Horizons Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Horizons Observatory clear sky chart from 2005-11-06 to 2024-07-31.

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
31%
23345
13%
89480
9%
23192
39%
23192
14%
61801

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
21%
752
13%
4378
6%
737
31%
737
8%
2849
January11th to 20thNight
21%
758
10%
4392
4%
740
30%
740
5%
3245
January21st to endNight
22%
830
8%
4872
6%
814
33%
814
7%
3748
February1st to 10thNight
16%
760
13%
2492
4%
760
28%
760
12%
1829
February11th to 20thNight
18%
743
14%
2220
4%
740
31%
740
14%
1591
February21st to endNight
22%
628
15%
1884
6%
628
34%
628
15%
1330
March1st to 10thNight
22%
756
13%
3712
8%
744
33%
744
9%
2649
March11th to 20thNight
22%
755
14%
3696
6%
740
31%
740
9%
2482
March21st to endNight
25%
833
13%
3391
6%
814
35%
814
12%
2331
April1st to 10thNight
25%
760
17%
1900
5%
760
31%
760
18%
1398
April11th to 20thNight
26%
751
14%
1875
5%
751
31%
751
19%
1368
April21st to endNight
24%
589
16%
1729
6%
589
33%
589
18%
1221
May1st to 10thNight
26%
570
11%
2825
8%
566
31%
566
9%
2026
May11th to 20thNight
32%
387
10%
2496
14%
382
43%
382
8%
1755
May21st to endNight
42%
408
10%
2721
13%
399
49%
399
11%
1518
June1st to 10thNight
37%
380
12%
1520
11%
380
47%
380
14%
1020
June11th to 20thNight
42%
380
11%
1520
12%
380
49%
380
17%
975
June21st to endNight
43%
380
11%
1520
12%
380
49%
380
16%
1001
July1st to 10thNight
53%
370
13%
1480
19%
370
58%
370
22%
896
July11th to 20thNight
54%
368
13%
1472
18%
368
59%
368
24%
863
July21st to endNight
53%
416
16%
1574
20%
394
59%
394
24%
915
August1st to 10thNight
58%
380
17%
1444
18%
378
61%
378
24%
853
August11th to 20thNight
49%
598
16%
1662
12%
598
51%
598
28%
1093
August21st to endNight
52%
792
19%
1980
9%
792
54%
792
32%
1299
September1st to 10thNight
49%
720
17%
1800
8%
720
54%
720
35%
1152
September11th to 20thNight
50%
720
23%
1928
15%
720
54%
720
33%
1197
September21st to endNight
45%
720
23%
1980
16%
720
51%
720
30%
1256
October1st to 10thNight
39%
720
15%
2010
10%
720
45%
720
19%
1411
October11th to 20thNight
38%
720
15%
2222
9%
720
45%
720
20%
1521
October21st to endNight
27%
792
12%
2574
7%
792
35%
792
13%
1898
November1st to 10thNight
32%
740
17%
2407
12%
740
41%
740
18%
1604
November11th to 20thNight
30%
760
20%
2470
13%
760
41%
760
18%
1653
November21st to endNight
28%
760
14%
2579
8%
760
38%
760
15%
1805
December1st to 10thNight
19%
760
10%
2660
5%
760
26%
760
9%
2046
December11th to 20thNight
22%
760
11%
3243
5%
759
30%
759
9%
2316
December21st to endNight
16%
829
7%
4852
3%
817
25%
817
6%
3687

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
15%9%38%5%5%5%46%7%33%
89834
Night
15%9%38%5%5%5%46%7%34%
89999
TransparencyDay
49%8%13%19%11%1
89553
Night
49%8%13%19%10%1
89834
SeeingDay
24%17%29%19%9%3
14453
Night
21%14%28%28%9%1
23388

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
10%11%5%9%7%6%5%5%6%5%32%
2517
11th to 20thDay
14%11%412%8%6%5%35%6%25%
2520
21st to endDay
13%14%7%11%6%5%5%45%6%23%
2772
February1st to 10thDay
11%12%310%6%7%5%46%7%29%
1683
11th to 20thDay
9%9%5%13%7%5%6%46%6%30%
1848
21st to endDay
15%12%48%6%5%4446%31%
1570
March1st to 10thDay
18%15%5%10%5%45%44425%
3116
11th to 20thDay
13%10%5%10%6%6%6%5%6%6%28%
3246
21st to endDay
16%12%39%6%5%4345%32%
4161
April1st to 10thDay
18%10%39%5%4435%6%34%
2280
11th to 20thDay
17%8%27%6%45%46%6%35%
2254
21st to endDay
21%10%38%5%4445%6%31%
2451
May1st to 10thDay
21%11%49%5%45%45%6%27%
4106
11th to 20thDay
17%8%38%5%5%6%5%7%7%30%
4396
21st to endDay
12%5%26%45%6%47%9%42%
4837
June1st to 10thDay
18%7%38%5%5%6%46%8%29%
2660
11th to 20thDay
16%6%26%5%5%5%46%9%35%
2660
21st to endDay
18%8%38%6%6%5%5%6%7%29%
2660
July1st to 10thDay
15%7%26%5%5%5%46%8%38%
2660
11th to 20thDay
10%6%36%6%6%7%5%7%9%35%
2660
21st to endDay
13%6%26%5%5%5%48%8%36%
2887
August1st to 10thDay
11%6%38%5%6%7%5%7%9%35%
2466
11th to 20thDay
12%5%39%6%5%5%5%8%9%35%
2235
21st to endDay
11%5%26%5%5%6%45%9%42%
2376
September1st to 10thDay
13%5%16%5%45%46%9%43%
2160
11th to 20thDay
13%6%25%44435%6%48%
2030
21st to endDay
15%6%25%44435%8%44%
1980
October1st to 10thDay
22%6%37%5%5%436%5%35%
1948
11th to 20thDay
13%9%38%5%6%436%7%37%
1738
21st to endDay
23%10%38%45%435%6%29%
1782
November1st to 10thDay
12%8%36%44447%7%39%
1671
11th to 20thDay
15%9%37%5%5%45%5%7%36%
1710
21st to endDay
17%10%39%5%4446%7%31%
1596
December1st to 10thDay
16%13%411%6%5%5%47%6%24%
1520
11th to 20thDay
20%12%37%45%5%446%31%
1906
21st to endDay
20%15%310%6%5%5%345%24%
2772
January1st to 10thNight
15%9%37%5%45%46%7%35%
4429
11th to 20thNight
11%12%5%13%7%5%5%46%8%25%
4440
21st to endNight
16%13%5%11%6%6%5%45%424%
4884
February1st to 10thNight
11%10%410%6%6%6%5%7%8%28%
2492
11th to 20thNight
9%8%49%7%7%6%5%6%8%30%
2218
21st to endNight
13%10%38%5%5%5%5%6%6%34%
1884
March1st to 10thNight
21%12%27%6%4446%5%28%
3724
11th to 20thNight
12%10%49%6%6%6%45%6%33%
3713
21st to endNight
16%12%39%5%4435%5%34%
3466
April1st to 10thNight
16%8%38%5%6%5%45%7%34%
1900
11th to 20thNight
16%9%37%5%5%445%7%36%
1875
21st to endNight
20%8%37%4435%6%6%34%
1729
May1st to 10thNight
23%12%36%5%4445%7%26%
2834
11th to 20thNight
16%9%310%6%5%446%6%30%
2513
21st to endNight
13%427%445%5%7%9%40%
2738
June1st to 10thNight
17%6%27%5%5%5%5%6%8%35%
1520
11th to 20thNight
18%5%26%44445%7%40%
1520
21st to endNight
16%6%26%5%5%5%47%8%36%
1520
July1st to 10thNight
14%5%25%5%45%46%8%43%
1520
11th to 20thNight
13%5%25%44347%9%44%
1520
21st to endNight
12%5%25%5%4447%8%45%
1662
August1st to 10thNight
11%426%5%45%36%8%46%
1454
11th to 20thNight
12%5%27%5%5%5%36%8%42%
1662
21st to endNight
13%6%25%43436%8%46%
1980
September1st to 10thNight
12%6%25%33436%8%49%
1800
11th to 20thNight
12%6%26%343447%50%
1928
21st to endNight
14%6%2443436%7%47%
1980
October1st to 10thNight
20%7%27%5%4446%7%36%
2010
11th to 20thNight
15%7%26%5%5%6%5%5%8%36%
2222
21st to endNight
21%9%27%5%45%47%6%31%
2574
November1st to 10thNight
13%8%27%46%5%46%5%39%
2407
11th to 20thNight
15%8%27%45%5%46%7%37%
2470
21st to endNight
13%9%37%6%5%437%7%34%
2579
December1st to 10thNight
16%12%411%6%6%5%45%7%24%
2660
11th to 20thNight
18%9%49%45%5%45%7%30%
3288
21st to endNight
20%12%412%6%5%435%5%23%
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
48%10%14%23%5%
2518
11th to 20thDay
60%8%10%16%5%1
2520
21st to endDay
60%9%11%14%6%1
2772
February1st to 10thDay
54%7%14%16%8%1
1683
11th to 20thDay
53%8%12%17%9%2
1857
21st to endDay
53%6%12%15%12%1
1570
March1st to 10thDay
62%6%9%11%10%2
3119
11th to 20thDay
50%9%16%13%10%2
3246
21st to endDay
52%5%9%13%18%3
4161
April1st to 10thDay
51%6%10%20%11%3
2280
11th to 20thDay
47%7%11%18%15%1
2254
21st to endDay
54%6%10%17%12%2
2451
May1st to 10thDay
57%7%10%18%7%1
4120
11th to 20thDay
49%10%15%15%9%1
4400
21st to endDay
36%9%14%26%13%1
4837
June1st to 10thDay
51%9%14%17%9%
2660
11th to 20thDay
45%7%13%19%14%2
2660
21st to endDay
52%9%11%17%10%1
2660
July1st to 10thDay
42%8%12%22%15%1
2590
11th to 20thDay
42%10%15%24%9%1
2585
21st to endDay
40%10%15%23%11%1
2734
August1st to 10thDay
41%11%16%22%10%
2452
11th to 20thDay
44%9%15%22%9%
2235
21st to endDay
38%8%15%26%13%1
2376
September1st to 10thDay
37%8%13%27%14%1
2160
11th to 20thDay
37%6%10%28%18%2
2030
21st to endDay
37%7%15%26%13%2
1980
October1st to 10thDay
49%8%11%18%13%2
1948
11th to 20thDay
44%7%15%25%7%1
1738
21st to endDay
57%8%10%16%8%2
1782
November1st to 10thDay
42%8%14%23%13%
1671
11th to 20thDay
47%8%13%17%13%2
1710
21st to endDay
52%7%18%16%6%1
1596
December1st to 10thDay
60%8%14%12%5%1
1520
11th to 20thDay
55%6%12%16%9%1
1906
21st to endDay
64%7%9%12%8%
2772
January1st to 10thNight
48%10%16%18%8%1
4429
11th to 20thNight
56%11%13%12%7%1
4440
21st to endNight
60%9%10%13%7%1
4884
February1st to 10thNight
51%10%13%16%8%1
2492
11th to 20thNight
49%9%13%19%10%1
2220
21st to endNight
48%8%14%20%8%3
1884
March1st to 10thNight
56%6%12%16%9%1
3734
11th to 20thNight
51%8%12%17%10%1
3714
21st to endNight
51%5%9%14%18%3
3466
April1st to 10thNight
50%6%10%20%12%2
1900
11th to 20thNight
47%7%10%21%13%1
1875
21st to endNight
48%7%11%18%13%4
1729
May1st to 10thNight
55%9%11%16%9%1
2840
11th to 20thNight
53%10%12%16%8%1
2514
21st to endNight
38%11%17%20%13%2
2739
June1st to 10thNight
45%11%14%18%11%1
1520
11th to 20thNight
42%10%15%19%13%2
1520
21st to endNight
45%9%13%22%10%1
1520
July1st to 10thNight
38%9%18%21%13%1
1480
11th to 20thNight
35%9%18%27%11%
1472
21st to endNight
36%8%16%28%11%1
1574
August1st to 10thNight
35%9%15%28%12%
1444
11th to 20thNight
39%8%17%25%11%
1662
21st to endNight
35%8%14%28%14%
1980
September1st to 10thNight
33%8%17%29%13%1
1800
11th to 20thNight
33%7%12%29%18%1
1928
21st to endNight
36%7%13%27%15%2
1980
October1st to 10thNight
47%7%12%21%10%2
2010
11th to 20thNight
45%9%15%23%8%1
2222
21st to endNight
53%9%12%17%8%1
2574
November1st to 10thNight
44%8%14%21%11%2
2407
11th to 20thNight
45%7%14%20%12%2
2470
21st to endNight
48%9%16%19%7%
2579
December1st to 10thNight
59%9%13%15%4
2660
11th to 20thNight
53%8%14%14%11%1
3288
21st to endNight
62%7%10%16%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
27%21%28%18%6%1
180
11th to 20thDay
33%22%28%13%21
180
21st to endDay
20%26%32%15%7%1
198
February1st to 10thDay
30%28%31%9%2
268
11th to 20thDay
27%25%31%14%2
456
21st to endDay
29%20%31%16%3
382
March1st to 10thDay
29%16%34%18%3
460
11th to 20thDay
32%25%27%12%41
460
21st to endDay
32%21%25%13%7%3
506
April1st to 10thDay
36%17%26%12%7%2
460
11th to 20thDay
33%21%25%12%9%1
452
21st to endDay
31%21%28%12%6%2
460
May1st to 10thDay
32%21%27%13%6%2
463
11th to 20thDay
19%19%34%20%6%2
487
21st to endDay
19%16%31%24%8%1
529
June1st to 10thDay
21%16%38%16%8%1
490
11th to 20thDay
19%15%31%22%10%2
490
21st to endDay
22%18%31%18%9%2
490
July1st to 10thDay
15%12%34%24%12%4
485
11th to 20thDay
13%9%35%27%12%3
484
21st to endDay
14%8%39%25%11%2
529
August1st to 10thDay
13%10%32%23%19%4
446
11th to 20thDay
15%10%28%26%14%7%
439
21st to endDay
13%16%25%26%14%6%
495
September1st to 10thDay
16%12%24%24%16%7%
450
11th to 20thDay
16%14%26%17%17%10%
450
21st to endDay
18%13%27%22%12%6%
450
October1st to 10thDay
25%14%23%18%13%5%
450
11th to 20thDay
19%19%25%24%12%2
419
21st to endDay
36%13%23%20%8%
396
November1st to 10thDay
22%17%23%24%13%1
365
11th to 20thDay
29%16%28%18%8%1
370
21st to endDay
30%18%27%19%41
256
December1st to 10thDay
28%22%29%14%6%1
180
11th to 20thDay
34%21%32%8%5%
180
21st to endDay
32%20%26%17%41
198
January1st to 10thNight
21%16%36%24%3
760
11th to 20thNight
24%17%31%25%3
760
21st to endNight
18%18%34%26%4
836
February1st to 10thNight
18%21%35%23%2
760
11th to 20thNight
20%17%37%23%3
743
21st to endNight
22%19%32%23%3
628
March1st to 10thNight
23%16%32%25%3
760
11th to 20thNight
23%20%29%23%6%
760
21st to endNight
26%15%29%22%7%1
836
April1st to 10thNight
23%20%27%25%5%
760
11th to 20thNight
21%25%26%21%8%
751
21st to endNight
27%15%29%22%6%
589
May1st to 10thNight
27%14%28%24%8%
570
11th to 20thNight
16%15%29%33%7%1
392
21st to endNight
15%9%28%36%11%
411
June1st to 10thNight
18%10%28%34%9%
380
11th to 20thNight
19%9%24%35%13%
380
21st to endNight
17%6%26%37%12%1
380
July1st to 10thNight
15%5%20%45%15%1
370
11th to 20thNight
15%323%39%19%1
368
21st to endNight
13%5%20%44%17%1
416
August1st to 10thNight
12%417%43%23%1
380
11th to 20thNight
14%6%23%35%19%3
598
21st to endNight
14%6%19%37%22%2
792
September1st to 10thNight
15%7%24%32%20%2
720
11th to 20thNight
16%8%21%30%21%4
720
21st to endNight
17%8%24%32%18%2
720
October1st to 10thNight
25%12%17%28%16%2
720
11th to 20thNight
20%12%23%31%12%1
720
21st to endNight
28%14%25%26%7%
792
November1st to 10thNight
19%16%27%29%9%
740
11th to 20thNight
21%15%29%26%8%1
760
21st to endNight
22%18%26%27%6%1
760
December1st to 10thNight
25%16%33%21%4
760
11th to 20thNight
24%18%30%24%4
760
21st to endNight
27%16%33%20%3
836
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