Dorval
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Dorval 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
32%
14499
12%
46477
8%
14469
37%
14469
16%
33538

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
19%
720
8%
2506
4%
716
24%
716
9%
1983
January16th to endNight
22%
768
10%
2688
5%
768
29%
768
11%
2021
February1st to 15thNight
21%
703
7%
2306
5%
701
26%
701
10%
1835
February16th to endNight
25%
631
12%
1893
6%
631
33%
631
14%
1422
March1st to 15thNight
25%
720
13%
2160
9%
720
31%
720
14%
1634
March16th to endNight
33%
768
18%
1997
10%
768
39%
768
22%
1423
April1st to 15thNight
29%
718
16%
1796
6%
718
34%
718
21%
1350
April16th to endNight
30%
624
16%
1704
6%
624
37%
624
21%
1221
May1st to 15thNight
33%
396
16%
1476
14%
396
42%
396
17%
987
May16th to endNight
37%
374
12%
1497
12%
374
44%
374
16%
997
June1st to 15thNight
36%
360
8%
1440
9%
360
43%
360
13%
1025
June16th to endNight
44%
360
8%
1440
10%
360
48%
360
19%
928
July1st to 15thNight
53%
340
13%
1360
19%
340
58%
340
24%
789
July16th to endNight
54%
376
12%
1418
16%
354
58%
354
24%
833
August1st to 15thNight
58%
354
15%
1403
16%
352
60%
352
21%
856
August16th to endNight
55%
698
18%
1748
10%
698
58%
698
34%
1157
September1st to 15thNight
50%
660
14%
1650
8%
660
52%
660
28%
1157
September16th to endNight
49%
657
19%
1807
13%
657
53%
657
29%
1181
October1st to 15thNight
34%
660
18%
1943
10%
660
43%
660
20%
1364
October16th to endNight
27%
704
10%
2255
7%
704
32%
704
14%
1678
November1st to 15thNight
26%
700
16%
2277
11%
700
33%
700
15%
1651
November16th to endNight
23%
720
12%
2505
8%
720
29%
720
11%
1880
December1st to 15thNight
18%
720
7%
2520
3%
720
23%
720
8%
2016
December16th to endNight
14%
768
8%
2688
4%
768
20%
768
8%
2150

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
17%8%36%5%35%45%7%33%
47508
Night
19%9%46%5%34446%31%
46646
Cloud CoverDay
18%8%36%5%35%45%6%32%
47517
Night
19%9%46%5%34445%31%
46652
TransparencyDay
49%6%12%19%9%1
47195
Night
53%5%11%18%9%1
46477
SeeingDay
21%7%26%25%14%4
5437
Night
23%7%28%28%10%
14501

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%15%5%9%6%35%43421%
1432
16th to endDay
14%14%5%7%6%46%45%6%25%
1536
February1st to 15thDay
14%12%5%10%7%5%5%435%24%
1554
16th to endDay
17%13%47%7%34445%27%
1580
March1st to 15thDay
18%10%5%8%6%36%445%27%
1790
16th to endDay
19%7%36%42435%6%37%
2225
April1st to 15thDay
20%8%37%5%24345%32%
2152
16th to endDay
19%7%25%425%345%38%
2256
May1st to 15thDay
19%6%26%5%34446%33%
2486
16th to endDay
18%6%25%5%2445%8%34%
2618
June1st to 15thDay
22%7%35%5%3445%8%31%
2520
16th to endDay
17%6%25%5%35%5%5%8%33%
2520
July1st to 15thDay
11%5%35%5%35%6%6%8%40%
2520
16th to endDay
11%4245%35%6%7%9%37%
2654
August1st to 15thDay
11%5%36%5%35%5%6%8%36%
2449
16th to endDay
9%3144245%7%9%45%
2100
September1st to 15thDay
15%5%2342445%8%42%
1980
16th to endDay
14%5%24423447%46%
1823
October1st to 15thDay
20%7%35%5%35%445%34%
1686
16th to endDay
23%10%36%5%34446%26%
1617
November1st to 15thDay
17%10%46%5%25%5%5%7%30%
1581
16th to endDay
23%9%47%6%3444427%
1453
December1st to 15thDay
25%13%5%7%5%35%5%6%5%17%
1440
16th to endDay
26%13%5%9%5%25%445%18%
1536
January1st to 15thNight
18%14%6%10%6%34445%20%
2506
16th to endNight
13%14%6%7%7%45%5%45%25%
2688
February1st to 15thNight
14%14%6%11%7%36%44421%
2304
16th to endNight
17%11%5%8%6%35%44429%
1893
March1st to 15thNight
20%11%47%5%3434428%
2147
16th to endNight
18%9%37%433335%38%
1997
April1st to 15thNight
26%8%35%424345%31%
1796
16th to endNight
22%7%25%423346%36%
1704
May1st to 15thNight
21%6%45%5%33335%36%
1476
16th to endNight
19%6%36%5%34447%35%
1495
June1st to 15thNight
25%6%35%4245%5%7%29%
1440
16th to endNight
16%5%25%5%25%5%7%8%35%
1440
July1st to 15thNight
11%2246%26%5%7%9%40%
1440
16th to endNight
11%425%425%5%6%8%42%
1514
August1st to 15thNight
12%425%425%46%9%40%
1413
16th to endNight
12%42442445%8%45%
1748
September1st to 15thNight
17%6%26%5%23446%41%
1650
16th to endNight
19%424324335%47%
1807
October1st to 15thNight
18%6%27%5%245%46%35%
1943
16th to endNight
26%8%36%5%34345%29%
2255
November1st to 15thNight
21%9%36%5%2445%5%31%
2277
16th to endNight
22%13%48%5%34345%26%
2505
December1st to 15thNight
24%14%6%7%5%34345%20%
2520
16th to endNight
25%16%5%7%5%24345%19%
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%15%6%8%6%35%42321%
1432
16th to endDay
14%15%47%6%36%45%6%25%
1536
February1st to 15thDay
13%13%5%10%8%46%43424%
1554
16th to endDay
17%13%5%6%8%24445%27%
1580
March1st to 15thDay
18%11%48%7%36%345%26%
1800
16th to endDay
18%8%36%423447%36%
2224
April1st to 15thDay
21%9%45%6%2334432%
2152
16th to endDay
19%7%35%5%23445%38%
2256
May1st to 15thDay
21%6%35%5%35%45%5%33%
2486
16th to endDay
19%6%35%42446%7%34%
2618
June1st to 15thDay
23%6%25%5%3445%8%30%
2520
16th to endDay
18%6%25%5%36%5%6%7%33%
2520
July1st to 15thDay
13%425%6%25%5%5%8%39%
2520
16th to endDay
12%425%5%45%5%8%9%37%
2654
August1st to 15thDay
12%6%36%6%35%5%7%8%35%
2449
16th to endDay
10%4144345%6%8%45%
2100
September1st to 15thDay
16%5%34425%45%7%42%
1980
16th to endDay
15%42432345%6%46%
1823
October1st to 15thDay
21%7%25%5%3445%5%34%
1686
16th to endDay
25%9%35%6%34346%26%
1617
November1st to 15thDay
18%10%36%435%45%6%31%
1581
16th to endDay
23%10%37%6%2444427%
1453
December1st to 15thDay
26%13%47%6%3445%6%17%
1440
16th to endDay
27%13%47%6%35%34319%
1536
January1st to 15thNight
18%15%6%9%6%34345%20%
2506
16th to endNight
14%13%6%8%6%45%44426%
2688
February1st to 15thNight
15%14%5%10%7%46%445%21%
2297
16th to endNight
17%11%5%7%6%35%345%28%
1893
March1st to 15thNight
21%11%46%6%34345%28%
2160
16th to endNight
18%9%46%5%3334438%
1997
April1st to 15thNight
26%8%35%43334431%
1796
16th to endNight
22%8%24423436%36%
1704
May1st to 15thNight
21%7%36%333335%36%
1476
16th to endNight
20%5%36%5%3445%6%35%
1495
June1st to 15thNight
26%6%45%42445%7%29%
1440
16th to endNight
17%435%435%5%7%8%35%
1440
July1st to 15thNight
11%3246%35%5%7%8%41%
1440
16th to endNight
11%425%5%25%47%9%40%
1514
August1st to 15thNight
13%425%425%5%7%8%40%
1413
16th to endNight
13%4145%2445%7%45%
1748
September1st to 15thNight
17%5%25%5%33436%41%
1650
16th to endNight
19%423323335%47%
1807
October1st to 15thNight
19%6%36%5%34436%36%
1943
16th to endNight
26%7%46%5%2434429%
2255
November1st to 15thNight
21%9%36%6%2445%5%30%
2277
16th to endNight
22%12%47%6%24335%27%
2505
December1st to 15thNight
24%14%5%7%5%35%345%20%
2520
16th to endNight
25%17%5%6%43334420%
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
66%5%8%13%5%
1433
16th to endDay
57%7%11%15%7%
1536
February1st to 15thDay
62%410%18%2
1556
16th to endDay
58%413%14%8%1
1580
March1st to 15thDay
59%5%11%15%7%1
1800
16th to endDay
47%5%11%18%14%3
2225
April1st to 15thDay
53%5%8%18%11%2
2152
16th to endDay
47%5%9%19%14%3
2256
May1st to 15thDay
50%5%10%18%13%1
2486
16th to endDay
46%7%12%20%11%1
2622
June1st to 15thDay
50%7%11%18%10%
2520
16th to endDay
47%7%12%21%9%1
2520
July1st to 15thDay
38%8%14%25%11%
2371
16th to endDay
39%8%18%24%8%
2487
August1st to 15thDay
43%9%17%22%7%
2435
16th to endDay
31%8%20%25%13%
2100
September1st to 15thDay
39%6%15%26%12%
1980
16th to endDay
37%6%12%31%11%
1823
October1st to 15thDay
50%6%12%22%8%
1686
16th to endDay
58%5%13%16%5%
1617
November1st to 15thDay
51%6%15%17%9%
1581
16th to endDay
59%6%10%16%7%
1453
December1st to 15thDay
65%8%11%11%1
1440
16th to endDay
68%412%9%5%
1536
January1st to 15thNight
64%5%11%13%4
2506
16th to endNight
58%6%10%15%7%
2688
February1st to 15thNight
64%5%11%14%3
2306
16th to endNight
57%5%11%15%9%
1893
March1st to 15thNight
58%5%9%16%8%1
2160
16th to endNight
49%49%17%14%3
1997
April1st to 15thNight
56%27%18%13%1
1796
16th to endNight
50%48%17%15%4
1704
May1st to 15thNight
51%38%16%15%4
1476
16th to endNight
48%6%10%18%13%2
1497
June1st to 15thNight
53%7%9%19%8%
1440
16th to endNight
43%8%15%17%13%1
1440
July1st to 15thNight
35%8%14%26%14%
1360
16th to endNight
36%7%18%23%12%
1418
August1st to 15thNight
38%8%19%20%12%
1403
16th to endNight
35%7%14%28%13%
1748
September1st to 15thNight
44%5%11%25%12%
1650
16th to endNight
40%5%10%29%14%
1807
October1st to 15thNight
49%5%10%24%10%
1943
16th to endNight
58%5%13%16%6%
2255
November1st to 15thNight
54%6%12%16%8%2
2277
16th to endNight
60%5%11%16%5%
2505
December1st to 15thNight
66%5%12%12%2
2520
16th to endNight
67%411%10%5%
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
21%18%25%23%10%1
60
16th to endDay
42%7%25%17%43
64
February1st to 15thDay
30%12%42%12%2
89
16th to endDay
27%6%32%27%5%
239
March1st to 15thDay
22%12%35%22%7%
270
16th to endDay
27%5%28%22%12%3
288
April1st to 15thDay
28%8%31%20%8%2
270
16th to endDay
21%11%26%20%15%4
270
May1st to 15thDay
28%7%26%20%12%5%
288
16th to endDay
19%8%30%20%16%3
331
June1st to 15thDay
26%7%23%27%11%2
315
16th to endDay
17%426%33%15%2
315
July1st to 15thDay
12%425%31%19%6%
305
16th to endDay
13%217%36%23%6%
326
August1st to 15thDay
15%218%33%21%9%
297
16th to endDay
12%223%27%21%13%
268
September1st to 15thDay
15%7%18%24%20%14%
255
16th to endDay
18%7%24%18%20%10%
254
October1st to 15thDay
19%7%25%25%17%5%
255
16th to endDay
30%9%26%22%10%
246
November1st to 15thDay
22%11%30%20%12%2
235
16th to endDay
34%9%24%21%9%
73
December1st to 15thDay
28%335%28%31
60
16th to endDay
35%14%34%15%
64
January1st to 15thNight
24%13%30%25%5%
720
16th to endNight
22%9%35%23%8%
768
February1st to 15thNight
21%9%36%26%6%
704
16th to endNight
23%8%34%28%6%
631
March1st to 15thNight
26%9%31%27%5%
720
16th to endNight
23%6%30%30%9%1
768
April1st to 15thNight
28%7%29%30%4
718
16th to endNight
25%9%29%23%11%
624
May1st to 15thNight
25%5%28%28%11%1
396
16th to endNight
19%430%34%11%
375
June1st to 15thNight
27%324%34%9%
360
16th to endNight
18%327%36%13%1
360
July1st to 15thNight
11%225%37%21%1
340
16th to endNight
12%125%42%17%1
376
August1st to 15thNight
14%117%42%22%1
354
16th to endNight
15%220%36%23%1
698
September1st to 15thNight
18%320%38%18%1
660
16th to endNight
20%421%30%21%1
657
October1st to 15thNight
21%6%30%27%13%
660
16th to endNight
28%9%28%26%6%
704
November1st to 15thNight
25%9%30%23%10%
700
16th to endNight
26%12%29%18%11%
720
December1st to 15thNight
33%10%30%21%4
720
16th to endNight
33%11%31%17%41
768
Page generated at 2017-08-13 14:16:31. Version ac04

Copyright 2017 A. Danko


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