Horizontal Heavens Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Horizontal Heavens Observatory clear sky chart from 2005-11-05 to 2017-12-01.

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
49%
15698
26%
47847
18%
15652
57%
15652
33%
27744

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 15thNight
37%
898
26%
2327
11%
895
46%
895
32%
1461
January16th to endNight
36%
867
31%
2403
12%
867
48%
867
32%
1467
February1st to 15thNight
34%
704
28%
2106
13%
698
47%
698
30%
1222
February16th to endNight
25%
632
28%
1894
9%
632
41%
632
29%
1099
March1st to 15thNight
31%
567
28%
2007
13%
567
47%
567
26%
1167
March16th to endNight
31%
576
23%
2112
15%
576
42%
576
24%
1222
April1st to 15thNight
27%
539
23%
1977
12%
539
44%
539
25%
1134
April16th to endNight
29%
540
25%
1764
13%
540
46%
540
25%
1105
May1st to 15thNight
33%
539
22%
1620
15%
539
43%
539
28%
1001
May16th to endNight
48%
562
24%
1684
19%
561
58%
561
31%
1039
June1st to 15thNight
64%
540
23%
1620
23%
540
71%
540
49%
825
June16th to endNight
65%
540
22%
1620
22%
540
70%
540
38%
924
July1st to 15thNight
76%
510
20%
1530
21%
510
79%
510
40%
860
July16th to endNight
80%
564
23%
1594
26%
531
84%
531
43%
873
August1st to 15thNight
87%
539
29%
1694
34%
536
87%
536
52%
798
August16th to endNight
76%
573
21%
1964
25%
573
77%
573
31%
1144
September1st to 15thNight
75%
540
20%
1980
24%
540
76%
540
31%
1110
September16th to endNight
66%
537
24%
1980
26%
537
69%
537
28%
1149
October1st to 15thNight
68%
540
32%
1980
32%
540
71%
540
37%
1006
October16th to endNight
61%
762
37%
2298
25%
762
68%
762
48%
1145
November1st to 15thNight
51%
789
26%
2309
16%
789
57%
789
34%
1360
November16th to endNight
46%
975
32%
2535
15%
975
55%
975
39%
1487
December1st to 15thNight
40%
905
29%
2353
15%
905
47%
905
31%
1511
December16th to endNight
37%
960
25%
2496
11%
960
44%
960
30%
1635

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
13%5%24424447%45%
48567
Night
12%413323347%52%
48049
Cloud CoverDay
12%5%25%42445%6%44%
48565
Night
12%423323347%52%
48049
TransparencyDay
38%49%19%18%9%
48265
Night
32%411%20%18%11%
47854
SeeingDay
14%417%31%25%6%
5590
Night
15%5%24%35%17%2
15702

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 15thDay
15%5%24322336%49%
1611
16th to endDay
16%5%1331233453%
1830
February1st to 15thDay
15%5%24423335%50%
1750
16th to endDay
17%5%23222335%50%
1580
March1st to 15thDay
20%5%24313245%45%
1951
16th to endDay
17%5%33313346%47%
2112
April1st to 15thDay
15%425%423446%45%
1971
16th to endDay
15%6%25%5%24335%43%
2193
May1st to 15thDay
17%5%36%4345%5%6%36%
2331
16th to endDay
19%6%25%435%45%7%34%
2430
June1st to 15thDay
5%4245%37%5%7%8%44%
2340
16th to endDay
12%6%35%5%46%5%6%9%34%
2340
July1st to 15thDay
10%4346%36%6%6%8%38%
2340
16th to endDay
8%4245%36%5%6%11%41%
2466
August1st to 15thDay
221332446%10%58%
2252
16th to endDay
8%5%25%8%46%5%6%9%38%
2238
September1st to 15thDay
10%6%35%7%45%6%5%7%39%
1980
16th to endDay
12%426%5%345%5%7%42%
1980
October1st to 15thDay
10%414424445%52%
1973
16th to endDay
8%213212335%63%
1920
November1st to 15thDay
15%6%26%6%24346%42%
1875
16th to endDay
14%42431423455%
1755
December1st to 15thDay
19%6%24322246%44%
1621
16th to endDay
14%8%2442333446%
1728
January1st to 15thNight
17%413423336%49%
2327
16th to endNight
16%423323335%51%
2403
February1st to 15thNight
15%5%2421222454%
2102
16th to endNight
15%413323345%51%
1894
March1st to 15thNight
16%6%14312235%50%
2007
16th to endNight
17%425%213346%47%
2112
April1st to 15thNight
15%5%24322346%48%
1977
16th to endNight
18%5%24324335%46%
1764
May1st to 15thNight
14%5%23424347%46%
1619
16th to endNight
12%42342336%9%46%
1683
June1st to 15thNight
4112414348%62%
1620
16th to endNight
7%32342446%8%52%
1620
July1st to 15thNight
6%2233236%6%11%51%
1620
16th to endNight
6%31231446%11%54%
1702
August1st to 15thNight
21113135%5%10%63%
1704
16th to endNight
8%313435%45%10%48%
1964
September1st to 15thNight
7%5%2442445%9%49%
1980
16th to endNight
10%5%25%424448%46%
1980
October1st to 15thNight
10%312323346%58%
1980
16th to endNight
8%312212335%66%
2298
November1st to 15thNight
11%6%14313346%51%
2309
16th to endNight
13%413212235%59%
2535
December1st to 15thNight
17%5%23413245%49%
2353
16th to endNight
16%7%24423335%47%
2496

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 15thDay
15%5%24313335%49%
1611
16th to endDay
15%5%23212425%53%
1830
February1st to 15thDay
14%5%24323335%50%
1750
16th to endDay
16%5%33213335%50%
1580
March1st to 15thDay
19%6%14423235%45%
1951
16th to endDay
16%6%34313345%47%
2110
April1st to 15thDay
15%5%25%424345%45%
1971
16th to endDay
14%6%25%5%23446%43%
2193
May1st to 15thDay
16%7%36%5%345%5%6%36%
2331
16th to endDay
17%7%36%5%35%45%7%34%
2430
June1st to 15thDay
5%5%25%5%37%6%6%9%44%
2340
16th to endDay
10%6%36%6%36%5%6%8%34%
2340
July1st to 15thDay
9%5%25%6%46%6%6%9%37%
2340
16th to endDay
7%4245%36%6%6%10%40%
2466
August1st to 15thDay
3212325%45%10%57%
2252
16th to endDay
7%426%7%46%5%6%9%37%
2238
September1st to 15thDay
9%5%36%7%45%6%5%6%38%
1980
16th to endDay
11%426%6%3446%6%41%
1980
October1st to 15thDay
10%5%24424446%51%
1973
16th to endDay
8%312323345%62%
1920
November1st to 15thDay
13%6%36%5%2445%5%41%
1875
16th to endDay
13%5%23413235%55%
1755
December1st to 15thDay
19%6%25%322335%44%
1621
16th to endDay
14%7%25%42333446%
1728
January1st to 15thNight
16%5%13323345%49%
2327
16th to endNight
16%423323335%51%
2403
February1st to 15thNight
15%6%2321223454%
2102
16th to endNight
15%5%13323345%51%
1894
March1st to 15thNight
16%6%24412235%49%
2007
16th to endNight
16%6%24313446%47%
2112
April1st to 15thNight
14%5%24423346%48%
1977
16th to endNight
17%6%24323345%45%
1764
May1st to 15thNight
14%5%24323447%46%
1619
16th to endNight
11%42442436%8%45%
1683
June1st to 15thNight
423414349%61%
1620
16th to endNight
7%32432446%9%52%
1620
July1st to 15thNight
6%3233245%6%11%50%
1620
16th to endNight
5%31231446%11%54%
1702
August1st to 15thNight
211121346%10%63%
1704
16th to endNight
8%413435%45%11%46%
1964
September1st to 15thNight
7%5%24425%45%9%49%
1980
16th to endNight
9%5%35%4245%47%46%
1980
October1st to 15thNight
9%313323346%58%
1980
16th to endNight
7%312212235%65%
2298
November1st to 15thNight
10%6%14413436%51%
2309
16th to endNight
13%5%12212235%59%
2535
December1st to 15thNight
16%5%14413245%49%
2353
16th to endNight
15%6%25%423335%47%
2496

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 15thDay
38%3418%20%14%
1612
16th to endDay
35%26%15%25%15%
1830
February1st to 15thDay
37%35%14%21%17%
1751
16th to endDay
37%25%18%21%14%
1580
March1st to 15thDay
41%37%13%20%13%
1951
16th to endDay
39%37%17%20%11%
2112
April1st to 15thDay
40%46%16%19%13%
1971
16th to endDay
43%4419%16%11%
2193
May1st to 15thDay
48%5%7%14%18%6%
2331
16th to endDay
48%5%8%16%16%3
2434
June1st to 15thDay
35%5%10%22%20%4
2340
16th to endDay
45%5%11%20%14%2
2340
July1st to 15thDay
38%7%13%25%13%1
2201
16th to endDay
33%7%17%29%10%
2311
August1st to 15thDay
22%6%15%37%16%1
2238
16th to endDay
41%6%14%24%10%2
2238
September1st to 15thDay
42%6%9%19%16%4
1980
16th to endDay
40%5%10%17%16%8%
1980
October1st to 15thDay
33%411%19%18%12%
1973
16th to endDay
24%4415%29%21%
1920
November1st to 15thDay
43%38%18%15%10%
1875
16th to endDay
33%2416%22%20%
1755
December1st to 15thDay
43%25%18%19%10%
1621
16th to endDay
41%37%14%21%10%
1728
January1st to 15thNight
37%25%16%19%17%
2327
16th to endNight
36%2415%23%17%
2403
February1st to 15thNight
35%25%14%21%21%
2112
16th to endNight
35%35%17%22%16%
1894
March1st to 15thNight
39%28%14%19%15%
2007
16th to endNight
37%49%15%20%12%
2112
April1st to 15thNight
38%28%16%17%16%
1977
16th to endNight
41%48%17%16%12%
1764
May1st to 15thNight
38%410%19%17%9%
1620
16th to endNight
35%6%13%25%15%4
1685
June1st to 15thNight
21%413%30%26%4
1620
16th to endNight
28%6%18%27%15%3
1620
July1st to 15thNight
24%8%21%30%13%
1530
16th to endNight
22%8%27%31%10%
1594
August1st to 15thNight
15%6%20%40%15%
1694
16th to endNight
31%7%21%27%8%3
1964
September1st to 15thNight
31%6%16%23%16%5%
1980
16th to endNight
35%6%14%19%15%8%
1980
October1st to 15thNight
27%412%25%18%12%
1980
16th to endNight
22%16%17%28%23%
2298
November1st to 15thNight
34%310%20%18%12%
2309
16th to endNight
30%27%16%22%20%
2535
December1st to 15thNight
38%36%18%21%11%
2353
16th to endNight
40%35%13%22%13%
2496

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 15thDay
17%10%33%26%12%
90
16th to endDay
17%13%33%26%9%
96
February1st to 15thDay
10%22%30%26%10%1
90
16th to endDay
14%14%44%23%2
81
March1st to 15thDay
25%5%37%18%12%
90
16th to endDay
11%438%32%12%1
96
April1st to 15thDay
17%11%43%23%4
90
16th to endDay
14%12%38%27%6%
303
May1st to 15thDay
18%11%36%25%6%
468
16th to endDay
26%422%33%11%1
517
June1st to 15thDay
7%112%44%28%4
495
16th to endDay
14%110%35%30%7%
495
July1st to 15thDay
9%110%38%33%7%
475
16th to endDay
7%1327%49%11%
515
August1st to 15thDay
4332%43%16%
406
16th to endDay
13%434%29%16%
278
September1st to 15thDay
5%1220%51%19%
135
16th to endDay
13%121%46%17%
135
October1st to 15thDay
25%31%40%19%
135
16th to endDay
15%417%27%29%5%
144
November1st to 15thDay
31%219%25%17%3
135
16th to endDay
17%8%28%22%15%5%
135
December1st to 15thDay
24%6%20%23%17%7%
90
16th to endDay
25%9%37%22%41
96
January1st to 15thNight
19%10%27%32%9%
900
16th to endNight
19%10%30%31%7%
867
February1st to 15thNight
20%12%28%30%7%
704
16th to endNight
21%10%40%24%2
632
March1st to 15thNight
20%8%35%29%6%
567
16th to endNight
19%8%36%28%5%
576
April1st to 15thNight
17%9%41%29%2
539
16th to endNight
20%6%38%24%8%
540
May1st to 15thNight
17%6%39%32%4
540
16th to endNight
14%330%38%12%
563
June1st to 15thNight
5%24%46%21%1
540
16th to endNight
8%18%48%20%3
540
July1st to 15thNight
7%19%48%30%2
510
16th to endNight
6%450%33%4
564
August1st to 15thNight
2346%41%6%
539
16th to endNight
12%340%38%5%
573
September1st to 15thNight
9%5%37%40%6%
540
16th to endNight
12%110%32%38%5%
537
October1st to 15thNight
12%112%39%28%5%
540
16th to endNight
9%5%21%34%26%3
762
November1st to 15thNight
14%423%36%19%1
789
16th to endNight
17%7%24%33%15%1
975
December1st to 15thNight
21%8%26%31%11%1
905
16th to endNight
21%7%28%33%8%
960
Page generated at 2017-12-02 08:14:22. Version ac04

Copyright 2017 A. Danko


Horizontal Heavens Observatory Clear Sky Chart |Charts Home