Weather and Sky Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Weather and Sky 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
36%
14890
14%
46953
10%
14853
42%
14853
18%
33050

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
13%
716
9%
2452
4%
716
20%
716
7%
1999
January16th to endNight
15%
768
10%
2496
3%
768
24%
768
8%
2022
February1st to 15thNight
16%
702
7%
2287
2%
701
23%
701
9%
1814
February16th to endNight
22%
631
10%
2004
4%
631
31%
631
12%
1551
March1st to 15thNight
26%
720
12%
1980
7%
720
30%
720
15%
1533
March16th to endNight
34%
768
19%
2112
10%
768
42%
768
23%
1477
April1st to 15thNight
30%
718
15%
1823
6%
718
35%
718
22%
1333
April16th to endNight
44%
567
21%
1647
10%
567
52%
567
27%
1072
May1st to 15thNight
48%
510
17%
1590
13%
510
52%
510
24%
1051
May16th to endNight
53%
374
17%
1497
20%
374
59%
374
23%
863
June1st to 15thNight
48%
360
13%
1440
14%
360
55%
360
20%
886
June16th to endNight
56%
360
13%
1440
17%
360
61%
360
24%
828
July1st to 15thNight
65%
340
17%
1360
24%
340
68%
340
32%
694
July16th to endNight
64%
513
15%
1544
16%
480
66%
480
32%
899
August1st to 15thNight
70%
539
17%
1609
15%
536
71%
536
34%
968
August16th to endNight
64%
627
23%
1773
16%
627
67%
627
37%
1034
September1st to 15thNight
57%
720
21%
1815
12%
720
61%
720
33%
1195
September16th to endNight
58%
685
22%
1911
17%
685
60%
685
32%
1209
October1st to 15thNight
44%
660
18%
1980
16%
660
48%
660
22%
1348
October16th to endNight
25%
704
12%
2112
7%
704
32%
704
15%
1576
November1st to 15thNight
36%
700
18%
2353
15%
700
42%
700
20%
1571
November16th to endNight
25%
720
11%
2520
8%
720
31%
720
12%
1868
December1st to 15thNight
17%
720
6%
2520
2%
720
21%
720
8%
2027
December16th to endNight
12%
768
6%
2688
1%
768
18%
768
6%
2232

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%8%36%6%35%5%5%7%33%
47921
Night
15%8%46%5%3445%7%34%
47135
Cloud CoverDay
13%8%36%6%35%5%5%7%33%
47931
Night
15%8%47%6%3445%6%33%
47139
TransparencyDay
48%6%13%19%10%1
47621
Night
49%7%11%19%10%1
46953
SeeingDay
17%6%22%26%18%8%
5379
Night
19%8%26%28%14%2
14897

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
16%14%48%8%46%6%5%6%18%
1483
16th to endDay
12%14%5%13%10%5%6%45%415%
1728
February1st to 15thDay
16%16%6%9%8%35%46%5%18%
1575
16th to endDay
15%14%5%10%8%35%446%20%
1467
March1st to 15thDay
15%11%6%8%8%35%5%45%25%
1969
16th to endDay
14%9%36%5%35%447%34%
2112
April1st to 15thDay
19%9%46%5%34446%31%
2124
16th to endDay
16%6%25%5%34346%39%
2322
May1st to 15thDay
13%7%26%6%35%5%5%7%36%
2361
16th to endDay
10%6%25%435%5%5%8%42%
2618
June1st to 15thDay
13%5%25%435%5%5%8%39%
2520
16th to endDay
11%325%425%6%7%9%40%
2520
July1st to 15thDay
8%5%235%35%5%6%9%44%
2520
16th to endDay
9%5%26%5%35%6%7%9%38%
2515
August1st to 15thDay
7%4345%45%6%6%10%40%
2340
16th to endDay
10%5%15%5%3445%9%44%
2416
September1st to 15thDay
13%5%25%5%35%5%5%8%40%
2142
16th to endDay
13%6%25%6%2445%6%41%
1862
October1st to 15thDay
17%9%26%5%25%35%6%35%
1650
16th to endDay
22%9%38%6%35%5%45%24%
1760
November1st to 15thDay
12%8%37%6%3445%8%35%
1501
16th to endDay
17%12%46%6%35%45%8%25%
1440
December1st to 15thDay
17%13%5%9%7%4445%5%21%
1440
16th to endDay
19%15%8%10%7%35%5%5%5%13%
1536
January1st to 15thNight
18%14%6%9%7%45%345%19%
2452
16th to endNight
14%13%6%10%8%35%5%5%7%19%
2496
February1st to 15thNight
14%14%7%9%7%35%446%23%
2285
16th to endNight
14%12%5%9%6%34446%26%
2004
March1st to 15thNight
19%10%47%7%35%5%5%6%25%
1968
16th to endNight
14%8%45%5%25%435%39%
2112
April1st to 15thNight
18%8%35%5%34346%34%
1823
16th to endNight
11%7%34435%45%6%42%
1647
May1st to 15thNight
16%5%35%5%235%48%38%
1590
16th to endNight
11%5%245%24447%46%
1495
June1st to 15thNight
12%5%25%5%35%46%8%38%
1440
16th to endNight
9%425%5%45%5%5%8%44%
1440
July1st to 15thNight
6%22342345%8%54%
1440
16th to endNight
7%3344245%6%8%49%
1651
August1st to 15thNight
7%214435%5%8%9%47%
1620
16th to endNight
7%415%43445%7%51%
1773
September1st to 15thNight
14%424433447%45%
1815
16th to endNight
11%5%16%434438%46%
1911
October1st to 15thNight
18%7%26%5%2335%6%38%
1980
16th to endNight
23%8%36%5%335%5%6%27%
2112
November1st to 15thNight
15%6%26%4345%48%37%
2353
16th to endNight
19%9%46%5%35%45%6%28%
2520
December1st to 15thNight
20%14%5%7%6%34447%21%
2520
16th to endNight
20%13%8%9%7%345%5%5%14%
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
16%14%49%8%47%6%46%17%
1483
16th to endDay
12%14%6%15%9%5%6%5%5%5%14%
1728
February1st to 15thDay
15%17%5%10%7%35%5%6%5%16%
1575
16th to endDay
16%13%49%8%35%5%47%18%
1467
March1st to 15thDay
15%10%5%9%9%35%5%5%5%24%
1980
16th to endDay
14%8%36%6%35%446%35%
2111
April1st to 15thDay
18%9%45%6%345%46%31%
2124
16th to endDay
16%6%35%5%34446%40%
2322
May1st to 15thDay
13%5%36%6%35%5%5%7%36%
2361
16th to endDay
10%5%245%35%6%5%8%42%
2618
June1st to 15thDay
13%5%25%5%35%5%5%8%39%
2520
16th to endDay
11%3245%25%5%7%10%41%
2520
July1st to 15thDay
7%4245%35%5%7%9%45%
2520
16th to endDay
9%425%6%36%7%6%10%38%
2515
August1st to 15thDay
6%5%25%5%45%6%7%10%40%
2340
16th to endDay
10%325%5%345%5%8%45%
2416
September1st to 15thDay
13%5%246%345%5%7%40%
2142
16th to endDay
14%425%6%3445%6%41%
1862
October1st to 15thDay
18%7%26%5%25%45%6%35%
1650
16th to endDay
22%10%38%7%35%445%24%
1760
November1st to 15thDay
12%8%36%5%4446%7%34%
1501
16th to endDay
17%12%46%7%35%5%5%7%25%
1440
December1st to 15thDay
16%13%6%10%6%45%35%6%20%
1440
16th to endDay
19%15%8%9%8%45%5%45%12%
1536
January1st to 15thNight
18%15%6%9%8%45%34418%
2452
16th to endNight
14%13%5%11%7%5%5%5%5%6%18%
2496
February1st to 15thNight
14%14%5%10%7%35%446%22%
2278
16th to endNight
15%11%5%9%8%35%45%6%23%
2004
March1st to 15thNight
19%10%47%7%35%45%5%25%
1980
16th to endNight
15%8%47%5%24435%38%
2112
April1st to 15thNight
18%9%36%5%34446%33%
1823
16th to endNight
12%7%35%434447%42%
1647
May1st to 15thNight
16%6%36%43445%8%38%
1589
16th to endNight
11%5%245%245%47%45%
1495
June1st to 15thNight
11%5%35%6%35%46%9%38%
1440
16th to endNight
8%325%5%35%5%49%43%
1440
July1st to 15thNight
5%31443445%8%53%
1440
16th to endNight
7%3244245%6%8%48%
1651
August1st to 15thNight
7%34435%6%7%8%47%
1620
16th to endNight
7%42442446%6%51%
1773
September1st to 15thNight
14%5%24423447%45%
1815
16th to endNight
11%425%5%34447%45%
1911
October1st to 15thNight
18%6%36%434346%38%
1980
16th to endNight
24%8%36%5%34446%27%
2112
November1st to 15thNight
15%6%36%5%245%47%37%
2353
16th to endNight
20%9%46%5%35%45%6%27%
2520
December1st to 15thNight
19%15%5%8%5%35%5%5%5%20%
2520
16th to endNight
20%13%8%10%7%445%45%14%
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
64%8%9%12%4
1484
16th to endDay
70%7%8%9%3
1728
February1st to 15thDay
66%8%9%12%2
1577
16th to endDay
63%6%10%13%4
1467
March1st to 15thDay
60%6%12%15%5%
1980
16th to endDay
49%5%11%21%10%1
2112
April1st to 15thDay
53%412%18%11%
2124
16th to endDay
46%311%19%15%2
2322
May1st to 15thDay
45%6%12%23%11%
2361
16th to endDay
38%7%14%24%13%1
2622
June1st to 15thDay
40%7%13%24%12%1
2520
16th to endDay
37%6%16%23%14%
2520
July1st to 15thDay
33%6%15%24%19%1
2371
16th to endDay
39%8%15%23%11%1
2359
August1st to 15thDay
36%7%18%21%14%1
2327
16th to endDay
37%415%27%13%1
2416
September1st to 15thDay
41%6%15%23%12%
2142
16th to endDay
41%5%15%25%11%
1862
October1st to 15thDay
49%412%20%11%1
1650
16th to endDay
60%7%13%13%3
1760
November1st to 15thDay
46%6%17%19%8%
1501
16th to endDay
58%6%14%13%7%
1440
December1st to 15thDay
61%9%16%10%2
1440
16th to endDay
71%9%8%9%1
1536
January1st to 15thNight
66%7%8%11%5%
2452
16th to endNight
62%10%9%12%4
2496
February1st to 15thNight
63%6%11%13%4
2287
16th to endNight
58%8%10%16%6%
2004
March1st to 15thNight
57%6%11%17%5%
1980
16th to endNight
47%5%8%22%14%1
2112
April1st to 15thNight
50%410%20%13%
1823
16th to endNight
41%410%22%16%3
1647
May1st to 15thNight
43%5%10%25%11%2
1590
16th to endNight
38%5%12%25%15%2
1497
June1st to 15thNight
40%8%10%22%15%1
1440
16th to endNight
37%6%14%24%16%1
1440
July1st to 15thNight
27%7%11%25%25%3
1360
16th to endNight
31%7%14%25%17%3
1544
August1st to 15thNight
29%9%16%28%14%1
1609
16th to endNight
31%5%13%31%17%1
1773
September1st to 15thNight
38%5%13%26%15%2
1815
16th to endNight
37%5%11%29%14%1
1911
October1st to 15thNight
45%6%10%22%13%
1980
16th to endNight
55%6%13%17%6%
2112
November1st to 15thNight
44%7%13%20%12%
2353
16th to endNight
55%8%13%17%6%
2520
December1st to 15thNight
61%10%11%12%3
2520
16th to endNight
69%7%11%9%2
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
17%18%40%11%10%1
69
16th to endDay
30%13%34%19%2
96
February1st to 15thDay
16%16%48%17%
90
16th to endDay
18%15%34%29%3
93
March1st to 15thDay
20%10%33%25%9%
270
16th to endDay
22%5%30%28%10%2
288
April1st to 15thDay
21%10%30%26%8%2
270
16th to endDay
20%5%27%27%14%5%
270
May1st to 15thDay
20%421%28%19%5%
288
16th to endDay
16%421%28%20%9%
331
June1st to 15thDay
16%420%36%18%4
315
16th to endDay
14%420%29%23%6%
315
July1st to 15thDay
14%111%31%26%13%
305
16th to endDay
11%17%33%22%14%
326
August1st to 15thDay
10%113%28%26%18%
313
16th to endDay
14%316%19%25%20%
332
September1st to 15thDay
11%316%22%30%15%
315
16th to endDay
19%212%23%22%19%
278
October1st to 15thDay
16%6%24%23%21%7%
255
16th to endDay
29%11%26%23%9%
272
November1st to 15thDay
11%18%24%29%10%5%
104
16th to endDay
18%16%28%20%16%
60
December1st to 15thDay
13%28%33%16%8%
60
16th to endDay
29%18%42%9%
64
January1st to 15thNight
26%18%31%18%4
720
16th to endNight
20%13%37%21%6%
768
February1st to 15thNight
21%17%35%21%3
704
16th to endNight
21%11%32%30%4
631
March1st to 15thNight
24%12%29%26%7%
720
16th to endNight
18%7%32%28%11%1
768
April1st to 15thNight
22%8%31%29%7%
718
16th to endNight
14%5%28%35%13%1
567
May1st to 15thNight
17%321%34%21%2
510
16th to endNight
14%422%36%20%2
375
June1st to 15thNight
15%224%34%21%1
360
16th to endNight
10%221%44%20%1
360
July1st to 15thNight
6%118%46%25%2
340
16th to endNight
10%213%42%27%3
513
August1st to 15thNight
8%111%40%32%5%
539
16th to endNight
8%116%33%35%4
627
September1st to 15thNight
15%315%33%24%7%
720
16th to endNight
13%218%27%30%8%
685
October1st to 15thNight
20%7%22%27%20%3
660
16th to endNight
28%9%30%24%6%
704
November1st to 15thNight
18%8%30%26%13%2
700
16th to endNight
24%14%30%20%8%1
720
December1st to 15thNight
29%14%31%19%4
720
16th to endNight
25%14%36%19%4
768
Page generated at 2017-09-23 06:43:38. Version ac04

Copyright 2017 A. Danko


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