Observatoire astronomique de Châteauguay
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Observatoire astronomique de Châteauguay clear sky chart from 2005-11-06 to 2024-01-27.

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
30%
22615
13%
74917
8%
22574
36%
22574
15%
54109

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
16%
760
9%
2649
4%
757
22%
757
7%
2134
January11th to 20thNight
16%
760
9%
2660
3%
760
23%
760
9%
2108
January21st to endNight
21%
820
11%
2868
6%
820
29%
820
12%
2132
February1st to 10thNight
17%
720
9%
2372
3%
720
24%
720
9%
1888
February11th to 20thNight
21%
702
11%
2177
5%
700
31%
700
14%
1613
February21st to endNight
21%
592
16%
1776
6%
592
33%
592
15%
1312
March1st to 10thNight
23%
720
14%
2160
9%
720
30%
720
14%
1637
March11th to 20thNight
23%
720
14%
2094
7%
720
31%
720
16%
1547
March21st to endNight
28%
792
20%
1980
8%
792
36%
792
22%
1452
April1st to 10thNight
25%
720
15%
1800
4%
720
30%
720
18%
1374
April11th to 20thNight
26%
711
14%
1775
4%
711
31%
711
21%
1320
April21st to endNight
28%
576
16%
1656
8%
576
35%
576
18%
1208
May1st to 10thNight
28%
419
15%
1499
10%
419
37%
419
16%
1023
May11th to 20thNight
33%
354
18%
1416
16%
354
44%
354
18%
910
May21st to endNight
37%
388
13%
1551
12%
388
47%
388
19%
975
June1st to 10thNight
33%
360
10%
1440
10%
360
43%
360
15%
986
June11th to 20thNight
38%
360
10%
1440
11%
360
46%
360
20%
895
June21st to endNight
42%
360
10%
1440
12%
360
49%
360
16%
957
July1st to 10thNight
50%
350
13%
1400
17%
350
57%
350
23%
829
July11th to 20thNight
52%
348
13%
1392
18%
348
56%
348
24%
827
July21st to endNight
51%
394
13%
1486
14%
372
55%
372
21%
889
August1st to 10thNight
59%
358
17%
1422
19%
356
62%
356
22%
841
August11th to 20thNight
51%
539
17%
1603
12%
539
55%
539
28%
1057
August21st to endNight
51%
792
18%
1980
9%
792
54%
792
31%
1335
September1st to 10thNight
49%
720
15%
1800
7%
720
51%
720
30%
1226
September11th to 20thNight
52%
720
19%
1876
12%
720
54%
720
32%
1221
September21st to endNight
41%
720
20%
1980
14%
720
46%
720
26%
1337
October1st to 10thNight
35%
720
17%
2096
10%
720
42%
720
22%
1471
October11th to 20thNight
31%
720
15%
2196
9%
720
39%
720
17%
1580
October21st to endNight
25%
792
11%
2574
7%
792
32%
792
13%
1928
November1st to 10thNight
25%
740
15%
2407
9%
740
33%
740
15%
1708
November11th to 20thNight
24%
760
16%
2535
10%
760
33%
760
15%
1819
November21st to endNight
19%
760
10%
2660
4%
760
26%
760
10%
2050
December1st to 10thNight
16%
760
7%
2660
3%
760
20%
760
8%
2174
December11th to 20thNight
14%
760
10%
3247
4%
759
22%
759
7%
2523
December21st to endNight
14%
828
5%
4850
3%
817
20%
817
5%
3823

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%36%6%35%5%5%7%33%
74876
Night
19%10%47%6%3445%6%31%
75164
TransparencyDay
49%8%12%20%10%1
74581
Night
53%7%11%18%10%1
74993
SeeingDay
21%13%29%23%11%3
13506
Night
23%10%30%27%9%1
22625

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
20%16%6%10%8%4433420%
1517
11th to 20thDay
19%15%5%10%7%5%4445%23%
1520
21st to endDay
13%14%48%7%46%6%5%7%25%
1632
February1st to 10thDay
16%15%5%11%8%46%345%25%
1584
11th to 20thDay
15%12%310%8%46%5%5%5%28%
1670
21st to endDay
19%14%5%7%7%34445%27%
1480
March1st to 10thDay
16%12%5%7%7%47%5%46%28%
1797
11th to 20thDay
16%10%47%6%45%45%7%32%
1862
21st to endDay
19%10%37%5%3345%7%35%
2375
April1st to 10thDay
22%10%46%5%35%345%33%
2160
11th to 20thDay
19%8%36%5%345%46%38%
2134
21st to endDay
22%9%45%6%35%5%46%32%
2303
May1st to 10thDay
20%7%37%5%45%5%46%35%
2475
11th to 20thDay
18%7%35%5%3445%7%39%
2478
21st to endDay
17%6%26%6%35%5%6%8%36%
2725
June1st to 10thDay
22%7%346%36%5%6%8%31%
2520
11th to 20thDay
16%6%26%5%35%5%5%8%40%
2520
21st to endDay
19%6%37%6%36%5%5%7%33%
2520
July1st to 10thDay
12%5%26%5%36%46%8%43%
2520
11th to 20thDay
13%5%25%6%35%5%8%10%38%
2520
21st to endDay
13%5%25%5%36%6%7%10%38%
2744
August1st to 10thDay
13%7%36%5%36%6%6%9%38%
2506
11th to 20thDay
11%5%25%435%5%7%10%43%
2274
21st to endDay
13%5%25%5%35%5%7%9%42%
2376
September1st to 10thDay
12%5%25%6%35%7%6%8%43%
2160
11th to 20thDay
15%6%2332445%7%49%
2084
21st to endDay
15%6%25%5%345%6%8%41%
1980
October1st to 10thDay
20%8%36%6%35%45%6%33%
1862
11th to 20thDay
19%8%46%7%45%6%6%6%29%
1763
21st to endDay
28%10%445%34445%29%
1782
November1st to 10thDay
19%11%36%6%345%5%6%32%
1671
11th to 20thDay
18%11%5%9%6%36%5%5%6%27%
1644
21st to endDay
22%12%48%7%45%5%5%5%22%
1520
December1st to 10thDay
24%15%6%9%7%3345%6%17%
1520
11th to 20thDay
22%14%5%8%6%35%5%6%5%21%
1906
21st to endDay
28%14%5%9%7%45%34418%
2772
January1st to 10thNight
18%17%6%9%8%45%445%19%
2649
11th to 20thNight
18%16%6%9%6%45%5%5%421%
2660
21st to endNight
15%13%6%9%8%45%445%28%
2868
February1st to 10thNight
16%15%7%12%8%45%445%20%
2372
11th to 20thNight
16%11%49%7%46%45%7%28%
2175
21st to endNight
18%12%5%9%6%3444432%
1776
March1st to 10thNight
18%13%47%7%35%445%28%
2150
11th to 20thNight
18%13%5%6%7%34445%31%
2094
21st to endNight
19%10%46%6%3435%437%
1980
April1st to 10thNight
24%9%35%6%3446%6%30%
1800
11th to 20thNight
24%9%35%5%25%446%34%
1775
21st to endNight
22%9%36%5%34445%34%
1656
May1st to 10thNight
22%8%37%5%3435%6%35%
1499
11th to 20thNight
19%7%36%5%4445%6%38%
1415
21st to endNight
16%6%35%44446%7%40%
1550
June1st to 10thNight
23%6%45%5%35%47%8%31%
1440
11th to 20thNight
18%5%26%4345%6%7%40%
1440
21st to endNight
18%6%36%6%45%6%5%8%34%
1440
July1st to 10thNight
12%425%5%36%6%6%8%43%
1440
11th to 20thNight
11%5%25%5%36%6%7%10%40%
1440
21st to endNight
12%5%35%5%35%5%6%10%40%
1574
August1st to 10thNight
13%5%25%435%46%10%44%
1432
11th to 20thNight
13%6%35%5%35%5%5%8%42%
1603
21st to endNight
14%6%25%5%35%45%8%43%
1980
September1st to 10thNight
14%5%26%6%345%6%5%44%
1800
11th to 20thNight
16%5%35%42436%6%47%
1876
21st to endNight
18%6%25%5%3445%7%41%
1980
October1st to 10thNight
19%7%26%5%35%5%5%7%36%
2096
11th to 20thNight
22%7%37%5%35%45%6%33%
2196
21st to endNight
24%10%37%5%4445%5%29%
2574
November1st to 10thNight
21%9%37%6%3445%6%31%
2407
11th to 20thNight
20%10%47%6%34445%32%
2535
21st to endNight
23%13%49%6%45%445%24%
2660
December1st to 10thNight
23%16%7%9%6%35%44418%
2660
11th to 20thNight
23%16%5%8%6%34345%23%
3288
21st to endNight
29%15%5%9%5%3344420%
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
68%410%14%4
1518
11th to 20thDay
64%5%10%11%9%
1520
21st to endDay
55%8%13%15%8%1
1632
February1st to 10thDay
63%5%9%18%42
1584
11th to 20thDay
57%7%12%17%7%1
1679
21st to endDay
59%6%10%14%9%1
1480
March1st to 10thDay
56%7%10%17%8%2
1800
11th to 20thDay
51%6%13%15%12%2
1862
21st to endDay
50%6%10%19%12%3
2375
April1st to 10thDay
53%6%8%19%11%2
2160
11th to 20thDay
46%7%9%22%15%2
2134
21st to endDay
51%7%10%17%11%3
2303
May1st to 10thDay
49%7%9%19%15%2
2475
11th to 20thDay
44%7%11%20%16%2
2482
21st to endDay
44%9%13%19%13%2
2725
June1st to 10thDay
50%8%12%18%11%1
2520
11th to 20thDay
42%7%11%24%13%3
2520
21st to endDay
50%8%12%19%10%1
2520
July1st to 10thDay
37%9%14%27%12%1
2450
11th to 20thDay
38%9%16%28%9%
2445
21st to endDay
40%10%16%25%9%
2591
August1st to 10thDay
40%10%17%25%8%
2492
11th to 20thDay
36%9%19%26%10%
2274
21st to endDay
37%8%16%25%13%1
2376
September1st to 10thDay
37%9%15%27%13%
2160
11th to 20thDay
35%7%13%31%14%1
2084
21st to endDay
40%8%15%27%9%1
1980
October1st to 10thDay
50%7%13%20%9%
1862
11th to 20thDay
52%9%15%18%6%
1763
21st to endDay
57%5%13%17%7%1
1782
November1st to 10thDay
50%8%14%20%8%1
1671
11th to 20thDay
56%7%12%15%10%
1644
21st to endDay
60%10%12%13%4
1520
December1st to 10thDay
68%8%10%11%3
1520
11th to 20thDay
62%9%13%12%5%
1906
21st to endDay
68%8%10%9%4
2772
January1st to 10thNight
67%6%9%13%5%
2649
11th to 20thNight
63%8%11%12%6%1
2660
21st to endNight
57%7%10%15%10%1
2868
February1st to 10thNight
66%5%9%14%5%1
2372
11th to 20thNight
56%6%13%18%7%1
2177
21st to endNight
56%5%8%18%12%1
1776
March1st to 10thNight
57%7%9%16%10%2
2160
11th to 20thNight
55%5%10%16%11%2
2095
21st to endNight
50%5%8%19%13%4
1980
April1st to 10thNight
54%6%9%17%12%2
1800
11th to 20thNight
51%6%9%19%13%2
1775
21st to endNight
52%6%8%17%13%4
1656
May1st to 10thNight
51%5%8%16%15%5%
1499
11th to 20thNight
46%6%10%16%18%4
1416
21st to endNight
43%7%12%19%16%3
1551
June1st to 10thNight
49%8%12%20%10%1
1440
11th to 20thNight
42%7%14%21%14%3
1440
21st to endNight
47%8%14%19%12%
1440
July1st to 10thNight
36%9%14%24%16%1
1400
11th to 20thNight
36%9%16%27%12%
1392
21st to endNight
37%9%19%23%10%
1486
August1st to 10thNight
36%9%18%25%12%
1422
11th to 20thNight
39%9%17%22%13%1
1603
21st to endNight
38%8%15%25%12%1
1980
September1st to 10thNight
39%8%12%25%15%1
1800
11th to 20thNight
38%6%13%31%12%1
1876
21st to endNight
42%7%12%25%14%
1980
October1st to 10thNight
47%7%11%23%12%1
2096
11th to 20thNight
51%7%12%22%8%
2196
21st to endNight
57%7%12%15%9%1
2574
November1st to 10thNight
53%6%12%19%9%1
2407
11th to 20thNight
54%6%12%18%9%1
2535
21st to endNight
61%7%12%15%5%
2660
December1st to 10thNight
68%6%11%12%3
2660
11th to 20thNight
64%6%10%14%6%
3288
21st to endNight
67%6%10%13%3
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
26%18%27%19%9%1
180
11th to 20thDay
28%17%33%18%31
180
21st to endDay
22%15%36%19%7%1
188
February1st to 10thDay
31%23%37%9%
224
11th to 20thDay
24%17%38%19%21
338
21st to endDay
27%16%29%26%2
346
March1st to 10thDay
24%14%35%21%6%
420
11th to 20thDay
18%21%35%19%6%1
420
21st to endDay
28%13%31%17%10%1
462
April1st to 10thDay
26%18%29%17%9%1
420
11th to 20thDay
27%17%25%17%10%3
412
21st to endDay
26%15%29%21%9%2
420
May1st to 10thDay
25%11%34%19%9%2
423
11th to 20thDay
17%12%34%23%11%3
447
21st to endDay
18%12%32%25%12%2
485
June1st to 10thDay
24%12%25%28%9%2
450
11th to 20thDay
15%10%28%29%15%2
450
21st to endDay
21%10%28%23%14%4
450
July1st to 10thDay
11%7%29%28%20%5%
445
11th to 20thDay
13%6%31%30%16%5%
444
21st to endDay
13%7%28%32%17%3
488
August1st to 10thDay
13%6%25%31%19%6%
446
11th to 20thDay
10%8%20%35%17%10%
439
21st to endDay
12%10%25%26%18%9%
495
September1st to 10thDay
11%8%26%28%17%9%
450
11th to 20thDay
16%10%20%25%18%12%
450
21st to endDay
15%10%29%22%18%6%
450
October1st to 10thDay
21%12%28%20%15%4
450
11th to 20thDay
20%16%30%19%13%1
431
21st to endDay
34%13%26%22%6%
396
November1st to 10thDay
27%15%24%24%8%1
365
11th to 20thDay
24%21%31%16%6%1
304
21st to endDay
27%16%31%21%5%1
180
December1st to 10thDay
34%11%24%25%6%1
180
11th to 20thDay
25%17%40%15%31
180
21st to endDay
31%20%25%18%42
198
January1st to 10thNight
26%14%32%23%5%
760
11th to 20thNight
26%11%37%21%5%
760
21st to endNight
21%10%36%25%6%1
820
February1st to 10thNight
22%12%36%24%5%1
720
11th to 20thNight
21%12%39%24%4
703
21st to endNight
24%11%36%25%4
592
March1st to 10thNight
24%13%32%26%5%
720
11th to 20thNight
26%12%32%24%6%
720
21st to endNight
22%11%30%27%8%1
792
April1st to 10thNight
27%13%29%26%5%
720
11th to 20thNight
28%12%30%24%6%
711
21st to endNight
26%12%30%24%8%
576
May1st to 10thNight
25%8%32%25%9%1
419
11th to 20thNight
20%6%34%31%8%1
354
21st to endNight
16%11%31%30%12%
389
June1st to 10thNight
25%7%27%31%9%1
360
11th to 20thNight
18%6%29%34%13%
360
21st to endNight
16%6%30%33%15%1
360
July1st to 10thNight
12%5%29%36%18%
350
11th to 20thNight
11%326%42%17%1
348
21st to endNight
12%325%42%17%1
394
August1st to 10thNight
13%316%48%18%1
358
11th to 20thNight
14%323%35%22%2
539
21st to endNight
16%423%35%21%2
792
September1st to 10thNight
14%5%25%39%16%1
720
11th to 20thNight
18%5%21%34%20%2
720
21st to endNight
19%6%28%28%16%2
720
October1st to 10thNight
21%8%29%27%14%1
720
11th to 20thNight
24%10%29%28%9%
720
21st to endNight
27%11%30%27%4
792
November1st to 10thNight
25%13%29%24%9%
740
11th to 20thNight
23%14%33%19%10%1
760
21st to endNight
28%15%29%20%8%
760
December1st to 10thNight
32%12%32%20%4
760
11th to 20thNight
29%15%32%17%6%1
760
21st to endNight
33%14%31%17%4
836
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