Willowbrook Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Willowbrook Observatory clear sky chart from 2005-11-05 to 2017-08-12.

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
26%
14669
13%
46231
7%
14631
33%
14631
16%
32887

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
8%
719
8%
2467
1%
716
15%
716
5%
2011
January16th to endNight
13%
768
8%
2496
3%
768
20%
768
7%
2009
February1st to 15thNight
9%
701
9%
2287
1%
701
18%
701
8%
1789
February16th to endNight
14%
631
12%
2012
3%
631
24%
631
10%
1507
March1st to 15thNight
15%
720
13%
1986
4%
720
22%
720
12%
1510
March16th to endNight
22%
768
20%
2112
7%
768
33%
768
20%
1438
April1st to 15thNight
18%
717
15%
1828
4%
717
24%
717
19%
1332
April16th to endNight
28%
540
19%
1620
7%
540
37%
540
22%
1075
May1st to 15thNight
37%
501
16%
1581
10%
501
45%
501
23%
1051
May16th to endNight
38%
374
17%
1497
12%
374
53%
374
25%
830
June1st to 15thNight
39%
360
13%
1420
10%
360
52%
360
19%
862
June16th to endNight
42%
360
15%
1260
13%
360
51%
360
27%
733
July1st to 15thNight
50%
340
18%
1246
19%
340
58%
340
33%
654
July16th to endNight
48%
497
16%
1529
12%
465
54%
465
29%
901
August1st to 15thNight
53%
527
17%
1575
11%
524
59%
524
34%
916
August16th to endNight
50%
557
21%
1607
11%
557
56%
557
33%
976
September1st to 15thNight
44%
660
20%
1675
8%
660
50%
660
32%
1078
September16th to endNight
46%
657
20%
1826
13%
657
51%
657
30%
1152
October1st to 15thNight
28%
660
16%
1980
11%
660
35%
660
19%
1374
October16th to endNight
16%
704
9%
2112
3%
704
23%
704
10%
1646
November1st to 15thNight
25%
700
16%
2387
12%
700
32%
700
15%
1672
November16th to endNight
19%
720
10%
2520
5%
720
26%
720
9%
1951
December1st to 15thNight
11%
720
5%
2520
1%
720
15%
720
5%
2096
December16th to endNight
6%
768
5%
2688
0%
768
11%
768
4%
2324

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
14%9%47%6%35%5%5%7%30%
47738
Night
13%8%47%6%35%45%7%33%
46407
Cloud CoverDay
14%9%47%6%35%5%5%7%30%
47748
Night
13%8%47%6%35%45%6%33%
46411
TransparencyDay
52%7%11%17%9%
47432
Night
50%7%11%19%10%1
46231
SeeingDay
17%17%31%20%9%4
5473
Night
18%18%32%21%8%1
14674

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
13%13%7%12%10%46%45%5%16%
1470
16th to endDay
9%17%7%13%11%46%5%5%5%13%
1728
February1st to 15thDay
11%15%7%12%10%5%6%5%45%15%
1575
16th to endDay
16%13%5%8%7%45%45%5%23%
1458
March1st to 15thDay
13%10%6%9%8%45%445%27%
1963
16th to endDay
11%12%5%7%6%35%44435%
2112
April1st to 15thDay
17%10%5%6%5%3445%7%30%
2131
16th to endDay
17%8%25%42335%6%37%
2340
May1st to 15thDay
16%7%36%5%25%447%35%
2370
16th to endDay
12%5%25%5%35%5%5%8%39%
2618
June1st to 15thDay
16%5%25%425%5%6%9%35%
2536
16th to endDay
13%425%5%45%6%6%9%36%
2700
July1st to 15thDay
9%426%6%45%5%6%9%38%
2635
16th to endDay
11%5%37%6%346%6%9%35%
2531
August1st to 15thDay
9%5%36%6%45%5%7%8%37%
2288
16th to endDay
12%5%25%6%36%6%6%8%35%
2231
September1st to 15thDay
17%7%36%5%35%35%7%35%
1953
16th to endDay
15%5%36%7%345%5%7%35%
1804
October1st to 15thDay
18%8%37%7%35%345%32%
1650
16th to endDay
23%16%47%6%2445%420%
1760
November1st to 15thDay
12%12%48%5%45%5%5%7%28%
1469
16th to endDay
14%14%7%10%8%4445%8%16%
1440
December1st to 15thDay
15%16%9%12%10%35%45%5%12%
1440
16th to endDay
17%18%7%10%8%35%5%5%5%11%
1536
January1st to 15thNight
16%14%5%11%9%46%45%5%16%
2467
16th to endNight
10%10%6%12%11%5%7%5%5%7%17%
2496
February1st to 15thNight
12%11%5%9%9%45%5%6%6%22%
2285
16th to endNight
9%12%5%10%8%45%5%5%6%26%
2012
March1st to 15thNight
15%10%5%9%6%34445%30%
1974
16th to endNight
11%8%46%5%3335%6%40%
2112
April1st to 15thNight
16%9%45%5%33447%34%
1828
16th to endNight
13%8%25%5%24446%40%
1620
May1st to 15thNight
14%4245%35%5%5%8%39%
1581
16th to endNight
11%41432445%7%49%
1495
June1st to 15thNight
12%3246%35%46%8%42%
1420
16th to endNight
10%315%33446%8%47%
1260
July1st to 15thNight
6%324423446%57%
1321
16th to endNight
8%4144345%5%9%48%
1635
August1st to 15thNight
6%3133245%6%10%50%
1586
16th to endNight
9%425%33345%7%51%
1607
September1st to 15thNight
13%42423435%8%47%
1675
16th to endNight
13%5%25%424347%47%
1826
October1st to 15thNight
17%6%36%6%33446%37%
1980
16th to endNight
19%11%37%6%35%5%5%7%24%
2112
November1st to 15thNight
15%8%46%5%36%5%5%7%32%
2387
16th to endNight
16%12%5%9%6%35%45%7%23%
2520
December1st to 15thNight
16%15%6%9%8%45%5%5%6%15%
2520
16th to endNight
17%12%9%13%9%45%5%5%411%
2688

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
13%14%7%12%10%36%45%5%15%
1470
16th to endDay
10%16%8%13%9%46%5%5%6%13%
1728
February1st to 15thDay
11%15%7%11%11%46%5%5%415%
1575
16th to endDay
16%14%5%9%7%345%5%5%22%
1458
March1st to 15thDay
13%11%410%9%3445%5%26%
1974
16th to endDay
12%12%5%7%6%35%335%35%
2111
April1st to 15thDay
17%11%5%6%5%24446%30%
2131
16th to endDay
17%9%25%5%23446%37%
2340
May1st to 15thDay
16%7%36%4245%47%36%
2370
16th to endDay
12%5%25%5%25%5%6%8%39%
2618
June1st to 15thDay
16%5%25%4245%6%9%35%
2536
16th to endDay
13%4245%36%6%5%9%36%
2700
July1st to 15thDay
9%425%6%35%6%5%10%39%
2635
16th to endDay
10%5%36%6%45%5%6%9%35%
2531
August1st to 15thDay
8%6%27%6%35%5%7%9%37%
2288
16th to endDay
11%6%25%5%45%6%7%8%35%
2231
September1st to 15thDay
17%6%26%5%34447%35%
1953
16th to endDay
15%6%25%6%45%45%8%35%
1804
October1st to 15thDay
18%8%37%6%35%436%32%
1650
16th to endDay
24%15%5%6%5%24445%20%
1760
November1st to 15thDay
12%12%5%9%5%35%6%5%5%29%
1469
16th to endDay
14%15%6%10%7%345%6%7%17%
1440
December1st to 15thDay
15%15%8%13%8%36%45%5%12%
1440
16th to endDay
17%17%8%9%7%46%45%5%12%
1536
January1st to 15thNight
16%14%5%11%8%45%45%5%17%
2467
16th to endNight
10%11%6%11%11%47%5%5%7%16%
2496
February1st to 15thNight
12%11%5%9%9%46%5%6%7%21%
2278
16th to endNight
10%12%5%11%8%36%5%5%6%24%
2012
March1st to 15thNight
15%9%5%9%6%35%33430%
1986
16th to endNight
12%8%37%5%23345%40%
2112
April1st to 15thNight
16%9%36%6%2445%6%34%
1828
16th to endNight
13%8%26%6%33446%40%
1620
May1st to 15thNight
14%5%25%6%245%6%8%39%
1580
16th to endNight
11%424334447%49%
1495
June1st to 15thNight
13%3245%45%5%5%8%42%
1420
16th to endNight
10%3243145%6%8%48%
1260
July1st to 15thNight
6%315%42435%5%57%
1321
16th to endNight
7%3244345%6%8%48%
1635
August1st to 15thNight
6%314325%5%6%9%50%
1586
16th to endNight
9%4235%23448%51%
1607
September1st to 15thNight
13%5%1332446%7%47%
1675
16th to endNight
13%5%25%423347%47%
1826
October1st to 15thNight
17%6%36%6%24445%38%
1980
16th to endNight
19%11%37%6%35%5%6%5%23%
2112
November1st to 15thNight
15%9%35%6%35%5%5%7%32%
2387
16th to endNight
16%12%5%8%6%35%5%5%6%23%
2520
December1st to 15thNight
17%14%6%10%6%46%5%6%5%15%
2520
16th to endNight
18%13%9%13%8%46%45%411%
2688

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
68%6%10%10%3
1471
16th to endDay
70%6%11%7%3
1728
February1st to 15thDay
70%6%7%12%2
1577
16th to endDay
61%6%10%13%7%1
1458
March1st to 15thDay
58%7%10%14%7%
1974
16th to endDay
51%5%9%17%14%1
2112
April1st to 15thDay
54%5%9%19%10%
2131
16th to endDay
46%5%10%21%14%1
2340
May1st to 15thDay
47%7%8%20%14%
2370
16th to endDay
41%8%12%21%14%1
2622
June1st to 15thDay
42%8%13%22%12%
2536
16th to endDay
42%8%12%23%12%
2700
July1st to 15thDay
38%9%13%22%15%1
2481
16th to endDay
44%8%13%21%10%1
2374
August1st to 15thDay
41%9%14%19%13%1
2275
16th to endDay
42%8%14%23%8%1
2231
September1st to 15thDay
47%7%13%22%8%
1953
16th to endDay
47%7%11%24%8%
1804
October1st to 15thDay
54%6%10%17%10%
1650
16th to endDay
66%6%12%9%4
1760
November1st to 15thDay
53%8%14%15%8%
1469
16th to endDay
65%7%12%9%3
1440
December1st to 15thDay
72%7%11%6%2
1440
16th to endDay
73%7%8%8%2
1536
January1st to 15thNight
67%6%9%9%5%
2467
16th to endNight
64%7%11%11%3
2496
February1st to 15thNight
60%8%10%15%4
2287
16th to endNight
57%6%12%15%7%
2012
March1st to 15thNight
57%5%10%18%7%
1986
16th to endNight
45%5%9%21%15%2
2112
April1st to 15thNight
50%410%19%14%
1828
16th to endNight
43%5%11%19%17%2
1620
May1st to 15thNight
42%6%10%21%16%2
1581
16th to endNight
34%6%14%27%14%3
1497
June1st to 15thNight
39%7%11%24%15%1
1420
16th to endNight
33%8%13%28%16%
1260
July1st to 15thNight
27%6%10%29%23%2
1246
16th to endNight
31%9%13%26%15%2
1529
August1st to 15thNight
28%8%17%29%14%1
1575
16th to endNight
31%8%11%31%16%1
1607
September1st to 15thNight
34%7%15%29%12%
1675
16th to endNight
36%5%13%31%12%
1826
October1st to 15thNight
47%7%10%22%11%1
1980
16th to endNight
58%8%13%13%6%
2112
November1st to 15thNight
49%7%13%18%9%1
2387
16th to endNight
59%8%13%14%3
2520
December1st to 15thNight
67%9%12%8%2
2520
16th to endNight
74%6%9%7%2
2688

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
13%38%31%11%4
67
16th to endDay
28%31%31%8%1
96
February1st to 15thDay
21%43%30%5%
90
16th to endDay
23%39%33%3
81
March1st to 15thDay
18%29%31%16%4
264
16th to endDay
22%19%32%16%7%1
288
April1st to 15thDay
25%22%29%12%7%1
270
16th to endDay
22%21%28%13%10%3
270
May1st to 15thDay
20%15%31%18%10%4
288
16th to endDay
15%17%33%20%10%2
331
June1st to 15thDay
19%15%30%24%8%
331
16th to endDay
13%16%36%22%8%1
495
July1st to 15thDay
12%9%34%27%10%6%
415
16th to endDay
10%9%34%26%12%6%
326
August1st to 15thDay
8%7%32%25%15%10%
297
16th to endDay
13%7%24%26%17%11%
268
September1st to 15thDay
13%9%24%28%11%12%
255
16th to endDay
16%7%27%23%18%7%
254
October1st to 15thDay
18%16%30%20%11%3
255
16th to endDay
32%25%26%15%
272
November1st to 15thDay
9%23%39%21%32
76
16th to endDay
30%18%40%10%1
60
December1st to 15thDay
25%36%25%11%1
60
16th to endDay
32%45%20%1
64
January1st to 15thNight
25%29%32%10%3
720
16th to endNight
17%22%37%18%4
768
February1st to 15thNight
21%28%34%12%2
704
16th to endNight
21%22%36%16%2
631
March1st to 15thNight
23%26%31%16%2
720
16th to endNight
18%19%35%18%7%
768
April1st to 15thNight
23%20%34%18%3
717
16th to endNight
18%15%34%22%8%
540
May1st to 15thNight
16%11%29%30%10%2
501
16th to endNight
11%10%37%29%11%
375
June1st to 15thNight
13%10%31%34%10%
360
16th to endNight
10%6%36%33%12%
360
July1st to 15thNight
5%5%32%36%18%1
340
16th to endNight
8%7%31%34%17%1
497
August1st to 15thNight
6%330%36%21%1
527
16th to endNight
11%5%28%31%22%2
557
September1st to 15thNight
15%9%27%24%16%6%
660
16th to endNight
15%8%26%28%17%3
657
October1st to 15thNight
20%14%32%20%11%1
660
16th to endNight
25%22%29%16%4
704
November1st to 15thNight
20%20%29%19%8%1
700
16th to endNight
22%24%29%18%5%
720
December1st to 15thNight
26%28%29%12%1
720
16th to endNight
26%26%33%11%2
768
Page generated at 2017-08-13 14:16:39. Version ac04

Copyright 2017 A. Danko


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