Willow Oak Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Willow Oak Observatory clear sky chart from 2005-11-05 to 2017-09-22.

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
35%
15203
20%
46934
10%
15163
46%
15163
24%
29602

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
18%
718
21%
2443
7%
716
35%
716
16%
1559
January16th to endNight
15%
768
22%
2496
5%
768
36%
768
19%
1563
February1st to 15thNight
17%
703
18%
2287
3%
701
35%
701
18%
1418
February16th to endNight
13%
631
23%
1916
4%
631
36%
631
18%
1231
March1st to 15thNight
22%
720
20%
2018
8%
720
35%
720
21%
1321
March16th to endNight
20%
768
21%
2079
6%
768
32%
768
23%
1388
April1st to 15thNight
23%
718
18%
1796
4%
718
32%
718
26%
1219
April16th to endNight
35%
720
19%
1800
6%
720
41%
720
29%
1208
May1st to 15thNight
35%
534
17%
1614
9%
534
43%
534
22%
1089
May16th to endNight
46%
374
17%
1497
14%
374
54%
374
21%
902
June1st to 15thNight
48%
360
19%
1440
18%
360
57%
360
25%
820
June16th to endNight
59%
360
18%
1440
18%
360
66%
360
26%
821
July1st to 15thNight
54%
340
12%
1360
13%
340
57%
340
17%
881
July16th to endNight
68%
517
17%
1548
14%
484
73%
484
36%
867
August1st to 15thNight
65%
539
18%
1609
14%
536
66%
536
30%
1014
August16th to endNight
62%
756
20%
1902
13%
756
63%
756
35%
1227
September1st to 15thNight
55%
720
25%
1800
13%
720
60%
720
35%
1156
September16th to endNight
53%
685
24%
1876
20%
685
56%
685
30%
1236
October1st to 15thNight
43%
660
23%
1980
19%
660
52%
660
26%
1273
October16th to endNight
34%
704
27%
2112
14%
704
51%
704
30%
1225
November1st to 15thNight
34%
700
25%
2241
14%
700
49%
700
25%
1354
November16th to endNight
26%
720
21%
2472
11%
720
41%
720
19%
1496
December1st to 15thNight
21%
720
17%
2520
6%
720
36%
720
16%
1640
December16th to endNight
17%
768
20%
2688
5%
768
35%
768
17%
1694

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%5%25%5%3445%7%40%
47935
Night
14%5%2442445%7%43%
47116
Cloud CoverDay
14%5%25%5%3445%7%41%
47945
Night
14%5%2442445%7%43%
47120
TransparencyDay
41%5%12%22%15%2
47635
Night
39%5%11%23%16%2
46934
SeeingDay
16%10%25%24%16%5%
5685
Night
17%11%30%26%12%1
15207

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
14%8%25%5%345%46%39%
1492
16th to endDay
13%8%35%43345%5%41%
1728
February1st to 15thDay
13%6%36%5%345%5%6%37%
1575
16th to endDay
14%7%35%5%34346%41%
1557
March1st to 15thDay
14%6%35%424436%42%
1930
16th to endDay
16%6%36%6%23445%39%
2143
April1st to 15thDay
14%5%25%5%2345%7%41%
2152
16th to endDay
17%5%15%424346%44%
2160
May1st to 15thDay
16%5%35%5%245%5%8%35%
2345
16th to endDay
15%5%25%5%35%449%37%
2618
June1st to 15thDay
12%414436%46%10%39%
2520
16th to endDay
10%4245%25%46%9%43%
2520
July1st to 15thDay
9%5%35%6%45%6%7%9%38%
2520
16th to endDay
7%425%5%47%7%8%10%37%
2510
August1st to 15thDay
12%425%5%36%5%6%8%37%
2334
16th to endDay
11%5%244246%6%7%42%
2292
September1st to 15thDay
15%324325%447%45%
2160
16th to endDay
16%5%2443445%7%40%
1893
October1st to 15thDay
18%4134235%46%44%
1650
16th to endDay
14%5%25%42345%5%46%
1760
November1st to 15thDay
13%6%3432346%6%45%
1615
16th to endDay
17%6%24324445%42%
1485
December1st to 15thDay
16%5%25%5%245%5%8%38%
1440
16th to endDay
16%6%25%5%2435%8%38%
1536
January1st to 15thNight
17%5%25%423447%39%
2443
16th to endNight
16%6%24424446%42%
2496
February1st to 15thNight
14%6%245%2335%7%43%
2285
16th to endNight
13%6%35%5%24447%41%
1916
March1st to 15thNight
15%5%243245%46%42%
2006
16th to endNight
14%6%345%24336%42%
2079
April1st to 15thNight
13%6%245%3445%8%40%
1796
16th to endNight
15%5%2442346%7%43%
1800
May1st to 15thNight
18%6%35%325%446%38%
1614
16th to endNight
12%6%25%43446%8%42%
1495
June1st to 15thNight
11%3244345%6%7%45%
1440
16th to endNight
9%424335%6%5%10%43%
1440
July1st to 15thNight
8%5%26%6%36%6%7%8%36%
1440
16th to endNight
6%313425%6%6%13%45%
1655
August1st to 15thNight
10%4244345%6%9%44%
1620
16th to endNight
10%4244245%5%9%45%
1902
September1st to 15thNight
12%5%25%423448%47%
1800
16th to endNight
14%6%25%5%33346%45%
1876
October1st to 15thNight
18%5%2342435%5%44%
1980
16th to endNight
14%31322445%6%51%
2112
November1st to 15thNight
13%6%25%423335%50%
2241
16th to endNight
16%424424448%42%
2472
December1st to 15thNight
16%5%35%5%35%447%37%
2520
16th to endNight
16%6%25%42446%7%39%
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
15%7%35%5%35%5%46%39%
1492
16th to endDay
13%7%35%5%24445%41%
1728
February1st to 15thDay
13%7%36%5%345%5%6%38%
1575
16th to endDay
14%7%36%5%34246%41%
1557
March1st to 15thDay
14%7%25%434337%42%
1941
16th to endDay
16%6%35%5%33346%39%
2142
April1st to 15thDay
14%5%25%5%2445%7%41%
2152
16th to endDay
17%434424346%44%
2160
May1st to 15thDay
17%5%25%5%3445%8%36%
2345
16th to endDay
15%5%25%5%35%45%8%38%
2618
June1st to 15thDay
13%3245%36%46%10%39%
2520
16th to endDay
11%4145%3446%9%43%
2520
July1st to 15thDay
9%425%5%35%6%7%9%39%
2520
16th to endDay
8%325%5%46%7%7%10%38%
2510
August1st to 15thDay
12%425%5%46%6%6%8%38%
2334
16th to endDay
12%425%4245%6%7%42%
2292
September1st to 15thDay
15%324334447%45%
2160
16th to endDay
16%6%1442445%7%41%
1893
October1st to 15thDay
18%414323446%44%
1650
16th to endDay
14%5%25%42345%5%46%
1760
November1st to 15thDay
13%6%3422345%6%45%
1615
16th to endDay
16%6%24434345%42%
1485
December1st to 15thDay
16%425%5%245%5%7%39%
1440
16th to endDay
17%7%245%2435%7%38%
1536
January1st to 15thNight
17%5%35%42445%8%39%
2443
16th to endNight
16%5%24424446%42%
2496
February1st to 15thNight
14%6%2442435%6%43%
2278
16th to endNight
13%6%35%5%24447%42%
1916
March1st to 15thNight
15%6%24334446%43%
2018
16th to endNight
15%6%345%24336%43%
2079
April1st to 15thNight
13%6%245%25%45%8%41%
1796
16th to endNight
16%5%2441345%7%43%
1800
May1st to 15thNight
19%6%35%325%45%6%38%
1613
16th to endNight
12%6%245%2445%8%42%
1495
June1st to 15thNight
12%314435%5%6%7%45%
1440
16th to endNight
9%423435%5%6%10%43%
1440
July1st to 15thNight
9%5%27%6%46%6%6%8%37%
1440
16th to endNight
6%323325%6%5%13%46%
1655
August1st to 15thNight
10%425%4245%6%9%44%
1620
16th to endNight
10%4244245%5%9%45%
1902
September1st to 15thNight
12%425%4135%47%48%
1800
16th to endNight
14%6%15%5%33346%44%
1876
October1st to 15thNight
18%5%23423445%45%
1980
16th to endNight
14%31422345%6%51%
2112
November1st to 15thNight
12%6%35%323335%50%
2241
16th to endNight
16%4245%25%447%42%
2472
December1st to 15thNight
16%5%25%5%25%5%46%37%
2520
16th to endNight
16%6%25%4245%5%6%40%
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
45%413%21%12%3
1493
16th to endDay
44%312%24%11%3
1728
February1st to 15thDay
44%412%25%10%2
1577
16th to endDay
44%313%25%11%2
1557
March1st to 15thDay
43%411%23%15%1
1941
16th to endDay
46%410%18%17%2
2143
April1st to 15thDay
41%5%10%21%17%3
2152
16th to endDay
41%5%7%19%20%5%
2160
May1st to 15thDay
45%6%11%21%14%1
2345
16th to endDay
43%6%13%21%12%1
2622
June1st to 15thDay
38%7%13%25%13%
2520
16th to endDay
37%7%14%22%17%1
2520
July1st to 15thDay
38%9%13%21%14%2
2371
16th to endDay
36%9%14%23%13%2
2354
August1st to 15thDay
41%7%12%20%16%1
2321
16th to endDay
37%7%12%24%16%2
2292
September1st to 15thDay
38%5%11%20%21%2
2160
16th to endDay
41%48%21%20%2
1893
October1st to 15thDay
39%411%24%17%3
1650
16th to endDay
38%311%26%19%
1760
November1st to 15thDay
38%412%21%17%5%
1615
16th to endDay
42%411%25%14%2
1485
December1st to 15thDay
43%415%22%11%2
1440
16th to endDay
44%5%13%22%13%
1536
January1st to 15thNight
43%5%10%21%15%3
2443
16th to endNight
42%311%26%14%1
2496
February1st to 15thNight
40%413%23%15%2
2287
16th to endNight
42%410%25%16%1
1916
March1st to 15thNight
41%5%11%27%13%1
2018
16th to endNight
42%411%21%16%2
2079
April1st to 15thNight
40%5%11%20%18%3
1796
16th to endNight
39%5%9%20%18%6%
1800
May1st to 15thNight
45%5%10%21%14%2
1614
16th to endNight
39%7%15%20%14%2
1497
June1st to 15thNight
35%7%12%27%16%
1440
16th to endNight
34%7%13%24%17%2
1440
July1st to 15thNight
43%9%13%17%11%3
1360
16th to endNight
27%7%19%23%18%2
1548
August1st to 15thNight
34%8%12%23%17%2
1609
16th to endNight
34%7%11%24%19%3
1902
September1st to 15thNight
35%5%11%21%22%3
1800
16th to endNight
40%5%8%21%21%3
1876
October1st to 15thNight
41%49%24%17%2
1980
16th to endNight
33%311%29%20%1
2112
November1st to 15thNight
37%47%23%20%6%
2241
16th to endNight
40%310%25%16%3
2472
December1st to 15thNight
46%412%23%12%1
2520
16th to endNight
42%5%15%22%12%1
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
22%34%27%12%2
70
16th to endDay
17%34%27%20%
96
February1st to 15thDay
20%33%35%11%
90
16th to endDay
18%21%42%15%
216
March1st to 15thDay
17%22%31%23%4
270
16th to endDay
19%15%38%19%42
288
April1st to 15thDay
18%15%31%21%10%2
270
16th to endDay
19%11%29%17%16%4
270
May1st to 15thDay
26%17%23%16%11%3
288
16th to endDay
18%6%22%24%23%4
331
June1st to 15thDay
14%7%26%26%19%5%
315
16th to endDay
12%6%28%30%16%4
315
July1st to 15thDay
11%325%28%22%7%
305
16th to endDay
7%312%37%29%9%
326
August1st to 15thDay
13%315%31%24%10%
313
16th to endDay
10%12%29%29%17%
332
September1st to 15thDay
14%5%21%19%25%13%
315
16th to endDay
19%215%28%19%14%
278
October1st to 15thDay
20%7%20%27%18%5%
255
16th to endDay
14%13%30%30%9%1
272
November1st to 15thDay
17%9%28%24%18%2
241
16th to endDay
21%16%32%19%9%
105
December1st to 15thDay
20%28%25%15%6%5%
60
16th to endDay
25%18%35%17%3
64
January1st to 15thNight
21%18%36%20%2
720
16th to endNight
20%20%40%16%1
768
February1st to 15thNight
20%19%37%21%1
704
16th to endNight
19%23%38%16%1
631
March1st to 15thNight
21%17%35%22%3
720
16th to endNight
18%16%41%17%5%
768
April1st to 15thNight
17%19%38%21%4
718
16th to endNight
18%10%31%26%10%1
720
May1st to 15thNight
20%8%32%27%10%
534
16th to endNight
16%6%21%36%18%1
375
June1st to 15thNight
10%229%33%23%
360
16th to endNight
10%221%46%18%
360
July1st to 15thNight
10%219%41%26%
340
16th to endNight
6%115%43%28%4
517
August1st to 15thNight
12%113%45%25%1
539
16th to endNight
12%115%36%28%4
756
September1st to 15thNight
13%220%31%28%2
720
16th to endNight
16%318%30%25%4
685
October1st to 15thNight
20%8%22%28%17%2
660
16th to endNight
17%12%32%26%10%
704
November1st to 15thNight
15%12%31%27%10%1
700
16th to endNight
19%18%32%21%7%
720
December1st to 15thNight
21%14%36%18%7%
720
16th to endNight
20%17%40%19%1
768
Page generated at 2017-09-23 06:43:36. Version ac04

Copyright 2017 A. Danko


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