Trottier Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Trottier Observatory clear sky chart from 2005-11-05 to 2017-09-15.

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
36%
14466
9%
46699
8%
14438
38%
14438
13%
34558

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
26%
750
10%
2686
9%
746
29%
746
9%
2164
January16th to endNight
27%
779
8%
2745
8%
779
29%
779
9%
2148
February1st to 15thNight
25%
700
6%
2288
5%
700
28%
700
8%
1856
February16th to endNight
30%
632
12%
2041
10%
632
35%
632
12%
1530
March1st to 15thNight
21%
720
4%
2061
3%
720
22%
720
7%
1751
March16th to endNight
23%
768
6%
2073
3%
768
24%
768
9%
1750
April1st to 15thNight
31%
718
7%
1798
3%
718
33%
718
14%
1437
April16th to endNight
32%
501
8%
1581
5%
501
35%
501
11%
1213
May1st to 15thNight
45%
359
11%
1439
17%
359
49%
359
15%
912
May16th to endNight
45%
374
6%
1460
8%
374
48%
374
14%
970
June1st to 15thNight
46%
360
2%
1260
1%
360
46%
360
16%
866
June16th to endNight
44%
360
4%
1260
1%
360
44%
360
16%
863
July1st to 15thNight
65%
340
9%
1190
8%
340
66%
340
32%
628
July16th to endNight
68%
376
15%
1328
18%
354
76%
354
38%
619
August1st to 15thNight
70%
427
16%
1499
22%
425
70%
425
37%
721
August16th to endNight
64%
612
18%
1758
15%
612
66%
612
35%
1018
September1st to 15thNight
68%
719
21%
1799
13%
719
69%
719
42%
1050
September16th to endNight
43%
656
16%
1812
11%
656
45%
656
20%
1281
October1st to 15thNight
40%
660
11%
1980
10%
660
42%
660
18%
1389
October16th to endNight
32%
704
5%
2202
5%
704
33%
704
10%
1758
November1st to 15thNight
20%
700
5%
2450
5%
700
22%
700
6%
2081
November16th to endNight
22%
720
5%
2520
3%
720
26%
720
7%
2019
December1st to 15thNight
25%
731
8%
2589
6%
731
27%
731
8%
2111
December16th to endNight
18%
800
6%
2880
4%
800
22%
800
6%
2423

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
28%8%35%5%34345%28%
48060
Night
30%8%35%423345%29%
46853
Cloud CoverDay
30%8%35%423335%28%
48060
Night
31%8%25%423345%28%
46856
TransparencyDay
57%5%13%16%6%1
47717
Night
57%413%15%6%1
46700
SeeingDay
25%12%15%25%16%4
6123
Night
29%16%8%21%19%4
14472

By Month

Percentage Cloud Cover NW (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
45%9%4442233315%
1253
16th to endDay
38%10%3442433418%
1477
February1st to 15thDay
38%11%36%5%33345%15%
1576
16th to endDay
27%12%47%5%44245%21%
1432
March1st to 15thDay
44%11%45%5%3432312%
1896
16th to endDay
36%11%47%6%3433215%
2151
April1st to 15thDay
30%11%47%7%46%43416%
2149
16th to endDay
29%10%46%5%45%445%19%
2379
May1st to 15thDay
22%8%35%6%34346%29%
2511
16th to endDay
22%8%37%6%35%45%5%24%
2648
June1st to 15thDay
26%9%36%5%34446%25%
2700
16th to endDay
23%9%37%5%45%346%26%
2700
July1st to 15thDay
12%424424346%50%
2700
16th to endDay
12%423323445%52%
2748
August1st to 15thDay
12%5%24323346%51%
2451
16th to endDay
14%42442445%6%45%
2444
September1st to 15thDay
15%5%24423245%48%
2150
16th to endDay
31%6%25%43333430%
1816
October1st to 15thDay
31%7%25%43435%5%27%
1650
16th to endDay
40%8%35%5%23336%17%
1669
November1st to 15thDay
52%7%3442322212%
1408
16th to endDay
42%8%35%42332417%
1440
December1st to 15thDay
36%13%45%31332222%
1368
16th to endDay
46%11%3442322214%
1344
January1st to 15thNight
43%9%2431323421%
2686
16th to endNight
36%10%35%422346%20%
2745
February1st to 15thNight
38%9%36%423445%18%
2276
16th to endNight
30%10%35%42334426%
2041
March1st to 15thNight
43%10%345%2332416%
2061
16th to endNight
37%9%36%6%25%33416%
2069
April1st to 15thNight
28%9%36%6%34446%22%
1798
16th to endNight
26%10%45%6%34435%24%
1581
May1st to 15thNight
19%5%26%4345%6%5%37%
1439
16th to endNight
17%7%36%5%35%5%5%7%32%
1460
June1st to 15thNight
23%7%345%25%345%34%
1260
16th to endNight
17%7%36%6%35%45%7%32%
1260
July1st to 15thNight
8%323324336%58%
1260
16th to endNight
8%123213446%60%
1420
August1st to 15thNight
7%31322325%461%
1507
16th to endNight
9%5%1342345%8%50%
1758
September1st to 15thNight
9%213423345%59%
1799
16th to endNight
26%7%25%323335%35%
1812
October1st to 15thNight
26%6%2442344436%
1980
16th to endNight
39%7%35%32334420%
2202
November1st to 15thNight
48%8%25%42323315%
2450
16th to endNight
38%9%36%5%2332320%
2520
December1st to 15thNight
38%9%35%42432420%
2589
16th to endNight
45%13%25%31222316%
2880

Percentage Cloud Cover SW (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
47%8%35%32223416%
1253
16th to endDay
39%10%25%413335%19%
1477
February1st to 15thDay
40%11%36%423335%15%
1576
16th to endDay
29%13%46%5%3333422%
1432
March1st to 15thDay
46%11%45%43322312%
1896
16th to endDay
38%12%38%5%3423314%
2151
April1st to 15thDay
32%12%47%6%35%43414%
2149
16th to endDay
32%9%37%5%3444417%
2379
May1st to 15thDay
25%8%36%5%24445%29%
2511
16th to endDay
26%8%37%6%35%445%24%
2648
June1st to 15thDay
30%9%26%5%34345%25%
2700
16th to endDay
26%9%36%5%44346%25%
2700
July1st to 15thDay
14%424423246%49%
2700
16th to endDay
15%423323345%51%
2748
August1st to 15thDay
13%424413346%50%
2451
16th to endDay
14%5%2442345%6%44%
2444
September1st to 15thDay
17%6%24323335%48%
2150
16th to endDay
32%6%25%5%2423430%
1816
October1st to 15thDay
33%6%25%422445%27%
1650
16th to endDay
43%8%35%422236%17%
1669
November1st to 15thDay
53%8%3442322212%
1408
16th to endDay
43%9%35%32232418%
1440
December1st to 15thDay
37%13%35%21322222%
1368
16th to endDay
47%13%35%42222214%
1344
January1st to 15thNight
44%8%2422223421%
2685
16th to endNight
37%11%25%412335%21%
2745
February1st to 15thNight
38%10%35%423345%18%
2276
16th to endNight
31%11%36%413245%26%
2041
March1st to 15thNight
44%10%25%42423315%
2061
16th to endNight
38%10%36%5%2443416%
2073
April1st to 15thNight
29%10%37%5%34446%21%
1798
16th to endNight
28%9%46%5%33435%23%
1581
May1st to 15thNight
20%436%5%2435%6%37%
1439
16th to endNight
18%8%36%6%35%36%6%32%
1460
June1st to 15thNight
23%8%245%35%335%33%
1260
16th to endNight
17%7%46%5%35%45%5%32%
1260
July1st to 15thNight
8%314423236%57%
1260
16th to endNight
8%213323346%60%
1420
August1st to 15thNight
8%31232235%5%60%
1507
16th to endNight
10%41432336%8%50%
1758
September1st to 15thNight
9%313332345%59%
1799
16th to endNight
28%6%25%422345%34%
1812
October1st to 15thNight
28%5%25%42334435%
1980
16th to endNight
40%8%25%32334420%
2202
November1st to 15thNight
50%7%25%42222314%
2450
16th to endNight
40%9%35%42333321%
2520
December1st to 15thNight
39%10%25%42333420%
2589
16th to endNight
46%13%25%31212316%
2880

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
73%46%9%5%
1253
16th to endDay
68%5%9%12%21
1477
February1st to 15thDay
71%410%6%32
1576
16th to endDay
63%5%12%13%6%
1432
March1st to 15thDay
77%38%7%1
1896
16th to endDay
74%48%11%1
2151
April1st to 15thDay
69%6%11%11%1
2149
16th to endDay
64%6%12%12%4
2379
May1st to 15thDay
54%5%15%17%6%
2511
16th to endDay
58%6%17%15%2
2652
June1st to 15thDay
58%417%13%41
2700
16th to endDay
56%417%14%5%
2700
July1st to 15thDay
35%417%25%12%3
2541
16th to endDay
31%5%20%28%12%1
2577
August1st to 15thDay
33%6%18%29%10%1
2434
16th to endDay
36%5%21%27%7%1
2444
September1st to 15thDay
37%416%22%15%2
2150
16th to endDay
57%412%17%7%1
1816
October1st to 15thDay
58%414%14%7%
1650
16th to endDay
69%412%10%3
1669
November1st to 15thDay
78%36%7%11
1408
16th to endDay
70%47%9%7%
1440
December1st to 15thDay
69%46%11%7%1
1368
16th to endDay
76%36%8%21
1344
January1st to 15thNight
68%46%14%5%
2686
16th to endNight
65%410%13%32
2745
February1st to 15thNight
67%5%12%8%32
2288
16th to endNight
58%411%16%6%1
2041
March1st to 15thNight
72%410%10%2
2061
16th to endNight
70%412%10%2
2073
April1st to 15thNight
62%413%16%3
1798
16th to endNight
61%413%14%4
1581
May1st to 15thNight
45%5%18%19%9%1
1440
16th to endNight
50%423%17%4
1460
June1st to 15thNight
51%322%14%42
1260
16th to endNight
49%6%18%17%6%2
1260
July1st to 15thNight
28%5%18%29%15%3
1190
16th to endNight
21%5%21%33%14%3
1328
August1st to 15thNight
24%324%30%12%3
1499
16th to endNight
29%6%23%26%10%3
1758
September1st to 15thNight
26%418%25%21%4
1799
16th to endNight
50%413%22%7%2
1812
October1st to 15thNight
51%315%19%9%
1980
16th to endNight
65%5%13%11%3
2202
November1st to 15thNight
75%38%7%22
2450
16th to endNight
68%310%9%7%
2520
December1st to 15thNight
69%48%10%5%1
2589
16th to endNight
74%37%8%5%1
2880

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
25%16%26%20%20%
60
16th to endDay
56%6%212%20%1
74
February1st to 15thDay
42%15%7%17%15%1
236
16th to endDay
30%17%15%18%14%3
213
March1st to 15thDay
48%15%18%13%4
270
16th to endDay
38%15%18%17%9%1
288
April1st to 15thDay
32%15%16%18%13%3
269
16th to endDay
27%17%17%24%11%1
270
May1st to 15thDay
23%13%19%28%13%2
288
16th to endDay
19%15%15%23%17%7%
363
June1st to 15thDay
19%16%17%29%12%3
495
16th to endDay
17%17%20%26%16%2
495
July1st to 15thDay
12%12%12%36%24%2
475
16th to endDay
8%13%13%34%25%4
417
August1st to 15thDay
14%7%10%32%25%9%
315
16th to endDay
13%6%17%32%23%7%
332
September1st to 15thDay
18%8%19%26%22%5%
312
16th to endDay
26%10%14%24%18%4
255
October1st to 15thDay
37%8%15%20%14%2
255
16th to endDay
42%8%10%22%12%4
197
November1st to 15thDay
63%8%8%16%3
60
16th to endDay
50%5%18%8%18%
60
December1st to 15thDay
56%311%13%13%1
60
16th to endDay
43%49%25%10%6%
64
January1st to 15thNight
39%18%9%16%13%2
750
16th to endNight
35%18%8%21%12%3
779
February1st to 15thNight
36%18%10%20%12%1
704
16th to endNight
30%19%11%23%13%2
632
March1st to 15thNight
40%20%10%16%10%1
720
16th to endNight
36%19%9%19%11%3
768
April1st to 15thNight
27%20%10%22%16%3
718
16th to endNight
26%20%8%29%12%3
501
May1st to 15thNight
19%17%5%26%26%4
360
16th to endNight
16%18%324%32%3
375
June1st to 15thNight
18%18%6%24%26%5%
360
16th to endNight
16%18%8%25%27%3
360
July1st to 15thNight
8%12%426%40%6%
340
16th to endNight
6%17%122%41%10%
376
August1st to 15thNight
10%10%118%43%15%
427
16th to endNight
12%10%5%20%41%9%
612
September1st to 15thNight
10%9%327%35%13%
719
16th to endNight
27%12%6%23%23%6%
656
October1st to 15thNight
29%10%12%23%20%2
660
16th to endNight
39%11%7%23%14%2
704
November1st to 15thNight
44%17%10%21%6%
700
16th to endNight
36%21%10%17%11%2
720
December1st to 15thNight
39%19%9%21%10%
731
16th to endNight
44%19%10%17%7%
800
Page generated at 2017-09-15 21:58:33. Version ac04

Copyright 2017 A. Danko


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