Deneb Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Deneb Observatory clear sky chart from 2005-11-06 to 2024-01-16.

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
38%
25007
16%
74827
9%
24956
44%
24956
25%
49256

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%
948
13%
2685
5%
946
34%
946
18%
1922
January11th to 20thNight
26%
930
11%
2604
5%
930
30%
930
16%
1979
January21st to endNight
24%
990
13%
2772
5%
990
30%
990
15%
2104
February1st to 10thNight
30%
900
11%
2493
5%
900
35%
900
18%
1859
February11th to 20thNight
27%
770
14%
2169
5%
770
34%
770
20%
1535
February21st to endNight
38%
592
19%
1776
9%
592
47%
592
24%
1205
March1st to 10thNight
31%
720
9%
2160
5%
720
34%
720
16%
1613
March11th to 20thNight
35%
604
20%
2044
13%
604
45%
604
23%
1313
March21st to endNight
38%
594
13%
2098
10%
594
45%
594
19%
1429
April1st to 10thNight
32%
540
18%
1800
11%
540
44%
540
23%
1144
April11th to 20thNight
36%
533
17%
1653
14%
533
44%
533
22%
1103
April21st to endNight
38%
540
17%
1620
12%
540
45%
540
24%
1072
May1st to 10thNight
45%
540
15%
1609
8%
540
48%
540
28%
1011
May11th to 20thNight
46%
531
16%
1417
7%
531
48%
531
31%
919
May21st to endNight
45%
584
12%
1420
5%
582
47%
582
32%
963
June1st to 10thNight
50%
540
15%
1260
7%
540
53%
540
37%
818
June11th to 20thNight
41%
540
11%
1260
5%
540
44%
540
35%
861
June21st to endNight
56%
540
13%
1260
7%
540
58%
540
46%
778
July1st to 10thNight
62%
525
15%
1247
9%
525
64%
525
48%
746
July11th to 20thNight
54%
522
17%
1392
8%
522
58%
522
50%
770
July21st to endNight
62%
591
20%
1488
11%
558
64%
558
50%
790
August1st to 10thNight
62%
537
23%
1441
15%
534
66%
534
46%
818
August11th to 20thNight
58%
534
25%
1600
18%
534
65%
534
40%
878
August21st to endNight
55%
594
29%
1836
26%
594
64%
594
46%
932
September1st to 10thNight
55%
540
24%
1800
22%
540
65%
540
41%
912
September11th to 20thNight
49%
540
24%
1968
20%
540
59%
540
34%
1021
September21st to endNight
41%
540
25%
1980
20%
540
52%
540
28%
1108
October1st to 10thNight
37%
540
22%
1980
17%
540
45%
540
22%
1213
October11th to 20thNight
38%
858
20%
2298
10%
858
44%
858
30%
1483
October21st to endNight
31%
990
19%
2574
6%
990
39%
990
27%
1740
November1st to 10thNight
31%
925
19%
2405
7%
925
39%
925
25%
1662
November11th to 20thNight
32%
950
19%
2527
10%
950
39%
950
22%
1785
November21st to endNight
30%
950
14%
2821
6%
950
38%
950
22%
1910
December1st to 10thNight
29%
950
10%
2850
5%
950
35%
950
18%
2073
December11th to 20thNight
24%
948
11%
3449
5%
948
32%
948
13%
2419
December21st to endNight
24%
1037
9%
5071
5%
1026
32%
1026
9%
3368

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
10%8%37%6%35%5%6%8%39%
74600
Night
8%8%47%6%45%5%6%7%40%
75226
TransparencyDay
42%8%15%24%10%1
74299
Night
41%8%15%24%12%1
75049
SeeingDay
12%9%33%30%13%3
14928
Night
11%9%35%34%10%1
25023

By Month

Percentage Cloud Cover SW (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
411%5%10%11%46%6%8%8%27%
1475
11th to 20thDay
5%10%6%11%11%5%6%7%6%9%24%
1484
21st to endDay
6%11%6%12%10%6%8%6%7%7%20%
1584
February1st to 10thDay
7%10%5%11%9%5%7%7%6%8%25%
1466
11th to 20thDay
6%12%5%10%10%5%8%6%7%6%25%
1692
21st to endDay
7%11%6%13%10%47%5%5%7%25%
1480
March1st to 10thDay
9%13%6%11%10%5%6%6%5%6%24%
1800
11th to 20thDay
8%8%48%8%5%6%5%5%8%34%
1916
21st to endDay
12%11%411%7%45%5%6%7%29%
2254
April1st to 10thDay
11%9%37%7%45%5%5%7%37%
2160
11th to 20thDay
16%11%48%6%4445%6%33%
2255
21st to endDay
16%10%36%5%35%5%5%7%35%
2340
May1st to 10thDay
16%8%36%5%345%47%39%
2348
11th to 20thDay
13%7%35%5%35%46%8%41%
2478
21st to endDay
14%8%35%5%35%5%6%8%37%
2856
June1st to 10thDay
13%6%24425%5%6%8%44%
2700
11th to 20thDay
11%5%35%5%45%5%7%8%41%
2700
21st to endDay
8%425%5%446%6%9%47%
2700
July1st to 10thDay
6%3145%35%5%6%9%52%
2673
11th to 20thDay
5%32443447%9%54%
2520
21st to endDay
43245%345%6%8%56%
2742
August1st to 10thDay
7%5%25%6%45%6%6%8%48%
2490
11th to 20thDay
10%5%244345%5%7%51%
2301
21st to endDay
7%5%25%42345%6%57%
2518
September1st to 10thDay
11%4243345%5%7%53%
2160
11th to 20thDay
10%5%2442345%7%53%
1991
21st to endDay
11%7%25%5%2335%6%50%
1980
October1st to 10thDay
17%9%46%5%23347%40%
1972
11th to 20thDay
11%9%47%5%3445%8%40%
1651
21st to endDay
12%10%48%7%345%5%7%35%
1782
November1st to 10thDay
11%10%49%6%35%5%48%35%
1673
11th to 20thDay
7%9%5%11%9%5%6%45%6%33%
1650
21st to endDay
6%9%5%11%9%35%5%6%7%33%
1354
December1st to 10thDay
5%11%5%9%10%5%7%7%7%10%23%
1330
11th to 20thDay
7%11%6%10%9%35%5%7%9%26%
1683
21st to endDay
10%10%47%10%45%6%7%7%31%
2442
January1st to 10thNight
7%10%5%9%7%46%6%7%9%30%
2685
11th to 20thNight
6%10%5%10%9%47%7%7%7%27%
2604
21st to endNight
411%5%11%10%6%7%7%6%8%25%
2772
February1st to 10thNight
6%10%5%10%8%5%8%7%7%7%27%
2493
11th to 20thNight
6%11%49%7%46%6%7%7%33%
2169
21st to endNight
7%10%48%6%46%5%5%7%38%
1776
March1st to 10thNight
8%11%5%11%9%46%6%6%7%27%
2160
11th to 20thNight
11%9%5%5%6%3445%8%41%
2044
21st to endNight
10%10%37%5%46%6%5%8%35%
2098
April1st to 10thNight
7%7%47%7%46%6%6%7%40%
1800
11th to 20thNight
12%9%46%6%35%5%6%7%37%
1653
21st to endNight
14%9%45%4346%7%7%39%
1620
May1st to 10thNight
10%7%26%5%35%5%6%7%42%
1609
11th to 20thNight
10%6%26%5%35%5%6%8%44%
1417
21st to endNight
12%5%25%4445%5%10%42%
1420
June1st to 10thNight
9%5%24425%5%7%9%48%
1260
11th to 20thNight
9%5%35%5%36%5%6%10%44%
1260
21st to endNight
4313435%6%6%9%55%
1260
July1st to 10thNight
5%313415%5%7%8%57%
1283
11th to 20thNight
5%32333345%8%61%
1440
21st to endNight
432432445%9%61%
1576
August1st to 10thNight
3415%4245%6%8%58%
1449
11th to 20thNight
5%3235%345%5%7%57%
1600
21st to endNight
33233235%5%7%63%
1836
September1st to 10thNight
6%41332345%7%62%
1800
11th to 20thNight
7%31443445%9%55%
1968
21st to endNight
12%6%2442435%6%52%
1980
October1st to 10thNight
12%8%36%5%3445%7%45%
1980
11th to 20thNight
12%8%3443445%7%46%
2298
21st to endNight
11%7%37%6%3446%7%41%
2574
November1st to 10thNight
8%8%47%6%45%6%6%8%39%
2405
11th to 20thNight
6%9%49%8%5%5%5%5%7%36%
2527
21st to endNight
6%6%48%8%46%6%6%8%38%
2821
December1st to 10thNight
7%9%5%10%8%5%6%7%7%7%29%
2850
11th to 20thNight
7%11%410%8%46%6%6%8%30%
3514
21st to endNight
11%11%5%8%9%45%5%5%5%33%
5225

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
51%9%17%14%7%1
1476
11th to 20thDay
56%7%13%16%7%1
1484
21st to endDay
61%6%13%13%7%
1584
February1st to 10thDay
56%7%13%15%8%1
1466
11th to 20thDay
57%8%12%16%7%1
1702
21st to endDay
58%6%13%15%8%
1480
March1st to 10thDay
60%8%14%12%5%
1800
11th to 20thDay
47%8%14%21%9%
1916
21st to endDay
54%9%15%16%5%
2254
April1st to 10thDay
44%7%15%19%13%1
2160
11th to 20thDay
52%6%12%18%11%1
2258
21st to endDay
47%8%16%23%6%1
2340
May1st to 10thDay
44%6%14%26%10%1
2348
11th to 20thDay
40%7%14%27%11%
2482
21st to endDay
43%9%17%25%6%
2856
June1st to 10thDay
37%9%16%28%9%1
2700
11th to 20thDay
39%9%15%25%10%1
2700
21st to endDay
31%9%21%26%12%1
2700
July1st to 10thDay
26%8%20%31%12%2
2600
11th to 20thDay
25%9%17%29%16%4
2445
21st to endDay
25%8%15%33%16%2
2590
August1st to 10thDay
32%9%16%27%14%2
2474
11th to 20thDay
31%8%16%32%12%1
2301
21st to endDay
28%7%12%32%18%3
2518
September1st to 10thDay
30%7%14%31%17%1
2160
11th to 20thDay
31%6%16%31%16%1
1991
21st to endDay
34%7%14%30%14%1
1980
October1st to 10thDay
44%6%15%23%12%1
1972
11th to 20thDay
42%6%13%27%11%1
1651
21st to endDay
48%8%15%20%9%1
1782
November1st to 10thDay
47%8%13%20%11%1
1673
11th to 20thDay
51%7%15%18%8%1
1650
21st to endDay
49%8%16%19%8%1
1354
December1st to 10thDay
53%10%15%16%42
1330
11th to 20thDay
50%10%17%19%5%
1680
21st to endDay
49%7%12%21%10%1
2442
January1st to 10thNight
48%8%14%20%9%
2685
11th to 20thNight
53%10%14%15%8%1
2604
21st to endNight
55%8%14%15%7%
2772
February1st to 10thNight
53%7%13%16%10%
2493
11th to 20thNight
47%8%14%20%10%2
2169
21st to endNight
45%6%14%21%12%2
1776
March1st to 10thNight
54%8%14%18%7%
2160
11th to 20thNight
43%6%13%26%12%
2044
21st to endNight
45%9%18%19%8%1
2098
April1st to 10thNight
41%7%13%22%15%2
1800
11th to 20thNight
45%6%12%22%14%2
1653
21st to endNight
41%8%14%25%11%1
1620
May1st to 10thNight
37%7%14%27%13%2
1609
11th to 20thNight
36%7%15%24%16%1
1418
21st to endNight
36%9%20%26%9%1
1420
June1st to 10thNight
30%9%17%31%12%2
1260
11th to 20thNight
34%9%16%26%11%2
1260
21st to endNight
23%10%22%31%13%2
1260
July1st to 10thNight
22%8%16%32%19%3
1247
11th to 20thNight
21%6%16%34%18%5%
1392
21st to endNight
19%7%18%30%23%3
1488
August1st to 10thNight
22%8%15%35%15%5%
1441
11th to 20thNight
24%8%14%34%17%3
1600
21st to endNight
20%6%12%36%22%3
1836
September1st to 10thNight
22%7%12%34%21%3
1800
11th to 20thNight
26%8%18%29%18%1
1968
21st to endNight
33%6%12%32%16%2
1980
October1st to 10thNight
39%7%14%23%15%2
1980
11th to 20thNight
38%5%13%29%13%3
2298
21st to endNight
41%7%15%25%10%1
2574
November1st to 10thNight
40%9%14%20%15%2
2405
11th to 20thNight
47%7%15%21%10%1
2527
21st to endNight
42%9%20%21%8%1
2821
December1st to 10thNight
50%8%17%17%6%
2850
11th to 20thNight
49%9%18%18%5%
3516
21st to endNight
51%7%10%21%11%1
5225

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
9%11%37%31%9%3
270
11th to 20thDay
9%9%42%28%9%2
257
21st to endDay
13%12%39%25%10%2
264
February1st to 10thDay
10%10%47%27%6%
240
11th to 20thDay
13%7%46%28%6%
240
21st to endDay
12%8%34%35%10%
198
March1st to 10thDay
18%8%40%25%9%1
240
11th to 20thDay
10%9%34%33%13%2
240
21st to endDay
12%11%35%28%11%3
340
April1st to 10thDay
16%12%38%26%8%
420
11th to 20thDay
23%13%30%23%10%1
536
21st to endDay
20%11%29%27%12%2
600
May1st to 10thDay
17%9%30%30%13%1
603
11th to 20thDay
16%10%28%30%12%4
625
21st to endDay
15%9%32%24%15%5%
679
June1st to 10thDay
13%10%26%30%16%6%
630
11th to 20thDay
12%10%39%25%12%3
630
21st to endDay
7%11%29%36%15%3
630
July1st to 10thDay
6%427%40%20%2
621
11th to 20thDay
7%5%33%36%17%2
618
21st to endDay
4427%42%19%4
684
August1st to 10thDay
6%324%40%22%5%
626
11th to 20thDay
9%425%33%24%6%
615
21st to endDay
8%7%28%33%20%4
637
September1st to 10thDay
11%9%32%33%13%3
450
11th to 20thDay
13%8%30%34%12%2
281
21st to endDay
16%13%33%28%10%1
270
October1st to 10thDay
20%12%30%24%10%2
270
11th to 20thDay
15%14%36%25%9%2
270
21st to endDay
17%13%44%19%5%2
297
November1st to 10thDay
19%10%35%26%9%1
270
11th to 20thDay
9%17%37%30%7%1
270
21st to endDay
11%13%40%27%9%1
270
December1st to 10thDay
9%9%47%25%9%1
270
11th to 20thDay
15%11%43%24%6%1
270
21st to endDay
14%12%38%26%6%4
297
January1st to 10thNight
11%11%41%29%8%
950
11th to 20thNight
12%11%38%30%8%
930
21st to endNight
11%12%41%28%8%
990
February1st to 10thNight
11%11%37%33%7%1
900
11th to 20thNight
12%9%41%28%8%2
774
21st to endNight
11%7%33%37%9%2
592
March1st to 10thNight
11%11%35%31%10%2
720
11th to 20thNight
12%10%36%29%12%1
604
21st to endNight
15%6%33%32%12%1
594
April1st to 10thNight
9%10%40%33%7%
540
11th to 20thNight
15%9%33%33%9%
533
21st to endNight
18%9%29%31%13%1
540
May1st to 10thNight
12%6%32%34%15%
540
11th to 20thNight
12%6%33%35%14%
531
21st to endNight
14%7%30%36%12%
584
June1st to 10thNight
10%9%26%41%13%1
540
11th to 20thNight
11%7%36%36%10%
540
21st to endNight
5%431%48%13%
540
July1st to 10thNight
6%226%49%16%
525
11th to 20thNight
4434%45%13%
522
21st to endNight
5%426%49%16%1
591
August1st to 10thNight
4325%51%15%1
537
11th to 20thNight
5%230%44%18%1
534
21st to endNight
5%5%31%43%15%1
594
September1st to 10thNight
8%430%41%14%2
540
11th to 20thNight
7%6%35%38%13%1
540
21st to endNight
13%8%34%32%11%1
540
October1st to 10thNight
16%10%32%33%8%
540
11th to 20thNight
17%10%29%33%9%1
858
21st to endNight
16%11%37%26%9%1
990
November1st to 10thNight
12%12%39%29%8%
925
11th to 20thNight
10%12%38%30%9%1
950
21st to endNight
9%11%41%33%5%1
950
December1st to 10thNight
11%10%39%32%8%1
950
11th to 20thNight
14%13%36%30%7%1
950
21st to endNight
17%10%36%28%9%
1045
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