HRH Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the HRH 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
64%
24446
20%
74522
17%
24394
67%
24394
30%
46495

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
51%
758
21%
2459
17%
757
59%
757
28%
1472
January11th to 20thNight
56%
727
24%
2360
21%
727
62%
727
32%
1351
January21st to endNight
46%
792
23%
2390
17%
792
53%
792
28%
1496
February1st to 10thNight
48%
720
23%
2160
17%
720
54%
720
31%
1311
February11th to 20thNight
56%
700
24%
2099
18%
700
63%
700
38%
1186
February21st to endNight
63%
592
28%
1721
21%
592
69%
592
37%
1001
March1st to 10thNight
56%
720
32%
1980
21%
720
66%
720
51%
1027
March11th to 20thNight
61%
720
28%
1980
19%
720
68%
720
42%
1123
March21st to endNight
60%
792
30%
2178
22%
792
65%
792
44%
1209
April1st to 10thNight
63%
720
23%
1980
18%
720
66%
720
44%
1123
April11th to 20thNight
60%
711
20%
1953
15%
711
64%
711
38%
1161
April21st to endNight
65%
720
29%
1854
20%
720
68%
720
47%
1062
May1st to 10thNight
79%
547
32%
1627
27%
547
82%
547
54%
789
May11th to 20thNight
71%
531
18%
1592
15%
531
73%
531
33%
978
May21st to endNight
77%
582
23%
1745
21%
582
77%
582
35%
1043
June1st to 10thNight
71%
540
15%
1620
14%
540
71%
540
26%
1093
June11th to 20thNight
80%
540
11%
1620
10%
540
81%
540
29%
1046
June21st to endNight
77%
540
9%
1620
8%
540
77%
540
24%
1131
July1st to 10thNight
76%
525
5%
1575
5%
525
76%
525
21%
1138
July11th to 20thNight
79%
522
4%
1566
5%
522
79%
522
19%
1179
July21st to endNight
76%
591
8%
1672
8%
558
77%
558
22%
1210
August1st to 10thNight
78%
537
3%
1643
4%
534
79%
534
23%
1160
August11th to 20thNight
76%
534
5%
1776
6%
534
76%
534
19%
1254
August21st to endNight
64%
783
8%
2169
8%
783
64%
783
16%
1671
September1st to 10thNight
72%
720
7%
1980
8%
720
72%
720
20%
1440
September11th to 20thNight
65%
720
8%
1980
8%
720
65%
720
19%
1469
September21st to endNight
64%
720
14%
1980
14%
720
64%
720
21%
1429
October1st to 10thNight
66%
720
20%
1980
19%
720
68%
720
29%
1297
October11th to 20thNight
66%
720
26%
2107
27%
720
69%
720
35%
1268
October21st to endNight
62%
792
26%
2376
25%
792
67%
792
33%
1413
November1st to 10thNight
60%
740
25%
2221
22%
740
66%
740
36%
1299
November11th to 20thNight
59%
760
28%
2280
25%
760
65%
760
33%
1357
November21st to endNight
56%
760
26%
2403
24%
760
62%
760
32%
1401
December1st to 10thNight
60%
760
26%
2470
23%
760
65%
760
32%
1428
December11th to 20thNight
57%
759
24%
2939
22%
758
61%
758
25%
1674
December21st to endNight
55%
831
15%
4467
24%
817
60%
817
16%
2806

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
7%5%26%6%46%7%8%10%40%
74815
Night
7%5%25%6%46%6%7%10%43%
74897
TransparencyDay
36%11%17%18%13%5%
74538
Night
33%10%17%20%14%5%
74693
SeeingDay
7%416%26%27%20%
14354
Night
8%316%34%30%10%
24459

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
9%6%25%4246%7%11%43%
1706
11th to 20thDay
6%5%24436%6%8%9%49%
1629
21st to endDay
11%425%5%35%5%6%8%46%
1962
February1st to 10thDay
10%5%35%445%5%7%8%46%
1800
11th to 20thDay
5%4245%2446%8%57%
1748
21st to endDay
6%424435%45%8%54%
1534
March1st to 10thDay
4313435%46%8%60%
1980
11th to 20thDay
6%3134245%7%9%56%
1980
21st to endDay
6%41442445%9%56%
2178
April1st to 10thDay
5%224425%5%8%10%52%
1980
11th to 20thDay
6%425%5%36%5%7%10%47%
1956
21st to endDay
4313335%6%7%9%56%
2110
May1st to 10thDay
331332345%10%62%
2340
11th to 20thDay
7%4245%46%7%9%10%41%
2301
21st to endDay
5%5%36%6%5%7%7%7%10%40%
2531
June1st to 10thDay
9%7%37%8%47%7%9%13%27%
2340
11th to 20thDay
5%426%7%5%9%8%10%12%29%
2340
21st to endDay
7%6%37%7%5%8%10%10%11%27%
2340
July1st to 10thDay
8%5%36%7%5%9%9%11%13%24%
2340
11th to 20thDay
5%5%38%9%7%9%8%10%13%23%
2340
21st to endDay
6%5%38%9%6%10%9%10%13%22%
2547
August1st to 10thDay
5%427%8%7%10%9%10%12%25%
2277
11th to 20thDay
5%5%28%8%7%11%11%11%11%22%
2112
21st to endDay
9%5%38%9%6%9%10%9%11%21%
2178
September1st to 10thDay
7%5%26%8%6%9%10%10%12%25%
1980
11th to 20thDay
9%6%36%7%6%9%8%9%11%25%
1980
21st to endDay
5%6%38%8%5%7%8%9%10%32%
1980
October1st to 10thDay
7%6%37%8%45%5%7%9%38%
1980
11th to 20thDay
5%5%25%5%46%5%7%10%47%
1850
21st to endDay
7%5%26%436%5%6%9%48%
1980
November1st to 10thDay
6%4245%35%5%6%8%51%
1857
11th to 20thDay
7%5%35%5%35%47%8%47%
1900
21st to endDay
7%5%245%35%6%6%9%48%
1772
December1st to 10thDay
5%425%6%35%5%6%10%47%
1710
11th to 20thDay
9%5%35%6%35%5%6%7%45%
2153
21st to endDay
10%8%48%8%46%5%5%7%36%
3124
January1st to 10thNight
9%5%35%5%46%5%7%10%42%
2459
11th to 20thNight
7%5%35%5%345%6%10%46%
2360
21st to endNight
11%5%36%5%35%5%6%8%43%
2390
February1st to 10thNight
9%6%36%5%35%5%5%8%47%
2160
11th to 20thNight
8%4245%3445%7%53%
2098
21st to endNight
5%5%24436%6%7%9%50%
1721
March1st to 10thNight
321433346%9%61%
1980
11th to 20thNight
4424435%5%6%10%52%
1980
21st to endNight
5%32442445%7%58%
2178
April1st to 10thNight
5%3145%3447%9%54%
1980
11th to 20thNight
6%4245%35%6%8%9%48%
1953
21st to endNight
43244345%6%9%56%
1854
May1st to 10thNight
221232446%9%64%
1627
11th to 20thNight
5%5%26%5%37%5%6%9%47%
1592
21st to endNight
4425%6%25%6%6%9%51%
1744
June1st to 10thNight
9%5%27%6%46%6%6%10%38%
1620
11th to 20thNight
5%325%6%47%7%9%13%39%
1620
21st to endNight
5%5%36%6%45%7%10%15%34%
1620
July1st to 10thNight
7%5%35%6%5%6%7%9%13%34%
1620
11th to 20thNight
4436%7%5%8%9%10%15%30%
1620
21st to endNight
4436%7%5%8%9%10%13%31%
1771
August1st to 10thNight
4435%6%47%8%10%15%33%
1654
11th to 20thNight
5%5%36%7%48%9%10%15%30%
1776
21st to endNight
9%6%37%8%5%7%7%8%12%27%
2169
September1st to 10thNight
45%36%7%5%7%8%11%15%29%
1980
11th to 20thNight
9%6%27%9%5%7%7%8%11%29%
1980
21st to endNight
7%7%38%8%46%7%8%11%30%
1980
October1st to 10thNight
6%5%37%6%36%6%8%10%40%
1980
11th to 20thNight
7%5%36%5%345%6%8%49%
2107
21st to endNight
7%5%26%5%35%5%6%9%47%
2376
November1st to 10thNight
6%326%6%45%45%9%50%
2221
11th to 20thNight
7%5%35%5%35%45%9%48%
2280
21st to endNight
7%435%5%35%5%5%10%47%
2403
December1st to 10thNight
6%425%6%46%5%6%9%47%
2470
11th to 20thNight
10%5%35%5%35%45%8%46%
3042
21st to endNight
12%7%37%6%46%6%6%9%35%
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
34%6%13%24%16%7%
1707
11th to 20thDay
30%6%10%26%21%8%
1629
21st to endDay
36%7%9%19%21%8%
1962
February1st to 10thDay
35%8%11%20%17%10%
1800
11th to 20thDay
25%5%12%22%25%10%
1758
21st to endDay
29%5%14%18%26%8%
1534
March1st to 10thDay
22%5%12%26%25%9%
1980
11th to 20thDay
24%6%13%30%19%8%
1980
21st to endDay
25%5%12%27%24%6%
2178
April1st to 10thDay
26%9%19%24%17%6%
1980
11th to 20thDay
31%9%16%17%21%5%
1956
21st to endDay
25%7%14%23%25%6%
2110
May1st to 10thDay
19%5%16%29%23%8%
2340
11th to 20thDay
34%11%18%20%16%1
2305
21st to endDay
37%10%16%21%15%1
2531
June1st to 10thDay
46%13%22%16%3
2340
11th to 20thDay
41%14%25%17%2
2340
21st to endDay
46%16%24%12%2
2340
July1st to 10thDay
47%19%26%7%2
2275
11th to 20thDay
49%18%25%8%
2271
21st to endDay
47%17%25%11%1
2405
August1st to 10thDay
45%18%30%8%
2264
11th to 20thDay
47%21%24%8%
2112
21st to endDay
50%18%22%9%1
2178
September1st to 10thDay
46%18%24%11%1
1980
11th to 20thDay
47%16%20%13%3
1980
21st to endDay
42%14%17%16%8%2
1980
October1st to 10thDay
40%11%13%16%13%7%
1980
11th to 20thDay
31%7%13%22%23%4
1850
21st to endDay
32%7%10%21%19%11%
1980
November1st to 10thDay
29%7%12%22%18%12%
1857
11th to 20thDay
34%6%11%22%19%8%
1900
21st to endDay
31%7%14%20%20%8%
1772
December1st to 10thDay
32%7%14%26%17%5%
1710
11th to 20thDay
35%6%11%19%17%11%
2150
21st to endDay
46%7%11%18%14%3
3124
January1st to 10thNight
36%6%13%22%17%6%
2459
11th to 20thNight
33%5%12%24%19%6%
2360
21st to endNight
38%6%11%20%17%7%
2390
February1st to 10thNight
34%7%11%20%20%8%
2160
11th to 20thNight
29%6%11%22%22%10%
2099
21st to endNight
29%7%14%19%25%6%
1721
March1st to 10thNight
19%6%14%30%24%8%
1980
11th to 20thNight
26%7%15%27%20%6%
1980
21st to endNight
25%6%12%26%25%6%
2178
April1st to 10thNight
24%8%20%27%15%7%
1980
11th to 20thNight
27%9%17%22%20%5%
1953
21st to endNight
24%6%14%26%25%4
1854
May1st to 10thNight
16%6%13%33%23%9%
1627
11th to 20thNight
32%9%19%24%15%1
1593
21st to endNight
28%10%17%25%19%1
1745
June1st to 10thNight
40%10%22%22%6%
1620
11th to 20thNight
32%14%31%20%4
1620
21st to endNight
34%19%27%15%5%
1620
July1st to 10thNight
38%19%33%9%1
1575
11th to 20thNight
40%20%29%10%
1566
21st to endNight
36%21%31%12%
1672
August1st to 10thNight
33%21%34%11%1
1643
11th to 20thNight
37%20%34%9%
1776
21st to endNight
47%17%23%12%2
2169
September1st to 10thNight
38%19%28%13%2
1980
11th to 20thNight
45%15%23%15%2
1980
21st to endNight
42%14%19%15%9%1
1980
October1st to 10thNight
35%12%17%17%14%7%
1980
11th to 20thNight
32%7%13%23%21%4
2107
21st to endNight
32%7%12%20%21%9%
2376
November1st to 10thNight
30%7%12%20%20%11%
2221
11th to 20thNight
33%7%12%22%19%8%
2280
21st to endNight
32%7%13%22%19%7%
2403
December1st to 10thNight
32%7%13%27%17%4
2470
11th to 20thNight
34%6%11%21%17%11%
3044
21st to endNight
43%9%14%16%14%4
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
10%10%37%30%13%1
270
11th to 20thDay
7%10%30%35%14%5%
243
21st to endDay
11%14%44%24%7%1
264
February1st to 10thDay
9%13%28%33%15%3
240
11th to 20thDay
6%5%29%37%20%4
240
21st to endDay
6%428%39%20%3
252
March1st to 10thDay
45%37%35%17%1
420
11th to 20thDay
7%6%27%32%25%2
420
21st to endDay
5%428%36%23%4
462
April1st to 10thDay
5%6%31%35%21%2
420
11th to 20thDay
8%7%22%38%21%4
412
21st to endDay
5%221%35%33%4
420
May1st to 10thDay
5%216%36%31%10%
423
11th to 20thDay
9%312%29%30%16%
447
21st to endDay
6%28%24%34%26%
485
June1st to 10thDay
11%210%23%30%23%
450
11th to 20thDay
8%15%20%31%34%
450
21st to endDay
5%1411%30%48%
450
July1st to 10thDay
6%28%29%54%
445
11th to 20thDay
5%137%36%50%
444
21st to endDay
9%2412%28%45%
488
August1st to 10thDay
7%1310%34%46%
446
11th to 20thDay
5%1310%29%52%
439
21st to endDay
9%15%16%31%38%
495
September1st to 10thDay
7%1413%32%42%
450
11th to 20thDay
7%26%19%35%30%
450
21st to endDay
5%25%18%38%32%
450
October1st to 10thDay
10%213%32%33%10%
450
11th to 20thDay
5%215%33%32%13%
450
21st to endDay
7%5%15%34%33%6%
495
November1st to 10thDay
7%418%33%32%6%
455
11th to 20thDay
11%5%22%35%22%6%
460
21st to endDay
8%5%24%35%23%5%
332
December1st to 10thDay
5%9%29%34%17%7%
270
11th to 20thDay
7%7%30%30%23%3
270
21st to endDay
5%7%28%37%20%4
297
January1st to 10thNight
10%6%26%40%17%1
760
11th to 20thNight
11%6%22%44%17%1
727
21st to endNight
11%7%29%40%13%
792
February1st to 10thNight
12%6%28%38%17%
720
11th to 20thNight
9%5%23%43%19%1
703
21st to endNight
7%421%47%21%
592
March1st to 10thNight
4431%45%15%1
720
11th to 20thNight
7%424%45%19%2
720
21st to endNight
5%326%45%20%1
792
April1st to 10thNight
5%323%47%21%1
720
11th to 20thNight
7%422%47%18%1
711
21st to endNight
5%222%47%23%1
720
May1st to 10thNight
3112%45%37%2
547
11th to 20thNight
8%111%39%37%5%
531
21st to endNight
6%16%34%45%8%
584
June1st to 10thNight
10%424%47%13%
540
11th to 20thNight
42313%52%26%
540
21st to endNight
5%1310%46%34%
540
July1st to 10thNight
7%135%45%40%
525
11th to 20thNight
5%127%40%45%
522
21st to endNight
437%47%39%
591
August1st to 10thNight
439%47%37%
537
11th to 20thNight
7%18%36%47%
534
21st to endNight
11%17%13%40%28%
783
September1st to 10thNight
6%1415%49%25%
720
11th to 20thNight
10%26%20%42%21%
720
21st to endNight
11%16%25%42%15%
720
October1st to 10thNight
8%213%40%32%6%
720
11th to 20thNight
8%212%37%37%4
720
21st to endNight
10%216%40%28%4
792
November1st to 10thNight
7%224%43%21%3
740
11th to 20thNight
7%422%43%22%3
760
21st to endNight
8%323%45%20%1
760
December1st to 10thNight
9%320%39%28%1
760
11th to 20thNight
10%422%39%23%2
760
21st to endNight
10%424%40%22%1
836
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