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 2005-11-05 to 2017-10-06.

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%
15841
20%
47108
12%
15803
50%
15803
23%
30636

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
19%
718
20%
2327
8%
716
34%
716
14%
1535
January16th to endNight
17%
768
21%
2496
7%
768
35%
768
16%
1623
February1st to 15thNight
17%
701
20%
2287
6%
701
35%
701
17%
1464
February16th to endNight
17%
631
22%
1869
7%
631
32%
631
16%
1262
March1st to 15thNight
27%
720
20%
1980
10%
720
36%
720
21%
1352
March16th to endNight
28%
768
16%
2112
9%
768
34%
768
21%
1472
April1st to 15thNight
31%
718
18%
1888
6%
718
40%
718
25%
1249
April16th to endNight
39%
627
20%
1707
9%
627
47%
627
26%
1128
May1st to 15thNight
40%
540
15%
1620
7%
540
45%
540
22%
1083
May16th to endNight
58%
561
14%
1684
9%
561
61%
561
27%
1100
June1st to 15thNight
63%
540
15%
1620
12%
540
66%
540
30%
1030
June16th to endNight
69%
540
16%
1620
13%
540
72%
540
32%
997
July1st to 15thNight
68%
510
12%
1530
10%
510
70%
510
27%
1010
July16th to endNight
76%
564
15%
1595
13%
531
78%
531
32%
988
August1st to 15thNight
69%
539
17%
1609
15%
536
69%
536
28%
1070
August16th to endNight
69%
663
19%
1809
15%
663
70%
663
30%
1194
September1st to 15thNight
63%
720
23%
1908
17%
720
65%
720
31%
1239
September16th to endNight
53%
717
21%
1980
17%
717
56%
717
27%
1314
October1st to 15thNight
50%
684
25%
2027
23%
684
57%
684
29%
1247
October16th to endNight
41%
704
29%
2112
19%
704
55%
704
29%
1251
November1st to 15thNight
44%
700
26%
2152
18%
700
55%
700
29%
1284
November16th to endNight
34%
720
25%
2340
15%
720
46%
720
23%
1415
December1st to 15thNight
26%
720
20%
2340
9%
720
39%
720
16%
1567
December16th to endNight
21%
768
19%
2496
8%
768
35%
768
13%
1762

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%6%36%6%35%5%5%7%36%
48076
Night
11%5%25%5%35%5%5%7%41%
47301
Cloud CoverDay
12%6%36%6%35%5%5%7%36%
48086
Night
11%6%25%5%345%5%8%41%
47305
TransparencyDay
44%7%11%19%13%2
47787
Night
39%7%12%22%15%2
47108
SeeingDay
17%10%20%21%18%10%
5677
Night
16%11%23%28%17%2
15848

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 15thDay
14%7%36%5%3445%7%36%
1611
16th to endDay
12%7%36%5%445%46%39%
1728
February1st to 15thDay
11%7%36%5%45%6%5%5%36%
1575
16th to endDay
13%5%47%7%346%5%7%34%
1601
March1st to 15thDay
14%5%26%5%345%46%42%
1969
16th to endDay
17%7%45%6%24346%36%
2112
April1st to 15thDay
10%5%35%5%23348%45%
2059
16th to endDay
14%7%25%6%35%5%5%7%37%
2262
May1st to 15thDay
15%7%36%6%446%6%7%30%
2331
16th to endDay
10%6%26%6%46%5%5%9%33%
2431
June1st to 15thDay
8%5%26%6%48%6%7%9%34%
2340
16th to endDay
7%435%6%38%6%7%9%36%
2340
July1st to 15thDay
10%6%38%7%47%7%7%9%28%
2340
16th to endDay
8%426%7%5%8%7%7%10%29%
2465
August1st to 15thDay
12%6%26%7%47%7%7%9%27%
2340
16th to endDay
10%6%35%6%46%7%7%10%32%
2381
September1st to 15thDay
13%6%25%7%25%46%7%39%
2049
16th to endDay
17%6%26%5%35%5%5%6%34%
1980
October1st to 15thDay
15%5%24424346%47%
1730
16th to endDay
12%6%245%23446%46%
1760
November1st to 15thDay
10%5%23323448%49%
1704
16th to endDay
13%7%26%5%24345%43%
1620
December1st to 15thDay
13%6%25%5%35%45%7%40%
1620
16th to endDay
16%9%35%4345%5%6%35%
1728
January1st to 15thNight
13%6%25%6%35%5%5%7%37%
2327
16th to endNight
13%7%35%5%345%5%7%37%
2496
February1st to 15thNight
13%6%35%5%25%5%5%7%40%
2285
16th to endNight
11%5%35%5%36%6%6%6%37%
1869
March1st to 15thNight
13%5%35%5%345%6%6%39%
1968
16th to endNight
14%6%35%435%45%6%39%
2112
April1st to 15thNight
11%5%2443446%9%41%
1888
16th to endNight
12%5%25%5%35%447%42%
1707
May1st to 15thNight
13%6%35%6%35%5%5%7%40%
1620
16th to endNight
9%5%25%5%345%6%9%41%
1682
June1st to 15thNight
7%415%5%25%6%5%10%45%
1620
16th to endNight
5%325%445%5%6%10%47%
1620
July1st to 15thNight
6%4245%46%5%6%9%42%
1620
16th to endNight
4224437%6%9%12%41%
1703
August1st to 15thNight
9%426%5%245%6%10%41%
1620
16th to endNight
8%4146%36%5%6%11%40%
1809
September1st to 15thNight
8%5%26%5%35%446%48%
1908
16th to endNight
13%5%25%4345%6%6%41%
1980
October1st to 15thNight
12%5%25%423336%49%
2027
16th to endNight
9%5%2442345%7%47%
2112
November1st to 15thNight
12%5%25%322445%50%
2152
16th to endNight
12%5%24424447%46%
2340
December1st to 15thNight
14%7%37%5%35%45%6%37%
2340
16th to endNight
14%9%47%5%35%45%7%31%
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
13%8%36%5%345%5%7%36%
1611
16th to endDay
12%7%36%6%35%445%39%
1728
February1st to 15thDay
10%8%37%7%46%446%36%
1575
16th to endDay
12%6%47%7%445%5%7%34%
1601
March1st to 15thDay
14%5%35%5%35%445%42%
1980
16th to endDay
16%8%45%5%235%46%36%
2111
April1st to 15thDay
10%6%35%5%23348%45%
2059
16th to endDay
14%6%25%6%35%5%5%6%37%
2262
May1st to 15thDay
16%6%37%6%36%6%5%7%30%
2331
16th to endDay
10%6%36%6%46%5%6%9%33%
2431
June1st to 15thDay
8%5%25%6%5%7%6%7%10%34%
2340
16th to endDay
7%5%36%6%37%6%8%8%36%
2340
July1st to 15thDay
10%6%37%7%5%6%7%6%10%28%
2340
16th to endDay
7%5%26%8%5%8%7%8%10%29%
2465
August1st to 15thDay
12%6%26%8%5%7%7%7%9%27%
2340
16th to endDay
10%6%26%6%45%7%6%9%33%
2381
September1st to 15thDay
12%6%26%6%345%5%7%39%
2049
16th to endDay
17%6%26%6%35%6%5%6%33%
1980
October1st to 15thDay
15%6%24323337%46%
1730
16th to endDay
12%7%25%424345%47%
1760
November1st to 15thDay
11%5%13424348%48%
1704
16th to endDay
12%7%26%5%33446%43%
1620
December1st to 15thDay
13%6%26%5%25%45%7%39%
1620
16th to endDay
16%10%26%4245%5%6%35%
1728
January1st to 15thNight
12%6%36%5%45%5%5%7%36%
2327
16th to endNight
13%8%35%5%35%5%6%7%36%
2496
February1st to 15thNight
12%7%36%5%2445%6%40%
2278
16th to endNight
10%7%36%6%36%5%5%8%36%
1869
March1st to 15thNight
12%6%36%5%345%5%7%39%
1980
16th to endNight
13%8%35%435%45%7%38%
2112
April1st to 15thNight
11%5%246%245%6%8%41%
1888
16th to endNight
13%5%25%5%245%47%43%
1707
May1st to 15thNight
12%6%35%5%25%5%5%6%40%
1619
16th to endNight
8%426%5%345%6%9%41%
1682
June1st to 15thNight
8%3245%35%5%6%11%44%
1620
16th to endNight
5%225%435%5%7%11%47%
1620
July1st to 15thNight
7%4245%45%6%7%9%42%
1620
16th to endNight
4215%437%7%8%12%42%
1703
August1st to 15thNight
7%5%26%5%346%6%10%41%
1620
16th to endNight
8%3146%46%5%6%10%40%
1809
September1st to 15thNight
7%5%26%6%34446%48%
1908
16th to endNight
13%435%6%345%5%6%41%
1980
October1st to 15thNight
12%5%25%423236%49%
2027
16th to endNight
10%5%25%5%1345%6%48%
2112
November1st to 15thNight
12%5%24323346%50%
2152
16th to endNight
12%6%34424446%46%
2340
December1st to 15thNight
13%7%37%5%35%45%7%36%
2340
16th to endNight
14%9%47%6%35%5%5%6%31%
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
45%6%10%22%12%2
1612
16th to endDay
44%5%9%24%14%2
1728
February1st to 15thDay
47%5%9%20%15%1
1577
16th to endDay
47%6%12%19%12%1
1601
March1st to 15thDay
42%6%9%22%15%3
1980
16th to endDay
48%7%8%21%11%2
2112
April1st to 15thDay
37%5%12%25%13%5%
2059
16th to endDay
44%7%9%19%13%4
2262
May1st to 15thDay
50%7%13%16%9%1
2331
16th to endDay
46%8%14%20%7%1
2435
June1st to 15thDay
41%11%16%19%10%
2340
16th to endDay
41%11%15%17%12%2
2340
July1st to 15thDay
48%10%13%14%10%1
2201
16th to endDay
46%11%16%16%9%
2310
August1st to 15thDay
48%11%13%17%8%
2327
16th to endDay
44%10%16%17%9%1
2381
September1st to 15thDay
43%7%10%18%16%2
2049
16th to endDay
49%6%9%19%14%1
1980
October1st to 15thDay
38%411%24%16%4
1730
16th to endDay
38%48%25%18%4
1760
November1st to 15thDay
34%49%19%26%6%
1704
16th to endDay
42%48%22%20%2
1620
December1st to 15thDay
42%7%11%18%16%4
1620
16th to endDay
47%6%10%18%14%2
1728
January1st to 15thNight
43%6%11%17%15%5%
2327
16th to endNight
43%6%10%21%15%1
2496
February1st to 15thNight
42%49%24%15%3
2287
16th to endNight
43%7%11%24%12%1
1869
March1st to 15thNight
42%6%10%24%12%2
1980
16th to endNight
44%7%11%22%13%1
2112
April1st to 15thNight
38%5%12%22%15%4
1888
16th to endNight
39%6%12%20%15%4
1707
May1st to 15thNight
42%7%16%20%11%
1620
16th to endNight
37%9%20%22%8%1
1684
June1st to 15thNight
33%10%20%23%11%
1620
16th to endNight
29%12%18%23%14%2
1620
July1st to 15thNight
35%12%16%19%12%3
1530
16th to endNight
28%13%21%23%11%1
1595
August1st to 15thNight
36%10%17%20%13%
1609
16th to endNight
36%9%16%24%11%1
1809
September1st to 15thNight
36%7%12%23%16%3
1908
16th to endNight
41%5%11%21%18%1
1980
October1st to 15thNight
37%49%26%18%3
2027
16th to endNight
34%48%27%19%4
2112
November1st to 15thNight
36%38%19%23%8%
2152
16th to endNight
37%48%25%19%3
2340
December1st to 15thNight
46%6%9%20%13%3
2340
16th to endNight
50%6%10%16%13%1
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
31%28%21%12%6%
90
16th to endDay
36%25%22%13%2
96
February1st to 15thDay
20%35%36%6%1
90
16th to endDay
25%28%27%12%5%
111
March1st to 15thDay
21%21%28%20%8%
270
16th to endDay
19%19%29%22%7%1
288
April1st to 15thDay
20%17%21%22%17%1
270
16th to endDay
20%9%24%17%19%8%
270
May1st to 15thDay
24%14%26%17%12%5%
288
16th to endDay
14%324%21%18%16%
331
June1st to 15thDay
9%5%19%26%24%14%
315
16th to endDay
12%5%25%27%18%10%
315
July1st to 15thDay
15%220%31%17%13%
305
16th to endDay
8%39%23%34%20%
326
August1st to 15thDay
17%115%23%21%19%
313
16th to endDay
10%210%21%25%30%
332
September1st to 15thDay
15%5%13%19%29%17%
315
16th to endDay
17%5%14%23%24%14%
314
October1st to 15thDay
20%7%15%23%26%6%
278
16th to endDay
15%13%24%23%18%4
272
November1st to 15thDay
22%8%20%23%20%3
212
16th to endDay
24%21%17%23%12%1
90
December1st to 15thDay
26%25%25%11%10%1
90
16th to endDay
36%19%21%18%3
96
January1st to 15thNight
25%22%28%19%3
720
16th to endNight
23%23%32%19%1
768
February1st to 15thNight
25%21%31%20%1
704
16th to endNight
20%26%31%19%2
631
March1st to 15thNight
20%17%32%25%3
720
16th to endNight
19%14%33%24%8%
768
April1st to 15thNight
19%17%29%26%7%
718
16th to endNight
15%11%26%31%14%
627
May1st to 15thNight
15%9%28%30%15%
540
16th to endNight
11%416%30%31%4
563
June1st to 15thNight
10%117%35%31%4
540
16th to endNight
6%114%37%35%5%
540
July1st to 15thNight
10%10%38%36%3
510
16th to endNight
6%6%33%44%7%
564
August1st to 15thNight
10%19%39%34%4
539
16th to endNight
10%7%30%44%6%
663
September1st to 15thNight
9%212%32%37%4
720
16th to endNight
16%316%31%27%3
717
October1st to 15thNight
16%8%19%29%22%4
684
16th to endNight
14%12%26%30%14%1
704
November1st to 15thNight
16%11%22%31%15%1
700
16th to endNight
20%16%24%25%10%
720
December1st to 15thNight
21%18%27%25%7%
720
16th to endNight
22%19%30%23%3
768
Page generated at 2017-10-07 14:07:49. Version ac04

Copyright 2017 A. Danko


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