7%
1664
February1st to 15thDay
61%5%10%17%3
1712
16th to endDay
59%5%9%14%8%1
1694
March1st to 15thDay
62%5%10%13%7%1
1960
16th to endDay
46%5%9%17%15%4
2288
April1st to 15thDay
53%47%19%12%2
2152
16th to endDay
49%5%8%18%14%4
2235
May1st to 15thDay
50%6%9%17%13%2
2511
16th to endDay
47%6%12%20%11%1
2622
June1st to 15thDay
51%6%11%19%10%
2520
16th to endDay
48%8%13%18%9%
2520
July1st to 15thDay
38%9%14%25%12%
2371
16th to endDay
41%10%17%22%8%
2487
August1st to 15thDay
43%9%16%22%7%
2478
16th to endDay
33%9%17%26%12%
2294
September1st to 15thDay
40%7%13%26%12%
2160
16th to endDay
36%413%33%11%
2007
October1st to 15thDay
49%6%11%23%9%
1813
16th to endDay
57%7%13%14%6%
1782
November1st to 15thDay
49%8%14%18%8%
1706
16th to endDay
56%8%11%16%5%
1560
December1st to 15thDay
64%9%12%10%3
1560
16th to endDay
66%6%9%10%6%
1664
January1st to 15thNight
65%7%10%11%4
2716
16th to endNight
60%5%11%15%6%
2912
February1st to 15thNight
62%6%10%16%3
2482
16th to endNight
56%5%10%16%10%
2064
March1st to 15thNight
60%5%9%16%7%1
2330
16th to endNight
46%39%19%15%4
2064
April1st to 15thNight
53%47%19%13%1
1796
16th to endNight
48%5%7%18%15%3
1725
May1st to 15thNight
50%5%8%16%15%4
1452
16th to endNight
46%7%11%19%13%1
1497
June1st to 15thNight
53%6%11%19%8%
1440
16th to endNight
42%9%13%18%14%
1440
July1st to 15thNight
33%9%13%28%15%
1360
16th to endNight
36%7%18%22%13%
1418
August1st to 15thNight
36%9%19%22%11%
1457
16th to endNight
34%7%15%29%12%
1896
September1st to 15thNight
42%6%11%25%13%
1800
16th to endNight
36%5%11%32%13%
1953
October1st to 15thNight
46%5%11%25%10%
2145
16th to endNight
56%6%14%17%5%
2442
November1st to 15thNight
52%6%13%17%8%1
2479
16th to endNight
58%6%13%16%4
2730
December1st to 15thNight
64%6%11%14%2
2730
16th to endNight
66%5%9%12%5%
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
26%18%25%24%4
90
16th to endDay
29%21%32%13%3
96
February1st to 15thDay
33%28%32%5%
135
16th to endDay
30%23%28%15%2
275
March1st to 15thDay
27%27%26%14%3
330
16th to endDay
21%27%26%16%5%2
311
April1st to 15thDay
25%28%27%11%5%1
270
16th to endDay
18%27%25%17%9%1
270
May1st to 15thDay
22%27%24%15%6%2
288
16th to endDay
19%25%30%14%8%1
331
June1st to 15thDay
24%20%31%18%41
315
16th to endDay
19%17%31%21%8%1
315
July1st to 15thDay
14%15%29%23%14%1
305
16th to endDay
14%13%25%29%13%3
326
August1st to 15thDay
12%11%26%32%12%5%
313
16th to endDay
10%10%29%25%16%8%
332
September1st to 15thDay
15%12%23%21%18%8%
315
16th to endDay
12%15%22%19%19%9%
314
October1st to 15thDay
21%18%22%20%15%1
315
16th to endDay
24%25%23%19%5%
301
November1st to 15thDay
20%24%24%20%9%1
270
16th to endDay
30%23%26%17%2
90
December1st to 15thDay
25%17%36%16%3
90
16th to endDay
39%20%30%9%
96
January1st to 15thNight
20%30%30%15%3
780
16th to endNight
20%30%27%17%4
832
February1st to 15thNight
19%29%29%18%3
764
16th to endNight
20%29%26%19%4
683
March1st to 15thNight
26%27%25%16%3
780
16th to endNight
18%27%23%25%4
792
April1st to 15thNight
23%28%23%21%3
718
16th to endNight
22%27%24%17%7%
645
May1st to 15thNight
21%24%22%22%7%2
372
16th to endNight
16%22%31%21%8%
375
June1st to 15thNight
21%23%22%27%5%
360
16th to endNight
15%21%24%24%13%1
360
July1st to 15thNight
11%16%24%29%16%
340
16th to endNight
11%18%24%31%13%1
376
August1st to 15thNight
12%11%19%34%20%1
386
16th to endNight
12%14%23%27%19%2
750
September1st to 15thNight
16%13%22%31%14%1
720
16th to endNight
17%14%19%23%22%2
717
October1st to 15thNight
20%18%25%23%11%
720
16th to endNight
25%20%25%21%5%
768
November1st to 15thNight
20%24%25%21%8%
760
16th to endNight
21%31%24%14%8%
780
December1st to 15thNight
27%30%22%16%2
780
16th to endNight
28%30%23%11%4
832
Page generated at 2018-03-22 11:34:23. Version ac04

Copyright 2018 A. Danko


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