Guelph
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Guelph clear sky chart from 2005-11-05 to 2017-12-20.

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%
15234
15%
47841
8%
15196
38%
15196
18%
33042

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
10%
716
10%
2434
3%
716
18%
716
7%
1945
January16th to endNight
13%
768
11%
2496
3%
768
22%
768
8%
1975
February1st to 15thNight
14%
703
9%
2287
2%
701
21%
701
9%
1783
February16th to endNight
18%
631
12%
1995
3%
631
26%
631
12%
1474
March1st to 15thNight
20%
720
14%
1989
6%
720
27%
720
16%
1473
March16th to endNight
27%
768
21%
2112
10%
768
37%
768
22%
1442
April1st to 15thNight
23%
712
16%
1835
5%
712
30%
712
20%
1323
April16th to endNight
33%
540
21%
1620
10%
540
42%
540
24%
1039
May1st to 15thNight
38%
531
18%
1611
10%
531
46%
531
23%
1062
May16th to endNight
43%
374
17%
1497
14%
374
56%
374
24%
843
June1st to 15thNight
42%
360
14%
1440
10%
360
54%
360
21%
837
June16th to endNight
45%
360
16%
1263
14%
360
56%
360
29%
725
July1st to 15thNight
57%
340
19%
1265
21%
340
65%
340
38%
624
July16th to endNight
51%
535
15%
1566
11%
502
56%
502
33%
910
August1st to 15thNight
58%
539
19%
1609
11%
536
65%
536
38%
910
August16th to endNight
56%
604
24%
1750
14%
604
62%
604
36%
1041
September1st to 15thNight
50%
720
22%
1844
11%
720
56%
720
35%
1165
September16th to endNight
51%
717
24%
1983
16%
717
56%
717
33%
1208
October1st to 15thNight
38%
720
18%
2160
14%
720
44%
720
23%
1436
October16th to endNight
23%
768
16%
2304
9%
768
33%
768
15%
1665
November1st to 15thNight
30%
760
17%
2566
13%
760
38%
760
18%
1713
November16th to endNight
20%
780
11%
2730
5%
780
27%
780
11%
2005
December1st to 15thNight
11%
780
7%
2730
1%
780
18%
780
7%
2155
December16th to endNight
8%
788
6%
2755
1%
788
14%
788
6%
2289

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
15%9%47%6%35%45%7%30%
48993
Night
15%9%46%5%3445%6%33%
48019
Cloud CoverDay
15%9%47%6%35%45%7%30%
49003
Night
15%9%47%5%3445%6%33%
48023
TransparencyDay
52%412%19%10%1
48690
Night
50%411%21%11%1
47841
SeeingDay
18%10%27%23%13%5%
5867
Night
19%13%28%25%11%1
15240

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
19%16%7%9%8%44435%16%
1501
16th to endDay
12%18%7%12%10%5%4435%15%
1728
February1st to 15thDay
16%16%6%11%7%46%445%15%
1575
16th to endDay
17%14%5%10%7%45%45%6%18%
1476
March1st to 15thDay
16%12%5%8%6%36%445%26%
1960
16th to endDay
15%9%5%6%5%35%446%32%
2112
April1st to 15thDay
20%10%45%43445%6%29%
2122
16th to endDay
18%9%345%23346%37%
2340
May1st to 15thDay
17%7%36%5%2346%7%35%
2340
16th to endDay
12%6%25%5%35%45%7%38%
2618
June1st to 15thDay
15%6%25%435%5%6%8%36%
2520
16th to endDay
13%5%25%6%35%5%6%10%36%
2694
July1st to 15thDay
9%425%6%36%5%7%9%39%
2616
16th to endDay
10%6%25%6%35%6%7%9%35%
2493
August1st to 15thDay
10%5%26%6%45%6%6%8%36%
2340
16th to endDay
12%6%36%5%35%46%9%37%
2438
September1st to 15thDay
15%5%25%6%2446%8%38%
2115
16th to endDay
14%5%26%43445%8%40%
1977
October1st to 15thDay
20%9%36%5%3444432%
1800
16th to endDay
25%10%37%6%24445%23%
1920
November1st to 15thDay
16%10%37%5%245%6%7%29%
1620
16th to endDay
19%14%7%8%6%3345%6%21%
1560
December1st to 15thDay
17%16%6%11%6%36%45%5%14%
1560
16th to endDay
21%17%9%10%7%4444312%
1568
January1st to 15thNight
21%13%7%9%7%34435%18%
2434
16th to endNight
15%13%8%11%8%35%446%18%
2496
February1st to 15thNight
15%13%6%10%8%36%45%420%
2285
16th to endNight
15%11%6%8%7%35%5%5%6%24%
1995
March1st to 15thNight
17%9%5%6%6%35%5%5%5%28%
1977
16th to endNight
15%9%45%42434441%
2112
April1st to 15thNight
19%9%345%24446%34%
1835
16th to endNight
12%7%36%6%3335%6%41%
1620
May1st to 15thNight
15%6%25%435%45%7%39%
1611
16th to endNight
10%4245%3446%7%46%
1495
June1st to 15thNight
12%5%24436%5%7%8%41%
1440
16th to endNight
10%42443435%7%46%
1263
July1st to 15thNight
5%31332345%8%56%
1341
16th to endNight
8%423425%5%5%9%47%
1673
August1st to 15thNight
6%3145%3347%9%50%
1620
16th to endNight
8%43442445%6%51%
1750
September1st to 15thNight
12%41442346%7%47%
1844
16th to endNight
13%42433346%8%46%
1983
October1st to 15thNight
17%6%36%423447%39%
2160
16th to endNight
20%8%36%6%34446%30%
2304
November1st to 15thNight
15%8%36%5%2446%7%33%
2566
16th to endNight
19%11%47%5%35%446%26%
2730
December1st to 15thNight
19%15%5%9%6%35%45%6%17%
2730
16th to endNight
21%15%8%10%7%3445%5%12%
2755

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
18%15%8%9%8%35%33417%
1501
16th to endDay
12%19%7%12%9%5%5%44414%
1728
February1st to 15thDay
16%15%7%12%8%45%445%15%
1575
16th to endDay
17%14%5%10%7%45%45%6%19%
1476
March1st to 15thDay
17%13%5%8%6%35%445%26%
1971
16th to endDay
15%9%5%6%6%3445%5%32%
2111
April1st to 15thDay
21%10%35%43445%6%29%
2122
16th to endDay
18%10%35%5%23346%37%
2340
May1st to 15thDay
17%8%36%5%3435%7%34%
2340
16th to endDay
12%6%26%5%35%45%8%38%
2618
June1st to 15thDay
15%6%25%435%5%5%9%35%
2520
16th to endDay
13%5%16%6%35%5%6%10%35%
2694
July1st to 15thDay
8%5%26%6%36%5%7%9%39%
2616
16th to endDay
10%6%26%5%46%6%6%10%34%
2493
August1st to 15thDay
9%6%26%7%46%6%5%8%36%
2340
16th to endDay
11%6%27%5%35%46%9%36%
2438
September1st to 15thDay
15%6%36%5%2446%8%37%
2115
16th to endDay
15%5%25%42445%8%39%
1977
October1st to 15thDay
20%9%37%435%34433%
1800
16th to endDay
25%10%48%5%33445%23%
1920
November1st to 15thDay
16%11%37%5%345%5%8%28%
1620
16th to endDay
19%14%6%9%5%3445%6%20%
1560
December1st to 15thDay
16%17%7%10%7%345%5%5%15%
1560
16th to endDay
21%17%8%11%8%45%43411%
1568
January1st to 15thNight
21%13%7%9%7%34445%18%
2434
16th to endNight
14%13%8%12%8%34445%18%
2496
February1st to 15thNight
14%14%6%10%8%36%5%5%420%
2278
16th to endNight
15%11%6%9%7%3446%6%24%
1995
March1st to 15thNight
17%9%47%6%445%5%5%28%
1989
16th to endNight
14%9%46%42434440%
2112
April1st to 15thNight
19%9%445%24446%34%
1835
16th to endNight
12%7%35%6%24346%41%
1620
May1st to 15thNight
15%6%26%43445%7%38%
1610
16th to endNight
10%425%425%5%47%46%
1495
June1st to 15thNight
11%5%25%426%5%6%8%41%
1440
16th to endNight
10%5%25%6%23448%46%
1263
July1st to 15thNight
5%3134235%49%55%
1341
16th to endNight
8%424425%45%9%47%
1673
August1st to 15thNight
6%3144345%7%8%50%
1620
16th to endNight
8%42443345%6%50%
1750
September1st to 15thNight
11%5%144235%5%7%48%
1844
16th to endNight
12%5%2442335%8%45%
1983
October1st to 15thNight
17%6%36%42345%7%39%
2160
16th to endNight
19%9%47%5%3435%6%29%
2304
November1st to 15thNight
15%8%36%5%2445%7%34%
2566
16th to endNight
20%10%47%5%35%5%47%25%
2730
December1st to 15thNight
20%15%49%6%35%5%45%18%
2730
16th to endNight
21%14%8%11%7%34445%12%
2755

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
70%38%11%5%
1502
16th to endDay
72%38%9%6%
1728
February1st to 15thDay
70%37%13%3
1577
16th to endDay
66%29%14%6%1
1476
March1st to 15thDay
61%311%15%7%
1971
16th to endDay
52%410%19%11%1
2112
April1st to 15thDay
54%310%19%11%
2122
16th to endDay
48%39%21%14%2
2340
May1st to 15thDay
48%311%24%11%
2340
16th to endDay
43%414%21%14%1
2622
June1st to 15thDay
44%413%24%12%
2520
16th to endDay
43%5%14%23%12%
2694
July1st to 15thDay
38%412%25%16%1
2464
16th to endDay
42%7%13%23%10%2
2337
August1st to 15thDay
43%5%15%21%12%1
2327
16th to endDay
42%414%26%11%
2438
September1st to 15thDay
43%414%24%11%1
2115
16th to endDay
41%5%12%29%10%
1977
October1st to 15thDay
53%311%18%11%1
1800
16th to endDay
61%413%14%5%
1920
November1st to 15thDay
52%5%14%18%7%
1620
16th to endDay
63%412%13%6%
1560
December1st to 15thDay
69%411%11%3
1560
16th to endDay
75%38%9%2
1568
January1st to 15thNight
68%39%12%6%
2434
16th to endNight
66%310%13%5%
2496
February1st to 15thNight
63%410%15%5%
2287
16th to endNight
59%212%16%8%
1995
March1st to 15thNight
55%311%20%7%
1989
16th to endNight
47%28%22%15%2
2112
April1st to 15thNight
50%38%21%13%
1835
16th to endNight
43%39%22%18%3
1620
May1st to 15thNight
45%39%25%13%2
1611
16th to endNight
36%413%26%14%3
1497
June1st to 15thNight
39%5%11%26%16%1
1440
16th to endNight
35%414%27%16%1
1263
July1st to 15thNight
25%411%31%24%2
1265
16th to endNight
33%416%25%16%3
1566
August1st to 15thNight
29%5%18%32%12%2
1609
16th to endNight
32%410%32%17%1
1750
September1st to 15thNight
33%5%13%30%15%1
1844
16th to endNight
35%412%31%15%
1983
October1st to 15thNight
44%49%25%14%
2160
16th to endNight
54%312%20%7%
2304
November1st to 15thNight
47%413%20%12%1
2566
16th to endNight
58%415%15%6%
2730
December1st to 15thNight
66%412%13%3
2730
16th to endNight
72%311%9%3
2755

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%28%36%14%2
71
16th to endDay
29%15%40%14%
96
February1st to 15thDay
23%23%42%10%1
90
16th to endDay
24%27%39%7%1
81
March1st to 15thDay
26%16%28%21%6%
261
16th to endDay
23%10%34%22%6%2
288
April1st to 15thDay
23%17%28%20%9%1
270
16th to endDay
19%12%32%18%12%3
270
May1st to 15thDay
21%9%26%24%11%6%
288
16th to endDay
16%10%32%23%10%6%
331
June1st to 15thDay
18%10%26%28%13%3
315
16th to endDay
15%10%30%30%11%2
489
July1st to 15thDay
10%6%28%29%17%7%
398
16th to endDay
11%6%26%27%20%8%
326
August1st to 15thDay
12%321%27%22%12%
313
16th to endDay
15%318%25%21%15%
332
September1st to 15thDay
12%5%18%26%24%12%
315
16th to endDay
16%7%18%21%20%16%
314
October1st to 15thDay
21%11%24%20%15%6%
315
16th to endDay
27%14%30%20%6%
336
November1st to 15thDay
22%12%33%20%8%2
116
16th to endDay
26%16%35%13%43
90
December1st to 15thDay
25%27%30%13%3
90
16th to endDay
36%15%43%5%
72
January1st to 15thNight
27%22%32%15%2
720
16th to endNight
21%19%35%17%5%
768
February1st to 15thNight
22%22%31%19%3
704
16th to endNight
20%19%33%20%5%
631
March1st to 15thNight
23%17%32%21%4
720
16th to endNight
19%15%31%23%9%
768
April1st to 15thNight
24%16%31%22%4
712
16th to endNight
13%13%32%29%9%1
540
May1st to 15thNight
16%10%27%29%13%1
531
16th to endNight
12%6%31%30%19%
375
June1st to 15thNight
12%9%28%34%13%
360
16th to endNight
11%431%38%14%
360
July1st to 15thNight
4427%41%20%2
340
16th to endNight
10%227%36%20%2
535
August1st to 15thNight
6%420%40%25%2
539
16th to endNight
11%420%33%26%3
604
September1st to 15thNight
13%6%22%31%20%5%
720
16th to endNight
15%6%19%27%24%7%
717
October1st to 15thNight
19%10%25%26%15%2
720
16th to endNight
23%16%31%23%4
768
November1st to 15thNight
19%13%29%25%10%1
760
16th to endNight
26%20%26%18%6%
780
December1st to 15thNight
29%21%30%15%4
780
16th to endNight
29%21%32%14%3
788
Page generated at 2017-12-21 07:04:51. Version ac04

Copyright 2017 A. Danko


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