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 2017-08-12.

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%
14863
18%
46458
9%
14832
42%
14832
20%
30309

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
18%
720
19%
2506
8%
716
33%
716
14%
1608
January16th to endNight
16%
768
19%
2664
6%
768
34%
768
15%
1678
February1st to 15thNight
15%
704
17%
2162
4%
701
30%
701
16%
1456
February16th to endNight
19%
631
22%
1893
7%
631
39%
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
34%
718
22%
1253
April16th to endNight
32%
645
17%
1725
4%
645
39%
645
22%
1188
May1st to 15thNight
34%
540
16%
1620
7%
540
42%
540
18%
1123
May16th to endNight
38%
407
12%
1530
10%
407
44%
407
14%
1004
June1st to 15thNight
44%
360
14%
1440
16%
360
50%
360
17%
919
June16th to endNight
53%
360
13%
1440
14%
360
61%
360
23%
812
July1st to 15thNight
51%
340
12%
1360
16%
340
53%
340
19%
834
July16th to endNight
52%
376
16%
1418
16%
354
61%
354
20%
847
August1st to 15thNight
59%
517
18%
1566
12%
515
62%
515
29%
951
August16th to endNight
63%
700
18%
1750
10%
700
65%
700
32%
1149
September1st to 15thNight
51%
660
22%
1683
13%
660
54%
660
29%
1123
September16th to endNight
50%
657
23%
1815
14%
657
57%
657
29%
1159
October1st to 15thNight
35%
660
22%
1864
14%
660
44%
660
23%
1246
October16th to endNight
27%
704
16%
2251
6%
704
40%
704
20%
1474
November1st to 15thNight
29%
700
19%
2277
11%
700
41%
700
18%
1484
November16th to endNight
24%
720
19%
2340
9%
720
36%
720
17%
1546
December1st to 15thNight
21%
720
15%
2499
6%
720
33%
720
12%
1680
December16th to endNight
15%
768
18%
2688
5%
768
31%
768
12%
1810

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%
47517
Night
17%6%25%42445%7%39%
46628
Cloud CoverDay
15%5%2442445%7%43%
47527
Night
17%6%25%42445%7%39%
46633
TransparencyDay
39%5%13%23%16%2
47205
Night
44%412%21%15%2
46458
SeeingDay
18%13%21%26%16%4
5753
Night
18%20%22%24%11%1
14864

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%24434447%38%
1432
16th to endDay
15%7%35%42445%7%37%
1558
February1st to 15thDay
17%5%35%4345%6%7%37%
1699
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
10%4233345%6%7%48%
2283
16th to endDay
10%42443445%8%48%
2100
September1st to 15thDay
12%5%1342345%6%50%
1945
16th to endDay
13%6%14423347%47%
1815
October1st to 15thDay
19%5%24323446%44%
1766
16th to endDay
16%424425%45%7%42%
1620
November1st to 15thDay
19%6%23423335%44%
1581
16th to endDay
14%7%244245%5%6%40%
1620
December1st to 15thDay
18%7%35%42446%7%35%
1458
16th to endDay
22%8%26%32335%5%34%
1536
January1st to 15thNight
18%6%25%42345%6%38%
2506
16th to endNight
18%8%35%323447%39%
2664
February1st to 15thNight
16%8%36%5%2445%7%36%
2161
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%6%6%8%44%
1576
16th to endNight
10%5%244245%5%8%45%
1750
September1st to 15thNight
14%5%245%3445%6%43%
1683
16th to endNight
14%5%24423346%47%
1815
October1st to 15thNight
17%7%3442334443%
1864
16th to endNight
18%7%25%423336%40%
2251
November1st to 15thNight
19%7%2442433441%
2277
16th to endNight
18%5%35%5%2345%7%37%
2340
December1st to 15thNight
20%8%36%43445%6%32%
2499
16th to endNight
20%7%35%425%45%6%33%
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
17%6%245%345%5%6%38%
1432
16th to endDay
15%7%35%43445%6%39%
1558
February1st to 15thDay
16%5%25%4345%5%8%36%
1699
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%9%47%
2283
16th to endDay
10%42443445%7%48%
2100
September1st to 15thDay
12%5%2342335%6%50%
1945
16th to endDay
13%5%24423346%48%
1815
October1st to 15thDay
19%5%2441345%5%43%
1766
16th to endDay
14%5%25%5%2445%6%43%
1620
November1st to 15thDay
20%5%2332433444%
1581
16th to endDay
15%5%245%25%45%7%40%
1620
December1st to 15thDay
18%8%35%42445%7%36%
1458
16th to endDay
22%8%35%32335%6%34%
1536
January1st to 15thNight
19%7%25%42445%6%38%
2506
16th to endNight
17%9%34423447%39%
2664
February1st to 15thNight
15%8%26%5%2435%7%37%
2154
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%
1576
16th to endNight
10%434325%5%6%8%46%
1750
September1st to 15thNight
15%5%145%3445%7%42%
1683
16th to endNight
14%5%25%433346%46%
1815
October1st to 15thNight
16%6%345%23345%43%
1864
16th to endNight
18%6%25%5%23346%39%
2251
November1st to 15thNight
19%6%2442443442%
2277
16th to endNight
18%7%35%42445%7%38%
2340
December1st to 15thNight
19%8%36%5%3435%6%33%
2499
16th to endNight
20%7%36%5%2445%6%33%
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
44%5%12%23%11%2
1433
16th to endDay
43%310%25%13%2
1558
February1st to 15thDay
43%413%22%13%
1702
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
32%6%17%24%17%1
2269
16th to endDay
35%5%10%26%20%2
2100
September1st to 15thDay
34%6%14%22%20%2
1945
16th to endDay
36%47%25%23%2
1815
October1st to 15thDay
41%410%22%18%2
1766
16th to endDay
40%411%25%16%1
1620
November1st to 15thDay
42%413%19%18%2
1581
16th to endDay
41%5%13%24%13%2
1620
December1st to 15thDay
46%317%17%13%
1458
16th to endDay
49%414%19%11%
1536
January1st to 15thNight
45%412%22%13%1
2506
16th to endNight
44%310%23%15%1
2664
February1st to 15thNight
46%411%22%12%1
2162
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
33%7%15%25%16%1
1566
16th to endNight
36%5%11%27%17%1
1750
September1st to 15thNight
40%410%25%17%
1683
16th to endNight
39%37%22%25%2
1815
October1st to 15thNight
45%38%21%17%2
1864
16th to endNight
45%414%20%14%1
2251
November1st to 15thNight
45%39%20%17%3
2277
16th to endNight
44%5%11%24%11%2
2340
December1st to 15thNight
50%315%18%12%
2499
16th to endNight
49%412%20%11%1
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
16%15%36%25%6%
60
16th to endDay
20%7%50%19%2
68
February1st to 15thDay
20%21%35%18%4
215
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
12%7%8%32%30%8%
297
16th to endDay
10%7%8%25%31%17%
268
September1st to 15thDay
17%8%16%24%22%10%
255
16th to endDay
17%7%15%24%22%12%
254
October1st to 15thDay
21%9%19%21%20%7%
255
16th to endDay
15%12%32%26%11%
247
November1st to 15thDay
21%19%20%24%12%1
235
16th to endDay
15%25%22%25%11%
240
December1st to 15thDay
21%19%26%23%7%1
78
16th to endDay
39%6%37%17%
64
January1st to 15thNight
18%30%29%17%3
720
16th to endNight
19%28%30%19%1
768
February1st to 15thNight
17%26%36%18%1
704
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
9%10%10%41%25%2
517
16th to endNight
10%10%8%29%33%7%
700
September1st to 15thNight
15%10%13%30%22%6%
660
16th to endNight
15%10%14%29%22%7%
657
October1st to 15thNight
20%14%23%25%14%2
660
16th to endNight
22%16%30%20%9%
704
November1st to 15thNight
18%22%24%21%10%2
700
16th to endNight
18%28%24%19%8%
720
December1st to 15thNight
22%27%22%20%5%1
720
16th to endNight
22%29%27%18%2
768
Page generated at 2017-08-13 14:16:26. Version ac04

Copyright 2017 A. Danko


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