3 Mile Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the 3 Mile 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
63%
15428
31%
47295
23%
15386
71%
15386
45%
24110

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
56%
716
35%
2148
23%
716
68%
716
45%
1110
January16th to endNight
54%
768
38%
2304
24%
768
66%
768
46%
1155
February1st to 15thNight
57%
702
37%
2106
22%
696
72%
696
54%
981
February16th to endNight
46%
632
34%
1894
18%
632
60%
632
50%
919
March1st to 15thNight
52%
720
35%
2130
19%
720
65%
720
49%
1056
March16th to endNight
50%
654
37%
1998
22%
654
65%
654
50%
955
April1st to 15thNight
46%
539
34%
1798
21%
539
64%
539
43%
857
April16th to endNight
50%
540
36%
1800
24%
540
69%
540
51%
793
May1st to 15thNight
50%
539
29%
1799
21%
539
66%
539
46%
849
May16th to endNight
60%
561
26%
1790
25%
561
68%
561
40%
938
June1st to 15thNight
73%
540
24%
1620
23%
540
78%
540
49%
818
June16th to endNight
77%
540
24%
1646
25%
540
79%
540
40%
890
July1st to 15thNight
79%
510
18%
1700
21%
510
80%
510
37%
923
July16th to endNight
80%
564
22%
1771
25%
531
82%
531
34%
995
August1st to 15thNight
84%
539
23%
1789
28%
536
85%
536
39%
933
August16th to endNight
78%
573
20%
1910
24%
573
78%
573
28%
1145
September1st to 15thNight
76%
540
20%
1806
23%
540
76%
540
32%
1030
September16th to endNight
74%
537
26%
1980
28%
537
76%
537
30%
1120
October1st to 15thNight
70%
538
32%
1906
31%
538
74%
538
42%
922
October16th to endNight
72%
704
37%
2112
26%
704
77%
704
57%
985
November1st to 15thNight
69%
766
38%
2166
25%
766
75%
766
61%
1009
November16th to endNight
64%
900
38%
2340
21%
900
73%
900
56%
1195
December1st to 15thNight
60%
900
38%
2340
23%
900
67%
900
52%
1266
December16th to endNight
54%
906
38%
2442
21%
906
65%
906
50%
1266

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
431332445%7%59%
47860
Night
321332445%8%59%
47521
Cloud CoverDay
431332445%7%59%
47858
Night
321332445%8%60%
47521
TransparencyDay
23%5%8%17%25%19%
47580
Night
21%5%9%18%24%20%
47303
SeeingDay
5%5%19%29%28%11%
5245
Night
4322%41%24%2
15437

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
6%312323436%62%
1790
16th to endDay
4422213345%64%
1920
February1st to 15thDay
2112224445%68%
1750
16th to endDay
222213246%70%
1580
March1st to 15thDay
131432345%7%61%
1827
16th to endDay
2312213346%68%
2224
April1st to 15thDay
2212314446%66%
2150
16th to endDay
212212245%73%
2160
May1st to 15thDay
2212323345%69%
2151
16th to endDay
22221346%7%65%
2318
June1st to 15thDay
1222345%8%67%
2340
16th to endDay
3213436%5%8%9%51%
2309
July1st to 15thDay
6%425%5%36%7%8%8%41%
2160
16th to endDay
5%325%6%47%7%7%11%37%
2277
August1st to 15thDay
5%3146%5%7%7%7%10%40%
2160
16th to endDay
7%426%7%47%6%9%8%35%
2292
September1st to 15thDay
9%5%25%6%46%6%5%7%39%
2154
16th to endDay
7%4245%345%5%7%48%
1980
October1st to 15thDay
43231346%5%65%
1849
16th to endDay
33221222473%
1760
November1st to 15thDay
321222345%71%
1678
16th to endDay
421231224471%
1620
December1st to 15thDay
331321335%6%64%
1620
16th to endDay
331232335%7%63%
1791
January1st to 15thNight
441432435%6%61%
2148
16th to endNight
5%21232336%7%61%
2304
February1st to 15thNight
2212223346%67%
2102
16th to endNight
3212223447%65%
1894
March1st to 15thNight
331222335%8%62%
2130
16th to endNight
3312223336%66%
1998
April1st to 15thNight
331232345%6%64%
1798
16th to endNight
211221433470%
1800
May1st to 15thNight
321332345%7%64%
1799
16th to endNight
3213325%5%6%8%57%
1789
June1st to 15thNight
212325%7%7%10%55%
1620
16th to endNight
3213325%7%8%10%50%
1646
July1st to 15thNight
3213425%8%6%11%49%
1800
16th to endNight
42146%35%5%7%12%46%
1891
August1st to 15thNight
32133346%11%15%44%
1800
16th to endNight
3315%5%5%6%7%8%11%41%
1910
September1st to 15thNight
5%3135%246%8%11%45%
1806
16th to endNight
5%3145%45%6%7%11%44%
1980
October1st to 15thNight
421231445%7%60%
1906
16th to endNight
321221244471%
2112
November1st to 15thNight
322212246%71%
2166
16th to endNight
3212223235%69%
2340
December1st to 15thNight
4312224346%62%
2340
16th to endNight
3313223335%67%
2442

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
5%413314336%61%
1790
16th to endDay
441322335%5%64%
1920
February1st to 15thDay
2112223445%68%
1750
16th to endDay
222213336%70%
1580
March1st to 15thDay
131432345%7%61%
1827
16th to endDay
2312213346%68%
2222
April1st to 15thDay
2212324446%66%
2150
16th to endDay
212212245%73%
2160
May1st to 15thDay
223223435%68%
2151
16th to endDay
321121345%7%66%
2318
June1st to 15thDay
1113235%5%8%67%
2340
16th to endDay
3123435%6%8%9%51%
2309
July1st to 15thDay
7%4245%37%6%7%9%40%
2160
16th to endDay
6%425%6%46%6%9%10%36%
2277
August1st to 15thDay
6%315%6%47%8%7%10%40%
2160
16th to endDay
7%5%36%6%47%6%7%9%35%
2292
September1st to 15thDay
10%5%245%45%5%6%8%39%
2154
16th to endDay
8%414435%45%7%49%
1980
October1st to 15thDay
5%31221345%5%65%
1849
16th to endDay
321221212474%
1760
November1st to 15thDay
3112212236%71%
1678
16th to endDay
43221223471%
1620
December1st to 15thDay
4312313347%63%
1620
16th to endDay
3312223446%64%
1791
January1st to 15thNight
4423324446%61%
2148
16th to endNight
5%21242335%6%62%
2304
February1st to 15thNight
222223347%67%
2102
16th to endNight
3212323346%65%
1894
March1st to 15thNight
331331335%8%62%
2130
16th to endNight
3312323336%67%
1998
April1st to 15thNight
4212323446%64%
1798
16th to endNight
211221434471%
1800
May1st to 15thNight
311232445%6%64%
1799
16th to endNight
4213325%46%8%58%
1789
June1st to 15thNight
2112325%6%6%10%56%
1620
16th to endNight
3113335%7%8%11%50%
1646
July1st to 15thNight
3314326%6%8%10%49%
1800
16th to endNight
4314435%5%7%12%47%
1891
August1st to 15thNight
4213325%6%11%14%44%
1800
16th to endNight
3225%5%47%6%8%11%41%
1910
September1st to 15thNight
6%3234345%8%11%46%
1806
16th to endNight
5%314435%7%7%10%45%
1980
October1st to 15thNight
421332345%7%60%
1906
16th to endNight
32221334471%
2112
November1st to 15thNight
222212246%72%
2166
16th to endNight
3212223335%69%
2340
December1st to 15thNight
431231335%6%62%
2340
16th to endNight
3313223335%67%
2442

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
24%3417%31%17%
1791
16th to endDay
21%35%14%35%19%
1920
February1st to 15thDay
16%35%14%25%33%
1751
16th to endDay
15%2313%36%27%
1580
March1st to 15thDay
21%3415%27%27%
1827
16th to endDay
17%25%11%28%33%
2224
April1st to 15thDay
18%37%9%28%32%
2150
16th to endDay
14%249%29%39%
2160
May1st to 15thDay
18%35%13%26%34%
2151
16th to endDay
16%48%17%26%26%
2321
June1st to 15thDay
13%410%19%29%21%
2340
16th to endDay
25%7%12%20%22%12%
2309
July1st to 15thDay
35%9%13%23%16%2
2031
16th to endDay
34%10%17%24%10%1
2134
August1st to 15thDay
34%9%15%25%14%1
2147
16th to endDay
39%10%17%17%12%2
2292
September1st to 15thDay
39%7%14%20%13%4
2154
16th to endDay
32%7%10%20%21%7%
1980
October1st to 15thDay
19%410%25%22%18%
1849
16th to endDay
17%1416%29%30%
1760
November1st to 15thDay
15%37%20%30%23%
1678
16th to endDay
17%26%14%32%26%
1620
December1st to 15thDay
21%35%19%32%17%
1620
16th to endDay
19%35%21%33%14%
1791
January1st to 15thNight
24%36%16%29%20%
2148
16th to endNight
22%35%15%35%17%
2304
February1st to 15thNight
17%26%12%26%33%
2112
16th to endNight
19%36%9%32%30%
1894
March1st to 15thNight
20%2416%26%29%
2130
16th to endNight
18%36%14%27%30%
1998
April1st to 15thNight
20%47%10%25%30%
1798
16th to endNight
17%26%11%24%37%
1800
May1st to 15thNight
19%47%15%21%30%
1800
16th to endNight
23%7%9%18%18%22%
1791
June1st to 15thNight
19%6%13%23%23%13%
1620
16th to endNight
23%8%14%20%20%13%
1646
July1st to 15thNight
25%8%18%30%15%2
1700
16th to endNight
26%10%19%28%13%1
1771
August1st to 15thNight
22%8%21%29%15%1
1789
16th to endNight
31%11%17%23%13%3
1910
September1st to 15thNight
29%9%20%23%11%5%
1806
16th to endNight
30%9%15%20%16%7%
1980
October1st to 15thNight
21%6%12%23%18%17%
1906
16th to endNight
16%35%14%28%31%
2112
November1st to 15thNight
14%26%18%34%22%
2166
16th to endNight
17%2413%29%31%
2340
December1st to 15thNight
22%35%19%28%20%
2340
16th to endNight
20%25%19%35%16%
2442

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
12%5%30%35%15%1
90
16th to endDay
6%18%32%21%13%7%
96
February1st to 15thDay
18%23%22%36%7%
90
16th to endDay
8%14%20%33%18%3
81
March1st to 15thDay
434%39%13%7%
117
16th to endDay
313%42%23%11%4
288
April1st to 15thDay
5%12%45%21%12%2
269
16th to endDay
216%39%29%8%3
270
May1st to 15thDay
210%41%32%11%1
288
16th to endDay
4326%41%21%1
405
June1st to 15thDay
111%41%36%9%
495
16th to endDay
38%29%46%11%
464
July1st to 15thDay
6%5%26%33%25%
305
16th to endDay
4424%41%23%
327
August1st to 15thDay
8%416%41%27%
315
16th to endDay
9%1325%37%21%
332
September1st to 15thDay
7%17%30%36%16%
309
16th to endDay
11%14%27%33%12%
135
October1st to 15thDay
14%12%19%37%14%
107
16th to endDay
4213%25%30%25%
96
November1st to 15thDay
11%5%27%26%21%7%
90
16th to endDay
12%13%25%28%14%5%
90
December1st to 15thDay
7%5%16%45%21%3
90
16th to endDay
7%12%34%30%14%1
96
January1st to 15thNight
6%327%42%18%2
720
16th to endNight
5%5%29%40%16%1
768
February1st to 15thNight
36%30%38%19%1
704
16th to endNight
46%38%41%9%
632
March1st to 15thNight
5%7%31%39%14%1
720
16th to endNight
37%34%36%15%2
654
April1st to 15thNight
38%37%43%6%
539
16th to endNight
27%37%42%9%
540
May1st to 15thNight
45%37%42%10%
540
16th to endNight
3327%48%16%
563
June1st to 15thNight
216%57%22%
540
16th to endNight
410%54%29%
540
July1st to 15thNight
36%47%40%1
510
16th to endNight
3444%43%3
564
August1st to 15thNight
3130%57%7%
539
16th to endNight
45%32%52%5%
573
September1st to 15thNight
5%6%38%43%5%
540
16th to endNight
5%9%45%35%2
537
October1st to 15thNight
5%17%46%27%2
538
16th to endNight
4217%37%29%8%
704
November1st to 15thNight
4220%41%26%5%
766
16th to endNight
4421%40%25%4
900
December1st to 15thNight
6%425%40%20%2
900
16th to endNight
5%7%27%41%14%2
906
Page generated at 2017-10-07 13:07:55. Version ac04

Copyright 2017 A. Danko


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