James C. Veen Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the James C. Veen Observatory clear sky chart from 2005-11-05 to 2017-08-12.

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
32%
15857
15%
46450
9%
15816
38%
15816
20%
32780

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
14%
900
7%
2506
2%
895
18%
895
9%
2082
January16th to endNight
17%
960
7%
2688
2%
960
21%
960
10%
2251
February1st to 15thNight
15%
707
9%
2205
3%
707
22%
707
10%
1740
February16th to endNight
19%
631
12%
1893
5%
631
27%
631
12%
1454
March1st to 15thNight
26%
720
15%
2130
9%
720
31%
720
18%
1486
March16th to endNight
27%
735
19%
2079
10%
735
35%
735
22%
1430
April1st to 15thNight
24%
539
13%
1797
7%
539
32%
539
17%
1253
April16th to endNight
30%
540
21%
1636
9%
540
40%
540
24%
1051
May1st to 15thNight
30%
540
14%
1620
7%
540
37%
540
22%
1102
May16th to endNight
41%
561
15%
1565
9%
561
46%
561
28%
1036
June1st to 15thNight
45%
540
15%
1440
9%
540
50%
540
34%
904
June16th to endNight
48%
540
15%
1440
9%
540
53%
540
33%
912
July1st to 15thNight
58%
510
16%
1360
12%
510
60%
510
44%
773
July16th to endNight
58%
564
21%
1417
15%
531
61%
531
41%
837
August1st to 15thNight
61%
527
22%
1570
15%
524
65%
524
39%
876
August16th to endNight
60%
525
20%
1665
17%
525
65%
525
33%
968
September1st to 15thNight
56%
564
26%
1719
21%
564
62%
564
34%
989
September16th to endNight
53%
657
23%
1815
15%
657
58%
657
34%
1111
October1st to 15thNight
41%
660
22%
1851
16%
660
47%
660
29%
1189
October16th to endNight
24%
704
14%
2277
7%
704
33%
704
15%
1606
November1st to 15thNight
31%
700
19%
2276
16%
700
37%
700
20%
1523
November16th to endNight
23%
720
13%
2340
8%
720
30%
720
12%
1770
December1st to 15thNight
13%
853
5%
2473
2%
853
16%
853
7%
2117
December16th to endNight
11%
960
6%
2688
0%
960
16%
960
7%
2320

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%9%47%6%3445%7%33%
47531
Night
15%9%47%5%34446%33%
46619
Cloud CoverDay
14%9%47%6%35%45%7%33%
47540
Night
15%9%47%5%34446%33%
46624
TransparencyDay
50%411%21%11%1
47218
Night
50%411%20%11%1
46450
SeeingDay
17%12%29%22%11%5%
5826
Night
19%11%29%27%11%1
15862

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
18%13%7%10%11%46%435%13%
1432
16th to endDay
13%20%11%11%11%4433311%
1536
February1st to 15thDay
13%15%8%12%11%4442417%
1654
16th to endDay
15%13%7%11%6%35%45%5%20%
1580
March1st to 15thDay
15%10%46%6%45%45%6%29%
1820
16th to endDay
16%10%47%5%23446%33%
2154
April1st to 15thDay
21%12%46%41435%5%31%
2151
16th to endDay
17%7%25%42445%6%38%
2323
May1st to 15thDay
20%7%35%5%3445%7%33%
2331
16th to endDay
11%6%26%5%25%5%47%40%
2548
June1st to 15thDay
11%6%245%35%5%5%7%41%
2520
16th to endDay
10%5%25%6%35%5%6%9%40%
2520
July1st to 15thDay
6%5%245%35%5%6%9%44%
2520
16th to endDay
8%424435%6%6%9%44%
2655
August1st to 15thDay
9%5%25%5%45%5%7%9%39%
2294
16th to endDay
12%425%5%25%5%7%8%40%
2184
September1st to 15thDay
12%5%25%6%2445%7%42%
1900
16th to endDay
11%435%5%35%45%8%40%
1815
October1st to 15thDay
17%7%37%5%3433439%
1779
16th to endDay
23%8%38%6%45%346%26%
1592
November1st to 15thDay
15%10%35%424346%38%
1582
16th to endDay
19%17%5%8%5%335%46%22%
1620
December1st to 15thDay
16%18%9%12%8%44235%14%
1485
16th to endDay
20%19%9%10%7%3435%312%
1536
January1st to 15thNight
20%14%7%10%8%3443418%
2506
16th to endNight
13%14%9%11%9%45%45%5%16%
2688
February1st to 15thNight
13%15%7%9%8%44345%22%
2203
16th to endNight
14%12%5%9%7%35%5%46%24%
1893
March1st to 15thNight
15%10%36%5%24347%34%
2117
16th to endNight
15%8%36%5%3435%6%37%
2079
April1st to 15thNight
19%9%35%5%25%445%34%
1797
16th to endNight
17%6%35%32445%6%41%
1636
May1st to 15thNight
19%5%35%5%35%447%36%
1620
16th to endNight
11%6%26%6%3445%8%40%
1563
June1st to 15thNight
10%425%5%3436%8%45%
1440
16th to endNight
7%425%5%25%6%7%10%42%
1440
July1st to 15thNight
6%4235%2345%9%53%
1440
16th to endNight
6%226%41445%8%53%
1513
August1st to 15thNight
6%31442437%8%51%
1580
16th to endNight
11%32442435%7%50%
1665
September1st to 15thNight
12%313324447%52%
1719
16th to endNight
9%5%35%325%337%48%
1815
October1st to 15thNight
17%6%24423347%43%
1851
16th to endNight
22%7%37%5%34346%32%
2277
November1st to 15thNight
14%10%35%43346%6%36%
2276
16th to endNight
19%14%47%5%3445%5%24%
2340
December1st to 15thNight
20%16%8%10%7%4443414%
2473
16th to endNight
21%17%9%10%7%3434413%
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
18%13%8%10%10%5%7%43413%
1432
16th to endDay
13%19%11%12%11%4443311%
1536
February1st to 15thDay
12%14%8%12%10%45%42417%
1654
16th to endDay
15%13%7%11%7%36%45%5%20%
1580
March1st to 15thDay
15%10%47%6%35%5%5%6%29%
1830
16th to endDay
16%10%47%5%24445%33%
2153
April1st to 15thDay
20%12%46%5%1335%5%31%
2151
16th to endDay
17%7%26%42445%7%38%
2323
May1st to 15thDay
19%7%25%5%3445%7%33%
2331
16th to endDay
11%6%36%5%35%5%5%7%40%
2548
June1st to 15thDay
10%6%35%5%35%5%5%8%40%
2520
16th to endDay
10%435%6%35%5%6%9%39%
2520
July1st to 15thDay
6%5%245%36%5%6%9%44%
2520
16th to endDay
8%425%435%5%6%9%44%
2655
August1st to 15thDay
8%5%25%5%45%5%7%9%40%
2294
16th to endDay
11%5%25%435%5%6%9%40%
2184
September1st to 15thDay
11%5%346%345%5%7%42%
1900
16th to endDay
11%436%5%3445%9%40%
1815
October1st to 15thDay
16%7%36%6%33345%38%
1779
16th to endDay
22%8%37%6%45%346%26%
1592
November1st to 15thDay
14%11%35%424346%38%
1582
16th to endDay
19%16%48%5%34446%22%
1620
December1st to 15thDay
15%18%9%13%8%34225%14%
1485
16th to endDay
20%19%9%9%7%3434412%
1536
January1st to 15thNight
19%15%7%10%8%3343418%
2506
16th to endNight
12%15%9%11%8%5%5%45%6%15%
2688
February1st to 15thNight
13%16%7%8%8%45%335%22%
2196
16th to endNight
14%12%6%8%8%445%46%24%
1893
March1st to 15thNight
16%10%36%5%24347%34%
2130
16th to endNight
15%7%36%5%34446%37%
2079
April1st to 15thNight
19%9%35%5%25%445%34%
1797
16th to endNight
17%7%24323446%41%
1636
May1st to 15thNight
18%5%35%435%446%36%
1619
16th to endNight
11%6%35%6%25%36%8%40%
1563
June1st to 15thNight
10%4245%3445%7%46%
1440
16th to endNight
7%425%435%7%6%11%42%
1440
July1st to 15thNight
6%3235%2345%8%54%
1440
16th to endNight
6%315%4135%6%9%52%
1513
August1st to 15thNight
6%41342447%8%51%
1580
16th to endNight
10%33442445%6%50%
1665
September1st to 15thNight
12%31431435%7%51%
1719
16th to endNight
9%5%35%334337%48%
1815
October1st to 15thNight
16%6%24423347%43%
1851
16th to endNight
21%8%36%5%24346%32%
2277
November1st to 15thNight
14%9%36%5%2445%6%36%
2276
16th to endNight
19%13%5%7%6%3445%5%25%
2340
December1st to 15thNight
20%16%8%10%7%35%335%14%
2473
16th to endNight
21%16%9%10%7%3433413%
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
73%5%9%8%2
1433
16th to endDay
77%37%8%2
1536
February1st to 15thDay
71%37%13%3
1656
16th to endDay
65%410%13%5%
1580
March1st to 15thDay
54%5%13%18%7%
1830
16th to endDay
51%412%23%6%1
2154
April1st to 15thDay
54%38%20%11%
2151
16th to endDay
46%413%18%14%3
2323
May1st to 15thDay
49%411%22%9%1
2331
16th to endDay
41%5%12%27%12%
2552
June1st to 15thDay
40%412%25%15%
2520
16th to endDay
39%6%12%21%18%1
2520
July1st to 15thDay
33%5%11%27%18%2
2371
16th to endDay
33%6%12%26%17%2
2488
August1st to 15thDay
38%5%11%29%12%2
2280
16th to endDay
37%5%13%26%15%2
2184
September1st to 15thDay
39%5%10%27%14%1
1900
16th to endDay
40%313%26%15%
1815
October1st to 15thDay
48%49%23%13%1
1779
16th to endDay
59%414%16%5%
1592
November1st to 15thDay
47%411%23%11%1
1582
16th to endDay
63%314%14%4
1620
December1st to 15thDay
73%310%10%2
1485
16th to endDay
74%49%8%1
1536
January1st to 15thNight
70%310%11%4
2506
16th to endNight
68%5%12%9%3
2688
February1st to 15thNight
64%48%16%4
2205
16th to endNight
60%411%15%6%
1893
March1st to 15thNight
50%412%21%9%1
2130
16th to endNight
47%412%24%10%2
2079
April1st to 15thNight
51%49%18%15%1
1797
16th to endNight
43%312%21%15%4
1636
May1st to 15thNight
48%313%20%11%2
1620
16th to endNight
42%412%22%15%2
1565
June1st to 15thNight
36%412%28%16%1
1440
16th to endNight
33%7%16%24%17%
1440
July1st to 15thNight
27%414%29%21%3
1360
16th to endNight
28%5%14%27%20%3
1417
August1st to 15thNight
28%5%13%32%17%2
1570
16th to endNight
33%49%28%19%3
1665
September1st to 15thNight
31%5%12%30%18%1
1719
16th to endNight
35%312%31%15%
1815
October1st to 15thNight
41%310%26%16%1
1851
16th to endNight
53%413%20%7%
2277
November1st to 15thNight
47%411%21%12%1
2276
16th to endNight
59%5%11%18%3
2340
December1st to 15thNight
73%48%8%4
2473
16th to endNight
74%49%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
16%24%43%13%2
90
16th to endDay
26%15%40%16%1
96
February1st to 15thDay
24%35%32%7%
90
16th to endDay
20%20%34%19%3
81
March1st to 15thDay
32%20%27%11%6%
120
16th to endDay
28%9%29%22%6%3
288
April1st to 15thDay
28%14%30%20%5%2
270
16th to endDay
21%16%32%15%9%5%
270
May1st to 15thDay
25%18%24%20%5%5%
288
16th to endDay
15%14%29%24%10%5%
448
June1st to 15thDay
13%10%37%25%9%3
495
16th to endDay
12%13%38%19%11%4
495
July1st to 15thDay
10%7%29%28%16%8%
475
16th to endDay
9%6%29%29%15%8%
515
August1st to 15thDay
10%5%21%24%22%16%
303
16th to endDay
17%419%26%19%12%
268
September1st to 15thDay
13%320%29%22%9%
255
16th to endDay
16%5%19%25%22%11%
254
October1st to 15thDay
22%9%23%25%15%3
255
16th to endDay
25%16%44%12%1
104
November1st to 15thDay
15%16%36%21%8%1
90
16th to endDay
33%21%27%12%5%
90
December1st to 15thDay
30%34%24%7%3
90
16th to endDay
29%42%25%21
96
January1st to 15thNight
26%19%29%16%6%
900
16th to endNight
19%15%34%27%3
960
February1st to 15thNight
21%18%34%21%4
710
16th to endNight
19%17%33%24%4
631
March1st to 15thNight
20%16%29%24%7%
720
16th to endNight
18%13%34%24%7%1
735
April1st to 15thNight
23%12%34%23%5%
539
16th to endNight
22%11%34%24%6%1
540
May1st to 15thNight
19%8%34%27%8%
540
16th to endNight
14%6%30%32%16%1
563
June1st to 15thNight
11%6%30%34%16%
540
16th to endNight
9%430%37%16%1
540
July1st to 15thNight
8%124%41%20%2
510
16th to endNight
6%125%35%27%2
564
August1st to 15thNight
8%119%40%25%3
527
16th to endNight
11%218%36%26%4
525
September1st to 15thNight
13%421%34%20%5%
564
16th to endNight
12%8%20%31%23%3
657
October1st to 15thNight
19%6%26%30%15%
660
16th to endNight
25%15%29%20%8%
704
November1st to 15thNight
20%12%30%25%11%
700
16th to endNight
26%16%29%22%5%
720
December1st to 15thNight
28%15%32%20%3
853
16th to endNight
28%15%34%17%3
960
Page generated at 2017-08-13 14:16:36. Version ac04

Copyright 2017 A. Danko


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