C. E. Scovil Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the C. E. Scovil Observatory clear sky chart from 2005-11-05 to 2017-12-02.

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
36%
15837
15%
47772
9%
15797
42%
15797
20%
33143

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
16%
898
10%
2506
3%
895
23%
895
12%
1977
January16th to endNight
19%
847
9%
2575
3%
847
25%
847
12%
2014
February1st to 15thNight
17%
701
9%
2281
1%
701
24%
701
10%
1747
February16th to endNight
23%
631
14%
1893
6%
631
30%
631
14%
1405
March1st to 15thNight
24%
720
14%
2067
8%
720
29%
720
17%
1508
March16th to endNight
35%
768
17%
2112
9%
768
41%
768
22%
1483
April1st to 15thNight
27%
602
14%
1806
7%
602
34%
602
19%
1264
April16th to endNight
35%
540
20%
1620
8%
540
45%
540
26%
1054
May1st to 15thNight
40%
540
14%
1620
8%
540
46%
540
22%
1102
May16th to endNight
50%
553
16%
1675
11%
552
56%
552
29%
1033
June1st to 15thNight
55%
360
17%
1260
17%
360
60%
360
28%
746
June16th to endNight
55%
368
18%
1268
17%
368
61%
368
30%
728
July1st to 15thNight
65%
510
16%
1360
14%
510
68%
510
40%
810
July16th to endNight
65%
564
16%
1508
14%
531
67%
531
37%
889
August1st to 15thNight
66%
539
18%
1609
14%
536
69%
536
34%
961
August16th to endNight
62%
573
24%
1719
17%
573
68%
573
36%
1002
September1st to 15thNight
57%
678
22%
1931
17%
678
61%
678
32%
1183
September16th to endNight
59%
717
22%
1980
17%
717
63%
717
32%
1240
October1st to 15thNight
44%
720
18%
2085
13%
720
49%
720
26%
1393
October16th to endNight
25%
768
14%
2394
9%
768
33%
768
15%
1754
November1st to 15thNight
34%
760
19%
2471
15%
760
41%
760
21%
1647
November16th to endNight
24%
780
13%
2656
8%
780
31%
780
14%
1903
December1st to 15thNight
15%
740
8%
2558
3%
740
21%
740
10%
1971
December16th to endNight
14%
960
8%
2818
2%
960
19%
960
9%
2329

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%8%47%6%35%5%5%7%32%
48660
Night
14%8%36%5%3445%6%35%
47951
Cloud CoverDay
13%8%47%6%35%5%5%7%32%
48670
Night
14%8%36%5%3445%6%35%
47955
TransparencyDay
49%7%12%19%10%1
48357
Night
47%6%12%20%11%1
47772
SeeingDay
16%8%27%23%15%7%
6074
Night
18%9%29%29%12%1
15843

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
15%14%7%11%8%36%445%18%
1432
16th to endDay
13%18%7%11%10%45%44415%
1647
February1st to 15thDay
14%13%6%11%9%46%5%45%17%
1581
16th to endDay
12%13%5%10%9%45%447%22%
1580
March1st to 15thDay
15%11%5%9%7%35%345%27%
1882
16th to endDay
15%11%5%8%7%35%44429%
2112
April1st to 15thDay
18%10%5%5%5%3445%6%30%
2150
16th to endDay
15%8%25%6%35%35%6%36%
2340
May1st to 15thDay
18%7%36%5%35%45%6%34%
2331
16th to endDay
14%5%25%5%35%45%9%38%
2437
June1st to 15thDay
10%5%36%5%35%6%6%9%38%
2697
16th to endDay
8%5%36%5%45%6%7%9%36%
2688
July1st to 15thDay
6%425%6%35%6%6%8%42%
2520
16th to endDay
8%425%6%46%5%7%9%38%
2555
August1st to 15thDay
8%5%27%7%46%6%6%9%35%
2340
16th to endDay
11%5%35%5%35%5%6%8%39%
2483
September1st to 15thDay
11%5%35%5%45%6%6%8%37%
2016
16th to endDay
13%5%24435%5%6%8%39%
1980
October1st to 15thDay
15%7%36%5%345%46%37%
1875
16th to endDay
19%10%37%5%34436%31%
1827
November1st to 15thDay
14%9%47%6%345%47%33%
1717
16th to endDay
14%13%49%6%35%45%6%26%
1631
December1st to 15thDay
14%14%6%12%7%3444422%
1436
16th to endDay
19%15%8%10%6%34345%18%
1403
January1st to 15thNight
22%11%5%7%7%34445%24%
2506
16th to endNight
12%12%8%9%8%45%45%6%22%
2575
February1st to 15thNight
14%13%5%8%6%36%5%45%26%
2279
16th to endNight
15%11%47%8%35%446%27%
1893
March1st to 15thNight
16%10%5%7%5%23445%33%
2055
16th to endNight
16%7%35%5%36%45%6%35%
2112
April1st to 15thNight
19%8%36%43345%6%33%
1806
16th to endNight
15%6%25%5%2345%7%40%
1620
May1st to 15thNight
16%5%26%5%35%45%9%34%
1620
16th to endNight
12%415%5%2445%8%45%
1673
June1st to 15thNight
8%4245%247%6%9%43%
1260
16th to endNight
8%6%2433446%9%46%
1268
July1st to 15thNight
6%415%43345%9%52%
1440
16th to endNight
5%324425%5%6%9%50%
1612
August1st to 15thNight
6%415%435%6%6%9%47%
1620
16th to endNight
9%5%24323448%51%
1719
September1st to 15thNight
11%414425%446%49%
1931
16th to endNight
9%6%25%5%2445%7%47%
1980
October1st to 15thNight
15%6%25%425%46%6%40%
2085
16th to endNight
20%7%37%6%35%45%5%31%
2394
November1st to 15thNight
14%8%25%5%3445%6%37%
2471
16th to endNight
19%10%46%5%345%5%7%27%
2656
December1st to 15thNight
18%13%5%8%6%3435%6%23%
2558
16th to endNight
19%15%6%10%7%24445%19%
2818

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
15%14%6%11%8%45%445%18%
1432
16th to endDay
13%16%8%12%8%45%5%4415%
1647
February1st to 15thDay
14%14%6%11%9%46%5%45%17%
1581
16th to endDay
12%14%5%9%9%45%447%23%
1580
March1st to 15thDay
16%11%6%8%7%35%445%27%
1893
16th to endDay
16%11%5%8%7%45%43429%
2111
April1st to 15thDay
18%10%5%5%5%2445%6%31%
2150
16th to endDay
16%8%35%5%3445%6%36%
2340
May1st to 15thDay
18%7%36%5%25%446%34%
2331
16th to endDay
14%5%35%5%35%45%8%38%
2437
June1st to 15thDay
10%5%35%5%35%5%6%9%39%
2697
16th to endDay
9%5%26%6%36%5%7%9%36%
2688
July1st to 15thDay
6%425%6%36%6%6%8%42%
2520
16th to endDay
8%435%6%36%6%7%9%38%
2555
August1st to 15thDay
8%5%36%7%46%6%7%8%36%
2340
16th to endDay
11%5%35%6%35%5%5%8%39%
2483
September1st to 15thDay
11%426%5%36%6%6%8%37%
2016
16th to endDay
13%5%25%425%6%6%8%39%
1980
October1st to 15thDay
14%7%36%5%345%5%5%37%
1875
16th to endDay
19%10%37%5%34436%31%
1827
November1st to 15thDay
15%9%36%6%34447%33%
1717
16th to endDay
14%12%48%6%45%45%6%26%
1631
December1st to 15thDay
15%13%6%12%8%3435%422%
1436
16th to endDay
18%15%8%10%6%34435%18%
1403
January1st to 15thNight
21%11%5%8%6%34445%24%
2506
16th to endNight
12%12%8%8%7%45%45%6%23%
2575
February1st to 15thNight
13%14%5%8%6%45%5%45%26%
2272
16th to endNight
15%11%48%7%36%346%27%
1893
March1st to 15thNight
16%10%5%7%5%24436%32%
2067
16th to endNight
15%7%35%5%35%45%6%36%
2112
April1st to 15thNight
18%8%36%42445%6%34%
1806
16th to endNight
15%6%25%5%2345%7%40%
1620
May1st to 15thNight
16%6%26%6%35%5%5%8%35%
1619
16th to endNight
12%3245%3445%7%46%
1673
June1st to 15thNight
8%4245%246%6%10%44%
1260
16th to endNight
8%5%25%43446%9%46%
1268
July1st to 15thNight
6%425%43345%9%52%
1440
16th to endNight
5%3235%246%5%9%50%
1612
August1st to 15thNight
6%3244346%7%9%47%
1620
16th to endNight
9%424423346%52%
1719
September1st to 15thNight
11%42443445%6%48%
1931
16th to endNight
9%5%3442445%6%47%
1980
October1st to 15thNight
15%5%25%4345%6%5%40%
2085
16th to endNight
20%7%36%5%3445%5%31%
2394
November1st to 15thNight
15%8%35%4345%5%6%37%
2471
16th to endNight
19%10%36%5%35%45%7%27%
2656
December1st to 15thNight
19%13%5%8%6%24446%23%
2558
16th to endNight
20%14%6%10%6%24445%19%
2818

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
66%6%9%12%4
1433
16th to endDay
71%6%8%9%4
1647
February1st to 15thDay
66%7%9%12%4
1583
16th to endDay
61%5%9%14%7%1
1580
March1st to 15thDay
58%6%11%16%7%
1893
16th to endDay
57%5%10%17%7%1
2112
April1st to 15thDay
52%6%10%19%10%
2150
16th to endDay
46%5%15%16%13%2
2340
May1st to 15thDay
50%5%11%20%10%2
2331
16th to endDay
42%6%12%22%13%1
2441
June1st to 15thDay
39%8%13%23%14%1
2697
16th to endDay
41%8%13%20%14%
2688
July1st to 15thDay
36%7%11%25%17%1
2371
16th to endDay
38%9%14%20%14%2
2396
August1st to 15thDay
40%9%14%21%12%1
2327
16th to endDay
39%8%11%25%13%1
2483
September1st to 15thDay
40%7%16%23%11%1
2016
16th to endDay
39%7%13%27%11%
1980
October1st to 15thDay
45%7%11%21%12%1
1875
16th to endDay
55%5%14%17%7%
1827
November1st to 15thDay
49%6%12%20%9%
1717
16th to endDay
57%6%13%16%5%
1631
December1st to 15thDay
64%7%12%11%3
1436
16th to endDay
67%413%10%3
1403
January1st to 15thNight
61%5%11%14%7%
2506
16th to endNight
61%6%13%14%3
2575
February1st to 15thNight
59%6%10%14%7%
2281
16th to endNight
56%5%11%18%7%
1893
March1st to 15thNight
53%411%19%9%1
2067
16th to endNight
48%5%12%22%10%1
2112
April1st to 15thNight
49%5%11%19%13%
1806
16th to endNight
41%5%11%19%18%3
1620
May1st to 15thNight
46%5%11%20%12%3
1620
16th to endNight
37%6%12%23%15%4
1675
June1st to 15thNight
33%8%13%26%16%1
1260
16th to endNight
34%5%14%24%18%1
1268
July1st to 15thNight
29%6%9%32%21%2
1360
16th to endNight
27%9%17%23%19%2
1508
August1st to 15thNight
29%9%14%30%12%2
1609
16th to endNight
31%5%12%31%16%2
1719
September1st to 15thNight
35%5%11%29%16%1
1931
16th to endNight
34%7%11%29%15%
1980
October1st to 15thNight
41%7%11%21%16%1
2085
16th to endNight
53%6%12%19%6%
2394
November1st to 15thNight
45%6%13%23%10%1
2471
16th to endNight
54%5%15%17%6%
2656
December1st to 15thNight
61%5%13%13%5%
2558
16th to endNight
66%6%10%11%4
2818

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%23%40%16%21
90
16th to endDay
34%13%35%15%1
96
February1st to 15thDay
18%23%43%13%1
90
16th to endDay
16%22%40%17%3
81
March1st to 15thDay
26%15%31%21%5%
183
16th to endDay
23%6%33%23%10%2
288
April1st to 15thDay
22%11%30%22%10%2
270
16th to endDay
22%9%30%16%14%5%
270
May1st to 15thDay
22%8%27%23%13%4
288
16th to endDay
16%9%26%23%16%7%
331
June1st to 15thDay
13%6%30%30%15%4
492
16th to endDay
10%9%30%26%17%5%
495
July1st to 15thDay
8%5%25%31%18%10%
475
16th to endDay
7%327%28%21%10%
415
August1st to 15thDay
9%317%27%23%18%
313
16th to endDay
14%5%16%20%23%18%
332
September1st to 15thDay
10%320%20%24%19%
315
16th to endDay
14%5%13%24%28%14%
314
October1st to 15thDay
18%6%24%26%17%6%
315
16th to endDay
21%15%34%21%6%
243
November1st to 15thDay
18%14%31%19%14%1
135
16th to endDay
25%21%35%12%4
106
December1st to 15thDay
17%20%39%19%3
63
16th to endDay
21%29%43%5%
74
January1st to 15thNight
25%14%35%18%41
900
16th to endNight
18%12%38%24%5%
847
February1st to 15thNight
20%16%39%20%3
704
16th to endNight
21%13%31%27%5%
631
March1st to 15thNight
22%11%34%25%5%
720
16th to endNight
19%10%30%30%8%1
768
April1st to 15thNight
22%12%33%27%4
602
16th to endNight
18%7%35%28%9%2
540
May1st to 15thNight
17%5%29%35%10%1
540
16th to endNight
13%425%37%17%1
554
June1st to 15thNight
10%127%48%11%
360
16th to endNight
10%326%44%15%
368
July1st to 15thNight
8%118%40%26%3
510
16th to endNight
6%218%46%22%2
564
August1st to 15thNight
7%116%42%28%4
539
16th to endNight
11%118%35%30%3
573
September1st to 15thNight
13%5%19%32%24%6%
678
16th to endNight
12%416%34%26%5%
717
October1st to 15thNight
18%6%24%30%17%2
720
16th to endNight
23%12%33%24%5%
768
November1st to 15thNight
19%10%30%25%12%1
760
16th to endNight
22%15%33%21%7%
780
December1st to 15thNight
25%13%37%19%4
740
16th to endNight
26%11%36%21%2
960
Page generated at 2017-12-03 11:16:36. Version ac04

Copyright 2017 A. Danko


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