Custer Institute
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Custer Institute 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
32%
15609
18%
48772
9%
15578
43%
15578
20%
31798

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
17%
780
19%
2716
7%
776
33%
776
15%
1721
January16th to endNight
17%
832
20%
2886
7%
832
34%
832
16%
1818
February1st to 15thNight
15%
760
17%
2333
4%
757
30%
757
16%
1585
February16th to endNight
20%
631
23%
1893
7%
631
40%
631
20%
1215
March1st to 15thNight
22%
720
17%
2160
7%
720
34%
720
17%
1435
March16th to endNight
26%
768
24%
2011
7%
768
39%
768
25%
1315
April1st to 15thNight
28%
718
17%
1796
5%
718
35%
718
22%
1253
April16th to endNight
32%
645
17%
1725
4%
645
39%
645
22%
1188
May1st to 15thNight
35%
540
16%
1620
7%
540
42%
540
18%
1123
May16th to endNight
39%
407
12%
1530
11%
407
44%
407
14%
1004
June1st to 15thNight
44%
360
14%
1440
16%
360
50%
360
17%
919
June16th to endNight
54%
360
13%
1440
14%
360
62%
360
23%
812
July1st to 15thNight
52%
340
12%
1360
17%
340
54%
340
19%
834
July16th to endNight
53%
376
16%
1418
16%
354
61%
354
20%
847
August1st to 15thNight
59%
531
17%
1602
12%
529
63%
529
29%
978
August16th to endNight
62%
764
20%
1910
11%
764
64%
764
33%
1246
September1st to 15thNight
50%
720
21%
1836
13%
720
53%
720
30%
1217
September16th to endNight
49%
717
23%
1980
14%
717
56%
717
27%
1289
October1st to 15thNight
36%
720
21%
2034
14%
720
45%
720
24%
1357
October16th to endNight
27%
768
18%
2456
8%
768
40%
768
21%
1587
November1st to 15thNight
30%
760
18%
2472
10%
760
42%
760
18%
1615
November16th to endNight
24%
780
21%
2535
10%
780
38%
780
18%
1645
December1st to 15thNight
22%
780
14%
2707
6%
780
33%
780
12%
1836
December16th to endNight
14%
832
17%
2912
4%
832
30%
832
12%
1959

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%5%2442445%7%43%
49306
Night
17%6%25%42445%7%39%
48942
Cloud CoverDay
15%5%2442445%7%43%
49316
Night
17%6%25%42445%7%39%
48947
TransparencyDay
39%5%13%23%16%2
48994
Night
44%412%21%15%2
48772
SeeingDay
18%13%21%26%16%4
6331
Night
18%20%23%24%11%1
15610

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%7%2343445%7%38%
1552
16th to endDay
16%7%35%42445%7%38%
1688
February1st to 15thDay
17%5%35%43446%7%37%
1829
16th to endDay
16%5%24435%347%41%
1580
March1st to 15thDay
18%5%15%5%3335%8%39%
1790
16th to endDay
16%6%24323445%45%
2208
April1st to 15thDay
18%6%1433335%6%43%
2152
16th to endDay
16%5%2332345%7%46%
2235
May1st to 15thDay
21%7%24323346%40%
2332
16th to endDay
15%5%2442345%8%43%
2593
June1st to 15thDay
19%42442445%9%39%
2520
16th to endDay
10%5%24425%5%5%9%45%
2520
July1st to 15thDay
10%415%4245%6%9%45%
2520
16th to endDay
12%4244245%6%8%43%
2654
August1st to 15thDay
11%4233345%6%7%48%
2332
16th to endDay
10%42443445%7%49%
2292
September1st to 15thDay
12%41442335%6%50%
2122
16th to endDay
13%6%14423347%47%
1980
October1st to 15thDay
18%5%24323446%44%
1926
16th to endDay
16%42442445%7%43%
1767
November1st to 15thDay
19%6%23423445%43%
1716
16th to endDay
14%6%244245%5%6%42%
1755
December1st to 15thDay
18%8%35%42346%7%35%
1579
16th to endDay
21%8%36%32336%6%33%
1664
January1st to 15thNight
18%6%25%42345%7%38%
2716
16th to endNight
18%8%24323447%39%
2886
February1st to 15thNight
17%8%36%5%2445%7%35%
2332
16th to endNight
17%6%23425%45%8%40%
1893
March1st to 15thNight
18%7%25%31445%6%40%
2147
16th to endNight
17%7%24223336%45%
2011
April1st to 15thNight
18%7%2442435%6%40%
1796
16th to endNight
19%6%26%424346%39%
1725
May1st to 15thNight
18%7%25%5%35%35%7%34%
1620
16th to endNight
19%6%245%3445%9%33%
1528
June1st to 15thNight
21%7%23424446%38%
1440
16th to endNight
12%424435%5%5%9%42%
1440
July1st to 15thNight
11%5%25%4345%6%9%38%
1440
16th to endNight
10%5%36%327%6%7%10%37%
1514
August1st to 15thNight
8%3245%45%5%6%8%43%
1612
16th to endNight
10%4244245%5%8%46%
1910
September1st to 15thNight
14%5%245%3445%6%43%
1836
16th to endNight
15%6%34423346%45%
1980
October1st to 15thNight
17%7%24423345%43%
2034
16th to endNight
18%7%25%423336%41%
2456
November1st to 15thNight
18%7%25%424335%41%
2472
16th to endNight
18%5%35%5%2345%7%39%
2535
December1st to 15thNight
20%8%36%43445%6%31%
2707
16th to endNight
19%7%35%425%45%7%32%
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
17%6%245%2445%7%39%
1552
16th to endDay
16%7%35%42445%6%39%
1688
February1st to 15thDay
17%5%25%5%345%5%7%36%
1829
16th to endDay
16%5%245%2445%6%42%
1580
March1st to 15thDay
18%6%2442435%8%39%
1800
16th to endDay
16%6%24313346%45%
2208
April1st to 15thDay
18%6%23323347%43%
2152
16th to endDay
16%5%2332335%7%45%
2235
May1st to 15thDay
21%7%25%312336%41%
2332
16th to endDay
15%5%2442345%8%43%
2593
June1st to 15thDay
19%5%23325%35%9%39%
2520
16th to endDay
10%4244245%6%9%45%
2520
July1st to 15thDay
10%415%4245%6%9%45%
2520
16th to endDay
12%414425%5%6%8%43%
2654
August1st to 15thDay
11%4134246%5%8%46%
2332
16th to endDay
10%42443445%7%49%
2292
September1st to 15thDay
11%5%2432335%6%50%
2122
16th to endDay
14%6%24423246%48%
1980
October1st to 15thDay
19%5%2441345%6%43%
1926
16th to endDay
15%425%42445%6%44%
1767
November1st to 15thDay
20%5%24324445%43%
1716
16th to endDay
14%5%245%2435%7%42%
1755
December1st to 15thDay
18%8%45%42445%7%35%
1579
16th to endDay
21%8%35%42335%7%33%
1664
January1st to 15thNight
18%7%2442345%7%38%
2716
16th to endNight
17%9%34323447%39%
2886
February1st to 15thNight
16%8%26%5%2445%7%36%
2325
16th to endNight
17%6%24425%35%7%40%
1893
March1st to 15thNight
18%7%25%324445%39%
2160
16th to endNight
16%8%23324335%45%
2010
April1st to 15thNight
17%7%25%42435%6%39%
1796
16th to endNight
20%6%25%423347%39%
1725
May1st to 15thNight
18%7%35%5%3445%7%34%
1620
16th to endNight
19%5%25%5%35%349%33%
1528
June1st to 15thNight
22%5%33424447%38%
1440
16th to endNight
12%5%235%25%45%8%44%
1440
July1st to 15thNight
12%5%25%5%245%6%10%38%
1440
16th to endNight
10%5%25%5%26%6%7%9%38%
1514
August1st to 15thNight
8%4246%35%5%6%9%43%
1612
16th to endNight
10%424325%5%5%8%46%
1910
September1st to 15thNight
15%415%5%3445%7%42%
1836
16th to endNight
15%6%25%433346%44%
1980
October1st to 15thNight
16%7%245%23345%43%
2034
16th to endNight
19%6%25%423346%40%
2456
November1st to 15thNight
19%6%24424435%41%
2472
16th to endNight
17%6%3442445%7%39%
2535
December1st to 15thNight
19%8%46%5%2435%6%32%
2707
16th to endNight
20%7%36%5%2445%6%33%
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
43%5%12%23%12%2
1553
16th to endDay
43%411%24%13%2
1688
February1st to 15thDay
44%413%22%12%1
1832
16th to endDay
41%212%26%15%1
1580
March1st to 15thDay
44%316%23%10%2
1800
16th to endDay
41%311%21%18%3
2208
April1st to 15thDay
40%411%22%17%3
2152
16th to endDay
37%410%24%19%3
2235
May1st to 15thDay
45%411%20%15%2
2332
16th to endDay
38%6%17%21%13%2
2597
June1st to 15thDay
42%5%14%23%12%
2520
16th to endDay
34%6%15%25%16%1
2520
July1st to 15thDay
35%7%13%24%17%1
2371
16th to endDay
36%6%14%25%13%3
2487
August1st to 15thDay
33%6%17%24%17%1
2318
16th to endDay
34%5%9%27%21%1
2292
September1st to 15thDay
33%5%13%23%21%2
2122
16th to endDay
37%5%7%24%23%2
1980
October1st to 15thDay
41%410%21%18%2
1926
16th to endDay
39%411%26%16%1
1767
November1st to 15thDay
43%413%20%16%1
1716
16th to endDay
40%412%25%14%2
1755
December1st to 15thDay
47%317%16%14%
1579
16th to endDay
49%5%14%18%11%
1664
January1st to 15thNight
44%413%22%13%1
2716
16th to endNight
44%311%23%15%1
2886
February1st to 15thNight
47%412%21%12%1
2333
16th to endNight
42%310%23%16%2
1893
March1st to 15thNight
45%314%19%13%3
2160
16th to endNight
40%310%21%19%5%
2011
April1st to 15thNight
45%410%20%16%2
1796
16th to endNight
46%39%19%17%4
1725
May1st to 15thNight
48%411%17%14%3
1620
16th to endNight
46%6%16%16%12%1
1530
June1st to 15thNight
46%412%19%13%2
1440
16th to endNight
36%415%24%16%1
1440
July1st to 15thNight
41%6%13%22%15%1
1360
16th to endNight
39%7%15%23%12%2
1418
August1st to 15thNight
34%7%15%25%15%1
1602
16th to endNight
36%5%11%26%18%
1910
September1st to 15thNight
39%410%25%17%1
1836
16th to endNight
41%37%20%25%2
1980
October1st to 15thNight
44%38%22%17%2
2034
16th to endNight
44%413%21%14%1
2456
November1st to 15thNight
45%410%20%17%2
2472
16th to endNight
43%5%11%25%12%2
2535
December1st to 15thNight
51%314%18%11%
2707
16th to endNight
49%412%20%11%
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
15%15%41%21%5%1
90
16th to endDay
23%5%46%17%6%
102
February1st to 15thDay
23%19%36%18%3
265
16th to endDay
13%22%33%24%5%
239
March1st to 15thDay
22%22%27%21%5%
270
16th to endDay
18%18%34%19%7%1
288
April1st to 15thDay
21%16%22%27%10%
270
16th to endDay
19%13%22%31%10%2
270
May1st to 15thDay
26%13%24%22%10%1
288
16th to endDay
19%11%17%27%19%4
331
June1st to 15thDay
19%10%19%26%19%5%
315
16th to endDay
14%10%14%37%16%5%
315
July1st to 15thDay
14%8%9%33%28%6%
305
16th to endDay
12%10%10%31%26%8%
326
August1st to 15thDay
13%6%8%33%29%8%
313
16th to endDay
9%6%9%30%30%13%
332
September1st to 15thDay
17%6%19%27%20%9%
315
16th to endDay
18%7%16%23%21%12%
314
October1st to 15thDay
21%8%18%22%20%7%
315
16th to endDay
17%12%28%25%14%1
298
November1st to 15thDay
21%17%23%23%13%1
280
16th to endDay
15%25%22%24%11%
285
December1st to 15thDay
22%19%27%20%9%
109
16th to endDay
32%14%39%13%
96
January1st to 15thNight
18%30%29%17%3
780
16th to endNight
20%26%32%18%1
832
February1st to 15thNight
19%24%36%18%1
760
16th to endNight
18%25%33%18%4
631
March1st to 15thNight
20%25%26%21%4
720
16th to endNight
18%22%29%20%8%1
768
April1st to 15thNight
19%20%26%24%7%
718
16th to endNight
18%19%24%24%11%1
645
May1st to 15thNight
18%18%21%27%11%1
540
16th to endNight
21%18%13%30%15%
408
June1st to 15thNight
18%19%11%28%20%1
360
16th to endNight
14%16%10%35%20%1
360
July1st to 15thNight
12%13%12%34%25%2
340
16th to endNight
10%16%11%35%22%3
376
August1st to 15thNight
10%9%10%41%25%2
531
16th to endNight
10%9%8%30%33%7%
764
September1st to 15thNight
15%9%16%30%21%5%
720
16th to endNight
17%9%14%28%21%7%
717
October1st to 15thNight
20%13%23%25%15%3
720
16th to endNight
22%15%30%19%11%
768
November1st to 15thNight
19%21%24%22%10%1
760
16th to endNight
17%27%25%19%8%
780
December1st to 15thNight
22%26%22%20%6%1
780
16th to endNight
22%28%28%17%2
832
Page generated at 2018-02-15 10:58:48. Version ac04

Copyright 2018 A. Danko


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