Observatoire Astronomique de Laval
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Observatoire Astronomique de Laval clear sky chart from 2005-11-05 to 2017-08-12.

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
35%
14492
14%
46477
9%
14460
41%
14460
18%
32867

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
22%
720
9%
2506
6%
716
27%
716
10%
1935
January16th to endNight
27%
768
11%
2688
7%
768
33%
768
12%
1984
February1st to 15thNight
25%
704
9%
2312
6%
701
31%
701
12%
1793
February16th to endNight
28%
631
14%
1893
7%
631
36%
631
16%
1408
March1st to 15thNight
28%
720
14%
2160
10%
720
34%
720
16%
1603
March16th to endNight
37%
768
20%
1998
10%
768
42%
768
25%
1405
April1st to 15thNight
30%
718
16%
1796
5%
718
36%
718
21%
1342
April16th to endNight
32%
620
17%
1700
6%
620
39%
620
23%
1201
May1st to 15thNight
34%
396
16%
1476
13%
396
43%
396
17%
987
May16th to endNight
36%
375
12%
1497
13%
374
43%
374
17%
997
June1st to 15thNight
41%
360
9%
1440
9%
360
47%
360
15%
996
June16th to endNight
45%
360
8%
1440
11%
360
49%
360
18%
924
July1st to 15thNight
55%
340
13%
1360
19%
340
60%
340
25%
779
July16th to endNight
57%
376
12%
1418
17%
354
60%
354
24%
820
August1st to 15thNight
57%
354
15%
1403
17%
352
59%
352
21%
854
August16th to endNight
57%
693
18%
1743
11%
693
59%
693
36%
1118
September1st to 15thNight
52%
660
14%
1650
7%
660
53%
660
29%
1148
September16th to endNight
51%
657
20%
1807
14%
657
55%
657
32%
1151
October1st to 15thNight
37%
660
20%
1941
12%
660
47%
660
23%
1324
October16th to endNight
31%
704
12%
2259
9%
704
36%
704
16%
1636
November1st to 15thNight
31%
700
18%
2277
12%
700
39%
700
18%
1610
November16th to endNight
27%
720
14%
2505
10%
720
34%
720
13%
1826
December1st to 15thNight
23%
720
9%
2520
4%
720
28%
720
11%
1934
December16th to endNight
17%
768
11%
2688
4%
768
25%
768
9%
2092

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%3445%6%33%
47503
Night
19%9%36%5%24446%33%
46646
Cloud CoverDay
18%8%36%5%3445%6%33%
47513
Night
19%9%36%5%34446%32%
46651
TransparencyDay
50%5%12%20%10%1
47190
Night
52%411%20%10%1
46477
SeeingDay
21%5%23%27%16%5%
5434
Night
23%5%27%31%11%
14492

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%7%6%4444423%
1432
16th to endDay
13%13%48%6%36%446%29%
1536
February1st to 15thDay
14%13%6%8%7%34445%25%
1548
16th to endDay
17%13%5%8%5%2444428%
1580
March1st to 15thDay
17%11%48%6%46%34428%
1790
16th to endDay
18%8%35%414335%39%
2224
April1st to 15thDay
22%8%35%6%2433433%
2152
16th to endDay
20%6%26%5%24435%38%
2259
May1st to 15thDay
22%5%36%5%3445%5%33%
2486
16th to endDay
20%5%25%5%25%46%7%32%
2618
June1st to 15thDay
22%6%25%5%345%46%31%
2520
16th to endDay
18%6%25%6%36%5%5%9%30%
2520
July1st to 15thDay
13%4246%36%6%6%7%38%
2520
16th to endDay
12%426%5%35%6%6%9%38%
2654
August1st to 15thDay
13%5%26%6%45%5%6%8%35%
2448
16th to endDay
10%324425%46%7%46%
2105
September1st to 15thDay
17%5%25%42445%7%41%
1980
16th to endDay
15%424424347%47%
1822
October1st to 15thDay
22%6%25%42445%6%34%
1688
16th to endDay
24%9%36%5%34445%28%
1612
November1st to 15thDay
16%9%46%5%2446%6%33%
1581
16th to endDay
23%9%35%5%24435%31%
1452
December1st to 15thDay
26%10%5%7%6%3444422%
1440
16th to endDay
26%13%46%5%3334424%
1536
January1st to 15thNight
18%15%6%8%6%34445%22%
2506
16th to endNight
12%13%5%8%7%35%45%5%27%
2688
February1st to 15thNight
13%13%6%10%8%35%445%23%
2310
16th to endNight
17%12%47%6%35%35%429%
1893
March1st to 15thNight
21%11%46%434436%29%
2147
16th to endNight
16%10%35%434335%39%
1998
April1st to 15thNight
26%7%36%423335%32%
1796
16th to endNight
23%6%24423345%37%
1700
May1st to 15thNight
21%6%27%5%14336%34%
1476
16th to endNight
20%6%34435%45%7%34%
1495
June1st to 15thNight
25%7%3443446%6%30%
1440
16th to endNight
17%5%25%4345%6%8%36%
1440
July1st to 15thNight
11%4145%25%5%7%8%42%
1440
16th to endNight
11%4245%35%5%6%10%41%
1514
August1st to 15thNight
13%5%35%5%2446%8%40%
1413
16th to endNight
13%31342435%7%47%
1743
September1st to 15thNight
16%426%434446%41%
1650
16th to endNight
16%424323346%48%
1807
October1st to 15thNight
17%6%35%424445%39%
1941
16th to endNight
25%8%36%424345%31%
2259
November1st to 15thNight
20%7%36%5%345%45%34%
2277
16th to endNight
22%11%38%5%2333429%
2505
December1st to 15thNight
23%13%47%6%24435%24%
2520
16th to endNight
23%16%5%7%5%2333423%
2688

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
21%14%5%8%6%25%43422%
1432
16th to endDay
13%13%48%6%35%446%29%
1536
February1st to 15thDay
14%13%5%8%7%35%445%24%
1548
16th to endDay
17%13%5%7%7%2445%428%
1580
March1st to 15thDay
18%9%48%7%36%35%428%
1800
16th to endDay
18%8%45%5%2434438%
2224
April1st to 15thDay
23%9%35%5%3434432%
2152
16th to endDay
20%6%36%424335%38%
2259
May1st to 15thDay
21%6%36%4345%5%5%33%
2486
16th to endDay
20%6%245%345%5%7%33%
2618
June1st to 15thDay
23%6%25%5%3345%8%31%
2520
16th to endDay
17%7%25%6%36%46%8%31%
2520
July1st to 15thDay
12%4246%36%6%6%8%38%
2520
16th to endDay
12%425%5%35%5%6%9%37%
2654
August1st to 15thDay
12%6%26%6%35%5%6%8%35%
2448
16th to endDay
10%423435%5%5%8%45%
2105
September1st to 15thDay
16%425%42445%7%41%
1980
16th to endDay
14%5%245%23347%46%
1822
October1st to 15thDay
20%7%35%5%3445%6%34%
1688
16th to endDay
23%9%36%6%3444428%
1612
November1st to 15thDay
17%8%46%5%2446%7%32%
1581
16th to endDay
24%8%36%5%25%435%31%
1452
December1st to 15thDay
27%10%47%6%34445%21%
1440
16th to endDay
26%14%46%423345%22%
1536
January1st to 15thNight
18%14%6%9%6%3444422%
2506
16th to endNight
13%13%6%7%7%45%45%5%27%
2688
February1st to 15thNight
14%13%7%10%8%35%44423%
2303
16th to endNight
17%12%5%7%5%25%44429%
1893
March1st to 15thNight
21%11%37%434345%29%
2160
16th to endNight
16%9%36%43424439%
1997
April1st to 15thNight
26%8%35%424335%32%
1796
16th to endNight
23%6%245%23445%36%
1700
May1st to 15thNight
21%6%37%5%34335%35%
1476
16th to endNight
20%5%35%425%5%5%7%34%
1495
June1st to 15thNight
25%6%35%425%45%7%30%
1440
16th to endNight
17%426%5%245%6%9%35%
1440
July1st to 15thNight
11%414435%5%6%9%41%
1440
16th to endNight
11%4245%345%7%10%41%
1514
August1st to 15thNight
13%5%25%5%25%46%8%40%
1413
16th to endNight
14%41342445%7%46%
1743
September1st to 15thNight
17%5%25%434447%41%
1650
16th to endNight
17%423323346%47%
1807
October1st to 15thNight
17%6%35%43445%6%38%
1941
16th to endNight
26%8%36%43333431%
2259
November1st to 15thNight
21%7%36%5%345%45%33%
2277
16th to endNight
22%12%47%423335%29%
2505
December1st to 15thNight
23%14%47%5%23345%24%
2520
16th to endNight
24%17%47%42433423%
2688

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
64%49%15%6%
1433
16th to endDay
55%5%12%17%8%
1536
February1st to 15thDay
59%411%18%4
1550
16th to endDay
58%410%15%9%1
1580
March1st to 15thDay
58%312%15%9%1
1800
16th to endDay
48%310%18%15%3
2224
April1st to 15thDay
54%37%19%12%2
2152
16th to endDay
49%48%19%14%3
2259
May1st to 15thDay
52%48%18%13%1
2486
16th to endDay
47%6%13%18%12%1
2622
June1st to 15thDay
51%6%11%19%10%1
2520
16th to endDay
49%6%11%21%8%1
2520
July1st to 15thDay
40%7%13%25%12%
2371
16th to endDay
40%7%17%24%8%
2487
August1st to 15thDay
44%7%17%22%7%
2434
16th to endDay
34%6%18%27%13%
2105
September1st to 15thDay
40%5%14%27%11%
1980
16th to endDay
38%412%32%12%
1822
October1st to 15thDay
48%5%13%22%9%
1688
16th to endDay
57%5%12%17%6%
1612
November1st to 15thDay
49%5%14%21%9%
1581
16th to endDay
55%5%12%18%7%
1452
December1st to 15thDay
63%412%15%3
1440
16th to endDay
64%411%11%8%
1536
January1st to 15thNight
63%410%16%4
2506
16th to endNight
57%5%10%17%8%
2688
February1st to 15thNight
63%49%16%5%
2312
16th to endNight
57%410%17%10%
1893
March1st to 15thNight
57%49%17%9%1
2160
16th to endNight
48%39%18%16%3
1998
April1st to 15thNight
54%26%20%14%1
1796
16th to endNight
49%37%19%15%4
1700
May1st to 15thNight
51%37%17%14%4
1476
16th to endNight
49%5%10%19%13%2
1497
June1st to 15thNight
52%5%10%19%10%
1440
16th to endNight
43%8%12%19%14%1
1440
July1st to 15thNight
35%6%14%27%15%
1360
16th to endNight
35%7%19%23%13%
1418
August1st to 15thNight
40%7%17%22%11%
1403
16th to endNight
35%5%16%29%12%
1743
September1st to 15thNight
42%5%12%25%12%
1650
16th to endNight
37%412%30%13%
1807
October1st to 15thNight
45%6%9%25%11%
1941
16th to endNight
55%412%20%7%
2259
November1st to 15thNight
52%412%19%9%2
2277
16th to endNight
58%310%19%7%
2505
December1st to 15thNight
62%411%17%3
2520
16th to endNight
65%39%13%6%
2688

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
18%11%23%31%15%
60
16th to endDay
28%7%29%23%7%3
64
February1st to 15thDay
35%11%23%26%3
88
16th to endDay
27%5%27%30%8%
239
March1st to 15thDay
22%9%29%28%9%
270
16th to endDay
22%429%27%12%3
288
April1st to 15thDay
26%12%24%23%9%3
270
16th to endDay
22%5%27%26%12%5%
270
May1st to 15thDay
26%5%26%23%12%5%
288
16th to endDay
21%7%26%23%16%3
331
June1st to 15thDay
27%6%21%26%13%4
315
16th to endDay
18%5%22%33%17%3
315
July1st to 15thDay
14%222%30%22%6%
305
16th to endDay
16%14%35%25%7%
326
August1st to 15thDay
14%217%28%25%11%
297
16th to endDay
9%217%29%23%17%
268
September1st to 15thDay
16%217%25%24%13%
255
16th to endDay
16%7%25%19%18%12%
254
October1st to 15thDay
21%5%23%27%17%4
255
16th to endDay
28%7%24%26%12%
245
November1st to 15thDay
24%6%26%24%15%2
235
16th to endDay
34%6%20%25%12%
72
December1st to 15thDay
28%333%25%6%3
60
16th to endDay
37%9%40%9%3
64
January1st to 15thNight
25%9%29%27%7%
720
16th to endNight
20%7%33%29%8%1
768
February1st to 15thNight
19%6%33%31%8%
704
16th to endNight
24%5%31%31%6%
631
March1st to 15thNight
27%5%31%29%6%
720
16th to endNight
21%428%34%10%
768
April1st to 15thNight
28%7%25%32%5%
718
16th to endNight
26%6%31%25%9%
620
May1st to 15thNight
23%5%29%30%10%
396
16th to endNight
22%330%34%8%
375
June1st to 15thNight
28%320%34%11%
360
16th to endNight
18%227%37%14%
360
July1st to 15thNight
10%323%40%21%
340
16th to endNight
12%220%46%17%1
376
August1st to 15thNight
15%214%40%24%1
354
16th to endNight
15%118%36%25%2
693
September1st to 15thNight
19%218%40%18%1
660
16th to endNight
18%322%33%20%1
657
October1st to 15thNight
21%5%28%32%11%
660
16th to endNight
30%6%25%28%8%
704
November1st to 15thNight
25%5%28%28%11%
700
16th to endNight
28%7%26%23%12%
720
December1st to 15thNight
29%7%28%27%6%
720
16th to endNight
32%7%32%21%5%
768
Page generated at 2017-08-13 14:16:59. Version ac04

Copyright 2017 A. Danko


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