Willow Grove Park
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Willow Grove Park clear sky chart from 2005-11-05 to 2017-09-23.

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
38%
14630
10%
46612
9%
14601
40%
14601
13%
34753

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
28%
720
8%
2506
7%
716
30%
716
8%
2061
January16th to endNight
32%
768
10%
2595
10%
768
34%
768
10%
2023
February1st to 15thNight
27%
700
5%
2288
5%
700
28%
700
7%
1920
February16th to endNight
32%
632
9%
2021
8%
632
34%
632
9%
1622
March1st to 15thNight
23%
720
5%
2035
4%
720
24%
720
8%
1764
March16th to endNight
23%
768
4%
2088
3%
768
23%
768
6%
1848
April1st to 15thNight
29%
719
5%
1799
2%
719
30%
719
10%
1545
April16th to endNight
35%
578
8%
1658
5%
578
38%
578
13%
1290
May1st to 15thNight
40%
382
12%
1462
11%
382
46%
382
14%
989
May16th to endNight
44%
375
8%
1496
11%
374
48%
374
9%
1112
June1st to 15thNight
44%
360
8%
1404
10%
360
45%
360
14%
974
June16th to endNight
48%
360
10%
1260
9%
360
54%
360
18%
815
July1st to 15thNight
64%
340
15%
1245
17%
340
69%
340
34%
635
July16th to endNight
65%
376
22%
1416
28%
354
76%
354
38%
631
August1st to 15thNight
71%
513
19%
1587
18%
511
73%
511
39%
795
August16th to endNight
63%
639
25%
1785
18%
639
66%
639
36%
1007
September1st to 15thNight
65%
720
24%
1800
14%
720
66%
720
44%
1034
September16th to endNight
50%
688
20%
1898
14%
688
54%
688
24%
1257
October1st to 15thNight
46%
660
16%
1980
15%
660
49%
660
20%
1362
October16th to endNight
32%
704
10%
2139
8%
704
36%
704
13%
1639
November1st to 15thNight
28%
700
6%
2422
6%
700
29%
700
7%
2042
November16th to endNight
29%
720
6%
2520
5%
720
31%
720
8%
2022
December1st to 15thNight
26%
720
8%
2520
7%
720
28%
720
8%
2040
December16th to endNight
19%
768
5%
2688
5%
768
19%
768
5%
2326

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
25%9%37%5%34345%27%
48314
Night
24%8%36%5%24445%30%
46775
Cloud CoverDay
24%10%47%5%34445%27%
48314
Night
23%8%36%5%24345%30%
46779
TransparencyDay
59%6%9%14%8%1
47980
Night
55%7%11%15%7%2
46613
SeeingDay
25%18%22%22%9%2
5962
Night
25%17%11%25%18%1
14635

By Month

Percentage Cloud Cover NW (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
39%11%5%5%5%3332316%
1432
16th to endDay
32%10%38%6%2333420%
1626
February1st to 15thDay
34%13%37%6%25%335%12%
1576
16th to endDay
33%11%37%7%2333318%
1452
March1st to 15thDay
39%13%48%6%2233311%
1923
16th to endDay
36%12%5%9%7%344339%
2136
April1st to 15thDay
31%10%48%6%35%445%14%
2149
16th to endDay
27%11%39%7%3444419%
2301
May1st to 15thDay
25%8%36%5%2335%5%30%
2487
16th to endDay
26%8%37%7%35%5%46%20%
2615
June1st to 15thDay
27%8%47%6%35%346%22%
2551
16th to endDay
19%8%37%6%3445%7%29%
2700
July1st to 15thDay
10%5%25%422346%52%
2637
16th to endDay
10%5%25%42335%7%48%
2657
August1st to 15thDay
9%5%25%5%2335%7%46%
2364
16th to endDay
12%5%25%423447%46%
2417
September1st to 15thDay
8%7%35%323447%50%
2160
16th to endDay
20%6%46%5%34446%33%
1896
October1st to 15thDay
25%9%46%5%2345%5%26%
1650
16th to endDay
32%7%37%6%24445%20%
1733
November1st to 15thDay
36%11%38%5%35%44411%
1436
16th to endDay
35%11%5%6%5%2333318%
1440
December1st to 15thDay
32%11%47%6%35%42416%
1440
16th to endDay
36%13%5%7%5%3323315%
1536
January1st to 15thNight
35%9%36%5%3433419%
2506
16th to endNight
31%9%36%424435%24%
2595
February1st to 15thNight
28%10%5%7%7%35%335%19%
2276
16th to endNight
27%10%47%5%3444423%
2021
March1st to 15thNight
34%11%37%6%2433417%
2035
16th to endNight
32%11%5%7%7%35%44413%
2084
April1st to 15thNight
24%10%48%7%45%5%35%21%
1799
16th to endNight
23%8%36%5%35%347%28%
1658
May1st to 15thNight
19%6%345%24446%36%
1462
16th to endNight
15%6%36%7%46%6%6%8%28%
1496
June1st to 15thNight
19%7%45%5%24336%36%
1404
16th to endNight
15%6%2442446%7%41%
1260
July1st to 15thNight
5%31332345%8%58%
1320
16th to endNight
3213212449%62%
1512
August1st to 15thNight
4212423448%61%
1595
16th to endNight
8%314323447%55%
1785
September1st to 15thNight
7%413312336%62%
1800
16th to endNight
14%4245%2445%7%42%
1898
October1st to 15thNight
21%5%25%5%23335%40%
1980
16th to endNight
27%7%36%6%33346%27%
2139
November1st to 15thNight
36%8%36%6%3444417%
2422
16th to endNight
31%8%37%5%2443423%
2520
December1st to 15thNight
30%10%36%5%35%445%20%
2520
16th to endNight
35%13%47%5%2433315%
2688

Percentage Cloud Cover SW (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
39%12%47%5%2322216%
1432
16th to endDay
32%10%47%6%34225%20%
1626
February1st to 15thDay
32%15%5%7%5%24336%12%
1576
16th to endDay
32%13%48%7%2233417%
1452
March1st to 15thDay
38%15%48%6%2232410%
1923
16th to endDay
34%13%5%9%7%45%4339%
2136
April1st to 15thDay
29%13%48%6%34445%14%
2149
16th to endDay
25%13%49%7%3434418%
2301
May1st to 15thDay
22%9%36%5%2335%5%30%
2487
16th to endDay
25%10%47%6%35%5%5%6%19%
2615
June1st to 15thDay
25%10%47%6%34446%21%
2551
16th to endDay
18%9%46%6%35%45%7%29%
2700
July1st to 15thDay
10%5%24413447%51%
2637
16th to endDay
10%6%35%423447%48%
2657
August1st to 15thDay
9%6%25%42335%8%46%
2364
16th to endDay
11%6%35%42335%7%46%
2417
September1st to 15thDay
8%6%24423448%49%
2160
16th to endDay
19%7%47%434435%34%
1896
October1st to 15thDay
26%9%36%435%35%426%
1650
16th to endDay
32%9%37%6%3344420%
1733
November1st to 15thDay
36%10%47%5%346%4411%
1436
16th to endDay
34%11%5%7%42344317%
1440
December1st to 15thDay
31%12%5%8%6%3443316%
1440
16th to endDay
37%13%48%5%2322314%
1536
January1st to 15thNight
36%9%36%5%2433418%
2506
16th to endNight
30%9%46%5%23335%24%
2595
February1st to 15thNight
28%11%5%8%6%45%34419%
2276
16th to endNight
27%11%36%6%35%34422%
2021
March1st to 15thNight
34%11%5%7%5%2434416%
2035
16th to endNight
31%13%48%6%34445%12%
2088
April1st to 15thNight
23%11%5%7%7%345%45%21%
1799
16th to endNight
21%11%45%5%3445%7%27%
1658
May1st to 15thNight
18%7%35%5%24445%37%
1462
16th to endNight
16%6%46%6%35%6%7%7%29%
1496
June1st to 15thNight
18%8%46%433336%35%
1404
16th to endNight
15%6%2442345%8%42%
1260
July1st to 15thNight
5%224413347%60%
1320
16th to endNight
4212322339%64%
1512
August1st to 15thNight
4312323348%63%
1595
16th to endNight
8%31432235%8%56%
1785
September1st to 15thNight
6%423212335%63%
1800
16th to endNight
14%6%245%345%47%42%
1898
October1st to 15thNight
21%6%25%5%23336%39%
1980
16th to endNight
27%7%37%6%33345%26%
2139
November1st to 15thNight
35%9%47%5%2444416%
2422
16th to endNight
30%9%37%5%34445%22%
2520
December1st to 15thNight
29%11%5%6%5%34435%20%
2520
16th to endNight
34%14%5%7%5%3433315%
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
74%36%6%7%1
1432
16th to endDay
67%5%7%9%6%3
1626
February1st to 15thDay
73%6%8%7%3
1576
16th to endDay
72%5%6%9%6%
1452
March1st to 15thDay
80%36%6%3
1923
16th to endDay
82%5%47%
2136
April1st to 15thDay
72%7%8%9%1
2149
16th to endDay
69%6%8%10%4
2301
May1st to 15thDay
55%7%12%17%6%
2487
16th to endDay
65%8%11%12%2
2619
June1st to 15thDay
63%8%14%10%4
2551
16th to endDay
53%9%14%16%5%1
2700
July1st to 15thDay
34%6%11%25%18%3
2483
16th to endDay
33%6%12%24%20%3
2490
August1st to 15thDay
34%8%14%25%14%4
2347
16th to endDay
37%6%11%27%13%2
2417
September1st to 15thDay
33%6%12%19%21%6%
2160
16th to endDay
51%6%12%14%11%2
1896
October1st to 15thDay
58%6%9%15%7%1
1650
16th to endDay
65%5%8%13%7%
1733
November1st to 15thDay
74%8%6%7%2
1436
16th to endDay
71%6%5%7%8%
1440
December1st to 15thDay
72%5%5%11%5%
1440
16th to endDay
76%36%8%4
1536
January1st to 15thNight
69%5%9%8%6%1
2506
16th to endNight
63%5%9%10%8%3
2595
February1st to 15thNight
68%7%9%10%3
2288
16th to endNight
64%6%9%11%7%
2021
March1st to 15thNight
72%6%7%11%2
2035
16th to endNight
75%6%8%8%1
2088
April1st to 15thNight
67%8%10%10%3
1799
16th to endNight
57%8%12%15%41
1658
May1st to 15thNight
47%8%14%19%9%1
1463
16th to endNight
52%13%14%15%4
1496
June1st to 15thNight
50%8%16%20%4
1404
16th to endNight
38%11%20%22%5%1
1260
July1st to 15thNight
23%8%19%26%17%5%
1245
16th to endNight
16%8%15%34%17%7%
1416
August1st to 15thNight
19%8%21%30%13%5%
1587
16th to endNight
26%7%16%31%13%4
1785
September1st to 15thNight
23%6%13%26%22%8%
1800
16th to endNight
40%7%14%20%12%4
1898
October1st to 15thNight
46%6%10%21%10%3
1980
16th to endNight
59%5%10%15%8%
2139
November1st to 15thNight
70%7%8%9%3
2422
16th to endNight
66%5%7%10%8%1
2520
December1st to 15thNight
66%6%7%12%6%
2520
16th to endNight
74%5%7%7%3
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
35%318%23%16%3
60
16th to endDay
40%16%16%16%8%1
154
February1st to 15thDay
45%19%14%13%6%
236
16th to endDay
40%22%17%12%5%
217
March1st to 15thDay
55%22%12%8%1
270
16th to endDay
38%27%18%10%31
288
April1st to 15thDay
30%23%23%14%6%1
269
16th to endDay
31%26%18%15%42
270
May1st to 15thDay
27%24%21%20%5%1
288
16th to endDay
23%19%24%17%11%3
330
June1st to 15thDay
26%20%21%22%7%1
346
16th to endDay
13%24%29%22%8%1
495
July1st to 15thDay
10%14%27%32%12%2
417
16th to endDay
6%15%28%34%13%2
327
August1st to 15thDay
10%12%18%40%13%5%
315
16th to endDay
9%10%23%34%15%5%
332
September1st to 15thDay
13%12%22%33%13%4
315
16th to endDay
19%15%24%28%10%1
284
October1st to 15thDay
31%16%20%20%9%
255
16th to endDay
44%12%18%15%8%1
245
November1st to 15thDay
30%9%33%21%31
65
16th to endDay
40%11%20%8%18%1
60
December1st to 15thDay
55%16%18%8%1
60
16th to endDay
34%12%25%17%9%1
64
January1st to 15thNight
36%19%9%20%12%2
720
16th to endNight
32%18%8%20%18%1
768
February1st to 15thNight
31%20%10%20%15%1
704
16th to endNight
29%20%10%24%13%
632
March1st to 15thNight
38%20%10%19%10%
720
16th to endNight
33%23%12%20%8%
768
April1st to 15thNight
25%23%13%20%15%1
719
16th to endNight
23%20%13%28%12%1
578
May1st to 15thNight
23%17%13%23%20%1
383
16th to endNight
16%18%11%27%23%2
375
June1st to 15thNight
15%17%12%32%21%1
360
16th to endNight
16%18%12%33%18%
360
July1st to 15thNight
412%13%39%30%
340
16th to endNight
317%11%37%29%1
376
August1st to 15thNight
6%10%6%33%38%5%
513
16th to endNight
10%9%9%34%32%2
639
September1st to 15thNight
9%10%10%29%35%4
720
16th to endNight
17%10%12%36%21%1
688
October1st to 15thNight
23%11%11%28%21%2
660
16th to endNight
32%13%12%22%17%2
704
November1st to 15thNight
35%17%11%25%9%
700
16th to endNight
31%21%10%21%13%1
720
December1st to 15thNight
34%18%12%20%11%2
720
16th to endNight
39%21%11%16%9%
768
Page generated at 2017-09-24 06:15:45. Version ac04

Copyright 2017 A. Danko


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