Blair Hill Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Blair Hill 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
42%
24257
19%
74519
12%
24209
50%
24209
23%
48550

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
19%
759
19%
2459
7%
757
34%
757
15%
1619
January11th to 20thNight
23%
744
20%
2417
9%
744
38%
744
15%
1647
January21st to endNight
18%
792
21%
2574
6%
792
35%
792
17%
1642
February1st to 10thNight
22%
720
19%
2340
8%
720
36%
720
19%
1517
February11th to 20thNight
14%
700
16%
2198
4%
699
28%
699
12%
1523
February21st to endNight
17%
592
23%
1728
8%
592
31%
592
18%
1203
March1st to 10thNight
25%
720
23%
1980
10%
720
37%
720
24%
1281
March11th to 20thNight
26%
720
16%
1980
7%
720
35%
720
18%
1428
March21st to endNight
26%
792
17%
2178
8%
792
32%
792
22%
1495
April1st to 10thNight
30%
720
18%
1939
6%
720
40%
720
24%
1289
April11th to 20thNight
35%
711
23%
1775
10%
711
41%
711
29%
1147
April21st to endNight
37%
581
22%
1661
10%
581
46%
581
26%
1068
May1st to 10thNight
40%
540
15%
1620
7%
540
46%
540
23%
1063
May11th to 20thNight
46%
531
14%
1593
8%
531
50%
531
23%
1092
May21st to endNight
60%
582
13%
1745
10%
582
62%
582
26%
1149
June1st to 10thNight
65%
540
15%
1620
13%
540
66%
540
28%
1042
June11th to 20thNight
62%
540
14%
1620
11%
540
65%
540
30%
1030
June21st to endNight
69%
540
17%
1620
14%
540
71%
540
31%
1012
July1st to 10thNight
70%
525
14%
1575
13%
525
72%
525
26%
1045
July11th to 20thNight
75%
522
14%
1566
12%
522
76%
522
33%
961
July21st to endNight
74%
591
15%
1672
13%
558
75%
558
28%
1073
August1st to 10thNight
67%
537
14%
1600
11%
534
67%
534
26%
1092
August11th to 20thNight
67%
534
19%
1598
17%
534
68%
534
27%
1070
August21st to endNight
67%
727
18%
1915
14%
727
68%
727
30%
1269
September1st to 10thNight
66%
720
21%
1872
15%
720
67%
720
31%
1247
September11th to 20thNight
60%
720
23%
1980
19%
720
61%
720
29%
1282
September21st to endNight
49%
720
20%
1980
15%
720
54%
720
25%
1345
October1st to 10thNight
51%
720
21%
2124
19%
720
56%
720
29%
1340
October11th to 20thNight
45%
720
25%
2160
19%
720
55%
720
27%
1327
October21st to endNight
43%
792
28%
2376
21%
792
55%
792
30%
1425
November1st to 10thNight
46%
740
28%
2221
20%
740
57%
740
29%
1363
November11th to 20thNight
39%
760
25%
2455
17%
760
52%
760
25%
1453
November21st to endNight
36%
760
25%
2470
16%
760
49%
760
24%
1484
December1st to 10thNight
25%
760
19%
2470
9%
760
38%
760
16%
1679
December11th to 20thNight
27%
759
21%
2976
10%
759
43%
759
15%
1929
December21st to endNight
25%
826
12%
4462
8%
817
37%
817
9%
2919

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
12%7%36%6%45%6%6%8%37%
74962
Night
12%7%36%6%35%5%6%8%40%
74861
TransparencyDay
43%8%12%20%14%3
74684
Night
40%8%13%22%15%3
74658
SeeingDay
19%13%23%22%16%7%
14119
Night
17%11%23%29%17%2
24274

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
14%8%37%6%35%45%8%38%
1706
11th to 20thDay
14%10%46%6%35%5%5%6%35%
1671
21st to endDay
12%9%47%6%345%45%40%
1782
February1st to 10thDay
11%9%48%7%46%5%5%6%35%
1620
11th to 20thDay
16%7%48%7%45%6%46%34%
1648
21st to endDay
17%7%47%7%445%5%6%35%
1526
March1st to 10thDay
13%6%35%5%35%45%7%46%
1980
11th to 20thDay
16%9%5%6%6%25%5%5%7%35%
1980
21st to endDay
15%9%46%6%445%5%6%36%
2178
April1st to 10thDay
9%7%35%6%3445%8%46%
2020
11th to 20thDay
12%7%35%435%5%6%7%42%
2134
21st to endDay
14%7%36%7%45%6%5%7%38%
2312
May1st to 10thDay
13%6%37%6%46%6%6%8%34%
2340
11th to 20thDay
16%7%26%7%45%6%5%10%31%
2301
21st to endDay
11%6%37%6%47%6%7%10%34%
2531
June1st to 10thDay
9%6%35%6%46%6%6%10%39%
2340
11th to 20thDay
7%5%35%7%47%6%8%10%37%
2340
21st to endDay
9%5%35%6%46%7%8%9%37%
2340
July1st to 10thDay
9%6%37%7%5%7%7%7%11%33%
2340
11th to 20thDay
7%5%36%7%47%8%8%11%34%
2340
21st to endDay
8%7%37%8%6%8%7%8%10%29%
2547
August1st to 10thDay
11%6%26%8%5%7%7%8%10%30%
2327
11th to 20thDay
11%6%36%7%5%6%8%8%10%29%
2303
21st to endDay
9%6%36%6%45%7%7%9%37%
2422
September1st to 10thDay
11%5%36%6%45%6%6%7%42%
2084
11th to 20thDay
15%5%35%5%45%6%6%8%38%
1980
21st to endDay
18%8%36%5%445%5%6%36%
1980
October1st to 10thDay
16%6%25%33335%6%48%
1835
11th to 20thDay
12%7%36%4345%5%6%45%
1800
21st to endDay
13%7%35%5%34447%45%
1980
November1st to 10thDay
11%6%245%3445%7%50%
1857
11th to 20thDay
10%7%25%5%345%5%8%46%
1721
21st to endDay
13%7%25%5%3445%6%46%
1710
December1st to 10thDay
14%9%47%6%45%6%5%7%34%
1710
11th to 20thDay
14%8%36%43435%8%42%
2153
21st to endDay
14%11%37%5%45%6%7%7%30%
3124
January1st to 10thNight
12%8%37%6%45%5%6%7%37%
2459
11th to 20thNight
15%7%47%6%45%6%5%7%34%
2417
21st to endNight
12%8%45%6%35%5%6%7%38%
2574
February1st to 10thNight
14%7%36%6%35%5%6%8%38%
2340
11th to 20thNight
18%8%36%6%45%5%5%6%34%
2197
21st to endNight
12%8%36%6%36%5%5%7%37%
1728
March1st to 10thNight
12%5%46%5%345%6%6%45%
1980
11th to 20thNight
15%9%46%5%36%6%6%7%34%
1980
21st to endNight
12%8%46%6%35%46%7%40%
2178
April1st to 10thNight
11%6%35%6%35%5%6%9%41%
1939
11th to 20thNight
13%425%6%245%5%7%47%
1775
21st to endNight
13%7%246%2445%6%45%
1661
May1st to 10thNight
11%7%36%5%35%5%5%7%42%
1620
11th to 20thNight
13%7%36%6%35%5%6%9%36%
1592
21st to endNight
11%5%26%6%346%7%10%39%
1744
June1st to 10thNight
7%5%25%5%36%6%7%12%42%
1620
11th to 20thNight
9%5%25%5%346%6%9%45%
1620
21st to endNight
6%325%5%46%5%8%11%45%
1620
July1st to 10thNight
8%5%25%5%45%6%7%10%42%
1620
11th to 20thNight
5%325%5%45%7%8%11%46%
1620
21st to endNight
4325%6%47%8%8%12%41%
1771
August1st to 10thNight
8%6%28%7%35%6%7%10%38%
1611
11th to 20thNight
9%5%35%6%46%6%7%10%39%
1598
21st to endNight
9%425%6%5%6%5%7%10%41%
1915
September1st to 10thNight
7%6%36%6%35%5%6%8%46%
1872
11th to 20thNight
11%6%36%6%35%46%8%42%
1980
21st to endNight
14%6%36%7%345%5%6%41%
1980
October1st to 10thNight
13%7%36%5%33447%46%
2124
11th to 20thNight
12%6%35%5%3446%6%46%
2160
21st to endNight
11%6%35%5%2445%7%47%
2376
November1st to 10thNight
12%6%35%434445%50%
2221
11th to 20thNight
10%7%25%5%3445%7%48%
2455
21st to endNight
12%5%36%435%5%5%7%46%
2470
December1st to 10thNight
14%9%47%6%35%46%7%35%
2470
11th to 20thNight
14%8%47%6%45%5%5%7%36%
3042
21st to endNight
14%10%48%8%46%6%5%7%28%
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
44%6%9%21%15%4
1707
11th to 20thDay
48%7%8%21%14%2
1671
21st to endDay
44%5%8%22%17%3
1782
February1st to 10thDay
48%6%11%19%14%2
1620
11th to 20thDay
49%7%9%19%13%3
1657
21st to endDay
49%6%11%14%16%5%
1526
March1st to 10thDay
38%5%9%25%19%5%
1980
11th to 20thDay
49%7%9%18%13%4
1980
21st to endDay
48%7%10%21%11%3
2178
April1st to 10thDay
37%7%11%24%17%5%
2020
11th to 20thDay
40%6%11%22%17%5%
2134
21st to endDay
45%8%10%23%11%3
2312
May1st to 10thDay
46%8%14%17%12%2
2340
11th to 20thDay
48%8%14%20%7%2
2305
21st to endDay
44%10%16%22%8%
2531
June1st to 10thDay
39%10%17%22%12%
2340
11th to 20thDay
39%12%16%18%13%2
2340
21st to endDay
40%11%16%20%12%2
2340
July1st to 10thDay
42%12%14%19%9%2
2275
11th to 20thDay
40%12%19%16%12%1
2271
21st to endDay
46%12%15%18%8%1
2405
August1st to 10thDay
45%13%15%20%7%
2314
11th to 20thDay
46%11%17%17%8%
2303
21st to endDay
40%11%18%17%10%3
2422
September1st to 10thDay
40%8%12%21%16%3
2084
11th to 20thDay
43%7%10%24%15%2
1980
21st to endDay
48%6%10%19%15%2
1980
October1st to 10thDay
37%5%12%24%18%4
1835
11th to 20thDay
38%6%11%23%19%4
1800
21st to endDay
37%6%8%26%18%5%
1980
November1st to 10thDay
33%5%10%22%25%5%
1857
11th to 20thDay
37%5%9%22%23%5%
1721
21st to endDay
38%5%11%21%22%3
1710
December1st to 10thDay
48%8%11%15%15%3
1710
11th to 20thDay
37%7%10%20%21%4
2150
21st to endDay
48%7%11%20%13%2
3124
January1st to 10thNight
44%7%12%17%16%5%
2459
11th to 20thNight
46%7%9%18%16%3
2417
21st to endNight
41%7%11%22%17%2
2574
February1st to 10thNight
43%6%13%23%14%1
2340
11th to 20thNight
50%5%8%19%14%5%
2199
21st to endNight
45%8%10%21%12%4
1728
March1st to 10thNight
39%6%9%23%19%4
1980
11th to 20thNight
46%10%10%22%12%1
1980
21st to endNight
44%7%11%21%14%3
2178
April1st to 10thNight
39%8%12%21%17%4
1939
11th to 20thNight
36%6%12%21%20%5%
1775
21st to endNight
39%6%13%27%14%2
1661
May1st to 10thNight
41%8%16%22%12%2
1620
11th to 20thNight
44%9%17%21%9%1
1593
21st to endNight
39%9%20%25%7%
1745
June1st to 10thNight
34%11%18%24%12%1
1620
11th to 20thNight
34%10%19%23%13%1
1620
21st to endNight
30%12%18%24%13%2
1620
July1st to 10thNight
36%13%16%20%11%4
1575
11th to 20thNight
29%12%24%21%12%3
1566
21st to endNight
30%16%21%24%9%
1672
August1st to 10thNight
38%13%20%19%9%1
1600
11th to 20thNight
37%11%18%21%11%1
1598
21st to endNight
36%9%17%25%10%2
1915
September1st to 10thNight
35%10%14%21%15%4
1872
11th to 20thNight
39%7%13%23%17%1
1980
21st to endNight
43%7%11%22%15%2
1980
October1st to 10thNight
38%5%12%23%18%3
2124
11th to 20thNight
37%6%10%25%18%4
2160
21st to endNight
35%5%10%24%21%6%
2376
November1st to 10thNight
37%5%9%21%23%6%
2221
11th to 20thNight
36%5%8%22%22%7%
2455
21st to endNight
37%5%9%26%19%4
2470
December1st to 10thNight
48%6%10%20%13%3
2470
11th to 20thNight
43%8%9%19%18%3
3044
21st to endNight
48%9%13%17%13%1
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
34%25%23%11%7%
270
11th to 20thDay
35%22%30%11%2
257
21st to endDay
36%24%25%12%4
264
February1st to 10thDay
30%25%31%11%3
240
11th to 20thDay
40%30%23%6%1
240
21st to endDay
35%23%29%11%2
244
March1st to 10thDay
25%23%28%19%5%
420
11th to 20thDay
28%22%26%17%7%1
420
21st to endDay
26%18%30%20%5%
462
April1st to 10thDay
22%22%24%22%10%1
420
11th to 20thDay
22%17%26%19%12%4
412
21st to endDay
25%15%28%17%11%4
420
May1st to 10thDay
20%19%30%17%11%4
423
11th to 20thDay
18%11%29%21%14%6%
447
21st to endDay
14%6%22%26%20%12%
485
June1st to 10thDay
13%6%24%28%20%10%
450
11th to 20thDay
10%8%23%28%21%11%
450
21st to endDay
11%7%26%27%20%9%
450
July1st to 10thDay
12%316%26%27%16%
445
11th to 20thDay
5%216%33%28%16%
444
21st to endDay
9%217%29%29%14%
488
August1st to 10thDay
16%418%23%24%16%
446
11th to 20thDay
11%414%26%27%18%
439
21st to endDay
10%312%28%25%21%
495
September1st to 10thDay
12%413%26%29%16%
450
11th to 20thDay
14%412%28%29%14%
450
21st to endDay
22%8%20%26%21%4
450
October1st to 10thDay
20%8%16%27%23%6%
450
11th to 20thDay
16%14%22%24%21%2
450
21st to endDay
19%14%27%22%15%4
495
November1st to 10thDay
18%12%25%27%14%4
455
11th to 20thDay
25%19%18%26%10%2
281
21st to endDay
22%21%24%24%7%1
270
December1st to 10thDay
26%26%26%16%5%
270
11th to 20thDay
30%18%26%18%8%1
270
21st to endDay
28%17%28%14%11%1
297
January1st to 10thNight
24%23%30%19%3
760
11th to 20thNight
24%20%29%24%3
744
21st to endNight
21%24%34%19%2
792
February1st to 10thNight
22%19%33%23%3
720
11th to 20thNight
31%21%30%17%1
703
21st to endNight
25%18%36%18%2
592
March1st to 10thNight
18%19%35%26%2
720
11th to 20thNight
25%17%28%23%8%
720
21st to endNight
19%14%35%26%5%
792
April1st to 10thNight
21%17%29%26%7%
720
11th to 20thNight
17%15%28%28%11%
711
21st to endNight
18%12%26%34%9%1
581
May1st to 10thNight
15%11%27%30%15%1
540
11th to 20thNight
15%7%22%34%20%2
531
21st to endNight
13%213%32%36%4
584
June1st to 10thNight
9%215%43%28%3
540
11th to 20thNight
10%316%33%34%4
540
21st to endNight
7%212%41%33%4
540
July1st to 10thNight
11%27%36%39%6%
525
11th to 20thNight
6%28%33%45%6%
522
21st to endNight
5%28%40%40%5%
591
August1st to 10thNight
10%38%37%37%5%
537
11th to 20thNight
10%27%34%40%6%
534
21st to endNight
11%210%32%39%8%
727
September1st to 10thNight
9%211%33%39%7%
720
11th to 20thNight
14%311%32%34%6%
720
21st to endNight
18%5%18%33%24%1
720
October1st to 10thNight
17%7%19%28%26%3
720
11th to 20thNight
16%9%24%33%17%1
720
21st to endNight
16%12%24%33%14%1
792
November1st to 10thNight
18%9%22%34%16%2
740
11th to 20thNight
17%15%24%34%10%
760
21st to endNight
18%15%27%30%10%1
760
December1st to 10thNight
23%17%31%24%6%
760
11th to 20thNight
18%18%31%28%6%
760
21st to endNight
23%16%28%27%5%
836
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