Peek Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Peek 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
46%
24026
18%
74517
13%
23974
51%
23974
21%
50174

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
26%
758
16%
2459
9%
757
37%
757
14%
1742
January11th to 20thNight
26%
735
17%
2386
10%
735
36%
735
14%
1709
January21st to endNight
25%
792
20%
2574
10%
792
38%
792
17%
1753
February1st to 10thNight
29%
720
18%
2264
11%
720
39%
720
19%
1551
February11th to 20thNight
19%
703
15%
2099
6%
700
28%
700
15%
1506
February21st to endNight
19%
592
20%
1776
8%
592
31%
592
16%
1273
March1st to 10thNight
29%
720
21%
2030
12%
720
36%
720
21%
1375
March11th to 20thNight
27%
720
15%
1980
7%
720
34%
720
18%
1437
March21st to endNight
30%
792
15%
2178
8%
792
35%
792
19%
1532
April1st to 10thNight
36%
720
19%
1980
10%
720
43%
720
25%
1287
April11th to 20thNight
39%
615
22%
1764
12%
615
46%
615
25%
1140
April21st to endNight
48%
540
23%
1620
14%
540
54%
540
27%
1026
May1st to 10thNight
47%
540
15%
1620
8%
540
52%
540
24%
1055
May11th to 20thNight
51%
531
13%
1593
9%
531
53%
531
18%
1128
May21st to endNight
67%
582
14%
1745
12%
582
67%
582
25%
1184
June1st to 10thNight
67%
540
13%
1620
11%
540
68%
540
24%
1087
June11th to 20thNight
70%
540
11%
1620
9%
540
72%
540
28%
1045
June21st to endNight
70%
540
15%
1620
14%
540
71%
540
27%
1050
July1st to 10thNight
66%
525
11%
1575
10%
525
67%
525
22%
1109
July11th to 20thNight
72%
522
11%
1566
9%
522
72%
522
20%
1120
July21st to endNight
70%
591
12%
1672
11%
558
71%
558
21%
1178
August1st to 10thNight
73%
537
11%
1600
10%
534
73%
534
21%
1125
August11th to 20thNight
66%
534
16%
1598
15%
534
67%
534
20%
1134
August21st to endNight
66%
637
15%
1935
14%
637
67%
637
22%
1330
September1st to 10thNight
64%
720
18%
1980
17%
720
65%
720
25%
1357
September11th to 20thNight
60%
720
19%
1980
18%
720
60%
720
24%
1394
September21st to endNight
55%
720
24%
1980
20%
720
59%
720
29%
1278
October1st to 10thNight
53%
720
23%
2011
19%
720
57%
720
29%
1294
October11th to 20thNight
48%
720
23%
2160
19%
720
56%
720
28%
1349
October21st to endNight
49%
792
29%
2376
23%
792
59%
792
29%
1423
November1st to 10thNight
54%
740
25%
2316
22%
740
60%
740
28%
1399
November11th to 20thNight
43%
760
25%
2470
20%
760
53%
760
26%
1490
November21st to endNight
41%
760
25%
2470
17%
760
52%
760
24%
1518
December1st to 10thNight
31%
760
16%
2470
11%
760
38%
760
14%
1775
December11th to 20thNight
34%
759
20%
2987
14%
759
44%
759
15%
2033
December21st to endNight
29%
829
10%
4443
9%
817
37%
817
8%
2988

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
11%8%37%6%46%5%6%8%36%
74905
Night
10%8%47%6%35%5%6%8%38%
74867
TransparencyDay
45%9%11%18%14%3
74628
Night
43%8%12%20%14%3
74663
SeeingDay
19%13%22%21%16%9%
13908
Night
18%10%20%29%20%3
24038

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
13%8%47%8%446%5%8%35%
1706
11th to 20thDay
12%9%5%9%8%45%446%33%
1647
21st to endDay
11%9%6%7%5%3435%7%39%
1782
February1st to 10thDay
13%9%49%6%45%5%47%34%
1692
11th to 20thDay
14%11%5%9%6%44446%33%
1748
21st to endDay
15%8%47%6%35%5%47%36%
1480
March1st to 10thDay
12%8%5%6%5%3445%7%42%
1930
11th to 20thDay
15%10%47%6%46%45%7%32%
1980
21st to endDay
13%11%47%6%46%5%5%7%34%
2178
April1st to 10thDay
10%6%35%5%36%5%6%8%42%
1980
11th to 20thDay
13%7%35%5%35%5%6%8%40%
2158
21st to endDay
11%7%36%6%445%6%7%41%
2340
May1st to 10thDay
10%7%36%6%5%6%5%6%8%38%
2340
11th to 20thDay
13%7%37%7%5%6%7%7%7%32%
2301
21st to endDay
9%6%37%7%46%5%8%11%34%
2531
June1st to 10thDay
10%7%35%7%5%7%7%8%8%34%
2340
11th to 20thDay
6%6%37%7%5%7%7%9%10%32%
2340
21st to endDay
8%6%36%7%5%7%7%9%9%33%
2340
July1st to 10thDay
9%7%49%8%5%8%7%9%10%26%
2340
11th to 20thDay
10%6%37%8%5%8%8%9%9%27%
2340
21st to endDay
8%6%48%7%5%10%9%7%10%26%
2547
August1st to 10thDay
9%7%37%9%6%7%8%9%10%25%
2327
11th to 20thDay
10%6%48%8%6%8%7%8%9%28%
2303
21st to endDay
9%6%37%9%46%7%8%9%31%
2398
September1st to 10thDay
9%6%48%7%46%7%7%8%36%
1980
11th to 20thDay
12%7%47%6%46%6%7%8%34%
1980
21st to endDay
11%6%37%7%45%5%5%7%40%
1980
October1st to 10thDay
11%6%36%5%3435%9%46%
1949
11th to 20thDay
9%8%35%5%2435%6%50%
1800
21st to endDay
12%7%26%32445%6%49%
1980
November1st to 10thDay
8%7%2442345%7%54%
1761
11th to 20thDay
10%7%36%5%3445%7%47%
1710
21st to endDay
12%8%35%5%24446%46%
1710
December1st to 10thDay
15%10%47%6%35%5%5%6%33%
1710
11th to 20thDay
13%10%46%4245%6%8%38%
2153
21st to endDay
11%12%5%8%6%45%5%5%8%33%
3124
January1st to 10thNight
13%8%48%9%45%5%5%6%33%
2459
11th to 20thNight
13%9%5%10%8%3445%6%31%
2386
21st to endNight
11%11%5%8%7%3445%437%
2574
February1st to 10thNight
12%8%5%8%6%345%6%6%36%
2264
11th to 20thNight
17%9%5%9%6%3445%6%34%
2098
21st to endNight
13%10%5%8%7%2446%7%34%
1776
March1st to 10thNight
12%9%46%5%45%45%6%40%
2030
11th to 20thNight
13%9%5%8%6%345%6%7%34%
1980
21st to endNight
10%10%47%6%46%45%8%36%
2178
April1st to 10thNight
10%7%47%4335%6%8%43%
1980
11th to 20thNight
9%8%25%6%35%45%7%46%
1764
21st to endNight
11%7%45%5%35%45%8%45%
1620
May1st to 10thNight
10%7%36%5%36%6%7%8%40%
1620
11th to 20thNight
10%6%37%8%46%6%7%9%35%
1592
21st to endNight
6%5%27%7%47%6%8%11%38%
1744
June1st to 10thNight
6%5%36%6%45%7%7%12%37%
1620
11th to 20thNight
5%5%25%5%45%7%10%13%40%
1620
21st to endNight
7%5%25%6%35%6%8%12%40%
1620
July1st to 10thNight
8%6%46%7%46%6%8%11%35%
1620
11th to 20thNight
7%436%7%47%8%9%12%33%
1620
21st to endNight
6%5%36%7%48%7%9%10%34%
1771
August1st to 10thNight
7%336%7%47%7%10%10%35%
1611
11th to 20thNight
8%7%36%7%47%6%7%10%36%
1598
21st to endNight
8%6%46%6%46%7%7%9%36%
1935
September1st to 10thNight
8%5%36%7%45%5%6%9%40%
1980
11th to 20thNight
11%7%37%6%346%6%8%38%
1980
21st to endNight
9%6%36%6%35%447%48%
1980
October1st to 10thNight
12%7%25%5%2445%7%47%
2011
11th to 20thNight
10%7%36%5%3445%7%47%
2160
21st to endNight
9%8%34434447%50%
2376
November1st to 10thNight
10%6%345%245%5%7%49%
2316
11th to 20thNight
10%8%35%43446%6%46%
2470
21st to endNight
11%8%36%4245%6%6%44%
2470
December1st to 10thNight
14%9%5%9%7%3447%7%30%
2470
11th to 20thNight
12%10%6%10%5%34445%36%
3042
21st to endNight
12%11%5%10%8%45%6%6%6%28%
4532

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
48%5%10%20%13%4
1707
11th to 20thDay
53%48%17%14%4
1647
21st to endDay
45%49%21%19%2
1782
February1st to 10thDay
51%5%10%18%15%2
1692
11th to 20thDay
54%49%16%14%3
1758
21st to endDay
48%6%9%16%15%6%
1480
March1st to 10thDay
42%5%7%21%19%6%
1930
11th to 20thDay
52%6%9%15%14%4
1980
21st to endDay
50%7%10%20%11%2
2178
April1st to 10thDay
39%7%11%22%17%3
1980
11th to 20thDay
40%7%9%18%21%5%
2158
21st to endDay
42%6%12%23%14%2
2340
May1st to 10thDay
43%10%13%21%12%2
2340
11th to 20thDay
49%12%13%17%9%1
2305
21st to endDay
42%11%14%22%10%1
2531
June1st to 10thDay
45%13%16%17%9%1
2340
11th to 20thDay
44%14%15%15%12%2
2340
21st to endDay
44%11%14%18%11%2
2340
July1st to 10thDay
49%13%14%14%9%1
2275
11th to 20thDay
49%14%14%11%10%2
2271
21st to endDay
47%16%14%15%7%1
2405
August1st to 10thDay
49%14%15%17%5%
2314
11th to 20thDay
50%13%15%14%8%1
2303
21st to endDay
46%13%12%17%9%3
2398
September1st to 10thDay
43%11%13%17%13%3
1980
11th to 20thDay
47%9%10%18%15%2
1980
21st to endDay
43%8%11%17%17%5%
1980
October1st to 10thDay
37%7%10%18%23%5%
1949
11th to 20thDay
36%5%8%23%20%7%
1800
21st to endDay
36%48%23%23%6%
1980
November1st to 10thDay
30%6%9%19%28%8%
1761
11th to 20thDay
36%5%7%22%25%6%
1710
21st to endDay
39%6%7%23%20%5%
1710
December1st to 10thDay
50%6%9%17%15%3
1710
11th to 20thDay
43%5%10%17%22%4
2150
21st to endDay
50%5%7%22%14%2
3124
January1st to 10thNight
53%5%8%16%15%3
2459
11th to 20thNight
54%5%9%14%14%3
2386
21st to endNight
50%47%20%17%2
2574
February1st to 10thNight
47%6%10%19%15%2
2264
11th to 20thNight
54%48%18%12%4
2099
21st to endNight
50%7%10%16%13%4
1776
March1st to 10thNight
45%5%9%16%20%5%
2030
11th to 20thNight
49%7%10%19%12%2
1980
21st to endNight
47%7%12%21%12%2
2178
April1st to 10thNight
38%7%13%23%17%2
1980
11th to 20thNight
39%7%9%22%19%4
1764
21st to endNight
39%6%13%27%14%1
1620
May1st to 10thNight
40%8%17%24%9%2
1620
11th to 20thNight
43%14%12%22%7%1
1593
21st to endNight
38%12%17%24%8%1
1745
June1st to 10thNight
36%14%19%21%9%
1620
11th to 20thNight
31%15%24%21%9%1
1620
21st to endNight
34%13%19%21%12%2
1620
July1st to 10thNight
41%13%17%18%10%1
1575
11th to 20thNight
38%18%19%15%7%2
1566
21st to endNight
39%16%18%19%7%
1672
August1st to 10thNight
37%18%20%18%7%
1600
11th to 20thNight
41%15%17%16%10%1
1598
21st to endNight
41%12%18%20%8%1
1935
September1st to 10thNight
40%11%13%20%13%3
1980
11th to 20thNight
42%11%12%19%15%1
1980
21st to endNight
37%8%11%24%17%4
1980
October1st to 10thNight
36%8%11%19%23%3
2011
11th to 20thNight
36%7%11%23%19%4
2160
21st to endNight
35%5%7%23%24%6%
2376
November1st to 10thNight
35%6%9%21%22%6%
2316
11th to 20thNight
37%5%8%21%23%7%
2470
21st to endNight
40%5%7%26%19%4
2470
December1st to 10thNight
52%7%9%16%12%3
2470
11th to 20thNight
50%5%8%16%18%3
3044
21st to endNight
52%7%11%18%10%1
4532

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
33%24%29%11%21
270
11th to 20thDay
32%20%31%15%2
249
21st to endDay
33%28%28%8%3
264
February1st to 10thDay
33%28%25%12%3
240
11th to 20thDay
45%25%24%5%1
240
21st to endDay
40%24%27%7%2
198
March1st to 10thDay
27%24%25%19%5%1
370
11th to 20thDay
30%22%22%19%5%1
420
21st to endDay
25%22%27%19%6%
462
April1st to 10thDay
24%18%24%23%10%2
420
11th to 20thDay
20%20%23%20%14%3
412
21st to endDay
23%16%23%20%13%5%
420
May1st to 10thDay
22%18%23%16%14%6%
423
11th to 20thDay
18%11%24%21%17%9%
447
21st to endDay
13%8%20%25%20%14%
485
June1st to 10thDay
14%7%24%24%19%12%
450
11th to 20thDay
8%7%24%23%22%16%
450
21st to endDay
11%8%25%24%17%15%
450
July1st to 10thDay
11%315%23%28%20%
445
11th to 20thDay
9%214%25%27%22%
444
21st to endDay
9%315%24%28%21%
488
August1st to 10thDay
17%417%20%23%17%
446
11th to 20thDay
10%5%15%25%26%18%
439
21st to endDay
14%313%24%25%22%
495
September1st to 10thDay
10%415%24%29%18%
450
11th to 20thDay
14%5%17%20%29%15%
450
21st to endDay
14%6%18%30%27%5%
450
October1st to 10thDay
16%9%18%26%21%9%
450
11th to 20thDay
15%14%22%25%21%4
450
21st to endDay
20%15%20%29%13%3
495
November1st to 10thDay
17%14%23%29%12%4
359
11th to 20thDay
22%17%33%18%9%1
270
21st to endDay
22%15%35%16%10%2
270
December1st to 10thDay
29%25%29%13%4
270
11th to 20thDay
23%23%27%20%7%1
270
21st to endDay
27%21%34%12%5%1
297
January1st to 10thNight
27%16%26%26%5%
760
11th to 20thNight
25%16%27%26%5%
735
21st to endNight
23%17%29%27%3
792
February1st to 10thNight
24%14%30%29%4
720
11th to 20thNight
28%20%29%21%1
703
21st to endNight
25%21%31%20%2
592
March1st to 10thNight
21%17%29%28%4
720
11th to 20thNight
26%15%27%24%8%1
720
21st to endNight
21%14%29%27%9%
792
April1st to 10thNight
22%15%24%29%10%1
720
11th to 20thNight
18%12%27%28%14%1
615
21st to endNight
17%10%20%38%14%1
540
May1st to 10thNight
16%10%22%32%18%1
540
11th to 20thNight
13%6%19%34%24%3
531
21st to endNight
9%312%33%38%5%
584
June1st to 10thNight
9%310%37%34%6%
540
11th to 20thNight
7%311%31%38%10%
540
21st to endNight
8%310%35%37%7%
540
July1st to 10thNight
11%112%24%44%9%
525
11th to 20thNight
9%26%25%44%14%
522
21st to endNight
7%28%25%45%14%
591
August1st to 10thNight
10%15%29%46%9%
537
11th to 20thNight
9%37%30%42%9%
534
21st to endNight
11%18%27%44%10%
637
September1st to 10thNight
11%211%27%41%8%
720
11th to 20thNight
14%411%27%37%7%
720
21st to endNight
16%416%30%31%3
720
October1st to 10thNight
16%7%17%27%25%7%
720
11th to 20thNight
16%9%22%34%17%3
720
21st to endNight
17%12%17%37%14%3
792
November1st to 10thNight
16%8%16%37%20%3
740
11th to 20thNight
21%11%22%33%12%2
760
21st to endNight
21%11%22%33%12%1
760
December1st to 10thNight
25%16%23%30%7%
760
11th to 20thNight
20%14%26%33%7%1
760
21st to endNight
26%14%25%29%6%
836
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