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 2017-04-24.

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%
14617
18%
45568
9%
14586
42%
14586
20%
29722

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
33%
627
17%
1671
5%
627
40%
627
22%
1142
May1st to 15thNight
35%
495
17%
1485
8%
495
43%
495
19%
1019
May16th to endNight
39%
372
11%
1399
9%
372
45%
372
14%
909
June1st to 15thNight
43%
330
14%
1320
16%
330
49%
330
16%
853
June16th to endNight
56%
330
12%
1320
15%
330
61%
330
22%
742
July1st to 15thNight
51%
310
13%
1240
17%
310
53%
310
20%
749
July16th to endNight
52%
348
15%
1306
16%
326
61%
326
21%
777
August1st to 15thNight
58%
487
18%
1468
13%
485
61%
485
28%
900
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%
46043
Night
17%6%25%42445%7%39%
45738
Cloud CoverDay
15%5%2442445%7%43%
46053
Night
17%6%25%42445%7%39%
45743
TransparencyDay
39%5%13%23%16%2
45731
Night
44%412%21%15%2
45568
SeeingDay
17%14%21%25%16%4
5322
Night
17%21%22%24%11%1
14618

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
15%42332345%7%47%
2149
May1st to 15thDay
21%7%25%313346%40%
2137
16th to endDay
14%5%2442345%8%44%
2371
June1st to 15thDay
19%42342445%9%38%
2310
16th to endDay
9%5%143245%5%10%46%
2310
July1st to 15thDay
10%41442446%10%45%
2310
16th to endDay
12%3245%245%6%8%43%
2458
August1st to 15thDay
10%4233335%6%7%49%
2138
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
18%6%36%424346%39%
1671
May1st to 15thNight
18%7%25%5%35%347%35%
1485
16th to endNight
19%6%245%3445%9%34%
1397
June1st to 15thNight
21%7%2342445%7%36%
1320
16th to endNight
12%424435%45%9%43%
1320
July1st to 15thNight
11%5%25%4345%6%9%39%
1320
16th to endNight
10%5%36%326%6%7%10%37%
1402
August1st to 15thNight
8%3245%46%6%7%7%43%
1478
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
15%42332335%7%47%
2149
May1st to 15thDay
21%7%25%312246%41%
2137
16th to endDay
14%5%2442345%8%44%
2371
June1st to 15thDay
19%423425%35%9%38%
2310
16th to endDay
10%4144245%6%9%46%
2310
July1st to 15thDay
10%4144245%6%9%46%
2310
16th to endDay
12%414425%5%6%9%43%
2458
August1st to 15thDay
11%4134236%5%9%47%
2138
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
19%6%25%423347%39%
1671
May1st to 15thNight
18%7%35%6%34347%35%
1485
16th to endNight
19%5%35%5%35%349%34%
1397
June1st to 15thNight
22%5%33434447%36%
1320
16th to endNight
12%5%235%25%45%8%44%
1320
July1st to 15thNight
12%5%26%5%245%6%10%38%
1320
16th to endNight
10%5%35%5%26%6%7%9%38%
1402
August1st to 15thNight
8%425%6%36%6%6%9%42%
1478
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
35%410%24%20%3
2149
May1st to 15thDay
45%411%20%15%2
2137
16th to endDay
37%7%18%20%14%2
2375
June1st to 15thDay
43%5%15%22%11%1
2310
16th to endDay
33%6%15%26%16%1
2310
July1st to 15thDay
34%6%14%25%16%1
2161
16th to endDay
35%6%15%25%13%3
2291
August1st to 15thDay
32%6%17%25%17%1
2124
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
45%39%19%17%4
1671
May1st to 15thNight
48%411%17%14%3
1485
16th to endNight
46%6%17%15%13%1
1399
June1st to 15thNight
47%5%13%19%11%2
1320
16th to endNight
36%415%25%16%1
1320
July1st to 15thNight
40%5%13%23%15%1
1240
16th to endNight
38%7%16%23%11%2
1306
August1st to 15thNight
33%7%14%25%15%1
1468
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
16%14%22%32%10%3
243
May1st to 15thDay
27%16%21%21%12%1
228
16th to endDay
15%13%17%28%20%5%
267
June1st to 15thDay
18%12%20%24%19%4
255
16th to endDay
12%12%8%39%20%7%
255
July1st to 15thDay
12%9%8%31%29%8%
245
16th to endDay
10%12%10%31%25%8%
270
August1st to 15thDay
11%8%8%32%31%7%
253
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
17%20%23%24%11%1
627
May1st to 15thNight
17%20%20%27%12%1
495
16th to endNight
20%19%10%31%16%1
373
June1st to 15thNight
18%20%11%26%20%1
330
16th to endNight
14%18%6%36%22%2
330
July1st to 15thNight
11%14%12%33%25%2
310
16th to endNight
8%18%10%35%22%4
348
August1st to 15thNight
9%10%10%41%24%2
487
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 2017-04-25 07:48:25. Version ac04

Copyright 2017 A. Danko


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