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 2017-04-30.

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%
15581
19%
45680
11%
15535
45%
15535
27%
29205

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
30%
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%
537
18%
1743
11%
537
43%
537
22%
1174
May1st to 15thNight
35%
495
17%
1419
10%
495
42%
495
25%
948
May16th to endNight
41%
515
17%
1369
7%
513
48%
513
29%
921
June1st to 15thNight
53%
495
17%
1320
8%
495
57%
495
34%
825
June16th to endNight
53%
495
17%
1320
9%
495
57%
495
40%
788
July1st to 15thNight
69%
465
19%
1240
12%
465
71%
465
48%
692
July16th to endNight
70%
522
24%
1387
18%
489
73%
489
47%
746
August1st to 15thNight
67%
494
22%
1475
22%
491
71%
491
40%
815
August16th to endNight
63%
525
20%
1692
23%
525
66%
525
34%
956
September1st to 15thNight
60%
495
28%
1650
28%
495
66%
495
35%
879
September16th to endNight
51%
544
26%
1809
18%
544
62%
544
37%
969
October1st to 15thNight
39%
660
27%
1980
17%
660
47%
660
32%
1181
October16th to endNight
33%
704
21%
2112
11%
704
43%
704
28%
1306
November1st to 15thNight
35%
740
26%
2295
14%
740
48%
740
31%
1316
November16th to endNight
28%
900
17%
2520
8%
900
36%
900
25%
1666
December1st to 15thNight
20%
900
14%
2520
4%
900
29%
900
18%
1836
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
14%7%36%5%3445%7%37%
46044
Night
12%7%35%424346%43%
45858
Cloud CoverDay
13%8%36%5%3445%7%37%
46053
Night
12%7%35%424446%43%
45862
TransparencyDay
45%413%22%12%1
45739
Night
41%313%24%14%2
45681
SeeingDay
15%7%26%28%16%4
5353
Night
15%8%31%30%12%1
15582

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%33446%30%
2193
May1st to 15thDay
20%7%37%6%3445%6%29%
2201
16th to endDay
14%6%26%5%345%5%6%37%
2393
June1st to 15thDay
14%5%35%5%35%5%5%7%37%
2310
16th to endDay
9%4245%25%6%6%9%42%
2310
July1st to 15thDay
8%425%5%345%6%9%44%
2310
16th to endDay
7%425%5%35%6%7%10%42%
2368
August1st to 15thDay
8%4245%36%6%7%9%41%
2145
16th to endDay
12%5%15%5%34448%44%
2157
September1st to 15thDay
12%424422448%49%
1980
16th to endDay
11%4245%2445%6%48%
1807
October1st to 15thDay
14%5%245%23335%47%
1650
16th to endDay
16%8%35%424345%41%
1760
November1st to 15thDay
10%6%24424447%47%
1559
16th to endDay
17%8%45%3123410%36%
1440
December1st to 15thDay
17%13%5%8%5%24436%26%
1440
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%6%23445%37%
1743
May1st to 15thNight
15%6%25%435%5%7%7%37%
1419
16th to endNight
13%425%5%35%45%6%43%
1368
June1st to 15thNight
9%5%2443346%9%45%
1320
16th to endNight
8%32432447%9%49%
1320
July1st to 15thNight
423325%47%10%54%
1320
16th to endNight
322443436%11%54%
1491
August1st to 15thNight
5%32432445%8%54%
1485
16th to endNight
9%32442445%6%52%
1692
September1st to 15thNight
10%5%22313346%55%
1650
16th to endNight
9%414324348%54%
1809
October1st to 15thNight
12%5%25%423235%51%
1980
16th to endNight
14%6%24423346%46%
2112
November1st to 15thNight
12%5%24323345%52%
2295
16th to endNight
16%8%35%423345%42%
2520
December1st to 15thNight
15%13%48%5%24435%33%
2520
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%
2193
May1st to 15thDay
20%8%37%6%3445%5%29%
2201
16th to endDay
14%6%26%5%345%6%7%37%
2393
June1st to 15thDay
13%6%25%6%35%5%5%7%36%
2310
16th to endDay
9%425%5%25%5%6%9%42%
2310
July1st to 15thDay
7%425%5%35%5%6%9%44%
2310
16th to endDay
6%425%5%25%6%7%10%42%
2368
August1st to 15thDay
7%4245%36%6%7%10%41%
2145
16th to endDay
12%5%25%5%24448%44%
2157
September1st to 15thDay
12%424422448%49%
1980
16th to endDay
11%425%42445%6%49%
1807
October1st to 15thDay
14%6%245%23245%47%
1650
16th to endDay
16%8%34424345%41%
1760
November1st to 15thDay
10%6%24324447%47%
1559
16th to endDay
17%9%45%412349%37%
1440
December1st to 15thDay
17%14%5%8%5%24446%26%
1440
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%23446%37%
1743
May1st to 15thNight
15%6%25%435%47%7%37%
1418
16th to endNight
12%5%25%5%26%35%5%43%
1368
June1st to 15thNight
9%5%2343346%9%45%
1320
16th to endNight
8%32432446%9%49%
1320
July1st to 15thNight
4113325%47%10%54%
1320
16th to endNight
3315%32445%10%54%
1491
August1st to 15thNight
5%32432436%8%54%
1485
16th to endNight
9%3245%2435%6%52%
1692
September1st to 15thNight
10%5%23223346%55%
1650
16th to endNight
9%314324348%54%
1809
October1st to 15thNight
13%5%26%322335%51%
1980
16th to endNight
14%7%25%323346%46%
2112
November1st to 15thNight
11%6%24323335%52%
2295
16th to endNight
16%8%25%423345%42%
2520
December1st to 15thNight
15%13%48%5%24335%33%
2520
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
2193
May1st to 15thDay
54%413%19%7%
2201
16th to endDay
44%5%13%22%11%2
2397
June1st to 15thDay
45%5%15%21%12%
2310
16th to endDay
36%6%14%24%16%2
2310
July1st to 15thDay
36%6%14%26%13%3
2161
16th to endDay
33%6%17%25%16%1
2209
August1st to 15thDay
34%7%14%26%15%1
2131
16th to endDay
38%411%26%15%3
2157
September1st to 15thDay
34%312%25%21%2
1980
16th to endDay
35%312%32%15%1
1807
October1st to 15thDay
39%39%25%20%1
1650
16th to endDay
45%311%25%12%1
1760
November1st to 15thDay
36%412%26%18%1
1559
16th to endDay
45%416%24%8%
1440
December1st to 15thDay
59%213%17%6%
1440
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
1743
May1st to 15thNight
43%5%16%21%12%
1419
16th to endNight
41%5%10%24%15%3
1370
June1st to 15thNight
34%5%17%26%13%2
1320
16th to endNight
29%5%12%27%20%4
1320
July1st to 15thNight
22%5%18%31%16%4
1240
16th to endNight
23%416%34%18%1
1387
August1st to 15thNight
26%5%15%33%16%2
1475
16th to endNight
32%411%24%20%6%
1692
September1st to 15thNight
30%38%28%26%3
1650
16th to endNight
30%212%32%19%2
1809
October1st to 15thNight
36%28%25%22%3
1980
16th to endNight
39%313%25%15%2
2112
November1st to 15thNight
34%311%27%19%3
2295
16th to endNight
44%213%27%11%
2520
December1st to 15thNight
54%312%21%8%
2520
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%28%22%9%2
311
May1st to 15thDay
22%14%32%20%8%2
393
16th to endDay
16%8%28%31%12%2
437
June1st to 15thDay
14%5%29%28%17%4
420
16th to endDay
9%427%34%19%5%
420
July1st to 15thDay
8%124%37%18%10%
400
16th to endDay
8%219%37%26%5%
445
August1st to 15thDay
10%215%34%32%4
418
16th to endDay
13%313%36%26%6%
325
September1st to 15thDay
14%219%30%26%7%
255
16th to endDay
8%7%28%30%19%4
146
October1st to 15thDay
7%14%27%31%10%8%
90
16th to endDay
16%9%30%33%10%
96
November1st to 15thDay
47%37%26%15%7%
90
16th to endDay
33%14%36%13%2
90
December1st to 15thDay
21%20%34%13%11%
90
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
20%7%35%25%8%1
537
May1st to 15thNight
19%7%33%31%6%1
495
16th to endNight
14%430%35%14%
515
June1st to 15thNight
12%5%21%46%13%
495
16th to endNight
9%228%42%16%1
495
July1st to 15thNight
5%118%45%27%2
465
16th to endNight
4115%47%28%3
522
August1st to 15thNight
6%114%44%30%2
494
16th to endNight
11%115%39%29%2
525
September1st to 15thNight
12%415%36%25%5%
495
16th to endNight
10%427%33%21%2
544
October1st to 15thNight
15%5%32%30%14%1
660
16th to endNight
17%8%34%28%10%
704
November1st to 15thNight
15%10%33%30%9%
740
16th to endNight
20%10%37%20%10%
900
December1st to 15thNight
21%12%36%24%4
900
16th to endNight
24%9%33%25%6%
960
Page generated at 2017-04-30 19:40:12. Version ac04

Copyright 2017 A. Danko


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