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 2005-11-06 to 2024-01-14.

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
38%
22510
11%
74297
10%
22466
41%
22466
15%
55181

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 10thNight
23%
760
6%
2646
5%
756
25%
756
7%
2264
January11th to 20thNight
30%
736
8%
2574
8%
736
32%
736
9%
2093
January21st to endNight
35%
792
10%
2632
10%
792
36%
792
10%
2102
February1st to 10thNight
27%
720
6%
2340
6%
720
29%
720
8%
1994
February11th to 20thNight
31%
700
8%
2274
5%
700
35%
700
11%
1833
February21st to endNight
28%
592
8%
1876
6%
592
31%
592
9%
1552
March1st to 10thNight
30%
720
8%
2063
6%
720
31%
720
11%
1708
March11th to 20thNight
29%
720
9%
1980
7%
720
32%
720
11%
1631
March21st to endNight
26%
792
5%
2142
4%
792
27%
792
8%
1872
April1st to 10thNight
28%
720
5%
1800
2%
720
29%
720
11%
1519
April11th to 20thNight
34%
680
10%
1748
5%
680
35%
680
14%
1409
April21st to endNight
38%
540
8%
1620
5%
540
40%
540
14%
1232
May1st to 10thNight
41%
395
12%
1475
11%
395
47%
395
17%
997
May11th to 20thNight
48%
354
11%
1416
12%
354
53%
354
15%
968
May21st to endNight
43%
389
8%
1552
10%
388
47%
388
13%
1110
June1st to 10thNight
41%
360
7%
1440
8%
360
45%
360
16%
970
June11th to 20thNight
42%
360
7%
1297
7%
360
45%
360
16%
912
June21st to endNight
54%
360
15%
1260
16%
360
61%
360
32%
699
July1st to 10thNight
61%
350
15%
1225
16%
350
67%
350
36%
650
July11th to 20thNight
54%
348
20%
1392
22%
348
67%
348
35%
672
July21st to endNight
68%
394
26%
1488
31%
372
80%
372
52%
582
August1st to 10thNight
69%
499
20%
1565
17%
497
74%
497
46%
760
August11th to 20thNight
70%
534
27%
1602
21%
534
74%
534
48%
788
August21st to endNight
56%
692
25%
1880
15%
692
60%
692
38%
1102
September1st to 10thNight
60%
720
24%
1800
14%
720
62%
720
44%
1063
September11th to 20thNight
55%
720
20%
1810
12%
720
57%
720
30%
1236
September21st to endNight
51%
720
19%
2058
15%
720
54%
720
28%
1327
October1st to 10thNight
48%
720
14%
2160
14%
720
50%
720
25%
1430
October11th to 20thNight
47%
720
17%
2160
17%
720
50%
720
21%
1522
October21st to endNight
37%
792
14%
2418
11%
792
41%
792
17%
1770
November1st to 10thNight
30%
740
8%
2544
9%
740
32%
740
9%
2073
November11th to 20thNight
26%
760
7%
2660
5%
760
29%
760
8%
2215
November21st to endNight
32%
760
6%
2660
6%
760
34%
760
10%
2109
December1st to 10thNight
32%
760
9%
2660
8%
760
33%
760
9%
2123
December11th to 20thNight
23%
759
7%
3233
5%
759
24%
759
6%
2721
December21st to endNight
15%
832
3%
4847
4%
817
17%
817
3%
4173

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
23%9%47%6%35%45%6%28%
75239
Night
23%9%37%6%35%446%30%
74535
TransparencyDay
56%7%11%15%9%2
74937
Night
55%8%12%15%8%2
74355
SeeingDay
23%14%27%25%10%2
13962
Night
25%8%20%30%15%1
22519

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 10thDay
40%14%5%7%7%4432410%
1519
11th to 20thDay
33%11%47%6%34345%19%
1464
21st to endDay
29%13%49%7%34345%18%
1721
February1st to 10thDay
33%13%47%7%35%446%13%
1620
11th to 20thDay
31%13%39%8%35%43418%
1575
21st to endDay
31%13%5%7%7%34345%17%
1379
March1st to 10thDay
30%14%48%8%35%5%5%414%
1894
11th to 20thDay
34%12%49%6%34335%17%
1980
21st to endDay
31%13%5%10%8%46%445%10%
2214
April1st to 10thDay
30%11%48%7%45%45%6%15%
2160
11th to 20thDay
29%11%48%6%445%4420%
2160
21st to endDay
24%10%38%8%45%5%5%5%22%
2340
May1st to 10thDay
20%7%46%7%45%5%5%7%31%
2484
11th to 20thDay
21%9%37%7%45%6%5%7%25%
2477
21st to endDay
24%8%37%8%46%6%5%6%23%
2723
June1st to 10thDay
24%8%47%7%35%5%46%27%
2520
11th to 20thDay
24%8%48%7%46%5%6%8%21%
2657
21st to endDay
12%7%36%6%45%5%5%7%41%
2700
July1st to 10thDay
9%6%26%5%3345%7%49%
2696
11th to 20thDay
10%425%5%3445%7%50%
2520
21st to endDay
7%424323347%59%
2741
August1st to 10thDay
8%5%26%5%35%35%8%50%
2364
11th to 20thDay
8%4134335%6%8%55%
2298
21st to endDay
11%6%36%5%35%45%8%45%
2476
September1st to 10thDay
8%6%35%4345%5%8%50%
2160
11th to 20thDay
13%8%37%5%345%5%7%39%
2147
21st to endDay
19%7%46%5%345%46%37%
1901
October1st to 10thDay
23%9%46%6%3445%5%32%
1800
11th to 20thDay
22%8%37%6%35%5%7%7%27%
1800
21st to endDay
30%9%47%6%34436%24%
1936
November1st to 10thDay
30%14%411%5%5%5%5%4413%
1534
11th to 20thDay
35%11%47%6%34345%16%
1520
21st to endDay
31%11%5%7%7%3444420%
1520
December1st to 10thDay
29%13%48%7%45%5%3419%
1520
11th to 20thDay
30%12%5%9%7%5%5%446%12%
1925
21st to endDay
32%15%5%11%8%35%4439%
2794
January1st to 10thNight
38%11%37%6%35%43415%
2646
11th to 20thNight
33%11%47%5%34445%20%
2574
21st to endNight
29%12%47%7%34345%22%
2632
February1st to 10thNight
29%13%5%8%6%46%44417%
2340
11th to 20thNight
25%12%47%6%45%445%24%
2272
21st to endNight
27%14%48%7%4444420%
1876
March1st to 10thNight
27%12%47%7%45%5%46%21%
2063
11th to 20thNight
28%10%48%6%36%445%21%
1979
21st to endNight
30%12%5%7%6%45%5%45%15%
2142
April1st to 10thNight
25%11%48%8%45%5%35%20%
1800
11th to 20thNight
23%10%38%6%44447%26%
1748
21st to endNight
22%10%46%6%35%45%7%28%
1620
May1st to 10thNight
17%8%35%6%35%5%38%37%
1475
11th to 20thNight
17%7%35%6%45%6%7%8%32%
1416
21st to endNight
13%6%48%8%46%6%5%8%32%
1552
June1st to 10thNight
17%6%46%7%35%446%37%
1440
11th to 20thNight
19%7%36%5%36%46%8%34%
1297
21st to endNight
10%5%2432345%7%55%
1260
July1st to 10thNight
6%32443446%9%56%
1260
11th to 20thNight
5%425%42345%7%59%
1440
21st to endNight
3312223448%70%
1576
August1st to 10thNight
432243445%9%61%
1573
11th to 20thNight
5%313323346%67%
1602
21st to endNight
9%424434448%53%
1880
September1st to 10thNight
9%424323346%60%
1800
11th to 20thNight
12%5%36%7%3445%6%45%
1810
21st to endNight
13%6%25%6%3445%8%45%
2058
October1st to 10thNight
20%6%35%5%33446%41%
2160
11th to 20thNight
20%7%36%6%34446%36%
2160
21st to endNight
26%8%36%6%33446%32%
2418
November1st to 10thNight
32%9%36%6%45%45%6%20%
2544
11th to 20thNight
34%10%47%6%35%5%45%19%
2660
21st to endNight
28%10%38%6%36%5%4423%
2660
December1st to 10thNight
29%11%47%6%35%446%21%
2660
11th to 20thNight
29%14%5%9%7%35%5%4415%
3280
21st to endNight
30%15%5%11%8%45%43411%
4862

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 10thDay
80%6%5%45%1
1519
11th to 20thDay
67%5%10%7%6%4
1464
21st to endDay
68%7%7%10%7%1
1721
February1st to 10thDay
72%7%9%8%4
1620
11th to 20thDay
72%7%7%8%5%1
1584
21st to endDay
72%7%8%7%6%1
1379
March1st to 10thDay
71%7%9%7%5%
1894
11th to 20thDay
73%47%12%31
1980
21st to endDay
78%8%7%8%
2214
April1st to 10thDay
71%8%9%10%1
2160
11th to 20thDay
67%7%8%10%7%1
2160
21st to endDay
65%7%11%11%5%1
2340
May1st to 10thDay
54%8%14%18%6%
2484
11th to 20thDay
58%9%14%14%5%1
2480
21st to endDay
60%11%13%14%2
2724
June1st to 10thDay
58%7%16%12%6%
2520
11th to 20thDay
61%9%16%11%3
2657
21st to endDay
41%9%16%22%9%2
2700
July1st to 10thDay
36%7%13%23%17%4
2622
11th to 20thDay
33%6%11%26%21%3
2445
21st to endDay
23%411%32%26%4
2589
August1st to 10thDay
31%8%15%26%17%3
2347
11th to 20thDay
26%8%15%29%17%5%
2298
21st to endDay
36%8%12%29%13%2
2476
September1st to 10thDay
32%8%13%23%18%7%
2160
11th to 20thDay
44%9%11%15%16%4
2147
21st to endDay
46%8%13%15%15%3
1901
October1st to 10thDay
53%7%8%19%11%2
1800
11th to 20thDay
54%9%13%14%10%1
1800
21st to endDay
62%5%7%14%11%
1936
November1st to 10thDay
72%8%7%9%3
1534
11th to 20thDay
69%7%9%7%5%2
1520
21st to endDay
68%7%5%9%12%
1520
December1st to 10thDay
69%6%6%10%8%1
1520
11th to 20thDay
70%8%10%10%2
1928
21st to endDay
75%9%9%5%2
2794
January1st to 10thNight
73%6%8%6%42
2646
11th to 20thNight
68%6%10%8%6%3
2574
21st to endNight
65%6%8%12%8%1
2632
February1st to 10thNight
71%7%7%11%31
2340
11th to 20thNight
63%8%11%9%7%2
2274
21st to endNight
68%7%10%9%6%
1876
March1st to 10thNight
65%8%10%13%4
2063
11th to 20thNight
64%9%10%12%5%1
1980
21st to endNight
70%10%10%9%1
2142
April1st to 10thNight
67%7%13%10%4
1800
11th to 20thNight
59%9%11%12%7%2
1748
21st to endNight
57%8%14%15%6%1
1620
May1st to 10thNight
47%9%15%21%8%1
1475
11th to 20thNight
47%11%17%18%5%2
1416
21st to endNight
48%13%17%17%5%
1552
June1st to 10thNight
48%9%17%21%5%
1440
11th to 20thNight
47%12%21%19%2
1297
21st to endNight
29%8%22%29%9%3
1260
July1st to 10thNight
23%10%19%26%17%5%
1225
11th to 20thNight
23%7%14%30%21%4
1392
21st to endNight
12%6%13%38%22%9%
1488
August1st to 10thNight
18%8%22%30%17%5%
1565
11th to 20thNight
18%7%18%34%16%7%
1602
21st to endNight
29%7%14%32%15%3
1880
September1st to 10thNight
26%6%14%27%19%8%
1800
11th to 20thNight
39%8%12%19%14%7%
1810
21st to endNight
38%8%15%19%17%4
2058
October1st to 10thNight
43%7%12%23%14%2
2160
11th to 20thNight
48%7%11%19%13%2
2160
21st to endNight
54%6%11%16%12%1
2418
November1st to 10thNight
66%8%9%11%6%1
2544
11th to 20thNight
68%8%7%8%5%3
2660
21st to endNight
64%7%7%9%12%1
2660
December1st to 10thNight
63%9%7%12%8%1
2660
11th to 20thNight
70%8%8%9%5%
3276
21st to endNight
72%11%8%7%2
4862

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 10thDay
40%19%21%10%7%2
180
11th to 20thDay
34%19%27%10%6%3
166
21st to endDay
33%12%20%25%8%2
313
February1st to 10thDay
46%11%16%17%10%
340
11th to 20thDay
41%14%24%16%41
335
21st to endDay
36%18%26%14%5%
302
March1st to 10thDay
34%19%27%15%6%
420
11th to 20thDay
31%17%25%18%8%1
420
21st to endDay
33%22%22%15%7%1
462
April1st to 10thDay
31%20%22%19%6%2
420
11th to 20thDay
32%19%24%15%6%2
412
21st to endDay
25%19%31%17%7%1
420
May1st to 10thDay
18%19%27%25%8%3
423
11th to 20thDay
29%11%26%22%10%2
448
21st to endDay
21%13%29%25%10%2
485
June1st to 10thDay
24%13%26%26%10%1
450
11th to 20thDay
18%22%33%21%5%1
587
21st to endDay
9%13%33%34%10%1
630
July1st to 10thDay
9%8%37%34%10%2
617
11th to 20thDay
9%10%32%35%12%2
444
21st to endDay
6%5%30%40%16%3
487
August1st to 10thDay
7%7%26%42%14%4
447
11th to 20thDay
5%426%42%19%5%
437
21st to endDay
8%10%33%33%12%3
495
September1st to 10thDay
10%8%31%37%12%2
450
11th to 20thDay
15%9%24%35%14%3
450
21st to endDay
18%15%22%30%12%2
450
October1st to 10thDay
24%12%23%25%14%2
450
11th to 20thDay
24%13%23%22%16%2
450
21st to endDay
31%16%23%17%10%3
451
November1st to 10thDay
28%22%26%17%7%1
203
11th to 20thDay
38%14%18%18%10%1
180
21st to endDay
29%22%21%17%11%1
180
December1st to 10thDay
33%19%20%19%7%3
180
11th to 20thDay
35%18%27%14%6%
180
21st to endDay
39%16%24%14%7%1
198
January1st to 10thNight
42%10%19%17%10%2
760
11th to 20thNight
38%10%15%22%14%1
736
21st to endNight
35%6%14%28%15%2
792
February1st to 10thNight
34%10%18%27%10%1
720
11th to 20thNight
30%8%21%27%13%1
704
21st to endNight
31%11%21%25%11%
592
March1st to 10thNight
30%10%21%24%13%1
720
11th to 20thNight
33%10%20%25%12%
720
21st to endNight
34%12%18%26%10%1
792
April1st to 10thNight
30%12%22%23%12%1
720
11th to 20thNight
24%11%23%28%12%2
680
21st to endNight
26%8%20%31%14%1
540
May1st to 10thNight
17%10%25%32%16%1
395
11th to 20thNight
19%5%23%34%17%2
354
21st to endNight
13%7%26%34%19%1
389
June1st to 10thNight
17%426%42%11%
360
11th to 20thNight
17%9%26%37%11%1
360
21st to endNight
11%328%42%17%
360
July1st to 10thNight
6%324%53%15%
350
11th to 20thNight
5%233%46%14%1
348
21st to endNight
4224%52%18%
394
August1st to 10thNight
5%216%48%27%2
499
11th to 20thNight
5%215%43%32%3
534
21st to endNight
10%423%41%20%2
692
September1st to 10thNight
11%320%43%21%1
720
11th to 20thNight
12%416%35%28%4
720
21st to endNight
15%5%21%38%20%2
720
October1st to 10thNight
20%418%36%20%2
720
11th to 20thNight
22%7%16%29%22%3
720
21st to endNight
30%8%17%26%18%1
792
November1st to 10thNight
32%10%19%28%10%1
740
11th to 20thNight
36%11%18%23%11%1
760
21st to endNight
31%10%18%27%13%2
760
December1st to 10thNight
31%9%19%25%13%2
760
11th to 20thNight
37%11%20%20%10%1
760
21st to endNight
41%14%22%16%7%1
836
Page generated at 2024-01-11 10:32:02. Version ac07

Copyright 2024 Attilla Danko


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