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 2018-02-14.

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%
15400
13%
48547
7%
15362
33%
15362
16%
34503

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
9%
779
8%
2674
2%
776
16%
776
6%
2161
January16th to endNight
13%
832
8%
2704
3%
832
20%
832
7%
2169
February1st to 15thNight
9%
757
9%
2468
1%
757
19%
757
7%
1930
February16th to endNight
13%
631
12%
2012
3%
631
24%
631
10%
1507
March1st to 15thNight
15%
720
13%
1986
4%
720
23%
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
27%
540
19%
1620
7%
540
37%
540
22%
1075
May1st to 15thNight
38%
501
16%
1581
10%
501
46%
501
23%
1051
May16th to endNight
38%
374
17%
1497
12%
374
53%
374
25%
830
June1st to 15thNight
38%
360
13%
1420
10%
360
51%
360
19%
862
June16th to endNight
42%
360
15%
1260
13%
360
50%
360
27%
733
July1st to 15thNight
51%
340
18%
1246
19%
340
58%
340
33%
654
July16th to endNight
48%
497
16%
1529
12%
465
55%
465
29%
901
August1st to 15thNight
53%
539
17%
1609
11%
536
59%
536
34%
936
August16th to endNight
50%
608
23%
1754
11%
608
58%
608
33%
1057
September1st to 15thNight
45%
720
20%
1827
8%
720
51%
720
33%
1160
September16th to endNight
47%
717
22%
1992
15%
717
52%
717
31%
1248
October1st to 15thNight
29%
720
16%
2160
11%
720
36%
720
19%
1498
October16th to endNight
16%
768
11%
2304
4%
768
25%
768
11%
1767
November1st to 15thNight
25%
760
15%
2592
11%
760
32%
760
14%
1837
November16th to endNight
18%
780
10%
2730
5%
780
26%
780
9%
2100
December1st to 15thNight
10%
780
5%
2730
1%
780
15%
780
5%
2267
December16th to endNight
6%
832
6%
2912
0%
832
11%
832
4%
2480

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%
49525
Night
13%8%47%6%35%45%7%33%
48723
Cloud CoverDay
14%9%47%6%35%5%5%7%30%
49535
Night
13%8%47%6%35%45%6%33%
48727
TransparencyDay
52%7%11%17%9%
49219
Night
50%7%12%19%10%1
48547
SeeingDay
17%18%30%19%9%4
6045
Night
18%18%31%21%8%1
15405

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%5%5%5%15%
1593
16th to endDay
9%16%6%13%10%46%5%5%6%13%
1872
February1st to 15thDay
12%15%7%12%10%5%6%5%4415%
1695
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%
2340
16th to endDay
12%5%25%6%36%6%6%8%35%
2435
September1st to 15thDay
16%7%26%5%35%36%7%36%
2130
16th to endDay
14%5%26%7%345%5%8%36%
1968
October1st to 15thDay
19%8%36%7%35%345%32%
1800
16th to endDay
22%15%37%5%2445%421%
1920
November1st to 15thDay
12%12%49%5%45%5%5%6%27%
1594
16th to endDay
14%14%7%10%8%445%5%8%17%
1560
December1st to 15thDay
14%16%9%12%10%35%5%5%5%13%
1560
16th to endDay
17%17%7%10%8%35%5%5%5%12%
1664
January1st to 15thNight
15%13%5%11%8%46%45%5%17%
2674
16th to endNight
10%10%6%11%11%5%7%5%5%7%17%
2704
February1st to 15thNight
12%11%5%9%9%45%5%6%6%22%
2466
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%3144245%6%10%49%
1620
16th to endNight
8%425%33345%7%52%
1754
September1st to 15thNight
13%42323335%7%48%
1827
16th to endNight
12%424424347%48%
1992
October1st to 15thNight
17%6%36%6%34346%37%
2160
16th to endNight
19%11%37%6%35%5%5%6%26%
2304
November1st to 15thNight
16%8%46%5%36%5%5%6%31%
2592
16th to endNight
15%11%5%8%6%35%45%7%24%
2730
December1st to 15thNight
16%14%6%9%9%45%5%5%6%15%
2730
16th to endNight
17%12%8%13%9%45%5%5%412%
2912

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%14%6%11%9%36%5%5%5%15%
1593
16th to endDay
10%16%8%12%9%46%5%5%6%14%
1872
February1st to 15thDay
12%15%7%11%11%46%5%5%415%
1695
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%6%9%37%
2340
16th to endDay
11%5%25%5%45%6%7%8%36%
2435
September1st to 15thDay
16%5%26%5%4445%7%36%
2130
16th to endDay
14%5%25%6%45%35%7%36%
1968
October1st to 15thDay
18%8%37%6%34436%32%
1800
16th to endDay
23%14%5%7%5%24445%21%
1920
November1st to 15thDay
11%12%5%9%6%35%6%5%5%27%
1594
16th to endDay
14%15%6%10%7%345%6%6%17%
1560
December1st to 15thDay
14%15%8%13%8%36%5%5%5%13%
1560
16th to endDay
17%16%8%9%7%46%45%5%13%
1664
January1st to 15thNight
16%14%5%11%8%45%5%5%5%17%
2674
16th to endNight
11%11%6%11%11%47%5%5%7%16%
2704
February1st to 15thNight
12%11%5%9%9%46%5%6%6%21%
2459
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%314425%5%6%9%49%
1620
16th to endNight
8%4245%23448%51%
1754
September1st to 15thNight
12%5%1332446%7%48%
1827
16th to endNight
12%5%15%423347%48%
1992
October1st to 15thNight
17%6%36%5%3445%5%38%
2160
16th to endNight
19%10%37%6%35%5%6%5%25%
2304
November1st to 15thNight
16%9%36%5%35%5%5%7%31%
2592
16th to endNight
16%12%48%6%35%5%5%6%23%
2730
December1st to 15thNight
16%14%6%11%7%46%5%6%5%15%
2730
16th to endNight
17%13%8%13%8%46%45%412%
2912

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%7%11%9%4
1594
16th to endDay
69%6%11%8%3
1872
February1st to 15thDay
69%6%7%12%2
1697
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
2327
16th to endDay
42%9%14%22%9%1
2435
September1st to 15thDay
46%7%12%23%9%1
2130
16th to endDay
46%7%10%25%9%
1968
October1st to 15thDay
54%5%10%18%9%
1800
16th to endDay
64%7%12%10%4
1920
November1st to 15thDay
54%8%13%15%7%
1594
16th to endDay
65%8%12%10%3
1560
December1st to 15thDay
70%8%11%7%2
1560
16th to endDay
72%7%8%8%2
1664
January1st to 15thNight
66%6%10%9%6%
2674
16th to endNight
64%7%12%11%3
2704
February1st to 15thNight
60%8%10%15%4
2468
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%9%17%29%14%1
1609
16th to endNight
31%8%11%31%16%1
1754
September1st to 15thNight
33%7%15%28%14%1
1827
16th to endNight
35%6%12%32%12%
1992
October1st to 15thNight
46%7%10%21%12%1
2160
16th to endNight
56%7%13%14%7%
2304
November1st to 15thNight
50%7%13%17%9%1
2592
16th to endNight
58%9%13%14%3
2730
December1st to 15thNight
66%9%12%8%2
2730
16th to endNight
72%6%9%7%3
2912

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
17%39%30%11%3
100
16th to endDay
29%29%32%6%1
144
February1st to 15thDay
23%43%28%3
130
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%6%32%25%16%9%
313
16th to endDay
13%9%24%25%16%10%
332
September1st to 15thDay
12%9%24%26%13%12%
315
16th to endDay
14%8%24%22%21%8%
314
October1st to 15thDay
19%17%28%19%10%4
315
16th to endDay
30%26%25%15%1
336
November1st to 15thDay
16%27%32%18%41
111
16th to endDay
26%22%41%7%2
90
December1st to 15thDay
18%37%33%8%1
90
16th to endDay
28%41%22%6%1
96
January1st to 15thNight
24%28%32%11%3
780
16th to endNight
17%22%36%18%4
832
February1st to 15thNight
21%29%34%11%2
760
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%329%36%21%1
539
16th to endNight
10%5%28%31%22%2
608
September1st to 15thNight
14%9%27%24%17%6%
720
16th to endNight
14%8%24%27%19%4
717
October1st to 15thNight
20%13%32%20%11%1
720
16th to endNight
25%22%30%17%4
768
November1st to 15thNight
21%20%28%20%8%1
760
16th to endNight
22%25%29%17%4
780
December1st to 15thNight
26%28%30%12%1
780
16th to endNight
25%27%33%11%2
832
Page generated at 2018-02-15 10:58:41. Version ac04

Copyright 2018 A. Danko


Willowbrook Observatory Clear Sky Chart |Charts Home