Eisenhower Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Eisenhower Observatory clear sky chart from 2005-11-06 to 2024-01-14.

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
31%
24854
16%
74594
8%
24797
39%
24797
22%
50152

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 10thNight
23%
950
13%
2648
4%
946
31%
946
18%
1945
January11th to 20thNight
23%
920
11%
2550
4%
920
28%
920
16%
1924
January21st to endNight
19%
990
11%
2574
3%
990
25%
990
15%
2010
February1st to 10thNight
21%
900
14%
2340
2%
900
28%
900
20%
1704
February11th to 20thNight
23%
879
17%
2273
5%
874
31%
874
24%
1571
February21st to endNight
25%
691
17%
1875
5%
691
34%
691
20%
1333
March1st to 10thNight
28%
594
13%
2034
9%
594
33%
594
18%
1382
March11th to 20thNight
28%
540
18%
1980
10%
540
40%
540
18%
1283
March21st to endNight
31%
594
16%
2161
13%
594
40%
594
16%
1471
April1st to 10thNight
27%
540
16%
1800
9%
540
39%
540
20%
1196
April11th to 20thNight
29%
533
18%
1755
14%
533
38%
533
18%
1213
April21st to endNight
28%
540
17%
1620
9%
540
38%
540
19%
1134
May1st to 10thNight
30%
540
15%
1530
8%
540
37%
540
24%
1037
May11th to 20thNight
34%
531
19%
1417
7%
531
41%
531
29%
937
May21st to endNight
41%
583
15%
1552
6%
582
45%
582
26%
1039
June1st to 10thNight
44%
540
16%
1440
5%
540
49%
540
37%
883
June11th to 20thNight
40%
540
14%
1440
4%
540
45%
540
32%
910
June21st to endNight
52%
540
19%
1311
8%
540
55%
540
44%
793
July1st to 10thNight
58%
525
18%
1395
8%
525
61%
525
48%
777
July11th to 20thNight
52%
522
21%
1392
8%
522
57%
522
43%
798
July21st to endNight
55%
591
23%
1521
11%
558
61%
558
48%
833
August1st to 10thNight
59%
537
21%
1601
16%
534
63%
534
36%
929
August11th to 20thNight
55%
534
23%
1600
19%
534
63%
534
37%
914
August21st to endNight
50%
594
24%
1885
20%
594
60%
594
36%
1032
September1st to 10thNight
49%
540
23%
1800
20%
540
58%
540
33%
985
September11th to 20thNight
42%
540
26%
1831
17%
540
56%
540
33%
1004
September21st to endNight
35%
602
21%
2042
12%
602
46%
602
28%
1187
October1st to 10thNight
35%
720
22%
2160
12%
720
42%
720
28%
1330
October11th to 20thNight
26%
720
20%
2160
8%
720
36%
720
22%
1432
October21st to endNight
27%
792
18%
2389
7%
792
37%
792
19%
1639
November1st to 10thNight
27%
857
21%
2523
8%
857
39%
857
24%
1657
November11th to 20thNight
23%
950
16%
2660
6%
950
31%
950
17%
1941
November21st to endNight
23%
950
16%
2660
6%
950
31%
950
19%
1867
December1st to 10thNight
25%
950
11%
2660
4%
950
31%
950
17%
1937
December11th to 20thNight
18%
949
12%
3210
3%
948
27%
948
12%
2375
December21st to endNight
14%
1036
6%
4805
2%
1026
22%
1026
7%
3750

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%37%
74811
Night
13%9%47%6%35%45%7%39%
74928
TransparencyDay
46%6%13%24%10%1
74514
Night
45%5%13%23%12%1
74745
SeeingDay
17%13%32%24%11%2
14837
Night
16%12%33%28%10%1
24865

By Month

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 10thDay
8%11%6%9%10%5%7%5%5%6%28%
1517
11th to 20thDay
10%16%7%11%8%5%6%446%23%
1486
21st to endDay
9%14%6%12%9%47%5%6%7%21%
1782
February1st to 10thDay
9%10%7%8%8%46%6%6%6%30%
1620
11th to 20thDay
9%11%6%9%7%46%5%5%7%31%
1574
21st to endDay
12%11%6%9%7%46%46%7%29%
1380
March1st to 10thDay
14%13%6%8%5%4435%7%31%
1943
11th to 20thDay
14%9%47%6%45%447%37%
1980
21st to endDay
19%11%46%6%35%5%5%7%30%
2193
April1st to 10thDay
16%10%47%6%3445%7%35%
2160
11th to 20thDay
20%11%36%5%34446%32%
2151
21st to endDay
22%10%36%6%35%45%5%31%
2340
May1st to 10thDay
18%7%37%6%445%5%7%34%
2428
11th to 20thDay
17%8%35%5%3445%7%39%
2479
21st to endDay
16%7%36%5%36%5%6%7%36%
2725
June1st to 10thDay
12%4244245%7%9%46%
2520
11th to 20thDay
13%5%36%6%45%5%6%9%39%
2520
21st to endDay
10%425%5%46%6%8%10%40%
2608
July1st to 10thDay
8%4245%35%5%7%10%48%
2520
11th to 20thDay
9%4145%35%5%6%8%51%
2520
21st to endDay
6%425%5%35%6%7%10%47%
2703
August1st to 10thDay
9%5%25%6%46%6%6%8%41%
2328
11th to 20thDay
14%5%244346%6%8%44%
2301
21st to endDay
9%415%5%235%5%7%53%
2470
September1st to 10thDay
12%5%35%43445%8%47%
2160
11th to 20thDay
13%5%25%425%5%6%7%45%
2124
21st to endDay
16%6%26%42445%7%43%
1900
October1st to 10thDay
18%7%35%5%335%5%5%42%
1800
11th to 20thDay
16%9%36%5%35%46%7%38%
1800
21st to endDay
18%10%46%5%33346%37%
1966
November1st to 10thDay
12%11%5%8%5%34447%37%
1552
11th to 20thDay
11%12%7%10%8%44446%29%
1520
21st to endDay
14%10%5%6%7%245%47%35%
1520
December1st to 10thDay
11%12%5%10%9%5%6%45%6%27%
1520
11th to 20thDay
12%12%7%10%8%35%5%46%29%
1918
21st to endDay
23%14%8%15%7%3333418%
2783
January1st to 10thNight
8%11%5%11%9%45%5%5%7%31%
2648
11th to 20thNight
11%13%6%10%7%36%5%6%7%27%
2550
21st to endNight
11%15%6%12%7%35%5%5%5%25%
2574
February1st to 10thNight
7%12%6%8%8%47%45%5%34%
2340
11th to 20thNight
7%9%47%7%5%5%5%6%7%39%
2273
21st to endNight
12%10%48%7%45%436%37%
1875
March1st to 10thNight
13%10%47%6%35%5%46%36%
2034
11th to 20thNight
13%10%37%5%45%445%39%
1980
21st to endNight
17%9%47%5%3445%7%35%
2161
April1st to 10thNight
15%10%36%4246%5%7%37%
1800
11th to 20thNight
21%10%47%6%24447%34%
1755
21st to endNight
19%9%46%445%45%7%35%
1620
May1st to 10thNight
18%6%36%5%35%45%7%39%
1530
11th to 20thNight
16%425%5%345%5%8%43%
1416
21st to endNight
12%5%36%5%36%5%5%8%42%
1551
June1st to 10thNight
10%4245%3446%9%49%
1440
11th to 20thNight
8%5%35%436%6%6%8%46%
1440
21st to endNight
7%4234335%6%10%54%
1311
July1st to 10thNight
4323335%5%6%9%57%
1435
11th to 20thNight
5%4244335%7%9%54%
1440
21st to endNight
5%31333446%9%59%
1611
August1st to 10thNight
6%3245%36%6%6%8%51%
1611
11th to 20thNight
11%3233235%5%8%53%
1600
21st to endNight
7%324434446%58%
1885
September1st to 10thNight
9%4245%3435%8%53%
1800
11th to 20thNight
10%41443445%7%53%
1831
21st to endNight
14%5%24423445%51%
2042
October1st to 10thNight
15%6%25%433447%47%
2160
11th to 20thNight
16%10%25%5%34446%41%
2160
21st to endNight
17%10%46%5%24446%38%
2389
November1st to 10thNight
14%8%36%4345%5%6%42%
2523
11th to 20thNight
13%11%5%10%6%345%45%34%
2660
21st to endNight
14%10%37%7%35%447%37%
2660
December1st to 10thNight
10%11%5%10%8%36%5%46%32%
2660
11th to 20thNight
11%13%5%10%8%45%45%6%28%
3279
21st to endNight
24%13%6%10%6%34445%21%
4884

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 10thDay
56%6%13%20%5%
1518
11th to 20thDay
64%410%13%8%
1486
21st to endDay
61%6%12%14%7%
1782
February1st to 10thDay
52%6%15%19%7%1
1620
11th to 20thDay
52%6%11%23%8%1
1583
21st to endDay
54%6%10%18%11%
1380
March1st to 10thDay
54%5%14%19%7%1
1943
11th to 20thDay
48%5%15%23%8%1
1980
21st to endDay
55%5%11%22%6%1
2193
April1st to 10thDay
49%5%13%21%11%1
2160
11th to 20thDay
54%410%19%11%2
2151
21st to endDay
55%412%20%8%1
2340
May1st to 10thDay
48%6%12%22%10%2
2428
11th to 20thDay
46%5%12%24%12%1
2483
21st to endDay
46%7%11%23%12%1
2725
June1st to 10thDay
34%7%15%31%12%1
2520
11th to 20thDay
41%7%14%23%14%1
2520
21st to endDay
36%8%17%28%9%2
2608
July1st to 10thDay
30%8%16%32%14%1
2451
11th to 20thDay
30%6%14%28%18%3
2445
21st to endDay
30%8%15%29%16%2
2553
August1st to 10thDay
38%7%15%27%9%3
2314
11th to 20thDay
36%7%15%31%11%
2301
21st to endDay
30%6%14%31%17%1
2470
September1st to 10thDay
35%6%14%31%14%
2160
11th to 20thDay
36%6%16%33%8%
2124
21st to endDay
41%5%12%33%9%
1900
October1st to 10thDay
44%412%28%12%
1800
11th to 20thDay
45%6%11%25%11%2
1800
21st to endDay
50%412%23%10%
1966
November1st to 10thDay
47%5%13%19%13%2
1552
11th to 20thDay
57%5%13%16%9%
1520
21st to endDay
49%5%17%21%6%1
1520
December1st to 10thDay
58%7%15%14%5%
1520
11th to 20thDay
57%6%18%15%4
1915
21st to endDay
71%5%9%10%5%
2783
January1st to 10thNight
53%5%12%20%9%1
2648
11th to 20thNight
57%5%15%16%7%1
2550
21st to endNight
60%5%12%15%8%1
2574
February1st to 10thNight
51%5%12%21%10%1
2340
11th to 20thNight
44%6%15%25%9%2
2274
21st to endNight
50%5%10%21%13%1
1875
March1st to 10thNight
49%5%15%22%8%
2034
11th to 20thNight
47%413%22%13%1
1980
21st to endNight
51%5%10%24%9%2
2161
April1st to 10thNight
47%410%24%14%1
1800
11th to 20thNight
52%48%20%14%2
1755
21st to endNight
50%5%8%21%14%2
1620
May1st to 10thNight
44%5%13%21%15%1
1530
11th to 20thNight
39%7%14%23%15%3
1417
21st to endNight
41%7%11%22%17%2
1552
June1st to 10thNight
31%6%15%28%18%1
1440
11th to 20thNight
32%9%15%24%18%3
1440
21st to endNight
26%6%17%31%17%3
1311
July1st to 10thNight
23%6%14%35%20%2
1395
11th to 20thNight
25%7%14%33%17%4
1392
21st to endNight
22%6%16%34%18%5%
1521
August1st to 10thNight
29%7%14%31%15%4
1601
11th to 20thNight
30%6%17%30%17%2
1600
21st to endNight
28%6%14%30%19%4
1885
September1st to 10thNight
30%5%13%34%18%1
1800
11th to 20thNight
30%6%16%30%18%
1831
21st to endNight
35%5%13%31%15%1
2042
October1st to 10thNight
38%5%10%31%15%2
2160
11th to 20thNight
44%5%13%23%13%2
2160
21st to endNight
49%412%22%12%1
2389
November1st to 10thNight
42%5%13%24%14%2
2523
11th to 20thNight
53%5%12%19%11%1
2660
21st to endNight
49%5%16%23%7%1
2660
December1st to 10thNight
53%5%14%21%6%
2660
11th to 20thNight
57%6%13%17%5%2
3281
21st to endNight
66%410%15%4
4884

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 10thDay
15%12%30%30%10%2
270
11th to 20thDay
19%12%36%27%41
249
21st to endDay
17%12%39%21%8%2
264
February1st to 10thDay
15%18%43%21%4
240
11th to 20thDay
16%22%34%22%5%1
240
21st to endDay
20%15%37%23%5%
198
March1st to 10thDay
20%16%37%19%8%
240
11th to 20thDay
19%13%36%22%8%2
240
21st to endDay
27%19%26%17%8%3
279
April1st to 10thDay
23%18%33%21%5%
420
11th to 20thDay
30%15%24%23%8%1
429
21st to endDay
26%17%30%18%8%2
600
May1st to 10thDay
21%18%33%21%6%1
603
11th to 20thDay
22%14%30%21%11%3
625
21st to endDay
18%12%30%26%12%3
679
June1st to 10thDay
10%10%32%32%15%2
630
11th to 20thDay
15%12%40%24%8%1
630
21st to endDay
11%13%34%26%14%3
630
July1st to 10thDay
9%5%28%34%18%6%
621
11th to 20thDay
11%9%31%30%16%3
618
21st to endDay
7%10%31%32%16%4
684
August1st to 10thDay
10%8%29%32%19%3
626
11th to 20thDay
15%8%23%30%18%5%
615
21st to endDay
11%9%30%32%15%4
589
September1st to 10thDay
15%13%29%26%14%2
450
11th to 20thDay
14%9%32%29%15%2
414
21st to endDay
20%18%31%19%11%2
270
October1st to 10thDay
21%13%29%21%13%3
270
11th to 20thDay
17%27%35%17%4
270
21st to endDay
28%19%37%11%5%
297
November1st to 10thDay
22%19%35%17%6%1
270
11th to 20thDay
19%20%42%16%3
270
21st to endDay
25%18%42%10%4
270
December1st to 10thDay
15%19%35%23%8%1
270
11th to 20thDay
22%20%33%21%4
270
21st to endDay
30%14%35%18%2
297
January1st to 10thNight
12%15%38%27%7%1
950
11th to 20thNight
16%13%37%27%5%1
920
21st to endNight
18%14%36%26%6%
990
February1st to 10thNight
13%15%39%26%6%1
900
11th to 20thNight
12%15%42%23%7%1
879
21st to endNight
17%16%34%25%8%1
691
March1st to 10thNight
17%13%34%27%8%1
594
11th to 20thNight
18%13%36%26%7%
540
21st to endNight
21%14%29%24%10%2
594
April1st to 10thNight
21%14%33%25%6%
540
11th to 20thNight
25%13%29%27%7%1
533
21st to endNight
23%12%32%23%8%1
540
May1st to 10thNight
20%12%33%29%5%1
540
11th to 20thNight
17%9%37%26%11%1
531
21st to endNight
15%11%28%34%13%1
584
June1st to 10thNight
12%5%34%39%11%
540
11th to 20thNight
10%7%39%38%7%
540
21st to endNight
9%6%28%40%16%1
540
July1st to 10thNight
5%329%37%25%1
525
11th to 20thNight
7%7%31%41%14%1
522
21st to endNight
5%432%40%17%2
591
August1st to 10thNight
6%424%43%20%1
537
11th to 20thNight
12%5%24%35%22%2
534
21st to endNight
7%6%32%32%22%1
594
September1st to 10thNight
10%7%29%37%14%3
540
11th to 20thNight
11%8%36%27%16%2
540
21st to endNight
14%14%33%25%13%1
602
October1st to 10thNight
18%12%31%25%13%
720
11th to 20thNight
23%14%33%22%8%1
720
21st to endNight
24%14%29%26%7%
792
November1st to 10thNight
19%14%33%26%7%
857
11th to 20thNight
19%18%33%23%7%
950
21st to endNight
18%19%34%23%6%
950
December1st to 10thNight
17%12%37%27%6%1
950
11th to 20thNight
19%14%35%25%5%1
950
21st to endNight
25%15%33%23%4
1045
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