W/P Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the W/P Observatory clear sky chart from 2005-11-05 to 2017-12-04.

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
37%
16325
19%
47865
11%
16279
45%
16279
27%
30411

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
27%
900
15%
2483
6%
895
35%
895
24%
1675
January16th to endNight
24%
960
17%
2496
5%
960
32%
960
21%
1791
February1st to 15thNight
23%
879
14%
2288
4%
876
29%
876
21%
1626
February16th to endNight
28%
705
20%
1967
7%
705
38%
705
26%
1267
March1st to 15thNight
31%
576
21%
2016
12%
576
42%
576
24%
1219
March16th to endNight
26%
576
11%
2094
7%
576
33%
576
15%
1449
April1st to 15thNight
27%
539
17%
1797
12%
539
37%
539
23%
1143
April16th to endNight
30%
540
18%
1752
11%
540
43%
540
21%
1183
May1st to 15thNight
36%
540
17%
1548
10%
540
43%
540
26%
1012
May16th to endNight
41%
563
19%
1497
8%
561
49%
561
29%
1002
June1st to 15thNight
56%
540
16%
1440
8%
540
59%
540
36%
880
June16th to endNight
52%
540
19%
1440
9%
540
57%
540
40%
857
July1st to 15thNight
70%
510
18%
1360
11%
510
72%
510
49%
757
July16th to endNight
70%
564
25%
1507
19%
531
73%
531
47%
803
August1st to 15thNight
68%
539
22%
1610
21%
536
71%
536
42%
878
August16th to endNight
63%
573
22%
1847
25%
573
67%
573
35%
1035
September1st to 15thNight
60%
540
28%
1800
27%
540
66%
540
36%
942
September16th to endNight
53%
594
28%
1974
21%
594
63%
594
37%
1055
October1st to 15thNight
38%
720
25%
2160
16%
720
45%
720
31%
1307
October16th to endNight
35%
768
23%
2304
13%
768
45%
768
29%
1410
November1st to 15thNight
34%
804
25%
2492
13%
804
46%
804
29%
1458
November16th to endNight
28%
975
19%
2730
9%
975
37%
975
27%
1778
December1st to 15thNight
21%
920
14%
2575
4%
920
29%
920
18%
1867
December16th to endNight
22%
960
16%
2688
5%
960
32%
960
16%
2017

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
13%7%36%5%3445%7%38%
48624
Night
12%7%35%424346%43%
48043
Cloud CoverDay
13%7%36%5%3445%7%38%
48633
Night
12%7%35%424446%43%
48047
TransparencyDay
44%413%23%12%1
48319
Night
40%313%25%14%2
47866
SeeingDay
15%7%27%29%16%4
6254
Night
15%7%31%31%12%1
16326

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
10%9%5%8%7%45%447%32%
1453
16th to endDay
13%12%6%8%9%34445%27%
1728
February1st to 15thDay
10%13%5%8%5%3345%6%32%
1575
16th to endDay
12%10%47%425%446%36%
1506
March1st to 15thDay
13%9%37%6%34447%33%
1942
16th to endDay
20%11%36%5%24346%30%
2130
April1st to 15thDay
18%8%45%424446%34%
2151
16th to endDay
23%9%36%5%23446%30%
2205
May1st to 15thDay
19%7%37%6%3445%6%32%
2402
16th to endDay
14%6%26%5%3445%6%37%
2617
June1st to 15thDay
13%5%25%5%35%5%5%7%40%
2520
16th to endDay
9%325%5%35%5%6%10%42%
2520
July1st to 15thDay
7%325%5%345%6%9%45%
2520
16th to endDay
6%325%5%346%6%10%44%
2555
August1st to 15thDay
7%4245%36%6%7%9%43%
2340
16th to endDay
12%5%15%5%34448%44%
2354
September1st to 15thDay
11%414422448%52%
2160
16th to endDay
11%4245%2445%6%48%
1971
October1st to 15thDay
15%6%25%5%23335%45%
1800
16th to endDay
15%8%35%5%24445%42%
1920
November1st to 15thDay
11%6%24424447%45%
1692
16th to endDay
17%8%35%313349%38%
1560
December1st to 15thDay
17%13%5%8%5%24436%27%
1467
16th to endDay
16%15%6%9%7%34235%24%
1536
January1st to 15thNight
10%9%46%5%3445%7%37%
2483
16th to endNight
11%12%48%5%33446%35%
2496
February1st to 15thNight
11%12%47%5%25%346%35%
2285
16th to endNight
12%8%35%42435%6%43%
1967
March1st to 15thNight
14%7%24424335%46%
2004
16th to endNight
16%9%46%5%25%5%45%32%
2094
April1st to 15thNight
14%8%35%42335%6%40%
1797
16th to endNight
19%7%26%5%23445%36%
1752
May1st to 15thNight
14%6%2443447%7%39%
1548
16th to endNight
12%425%5%35%35%6%43%
1496
June1st to 15thNight
8%5%2343346%10%47%
1440
16th to endNight
7%32432447%9%49%
1440
July1st to 15thNight
4113325%47%10%55%
1440
16th to endNight
321342436%10%54%
1611
August1st to 15thNight
5%32442445%9%54%
1620
16th to endNight
9%32442445%6%53%
1847
September1st to 15thNight
9%5%23313346%56%
1800
16th to endNight
9%414314348%54%
1974
October1st to 15thNight
13%5%25%423245%49%
2160
16th to endNight
14%6%24323346%47%
2304
November1st to 15thNight
12%6%24423345%50%
2492
16th to endNight
15%7%25%423346%44%
2730
December1st to 15thNight
15%12%48%5%24435%33%
2575
16th to endNight
19%11%47%6%2434430%
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
9%10%5%8%7%45%447%32%
1453
16th to endDay
13%12%6%8%8%44445%26%
1728
February1st to 15thDay
10%13%5%8%5%33446%32%
1575
16th to endDay
12%10%47%5%25%5%46%36%
1506
March1st to 15thDay
14%9%46%6%34448%33%
1953
16th to endDay
20%11%46%5%24346%30%
2128
April1st to 15thDay
19%7%45%424447%34%
2151
16th to endDay
23%9%25%6%23446%30%
2205
May1st to 15thDay
19%7%37%6%3445%6%32%
2402
16th to endDay
15%6%26%5%3445%6%37%
2617
June1st to 15thDay
12%6%25%5%35%5%5%7%40%
2520
16th to endDay
8%425%6%25%5%6%10%42%
2520
July1st to 15thDay
7%415%5%35%5%6%9%45%
2520
16th to endDay
6%425%5%25%5%7%10%43%
2555
August1st to 15thDay
7%4245%36%6%7%9%42%
2340
16th to endDay
12%5%25%5%24448%43%
2354
September1st to 15thDay
11%414422448%52%
2160
16th to endDay
11%5%15%5%2445%6%48%
1971
October1st to 15thDay
16%6%245%2334445%
1800
16th to endDay
15%8%35%424445%42%
1920
November1st to 15thDay
11%6%25%324447%45%
1692
16th to endDay
16%9%35%412348%39%
1560
December1st to 15thDay
17%13%5%8%5%24436%27%
1467
16th to endDay
16%16%6%9%7%33235%23%
1536
January1st to 15thNight
10%9%46%5%34447%38%
2483
16th to endNight
11%12%48%6%34346%34%
2496
February1st to 15thNight
11%12%47%5%24446%35%
2278
16th to endNight
12%8%35%42435%6%42%
1967
March1st to 15thNight
14%7%34423335%46%
2016
16th to endNight
16%9%47%435%5%45%32%
2094
April1st to 15thNight
15%8%36%42335%6%41%
1797
16th to endNight
19%7%26%6%23445%36%
1752
May1st to 15thNight
14%5%25%43447%7%39%
1547
16th to endNight
12%425%5%36%35%5%44%
1496
June1st to 15thNight
9%5%2333345%10%47%
1440
16th to endNight
8%32432446%9%49%
1440
July1st to 15thNight
3113325%47%10%55%
1440
16th to endNight
331432445%10%55%
1611
August1st to 15thNight
5%32442436%9%54%
1620
16th to endNight
9%3245%2435%6%53%
1847
September1st to 15thNight
9%5%23323346%56%
1800
16th to endNight
9%414323347%54%
1974
October1st to 15thNight
14%5%25%322345%49%
2160
16th to endNight
14%6%24323346%47%
2304
November1st to 15thNight
12%6%24323346%50%
2492
16th to endNight
16%7%25%423346%44%
2730
December1st to 15thNight
14%13%48%5%24335%33%
2575
16th to endNight
19%11%48%6%3333430%
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
52%313%18%11%
1454
16th to endDay
58%313%16%6%
1728
February1st to 15thDay
53%314%22%6%
1576
16th to endDay
48%413%21%9%1
1506
March1st to 15thDay
50%415%19%9%
1953
16th to endDay
55%414%18%6%1
2130
April1st to 15thDay
50%410%25%9%
2151
16th to endDay
55%311%18%9%1
2205
May1st to 15thDay
52%413%21%8%
2402
16th to endDay
45%5%13%21%11%2
2621
June1st to 15thDay
42%5%14%22%14%1
2520
16th to endDay
36%6%14%24%16%2
2520
July1st to 15thDay
35%6%14%26%13%3
2371
16th to endDay
32%5%16%26%17%1
2396
August1st to 15thDay
33%7%15%28%14%1
2326
16th to endDay
38%411%26%14%3
2354
September1st to 15thDay
31%313%27%21%2
2160
16th to endDay
35%312%32%14%1
1971
October1st to 15thDay
41%39%25%18%1
1800
16th to endDay
44%311%25%13%1
1920
November1st to 15thDay
38%412%26%17%1
1692
16th to endDay
44%316%25%9%
1560
December1st to 15thDay
58%213%17%6%
1467
16th to endDay
64%311%12%7%
1536
January1st to 15thNight
45%316%19%14%
2483
16th to endNight
50%314%20%9%
2496
February1st to 15thNight
50%313%23%9%
2288
16th to endNight
41%314%25%13%1
1967
March1st to 15thNight
40%312%27%14%1
2016
16th to endNight
52%5%14%18%8%1
2094
April1st to 15thNight
44%310%26%12%2
1797
16th to endNight
48%312%19%13%2
1752
May1st to 15thNight
41%5%15%22%13%
1548
16th to endNight
41%5%10%24%16%2
1498
June1st to 15thNight
32%5%17%26%15%2
1440
16th to endNight
29%5%12%27%20%4
1440
July1st to 15thNight
22%5%18%32%16%4
1360
16th to endNight
23%416%33%20%1
1507
August1st to 15thNight
26%5%15%34%15%3
1610
16th to endNight
31%412%25%19%6%
1847
September1st to 15thNight
29%38%30%25%2
1800
16th to endNight
29%211%33%19%2
1974
October1st to 15thNight
38%28%25%22%3
2160
16th to endNight
38%312%25%16%2
2304
November1st to 15thNight
35%311%27%18%3
2492
16th to endNight
42%212%28%11%1
2730
December1st to 15thNight
54%312%21%8%
2575
16th to endNight
57%311%17%9%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%11%42%20%22
90
16th to endDay
26%10%32%28%21
96
February1st to 15thDay
13%21%38%18%6%1
90
16th to endDay
24%6%39%17%8%3
81
March1st to 15thDay
16%24%36%18%3
90
16th to endDay
35%9%27%18%7%1
114
April1st to 15thDay
21%11%32%22%10%
270
16th to endDay
27%10%27%21%8%2
315
May1st to 15thDay
20%13%33%22%8%1
468
16th to endDay
17%9%29%28%12%2
517
June1st to 15thDay
11%5%27%31%19%4
495
16th to endDay
8%7%30%32%16%4
495
July1st to 15thDay
7%125%36%18%10%
475
16th to endDay
7%219%37%27%4
515
August1st to 15thDay
8%217%36%29%5%
493
16th to endDay
12%315%34%25%7%
394
September1st to 15thDay
11%318%31%26%7%
315
16th to endDay
10%6%29%33%17%3
196
October1st to 15thDay
20%11%28%25%8%5%
135
16th to endDay
17%9%35%25%11%
144
November1st to 15thDay
10%9%40%23%11%5%
135
16th to endDay
26%14%38%15%4
135
December1st to 15thDay
20%18%31%19%12%
100
16th to endDay
32%15%35%12%4
96
January1st to 15thNight
14%12%39%26%6%1
900
16th to endNight
16%11%37%28%6%
960
February1st to 15thNight
17%11%37%26%6%
880
16th to endNight
17%10%37%27%5%1
705
March1st to 15thNight
18%10%35%23%11%
576
16th to endNight
20%8%37%25%7%
576
April1st to 15thNight
17%12%36%28%5%
539
16th to endNight
21%7%35%25%8%1
540
May1st to 15thNight
18%7%32%33%6%1
540
16th to endNight
14%431%35%14%
563
June1st to 15thNight
11%5%20%46%15%
540
16th to endNight
8%230%42%15%
540
July1st to 15thNight
4117%46%27%2
510
16th to endNight
4115%48%27%3
564
August1st to 15thNight
5%114%44%30%2
539
16th to endNight
10%115%39%30%2
573
September1st to 15thNight
11%415%38%24%5%
540
16th to endNight
11%426%34%21%2
594
October1st to 15thNight
17%5%32%29%13%1
720
16th to endNight
16%9%33%30%9%
768
November1st to 15thNight
16%10%33%29%9%
804
16th to endNight
19%10%39%21%9%
975
December1st to 15thNight
20%12%36%24%5%
920
16th to endNight
24%9%33%25%6%
960
Page generated at 2017-12-05 09:19:03. Version ac04

Copyright 2017 A. Danko


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