Great Falls Observatory Park
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Great Falls Observatory Park clear sky chart from 2005-11-05 to 2018-02-14.

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%
15841
21%
48834
12%
15804
49%
15804
25%
30600

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%
777
22%
2612
8%
776
40%
776
20%
1629
January16th to endNight
22%
832
23%
2704
9%
832
40%
832
22%
1691
February1st to 15thNight
22%
757
19%
2468
5%
757
39%
757
20%
1525
February16th to endNight
19%
631
23%
1922
6%
631
39%
631
21%
1209
March1st to 15thNight
24%
720
21%
1998
8%
720
37%
720
26%
1284
March16th to endNight
23%
768
21%
2093
9%
768
33%
768
24%
1413
April1st to 15thNight
26%
718
19%
1796
5%
718
35%
718
28%
1193
April16th to endNight
41%
720
20%
1800
8%
720
46%
720
31%
1199
May1st to 15thNight
39%
559
17%
1639
9%
559
45%
559
23%
1111
May16th to endNight
55%
374
17%
1497
15%
374
62%
374
23%
871
June1st to 15thNight
56%
360
19%
1440
20%
360
63%
360
25%
826
June16th to endNight
59%
360
17%
1440
18%
360
65%
360
26%
811
July1st to 15thNight
55%
340
12%
1360
14%
340
58%
340
20%
846
July16th to endNight
72%
559
16%
1590
14%
526
76%
526
36%
911
August1st to 15thNight
65%
539
18%
1609
13%
536
66%
536
31%
1016
August16th to endNight
65%
750
19%
1896
13%
750
66%
750
34%
1261
September1st to 15thNight
55%
720
24%
1800
14%
720
59%
720
33%
1201
September16th to endNight
54%
717
24%
1977
20%
717
57%
717
29%
1304
October1st to 15thNight
46%
720
23%
2160
20%
720
53%
720
27%
1368
October16th to endNight
38%
768
30%
2304
18%
768
55%
768
32%
1351
November1st to 15thNight
38%
760
25%
2406
15%
760
52%
760
27%
1434
November16th to endNight
33%
780
24%
2681
13%
780
48%
780
24%
1572
December1st to 15thNight
28%
780
18%
2730
7%
780
42%
780
19%
1743
December16th to endNight
24%
832
21%
2912
9%
832
41%
832
19%
1831

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
13%5%25%43445%7%42%
49226
Night
13%5%2442445%7%43%
49016
Cloud CoverDay
14%5%2443445%7%41%
49236
Night
14%5%2442445%7%43%
49020
TransparencyDay
40%5%12%23%15%2
48926
Night
38%5%11%23%17%3
48834
SeeingDay
17%7%24%25%17%7%
6115
Night
17%9%28%29%13%1
15848

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%5%244345%5%6%41%
1653
16th to endDay
14%7%35%5%23445%43%
1872
February1st to 15thDay
14%6%26%43446%6%39%
1695
16th to endDay
14%7%35%5%3435%5%41%
1549
March1st to 15thDay
16%5%24425%345%44%
1950
16th to endDay
16%6%36%42345%6%40%
2130
April1st to 15thDay
13%5%3443345%6%44%
2152
16th to endDay
17%5%24423336%44%
2160
May1st to 15thDay
16%6%25%5%25%46%7%37%
2319
16th to endDay
15%5%15%5%345%6%7%40%
2618
June1st to 15thDay
10%325%425%5%6%10%41%
2520
16th to endDay
9%425%5%35%47%9%43%
2520
July1st to 15thDay
9%425%446%6%7%8%38%
2520
16th to endDay
7%3246%46%6%8%10%39%
2468
August1st to 15thDay
11%5%25%6%46%7%6%8%36%
2340
16th to endDay
11%424435%5%6%9%42%
2292
September1st to 15thDay
13%42443336%7%45%
2160
16th to endDay
14%6%25%43345%7%41%
1979
October1st to 15thDay
19%5%23323346%45%
1800
16th to endDay
13%424423346%50%
1920
November1st to 15thDay
15%5%143245%45%47%
1780
16th to endDay
16%314224446%46%
1605
December1st to 15thDay
15%4245%445%5%6%41%
1560
16th to endDay
17%7%25%434447%39%
1664
January1st to 15thNight
16%6%25%42435%6%42%
2612
16th to endNight
16%6%2332445%7%42%
2704
February1st to 15thNight
14%6%145%34446%43%
2466
16th to endNight
14%6%25%424346%44%
1922
March1st to 15thNight
16%5%2332445%6%44%
1986
16th to endNight
15%6%36%42345%5%41%
2093
April1st to 15thNight
13%6%245%15%5%5%6%43%
1796
16th to endNight
14%5%24425%45%7%43%
1800
May1st to 15thNight
18%6%25%42445%7%39%
1639
16th to endNight
12%5%234345%5%10%42%
1495
June1st to 15thNight
10%224336%6%6%8%45%
1440
16th to endNight
7%5%25%5%345%7%9%43%
1440
July1st to 15thNight
8%5%37%7%35%6%6%9%37%
1440
16th to endNight
5%324435%6%9%11%45%
1697
August1st to 15thNight
8%415%5%35%6%6%8%43%
1620
16th to endNight
9%425%5%35%5%6%8%43%
1896
September1st to 15thNight
12%5%245%3345%6%46%
1800
16th to endNight
12%6%26%5%24347%43%
1977
October1st to 15thNight
19%42442335%6%44%
2160
16th to endNight
13%313314446%52%
2304
November1st to 15thNight
14%5%24323335%50%
2406
16th to endNight
15%32342345%7%46%
2681
December1st to 15thNight
16%6%35%5%3445%6%38%
2730
16th to endNight
16%6%245%24447%40%
2912

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%3443446%6%41%
1653
16th to endDay
14%7%245%23346%43%
1872
February1st to 15thDay
15%6%26%5%2336%6%39%
1695
16th to endDay
13%7%36%5%3445%6%41%
1549
March1st to 15thDay
15%5%34434346%43%
1961
16th to endDay
16%6%35%424446%40%
2129
April1st to 15thDay
13%6%2442435%7%44%
2152
16th to endDay
17%5%14423347%43%
2160
May1st to 15thDay
16%6%245%35%46%7%37%
2319
16th to endDay
15%4244346%5%8%39%
2618
June1st to 15thDay
11%425%425%5%6%10%41%
2520
16th to endDay
9%4245%35%5%6%10%43%
2520
July1st to 15thDay
9%5%25%6%37%5%6%9%38%
2520
16th to endDay
7%3245%47%7%7%10%39%
2468
August1st to 15thDay
11%425%6%5%6%6%7%7%35%
2340
16th to endDay
12%325%4246%6%8%41%
2292
September1st to 15thDay
13%42443345%7%45%
2160
16th to endDay
15%5%25%4246%47%40%
1979
October1st to 15thDay
20%5%13323446%45%
1800
16th to endDay
13%424423445%50%
1920
November1st to 15thDay
15%5%23314445%47%
1780
16th to endDay
16%5%2332445%6%47%
1605
December1st to 15thDay
15%315%435%45%6%42%
1560
16th to endDay
17%6%26%424446%39%
1664
January1st to 15thNight
16%5%25%423347%42%
2612
16th to endNight
16%6%24424347%42%
2704
February1st to 15thNight
14%5%25%5%3445%6%43%
2459
16th to endNight
14%6%25%433445%44%
1922
March1st to 15thNight
16%4243335%5%6%44%
1998
16th to endNight
15%6%36%5%2436%6%41%
2093
April1st to 15thNight
14%5%245%2445%6%43%
1796
16th to endNight
14%5%24425%45%7%43%
1800
May1st to 15thNight
18%6%25%42445%7%38%
1638
16th to endNight
13%5%233245%6%9%42%
1495
June1st to 15thNight
10%3235%26%6%6%8%45%
1440
16th to endNight
7%427%425%5%6%10%42%
1440
July1st to 15thNight
8%5%37%6%36%5%6%9%37%
1440
16th to endNight
6%314325%6%8%12%44%
1697
August1st to 15thNight
9%415%6%35%5%6%8%43%
1620
16th to endNight
10%424445%5%6%8%43%
1896
September1st to 15thNight
12%5%245%3345%6%46%
1800
16th to endNight
12%6%246%2345%7%43%
1977
October1st to 15thNight
19%5%1342235%5%44%
2160
16th to endNight
14%314323446%52%
2304
November1st to 15thNight
14%5%2442333450%
2406
16th to endNight
15%31433445%6%46%
2681
December1st to 15thNight
15%6%26%5%345%5%6%38%
2730
16th to endNight
16%6%244245%47%40%
2912

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
41%5%12%22%13%4
1654
16th to endDay
42%311%24%14%3
1872
February1st to 15thDay
43%410%25%13%2
1697
16th to endDay
43%312%25%13%1
1549
March1st to 15thDay
41%410%24%15%3
1961
16th to endDay
45%410%19%17%2
2130
April1st to 15thDay
39%410%23%18%3
2152
16th to endDay
41%49%19%19%6%
2160
May1st to 15thDay
44%6%12%20%15%1
2319
16th to endDay
40%7%14%24%11%1
2622
June1st to 15thDay
35%8%14%26%14%1
2520
16th to endDay
34%8%14%24%16%2
2520
July1st to 15thDay
39%7%15%19%15%2
2371
16th to endDay
34%8%15%24%14%1
2312
August1st to 15thDay
43%7%11%21%14%1
2327
16th to endDay
36%7%14%23%14%2
2292
September1st to 15thDay
36%6%12%20%21%2
2160
16th to endDay
41%47%24%19%2
1979
October1st to 15thDay
40%310%25%17%3
1800
16th to endDay
36%37%29%21%1
1920
November1st to 15thDay
38%411%22%16%5%
1780
16th to endDay
37%49%27%17%2
1605
December1st to 15thDay
41%5%13%24%12%3
1560
16th to endDay
44%5%11%25%12%1
1664
January1st to 15thNight
41%410%19%18%5%
2612
16th to endNight
41%410%25%17%1
2704
February1st to 15thNight
40%411%24%15%2
2468
16th to endNight
40%410%25%17%1
1922
March1st to 15thNight
38%5%12%27%13%2
1998
16th to endNight
42%5%10%21%16%3
2093
April1st to 15thNight
40%49%21%20%3
1796
16th to endNight
37%6%10%21%16%7%
1800
May1st to 15thNight
43%7%10%22%13%3
1639
16th to endNight
35%8%17%21%15%2
1497
June1st to 15thNight
34%8%13%26%16%1
1440
16th to endNight
35%5%16%22%17%2
1440
July1st to 15thNight
41%8%16%17%12%4
1360
16th to endNight
26%9%18%25%18%1
1590
August1st to 15thNight
36%7%12%24%17%2
1609
16th to endNight
35%7%12%24%16%3
1896
September1st to 15thNight
37%5%10%21%20%3
1800
16th to endNight
40%6%7%22%19%3
1977
October1st to 15thNight
40%49%23%18%3
2160
16th to endNight
32%48%29%21%3
2304
November1st to 15thNight
37%38%23%20%6%
2406
16th to endNight
36%5%9%24%20%3
2681
December1st to 15thNight
44%5%11%22%13%2
2730
16th to endNight
42%5%12%24%12%1
2912

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
23%14%39%17%21
113
16th to endDay
18%22%35%18%4
144
February1st to 15thDay
30%16%39%13%
130
16th to endDay
16%18%39%22%1
208
March1st to 15thDay
20%13%35%25%5%
270
16th to endDay
19%13%35%21%7%3
288
April1st to 15thDay
17%14%28%24%12%2
270
16th to endDay
21%8%24%22%17%5%
270
May1st to 15thDay
23%12%28%15%14%5%
288
16th to endDay
21%320%26%20%6%
331
June1st to 15thDay
15%224%25%21%10%
315
16th to endDay
9%326%34%16%8%
315
July1st to 15thDay
13%216%32%25%8%
305
16th to endDay
9%111%30%34%12%
326
August1st to 15thDay
15%13%25%32%12%
313
16th to endDay
10%9%32%25%21%
332
September1st to 15thDay
14%5%17%20%22%19%
315
16th to endDay
21%214%27%18%16%
314
October1st to 15thDay
25%414%22%25%7%
315
16th to endDay
13%8%29%26%17%4
336
November1st to 15thDay
19%7%23%28%16%4
296
16th to endDay
16%11%34%22%12%2
135
December1st to 15thDay
17%12%35%18%11%4
90
16th to endDay
29%7%38%21%3
96
January1st to 15thNight
21%16%35%21%5%
780
16th to endNight
21%14%37%24%2
832
February1st to 15thNight
19%14%39%25%2
760
16th to endNight
20%19%36%21%1
631
March1st to 15thNight
21%12%38%24%2
720
16th to endNight
18%14%37%22%5%1
768
April1st to 15thNight
16%16%38%23%5%
718
16th to endNight
18%7%28%32%11%
720
May1st to 15thNight
21%6%26%32%11%
559
16th to endNight
14%319%39%21%1
375
June1st to 15thNight
9%125%38%24%
360
16th to endNight
8%219%49%18%1
360
July1st to 15thNight
10%118%44%24%1
340
16th to endNight
6%111%41%34%4
559
August1st to 15thNight
12%11%45%27%3
539
16th to endNight
13%111%38%30%3
750
September1st to 15thNight
15%217%29%31%3
720
16th to endNight
15%217%32%30%3
717
October1st to 15thNight
21%5%18%29%21%3
720
16th to endNight
16%10%31%26%14%
768
November1st to 15thNight
15%10%30%28%13%1
760
16th to endNight
17%14%30%27%9%
780
December1st to 15thNight
20%10%33%25%9%
780
16th to endNight
20%12%38%26%2
832
Page generated at 2018-02-15 10:58:51. Version ac04

Copyright 2018 A. Danko


Great Falls Observatory Park Clear Sky Chart |Charts Home