Hirsch Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Hirsch Observatory clear sky chart from 2005-11-06 to 2024-01-12.

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
28%
22867
14%
74699
8%
22826
36%
22826
16%
52731

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%
760
12%
2649
5%
757
25%
757
12%
1935
January11th to 20thNight
18%
727
9%
2541
4%
727
25%
727
9%
1954
January21st to endNight
15%
792
12%
2722
5%
792
25%
792
12%
1955
February1st to 10thNight
12%
720
10%
2340
2%
720
21%
720
10%
1803
February11th to 20thNight
14%
703
12%
2118
3%
700
24%
700
13%
1588
February21st to endNight
16%
592
13%
1776
6%
592
26%
592
12%
1339
March1st to 10thNight
16%
720
15%
2160
7%
720
28%
720
14%
1579
March11th to 20thNight
17%
720
14%
2091
6%
720
27%
720
14%
1567
March21st to endNight
21%
792
20%
1981
5%
792
31%
792
18%
1448
April1st to 10thNight
21%
720
15%
1800
3%
720
27%
720
18%
1351
April11th to 20thNight
22%
711
15%
1775
4%
711
27%
711
20%
1295
April21st to endNight
24%
632
16%
1712
4%
632
31%
632
18%
1238
May1st to 10thNight
24%
521
13%
1601
6%
521
31%
521
14%
1175
May11th to 20thNight
28%
354
17%
1416
11%
354
41%
354
15%
933
May21st to endNight
41%
388
13%
1551
12%
388
48%
388
18%
990
June1st to 10thNight
37%
360
15%
1440
14%
360
47%
360
18%
927
June11th to 20thNight
44%
360
13%
1440
15%
360
51%
360
19%
913
June21st to endNight
48%
360
13%
1440
16%
360
54%
360
22%
875
July1st to 10thNight
49%
350
13%
1400
16%
350
55%
350
22%
858
July11th to 20thNight
53%
348
15%
1392
17%
348
57%
348
21%
845
July21st to endNight
56%
394
18%
1486
20%
372
60%
372
24%
879
August1st to 10thNight
56%
358
17%
1422
17%
356
61%
356
25%
832
August11th to 20thNight
51%
694
14%
1758
8%
694
53%
694
26%
1237
August21st to endNight
49%
792
18%
1980
9%
792
52%
792
31%
1329
September1st to 10thNight
43%
720
17%
1800
7%
720
48%
720
29%
1246
September11th to 20thNight
46%
720
24%
1885
14%
720
52%
720
35%
1168
September21st to endNight
41%
720
22%
1980
14%
720
48%
720
28%
1330
October1st to 10thNight
35%
720
15%
2070
10%
720
40%
720
19%
1476
October11th to 20thNight
31%
720
16%
2160
9%
720
41%
720
20%
1500
October21st to endNight
22%
792
12%
2569
7%
792
30%
792
15%
1881
November1st to 10thNight
27%
740
18%
2407
11%
740
37%
740
20%
1597
November11th to 20thNight
25%
760
17%
2470
10%
760
34%
760
17%
1709
November21st to endNight
25%
760
13%
2607
7%
760
34%
760
14%
1841
December1st to 10thNight
14%
760
10%
2660
4%
760
22%
760
10%
2019
December11th to 20thNight
20%
760
10%
3255
3%
759
28%
759
8%
2374
December21st to endNight
14%
827
6%
4845
2%
817
24%
817
5%
3745

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
16%8%36%6%35%5%6%7%34%
74763
Night
17%9%47%6%35%5%5%7%33%
74942
TransparencyDay
48%8%12%19%11%1
74468
Night
50%7%12%18%11%1
74771
SeeingDay
22%16%28%21%10%3
13708
Night
22%14%30%25%8%1
22877

By Month

Percentage Cloud Cover NE (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
14%11%5%9%7%46%6%5%7%26%
1517
11th to 20thDay
18%13%38%7%47%6%6%6%24%
1448
21st to endDay
13%10%5%8%8%46%5%6%7%27%
1633
February1st to 10thDay
17%12%47%8%46%6%6%7%23%
1620
11th to 20thDay
13%14%49%7%45%5%5%7%27%
1730
21st to endDay
19%11%5%9%7%35%5%45%27%
1480
March1st to 10thDay
15%9%5%8%6%35%5%5%7%32%
1797
11th to 20thDay
17%11%47%6%45%35%6%33%
1867
21st to endDay
20%9%47%5%35%445%35%
2374
April1st to 10thDay
19%9%47%6%35%447%33%
2160
11th to 20thDay
16%8%46%6%35%5%5%6%37%
2134
21st to endDay
22%8%36%6%35%45%7%31%
2246
May1st to 10thDay
21%9%37%6%445%5%6%31%
2375
11th to 20thDay
18%6%34435%5%6%8%38%
2478
21st to endDay
13%5%36%5%36%5%6%9%39%
2725
June1st to 10thDay
15%6%36%6%46%6%7%8%33%
2520
11th to 20thDay
17%6%36%435%6%7%11%33%
2520
21st to endDay
15%6%38%5%35%6%6%9%34%
2520
July1st to 10thDay
12%5%35%6%45%6%7%9%38%
2520
11th to 20thDay
11%5%35%6%45%5%7%9%40%
2520
21st to endDay
12%5%36%6%46%6%7%9%36%
2744
August1st to 10thDay
9%5%26%6%45%6%7%9%39%
2498
11th to 20thDay
11%7%26%7%46%6%7%9%35%
2130
21st to endDay
13%5%24435%5%7%9%42%
2376
September1st to 10thDay
14%5%25%435%5%6%9%42%
2160
11th to 20thDay
14%6%24434448%47%
2073
21st to endDay
16%5%35%4345%6%7%42%
1980
October1st to 10thDay
20%9%38%5%3444435%
1886
11th to 20thDay
15%7%37%5%25%5%6%8%37%
1800
21st to endDay
25%9%45%6%345%5%7%27%
1785
November1st to 10thDay
15%7%35%4345%5%7%42%
1671
11th to 20thDay
17%8%38%5%35%5%5%6%36%
1710
21st to endDay
18%10%46%6%46%46%6%32%
1568
December1st to 10thDay
19%12%6%8%7%36%446%24%
1520
11th to 20thDay
23%13%46%5%23346%31%
1906
21st to endDay
24%15%6%9%6%35%44421%
2772
January1st to 10thNight
16%13%48%7%45%45%6%28%
2649
11th to 20thNight
18%15%48%7%45%447%24%
2541
21st to endNight
13%12%6%9%6%5%5%5%5%6%29%
2722
February1st to 10thNight
17%11%7%9%7%45%5%5%6%24%
2340
11th to 20thNight
13%11%6%9%8%45%5%5%6%27%
2116
21st to endNight
17%12%5%9%7%36%45%6%27%
1776
March1st to 10thNight
17%11%5%8%6%35%45%5%31%
2150
11th to 20thNight
18%10%37%8%44446%31%
2090
21st to endNight
18%9%36%6%34446%36%
1981
April1st to 10thNight
18%11%47%435%5%5%7%31%
1800
11th to 20thNight
21%6%37%5%4435%7%34%
1775
21st to endNight
20%7%45%5%35%5%5%6%35%
1712
May1st to 10thNight
21%11%47%6%34445%32%
1601
11th to 20thNight
17%6%46%6%45%45%7%34%
1415
21st to endNight
12%5%36%6%45%6%7%9%37%
1550
June1st to 10thNight
17%6%35%5%35%5%5%7%38%
1440
11th to 20thNight
17%5%25%445%47%8%39%
1440
21st to endNight
12%425%5%35%6%6%9%43%
1440
July1st to 10thNight
11%436%6%45%5%8%9%40%
1440
11th to 20thNight
14%426%5%36%6%6%8%41%
1440
21st to endNight
11%6%345%35%5%6%10%43%
1574
August1st to 10thNight
10%5%25%5%346%6%9%44%
1432
11th to 20thNight
12%7%37%6%45%5%7%7%38%
1758
21st to endNight
14%6%36%42446%8%43%
1980
September1st to 10thNight
14%8%36%4245%5%7%42%
1800
11th to 20thNight
15%5%23424448%50%
1885
21st to endNight
15%6%25%6%335%6%7%43%
1980
October1st to 10thNight
19%8%36%6%35%446%35%
2070
11th to 20thNight
16%6%36%6%35%5%5%7%38%
2160
21st to endNight
24%9%36%5%345%6%7%29%
2569
November1st to 10thNight
16%9%36%5%3445%5%40%
2407
11th to 20thNight
17%9%37%6%36%5%5%6%34%
2470
21st to endNight
17%10%47%5%45%45%6%33%
2607
December1st to 10thNight
21%12%5%7%6%35%5%5%6%25%
2660
11th to 20thNight
23%11%47%6%3445%6%26%
3288
21st to endNight
26%15%6%7%5%3444422%
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%7%13%17%8%
1518
11th to 20thDay
59%9%12%12%7%1
1448
21st to endDay
54%7%15%15%8%1
1633
February1st to 10thDay
59%8%13%13%7%1
1620
11th to 20thDay
56%7%11%17%8%1
1739
21st to endDay
58%6%10%14%10%1
1480
March1st to 10thDay
50%8%14%17%10%2
1800
11th to 20thDay
52%6%11%16%12%3
1867
21st to endDay
52%5%8%15%15%3
2374
April1st to 10thDay
52%5%9%20%11%2
2160
11th to 20thDay
48%6%10%19%15%2
2134
21st to endDay
53%6%9%16%13%3
2246
May1st to 10thDay
54%6%8%18%11%2
2375
11th to 20thDay
43%8%11%22%15%2
2482
21st to endDay
42%10%14%22%11%2
2725
June1st to 10thDay
47%10%14%19%10%1
2520
11th to 20thDay
45%9%13%20%12%1
2520
21st to endDay
45%9%14%20%11%1
2520
July1st to 10thDay
41%10%12%21%14%2
2450
11th to 20thDay
39%10%15%25%11%
2445
21st to endDay
41%10%15%20%12%1
2591
August1st to 10thDay
39%11%15%23%11%1
2484
11th to 20thDay
42%11%16%22%8%1
2130
21st to endDay
37%10%14%25%13%1
2376
September1st to 10thDay
39%8%14%24%14%1
2160
11th to 20thDay
36%7%12%23%21%1
2073
21st to endDay
39%8%12%25%14%2
1980
October1st to 10thDay
50%7%10%19%12%2
1886
11th to 20thDay
44%8%16%23%8%1
1800
21st to endDay
56%7%13%13%10%1
1785
November1st to 10thDay
41%7%15%24%12%1
1671
11th to 20thDay
50%6%11%18%13%1
1710
21st to endDay
51%7%16%17%6%1
1568
December1st to 10thDay
61%8%11%13%7%1
1520
11th to 20thDay
56%5%15%16%7%1
1906
21st to endDay
66%6%9%14%5%
2772
January1st to 10thNight
56%6%13%13%10%1
2649
11th to 20thNight
61%6%12%13%7%1
2541
21st to endNight
55%5%12%16%9%2
2722
February1st to 10thNight
60%7%11%13%7%1
2340
11th to 20thNight
56%6%13%15%8%1
2118
21st to endNight
57%7%12%15%9%2
1776
March1st to 10thNight
54%6%12%18%10%1
2160
11th to 20thNight
56%6%10%18%9%2
2091
21st to endNight
50%5%9%17%15%4
1981
April1st to 10thNight
52%6%11%19%10%2
1800
11th to 20thNight
49%6%10%19%14%1
1775
21st to endNight
48%9%10%17%13%4
1712
May1st to 10thNight
55%6%9%15%13%2
1601
11th to 20thNight
48%6%11%19%13%3
1416
21st to endNight
42%9%14%21%13%2
1551
June1st to 10thNight
45%7%12%23%12%1
1440
11th to 20thNight
42%9%15%22%12%
1440
21st to endNight
38%9%16%24%13%1
1440
July1st to 10thNight
38%9%14%23%14%2
1400
11th to 20thNight
38%10%15%27%10%
1392
21st to endNight
36%9%15%25%13%2
1486
August1st to 10thNight
34%10%17%28%10%1
1422
11th to 20thNight
41%10%15%23%10%
1758
21st to endNight
37%8%13%26%15%1
1980
September1st to 10thNight
42%7%11%25%14%1
1800
11th to 20thNight
33%6%13%25%22%1
1885
21st to endNight
40%7%10%24%17%1
1980
October1st to 10thNight
50%6%12%19%12%1
2070
11th to 20thNight
45%9%13%21%10%1
2160
21st to endNight
54%7%13%16%9%1
2569
November1st to 10thNight
44%7%12%19%16%2
2407
11th to 20thNight
50%6%13%18%11%2
2470
21st to endNight
51%7%15%18%8%
2607
December1st to 10thNight
59%8%13%15%6%
2660
11th to 20thNight
57%7%15%13%8%
3288
21st to endNight
64%7%10%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
24%28%27%16%41
180
11th to 20thDay
33%27%23%12%41
162
21st to endDay
22%25%31%15%6%1
198
February1st to 10thDay
35%26%30%8%1
240
11th to 20thDay
27%29%31%11%2
398
21st to endDay
30%23%29%16%2
346
March1st to 10thDay
25%21%33%17%3
420
11th to 20thDay
29%22%30%15%4
420
21st to endDay
32%21%24%14%6%2
462
April1st to 10thDay
31%22%22%18%5%1
420
11th to 20thDay
25%24%27%16%7%1
412
21st to endDay
30%17%29%18%42
420
May1st to 10thDay
30%16%30%14%9%2
423
11th to 20thDay
20%17%34%22%6%2
447
21st to endDay
18%11%29%26%11%4
485
June1st to 10thDay
16%14%34%21%13%2
450
11th to 20thDay
18%12%29%26%13%2
450
21st to endDay
16%14%31%24%12%3
450
July1st to 10thDay
13%9%27%31%15%5%
445
11th to 20thDay
14%6%29%32%17%3
444
21st to endDay
13%7%31%31%15%2
488
August1st to 10thDay
13%7%25%32%17%6%
446
11th to 20thDay
15%5%26%29%17%7%
439
21st to endDay
12%12%26%27%17%6%
495
September1st to 10thDay
18%10%22%24%17%9%
450
11th to 20thDay
17%11%24%22%15%12%
450
21st to endDay
19%11%25%24%14%6%
450
October1st to 10thDay
24%13%24%20%13%6%
450
11th to 20thDay
19%18%28%21%12%2
450
21st to endDay
31%16%25%22%6%1
397
November1st to 10thDay
23%16%24%22%13%2
365
11th to 20thDay
26%21%30%16%6%1
370
21st to endDay
26%21%27%21%4
228
December1st to 10thDay
24%24%34%11%6%1
180
11th to 20thDay
29%23%31%16%2
180
21st to endDay
26%27%30%14%21
198
January1st to 10thNight
22%19%37%19%2
760
11th to 20thNight
28%19%29%21%2
727
21st to endNight
20%21%36%21%3
792
February1st to 10thNight
26%19%36%17%2
720
11th to 20thNight
22%22%37%19%1
703
21st to endNight
27%19%33%18%3
592
March1st to 10thNight
26%21%34%17%2
720
11th to 20thNight
26%22%32%18%2
720
21st to endNight
24%21%29%20%5%
792
April1st to 10thNight
26%19%29%23%3
720
11th to 20thNight
24%18%31%20%6%
711
21st to endNight
24%18%31%22%5%
632
May1st to 10thNight
26%16%28%23%7%
521
11th to 20thNight
19%14%35%26%6%1
354
21st to endNight
15%9%30%37%9%1
389
June1st to 10thNight
19%6%32%30%12%1
360
11th to 20thNight
18%7%26%37%13%
360
21st to endNight
13%5%29%41%12%1
360
July1st to 10thNight
13%5%25%42%14%1
350
11th to 20thNight
13%424%40%18%1
348
21st to endNight
13%5%21%48%13%1
394
August1st to 10thNight
12%222%46%16%1
358
11th to 20thNight
14%5%22%36%19%4
694
21st to endNight
15%5%23%35%19%2
792
September1st to 10thNight
17%6%27%33%15%2
720
11th to 20thNight
19%6%25%28%17%4
720
21st to endNight
17%8%28%31%14%2
720
October1st to 10thNight
24%9%24%29%12%2
720
11th to 20thNight
20%13%30%29%9%
720
21st to endNight
31%13%30%22%5%
792
November1st to 10thNight
21%15%32%24%8%
740
11th to 20thNight
23%15%33%21%7%1
760
21st to endNight
23%17%31%24%5%
760
December1st to 10thNight
26%20%34%16%4
760
11th to 20thNight
28%17%29%23%3
760
21st to endNight
31%17%32%18%2
836
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