Hansen Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Hansen 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
41%
24591
24%
74005
14%
24530
51%
24530
34%
42826

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
35%
950
26%
2458
11%
946
46%
946
33%
1538
January11th to 20thNight
35%
930
25%
2418
11%
930
44%
930
32%
1547
January21st to endNight
34%
990
25%
2574
10%
990
44%
990
33%
1625
February1st to 10thNight
28%
900
18%
2340
6%
900
36%
900
28%
1570
February11th to 20thNight
30%
722
22%
2116
10%
718
39%
718
29%
1288
February21st to endNight
28%
592
28%
1776
8%
592
45%
592
31%
1019
March1st to 10thNight
30%
653
24%
2093
11%
653
42%
653
30%
1177
March11th to 20thNight
32%
540
25%
1980
16%
540
44%
540
26%
1129
March21st to endNight
25%
594
16%
2178
9%
594
37%
594
19%
1395
April1st to 10thNight
28%
540
19%
1931
11%
540
42%
540
25%
1110
April11th to 20thNight
29%
533
22%
1653
12%
533
44%
533
28%
992
April21st to endNight
34%
540
23%
1620
15%
540
47%
540
30%
980
May1st to 10thNight
31%
540
16%
1620
10%
540
40%
540
24%
1071
May11th to 20thNight
35%
531
18%
1594
12%
531
43%
531
27%
989
May21st to endNight
43%
584
13%
1746
8%
582
49%
582
28%
1103
June1st to 10thNight
58%
540
21%
1620
13%
540
63%
540
43%
861
June11th to 20thNight
47%
540
21%
1542
11%
540
53%
540
42%
858
June21st to endNight
53%
540
23%
1440
12%
540
59%
540
49%
792
July1st to 10thNight
61%
525
18%
1408
11%
525
63%
525
41%
834
July11th to 20thNight
68%
522
22%
1566
17%
522
72%
522
48%
789
July21st to endNight
69%
591
24%
1673
21%
558
74%
558
46%
869
August1st to 10thNight
69%
537
20%
1601
22%
534
71%
534
40%
879
August11th to 20thNight
63%
534
28%
1600
26%
534
71%
534
44%
847
August21st to endNight
64%
594
29%
1839
30%
594
69%
594
45%
950
September1st to 10thNight
59%
540
27%
1943
26%
540
68%
540
42%
922
September11th to 20thNight
51%
540
30%
1980
26%
540
63%
540
38%
984
September21st to endNight
47%
540
31%
1980
24%
540
58%
540
38%
966
October1st to 10thNight
43%
561
29%
2001
23%
561
52%
561
30%
1106
October11th to 20thNight
35%
720
29%
2160
13%
720
47%
720
38%
1164
October21st to endNight
40%
864
28%
2448
13%
864
49%
864
38%
1391
November1st to 10thNight
45%
925
33%
2405
13%
925
56%
925
49%
1292
November11th to 20thNight
36%
950
32%
2470
11%
950
50%
950
41%
1421
November21st to endNight
41%
950
28%
2470
13%
950
49%
950
39%
1455
December1st to 10thNight
35%
950
26%
2470
10%
950
46%
950
38%
1473
December11th to 20thNight
32%
949
30%
2848
11%
948
43%
948
31%
1605
December21st to endNight
33%
1040
18%
4444
11%
1026
46%
1026
19%
2835

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%6%25%5%3445%7%50%
75352
Night
9%5%2442445%7%53%
74482
TransparencyDay
34%6%13%27%17%3
75072
Night
31%6%13%27%19%4
74282
SeeingDay
12%9%31%31%15%2
14558
Night
11%9%34%35%10%1
24597

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%9%25%4345%46%49%
1707
11th to 20thDay
8%8%35%43446%8%46%
1670
21st to endDay
8%8%38%5%3445%6%45%
1782
February1st to 10thDay
9%9%46%6%3445%7%43%
1620
11th to 20thDay
6%8%26%5%445%6%8%46%
1745
21st to endDay
7%8%36%5%3445%6%50%
1480
March1st to 10thDay
7%6%25%42445%7%53%
1866
11th to 20thDay
15%8%35%423347%47%
1980
21st to endDay
13%10%46%5%35%45%6%39%
2178
April1st to 10thDay
13%6%25%42445%6%50%
2024
11th to 20thDay
13%6%26%5%345%5%8%43%
2258
21st to endDay
18%6%2442446%6%43%
2340
May1st to 10thDay
13%7%35%5%245%6%7%43%
2340
11th to 20thDay
15%5%26%5%3445%7%44%
2302
21st to endDay
14%5%25%5%36%6%7%8%39%
2531
June1st to 10thDay
5%3234345%6%8%57%
2340
11th to 20thDay
7%4245%35%5%6%9%52%
2411
21st to endDay
6%325%43445%8%56%
2520
July1st to 10thDay
7%3135%35%5%7%10%52%
2507
11th to 20thDay
43133245%6%9%60%
2340
21st to endDay
7%4145%35%5%6%9%51%
2546
August1st to 10thDay
7%425%5%45%5%7%10%46%
2328
11th to 20thDay
7%4145%2446%7%57%
2301
21st to endDay
7%41342345%7%59%
2515
September1st to 10thDay
14%424324436%54%
2016
11th to 20thDay
9%42332345%8%56%
1980
21st to endDay
9%32342335%6%60%
1980
October1st to 10thDay
16%5%23422335%55%
1940
11th to 20thDay
10%414423335%61%
1800
21st to endDay
13%6%2342334457%
1906
November1st to 10thDay
8%42332345%7%60%
1673
11th to 20thDay
7%5%345%34445%58%
1710
21st to endDay
11%7%44323338%52%
1710
December1st to 10thDay
8%7%25%6%3345%8%50%
1710
11th to 20thDay
8%7%46%5%2445%7%50%
2161
21st to endDay
12%10%411%7%34446%35%
3135
January1st to 10thNight
6%9%45%42445%7%49%
2458
11th to 20thNight
6%9%35%435%446%50%
2418
21st to endNight
7%7%35%5%345%5%8%49%
2574
February1st to 10thNight
9%10%45%43445%6%45%
2340
11th to 20thNight
7%7%35%5%35%5%6%8%46%
2116
21st to endNight
7%6%25%5%34446%54%
1776
March1st to 10thNight
9%6%35%5%23346%54%
2093
11th to 20thNight
13%7%24423336%52%
1980
21st to endNight
14%8%45%5%345%5%8%40%
2178
April1st to 10thNight
9%7%25%43345%7%50%
1931
11th to 20thNight
11%7%34335%5%6%7%47%
1653
21st to endNight
13%5%25%423447%51%
1620
May1st to 10thNight
16%6%35%5%345%6%8%40%
1620
11th to 20thNight
15%5%25%5%33446%48%
1593
21st to endNight
12%5%245%345%6%9%44%
1745
June1st to 10thNight
5%2134345%6%10%58%
1620
11th to 20thNight
7%32442445%9%56%
1542
21st to endNight
42133245%7%11%56%
1440
July1st to 10thNight
7%425%4245%6%11%50%
1448
11th to 20thNight
32134245%6%10%60%
1620
21st to endNight
43144345%8%10%55%
1772
August1st to 10thNight
4325%325%6%6%11%53%
1611
11th to 20thNight
6%2235%2446%9%57%
1600
21st to endNight
6%313424447%63%
1839
September1st to 10thNight
8%313232346%64%
1943
11th to 20thNight
8%4233245%5%7%58%
1980
21st to endNight
7%31432345%5%62%
1980
October1st to 10thNight
13%423424445%54%
2001
11th to 20thNight
11%5%24323345%61%
2160
21st to endNight
12%42432334459%
2448
November1st to 10thNight
6%323323347%65%
2405
11th to 20thNight
8%423324446%60%
2470
21st to endNight
9%5%24313446%57%
2470
December1st to 10thNight
6%6%244245%47%55%
2470
11th to 20thNight
8%6%34423347%55%
3036
21st to endNight
16%10%36%424347%41%
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
37%5%17%26%14%1
1708
11th to 20thDay
35%7%17%25%14%2
1670
21st to endDay
41%5%16%24%12%2
1782
February1st to 10thDay
41%5%17%22%12%3
1620
11th to 20thDay
37%6%13%27%14%3
1755
21st to endDay
35%5%13%26%17%3
1480
March1st to 10thDay
31%5%13%28%19%4
1866
11th to 20thDay
39%5%12%25%15%4
1980
21st to endDay
47%6%15%22%9%1
2178
April1st to 10thDay
35%5%14%25%18%3
2024
11th to 20thDay
39%5%14%24%14%3
2258
21st to endDay
42%6%11%26%14%1
2340
May1st to 10thDay
40%7%14%23%14%2
2340
11th to 20thDay
39%8%15%26%12%
2306
21st to endDay
41%10%15%22%9%2
2531
June1st to 10thDay
25%8%17%35%14%2
2340
11th to 20thDay
29%7%14%30%17%3
2411
21st to endDay
26%8%13%31%18%4
2520
July1st to 10thDay
27%8%15%30%19%2
2438
11th to 20thDay
21%7%14%30%23%4
2271
21st to endDay
28%7%15%32%17%1
2405
August1st to 10thDay
31%10%18%27%12%2
2314
11th to 20thDay
26%7%14%32%19%2
2301
21st to endDay
25%6%14%24%26%6%
2515
September1st to 10thDay
32%5%11%23%22%7%
2016
11th to 20thDay
27%6%12%28%25%3
1980
21st to endDay
26%5%10%31%27%1
1980
October1st to 10thDay
35%48%26%26%2
1940
11th to 20thDay
29%38%28%24%8%
1800
21st to endDay
33%310%26%23%5%
1906
November1st to 10thDay
25%412%31%23%4
1673
11th to 20thDay
29%6%9%25%28%3
1710
21st to endDay
34%5%10%27%21%4
1710
December1st to 10thDay
33%5%16%29%15%2
1710
11th to 20thDay
35%5%13%32%13%1
2159
21st to endDay
51%5%11%20%12%1
3135
January1st to 10thNight
35%5%13%29%16%2
2458
11th to 20thNight
37%413%27%17%1
2418
21st to endNight
35%5%17%27%14%3
2574
February1st to 10thNight
40%6%13%22%15%5%
2340
11th to 20thNight
35%6%13%29%14%3
2116
21st to endNight
32%413%22%21%7%
1776
March1st to 10thNight
33%411%26%20%6%
2093
11th to 20thNight
36%5%9%26%20%5%
1980
21st to endNight
43%7%15%21%12%2
2178
April1st to 10thNight
35%6%12%23%21%3
1931
11th to 20thNight
36%5%15%27%16%2
1653
21st to endNight
35%6%12%27%18%2
1620
May1st to 10thNight
41%8%13%21%14%2
1620
11th to 20thNight
38%6%15%25%15%1
1594
21st to endNight
35%9%19%24%11%2
1746
June1st to 10thNight
21%8%18%32%19%2
1620
11th to 20thNight
26%5%15%29%19%6%
1542
21st to endNight
21%7%16%30%22%5%
1440
July1st to 10thNight
27%8%15%26%19%5%
1408
11th to 20thNight
19%6%16%29%22%7%
1566
21st to endNight
20%8%18%32%20%2
1673
August1st to 10thNight
24%8%20%31%13%3
1601
11th to 20thNight
23%6%17%32%19%4
1600
21st to endNight
22%6%12%25%25%9%
1839
September1st to 10thNight
23%412%27%24%10%
1943
11th to 20thNight
25%6%11%27%26%6%
1980
21st to endNight
24%5%10%27%30%4
1980
October1st to 10thNight
31%6%9%25%24%4
2001
11th to 20thNight
28%48%26%26%8%
2160
21st to endNight
29%411%23%25%7%
2448
November1st to 10thNight
21%311%30%27%7%
2405
11th to 20thNight
26%5%9%29%26%5%
2470
21st to endNight
29%411%28%24%4
2470
December1st to 10thNight
29%6%14%30%19%2
2470
11th to 20thNight
30%5%11%36%17%1
3037
21st to endNight
44%6%12%25%11%2
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
13%10%38%29%8%2
270
11th to 20thDay
20%12%33%26%8%1
257
21st to endDay
13%9%46%28%4
264
February1st to 10thDay
13%14%45%25%4
240
11th to 20thDay
10%13%45%25%5%1
240
21st to endDay
9%15%54%18%5%
198
March1st to 10thDay
15%14%44%21%6%
240
11th to 20thDay
28%10%31%22%10%
240
21st to endDay
21%22%36%17%4
264
April1st to 10thDay
14%17%43%21%5%
284
11th to 20thDay
17%14%40%22%7%
536
21st to endDay
21%18%32%20%9%2
600
May1st to 10thDay
14%11%34%30%11%
603
11th to 20thDay
17%11%32%25%13%3
625
21st to endDay
15%9%32%29%12%2
679
June1st to 10thDay
46%25%35%24%6%
630
11th to 20thDay
9%436%35%15%2
630
21st to endDay
7%7%32%35%17%1
630
July1st to 10thDay
8%220%39%27%4
621
11th to 20thDay
5%218%46%24%6%
618
21st to endDay
7%217%42%28%5%
684
August1st to 10thDay
7%317%45%24%4
626
11th to 20thDay
7%417%44%25%3
615
21st to endDay
10%220%38%26%5%
634
September1st to 10thDay
16%6%22%37%18%2
306
11th to 20thDay
6%328%41%17%5%
270
21st to endDay
14%8%31%27%17%3
270
October1st to 10thDay
21%9%22%30%15%4
270
11th to 20thDay
7%15%41%24%12%1
270
21st to endDay
22%12%36%23%6%2
297
November1st to 10thDay
12%14%38%24%9%2
270
11th to 20thDay
10%17%43%21%7%2
270
21st to endDay
22%13%29%26%9%1
270
December1st to 10thDay
7%10%45%26%12%1
270
11th to 20thDay
17%9%41%26%7%
270
21st to endDay
18%15%39%25%2
297
January1st to 10thNight
12%11%35%33%8%1
950
11th to 20thNight
12%11%36%33%7%1
930
21st to endNight
10%11%40%32%7%
990
February1st to 10thNight
15%9%42%29%5%
900
11th to 20thNight
11%11%43%30%41
722
21st to endNight
10%15%42%28%41
592
March1st to 10thNight
12%14%41%28%5%
653
11th to 20thNight
16%11%39%28%6%1
540
21st to endNight
17%15%39%24%5%
594
April1st to 10thNight
14%13%41%27%4
540
11th to 20thNight
16%14%38%28%5%
533
21st to endNight
14%11%37%29%7%1
540
May1st to 10thNight
21%8%34%29%7%
540
11th to 20thNight
17%8%34%31%8%
531
21st to endNight
14%8%30%34%12%1
584
June1st to 10thNight
5%5%29%43%16%1
540
11th to 20thNight
7%5%35%41%12%
540
21st to endNight
4435%45%12%
540
July1st to 10thNight
7%321%42%26%
525
11th to 20thNight
3221%52%20%2
522
21st to endNight
4117%51%26%1
591
August1st to 10thNight
5%219%53%19%1
537
11th to 20thNight
8%222%47%20%1
534
21st to endNight
7%223%42%25%2
594
September1st to 10thNight
10%227%44%17%1
540
11th to 20thNight
9%333%37%16%1
540
21st to endNight
8%6%35%37%13%1
540
October1st to 10thNight
15%7%32%34%12%
561
11th to 20thNight
14%10%37%32%7%
720
21st to endNight
14%10%33%32%11%1
864
November1st to 10thNight
8%10%34%39%9%
925
11th to 20thNight
10%11%40%32%7%
950
21st to endNight
11%12%32%34%9%1
950
December1st to 10thNight
8%11%41%34%5%
950
11th to 20thNight
14%11%38%31%7%
950
21st to endNight
16%10%36%33%5%
1045
Page generated at 2024-01-13 18:28:32. Version ac07

Copyright 2024 Attilla Danko


Hansen Observatory Clear Sky Chart |Charts Home