Whisper Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Whisper Observatory clear sky chart from 2005-11-06 to 2024-01-27.

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
35%
24409
16%
74785
10%
24354
41%
24354
20%
52363

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
16%
950
13%
2648
4%
946
24%
946
13%
2074
January11th to 20thNight
15%
946
8%
2656
3%
946
19%
946
10%
2165
January21st to endNight
17%
820
9%
2664
3%
820
24%
820
10%
2102
February1st to 10thNight
17%
720
7%
2340
2%
720
22%
720
10%
1829
February11th to 20thNight
20%
700
14%
2108
6%
700
27%
700
13%
1580
February21st to endNight
15%
592
14%
1776
3%
592
25%
592
13%
1337
March1st to 10thNight
26%
720
16%
2129
9%
720
32%
720
17%
1536
March11th to 20thNight
26%
720
19%
1980
9%
720
35%
720
22%
1365
March21st to endNight
28%
792
18%
2178
9%
792
35%
792
19%
1566
April1st to 10thNight
26%
612
15%
1872
7%
612
34%
612
19%
1266
April11th to 20thNight
29%
533
20%
1656
9%
533
40%
533
20%
1132
April21st to endNight
31%
540
17%
1620
5%
540
40%
540
23%
1101
May1st to 10thNight
34%
540
12%
1620
6%
540
40%
540
18%
1190
May11th to 20thNight
38%
531
17%
1593
10%
531
45%
531
23%
1068
May21st to endNight
53%
583
15%
1745
12%
582
57%
582
30%
1091
June1st to 10thNight
49%
540
16%
1486
10%
540
56%
540
33%
945
June11th to 20thNight
52%
540
18%
1440
14%
540
57%
540
31%
939
June21st to endNight
53%
540
19%
1440
13%
540
60%
540
34%
899
July1st to 10thNight
64%
525
17%
1400
14%
525
66%
525
36%
853
July11th to 20thNight
59%
522
21%
1392
16%
522
63%
522
39%
838
July21st to endNight
66%
591
19%
1655
13%
558
71%
558
38%
957
August1st to 10thNight
66%
537
20%
1600
17%
534
68%
534
38%
919
August11th to 20thNight
64%
534
23%
1598
17%
534
69%
534
33%
965
August21st to endNight
63%
594
22%
1844
17%
594
68%
594
33%
1087
September1st to 10thNight
57%
641
21%
1901
17%
641
60%
641
31%
1163
September11th to 20thNight
60%
720
29%
1980
21%
720
63%
720
37%
1188
September21st to endNight
50%
720
23%
1980
18%
720
54%
720
30%
1268
October1st to 10thNight
47%
720
21%
2006
16%
720
53%
720
29%
1275
October11th to 20thNight
30%
720
17%
2160
10%
720
39%
720
20%
1482
October21st to endNight
28%
792
18%
2520
13%
792
36%
792
17%
1749
November1st to 10thNight
34%
740
21%
2406
16%
740
42%
740
23%
1509
November11th to 20thNight
24%
760
18%
2470
10%
760
35%
760
17%
1702
November21st to endNight
23%
760
14%
2470
10%
760
28%
760
14%
1830
December1st to 10thNight
20%
760
10%
2470
5%
760
25%
760
11%
1916
December11th to 20thNight
19%
815
10%
3111
3%
814
26%
814
10%
2334
December21st to endNight
14%
1039
4%
4871
2%
1026
20%
1026
5%
4143

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
14%9%47%6%35%5%5%7%34%
75050
Night
16%9%47%6%3445%7%35%
75015
TransparencyDay
48%7%12%20%12%1
74763
Night
49%6%12%20%12%1
74823
SeeingDay
18%12%29%23%13%5%
14472
Night
19%10%29%28%12%1
24419

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
18%14%7%11%7%5%5%5%45%20%
1517
11th to 20thDay
19%18%6%10%8%46%445%16%
1521
21st to endDay
17%18%8%10%7%345%35%18%
1836
February1st to 10thDay
15%15%6%10%10%5%5%5%5%5%20%
1620
11th to 20thDay
15%11%5%12%9%5%5%45%5%24%
1735
21st to endDay
18%13%5%9%7%34446%27%
1480
March1st to 10thDay
17%12%5%9%8%3445%7%26%
1831
11th to 20thDay
16%13%37%5%34446%34%
1980
21st to endDay
22%13%5%7%5%35%5%5%5%25%
2178
April1st to 10thDay
15%10%48%7%45%5%5%6%31%
2102
11th to 20thDay
19%8%36%5%35%46%7%34%
2253
21st to endDay
19%8%46%5%335%5%7%34%
2340
May1st to 10thDay
17%7%37%6%35%5%6%7%33%
2340
11th to 20thDay
18%8%37%6%35%46%7%34%
2301
21st to endDay
11%5%35%6%46%6%6%8%40%
2531
June1st to 10thDay
11%6%26%5%47%6%6%8%40%
2470
11th to 20thDay
10%6%35%6%45%5%5%8%43%
2520
21st to endDay
10%5%35%6%46%6%6%10%39%
2520
July1st to 10thDay
8%5%25%5%445%6%9%46%
2520
11th to 20thDay
8%424445%6%7%10%44%
2520
21st to endDay
7%5%35%7%46%5%7%9%43%
2565
August1st to 10thDay
7%5%25%6%45%6%7%10%44%
2327
11th to 20thDay
9%5%25%5%47%6%7%9%41%
2303
21st to endDay
11%425%6%25%6%7%10%44%
2509
September1st to 10thDay
13%5%36%6%245%6%8%42%
2040
11th to 20thDay
9%5%25%5%35%5%6%9%45%
1980
21st to endDay
15%7%344345%6%8%42%
1980
October1st to 10thDay
12%6%35%5%35%5%6%6%44%
1954
11th to 20thDay
19%8%36%5%35%45%6%36%
1800
21st to endDay
19%9%38%6%35%5%6%6%30%
1835
November1st to 10thDay
11%8%36%5%35%46%7%41%
1672
11th to 20thDay
17%13%47%6%34445%33%
1710
21st to endDay
20%13%5%7%5%25%5%5%6%28%
1710
December1st to 10thDay
17%16%8%11%7%44345%20%
1710
11th to 20thDay
14%16%6%9%6%5%45%5%7%23%
2057
21st to endDay
25%20%8%11%6%34445%12%
2783
January1st to 10thNight
18%13%6%8%7%3445%5%25%
2648
11th to 20thNight
19%14%6%10%7%45%45%5%20%
2656
21st to endNight
17%16%7%9%7%445%46%22%
2664
February1st to 10thNight
18%14%5%9%8%45%446%23%
2340
11th to 20thNight
15%14%5%9%6%45%45%6%27%
2107
21st to endNight
22%11%47%6%35%445%28%
1776
March1st to 10thNight
16%9%5%10%6%34446%33%
2129
11th to 20thNight
15%9%35%5%4446%6%39%
1980
21st to endNight
21%8%47%5%34446%34%
2178
April1st to 10thNight
15%9%45%5%345%5%8%36%
1872
11th to 20thNight
18%7%26%7%35%5%5%6%37%
1656
21st to endNight
16%10%35%4345%5%7%37%
1620
May1st to 10thNight
18%7%37%6%46%45%8%31%
1620
11th to 20thNight
19%7%35%43445%7%39%
1592
21st to endNight
9%5%45%5%35%5%7%9%43%
1744
June1st to 10thNight
11%425%6%346%7%8%45%
1486
11th to 20thNight
10%5%26%5%35%5%7%7%44%
1440
21st to endNight
11%425%435%6%7%11%44%
1440
July1st to 10thNight
8%4244245%5%9%51%
1440
11th to 20thNight
8%325%5%345%5%8%52%
1440
21st to endNight
6%4235%35%5%7%10%50%
1753
August1st to 10thNight
6%425%5%3447%9%51%
1611
11th to 20thNight
9%424445%6%6%8%48%
1598
21st to endNight
8%425%5%35%5%5%8%50%
1844
September1st to 10thNight
10%5%25%5%345%5%7%48%
1901
11th to 20thNight
9%5%345%3335%8%52%
1980
21st to endNight
12%6%36%5%335%5%6%46%
1980
October1st to 10thNight
13%7%246%3446%7%45%
2006
11th to 20thNight
17%9%36%6%345%46%37%
2160
21st to endNight
21%8%36%6%24445%35%
2520
November1st to 10thNight
13%8%36%5%3445%6%44%
2406
11th to 20thNight
16%10%46%5%345%7%7%33%
2470
21st to endNight
21%12%47%6%3445%7%27%
2470
December1st to 10thNight
17%14%6%9%7%45%35%6%24%
2470
11th to 20thNight
17%14%6%9%6%445%45%26%
3134
21st to endNight
28%19%7%8%6%3434413%
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
66%6%11%13%4
1518
11th to 20thDay
71%5%13%7%3
1521
21st to endDay
68%5%8%12%5%1
1836
February1st to 10thDay
66%6%11%12%5%
1620
11th to 20thDay
61%6%10%15%7%
1745
21st to endDay
56%6%13%16%6%2
1480
March1st to 10thDay
58%6%12%18%6%1
1831
11th to 20thDay
52%412%19%10%2
1980
21st to endDay
60%5%9%20%5%1
2178
April1st to 10thDay
52%7%12%16%13%
2102
11th to 20thDay
48%7%11%20%12%2
2253
21st to endDay
49%6%12%20%11%2
2340
May1st to 10thDay
50%8%11%19%11%1
2340
11th to 20thDay
49%8%10%18%14%2
2305
21st to endDay
40%9%13%23%12%2
2531
June1st to 10thDay
40%7%12%25%14%3
2470
11th to 20thDay
38%8%14%22%16%3
2520
21st to endDay
38%9%13%20%15%4
2520
July1st to 10thDay
35%8%12%24%18%3
2451
11th to 20thDay
31%9%15%25%18%2
2445
21st to endDay
36%7%14%24%17%2
2423
August1st to 10thDay
33%9%16%28%14%1
2314
11th to 20thDay
37%9%15%24%14%1
2303
21st to endDay
34%8%13%24%17%3
2509
September1st to 10thDay
39%8%15%20%17%3
2040
11th to 20thDay
34%7%11%29%18%1
1980
21st to endDay
39%7%13%25%16%1
1980
October1st to 10thDay
40%7%11%25%15%2
1954
11th to 20thDay
47%5%12%22%13%1
1800
21st to endDay
52%6%13%18%10%1
1835
November1st to 10thDay
42%6%12%23%16%1
1672
11th to 20thDay
54%5%11%17%11%1
1710
21st to endDay
57%6%13%17%7%
1710
December1st to 10thDay
67%6%10%14%4
1710
11th to 20thDay
60%7%12%16%4
2054
21st to endDay
76%6%8%7%3
2783
January1st to 10thNight
60%6%10%17%7%
2648
11th to 20thNight
65%6%13%11%5%
2656
21st to endNight
64%6%12%13%5%1
2664
February1st to 10thNight
63%5%12%14%41
2340
11th to 20thNight
57%6%11%18%8%1
2108
21st to endNight
57%6%10%17%8%2
1776
March1st to 10thNight
52%6%10%21%10%1
2129
11th to 20thNight
45%6%13%18%16%2
1980
21st to endNight
51%5%9%24%9%1
2178
April1st to 10thNight
46%7%12%19%14%2
1872
11th to 20thNight
47%7%10%18%17%2
1656
21st to endNight
46%6%11%20%15%2
1620
May1st to 10thNight
52%8%10%16%12%2
1620
11th to 20thNight
45%6%10%20%17%2
1593
21st to endNight
35%8%13%27%14%2
1745
June1st to 10thNight
34%8%13%25%17%3
1486
11th to 20thNight
35%10%12%25%16%3
1440
21st to endNight
32%9%14%25%19%1
1440
July1st to 10thNight
30%9%11%26%21%3
1400
11th to 20thNight
29%7%14%27%20%2
1392
21st to endNight
27%9%17%25%19%4
1655
August1st to 10thNight
28%8%16%30%17%2
1600
11th to 20thNight
31%10%14%27%16%2
1598
21st to endNight
32%7%13%24%21%3
1844
September1st to 10thNight
36%6%13%24%17%4
1901
11th to 20thNight
32%5%11%27%24%1
1980
21st to endNight
38%6%12%25%17%2
1980
October1st to 10thNight
38%6%12%23%20%1
2006
11th to 20thNight
47%6%11%20%16%1
2160
21st to endNight
50%5%10%23%11%1
2520
November1st to 10thNight
41%5%11%25%16%1
2406
11th to 20thNight
48%7%13%20%10%1
2470
21st to endNight
57%6%12%19%6%
2470
December1st to 10thNight
61%6%11%15%7%
2470
11th to 20thNight
59%6%14%13%7%1
3136
21st to endNight
74%6%10%8%2
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
23%24%29%21%21
270
11th to 20thDay
25%20%41%10%4
270
21st to endDay
30%17%29%19%4
282
February1st to 10thDay
29%19%40%12%1
240
11th to 20thDay
28%19%38%14%2
240
21st to endDay
27%23%39%9%2
198
March1st to 10thDay
25%18%36%16%4
271
11th to 20thDay
24%13%38%18%6%1
420
21st to endDay
32%14%31%17%7%
462
April1st to 10thDay
20%15%34%21%9%1
420
11th to 20thDay
22%21%26%19%9%3
412
21st to endDay
26%13%34%17%8%2
420
May1st to 10thDay
23%15%30%18%11%4
423
11th to 20thDay
19%13%31%20%11%5%
447
21st to endDay
13%8%28%24%19%8%
485
June1st to 10thDay
11%7%31%32%14%4
580
11th to 20thDay
12%10%29%31%14%4
630
21st to endDay
13%7%30%31%15%5%
630
July1st to 10thDay
8%5%21%33%20%12%
621
11th to 20thDay
11%6%24%31%18%10%
618
21st to endDay
8%426%31%22%9%
506
August1st to 10thDay
10%416%30%26%14%
446
11th to 20thDay
10%5%23%29%23%11%
439
21st to endDay
10%5%21%29%24%12%
495
September1st to 10thDay
15%6%19%29%20%11%
450
11th to 20thDay
10%320%26%27%13%
450
21st to endDay
16%11%23%23%19%8%
450
October1st to 10thDay
15%7%28%25%18%8%
450
11th to 20thDay
19%20%32%21%8%1
450
21st to endDay
26%15%34%19%5%1
350
November1st to 10thDay
14%18%36%20%9%4
270
11th to 20thDay
26%21%26%20%7%
270
21st to endDay
26%19%35%18%2
270
December1st to 10thDay
23%26%29%14%7%
270
11th to 20thDay
21%25%35%16%3
270
21st to endDay
29%21%32%15%3
297
January1st to 10thNight
24%17%34%20%5%
950
11th to 20thNight
27%14%35%18%5%1
946
21st to endNight
23%15%37%21%4
820
February1st to 10thNight
27%13%36%22%3
720
11th to 20thNight
24%13%36%24%3
703
21st to endNight
27%15%38%18%2
592
March1st to 10thNight
21%12%33%27%7%1
720
11th to 20thNight
18%17%35%24%6%1
720
21st to endNight
23%12%31%25%8%
792
April1st to 10thNight
19%15%35%26%4
612
11th to 20thNight
21%11%33%26%8%1
533
21st to endNight
22%9%36%26%8%1
540
May1st to 10thNight
21%8%30%31%9%1
540
11th to 20thNight
20%6%29%30%14%1
531
21st to endNight
10%327%36%21%3
584
June1st to 10thNight
12%328%41%15%
540
11th to 20thNight
12%5%25%40%18%1
540
21st to endNight
12%325%36%22%2
540
July1st to 10thNight
9%317%40%27%4
525
11th to 20thNight
9%321%39%23%4
522
21st to endNight
7%218%43%28%2
591
August1st to 10thNight
8%215%42%30%3
537
11th to 20thNight
10%315%36%31%5%
534
21st to endNight
9%216%37%32%4
594
September1st to 10thNight
13%419%34%25%4
641
11th to 20thNight
10%419%33%30%4
720
21st to endNight
14%7%22%31%23%3
720
October1st to 10thNight
14%8%25%31%19%3
720
11th to 20thNight
20%15%29%24%10%2
720
21st to endNight
25%14%28%25%8%
792
November1st to 10thNight
16%14%33%26%10%1
740
11th to 20thNight
22%14%34%21%8%1
760
21st to endNight
26%12%34%21%6%
760
December1st to 10thNight
24%16%35%21%5%
760
11th to 20thNight
26%14%35%21%4
815
21st to endNight
30%13%34%20%2
1045
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