Shirley Road Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Shirley Road Observatory clear sky chart from 2005-11-05 to 2017-03-31.

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
47%
15065
22%
45337
15%
15022
55%
15022
27%
28411

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
29%
720
23%
2327
10%
716
45%
716
20%
1443
January16th to endNight
27%
768
25%
2496
12%
768
44%
768
21%
1532
February1st to 15thNight
31%
704
26%
2125
10%
701
48%
701
26%
1279
February16th to endNight
26%
631
26%
1893
10%
631
41%
631
26%
1147
March1st to 15thNight
33%
720
24%
2023
13%
720
43%
720
28%
1266
March16th to endNight
38%
768
21%
2112
10%
768
46%
768
27%
1349
April1st to 15thNight
41%
644
26%
1797
16%
644
49%
644
32%
1083
April16th to endNight
46%
495
23%
1515
14%
495
54%
495
29%
912
May1st to 15thNight
47%
495
20%
1485
11%
495
55%
495
27%
948
May16th to endNight
59%
513
17%
1539
13%
513
62%
513
28%
974
June1st to 15thNight
65%
495
17%
1485
14%
495
67%
495
29%
951
June16th to endNight
73%
495
15%
1485
13%
495
74%
495
31%
927
July1st to 15thNight
65%
465
10%
1395
9%
465
65%
465
21%
996
July16th to endNight
73%
522
14%
1469
12%
489
75%
489
26%
982
August1st to 15thNight
67%
494
15%
1474
14%
491
68%
491
23%
1013
August16th to endNight
62%
547
14%
1723
13%
547
63%
547
19%
1225
September1st to 15thNight
65%
660
17%
1815
15%
660
67%
660
25%
1248
September16th to endNight
53%
657
21%
1815
15%
657
58%
657
28%
1200
October1st to 15thNight
51%
660
31%
1862
24%
660
59%
660
34%
1126
October16th to endNight
54%
704
34%
2112
26%
704
66%
704
39%
1150
November1st to 15thNight
52%
700
31%
2214
26%
700
62%
700
37%
1203
November16th to endNight
41%
720
27%
2340
19%
720
54%
720
29%
1355
December1st to 15thNight
39%
720
24%
2340
15%
720
50%
720
24%
1444
December16th to endNight
32%
768
22%
2496
15%
768
43%
768
18%
1658

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
13%5%245%3445%7%42%
45741
Night
13%42442445%7%45%
45535
TransparencyDay
38%7%11%19%17%5%
45442
Night
37%6%10%18%19%6%
45338
SeeingDay
17%5%19%22%21%13%
4792
Night
16%7%22%29%20%3
15067

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 15thDay
17%5%25%42433445%
1611
16th to endDay
18%424323346%47%
1728
February1st to 15thDay
14%5%145%2445%5%46%
1735
16th to endDay
17%5%25%412336%45%
1580
March1st to 15thDay
16%42342435%7%47%
1935
16th to endDay
16%424424446%43%
2106
April1st to 15thDay
12%425%43446%7%45%
1829
16th to endDay
10%3244345%46%50%
2115
May1st to 15thDay
11%425%5%35%5%6%8%41%
2136
16th to endDay
10%415%5%35%6%6%8%42%
2223
June1st to 15thDay
9%4246%46%6%8%11%35%
2145
16th to endDay
32136%48%7%8%11%41%
2145
July1st to 15thDay
10%5%36%7%46%7%7%11%29%
2145
16th to endDay
7%425%6%46%7%9%13%31%
2283
August1st to 15thDay
9%416%7%47%6%8%9%32%
2145
16th to endDay
14%6%27%7%46%6%6%8%27%
2113
September1st to 15thDay
11%6%25%5%36%6%6%7%37%
1815
16th to endDay
19%5%2342345%7%41%
1815
October1st to 15thDay
15%42321223456%
1766
16th to endDay
11%5%1432223360%
1760
November1st to 15thDay
12%41322334456%
1643
16th to endDay
15%5%23413345%49%
1620
December1st to 15thDay
18%7%24423346%42%
1620
16th to endDay
25%7%24412335%39%
1728
January1st to 15thNight
17%5%2442423447%
2327
16th to endNight
17%6%24422345%45%
2496
February1st to 15thNight
14%5%14424335%50%
2116
16th to endNight
16%423423345%48%
1893
March1st to 15thNight
14%424324445%48%
2023
16th to endNight
13%5%2441445%6%46%
2112
April1st to 15thNight
13%413423346%51%
1797
16th to endNight
11%324425%45%6%49%
1515
May1st to 15thNight
11%5%345%25%45%8%43%
1484
16th to endNight
8%415%6%35%6%5%8%43%
1538
June1st to 15thNight
8%3245%36%5%7%13%38%
1485
16th to endNight
4324435%8%9%11%42%
1485
July1st to 15thNight
9%6%26%5%46%6%8%11%31%
1485
16th to endNight
6%3146%47%8%9%14%31%
1577
August1st to 15thNight
10%425%6%48%6%8%11%31%
1485
16th to endNight
11%5%25%7%36%6%8%10%31%
1723
September1st to 15thNight
10%4146%47%7%7%10%34%
1815
16th to endNight
15%5%15%4235%5%6%42%
1815
October1st to 15thNight
15%423323345%53%
1862
16th to endNight
10%31332333460%
2112
November1st to 15thNight
11%314323225%59%
2214
16th to endNight
15%424422245%51%
2340
December1st to 15thNight
15%5%245%24345%45%
2340
16th to endNight
21%6%24424336%39%
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
41%48%15%24%5%
1612
16th to endDay
38%36%19%22%8%
1728
February1st to 15thDay
38%39%19%23%5%
1737
16th to endDay
39%46%22%20%7%
1580
March1st to 15thDay
37%48%20%19%8%
1935
16th to endDay
40%5%11%21%16%4
2107
April1st to 15thDay
36%5%10%20%21%6%
1829
16th to endDay
33%6%8%20%23%6%
2115
May1st to 15thDay
37%9%13%20%15%3
2136
16th to endDay
36%9%14%22%14%2
2227
June1st to 15thDay
38%10%20%20%8%1
2145
16th to endDay
30%12%18%21%14%3
2145
July1st to 15thDay
43%12%15%17%8%2
2006
16th to endDay
36%14%20%18%7%1
2128
August1st to 15thDay
42%10%15%19%9%1
2132
16th to endDay
49%10%15%13%9%1
2113
September1st to 15thDay
42%9%12%17%15%2
1815
16th to endDay
41%6%12%18%16%5%
1815
October1st to 15thDay
32%410%20%25%7%
1766
16th to endDay
29%35%20%29%12%
1760
November1st to 15thDay
30%37%17%28%11%
1643
16th to endDay
36%37%18%26%7%
1620
December1st to 15thDay
43%47%18%16%8%
1620
16th to endDay
48%37%17%15%6%
1728
January1st to 15thNight
41%36%15%25%6%
2327
16th to endNight
40%35%18%22%9%
2496
February1st to 15thNight
37%36%19%24%8%
2125
16th to endNight
36%37%22%22%7%
1893
March1st to 15thNight
36%49%17%21%9%
2023
16th to endNight
36%7%10%19%20%5%
2112
April1st to 15thNight
34%39%23%22%7%
1797
16th to endNight
33%6%9%19%19%10%
1515
May1st to 15thNight
38%6%14%18%18%2
1485
16th to endNight
35%9%14%26%11%2
1540
June1st to 15thNight
35%9%19%24%9%1
1485
16th to endNight
28%14%17%22%14%3
1485
July1st to 15thNight
42%12%18%16%7%1
1395
16th to endNight
34%14%22%18%8%1
1469
August1st to 15thNight
40%11%17%19%8%1
1474
16th to endNight
42%14%16%13%11%1
1723
September1st to 15thNight
38%12%16%17%11%3
1815
16th to endNight
39%7%12%18%15%6%
1815
October1st to 15thNight
33%5%9%18%25%8%
1862
16th to endNight
27%36%19%29%13%
2112
November1st to 15thNight
29%35%16%31%13%
2214
16th to endNight
35%36%17%27%10%
2340
December1st to 15thNight
40%48%19%18%8%
2340
16th to endNight
46%48%15%18%6%
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
27%5%34%27%4
90
16th to endDay
26%26%33%12%2
96
February1st to 15thDay
17%17%51%13%
90
16th to endDay
28%9%40%19%1
81
March1st to 15thDay
17%15%28%27%10%
225
16th to endDay
19%11%31%19%17%1
286
April1st to 15thDay
20%10%28%19%18%2
210
16th to endDay
11%430%24%21%7%
210
May1st to 15thDay
11%7%31%17%18%14%
228
16th to endDay
16%212%22%26%20%
267
June1st to 15thDay
12%310%26%24%23%
255
16th to endDay
5%14%23%26%29%
255
July1st to 15thDay
13%213%22%22%26%
245
16th to endDay
7%6%22%35%27%
270
August1st to 15thDay
16%7%25%28%22%
253
16th to endDay
17%19%21%28%21%
268
September1st to 15thDay
17%15%20%34%21%
255
16th to endDay
27%112%24%23%10%
254
October1st to 15thDay
21%418%27%21%6%
255
16th to endDay
14%419%26%29%5%
272
November1st to 15thDay
27%9%18%25%15%4
151
16th to endDay
14%430%32%15%3
90
December1st to 15thDay
31%8%35%16%7%
90
16th to endDay
38%15%26%13%6%
96
January1st to 15thNight
21%15%27%27%7%
720
16th to endNight
21%17%30%27%3
768
February1st to 15thNight
18%15%31%31%3
704
16th to endNight
17%15%36%25%4
631
March1st to 15thNight
19%11%32%29%6%
720
16th to endNight
16%11%29%28%14%
768
April1st to 15thNight
16%10%28%30%12%
644
16th to endNight
14%7%26%30%20%
495
May1st to 15thNight
13%426%33%20%2
495
16th to endNight
12%215%31%31%5%
515
June1st to 15thNight
10%113%32%33%9%
495
16th to endNight
6%13%26%39%13%
495
July1st to 15thNight
13%17%30%36%9%
465
16th to endNight
6%8%28%45%9%
522
August1st to 15thNight
12%7%28%41%9%
494
16th to endNight
15%17%26%39%10%
547
September1st to 15thNight
11%110%26%40%9%
660
16th to endNight
19%116%31%27%3
657
October1st to 15thNight
17%5%21%24%25%5%
660
16th to endNight
12%6%21%33%23%2
704
November1st to 15thNight
13%6%23%33%21%2
700
16th to endNight
18%10%25%29%15%1
720
December1st to 15thNight
18%10%24%32%11%
720
16th to endNight
25%10%25%30%8%
768
Page generated at 2017-04-01 08:23:19. Version ac04

Copyright 2017 A. Danko


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