Twentymile Rock
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Twentymile Rock clear sky chart from 2005-11-05 to 2017-09-21.

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
54%
15435
18%
46830
15%
15398
57%
15398
28%
29347

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
43%
731
9%
2521
8%
731
44%
731
13%
1907
January16th to endNight
45%
768
11%
2557
11%
768
47%
768
16%
1840
February1st to 15thNight
44%
699
9%
2288
8%
696
47%
696
16%
1650
February16th to endNight
42%
632
13%
1928
10%
632
47%
632
20%
1345
March1st to 15thNight
48%
720
12%
2049
10%
720
51%
720
23%
1435
March16th to endNight
45%
768
15%
2112
9%
768
49%
768
25%
1446
April1st to 15thNight
47%
643
17%
1807
10%
643
51%
643
28%
1189
April16th to endNight
49%
540
21%
1620
14%
540
52%
540
30%
1003
May1st to 15thNight
54%
539
19%
1619
15%
539
56%
539
35%
938
May16th to endNight
55%
561
17%
1597
15%
561
58%
561
31%
998
June1st to 15thNight
56%
369
17%
1269
15%
369
64%
369
31%
710
June16th to endNight
69%
360
29%
1260
32%
360
78%
360
54%
530
July1st to 15thNight
78%
340
23%
1282
30%
340
83%
340
50%
528
July16th to endNight
79%
488
30%
1519
34%
457
82%
457
60%
633
August1st to 15thNight
75%
539
28%
1611
25%
536
77%
536
52%
792
August16th to endNight
73%
573
38%
1719
29%
573
77%
573
56%
804
September1st to 15thNight
70%
679
29%
1911
21%
679
76%
679
52%
985
September16th to endNight
68%
680
32%
1881
25%
680
72%
680
47%
1006
October1st to 15thNight
60%
660
25%
1919
21%
660
64%
660
38%
1103
October16th to endNight
60%
704
23%
2181
20%
704
65%
704
33%
1284
November1st to 15thNight
53%
700
20%
2275
19%
700
55%
700
26%
1409
November16th to endNight
49%
882
14%
2502
10%
882
53%
882
26%
1677
December1st to 15thNight
40%
900
8%
2523
5%
900
43%
900
17%
1906
December16th to endNight
37%
960
8%
2880
5%
960
40%
960
14%
2229

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
6%6%25%5%2445%7%48%
48034
Night
7%7%35%5%3445%6%45%
47010
TransparencyDay
34%6%12%20%17%9%
47713
Night
37%6%14%19%14%7%
46831
SeeingDay
9%8%23%28%21%9%
5756
Night
11%421%38%21%2
15447

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
9%10%410%8%45%447%29%
1426
16th to endDay
12%12%5%8%8%35%45%6%28%
1665
February1st to 15thDay
6%9%5%10%7%35%6%6%8%30%
1575
16th to endDay
6%10%48%7%35%46%6%35%
1545
March1st to 15thDay
5%9%5%8%6%36%6%48%34%
1909
16th to endDay
7%10%48%6%445%5%6%35%
2112
April1st to 15thDay
7%8%35%5%35%5%6%7%40%
2137
16th to endDay
7%7%35%5%2445%7%44%
2340
May1st to 15thDay
5%6%2442435%8%50%
2331
16th to endDay
9%7%26%425%5%5%9%41%
2516
June1st to 15thDay
7%5%26%5%345%6%9%44%
2688
16th to endDay
2212212346%70%
2700
July1st to 15thDay
221222235%9%64%
2597
16th to endDay
2213213346%68%
2545
August1st to 15thDay
2212313446%66%
2340
16th to endDay
2212322346%67%
2483
September1st to 15thDay
4212212346%67%
2046
16th to endDay
6%3222235%7%63%
1871
October1st to 15thDay
10%6%24424446%50%
1711
16th to endDay
7%5%25%5%35%5%5%7%46%
1689
November1st to 15thDay
9%7%36%5%34446%43%
1584
16th to endDay
7%8%46%6%2446%7%41%
1446
December1st to 15thDay
9%12%5%11%9%345%45%27%
1434
16th to endDay
11%14%6%9%7%44337%26%
1344
January1st to 15thNight
9%12%5%9%7%45%446%29%
2521
16th to endNight
11%12%5%9%5%34446%33%
2557
February1st to 15thNight
9%10%47%6%46%5%6%7%32%
2277
16th to endNight
6%10%5%7%6%45%45%7%35%
1928
March1st to 15thNight
48%38%8%46%5%6%7%35%
2049
16th to endNight
8%8%36%5%36%46%7%36%
2112
April1st to 15thNight
8%8%35%5%245%6%8%40%
1807
16th to endNight
8%8%36%434445%46%
1620
May1st to 15thNight
46%35%425%5%46%50%
1619
16th to endNight
8%7%25%5%3445%8%44%
1596
June1st to 15thNight
7%425%4345%6%8%46%
1269
16th to endNight
2212312336%69%
1260
July1st to 15thNight
221132445%8%62%
1360
16th to endNight
122222446%69%
1624
August1st to 15thNight
32123145%5%8%61%
1620
16th to endNight
2213213446%67%
1719
September1st to 15thNight
5%313212345%65%
1911
16th to endNight
7%313323347%59%
1881
October1st to 15thNight
8%5%24323346%55%
1919
16th to endNight
7%5%245%35%45%6%49%
2181
November1st to 15thNight
10%8%3443345%5%45%
2275
16th to endNight
6%7%36%7%35%6%5%7%41%
2502
December1st to 15thNight
10%11%5%8%7%3445%6%31%
2523
16th to endNight
12%16%5%7%6%35%445%28%
2880

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
53%11%16%13%31
1426
16th to endDay
56%8%14%14%41
1665
February1st to 15thDay
49%12%17%16%4
1575
16th to endDay
47%9%17%20%41
1545
March1st to 15thDay
45%9%17%19%6%1
1909
16th to endDay
47%7%15%20%8%
2112
April1st to 15thDay
39%7%18%21%12%
2137
16th to endDay
37%5%11%28%16%1
2340
May1st to 15thDay
30%5%9%28%21%3
2331
16th to endDay
37%7%13%25%13%2
2519
June1st to 15thDay
34%5%11%25%18%4
2688
16th to endDay
14%28%19%34%20%
2700
July1st to 15thDay
16%26%17%29%26%
2446
16th to endDay
17%25%12%23%38%
2387
August1st to 15thDay
17%37%16%28%26%
2325
16th to endDay
16%36%19%30%23%
2483
September1st to 15thDay
18%38%20%33%16%
2046
16th to endDay
20%410%28%27%8%
1871
October1st to 15thDay
34%5%16%29%11%3
1711
16th to endDay
34%7%17%24%15%
1689
November1st to 15thDay
41%5%19%22%10%1
1584
16th to endDay
41%8%22%19%6%1
1446
December1st to 15thDay
56%8%15%15%3
1434
16th to endDay
58%7%16%11%3
1344
January1st to 15thNight
53%10%15%14%3
2521
16th to endNight
52%7%17%16%5%1
2557
February1st to 15thNight
50%9%17%17%4
2288
16th to endNight
46%8%18%19%5%1
1928
March1st to 15thNight
44%8%17%21%7%1
2049
16th to endNight
42%8%16%23%8%1
2112
April1st to 15thNight
39%7%15%23%12%1
1807
16th to endNight
37%412%24%19%
1620
May1st to 15thNight
31%5%11%26%21%3
1620
16th to endNight
35%6%13%24%15%3
1597
June1st to 15thNight
31%6%13%20%23%5%
1269
16th to endNight
15%26%14%33%26%
1260
July1st to 15thNight
16%47%16%26%29%
1282
16th to endNight
14%25%13%23%41%
1519
August1st to 15thNight
19%38%15%22%30%
1611
16th to endNight
17%26%14%29%29%
1719
September1st to 15thNight
20%37%13%32%22%
1911
16th to endNight
23%48%25%27%10%
1881
October1st to 15thNight
29%5%15%28%16%4
1919
16th to endNight
32%6%14%25%19%1
2181
November1st to 15thNight
38%6%16%25%10%2
2275
16th to endNight
41%7%20%19%9%1
2502
December1st to 15thNight
52%9%19%13%4
2523
16th to endNight
58%7%16%13%3
2880

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
16%11%8%21%21%20%
60
16th to endDay
22%111%29%28%7%
85
February1st to 15thDay
16%7%26%17%25%5%
90
16th to endDay
9%11%28%31%14%3
204
March1st to 15thDay
8%15%28%32%14%1
270
16th to endDay
13%17%33%25%10%
288
April1st to 15thDay
12%17%26%22%14%7%
269
16th to endDay
13%14%28%26%12%4
270
May1st to 15thDay
8%9%30%27%18%6%
288
16th to endDay
13%5%20%25%21%12%
417
June1st to 15thDay
9%9%20%28%23%7%
495
16th to endDay
45%24%30%25%8%
495
July1st to 15thDay
4317%32%29%12%
380
16th to endDay
2322%38%22%10%
327
August1st to 15thDay
5%223%31%27%9%
315
16th to endDay
2317%30%25%19%
332
September1st to 15thDay
47%20%24%27%15%
315
16th to endDay
7%7%21%24%27%11%
276
October1st to 15thDay
10%6%24%21%23%13%
151
16th to endDay
15%8%17%19%27%11%
96
November1st to 15thDay
13%6%24%36%18%
90
16th to endDay
21%320%44%10%1
90
December1st to 15thDay
20%7%30%31%8%1
89
16th to endDay
17%317%32%18%10%
64
January1st to 15thNight
15%6%19%36%20%2
735
16th to endNight
17%418%33%20%4
768
February1st to 15thNight
15%422%37%18%1
704
16th to endNight
9%6%28%38%12%2
632
March1st to 15thNight
7%6%25%43%15%1
720
16th to endNight
11%6%29%38%13%
768
April1st to 15thNight
11%6%26%37%16%1
643
16th to endNight
11%6%27%37%15%1
540
May1st to 15thNight
8%427%37%20%
540
16th to endNight
12%419%36%24%2
563
June1st to 15thNight
7%323%41%22%1
369
16th to endNight
3121%44%25%3
360
July1st to 15thNight
212%45%37%1
340
16th to endNight
312%46%33%3
488
August1st to 15thNight
4111%44%35%1
539
16th to endNight
3117%40%34%3
573
September1st to 15thNight
6%116%37%30%7%
679
16th to endNight
8%215%33%31%8%
680
October1st to 15thNight
11%218%37%27%2
660
16th to endNight
10%219%42%21%3
704
November1st to 15thNight
12%5%21%37%20%2
700
16th to endNight
10%6%24%38%18%2
882
December1st to 15thNight
16%7%21%40%12%1
900
16th to endNight
20%7%20%35%14%1
960
Page generated at 2017-09-22 05:35:32. Version ac04

Copyright 2017 A. Danko


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