York University Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the York University Observatory clear sky chart from 2005-11-06 to 2024-02-25.

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%
22973
14%
80474
10%
22884
42%
22884
18%
55902

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
19%
750
10%
4403
5%
737
28%
737
6%
3270
January11th to 20thNight
18%
757
8%
4172
3%
740
24%
740
5%
3219
January21st to endNight
23%
828
7%
4527
6%
814
31%
814
7%
3561
February1st to 10thNight
19%
760
11%
2470
4%
760
27%
760
12%
1850
February11th to 20thNight
21%
740
13%
2405
5%
740
31%
740
13%
1774
February21st to endNight
22%
611
13%
1927
5%
611
32%
611
16%
1375
March1st to 10thNight
26%
720
16%
1980
7%
720
34%
720
19%
1406
March11th to 20thNight
23%
720
17%
1980
8%
720
32%
720
19%
1426
March21st to endNight
32%
792
20%
2178
10%
792
39%
792
23%
1496
April1st to 10thNight
26%
720
15%
1836
5%
720
32%
720
21%
1341
April11th to 20thNight
32%
655
18%
1719
7%
655
38%
655
24%
1209
April21st to endNight
35%
540
17%
1620
7%
540
43%
540
22%
1108
May1st to 10thNight
43%
540
16%
1620
9%
540
49%
540
25%
1074
May11th to 20thNight
45%
372
23%
1434
17%
372
59%
372
25%
830
May21st to endNight
52%
388
17%
1551
18%
388
60%
388
29%
809
June1st to 10thNight
47%
360
15%
1440
15%
360
57%
360
23%
834
June11th to 20thNight
45%
360
16%
1440
16%
360
56%
360
25%
810
June21st to endNight
56%
360
17%
1313
20%
360
62%
360
26%
757
July1st to 10thNight
63%
350
17%
1386
23%
350
69%
350
35%
682
July11th to 20thNight
60%
348
20%
1392
25%
348
65%
348
30%
742
July21st to endNight
64%
563
18%
1646
16%
532
68%
532
35%
934
August1st to 10thNight
67%
537
17%
1600
14%
534
69%
534
35%
954
August11th to 20thNight
67%
534
24%
1598
17%
534
71%
534
39%
915
August21st to endNight
57%
670
22%
1858
12%
670
61%
670
35%
1136
September1st to 10thNight
55%
720
18%
1800
9%
720
58%
720
36%
1162
September11th to 20thNight
58%
720
28%
1905
16%
720
64%
720
39%
1131
September21st to endNight
48%
720
20%
2016
16%
720
52%
720
28%
1320
October1st to 10thNight
42%
720
17%
2160
15%
720
47%
720
23%
1465
October11th to 20thNight
35%
720
15%
2160
11%
720
41%
720
20%
1532
October21st to endNight
28%
792
16%
2376
10%
792
35%
792
16%
1760
November1st to 10thNight
37%
740
19%
2467
16%
740
45%
740
21%
1615
November11th to 20thNight
26%
760
14%
2660
8%
760
36%
760
16%
1865
November21st to endNight
26%
760
10%
2660
7%
760
32%
760
13%
1953
December1st to 10thNight
19%
760
9%
2660
5%
760
25%
760
10%
2051
December11th to 20thNight
20%
759
7%
3250
3%
758
27%
758
9%
2471
December21st to endNight
15%
827
4%
4865
1%
817
21%
817
5%
4065

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%45%5%6%7%32%
78468
Night
16%9%47%6%35%5%5%7%33%
80773
TransparencyDay
50%7%12%20%11%1
78183
Night
50%6%12%20%11%1
80592
SeeingDay
19%9%29%25%13%5%
13524
Night
19%7%30%30%12%1
23008

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
17%12%5%8%7%35%446%28%
2517
11th to 20thDay
11%17%7%14%11%46%445%18%
2753
21st to endDay
21%18%7%10%7%34445%17%
3124
February1st to 10thDay
15%15%5%10%7%5%6%5%46%22%
1710
11th to 20thDay
12%13%7%10%8%46%45%6%26%
1663
21st to endDay
18%14%6%8%7%35%5%45%26%
1418
March1st to 10thDay
15%10%5%9%6%45%5%5%7%30%
1977
11th to 20thDay
14%11%5%7%7%45%5%5%5%32%
1980
21st to endDay
18%10%38%6%35%45%5%32%
2178
April1st to 10thDay
17%11%47%7%36%5%5%7%29%
2124
11th to 20thDay
19%8%37%5%345%5%6%36%
2204
21st to endDay
21%7%46%7%3445%6%33%
2340
May1st to 10thDay
17%8%37%5%4446%8%33%
2340
11th to 20thDay
15%8%35%5%45%5%5%7%40%
2460
21st to endDay
12%5%25%5%45%5%5%8%44%
2725
June1st to 10thDay
14%6%25%6%35%5%6%9%38%
2520
11th to 20thDay
13%426%5%35%5%7%11%39%
2520
21st to endDay
13%5%25%5%45%6%6%9%40%
2616
July1st to 10thDay
8%425%6%47%6%6%9%44%
2520
11th to 20thDay
11%426%5%46%6%7%9%40%
2520
21st to endDay
9%5%36%7%46%6%8%11%35%
2570
August1st to 10thDay
9%5%35%6%36%6%9%9%40%
2327
11th to 20thDay
9%6%37%6%35%6%6%10%39%
2303
21st to endDay
11%6%26%6%36%5%7%9%39%
2480
September1st to 10thDay
13%5%36%6%35%6%6%8%41%
2160
11th to 20thDay
12%5%26%5%345%6%8%44%
2052
21st to endDay
13%5%37%6%45%6%6%9%36%
1944
October1st to 10thDay
19%10%35%5%345%5%7%34%
1800
11th to 20thDay
21%7%37%7%445%5%6%28%
1800
21st to endDay
24%11%47%7%45%5%5%6%22%
1980
November1st to 10thDay
14%8%5%7%6%35%5%5%8%33%
1605
11th to 20thDay
16%10%49%7%36%7%6%7%25%
1520
21st to endDay
19%13%5%7%7%5%5%5%5%7%22%
1520
December1st to 10thDay
17%14%7%10%7%5%6%5%6%6%17%
1520
11th to 20thDay
20%12%5%9%7%45%5%5%6%22%
1906
21st to endDay
29%17%6%11%7%3444412%
2772
January1st to 10thNight
18%14%6%9%7%45%5%5%6%22%
4429
11th to 20thNight
15%13%6%10%11%5%6%5%5%6%18%
4202
21st to endNight
21%12%6%8%9%45%45%6%19%
4532
February1st to 10thNight
14%14%5%9%8%45%5%46%25%
2470
11th to 20thNight
14%13%5%8%8%35%5%6%5%27%
2403
21st to endNight
14%11%5%9%7%35%5%5%6%31%
1927
March1st to 10thNight
13%8%5%8%7%5%5%5%5%5%35%
1970
11th to 20thNight
16%11%5%7%7%34346%34%
1979
21st to endNight
16%9%36%5%35%446%39%
2178
April1st to 10thNight
19%8%37%6%445%5%6%32%
1836
11th to 20thNight
16%7%37%6%35%5%5%6%39%
1719
21st to endNight
16%9%36%5%35%5%6%7%36%
1620
May1st to 10thNight
16%6%345%45%5%6%7%39%
1620
11th to 20thNight
14%5%35%335%46%7%46%
1433
21st to endNight
10%4235%3446%7%52%
1550
June1st to 10thNight
12%5%35%6%45%5%6%8%43%
1440
11th to 20thNight
9%6%35%5%25%5%6%9%44%
1440
21st to endNight
11%5%345%25%5%5%8%47%
1313
July1st to 10thNight
6%32432445%8%58%
1426
11th to 20thNight
10%424435%5%6%8%50%
1440
21st to endNight
7%326%435%5%6%8%51%
1743
August1st to 10thNight
7%425%6%4446%8%50%
1611
11th to 20thNight
7%5%244345%5%10%52%
1598
21st to endNight
10%425%5%345%5%7%48%
1858
September1st to 10thNight
9%5%26%5%25%6%6%7%47%
1800
11th to 20thNight
11%42442345%7%53%
1905
21st to endNight
14%6%36%6%3445%7%43%
2016
October1st to 10thNight
16%7%36%5%345%6%6%38%
2160
11th to 20thNight
18%8%37%6%3445%7%33%
2160
21st to endNight
21%10%37%6%35%5%6%7%28%
2376
November1st to 10thNight
17%8%36%5%35%45%6%38%
2467
11th to 20thNight
17%9%48%6%35%5%5%7%30%
2660
21st to endNight
20%10%5%6%6%445%5%7%27%
2660
December1st to 10thNight
20%14%5%8%6%45%5%5%6%22%
2660
11th to 20thNight
16%15%5%10%7%45%5%5%7%22%
3288
21st to endNight
30%14%5%9%7%34445%14%
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
55%7%17%12%7%1
2518
11th to 20thDay
68%6%10%11%4
2753
21st to endDay
69%6%10%9%5%1
3124
February1st to 10thDay
62%6%10%17%41
1710
11th to 20thDay
58%5%13%15%8%1
1672
21st to endDay
60%7%9%16%8%1
1418
March1st to 10thDay
53%7%13%17%10%1
1980
11th to 20thDay
54%6%9%18%12%2
1980
21st to endDay
53%5%9%17%13%2
2178
April1st to 10thDay
55%411%18%11%1
2124
11th to 20thDay
49%5%10%21%15%1
2204
21st to endDay
52%5%11%18%12%3
2340
May1st to 10thDay
49%5%11%20%14%1
2340
11th to 20thDay
43%6%11%24%14%1
2464
21st to endDay
38%6%15%24%14%2
2725
June1st to 10thDay
43%7%12%22%15%1
2520
11th to 20thDay
38%8%12%25%16%2
2520
21st to endDay
39%7%14%24%15%1
2616
July1st to 10thDay
34%8%12%25%19%3
2450
11th to 20thDay
37%8%13%28%12%1
2445
21st to endDay
41%9%14%21%13%1
2426
August1st to 10thDay
37%9%16%23%14%2
2314
11th to 20thDay
36%8%17%26%12%1
2303
21st to endDay
38%8%13%27%12%1
2480
September1st to 10thDay
39%7%14%27%11%1
2160
11th to 20thDay
36%6%14%29%15%1
2052
21st to endDay
44%7%13%26%9%
1944
October1st to 10thDay
48%6%14%19%12%1
1800
11th to 20thDay
54%7%12%20%7%1
1800
21st to endDay
61%6%12%15%6%1
1980
November1st to 10thDay
48%8%13%21%9%1
1605
11th to 20thDay
55%8%14%14%8%1
1520
21st to endDay
59%7%15%15%3
1520
December1st to 10thDay
65%7%13%11%4
1520
11th to 20thDay
60%8%16%11%6%
1906
21st to endDay
74%6%8%10%3
2772
January1st to 10thNight
59%9%12%14%5%1
4429
11th to 20thNight
63%9%12%12%41
4202
21st to endNight
64%8%12%11%5%1
4532
February1st to 10thNight
59%5%10%19%6%1
2470
11th to 20thNight
56%6%11%17%9%1
2405
21st to endNight
53%5%14%19%9%1
1927
March1st to 10thNight
50%6%13%20%10%2
1980
11th to 20thNight
53%39%20%12%2
1980
21st to endNight
47%5%9%21%16%3
2178
April1st to 10thNight
51%5%10%18%15%1
1836
11th to 20thNight
48%49%21%15%2
1719
21st to endNight
46%6%11%19%15%3
1620
May1st to 10thNight
43%6%9%21%17%3
1620
11th to 20thNight
38%5%10%25%18%4
1434
21st to endNight
30%6%16%25%18%5%
1551
June1st to 10thNight
38%6%11%25%18%2
1440
11th to 20thNight
37%5%12%23%22%2
1440
21st to endNight
36%6%13%26%18%1
1313
July1st to 10thNight
26%5%11%27%26%4
1386
11th to 20thNight
30%6%12%31%18%2
1392
21st to endNight
30%6%15%30%16%3
1646
August1st to 10thNight
32%7%17%29%14%2
1600
11th to 20thNight
27%7%15%34%14%2
1598
21st to endNight
33%6%14%29%16%1
1858
September1st to 10thNight
34%7%16%28%14%1
1800
11th to 20thNight
31%5%10%36%18%1
1905
21st to endNight
40%6%12%29%12%1
2016
October1st to 10thNight
44%7%11%21%15%1
2160
11th to 20thNight
50%6%12%24%8%
2160
21st to endNight
55%6%11%16%11%1
2376
November1st to 10thNight
45%5%12%24%12%1
2467
11th to 20thNight
51%7%14%18%10%1
2660
21st to endNight
56%7%17%16%4
2660
December1st to 10thNight
60%7%11%15%6%
2660
11th to 20thNight
61%6%14%14%4
3288
21st to endNight
71%7%11%8%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
21%16%30%26%7%2
180
11th to 20thDay
23%18%36%14%7%2
247
21st to endDay
25%13%36%20%6%1
297
February1st to 10thDay
24%14%44%14%3
270
11th to 20thDay
23%16%44%14%3
270
21st to endDay
32%20%30%17%
224
March1st to 10thDay
20%14%37%20%8%
420
11th to 20thDay
21%12%36%23%6%1
420
21st to endDay
27%10%32%21%8%1
462
April1st to 10thDay
21%14%35%22%8%1
420
11th to 20thDay
26%11%32%18%8%4
412
21st to endDay
24%10%32%20%10%3
420
May1st to 10thDay
23%8%29%24%10%6%
423
11th to 20thDay
18%5%29%29%13%6%
447
21st to endDay
15%6%24%30%16%8%
485
June1st to 10thDay
14%7%30%33%13%3
450
11th to 20thDay
16%9%22%30%19%4
450
21st to endDay
15%6%25%33%15%5%
546
July1st to 10thDay
10%220%33%26%8%
445
11th to 20thDay
14%321%30%22%9%
444
21st to endDay
10%224%37%18%9%
488
August1st to 10thDay
11%320%29%23%13%
446
11th to 20thDay
10%318%29%26%13%
439
21st to endDay
12%6%22%27%21%12%
495
September1st to 10thDay
15%5%22%28%18%12%
450
11th to 20thDay
11%5%22%27%19%17%
450
21st to endDay
13%8%28%27%16%8%
450
October1st to 10thDay
21%8%26%22%15%7%
450
11th to 20thDay
20%11%37%22%8%2
450
21st to endDay
29%10%31%24%6%1
495
November1st to 10thDay
18%17%29%24%10%1
261
11th to 20thDay
23%14%34%22%6%2
180
21st to endDay
24%10%41%22%3
180
December1st to 10thDay
26%12%39%18%6%1
180
11th to 20thDay
27%20%28%20%6%
180
21st to endDay
28%14%40%16%2
198
January1st to 10thNight
24%11%35%24%6%1
760
11th to 20thNight
27%11%31%25%5%
760
21st to endNight
20%7%37%28%7%
836
February1st to 10thNight
20%12%41%24%3
760
11th to 20thNight
21%12%36%25%5%
743
21st to endNight
20%11%40%24%5%
611
March1st to 10thNight
19%10%37%26%8%
720
11th to 20thNight
22%12%35%23%7%1
720
21st to endNight
21%9%32%28%10%1
792
April1st to 10thNight
23%10%33%28%5%
720
11th to 20thNight
19%9%34%27%9%1
655
21st to endNight
22%6%29%31%11%
540
May1st to 10thNight
18%427%33%16%2
540
11th to 20thNight
16%231%35%14%1
372
21st to endNight
11%426%37%22%1
389
June1st to 10thNight
13%328%39%17%1
360
11th to 20thNight
14%428%37%17%1
360
21st to endNight
11%5%20%45%18%1
360
July1st to 10thNight
6%321%45%23%2
350
11th to 20thNight
11%121%43%22%2
348
21st to endNight
8%218%46%24%2
563
August1st to 10thNight
10%113%42%31%3
537
11th to 20thNight
7%114%43%31%3
534
21st to endNight
12%320%36%26%3
670
September1st to 10thNight
12%322%36%21%6%
720
11th to 20thNight
14%219%33%27%5%
720
21st to endNight
17%421%33%22%4
720
October1st to 10thNight
19%5%24%29%19%2
720
11th to 20thNight
21%6%29%32%10%2
720
21st to endNight
27%7%29%31%6%
792
November1st to 10thNight
20%10%26%31%11%1
740
11th to 20thNight
22%9%33%26%9%2
760
21st to endNight
26%8%32%26%8%
760
December1st to 10thNight
28%8%36%22%6%
760
11th to 20thNight
22%11%39%24%4
760
21st to endNight
32%8%34%23%3
836
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