DeLan Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the DeLan Observatory clear sky chart from 2005-11-05 to 2017-09-19.

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
27%
14635
12%
46851
7%
14604
33%
14604
14%
34638

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
16%
721
10%
2515
4%
717
23%
717
7%
2007
January16th to endNight
18%
768
12%
2688
3%
768
29%
768
10%
2008
February1st to 15thNight
20%
703
10%
2362
4%
701
28%
701
12%
1784
February16th to endNight
19%
631
12%
1893
5%
631
26%
631
12%
1481
March1st to 15thNight
22%
720
14%
2144
9%
720
29%
720
15%
1614
March16th to endNight
32%
768
17%
2046
8%
768
38%
768
21%
1488
April1st to 15thNight
27%
697
16%
1775
7%
697
32%
697
18%
1363
April16th to endNight
28%
540
15%
1620
8%
540
34%
540
18%
1147
May1st to 15thNight
34%
477
12%
1557
10%
477
39%
477
16%
1129
May16th to endNight
30%
375
10%
1422
8%
374
37%
374
14%
1014
June1st to 15thNight
36%
360
6%
1260
4%
360
40%
360
13%
935
June16th to endNight
39%
360
7%
1260
7%
360
41%
360
15%
892
July1st to 15thNight
41%
340
8%
1276
9%
340
45%
340
19%
854
July16th to endNight
41%
376
9%
1418
11%
354
44%
354
16%
936
August1st to 15thNight
48%
434
11%
1505
11%
432
51%
432
19%
1017
August16th to endNight
41%
614
15%
1760
9%
614
45%
614
23%
1199
September1st to 15thNight
37%
720
11%
1863
6%
720
39%
720
24%
1335
September16th to endNight
37%
673
17%
1858
10%
673
42%
673
27%
1229
October1st to 15thNight
29%
660
16%
1905
7%
660
38%
660
20%
1362
October16th to endNight
22%
704
8%
2288
4%
704
27%
704
12%
1735
November1st to 15thNight
26%
700
15%
2277
10%
700
32%
700
13%
1675
November16th to endNight
22%
744
12%
2579
6%
744
30%
744
12%
1935
December1st to 15thNight
13%
750
6%
2700
1%
750
18%
750
6%
2216
December16th to endNight
14%
800
9%
2880
4%
800
22%
800
8%
2283

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
18%9%36%5%3445%6%30%
47965
Night
18%10%47%5%34445%29%
47018
TransparencyDay
52%6%12%19%8%
47651
Night
56%5%11%17%8%
46851
SeeingDay
24%10%27%23%10%2
6207
Night
25%10%29%25%8%
14636

By Month

Percentage Cloud Cover NE (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
16%15%5%8%7%3445%6%22%
1420
16th to endDay
10%12%6%9%6%46%45%6%26%
1536
February1st to 15thDay
9%13%6%11%6%46%5%45%26%
1497
16th to endDay
14%13%6%9%6%35%445%25%
1580
March1st to 15thDay
16%12%5%7%5%34436%29%
1805
16th to endDay
15%8%47%6%35%446%33%
2175
April1st to 15thDay
22%9%36%423436%32%
2172
16th to endDay
22%8%35%413345%36%
2340
May1st to 15thDay
22%8%35%5%34446%33%
2403
16th to endDay
21%6%36%6%35%5%5%6%28%
2690
June1st to 15thDay
20%6%26%5%26%6%6%8%28%
2700
16th to endDay
18%7%35%6%5%6%5%6%7%26%
2700
July1st to 15thDay
12%6%35%6%36%6%6%9%33%
2604
16th to endDay
16%7%35%5%35%6%6%7%32%
2654
August1st to 15thDay
14%6%36%6%36%6%6%9%30%
2433
16th to endDay
12%5%25%425%5%6%9%40%
2441
September1st to 15thDay
18%6%245%34336%41%
2097
16th to endDay
17%6%34323337%43%
1848
October1st to 15thDay
20%9%46%5%2345%5%32%
1721
16th to endDay
27%10%37%6%2344423%
1584
November1st to 15thDay
19%10%38%6%35%45%5%27%
1581
16th to endDay
20%16%6%7%5%22345%25%
1380
December1st to 15thDay
24%16%47%5%34336%20%
1260
16th to endDay
20%16%5%9%5%3434422%
1344
January1st to 15thNight
17%14%6%10%8%35%44420%
2515
16th to endNight
12%12%6%8%7%46%5%45%25%
2688
February1st to 15thNight
8%13%5%11%8%5%5%45%6%24%
2361
16th to endNight
15%15%7%11%6%34445%23%
1893
March1st to 15thNight
17%10%48%5%245%45%30%
2131
16th to endNight
14%12%46%5%34446%33%
2046
April1st to 15thNight
24%11%46%323445%29%
1775
16th to endNight
23%8%25%423345%35%
1620
May1st to 15thNight
24%9%345%34446%30%
1557
16th to endNight
24%7%25%6%4434432%
1420
June1st to 15thNight
23%8%247%36%45%5%28%
1260
16th to endNight
17%7%36%5%345%5%7%31%
1260
July1st to 15thNight
16%6%25%435%5%6%8%34%
1353
16th to endNight
14%6%45%6%346%6%7%34%
1514
August1st to 15thNight
15%6%36%5%335%6%8%35%
1515
16th to endNight
14%5%25%5%345%7%7%37%
1760
September1st to 15thNight
19%6%25%33445%6%37%
1863
16th to endNight
21%6%14313346%43%
1858
October1st to 15thNight
20%8%35%423346%34%
1905
16th to endNight
26%10%36%424445%26%
2288
November1st to 15thNight
18%10%46%6%2445%6%29%
2277
16th to endNight
20%14%5%8%5%23345%26%
2579
December1st to 15thNight
21%15%6%10%7%4433418%
2700
16th to endNight
22%15%49%7%33335%21%
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
61%5%10%13%7%
1421
16th to endDay
55%5%11%20%6%
1536
February1st to 15thDay
58%5%12%15%7%
1500
16th to endDay
59%5%7%17%8%1
1580
March1st to 15thDay
55%410%14%12%1
1815
16th to endDay
52%5%9%19%11%2
2175
April1st to 15thDay
54%49%18%10%3
2172
16th to endDay
50%410%17%13%2
2340
May1st to 15thDay
51%5%11%19%11%
2403
16th to endDay
55%7%10%18%6%
2694
June1st to 15thDay
50%8%13%17%7%1
2700
16th to endDay
54%8%11%18%8%
2700
July1st to 15thDay
43%10%14%24%6%
2453
16th to endDay
48%9%13%20%6%
2487
August1st to 15thDay
46%10%19%19%4
2419
16th to endDay
38%8%18%27%7%
2441
September1st to 15thDay
45%5%13%23%10%
2097
16th to endDay
41%413%27%11%
1848
October1st to 15thDay
52%412%22%7%
1721
16th to endDay
64%411%15%3
1584
November1st to 15thDay
58%5%11%18%6%
1581
16th to endDay
61%5%11%16%4
1380
December1st to 15thDay
65%5%10%13%4
1260
16th to endDay
66%47%14%7%
1344
January1st to 15thNight
67%49%12%6%
2515
16th to endNight
58%6%9%16%8%
2688
February1st to 15thNight
59%5%10%15%7%
2362
16th to endNight
62%48%14%9%
1893
March1st to 15thNight
56%6%8%15%11%1
2144
16th to endNight
51%410%20%10%2
2046
April1st to 15thNight
57%36%16%12%2
1775
16th to endNight
51%5%9%17%14%2
1620
May1st to 15thNight
54%5%9%16%12%1
1557
16th to endNight
56%411%16%10%
1422
June1st to 15thNight
56%7%10%16%6%1
1260
16th to endNight
51%7%12%17%9%
1260
July1st to 15thNight
45%8%16%21%7%1
1276
16th to endNight
46%8%15%20%8%
1418
August1st to 15thNight
45%8%18%23%3
1505
16th to endNight
42%8%16%25%6%
1760
September1st to 15thNight
47%6%13%21%10%
1863
16th to endNight
42%5%16%26%9%
1858
October1st to 15thNight
50%5%12%22%8%
1905
16th to endNight
59%5%12%16%5%
2288
November1st to 15thNight
54%7%13%15%8%
2277
16th to endNight
59%411%17%5%
2579
December1st to 15thNight
70%5%10%10%4
2700
16th to endNight
67%39%11%7%
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
21%20%43%11%3
60
16th to endDay
21%21%34%12%6%3
64
February1st to 15thDay
19%16%34%19%9%
139
16th to endDay
25%12%31%23%5%
239
March1st to 15thDay
24%15%27%24%6%
270
16th to endDay
21%10%26%23%13%4
288
April1st to 15thDay
31%11%25%21%7%1
270
16th to endDay
28%10%26%22%7%5%
270
May1st to 15thDay
32%7%26%21%9%2
288
16th to endDay
26%12%30%22%7%
403
June1st to 15thDay
24%8%26%27%12%
495
16th to endDay
23%6%28%27%13%1
495
July1st to 15thDay
20%8%27%28%11%3
387
16th to endDay
19%5%29%30%13%1
326
August1st to 15thDay
14%9%29%25%15%5%
313
16th to endDay
19%9%25%22%16%6%
332
September1st to 15thDay
23%9%26%25%12%3
315
16th to endDay
20%11%28%24%12%3
266
October1st to 15thDay
27%11%28%19%10%2
244
16th to endDay
38%11%25%14%8%
240
November1st to 15thDay
25%17%25%22%6%1
235
16th to endDay
32%14%22%22%5%1
144
December1st to 15thDay
40%13%25%15%5%1
60
16th to endDay
45%25%20%7%1
64
January1st to 15thNight
26%17%31%20%3
721
16th to endNight
19%15%35%23%5%
768
February1st to 15thNight
14%14%39%23%7%
704
16th to endNight
26%11%31%23%6%
631
March1st to 15thNight
24%10%36%23%5%
720
16th to endNight
20%9%29%29%8%2
768
April1st to 15thNight
30%10%24%24%8%
697
16th to endNight
28%8%28%24%9%
540
May1st to 15thNight
28%7%24%27%11%
477
16th to endNight
30%329%28%7%
375
June1st to 15thNight
26%323%36%10%
360
16th to endNight
18%5%28%33%12%
360
July1st to 15thNight
21%229%30%14%1
340
16th to endNight
17%328%38%11%1
376
August1st to 15thNight
17%420%37%18%1
434
16th to endNight
16%5%28%34%11%1
614
September1st to 15thNight
24%426%31%10%1
720
16th to endNight
23%8%27%28%11%
673
October1st to 15thNight
24%10%30%25%7%
660
16th to endNight
31%10%31%18%8%
704
November1st to 15thNight
26%15%26%24%6%1
700
16th to endNight
28%13%27%21%6%1
744
December1st to 15thNight
30%17%31%16%3
750
16th to endNight
33%14%28%17%41
800
Page generated at 2017-09-20 04:49:50. Version ac04

Copyright 2017 A. Danko


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