Port Perry Astronomical Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Port Perry Astronomical Observatory clear sky chart from 2005-11-05 to 2017-09-21.

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
33%
14796
13%
46930
8%
14761
39%
14761
18%
32924

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
14%
719
9%
2506
3%
716
21%
716
8%
1984
January16th to endNight
19%
768
10%
2496
4%
768
26%
768
10%
1947
February1st to 15thNight
15%
702
9%
2287
3%
701
23%
701
10%
1791
February16th to endNight
20%
631
12%
2024
4%
631
30%
631
13%
1482
March1st to 15thNight
20%
720
13%
1980
5%
720
27%
720
16%
1485
March16th to endNight
29%
768
20%
2112
9%
768
38%
768
24%
1411
April1st to 15thNight
25%
718
15%
1802
6%
718
30%
718
22%
1324
April16th to endNight
33%
579
19%
1659
8%
579
40%
579
26%
1103
May1st to 15thNight
44%
459
15%
1539
12%
459
50%
459
20%
1014
May16th to endNight
48%
374
16%
1497
16%
374
56%
374
25%
837
June1st to 15thNight
41%
360
11%
1440
11%
360
48%
360
19%
892
June16th to endNight
52%
360
11%
1440
14%
360
56%
360
23%
842
July1st to 15thNight
61%
340
15%
1360
21%
340
65%
340
32%
709
July16th to endNight
60%
450
16%
1486
16%
422
64%
422
31%
858
August1st to 15thNight
64%
539
16%
1609
11%
536
67%
536
35%
934
August16th to endNight
60%
636
21%
1782
13%
636
64%
636
35%
1067
September1st to 15thNight
51%
720
19%
1800
9%
720
55%
720
34%
1181
September16th to endNight
53%
681
21%
1902
15%
681
56%
681
30%
1221
October1st to 15thNight
36%
660
15%
1980
13%
660
41%
660
20%
1379
October16th to endNight
22%
704
11%
2112
5%
704
29%
704
13%
1633
November1st to 15thNight
31%
700
17%
2389
14%
700
38%
700
17%
1654
November16th to endNight
23%
720
10%
2520
6%
720
29%
720
10%
1947
December1st to 15thNight
14%
720
5%
2520
2%
720
18%
720
8%
2018
December16th to endNight
14%
768
6%
2688
2%
768
20%
768
7%
2211

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%45%6%32%
47929
Night
15%9%46%5%3445%6%34%
47107
Cloud CoverDay
16%8%36%6%35%45%7%32%
47939
Night
15%9%36%5%35%45%6%34%
47112
TransparencyDay
50%7%13%18%9%
47624
Night
49%7%13%18%10%1
46930
SeeingDay
18%10%28%24%12%5%
5377
Night
19%10%30%27%10%1
14800

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%14%5%9%6%24446%22%
1432
16th to endDay
13%14%7%10%9%45%445%19%
1725
February1st to 15thDay
13%14%7%12%8%35%44421%
1575
16th to endDay
16%13%5%9%7%35%335%25%
1446
March1st to 15thDay
15%11%48%7%45%445%28%
1969
16th to endDay
16%9%36%5%34435%37%
2112
April1st to 15thDay
21%10%35%5%23446%30%
2146
16th to endDay
18%8%345%23346%38%
2310
May1st to 15thDay
16%7%27%5%35%5%5%6%34%
2410
16th to endDay
13%6%25%5%35%5%5%6%40%
2618
June1st to 15thDay
17%6%35%435%46%7%34%
2520
16th to endDay
14%5%25%436%5%6%9%35%
2520
July1st to 15thDay
10%4246%35%5%7%8%40%
2520
16th to endDay
10%5%36%6%36%6%7%9%34%
2577
August1st to 15thDay
9%436%6%35%6%7%8%37%
2340
16th to endDay
12%5%245%35%5%6%8%39%
2408
September1st to 15thDay
16%5%25%5%35%45%7%38%
2160
16th to endDay
14%6%35%5%245%46%41%
1848
October1st to 15thDay
20%8%36%6%34445%32%
1650
16th to endDay
26%11%48%6%3434423%
1760
November1st to 15thDay
16%10%37%5%35%45%6%31%
1467
16th to endDay
17%14%5%8%6%34446%22%
1440
December1st to 15thDay
18%14%6%9%7%35%5%46%17%
1440
16th to endDay
19%14%6%10%7%35%446%16%
1536
January1st to 15thNight
18%15%6%8%7%44445%21%
2506
16th to endNight
13%14%6%9%7%45%5%45%22%
2496
February1st to 15thNight
13%15%6%8%7%46%445%24%
2285
16th to endNight
13%12%49%7%35%45%6%28%
2024
March1st to 15thNight
17%10%5%7%7%3433332%
1968
16th to endNight
14%6%36%424435%42%
2112
April1st to 15thNight
19%8%35%5%33347%34%
1802
16th to endNight
13%8%24424446%42%
1659
May1st to 15thNight
15%426%6%35%5%5%7%37%
1539
16th to endNight
11%4234245%6%7%47%
1495
June1st to 15thNight
12%5%246%35%45%9%39%
1440
16th to endNight
10%6%33435%5%5%8%42%
1440
July1st to 15thNight
5%32443446%7%53%
1440
16th to endNight
8%424435%46%8%47%
1588
August1st to 15thNight
7%41443447%9%49%
1620
16th to endNight
7%425%43445%6%51%
1782
September1st to 15thNight
13%5%24423446%47%
1800
16th to endNight
14%5%26%423347%46%
1902
October1st to 15thNight
18%6%25%6%3335%7%37%
1980
16th to endNight
22%9%36%6%35%45%6%26%
2112
November1st to 15thNight
17%7%36%5%2345%7%33%
2389
16th to endNight
20%10%47%6%45%5%5%6%23%
2520
December1st to 15thNight
18%15%5%9%6%345%5%6%19%
2520
16th to endNight
20%13%5%9%7%45%45%5%16%
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
19%14%5%8%6%33347%23%
1432
16th to endDay
14%14%7%9%9%35%5%45%19%
1725
February1st to 15thDay
13%14%8%11%7%35%435%21%
1575
16th to endDay
16%14%5%8%7%25%335%26%
1446
March1st to 15thDay
16%11%48%7%35%445%28%
1980
16th to endDay
16%8%45%434335%38%
2111
April1st to 15thDay
22%10%36%434446%30%
2146
16th to endDay
18%9%24433347%38%
2310
May1st to 15thDay
16%7%26%5%345%5%6%35%
2410
16th to endDay
13%5%346%245%5%7%40%
2618
June1st to 15thDay
17%5%35%435%5%6%7%34%
2520
16th to endDay
15%4345%35%5%6%8%36%
2520
July1st to 15thDay
10%415%5%35%5%6%8%41%
2520
16th to endDay
10%5%36%6%36%6%6%9%35%
2577
August1st to 15thDay
10%5%26%6%35%6%5%8%39%
2340
16th to endDay
13%5%244345%6%8%39%
2408
September1st to 15thDay
16%5%245%345%47%39%
2160
16th to endDay
14%6%35%5%2445%7%41%
1848
October1st to 15thDay
20%8%35%6%35%445%32%
1650
16th to endDay
26%10%46%6%34336%23%
1760
November1st to 15thDay
17%9%46%6%34446%31%
1467
16th to endDay
18%14%5%7%6%35%45%6%23%
1440
December1st to 15thDay
18%14%5%10%5%345%5%5%19%
1440
16th to endDay
20%14%6%9%7%35%436%17%
1536
January1st to 15thNight
19%14%6%8%6%35%34422%
2506
16th to endNight
14%14%6%9%6%35%446%22%
2496
February1st to 15thNight
13%15%5%9%7%35%445%24%
2278
16th to endNight
14%11%5%8%6%35%445%29%
2024
March1st to 15thNight
18%10%47%6%3433432%
1980
16th to endNight
15%6%45%434336%42%
2112
April1st to 15thNight
20%8%26%5%33346%34%
1802
16th to endNight
13%9%24424446%42%
1659
May1st to 15thNight
16%5%25%6%35%45%7%37%
1539
16th to endNight
12%42332445%9%46%
1495
June1st to 15thNight
13%425%6%35%45%9%39%
1440
16th to endNight
10%6%25%325%46%9%42%
1440
July1st to 15thNight
5%3234245%6%8%53%
1440
16th to endNight
8%324325%47%8%48%
1588
August1st to 15thNight
8%32342445%8%50%
1620
16th to endNight
6%5%25%33435%7%52%
1782
September1st to 15thNight
13%5%24424446%47%
1800
16th to endNight
14%5%245%13346%46%
1902
October1st to 15thNight
18%6%25%5%3435%7%37%
1980
16th to endNight
22%8%35%5%35%5%5%6%28%
2112
November1st to 15thNight
17%6%35%5%3445%7%34%
2389
16th to endNight
21%10%36%7%35%5%6%6%23%
2520
December1st to 15thNight
18%16%48%6%4445%6%20%
2520
16th to endNight
21%12%5%9%5%46%45%6%17%
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
61%6%12%12%5%
1433
16th to endDay
65%6%9%10%6%
1725
February1st to 15thDay
65%6%8%14%3
1577
16th to endDay
61%5%10%14%7%
1446
March1st to 15thDay
57%5%13%16%6%
1980
16th to endDay
47%5%9%17%17%2
2112
April1st to 15thDay
53%5%10%17%11%
2146
16th to endDay
46%5%10%21%15%1
2310
May1st to 15thDay
48%7%12%17%13%1
2410
16th to endDay
41%7%14%21%12%1
2622
June1st to 15thDay
47%7%14%20%10%
2520
16th to endDay
42%9%15%19%12%
2520
July1st to 15thDay
36%8%15%23%15%1
2371
16th to endDay
43%8%15%22%9%1
2416
August1st to 15thDay
40%9%16%19%12%1
2327
16th to endDay
39%7%16%24%10%1
2408
September1st to 15thDay
44%6%13%24%10%
2160
16th to endDay
41%8%12%27%9%
1848
October1st to 15thDay
53%7%11%18%8%1
1650
16th to endDay
63%5%12%12%4
1760
November1st to 15thDay
52%7%15%15%8%1
1467
16th to endDay
61%8%12%11%4
1440
December1st to 15thDay
63%8%16%9%1
1440
16th to endDay
67%7%10%11%2
1536
January1st to 15thNight
64%6%10%11%5%1
2506
16th to endNight
61%6%10%14%5%
2496
February1st to 15thNight
62%7%9%15%5%
2287
16th to endNight
55%5%13%17%6%
2024
March1st to 15thNight
56%412%18%7%
1980
16th to endNight
42%5%11%22%15%2
2112
April1st to 15thNight
50%5%10%18%15%
1802
16th to endNight
41%6%12%17%19%2
1659
May1st to 15thNight
45%6%11%18%15%2
1539
16th to endNight
33%8%15%25%15%2
1497
June1st to 15thNight
41%7%12%22%15%
1440
16th to endNight
37%6%15%23%15%1
1440
July1st to 15thNight
27%7%14%27%22%1
1360
16th to endNight
31%8%13%28%16%1
1486
August1st to 15thNight
30%6%22%25%13%
1609
16th to endNight
30%7%14%29%15%1
1782
September1st to 15thNight
36%6%16%25%13%1
1800
16th to endNight
37%6%11%30%13%
1902
October1st to 15thNight
46%7%13%19%11%
1980
16th to endNight
56%9%12%14%7%
2112
November1st to 15thNight
48%7%12%17%11%1
2389
16th to endNight
59%8%14%12%4
2520
December1st to 15thNight
62%9%14%10%2
2520
16th to endNight
67%8%11%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
18%31%30%15%5%
60
16th to endDay
31%15%35%14%2
95
February1st to 15thDay
18%36%35%8%
90
16th to endDay
26%23%34%12%2
105
March1st to 15thDay
20%18%38%19%3
270
16th to endDay
23%12%31%20%8%2
288
April1st to 15thDay
21%14%37%17%7%1
270
16th to endDay
18%14%30%20%11%4
270
May1st to 15thDay
22%11%25%24%10%6%
288
16th to endDay
16%8%31%27%12%4
331
June1st to 15thDay
21%6%27%31%11%1
315
16th to endDay
17%5%26%32%13%4
315
July1st to 15thDay
13%321%33%20%7%
305
16th to endDay
10%426%31%17%8%
326
August1st to 15thDay
10%420%32%19%12%
313
16th to endDay
15%319%24%22%13%
332
September1st to 15thDay
13%422%25%20%13%
315
16th to endDay
15%5%19%29%17%13%
275
October1st to 15thDay
19%6%33%21%14%5%
255
16th to endDay
28%16%29%21%4
272
November1st to 15thDay
15%10%33%30%8%
103
16th to endDay
26%16%36%13%6%
60
December1st to 15thDay
20%28%26%20%5%
60
16th to endDay
23%37%31%7%
64
January1st to 15thNight
23%20%33%15%4
720
16th to endNight
21%11%36%24%5%
768
February1st to 15thNight
20%13%42%18%3
704
16th to endNight
19%15%37%22%5%
631
March1st to 15thNight
22%16%34%23%2
720
16th to endNight
17%10%35%25%8%1
768
April1st to 15thNight
22%13%35%25%4
718
16th to endNight
16%10%34%26%9%1
579
May1st to 15thNight
18%5%25%34%13%2
459
16th to endNight
12%430%33%18%
375
June1st to 15thNight
13%8%27%36%13%
360
16th to endNight
11%226%43%15%
360
July1st to 15thNight
6%325%43%21%1
340
16th to endNight
9%122%43%22%1
450
August1st to 15thNight
8%117%43%25%3
539
16th to endNight
9%220%38%26%3
636
September1st to 15thNight
15%422%30%21%5%
720
16th to endNight
15%5%20%31%22%5%
681
October1st to 15thNight
21%7%27%28%13%1
660
16th to endNight
27%11%32%22%5%
704
November1st to 15thNight
21%11%31%24%9%1
700
16th to endNight
24%14%30%22%7%
720
December1st to 15thNight
26%17%34%17%2
720
16th to endNight
26%13%38%19%2
768
Page generated at 2017-09-22 07:35:30. Version ac04

Copyright 2017 A. Danko


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