The Decker-Grebner-Van Zandt Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the The Decker-Grebner-Van Zandt Observatory clear sky chart from 2005-11-05 to 2018-02-13.

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
38%
16613
19%
48791
11%
16569
45%
16569
25%
31855

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
24%
975
16%
2716
6%
970
33%
970
21%
1903
January16th to endNight
21%
1040
15%
2779
4%
1040
30%
1040
19%
2016
February1st to 15thNight
24%
905
14%
2412
4%
904
30%
904
20%
1739
February16th to endNight
25%
631
20%
1893
7%
631
36%
631
23%
1245
March1st to 15thNight
26%
692
16%
2132
8%
692
33%
692
19%
1442
March16th to endNight
28%
576
13%
2013
8%
576
36%
576
16%
1396
April1st to 15thNight
31%
539
17%
1797
13%
539
40%
539
22%
1159
April16th to endNight
36%
540
17%
1800
13%
540
45%
540
21%
1203
May1st to 15thNight
35%
540
17%
1557
10%
540
41%
540
22%
1087
May16th to endNight
48%
563
19%
1497
11%
561
53%
561
31%
993
June1st to 15thNight
56%
540
18%
1440
11%
540
59%
540
37%
890
June16th to endNight
50%
540
18%
1440
10%
540
54%
540
34%
916
July1st to 15thNight
64%
510
18%
1360
12%
510
66%
510
38%
821
July16th to endNight
74%
564
25%
1417
18%
531
77%
531
42%
822
August1st to 15thNight
74%
539
23%
1640
23%
536
75%
536
40%
885
August16th to endNight
70%
573
22%
1910
24%
573
74%
573
35%
1069
September1st to 15thNight
64%
540
29%
1800
30%
540
68%
540
36%
961
September16th to endNight
59%
675
29%
1938
20%
675
66%
675
38%
1125
October1st to 15thNight
46%
720
26%
2141
19%
720
54%
720
33%
1272
October16th to endNight
31%
768
21%
2373
13%
768
41%
768
26%
1481
November1st to 15thNight
31%
760
24%
2471
15%
760
43%
760
26%
1499
November16th to endNight
27%
868
22%
2623
10%
868
39%
868
24%
1672
December1st to 15thNight
22%
975
13%
2730
4%
975
30%
975
19%
1965
December16th to endNight
18%
1040
12%
2912
4%
1040
26%
1040
13%
2294

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%35%45%7%38%
49260
Night
13%7%35%5%24446%42%
48960
Cloud CoverDay
13%7%36%5%35%45%7%38%
49268
Night
13%7%35%5%24446%42%
48965
TransparencyDay
44%6%12%22%12%1
48946
Night
42%5%12%23%14%2
48792
SeeingDay
14%8%26%26%18%5%
6420
Night
16%8%30%30%12%1
16615

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
15%12%5%8%6%35%346%26%
1552
16th to endDay
14%13%6%10%5%35%446%26%
1794
February1st to 15thDay
15%11%5%8%5%36%346%30%
1714
16th to endDay
13%9%5%7%7%3445%5%32%
1580
March1st to 15thDay
14%12%37%5%34345%33%
1829
16th to endDay
17%10%37%5%35%45%7%28%
2211
April1st to 15thDay
16%9%45%5%33435%36%
2151
16th to endDay
19%8%26%5%34345%35%
2160
May1st to 15thDay
19%7%35%5%345%5%8%30%
2389
16th to endDay
15%6%25%5%35%5%5%6%37%
2617
June1st to 15thDay
10%5%25%6%45%5%5%7%42%
2520
16th to endDay
9%5%25%436%6%6%9%40%
2520
July1st to 15thDay
7%4145%36%6%7%9%43%
2520
16th to endDay
5%415%5%46%6%6%10%43%
2655
August1st to 15thDay
7%425%5%35%5%6%10%43%
2310
16th to endDay
8%425%5%35%5%6%7%45%
2292
September1st to 15thDay
12%314434347%49%
2160
16th to endDay
8%5%24425%45%8%48%
2010
October1st to 15thDay
17%6%25%322345%46%
1818
16th to endDay
16%8%245%24345%42%
1851
November1st to 15thDay
16%7%24333447%42%
1717
16th to endDay
16%10%5%5%323336%40%
1666
December1st to 15thDay
17%11%46%6%35%346%29%
1560
16th to endDay
19%13%5%7%5%34337%26%
1664
January1st to 15thNight
13%9%37%6%34345%37%
2716
16th to endNight
12%11%5%8%5%34346%34%
2779
February1st to 15thNight
15%10%46%5%34445%35%
2410
16th to endNight
14%9%36%423336%40%
1893
March1st to 15thNight
15%10%35%5%24435%38%
2119
16th to endNight
16%8%46%5%25%5%5%6%32%
2013
April1st to 15thNight
15%8%35%42345%6%40%
1797
16th to endNight
18%6%35%5%33346%38%
1800
May1st to 15thNight
18%5%25%5%3445%7%36%
1557
16th to endNight
9%425%6%445%7%8%42%
1496
June1st to 15thNight
10%5%14433347%50%
1440
16th to endNight
9%425%5%35%448%46%
1440
July1st to 15thNight
5%4144247%6%9%49%
1440
16th to endNight
42125%35%5%7%10%51%
1513
August1st to 15thNight
42135%3446%9%54%
1649
16th to endNight
7%3235%3435%7%53%
1910
September1st to 15thNight
12%313412435%57%
1800
16th to endNight
7%4144245%5%8%50%
1938
October1st to 15thNight
11%424423435%52%
2141
16th to endNight
14%6%25%423346%44%
2373
November1st to 15thNight
13%7%24423435%47%
2471
16th to endNight
16%6%35%42335%6%42%
2623
December1st to 15thNight
18%11%37%423345%34%
2730
16th to endNight
21%14%48%6%3433426%
2912

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%12%5%8%6%35%346%26%
1552
16th to endDay
13%14%5%9%6%35%446%26%
1794
February1st to 15thDay
15%11%6%8%5%36%336%30%
1714
16th to endDay
13%9%5%7%6%35%446%32%
1580
March1st to 15thDay
15%11%46%435%35%5%33%
1839
16th to endDay
16%10%37%5%345%5%7%28%
2209
April1st to 15thDay
17%8%5%5%5%33335%36%
2151
16th to endDay
19%8%26%5%34345%35%
2160
May1st to 15thDay
19%7%35%5%2445%8%30%
2389
16th to endDay
15%6%35%5%35%5%46%36%
2617
June1st to 15thDay
10%5%25%6%35%5%5%7%42%
2520
16th to endDay
10%425%436%6%7%9%39%
2520
July1st to 15thDay
7%4145%35%6%7%9%43%
2520
16th to endDay
5%415%5%46%6%6%10%43%
2655
August1st to 15thDay
7%4245%35%6%6%10%43%
2310
16th to endDay
8%4246%35%5%6%7%45%
2292
September1st to 15thDay
12%314424447%49%
2160
16th to endDay
8%5%24425%45%8%48%
2010
October1st to 15thDay
16%7%15%313246%46%
1818
16th to endDay
16%8%14433346%42%
1851
November1st to 15thDay
16%8%14433446%42%
1717
16th to endDay
16%9%45%313326%40%
1666
December1st to 15thDay
17%11%46%6%35%446%29%
1560
16th to endDay
19%13%47%5%34346%26%
1664
January1st to 15thNight
13%10%37%6%34335%37%
2716
16th to endNight
12%11%5%8%6%34345%34%
2779
February1st to 15thNight
14%11%46%6%34345%35%
2403
16th to endNight
14%9%45%433335%41%
1893
March1st to 15thNight
15%11%35%5%24345%37%
2132
16th to endNight
16%8%46%5%245%5%6%32%
2013
April1st to 15thNight
14%9%35%42335%6%40%
1797
16th to endNight
18%6%35%5%3335%6%38%
1800
May1st to 15thNight
18%6%35%5%2445%7%36%
1556
16th to endNight
9%425%5%345%6%8%42%
1496
June1st to 15thNight
10%5%24424446%50%
1440
16th to endNight
8%5%25%6%25%5%5%8%46%
1440
July1st to 15thNight
5%424425%6%6%9%49%
1440
16th to endNight
42135%25%5%6%10%51%
1513
August1st to 15thNight
431343446%9%54%
1649
16th to endNight
7%3145%2435%7%53%
1910
September1st to 15thNight
12%313412345%57%
1800
16th to endNight
7%4133245%6%8%51%
1938
October1st to 15thNight
12%424424335%52%
2141
16th to endNight
15%6%25%5%23346%44%
2373
November1st to 15thNight
13%7%24423436%47%
2471
16th to endNight
16%6%35%423446%42%
2623
December1st to 15thNight
18%12%36%423345%34%
2730
16th to endNight
21%14%48%6%3333426%
2912

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
58%410%15%9%
1553
16th to endDay
58%5%12%15%7%
1794
February1st to 15thDay
55%415%18%5%1
1716
16th to endDay
51%6%13%20%7%
1580
March1st to 15thDay
51%5%15%20%6%
1839
16th to endDay
53%6%14%19%41
2211
April1st to 15thDay
49%410%23%10%1
2151
16th to endDay
50%5%10%18%13%1
2160
May1st to 15thDay
50%5%13%21%8%
2389
16th to endDay
46%7%11%17%15%1
2621
June1st to 15thDay
39%6%12%23%15%1
2520
16th to endDay
37%8%13%21%15%2
2520
July1st to 15thDay
34%7%14%24%14%3
2371
16th to endDay
33%8%17%25%13%2
2488
August1st to 15thDay
33%8%14%25%16%1
2295
16th to endDay
34%7%11%25%17%3
2292
September1st to 15thDay
34%48%25%20%5%
2160
16th to endDay
33%5%11%28%19%2
2010
October1st to 15thDay
39%49%26%17%2
1818
16th to endDay
43%412%24%12%2
1851
November1st to 15thDay
42%412%23%15%2
1717
16th to endDay
46%5%13%25%9%
1666
December1st to 15thDay
55%5%13%18%7%
1560
16th to endDay
59%5%11%15%8%
1664
January1st to 15thNight
49%411%20%13%
2716
16th to endNight
51%416%19%8%
2779
February1st to 15thNight
51%313%19%10%1
2412
16th to endNight
45%313%26%10%
1893
March1st to 15thNight
47%5%12%22%10%1
2132
16th to endNight
49%7%12%19%8%1
2013
April1st to 15thNight
44%49%25%13%2
1797
16th to endNight
46%411%20%12%4
1800
May1st to 15thNight
46%6%11%20%14%
1557
16th to endNight
37%8%14%19%18%2
1498
June1st to 15thNight
33%412%24%19%4
1440
16th to endNight
35%6%12%21%19%4
1440
July1st to 15thNight
30%6%12%30%16%3
1360
16th to endNight
24%8%13%31%18%2
1417
August1st to 15thNight
24%7%13%31%19%3
1640
16th to endNight
29%6%11%26%20%5%
1910
September1st to 15thNight
29%39%27%24%5%
1800
16th to endNight
29%5%12%27%21%3
1938
October1st to 15thNight
34%49%25%21%5%
2141
16th to endNight
40%410%24%18%1
2373
November1st to 15thNight
38%410%23%18%4
2471
16th to endNight
42%312%27%13%
2623
December1st to 15thNight
52%412%21%9%
2730
16th to endNight
62%410%15%7%
2912

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
25%17%28%22%5%
135
16th to endDay
16%19%43%12%7%
144
February1st to 15thDay
23%19%39%13%4
126
16th to endDay
11%20%32%27%7%1
81
March1st to 15thDay
27%23%36%10%2
90
16th to endDay
26%15%25%23%8%1
195
April1st to 15thDay
18%12%34%24%9%
270
16th to endDay
21%8%30%21%15%3
270
May1st to 15thDay
21%12%29%26%6%3
360
16th to endDay
17%8%31%24%12%4
517
June1st to 15thDay
9%429%26%24%5%
495
16th to endDay
10%7%31%28%16%5%
495
July1st to 15thDay
11%321%33%19%11%
475
16th to endDay
5%20%33%30%9%
515
August1st to 15thDay
7%116%35%27%12%
445
16th to endDay
8%212%30%34%12%
332
September1st to 15thDay
10%314%31%33%6%
315
16th to endDay
6%5%19%31%33%4
314
October1st to 15thDay
21%9%21%23%20%3
153
16th to endDay
21%9%37%25%5%
144
November1st to 15thDay
13%8%34%22%17%4
135
16th to endDay
25%17%34%18%4
135
December1st to 15thDay
16%25%30%19%6%1
135
16th to endDay
23%17%49%9%
144
January1st to 15thNight
17%13%40%21%6%
975
16th to endNight
18%12%39%26%3
1040
February1st to 15thNight
20%12%36%26%4
907
16th to endNight
18%12%39%24%4
631
March1st to 15thNight
21%10%37%24%5%
692
16th to endNight
19%9%36%25%9%
576
April1st to 15thNight
19%12%33%27%7%
539
16th to endNight
21%7%29%30%10%
540
May1st to 15thNight
18%6%32%33%7%
540
16th to endNight
10%330%40%14%
563
June1st to 15thNight
12%321%44%16%1
540
16th to endNight
10%129%38%17%1
540
July1st to 15thNight
7%20%40%28%2
510
16th to endNight
5%111%43%35%2
564
August1st to 15thNight
5%12%43%31%6%
539
16th to endNight
7%113%39%35%2
573
September1st to 15thNight
13%215%35%28%4
540
16th to endNight
10%419%35%27%3
675
October1st to 15thNight
13%5%27%32%18%2
720
16th to endNight
19%9%35%25%8%
768
November1st to 15thNight
16%9%36%25%10%
760
16th to endNight
19%11%36%25%5%
868
December1st to 15thNight
23%12%35%22%5%
975
16th to endNight
27%11%34%23%2
1040
Page generated at 2018-02-13 19:46:06. Version ac04

Copyright 2018 A. Danko


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