Alsask Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Alsask Observatory clear sky chart from 2005-11-05 to 2017-09-10.

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
36%
15380
12%
46684
7%
15354
41%
15354
19%
31913

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
29%
895
8%
2685
5%
895
32%
895
12%
2027
January16th to endNight
25%
939
14%
2859
5%
939
34%
939
15%
2029
February1st to 15thNight
38%
701
9%
2307
8%
700
41%
700
14%
1643
February16th to endNight
40%
632
13%
2052
11%
632
45%
632
17%
1470
March1st to 15thNight
34%
720
13%
2069
8%
720
39%
720
20%
1468
March16th to endNight
33%
768
9%
2110
5%
767
37%
767
14%
1598
April1st to 15thNight
32%
539
12%
1709
6%
539
37%
539
16%
1203
April16th to endNight
34%
540
15%
1599
9%
540
41%
540
17%
1107
May1st to 15thNight
43%
537
12%
1438
8%
537
46%
537
26%
941
May16th to endNight
41%
374
7%
1309
6%
374
43%
374
17%
877
June1st to 15thNight
35%
360
0%
1260
0%
360
36%
360
14%
867
June16th to endNight
48%
360
0%
1260
0%
360
48%
360
22%
742
July1st to 15thNight
51%
340
1%
1190
0%
340
51%
340
28%
650
July16th to endNight
51%
376
10%
1239
9%
354
58%
354
36%
615
August1st to 15thNight
58%
500
12%
1478
11%
498
60%
498
31%
860
August16th to endNight
53%
573
20%
1719
13%
573
60%
573
36%
953
September1st to 15thNight
49%
525
18%
1723
14%
525
55%
525
29%
987
September16th to endNight
47%
653
21%
1827
14%
653
54%
653
35%
1107
October1st to 15thNight
38%
660
18%
1980
10%
660
45%
660
22%
1323
October16th to endNight
37%
753
16%
2265
9%
753
44%
753
22%
1520
November1st to 15thNight
32%
875
16%
2450
8%
875
38%
875
21%
1690
November16th to endNight
27%
900
12%
2576
5%
900
33%
900
15%
1883
December1st to 15thNight
25%
900
7%
2700
4%
900
28%
900
9%
2186
December16th to endNight
27%
960
10%
2880
4%
960
33%
960
12%
2167

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
10%8%37%6%35%5%5%7%35%
47982
Night
8%8%47%6%35%5%6%8%35%
46834
TransparencyDay
45%8%16%21%7%
47632
Night
45%7%16%21%8%1
46688
SeeingDay
13%14%23%26%16%4
6091
Night
10%19%24%29%14%2
15390

By Month

Percentage Cloud Cover NW (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
8%10%5%9%7%6%5%6%7%9%23%
1253
16th to endDay
411%6%10%8%46%6%5%6%30%
1374
February1st to 15thDay
8%12%6%9%9%5%46%5%6%26%
1554
16th to endDay
9%13%6%12%8%5%5%5%5%7%20%
1422
March1st to 15thDay
6%9%49%9%46%46%8%29%
1890
16th to endDay
11%14%6%10%6%35%45%7%23%
2111
April1st to 15thDay
10%9%48%6%47%5%5%6%31%
2236
16th to endDay
14%10%26%6%35%447%32%
2361
May1st to 15thDay
12%7%27%6%345%5%6%38%
2511
16th to endDay
18%9%36%5%3445%8%29%
2802
June1st to 15thDay
18%8%35%6%35%5%5%8%30%
2700
16th to endDay
14%6%25%6%35%5%6%8%36%
2700
July1st to 15thDay
7%414436%7%8%8%43%
2700
16th to endDay
6%313425%5%6%8%52%
2846
August1st to 15thDay
9%5%245%25%6%5%8%45%
2474
16th to endDay
7%42442446%10%49%
2483
September1st to 15thDay
13%5%245%23345%48%
2124
16th to endDay
10%424323349%50%
1800
October1st to 15thDay
10%9%45%6%3445%7%41%
1650
16th to endDay
6%8%47%6%347%5%8%35%
1596
November1st to 15thDay
6%10%6%7%6%35%6%6%7%33%
1408
16th to endDay
7%10%5%10%7%46%5%5%8%27%
1383
December1st to 15thDay
8%12%6%14%7%6%7%6%7%5%18%
1260
16th to endDay
6%13%5%10%9%5%6%6%6%9%20%
1344
January1st to 15thNight
5%10%5%8%9%7%7%5%6%9%24%
2685
16th to endNight
49%49%8%5%6%6%6%7%31%
2859
February1st to 15thNight
6%8%5%9%9%46%6%5%7%30%
2298
16th to endNight
6%10%5%9%8%45%5%6%9%28%
2052
March1st to 15thNight
7%6%37%7%45%7%7%8%32%
2069
16th to endNight
10%11%5%8%7%35%5%7%7%27%
2109
April1st to 15thNight
10%9%46%6%46%5%6%8%30%
1709
16th to endNight
12%9%36%5%35%6%5%5%37%
1599
May1st to 15thNight
11%7%343245%6%8%41%
1437
16th to endNight
15%5%36%5%2446%6%37%
1309
June1st to 15thNight
16%7%25%6%346%6%7%32%
1260
16th to endNight
10%315%4245%6%9%44%
1260
July1st to 15thNight
6%41342336%9%52%
1260
16th to endNight
421332446%9%58%
1323
August1st to 15thNight
7%2244345%7%10%47%
1485
16th to endNight
6%31342346%8%53%
1719
September1st to 15thNight
10%415%42235%6%52%
1723
16th to endNight
8%4134346%7%7%48%
1827
October1st to 15thNight
8%9%36%5%345%6%7%38%
1980
16th to endNight
10%8%36%6%345%6%8%37%
2265
November1st to 15thNight
8%9%46%5%345%5%8%38%
2450
16th to endNight
7%11%5%8%8%35%447%32%
2576
December1st to 15thNight
7%13%6%11%10%6%6%5%5%7%19%
2700
16th to endNight
5%10%5%9%9%5%7%6%5%8%25%
2880

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
54%11%14%15%4
1253
16th to endDay
52%7%14%17%7%1
1374
February1st to 15thDay
57%7%13%16%5%
1554
16th to endDay
61%6%13%10%7%
1422
March1st to 15thDay
51%9%19%16%3
1890
16th to endDay
59%8%14%14%3
2112
April1st to 15thDay
50%7%16%17%7%
2236
16th to endDay
51%7%12%20%6%1
2361
May1st to 15thDay
44%7%14%24%8%
2511
16th to endDay
52%6%12%18%8%
2807
June1st to 15thDay
50%9%14%19%6%
2700
16th to endDay
43%9%19%20%6%
2700
July1st to 15thDay
31%11%21%28%6%
2541
16th to endDay
27%8%22%30%9%
2667
August1st to 15thDay
35%9%24%23%7%
2456
16th to endDay
29%7%19%30%12%1
2483
September1st to 15thDay
38%5%14%24%12%4
2124
16th to endDay
31%6%17%33%9%2
1800
October1st to 15thDay
42%5%19%24%6%
1650
16th to endDay
43%9%17%21%7%
1596
November1st to 15thDay
46%8%17%20%5%
1408
16th to endDay
54%8%15%17%3
1383
December1st to 15thDay
64%9%13%10%2
1260
16th to endDay
59%7%13%14%4
1344
January1st to 15thNight
54%8%17%15%4
2685
16th to endNight
49%7%16%18%6%1
2859
February1st to 15thNight
51%7%16%16%8%
2309
16th to endNight
50%7%15%17%8%
2052
March1st to 15thNight
46%8%20%17%6%
2069
16th to endNight
53%8%14%18%4
2112
April1st to 15thNight
49%7%12%21%8%
1709
16th to endNight
47%6%11%19%13%2
1599
May1st to 15thNight
38%6%11%27%15%
1438
16th to endNight
45%7%12%21%10%1
1309
June1st to 15thNight
46%10%15%19%8%
1260
16th to endNight
33%9%21%26%9%
1260
July1st to 15thNight
28%8%22%29%10%
1190
16th to endNight
21%7%23%33%12%2
1239
August1st to 15thNight
29%8%23%28%8%1
1478
16th to endNight
26%6%15%30%17%2
1719
September1st to 15thNight
32%5%12%25%16%7%
1723
16th to endNight
30%8%13%29%17%2
1827
October1st to 15thNight
42%7%14%23%10%
1980
16th to endNight
43%6%17%24%6%
2265
November1st to 15thNight
44%7%17%21%8%1
2450
16th to endNight
51%7%17%15%7%
2576
December1st to 15thNight
63%8%14%11%2
2700
16th to endNight
54%8%16%14%4
2880

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
10%316%30%31%8%
60
16th to endDay
10%331%31%17%6%
64
February1st to 15thDay
17%9%27%32%12%1
87
16th to endDay
14%129%30%18%4
81
March1st to 15thDay
9%22%36%22%7%
231
16th to endDay
15%19%32%25%6%
288
April1st to 15thDay
14%23%30%20%8%1
269
16th to endDay
21%17%21%21%15%2
291
May1st to 15thDay
12%19%20%30%14%3
468
16th to endDay
17%18%17%21%16%8%
517
June1st to 15thDay
16%18%24%21%14%4
495
16th to endDay
12%17%24%25%15%4
495
July1st to 15thDay
10%11%17%29%23%6%
475
16th to endDay
8%16%23%25%22%4
515
August1st to 15thDay
10%9%16%29%23%10%
409
16th to endDay
8%11%20%33%22%4
332
September1st to 15thDay
18%11%23%25%16%5%
294
16th to endDay
10%14%25%29%16%4
239
October1st to 15thDay
30%428%25%8%2
90
16th to endDay
12%7%22%37%18%1
96
November1st to 15thDay
8%444%25%14%2
90
16th to endDay
9%7%30%23%16%12%
81
December1st to 15thDay
10%5%40%31%8%5%
60
16th to endDay
23%125%35%9%4
64
January1st to 15thNight
10%24%27%26%11%1
900
16th to endNight
8%24%30%26%8%1
939
February1st to 15thNight
8%21%18%33%15%2
704
16th to endNight
9%19%18%29%19%3
632
March1st to 15thNight
7%20%28%30%11%2
720
16th to endNight
13%22%19%28%13%2
768
April1st to 15thNight
11%20%28%27%9%1
539
16th to endNight
16%21%20%26%13%1
540
May1st to 15thNight
12%20%17%32%15%1
538
16th to endNight
13%21%15%26%20%2
375
June1st to 15thNight
17%19%20%26%15%1
360
16th to endNight
10%18%16%33%19%1
360
July1st to 15thNight
10%13%20%33%21%
340
16th to endNight
5%18%19%35%19%1
376
August1st to 15thNight
8%11%15%36%23%5%
500
16th to endNight
7%11%22%33%23%1
573
September1st to 15thNight
12%11%20%35%17%1
525
16th to endNight
6%12%30%31%16%2
653
October1st to 15thNight
13%13%26%30%14%2
660
16th to endNight
12%14%28%31%11%
753
November1st to 15thNight
13%19%29%26%9%2
875
16th to endNight
11%23%26%29%7%1
900
December1st to 15thNight
13%20%25%26%11%2
900
16th to endNight
9%20%24%28%13%3
960
Page generated at 2017-09-11 17:35:19. Version ac04

Copyright 2017 A. Danko


Alsask Observatory Clear Sky Chart | Charts Home