Grenville Base Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Grenville Base Observatory clear sky chart from 2005-11-05 to 2017-12-02.

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
39%
15566
20%
47844
12%
15530
49%
15530
25%
30151

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
22%
716
21%
2389
9%
716
38%
716
20%
1511
January16th to endNight
19%
768
21%
2496
7%
768
37%
768
21%
1574
February1st to 15thNight
21%
701
19%
2287
5%
701
38%
701
22%
1403
February16th to endNight
17%
631
22%
1934
6%
631
37%
631
21%
1218
March1st to 15thNight
24%
720
22%
1980
8%
720
38%
720
26%
1284
March16th to endNight
23%
768
20%
2103
8%
768
33%
768
23%
1419
April1st to 15thNight
28%
718
18%
1796
5%
718
36%
718
28%
1197
April16th to endNight
42%
720
19%
1800
8%
720
47%
720
29%
1214
May1st to 15thNight
40%
555
16%
1635
9%
555
45%
555
23%
1113
May16th to endNight
53%
385
17%
1508
16%
385
59%
385
23%
896
June1st to 15thNight
58%
360
18%
1440
19%
360
64%
360
25%
823
June16th to endNight
63%
360
17%
1440
19%
360
69%
360
26%
823
July1st to 15thNight
57%
360
12%
1380
13%
360
60%
360
20%
869
July16th to endNight
74%
564
17%
1595
13%
531
78%
531
37%
930
August1st to 15thNight
66%
539
18%
1609
14%
536
68%
536
29%
1038
August16th to endNight
66%
740
19%
1886
13%
740
66%
740
32%
1269
September1st to 15thNight
57%
720
23%
1800
14%
720
61%
720
32%
1206
September16th to endNight
55%
717
23%
1986
19%
717
57%
717
28%
1329
October1st to 15thNight
47%
720
22%
2160
20%
720
54%
720
27%
1365
October16th to endNight
38%
768
30%
2304
19%
768
54%
768
32%
1350
November1st to 15thNight
38%
760
24%
2385
15%
760
51%
760
28%
1429
November16th to endNight
33%
780
23%
2697
13%
780
48%
780
25%
1564
December1st to 15thNight
28%
728
18%
2546
7%
728
42%
728
19%
1628
December16th to endNight
25%
768
20%
2688
8%
768
41%
768
19%
1699

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
14%5%245%3445%7%41%
48591
Night
14%5%2442445%7%44%
48028
Cloud CoverDay
14%5%245%245%5%7%42%
48601
Night
14%5%2442445%7%44%
48032
TransparencyDay
39%6%12%22%15%2
48294
Night
38%6%12%22%17%3
47844
SeeingDay
17%7%23%25%18%8%
5913
Night
16%8%28%30%14%1
15574

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
16%425%43445%6%40%
1546
16th to endDay
14%7%35%5%245%37%41%
1728
February1st to 15thDay
13%7%25%5%346%6%6%38%
1575
16th to endDay
13%8%246%35%35%5%42%
1537
March1st to 15thDay
15%5%35%434326%45%
1966
16th to endDay
15%6%35%5%34446%39%
2120
April1st to 15thDay
13%5%35%5%3435%6%44%
2152
16th to endDay
18%425%424445%43%
2160
May1st to 15thDay
18%5%145%35%45%8%37%
2325
16th to endDay
15%425%43447%8%38%
2606
June1st to 15thDay
11%4244246%6%10%42%
2520
16th to endDay
8%435%4346%7%10%42%
2520
July1st to 15thDay
10%425%7%35%6%6%10%38%
2496
16th to endDay
7%3246%46%6%8%10%38%
2465
August1st to 15thDay
10%5%25%6%46%6%7%8%35%
2340
16th to endDay
12%424435%5%7%8%41%
2303
September1st to 15thDay
13%425%5%34447%45%
2160
16th to endDay
15%5%25%434447%43%
1971
October1st to 15thDay
21%423324436%44%
1800
16th to endDay
14%424433345%50%
1920
November1st to 15thDay
14%5%24423446%46%
1802
16th to endDay
17%42332435%6%46%
1592
December1st to 15thDay
16%5%245%35%45%6%41%
1451
16th to endDay
17%6%345%2435%7%38%
1536
January1st to 15thNight
16%6%345%24346%41%
2389
16th to endNight
16%6%34423446%42%
2496
February1st to 15thNight
13%5%245%245%5%6%44%
2285
16th to endNight
12%7%25%425%346%43%
1934
March1st to 15thNight
16%5%24324446%44%
1968
16th to endNight
16%6%35%5%2345%7%41%
2103
April1st to 15thNight
13%5%25%425%45%6%43%
1796
16th to endNight
14%5%2442445%7%43%
1800
May1st to 15thNight
18%6%14435%448%37%
1635
16th to endNight
13%5%2342436%9%42%
1506
June1st to 15thNight
9%5%13426%5%7%7%45%
1440
16th to endNight
8%424425%6%6%9%44%
1440
July1st to 15thNight
9%5%25%6%36%6%6%9%38%
1461
16th to endNight
5%213435%6%7%12%45%
1703
August1st to 15thNight
8%5%25%6%35%5%6%7%42%
1620
16th to endNight
10%5%245%25%5%6%10%41%
1886
September1st to 15thNight
12%425%5%35%45%6%46%
1800
16th to endNight
15%5%25%5%33446%43%
1986
October1st to 15thNight
19%5%1342334446%
2160
16th to endNight
13%413323346%53%
2304
November1st to 15thNight
13%6%24423335%50%
2385
16th to endNight
15%31322445%7%47%
2697
December1st to 15thNight
16%6%25%435%346%40%
2546
16th to endNight
16%5%3442445%7%40%
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
16%5%25%42445%6%41%
1546
16th to endDay
15%6%35%6%33346%41%
1728
February1st to 15thDay
14%6%25%5%245%6%6%39%
1575
16th to endDay
14%7%345%3445%442%
1537
March1st to 15thDay
15%5%245%34336%44%
1977
16th to endDay
16%6%35%434446%39%
2119
April1st to 15thDay
13%425%5%24446%45%
2152
16th to endDay
18%42442345%5%43%
2160
May1st to 15thDay
18%5%245%25%45%7%37%
2325
16th to endDay
16%414435%5%6%8%39%
2606
June1st to 15thDay
11%3244246%7%9%43%
2520
16th to endDay
8%425%5%25%5%6%10%43%
2520
July1st to 15thDay
10%425%6%36%6%6%9%38%
2496
16th to endDay
7%3246%36%6%9%11%38%
2465
August1st to 15thDay
11%5%26%5%46%5%8%8%35%
2340
16th to endDay
12%3145%26%5%7%8%42%
2303
September1st to 15thDay
12%425%4345%47%45%
2160
16th to endDay
15%6%244235%45%43%
1971
October1st to 15thDay
21%324423345%45%
1800
16th to endDay
14%414423345%50%
1920
November1st to 15thDay
14%6%24423446%47%
1802
16th to endDay
16%4233245%46%46%
1592
December1st to 15thDay
16%5%245%24446%42%
1451
16th to endDay
19%6%15%42445%6%38%
1536
January1st to 15thNight
16%5%25%424346%42%
2389
16th to endNight
17%6%2442345%6%42%
2496
February1st to 15thNight
13%5%245%345%46%45%
2278
16th to endNight
13%7%25%42435%6%44%
1934
March1st to 15thNight
16%5%23324448%44%
1980
16th to endNight
16%6%245%2445%6%41%
2103
April1st to 15thNight
14%5%2443445%6%43%
1796
16th to endNight
15%4243245%5%7%43%
1800
May1st to 15thNight
18%5%15%4245%5%8%37%
1634
16th to endNight
14%41443446%9%42%
1506
June1st to 15thNight
9%324425%5%6%8%46%
1440
16th to endNight
8%335%435%5%6%9%44%
1440
July1st to 15thNight
9%5%26%5%36%5%5%10%39%
1461
16th to endNight
5%3145%36%5%8%11%45%
1703
August1st to 15thNight
8%425%5%45%5%6%8%42%
1620
16th to endNight
10%4244345%6%9%41%
1886
September1st to 15thNight
12%5%245%35%45%5%45%
1800
16th to endNight
14%6%25%5%2435%5%43%
1986
October1st to 15thNight
19%5%24333435%46%
2160
16th to endNight
13%323323345%53%
2304
November1st to 15thNight
14%5%3442233450%
2385
16th to endNight
15%31332445%6%47%
2697
December1st to 15thNight
17%5%25%5%34446%41%
2546
16th to endNight
17%5%2443445%6%41%
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
42%5%13%20%14%3
1547
16th to endDay
43%311%23%14%3
1728
February1st to 15thDay
43%5%12%22%13%2
1577
16th to endDay
42%413%24%14%1
1537
March1st to 15thDay
42%410%23%15%3
1977
16th to endDay
45%5%11%17%16%2
2120
April1st to 15thDay
39%5%11%21%18%3
2152
16th to endDay
42%5%10%18%18%6%
2160
May1st to 15thDay
44%7%13%18%14%1
2325
16th to endDay
40%8%14%24%10%1
2610
June1st to 15thDay
35%9%15%25%13%1
2520
16th to endDay
33%10%16%22%15%1
2520
July1st to 15thDay
38%10%14%19%14%2
2349
16th to endDay
33%10%15%24%14%1
2310
August1st to 15thDay
41%9%14%19%14%
2327
16th to endDay
36%8%14%24%13%2
2303
September1st to 15thDay
36%7%12%19%20%3
2160
16th to endDay
40%6%8%24%18%1
1971
October1st to 15thDay
40%411%24%15%3
1800
16th to endDay
35%48%27%21%2
1920
November1st to 15thDay
37%412%21%17%5%
1802
16th to endDay
37%5%10%26%17%2
1592
December1st to 15thDay
40%5%13%23%12%3
1451
16th to endDay
44%5%11%23%12%1
1536
January1st to 15thNight
42%5%11%18%17%4
2389
16th to endNight
41%412%23%16%1
2496
February1st to 15thNight
39%5%12%23%16%3
2287
16th to endNight
40%411%25%16%2
1934
March1st to 15thNight
38%6%12%26%13%2
1980
16th to endNight
42%5%11%20%17%2
2103
April1st to 15thNight
39%6%10%21%19%3
1796
16th to endNight
38%7%10%20%16%7%
1800
May1st to 15thNight
43%7%11%20%13%2
1635
16th to endNight
37%7%17%21%14%1
1508
June1st to 15thNight
33%8%14%25%16%1
1440
16th to endNight
34%7%15%21%18%2
1440
July1st to 15thNight
39%10%15%17%13%3
1380
16th to endNight
27%9%19%24%18%1
1595
August1st to 15thNight
37%8%13%22%17%1
1609
16th to endNight
36%8%13%23%15%2
1886
September1st to 15thNight
37%6%11%20%19%3
1800
16th to endNight
42%5%7%21%19%3
1986
October1st to 15thNight
40%411%21%18%3
2160
16th to endNight
32%48%28%21%3
2304
November1st to 15thNight
37%38%22%19%7%
2385
16th to endNight
35%410%24%20%4
2697
December1st to 15thNight
44%5%10%21%14%2
2546
16th to endNight
42%6%13%21%13%2
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
27%22%32%15%1
79
16th to endDay
18%29%33%16%2
96
February1st to 15thDay
21%25%43%8%1
90
16th to endDay
20%17%36%24%1
196
March1st to 15thDay
17%17%32%26%5%
270
16th to endDay
18%14%35%21%6%3
288
April1st to 15thDay
15%15%25%28%12%1
270
16th to endDay
24%427%18%20%4
270
May1st to 15thDay
25%12%28%16%11%5%
288
16th to endDay
21%322%22%20%9%
331
June1st to 15thDay
12%424%25%22%10%
315
16th to endDay
10%224%33%21%8%
315
July1st to 15thDay
13%118%32%24%9%
305
16th to endDay
8%10%31%34%14%
326
August1st to 15thDay
16%13%23%30%14%
313
16th to endDay
12%18%30%28%18%
332
September1st to 15thDay
13%5%17%24%19%20%
315
16th to endDay
21%214%23%23%13%
314
October1st to 15thDay
23%415%24%23%8%
315
16th to endDay
13%8%30%25%17%4
336
November1st to 15thDay
16%9%26%28%16%3
300
16th to endDay
16%14%33%23%9%2
122
December1st to 15thDay
12%22%33%17%9%4
63
16th to endDay
34%14%26%23%1
64
January1st to 15thNight
20%15%36%22%4
720
16th to endNight
21%13%41%22%1
768
February1st to 15thNight
17%12%42%24%2
704
16th to endNight
19%19%38%20%2
631
March1st to 15thNight
20%13%38%24%3
720
16th to endNight
19%12%39%21%5%1
768
April1st to 15thNight
16%14%38%24%5%
718
16th to endNight
17%7%27%34%11%1
720
May1st to 15thNight
21%5%26%32%13%
555
16th to endNight
16%319%38%21%1
386
June1st to 15thNight
8%123%41%23%1
360
16th to endNight
8%218%51%18%
360
July1st to 15thNight
12%119%37%28%1
360
16th to endNight
6%10%40%35%5%
564
August1st to 15thNight
11%12%44%29%2
539
16th to endNight
11%19%43%29%4
740
September1st to 15thNight
15%118%30%31%2
720
16th to endNight
16%117%32%29%2
717
October1st to 15thNight
21%419%30%20%3
720
16th to endNight
15%10%32%27%13%
768
November1st to 15thNight
16%8%31%29%12%1
760
16th to endNight
17%13%31%27%9%
780
December1st to 15thNight
19%9%36%25%8%
728
16th to endNight
20%10%38%27%1
768
Page generated at 2017-12-03 11:16:38. Version ac04

Copyright 2017 A. Danko


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