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Blue Hills Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Blue Hills Observatory clear sky chart from 2005-11-05 to 2018-01-12.

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
73%
16129
35%
48356
30%
16090
79%
16090
53%
23108

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
68%
763
31%
2471
28%
760
76%
760
45%
1204
January16th to endNight
62%
768
35%
2496
33%
768
71%
768
41%
1285
February1st to 15thNight
67%
699
35%
2180
29%
699
78%
699
51%
1024
February16th to endNight
63%
632
35%
1895
32%
632
71%
632
45%
985
March1st to 15thNight
63%
720
38%
1994
29%
720
72%
720
52%
1004
March16th to endNight
66%
768
36%
2112
26%
768
74%
768
56%
1030
April1st to 15thNight
59%
716
30%
1975
17%
716
69%
716
52%
1015
April16th to endNight
61%
540
44%
1650
28%
540
75%
540
67%
662
May1st to 15thNight
70%
539
36%
1619
28%
539
76%
539
59%
718
May16th to endNight
73%
561
38%
1684
29%
561
78%
561
67%
697
June1st to 15thNight
79%
540
33%
1620
29%
540
80%
540
68%
674
June16th to endNight
85%
540
34%
1620
31%
540
86%
540
72%
655
July1st to 15thNight
86%
510
24%
1530
21%
510
86%
510
47%
781
July16th to endNight
80%
564
22%
1593
18%
531
83%
531
38%
908
August1st to 15thNight
87%
539
29%
1611
25%
536
87%
536
49%
797
August16th to endNight
83%
654
33%
1851
29%
654
84%
654
44%
1004
September1st to 15thNight
83%
720
30%
1980
26%
720
85%
720
51%
1043
September16th to endNight
81%
716
45%
1980
35%
716
87%
716
72%
866
October1st to 15thNight
76%
720
38%
2052
31%
720
81%
720
63%
931
October16th to endNight
81%
768
45%
2304
39%
768
86%
768
63%
1012
November1st to 15thNight
77%
760
37%
2365
32%
760
84%
760
57%
1069
November16th to endNight
73%
780
41%
2535
39%
780
81%
780
55%
1112
December1st to 15thNight
73%
780
35%
2535
33%
780
78%
780
47%
1213
December16th to endNight
64%
832
35%
2704
33%
832
71%
832
39%
1419

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
3313323446%63%
48977
Night
2212323346%67%
48553
TransparencyDay
20%45%15%28%25%
48672
Night
17%36%15%29%26%
48359
SeeingDay
49%21%31%22%8%
5842
Night
3313%35%35%8%
16138

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
4313333236%63%
1710
16th to endDay
6%413323346%60%
1728
February1st to 15thDay
35%24223346%60%
1680
16th to endDay
5%5%14423447%56%
1579
March1st to 15thDay
2313323346%64%
1965
16th to endDay
33123235%45%65%
2112
April1st to 15thDay
221332345%6%65%
1973
16th to endDay
122212235%72%
2307
May1st to 15thDay
2212313336%68%
2331
16th to endDay
321221333470%
2429
June1st to 15thDay
11221122479%
2340
16th to endDay
11121222380%
2340
July1st to 15thDay
22135%35%6%6%7%53%
2340
16th to endDay
5%426%7%47%8%7%9%34%
2467
August1st to 15thDay
43245%46%5%7%8%47%
2340
16th to endDay
44245%25%5%6%7%50%
2345
September1st to 15thDay
4213435%5%6%7%54%
1980
16th to endDay
2111222245%74%
1980
October1st to 15thDay
111221333473%
1908
16th to endDay
212222335%73%
1920
November1st to 15thDay
112323446%68%
1821
16th to endDay
4312212236%70%
1755
December1st to 15thDay
45%12424336%61%
1755
16th to endDay
6%5%24323347%55%
1872
January1st to 15thNight
3313433446%62%
2471
16th to endNight
5%424324235%61%
2496
February1st to 15thNight
2413222246%67%
2170
16th to endNight
441232445%7%60%
1895
March1st to 15thNight
232231325%6%66%
1994
16th to endNight
221221335%6%68%
2112
April1st to 15thNight
2223323346%66%
1975
16th to endNight
112112235%78%
1650
May1st to 15thNight
122212235%73%
1619
16th to endNight
1121122479%
1684
June1st to 15thNight
111212225%78%
1620
16th to endNight
1121212382%
1620
July1st to 15thNight
21133345%6%7%59%
1620
16th to endNight
1315%6%36%7%7%12%44%
1701
August1st to 15thNight
1124335%6%11%58%
1620
16th to endNight
2114335%5%7%8%55%
1851
September1st to 15thNight
221342346%8%59%
1980
16th to endNight
111111125%80%
1980
October1st to 15thNight
2111212235%74%
2052
16th to endNight
11221223476%
2304
November1st to 15thNight
1212223345%69%
2365
16th to endNight
3112212225%72%
2535
December1st to 15thNight
3312323445%64%
2535
16th to endNight
6%41332435%6%57%
2704

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
23%26%16%33%17%
1711
16th to endDay
25%45%18%29%16%
1728
February1st to 15thDay
25%26%19%24%22%
1680
16th to endDay
26%45%18%29%15%
1579
March1st to 15thDay
21%3315%36%20%
1965
16th to endDay
19%4418%36%17%
2112
April1st to 15thDay
18%3418%34%20%
1973
16th to endDay
14%2315%34%28%
2307
May1st to 15thDay
16%2415%31%28%
2331
16th to endDay
16%2311%34%31%
2432
June1st to 15thDay
10%126%26%51%
2340
16th to endDay
9%126%20%59%
2340
July1st to 15thDay
26%6%9%14%16%26%
2201
16th to endDay
36%12%14%16%15%5%
2312
August1st to 15thDay
29%6%9%17%18%18%
2325
16th to endDay
27%7%8%17%24%13%
2345
September1st to 15thDay
25%6%9%16%23%19%
1980
16th to endDay
13%2410%34%35%
1980
October1st to 15thDay
15%2314%28%35%
1908
16th to endDay
13%2412%34%32%
1920
November1st to 15thDay
15%3420%31%24%
1821
16th to endDay
18%1318%30%28%
1755
December1st to 15thDay
24%47%21%24%17%
1755
16th to endDay
28%37%22%27%11%
1872
January1st to 15thNight
22%36%16%33%17%
2471
16th to endNight
27%2416%30%18%
2496
February1st to 15thNight
19%27%19%27%23%
2181
16th to endNight
22%5%5%19%29%17%
1895
March1st to 15thNight
18%3413%36%23%
1994
16th to endNight
15%37%18%33%19%
2112
April1st to 15thNight
19%3414%35%22%
1975
16th to endNight
9%1414%36%33%
1650
May1st to 15thNight
13%3415%30%33%
1620
16th to endNight
10%1211%38%34%
1685
June1st to 15thNight
10%125%23%55%
1620
16th to endNight
9%12415%66%
1620
July1st to 15thNight
20%6%11%14%19%28%
1530
16th to endNight
25%9%17%20%18%7%
1593
August1st to 15thNight
18%6%12%19%21%22%
1611
16th to endNight
22%7%8%21%25%15%
1851
September1st to 15thNight
20%5%12%18%21%21%
1980
16th to endNight
9%1310%36%38%
1980
October1st to 15thNight
13%2416%27%36%
2052
16th to endNight
12%25%11%27%40%
2304
November1st to 15thNight
16%25%16%35%23%
2365
16th to endNight
16%2315%30%31%
2535
December1st to 15thNight
20%46%19%27%21%
2535
16th to endNight
26%47%20%28%12%
2704

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
13%7%19%34%21%2
123
16th to endDay
6%11%18%27%22%13%
96
February1st to 15thDay
414%26%29%18%5%
195
16th to endDay
10%21%31%20%13%2
238
March1st to 15thDay
419%24%30%15%5%
270
16th to endDay
421%30%24%15%3
288
April1st to 15thDay
7%19%30%24%9%8%
269
16th to endDay
425%34%22%8%3
270
May1st to 15thDay
5%17%31%27%11%5%
288
16th to endDay
5%10%29%32%17%5%
330
June1st to 15thDay
29%32%31%16%7%
315
16th to endDay
118%44%21%13%
315
July1st to 15thDay
2115%41%29%9%
305
16th to endDay
5%7%32%33%19%
327
August1st to 15thDay
6%18%32%38%12%
315
16th to endDay
427%34%36%15%
332
September1st to 15thDay
4217%39%28%8%
315
16th to endDay
5%8%20%34%23%6%
315
October1st to 15thDay
28%29%31%21%6%
243
16th to endDay
1213%34%31%16%
144
November1st to 15thDay
25%17%32%33%8%
135
16th to endDay
48%27%26%23%9%
135
December1st to 15thDay
37%14%37%32%4
135
16th to endDay
6%7%12%35%30%7%
144
January1st to 15thNight
4415%41%24%9%
764
16th to endNight
8%418%38%25%5%
768
February1st to 15thNight
4417%41%27%4
704
16th to endNight
5%5%21%43%19%3
632
March1st to 15thNight
47%20%35%28%3
720
16th to endNight
26%19%37%27%5%
768
April1st to 15thNight
46%24%37%24%3
716
16th to endNight
7%27%36%23%4
540
May1st to 15thNight
2417%42%29%3
540
16th to endNight
2315%40%32%4
563
June1st to 15thNight
2113%44%32%5%
540
16th to endNight
18%35%48%6%
540
July1st to 15thNight
2224%59%10%
510
16th to endNight
21116%58%19%
564
August1st to 15thNight
3116%60%18%
539
16th to endNight
2215%56%22%
654
September1st to 15thNight
25%23%53%15%
720
16th to endNight
112%31%42%10%
716
October1st to 15thNight
2215%38%35%5%
720
16th to endNight
1210%32%39%13%
768
November1st to 15thNight
3114%40%33%7%
760
16th to endNight
4314%35%33%8%
780
December1st to 15thNight
6%214%41%31%3
780
16th to endNight
9%413%40%25%5%
832
Page generated at 2018-01-12 20:01:31. Version ac04

Copyright 2018 A. Danko


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