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The Dilly Hill Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the The Dilly Hill Observatory clear sky chart from 2005-11-05 to 2017-09-10.

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
51%
15231
23%
46906
17%
15188
59%
15188
29%
28582

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
32%
719
24%
2273
11%
716
46%
716
23%
1395
January16th to endNight
27%
768
26%
2304
11%
768
41%
768
23%
1484
February1st to 15thNight
34%
704
26%
2111
12%
700
46%
700
28%
1278
February16th to endNight
30%
631
25%
1893
11%
631
41%
631
24%
1195
March1st to 15thNight
32%
720
23%
2154
11%
720
43%
720
28%
1317
March16th to endNight
40%
711
26%
2055
14%
711
49%
711
32%
1221
April1st to 15thNight
46%
539
25%
1797
20%
539
55%
539
30%
1016
April16th to endNight
50%
540
27%
1800
21%
540
62%
540
37%
964
May1st to 15thNight
53%
539
23%
1691
16%
539
61%
539
33%
988
May16th to endNight
68%
561
21%
1682
18%
561
71%
561
34%
1020
June1st to 15thNight
71%
540
14%
1620
13%
540
72%
540
28%
1039
June16th to endNight
74%
540
17%
1620
15%
540
74%
540
29%
1034
July1st to 15thNight
78%
510
10%
1530
10%
510
78%
510
27%
1003
July16th to endNight
74%
564
12%
1700
13%
531
75%
531
26%
1099
August1st to 15thNight
76%
539
11%
1788
13%
536
76%
536
21%
1215
August16th to endNight
67%
573
17%
1910
21%
573
67%
573
21%
1300
September1st to 15thNight
64%
604
18%
1829
17%
604
65%
604
25%
1214
September16th to endNight
63%
657
25%
1815
19%
657
66%
657
30%
1162
October1st to 15thNight
60%
660
33%
1815
23%
660
68%
660
42%
1018
October16th to endNight
54%
704
35%
2067
28%
704
62%
704
39%
1141
November1st to 15thNight
53%
700
31%
2276
26%
700
63%
700
37%
1195
November16th to endNight
46%
720
31%
2340
23%
720
59%
720
34%
1262
December1st to 15thNight
42%
720
25%
2340
20%
720
51%
720
28%
1371
December16th to endNight
33%
768
21%
2496
13%
768
43%
768
20%
1651

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
10%5%25%6%35%5%5%7%40%
47699
Night
10%5%2442446%8%45%
47117
TransparencyDay
41%7%10%16%17%6%
47411
Night
35%6%11%17%19%9%
46910
SeeingDay
14%5%17%23%24%15%
5166
Night
13%6%21%29%23%5%
15235

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
14%6%34324446%45%
1664
16th to endDay
17%5%2442334446%
1920
February1st to 15thDay
12%6%2443345%5%46%
1750
16th to endDay
16%6%35%424435%41%
1580
March1st to 15thDay
11%5%246%25%45%7%44%
1806
16th to endDay
11%5%25%5%3445%6%45%
2177
April1st to 15thDay
13%4345%3445%6%44%
2151
16th to endDay
9%5%246%245%5%6%47%
2160
May1st to 15thDay
10%425%5%35%5%6%6%42%
2257
16th to endDay
7%425%6%47%7%6%7%40%
2431
June1st to 15thDay
7%5%27%8%47%7%7%10%30%
2340
16th to endDay
6%446%7%6%7%7%7%9%32%
2340
July1st to 15thDay
9%5%37%8%5%9%7%8%9%25%
2340
16th to endDay
8%5%38%8%6%9%9%7%9%25%
2347
August1st to 15thDay
9%5%26%8%5%8%9%7%11%26%
2160
16th to endDay
11%8%37%8%47%6%7%7%27%
2292
September1st to 15thDay
9%5%28%8%47%6%6%7%32%
2004
16th to endDay
11%426%5%45%45%6%43%
1815
October1st to 15thDay
9%42442345%7%52%
1815
16th to endDay
9%423323236%57%
1800
November1st to 15thDay
9%414424236%55%
1582
16th to endDay
12%414323346%52%
1620
December1st to 15thDay
12%6%26%5%2434445%
1620
16th to endDay
20%8%36%432435%37%
1728
January1st to 15thNight
14%5%35%5%23346%46%
2273
16th to endNight
17%7%24423335%45%
2304
February1st to 15thNight
14%5%2431435%6%47%
2103
16th to endNight
14%6%34423445%46%
1893
March1st to 15thNight
11%6%35%42435%7%46%
2154
16th to endNight
10%5%34333347%51%
2055
April1st to 15thNight
11%423325%5%5%9%47%
1797
16th to endNight
7%32332445%7%54%
1800
May1st to 15thNight
9%5%2333345%9%48%
1691
16th to endNight
5%424436%5%7%9%46%
1682
June1st to 15thNight
5%425%5%45%6%10%14%35%
1620
16th to endNight
44245%36%6%8%12%39%
1620
July1st to 15thNight
4425%6%45%8%12%14%33%
1620
16th to endNight
5%325%5%46%7%9%12%36%
1817
August1st to 15thNight
5%4146%5%7%8%9%14%31%
1800
16th to endNight
9%5%35%6%35%7%7%11%33%
1910
September1st to 15thNight
8%6%27%5%35%7%6%8%38%
1829
16th to endNight
9%6%24425%5%7%8%43%
1815
October1st to 15thNight
8%413313446%57%
1815
16th to endNight
9%41332233459%
2067
November1st to 15thNight
8%413323346%57%
2276
16th to endNight
11%422313445%55%
2340
December1st to 15thNight
11%6%2432345%7%47%
2340
16th to endNight
18%8%25%5%35%345%38%
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
39%45%14%25%9%
1665
16th to endDay
40%35%15%25%10%
1920
February1st to 15thDay
38%46%15%24%10%
1752
16th to endDay
43%3414%22%9%
1580
March1st to 15thDay
38%5%7%18%22%8%
1806
16th to endDay
39%48%18%20%8%
2178
April1st to 15thDay
39%5%5%18%20%10%
2151
16th to endDay
35%5%9%20%22%6%
2160
May1st to 15thDay
38%7%9%16%20%6%
2257
16th to endDay
39%8%11%20%17%2
2435
June1st to 15thDay
45%10%11%20%10%1
2340
16th to endDay
44%10%15%17%10%1
2340
July1st to 15thDay
48%13%19%12%5%1
2201
16th to endDay
49%12%18%15%4
2202
August1st to 15thDay
47%12%17%15%6%
2148
16th to endDay
53%9%13%12%8%2
2292
September1st to 15thDay
46%9%14%12%12%4
2004
16th to endDay
39%6%10%18%18%6%
1815
October1st to 15thDay
31%410%16%26%11%
1815
16th to endDay
28%3414%28%19%
1800
November1st to 15thDay
31%35%18%26%14%
1582
16th to endDay
32%37%16%24%15%
1620
December1st to 15thDay
41%47%18%20%7%
1620
16th to endDay
49%35%19%15%5%
1728
January1st to 15thNight
39%36%15%25%9%
2273
16th to endNight
42%35%12%25%10%
2304
February1st to 15thNight
36%46%16%21%13%
2112
16th to endNight
39%5%5%14%21%13%
1893
March1st to 15thNight
36%49%17%23%8%
2154
16th to endNight
33%48%19%24%9%
2055
April1st to 15thNight
32%5%10%16%23%11%
1797
16th to endNight
27%5%9%27%20%9%
1800
May1st to 15thNight
32%6%12%18%22%8%
1692
16th to endNight
31%8%16%21%15%6%
1684
June1st to 15thNight
32%12%19%20%12%1
1620
16th to endNight
32%12%18%22%10%2
1620
July1st to 15thNight
32%14%31%14%5%1
1530
16th to endNight
34%13%27%20%4
1700
August1st to 15thNight
36%16%20%19%42
1788
16th to endNight
42%11%17%14%11%2
1910
September1st to 15thNight
39%9%14%18%12%5%
1829
16th to endNight
35%8%11%17%20%7%
1815
October1st to 15thNight
26%5%9%17%24%15%
1815
16th to endNight
28%3413%27%22%
2067
November1st to 15thNight
27%36%17%28%16%
2276
16th to endNight
29%37%16%24%18%
2340
December1st to 15thNight
36%48%19%23%7%
2340
16th to endNight
46%36%18%17%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
24%11%33%18%7%4
90
16th to endDay
20%23%36%9%8%1
96
February1st to 15thDay
15%24%33%22%4
90
16th to endDay
22%16%35%24%1
81
March1st to 15thDay
16%23%40%16%2
96
16th to endDay
15%7%30%26%16%2
288
April1st to 15thDay
22%11%23%22%17%2
270
16th to endDay
10%6%27%27%20%8%
270
May1st to 15thDay
12%8%22%24%23%8%
288
16th to endDay
12%217%24%22%19%
331
June1st to 15thDay
14%7%24%28%22%
315
16th to endDay
12%6%22%28%28%
315
July1st to 15thDay
6%1416%37%33%
305
16th to endDay
8%5%19%36%29%
326
August1st to 15thDay
13%5%20%33%27%
313
16th to endDay
15%421%30%27%
332
September1st to 15thDay
12%210%19%29%24%
294
16th to endDay
14%111%32%26%13%
254
October1st to 15thDay
10%318%31%27%9%
255
16th to endDay
15%323%22%24%10%
191
November1st to 15thDay
20%230%33%11%3
90
16th to endDay
7%15%41%22%11%2
90
December1st to 15thDay
21%14%37%18%7%
90
16th to endDay
32%10%22%22%11%
96
January1st to 15thNight
19%15%28%29%7%
720
16th to endNight
22%14%30%26%6%
768
February1st to 15thNight
17%13%31%30%7%
704
16th to endNight
18%14%33%27%6%
631
March1st to 15thNight
14%13%34%27%8%
720
16th to endNight
14%11%29%29%14%1
711
April1st to 15thNight
13%8%27%33%16%
539
16th to endNight
9%6%28%35%18%1
540
May1st to 15thNight
12%324%33%24%2
540
16th to endNight
8%113%36%36%4
563
June1st to 15thNight
8%7%35%37%10%
540
16th to endNight
7%6%23%45%16%
540
July1st to 15thNight
4425%42%21%
510
16th to endNight
5%1422%49%17%
564
August1st to 15thNight
7%421%45%22%
539
16th to endNight
12%6%21%40%18%
573
September1st to 15thNight
12%10%25%35%14%
604
16th to endNight
11%114%34%31%5%
657
October1st to 15thNight
10%121%34%28%3
660
16th to endNight
13%5%21%28%27%3
704
November1st to 15thNight
12%424%34%20%4
700
16th to endNight
14%10%25%33%15%
720
December1st to 15thNight
14%11%27%32%13%1
720
16th to endNight
23%10%29%27%9%
768
Page generated at 2017-09-11 18:35:23. Version ac04

Copyright 2017 A. Danko


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