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 2005-11-05 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
37%
14687
15%
46449
10%
14651
44%
14651
20%
31875

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
18%
718
10%
2473
4%
716
26%
716
10%
1929
January16th to endNight
22%
768
12%
2496
5%
768
30%
768
13%
1901
February1st to 15thNight
19%
701
11%
2287
3%
701
27%
701
12%
1719
February16th to endNight
26%
631
13%
2014
6%
631
36%
631
16%
1462
March1st to 15thNight
24%
720
14%
1980
6%
720
31%
720
17%
1472
March16th to endNight
32%
768
21%
2112
12%
768
40%
768
24%
1431
April1st to 15thNight
30%
718
15%
1820
5%
718
36%
718
23%
1324
April16th to endNight
40%
561
20%
1641
9%
561
48%
561
27%
1066
May1st to 15thNight
49%
492
17%
1572
12%
492
56%
492
24%
1024
May16th to endNight
50%
374
17%
1497
17%
374
58%
374
27%
818
June1st to 15thNight
47%
360
13%
1440
13%
360
55%
360
22%
830
June16th to endNight
55%
360
14%
1355
18%
360
61%
360
26%
777
July1st to 15thNight
65%
340
17%
1351
24%
340
71%
340
35%
672
July16th to endNight
64%
488
17%
1521
17%
457
66%
457
32%
857
August1st to 15thNight
69%
527
18%
1575
14%
524
71%
524
38%
913
August16th to endNight
65%
572
23%
1622
15%
572
68%
572
37%
955
September1st to 15thNight
56%
660
22%
1660
11%
660
61%
660
35%
1070
September16th to endNight
57%
657
21%
1837
15%
657
60%
657
33%
1138
October1st to 15thNight
43%
660
18%
1980
16%
660
48%
660
23%
1351
October16th to endNight
28%
704
13%
2112
7%
704
35%
704
16%
1583
November1st to 15thNight
38%
700
19%
2376
16%
700
45%
700
20%
1584
November16th to endNight
29%
720
11%
2520
8%
720
34%
720
13%
1859
December1st to 15thNight
21%
720
8%
2520
4%
720
26%
720
10%
1965
December16th to endNight
17%
768
6%
2688
2%
768
22%
768
9%
2175

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%8%36%6%35%5%5%7%32%
47487
Night
15%8%36%5%3445%6%35%
46629
Cloud CoverDay
15%8%36%5%35%46%7%33%
47497
Night
15%8%36%5%3445%6%36%
46633
TransparencyDay
49%6%12%20%10%1
47185
Night
48%5%12%21%11%1
46449
SeeingDay
18%5%25%27%16%7%
5261
Night
19%6%27%31%13%1
14693

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
18%12%5%8%7%46%445%21%
1462
16th to endDay
13%14%6%10%9%36%5%5%5%17%
1728
February1st to 15thDay
14%14%6%11%6%5%6%445%20%
1575
16th to endDay
17%13%5%6%6%25%45%5%26%
1455
March1st to 15thDay
16%9%5%8%7%45%45%6%28%
1969
16th to endDay
16%9%37%5%35%45%5%34%
2112
April1st to 15thDay
20%9%45%5%25%45%7%29%
2128
16th to endDay
18%7%35%5%24445%38%
2328
May1st to 15thDay
15%7%25%6%35%46%7%35%
2379
16th to endDay
12%6%25%5%35%5%5%7%41%
2618
June1st to 15thDay
15%5%25%5%25%46%9%36%
2520
16th to endDay
13%425%5%45%5%6%10%36%
2583
July1st to 15thDay
8%426%5%47%6%6%8%40%
2520
16th to endDay
10%426%6%45%5%7%10%35%
2539
August1st to 15thDay
8%5%26%6%35%6%7%8%39%
2288
16th to endDay
10%5%25%6%25%5%6%8%39%
2217
September1st to 15thDay
15%6%25%5%3445%7%39%
1969
16th to endDay
14%5%37%43345%8%38%
1793
October1st to 15thDay
19%6%36%6%245%5%6%33%
1650
16th to endDay
24%10%36%6%345%46%22%
1760
November1st to 15thDay
14%9%5%7%5%25%45%7%33%
1478
16th to endDay
18%13%5%7%6%35%5%5%7%22%
1440
December1st to 15thDay
19%12%7%9%7%45%45%6%17%
1440
16th to endDay
20%14%6%8%6%245%5%6%17%
1536
January1st to 15thNight
20%14%48%6%345%45%21%
2473
16th to endNight
14%13%6%8%7%45%446%24%
2496
February1st to 15thNight
13%14%5%7%7%35%445%26%
2285
16th to endNight
14%10%5%8%7%35%45%5%29%
2014
March1st to 15thNight
18%8%47%6%44436%31%
1968
16th to endNight
15%8%36%434235%42%
2112
April1st to 15thNight
18%7%36%5%34447%34%
1820
16th to endNight
12%7%35%424445%44%
1641
May1st to 15thNight
15%4245%35%5%5%7%39%
1572
16th to endNight
12%42343346%6%48%
1495
June1st to 15thNight
12%5%244344410%41%
1440
16th to endNight
10%5%245%25%45%9%44%
1355
July1st to 15thNight
6%223325%45%7%56%
1431
16th to endNight
8%315%425%5%5%8%49%
1626
August1st to 15thNight
6%41443345%9%51%
1586
16th to endNight
9%3144345%46%52%
1622
September1st to 15thNight
11%4244245%5%6%47%
1660
16th to endNight
12%5%25%423447%48%
1837
October1st to 15thNight
18%6%35%5%2335%6%38%
1980
16th to endNight
21%8%36%6%345%5%6%28%
2112
November1st to 15thNight
16%7%26%5%3445%6%36%
2376
16th to endNight
21%9%46%6%3445%7%26%
2520
December1st to 15thNight
21%13%47%5%345%5%6%21%
2520
16th to endNight
21%12%6%9%8%3445%6%17%
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
18%12%48%7%45%446%23%
1462
16th to endDay
14%13%6%12%8%36%45%6%17%
1728
February1st to 15thDay
14%13%7%12%7%45%34422%
1575
16th to endDay
17%12%5%7%6%23446%27%
1455
March1st to 15thDay
16%8%47%6%35%46%5%28%
1980
16th to endDay
16%9%37%424446%34%
2111
April1st to 15thDay
20%10%35%5%3435%7%30%
2128
16th to endDay
17%7%25%434345%38%
2328
May1st to 15thDay
15%6%35%5%35%45%7%35%
2379
16th to endDay
12%5%25%5%35%5%5%8%41%
2618
June1st to 15thDay
15%5%145%35%46%8%37%
2520
16th to endDay
13%415%446%5%7%10%37%
2583
July1st to 15thDay
8%4245%46%5%7%9%40%
2520
16th to endDay
10%5%25%5%46%6%8%8%36%
2539
August1st to 15thDay
8%5%25%5%45%5%6%9%40%
2288
16th to endDay
11%5%245%35%5%6%9%41%
2217
September1st to 15thDay
16%5%244345%5%8%39%
1969
16th to endDay
13%5%36%5%3435%8%39%
1793
October1st to 15thDay
20%6%25%5%345%5%6%34%
1650
16th to endDay
25%9%37%6%25%5%5%6%22%
1760
November1st to 15thDay
14%9%46%7%2446%8%33%
1478
16th to endDay
18%12%47%5%35%45%7%24%
1440
December1st to 15thDay
19%13%5%8%8%35%446%19%
1440
16th to endDay
21%13%5%8%6%4446%6%18%
1536
January1st to 15thNight
21%13%5%7%6%3445%422%
2473
16th to endNight
15%12%5%8%7%45%45%6%24%
2496
February1st to 15thNight
14%14%5%7%6%35%45%426%
2278
16th to endNight
15%10%39%7%345%45%30%
2014
March1st to 15thNight
18%9%47%6%34436%31%
1980
16th to endNight
15%8%36%423335%42%
2112
April1st to 15thNight
19%7%35%6%34436%35%
1820
16th to endNight
13%6%35%434446%44%
1641
May1st to 15thNight
15%4246%245%5%8%39%
1571
16th to endNight
12%5%234235%5%6%48%
1495
June1st to 15thNight
13%5%25%435%45%10%41%
1440
16th to endNight
10%425%5%25%5%5%9%43%
1355
July1st to 15thNight
5%313325%447%56%
1431
16th to endNight
8%3144345%6%9%49%
1626
August1st to 15thNight
6%315%33445%9%50%
1586
16th to endNight
9%3233245%5%6%52%
1622
September1st to 15thNight
11%5%2432445%7%47%
1660
16th to endNight
12%42442435%7%48%
1837
October1st to 15thNight
18%6%25%5%2335%6%39%
1980
16th to endNight
21%9%36%5%34445%29%
2112
November1st to 15thNight
17%6%35%5%35%45%6%37%
2376
16th to endNight
21%10%45%5%345%5%6%27%
2520
December1st to 15thNight
21%13%46%5%35%5%5%5%23%
2520
16th to endNight
22%13%5%8%6%345%5%6%18%
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
62%5%11%14%6%
1463
16th to endDay
65%6%8%11%5%
1728
February1st to 15thDay
65%5%9%15%3
1577
16th to endDay
57%6%11%15%7%1
1455
March1st to 15thDay
55%6%11%17%8%
1980
16th to endDay
50%5%10%18%14%1
2112
April1st to 15thDay
54%310%19%11%
2128
16th to endDay
47%410%20%14%2
2328
May1st to 15thDay
47%5%10%22%13%
2379
16th to endDay
40%6%13%22%14%1
2622
June1st to 15thDay
43%7%11%24%12%
2520
16th to endDay
41%6%15%21%13%
2583
July1st to 15thDay
36%7%11%24%16%2
2371
16th to endDay
42%8%13%22%11%1
2381
August1st to 15thDay
39%7%15%20%14%1
2275
16th to endDay
37%7%13%28%10%1
2217
September1st to 15thDay
42%6%14%24%9%
1969
16th to endDay
43%5%12%28%9%
1793
October1st to 15thDay
49%5%11%21%10%1
1650
16th to endDay
60%7%13%13%4
1760
November1st to 15thDay
49%7%14%18%8%1
1478
16th to endDay
59%7%13%14%4
1440
December1st to 15thDay
65%7%13%12%1
1440
16th to endDay
66%5%12%12%3
1536
January1st to 15thNight
63%5%10%13%6%
2473
16th to endNight
60%5%11%16%5%
2496
February1st to 15thNight
59%5%10%18%6%
2287
16th to endNight
54%5%13%18%7%
2014
March1st to 15thNight
55%411%20%7%
1980
16th to endNight
45%49%22%16%2
2112
April1st to 15thNight
50%49%19%15%
1820
16th to endNight
40%5%10%22%17%3
1641
May1st to 15thNight
42%6%9%22%16%2
1572
16th to endNight
34%414%26%15%3
1497
June1st to 15thNight
39%412%25%16%1
1440
16th to endNight
36%412%26%18%1
1355
July1st to 15thNight
26%5%10%29%25%3
1351
16th to endNight
31%6%14%26%17%3
1521
August1st to 15thNight
29%6%18%29%14%1
1575
16th to endNight
31%5%11%32%16%1
1622
September1st to 15thNight
35%5%13%30%14%
1660
16th to endNight
36%412%32%13%
1837
October1st to 15thNight
46%5%10%23%13%
1980
16th to endNight
55%6%11%17%7%
2112
November1st to 15thNight
46%5%11%20%13%1
2376
16th to endNight
57%5%14%16%4
2520
December1st to 15thNight
62%6%12%13%4
2520
16th to endNight
67%5%12%10%3
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
15%18%40%15%9%1
65
16th to endDay
35%10%30%21%2
96
February1st to 15thDay
23%10%50%15%1
90
16th to endDay
20%11%42%24%2
90
March1st to 15thDay
20%11%32%24%10%
270
16th to endDay
22%6%31%26%9%2
288
April1st to 15thDay
25%8%30%27%7%1
270
16th to endDay
20%7%31%21%14%5%
270
May1st to 15thDay
20%326%25%13%10%
288
16th to endDay
15%328%28%18%6%
331
June1st to 15thDay
19%5%20%31%21%2
315
16th to endDay
14%424%32%19%5%
378
July1st to 15thDay
11%117%30%29%9%
305
16th to endDay
10%219%34%22%11%
326
August1st to 15thDay
9%117%26%27%16%
297
16th to endDay
13%211%23%26%22%
268
September1st to 15thDay
15%117%26%22%17%
255
16th to endDay
18%216%29%20%12%
254
October1st to 15thDay
18%5%25%25%16%8%
255
16th to endDay
27%7%28%26%8%
272
November1st to 15thDay
17%7%20%37%14%3
94
16th to endDay
26%8%28%28%8%
60
December1st to 15thDay
18%15%40%18%6%1
60
16th to endDay
29%17%40%12%
64
January1st to 15thNight
28%11%32%21%5%
720
16th to endNight
20%7%34%29%7%
768
February1st to 15thNight
21%9%38%25%4
704
16th to endNight
19%9%34%29%6%
631
March1st to 15thNight
23%9%34%26%6%
720
16th to endNight
19%8%32%26%11%1
768
April1st to 15thNight
22%8%32%29%6%
718
16th to endNight
16%6%29%33%11%1
561
May1st to 15thNight
16%323%34%18%2
492
16th to endNight
12%227%35%21%
375
June1st to 15thNight
15%326%35%17%
360
16th to endNight
10%320%43%20%
360
July1st to 15thNight
6%18%48%23%2
340
16th to endNight
8%116%45%25%2
488
August1st to 15thNight
8%11%44%31%3
527
16th to endNight
11%213%37%31%3
572
September1st to 15thNight
14%220%30%25%6%
660
16th to endNight
16%215%31%28%4
657
October1st to 15thNight
20%422%31%17%2
660
16th to endNight
25%6%30%30%6%
704
November1st to 15thNight
19%7%27%29%14%1
700
16th to endNight
27%7%26%27%9%1
720
December1st to 15thNight
28%8%35%22%5%
720
16th to endNight
27%7%37%23%3
768
Page generated at 2017-08-13 14:16:57. Version ac04

Copyright 2017 A. Danko


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