Northstar Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Northstar Observatory clear sky chart from 2005-11-05 to 2017-09-23.

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
48%
14962
13%
46914
12%
14930
50%
14930
20%
32528

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
28%
720
6%
2506
6%
716
29%
716
7%
2100
January16th to endNight
36%
768
6%
2688
7%
768
37%
768
10%
2115
February1st to 15thNight
35%
704
7%
2396
6%
700
38%
700
11%
1849
February16th to endNight
40%
632
13%
1895
13%
632
43%
632
15%
1424
March1st to 15thNight
39%
720
6%
2160
5%
720
41%
720
13%
1683
March16th to endNight
38%
768
8%
2025
5%
768
40%
768
14%
1625
April1st to 15thNight
48%
719
12%
1799
6%
719
49%
719
22%
1326
April16th to endNight
50%
546
15%
1626
12%
546
51%
546
21%
1112
May1st to 15thNight
56%
407
17%
1488
21%
407
60%
407
26%
846
May16th to endNight
57%
374
11%
1460
17%
374
60%
374
20%
921
June1st to 15thNight
59%
360
9%
1260
9%
360
60%
360
24%
770
June16th to endNight
63%
360
13%
1260
14%
360
64%
360
27%
727
July1st to 15thNight
79%
340
17%
1314
22%
340
82%
340
51%
527
July16th to endNight
84%
376
26%
1416
43%
354
86%
354
60%
505
August1st to 15thNight
85%
381
29%
1453
39%
379
87%
379
53%
569
August16th to endNight
78%
654
32%
1800
24%
654
80%
654
57%
898
September1st to 15thNight
73%
720
30%
1818
18%
720
75%
720
58%
959
September16th to endNight
62%
688
24%
1903
20%
688
64%
688
35%
1179
October1st to 15thNight
56%
660
19%
1918
17%
660
58%
660
28%
1232
October16th to endNight
49%
704
14%
2288
15%
704
51%
704
18%
1620
November1st to 15thNight
37%
700
9%
2275
9%
700
39%
700
12%
1783
November16th to endNight
37%
813
6%
2598
5%
813
38%
813
11%
2064
December1st to 15thNight
34%
900
5%
2700
4%
900
35%
900
10%
2225
December16th to endNight
25%
948
4%
2868
2%
948
27%
948
7%
2469

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%8%35%5%24446%39%
48004
Night
15%9%35%5%3445%6%37%
47082
Cloud CoverDay
14%8%35%5%24446%39%
48004
Night
16%9%35%5%24446%37%
47086
TransparencyDay
46%7%12%19%11%2
47675
Night
48%6%12%17%11%3
46914
SeeingDay
16%11%22%26%17%4
5735
Night
21%6%15%30%22%3
14964

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
27%15%5%8%5%2435%416%
1432
16th to endDay
23%16%5%8%6%34445%18%
1536
February1st to 15thDay
22%14%5%8%425%5%5%5%20%
1464
16th to endDay
18%18%6%8%6%2334322%
1579
March1st to 15thDay
23%16%5%7%6%2444419%
1800
16th to endDay
22%16%5%8%6%36%345%18%
2196
April1st to 15thDay
17%11%47%7%45%6%5%6%22%
2149
16th to endDay
15%8%47%6%35%5%5%6%31%
2334
May1st to 15thDay
9%6%35%6%35%5%5%8%40%
2463
16th to endDay
12%7%37%6%35%6%6%8%31%
2648
June1st to 15thDay
17%5%35%5%3445%7%38%
2700
16th to endDay
12%4145%35%45%6%46%
2700
July1st to 15thDay
4212323445%65%
2563
16th to endDay
111212345%74%
2657
August1st to 15thDay
322312347%68%
2499
16th to endDay
431231435%6%63%
2399
September1st to 15thDay
3313213346%65%
2142
16th to endDay
10%5%15%43445%7%47%
1893
October1st to 15thDay
17%8%35%42424442%
1710
16th to endDay
21%9%36%42434433%
1584
November1st to 15thDay
27%11%5%6%5%23435%25%
1584
16th to endDay
22%17%410%6%3344420%
1347
December1st to 15thDay
21%17%7%8%6%3334418%
1260
16th to endDay
25%20%6%7%5%3324316%
1365
January1st to 15thNight
27%15%5%8%5%2334417%
2506
16th to endNight
22%12%6%6%6%23336%24%
2688
February1st to 15thNight
20%16%46%5%33345%27%
2385
16th to endNight
16%14%35%5%3445%6%29%
1895
March1st to 15thNight
19%12%46%6%35%45%6%26%
2160
16th to endNight
19%10%46%6%45%5%5%6%25%
2021
April1st to 15thNight
11%8%37%6%35%5%6%7%33%
1799
16th to endNight
13%7%36%5%35%45%439%
1626
May1st to 15thNight
7%434435%46%8%45%
1487
16th to endNight
11%6%27%6%35%5%7%9%35%
1460
June1st to 15thNight
11%415%435%6%5%7%44%
1260
16th to endNight
9%5%246%25%5%47%46%
1260
July1st to 15thNight
211231235%5%70%
1394
16th to endNight
1111212336%74%
1512
August1st to 15thNight
21221235%7%69%
1461
16th to endNight
111232335%5%69%
1800
September1st to 15thNight
3212312247%67%
1818
16th to endNight
10%425%42445%8%48%
1903
October1st to 15thNight
14%426%434345%47%
1918
16th to endNight
17%8%36%43445%6%35%
2288
November1st to 15thNight
24%11%45%5%235%5%6%25%
2275
16th to endNight
25%12%46%434345%24%
2598
December1st to 15thNight
21%15%5%8%433446%21%
2700
16th to endNight
25%19%5%7%6%2344317%
2868

Percentage Cloud Cover SW (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
27%15%47%6%3435%5%15%
1432
16th to endDay
23%17%6%6%5%2445%5%18%
1536
February1st to 15thDay
21%16%5%6%5%345%46%19%
1464
16th to endDay
19%18%6%7%6%2334422%
1579
March1st to 15thDay
23%16%5%8%6%4444418%
1800
16th to endDay
23%15%48%6%35%5%35%18%
2196
April1st to 15thDay
17%10%38%7%46%5%5%6%23%
2149
16th to endDay
14%9%36%6%35%45%6%31%
2334
May1st to 15thDay
9%6%26%6%35%46%7%41%
2463
16th to endDay
13%6%36%6%36%5%6%9%32%
2648
June1st to 15thDay
16%5%35%6%2445%7%38%
2700
16th to endDay
13%4145%35%45%6%45%
2700
July1st to 15thDay
4212323445%66%
2563
16th to endDay
111213345%74%
2657
August1st to 15thDay
3212213347%68%
2499
16th to endDay
421232335%7%63%
2399
September1st to 15thDay
3313313346%65%
2142
16th to endDay
10%5%1343445%7%47%
1893
October1st to 15thDay
17%7%25%42433442%
1710
16th to endDay
22%9%26%5%24345%32%
1584
November1st to 15thDay
28%11%46%433335%24%
1584
16th to endDay
21%16%5%10%6%2433419%
1347
December1st to 15thDay
21%16%6%9%6%3435%6%17%
1260
16th to endDay
25%20%6%8%6%15%33315%
1365
January1st to 15thNight
28%15%49%5%3333417%
2506
16th to endNight
23%13%46%5%33346%23%
2688
February1st to 15thNight
21%15%5%6%423245%26%
2385
16th to endNight
16%13%47%5%3435%6%29%
1895
March1st to 15thNight
19%11%46%5%36%45%6%26%
2160
16th to endNight
19%10%46%6%36%6%6%6%24%
2025
April1st to 15thNight
11%8%37%6%45%5%6%7%33%
1799
16th to endNight
12%7%27%5%35%45%5%39%
1626
May1st to 15thNight
7%435%425%5%6%8%45%
1487
16th to endNight
10%6%36%6%35%5%6%8%36%
1460
June1st to 15thNight
10%434435%6%5%7%44%
1260
16th to endNight
10%4245%345%5%7%46%
1260
July1st to 15thNight
312212345%71%
1394
16th to endNight
112212335%75%
1512
August1st to 15thNight
21221235%7%70%
1461
16th to endNight
1212313445%69%
1800
September1st to 15thNight
322312247%67%
1818
16th to endNight
9%5%145%2435%8%48%
1903
October1st to 15thNight
14%426%334346%46%
1918
16th to endNight
18%7%36%5%3445%7%35%
2288
November1st to 15thNight
24%12%35%43445%6%25%
2275
16th to endNight
25%12%46%5%34446%23%
2598
December1st to 15thNight
20%16%5%7%5%24446%21%
2700
16th to endNight
25%19%5%7%5%2344416%
2868

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
71%8%10%6%2
1432
16th to endDay
66%7%14%7%2
1536
February1st to 15thDay
64%9%12%8%3
1464
16th to endDay
66%6%9%14%3
1579
March1st to 15thDay
69%8%11%8%1
1800
16th to endDay
69%6%12%9%1
2196
April1st to 15thDay
59%7%15%12%3
2149
16th to endDay
51%8%16%17%4
2334
May1st to 15thDay
40%8%14%25%10%
2463
16th to endDay
47%9%17%20%5%
2652
June1st to 15thDay
44%7%12%21%10%2
2700
16th to endDay
37%6%13%27%13%2
2700
July1st to 15thDay
21%410%28%24%11%
2414
16th to endDay
12%49%22%38%12%
2490
August1st to 15thDay
16%49%29%31%8%
2482
16th to endDay
20%410%30%27%6%
2399
September1st to 15thDay
20%411%29%26%7%
2142
16th to endDay
34%9%16%21%13%3
1893
October1st to 15thDay
45%5%13%25%9%
1710
16th to endDay
54%7%14%19%3
1584
November1st to 15thDay
62%7%12%11%31
1584
16th to endDay
67%8%10%9%3
1347
December1st to 15thDay
70%8%9%9%1
1260
16th to endDay
73%7%9%8%
1365
January1st to 15thNight
69%7%10%9%2
2506
16th to endNight
62%7%13%12%11
2688
February1st to 15thNight
60%7%15%11%4
2396
16th to endNight
55%7%14%15%7%
1895
March1st to 15thNight
58%7%15%13%4
2160
16th to endNight
58%9%12%14%5%
2025
April1st to 15thNight
48%8%14%20%7%
1799
16th to endNight
46%6%13%22%10%
1626
May1st to 15thNight
34%6%14%28%13%1
1488
16th to endNight
41%8%16%23%9%
1460
June1st to 15thNight
37%8%10%22%17%3
1260
16th to endNight
35%5%10%25%17%4
1260
July1st to 15thNight
16%36%24%31%18%
1314
16th to endNight
11%26%18%35%24%
1416
August1st to 15thNight
14%39%18%40%13%
1453
16th to endNight
15%48%27%30%13%
1800
September1st to 15thNight
17%39%27%29%11%
1818
16th to endNight
33%6%13%21%17%7%
1903
October1st to 15thNight
39%5%11%27%14%1
1918
16th to endNight
47%9%12%21%8%
2288
November1st to 15thNight
59%9%12%13%32
2275
16th to endNight
62%6%14%11%3
2598
December1st to 15thNight
64%8%13%11%1
2700
16th to endNight
71%7%11%8%1
2868

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
20%5%15%35%16%8%
60
16th to endDay
43%6%18%15%14%1
64
February1st to 15thDay
30%20%15%16%15%1
106
16th to endDay
29%18%20%20%8%2
238
March1st to 15thDay
35%21%21%15%4
270
16th to endDay
31%16%25%19%6%
288
April1st to 15thDay
18%18%28%20%8%4
269
16th to endDay
20%17%22%22%12%3
270
May1st to 15thDay
12%15%26%27%15%2
288
16th to endDay
14%7%17%28%24%7%
363
June1st to 15thDay
14%7%25%27%19%4
495
16th to endDay
11%10%21%28%22%5%
495
July1st to 15thDay
6%5%22%31%30%3
348
16th to endDay
18%25%38%18%6%
327
August1st to 15thDay
5%421%32%26%9%
315
16th to endDay
27%24%32%21%11%
332
September1st to 15thDay
8%9%27%22%22%8%
315
16th to endDay
13%14%21%25%22%3
284
October1st to 15thDay
20%12%27%22%14%2
235
16th to endDay
46%414%20%7%6%
96
November1st to 15thDay
34%12%8%30%14%
90
16th to endDay
33%9%19%20%14%3
63
December1st to 15thDay
40%18%16%18%5%1
60
16th to endDay
34%10%28%15%10%
64
January1st to 15thNight
37%7%15%24%12%2
720
16th to endNight
29%5%13%30%17%2
768
February1st to 15thNight
32%6%16%29%13%2
704
16th to endNight
24%8%17%27%19%2
632
March1st to 15thNight
26%9%16%30%15%1
720
16th to endNight
24%8%19%32%12%1
768
April1st to 15thNight
14%8%20%32%21%2
719
16th to endNight
17%5%19%34%20%2
546
May1st to 15thNight
11%420%29%27%6%
408
16th to endNight
13%414%28%32%6%
375
June1st to 15thNight
13%318%34%24%5%
360
16th to endNight
11%411%36%30%5%
360
July1st to 15thNight
4210%35%44%3
340
16th to endNight
118%40%42%5%
376
August1st to 15thNight
315%33%46%9%
381
16th to endNight
2112%33%40%8%
654
September1st to 15thNight
5%213%29%37%11%
720
16th to endNight
12%215%30%30%7%
688
October1st to 15thNight
16%5%13%36%25%2
660
16th to endNight
23%6%14%28%23%3
704
November1st to 15thNight
31%8%16%31%12%
700
16th to endNight
33%8%12%25%17%2
813
December1st to 15thNight
29%7%17%30%13%
900
16th to endNight
37%8%17%28%8%
948
Page generated at 2017-09-24 06:15:44. Version ac04

Copyright 2017 A. Danko


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