Van Vleck Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Van Vleck Observatory clear sky chart from 2005-11-06 to 2024-03-23.

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
26%
23299
19%
80745
8%
23206
39%
23206
18%
51982

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 10thNight
15%
750
25%
4333
6%
737
35%
737
12%
2342
January11th to 20thNight
17%
760
23%
4330
4%
740
34%
740
9%
2575
January21st to endNight
17%
832
13%
4809
6%
814
36%
814
8%
3312
February1st to 10thNight
16%
760
19%
2392
5%
760
33%
760
17%
1578
February11th to 20thNight
11%
743
21%
2220
4%
740
31%
740
16%
1465
February21st to endNight
15%
628
22%
1884
7%
628
34%
628
16%
1260
March1st to 10thNight
17%
760
20%
2280
6%
760
34%
760
18%
1486
March11th to 20thNight
18%
760
19%
2231
5%
760
33%
760
19%
1484
March21st to endNight
17%
801
23%
2001
5%
801
30%
801
20%
1369
April1st to 10thNight
19%
720
18%
1800
5%
720
26%
720
19%
1291
April11th to 20thNight
22%
711
17%
1775
4%
711
29%
711
23%
1232
April21st to endNight
19%
608
18%
1688
4%
608
28%
608
19%
1169
May1st to 10thNight
22%
540
13%
1620
6%
540
29%
540
15%
1165
May11th to 20thNight
20%
458
17%
1520
7%
458
30%
458
17%
1006
May21st to endNight
37%
389
13%
1551
11%
388
44%
388
18%
982
June1st to 10thNight
33%
360
15%
1440
12%
360
45%
360
17%
919
June11th to 20thNight
37%
360
14%
1440
13%
360
46%
360
19%
915
June21st to endNight
41%
360
14%
1440
12%
360
52%
360
21%
871
July1st to 10thNight
44%
350
13%
1400
15%
350
49%
350
21%
869
July11th to 20thNight
50%
348
13%
1392
14%
348
54%
348
21%
869
July21st to endNight
45%
394
17%
1486
15%
372
54%
372
23%
881
August1st to 10thNight
45%
438
16%
1502
11%
436
50%
436
25%
912
August11th to 20thNight
47%
675
15%
1739
8%
675
50%
675
25%
1213
August21st to endNight
48%
792
20%
1980
9%
792
52%
792
33%
1303
September1st to 10thNight
38%
720
20%
1800
8%
720
44%
720
29%
1233
September11th to 20thNight
42%
720
25%
1933
14%
720
50%
720
34%
1169
September21st to endNight
39%
720
23%
1980
14%
720
47%
720
27%
1326
October1st to 10thNight
35%
720
17%
1989
10%
720
41%
720
22%
1369
October11th to 20thNight
32%
720
22%
2186
10%
720
48%
720
25%
1354
October21st to endNight
23%
792
17%
2574
8%
792
35%
792
19%
1694
November1st to 10thNight
30%
740
23%
2407
10%
740
47%
740
24%
1435
November11th to 20thNight
23%
760
24%
2470
10%
760
40%
760
20%
1533
November21st to endNight
23%
760
23%
2483
8%
760
42%
760
20%
1523
December1st to 10thNight
21%
760
18%
2660
7%
760
35%
760
15%
1740
December11th to 20thNight
19%
760
17%
3198
4%
759
34%
759
12%
2001
December21st to endNight
14%
830
13%
4812
4%
817
34%
817
9%
3137

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
17%7%35%5%3445%7%40%
78566
Night
18%7%35%5%3445%6%41%
81313
TransparencyDay
44%6%12%22%15%2
78271
Night
44%5%12%22%15%2
81142
SeeingDay
23%18%31%19%8%2
14142
Night
22%15%32%24%7%1
23319

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 10thDay
15%6%25%435%45%5%45%
2517
11th to 20thDay
17%9%37%5%346%5%5%36%
2520
21st to endDay
23%12%36%5%3344433%
2816
February1st to 10thDay
19%7%35%5%36%5%5%6%37%
1786
11th to 20thDay
14%7%26%5%45%45%7%41%
1848
21st to endDay
21%8%34434445%37%
1570
March1st to 10thDay
17%7%35%5%35%35%7%40%
1897
11th to 20thDay
16%5%35%5%36%6%6%6%39%
1945
21st to endDay
20%8%36%5%34445%40%
2400
April1st to 10thDay
18%8%344245%5%8%38%
2160
11th to 20thDay
19%6%25%434447%42%
2134
21st to endDay
22%5%25%435%45%5%39%
2272
May1st to 10thDay
24%7%36%42445%7%33%
2340
11th to 20thDay
19%6%26%43445%7%40%
2389
21st to endDay
16%6%25%4345%5%7%44%
2725
June1st to 10thDay
18%6%35%5%35%5%6%9%34%
2520
11th to 20thDay
16%6%36%5%45%5%6%8%36%
2520
21st to endDay
14%6%36%5%35%5%6%10%38%
2520
July1st to 10thDay
13%6%35%6%45%5%5%8%40%
2520
11th to 20thDay
12%5%25%5%35%7%6%9%40%
2520
21st to endDay
13%426%6%36%6%7%9%38%
2744
August1st to 10thDay
11%7%25%7%46%5%8%10%35%
2406
11th to 20thDay
12%6%35%6%36%5%6%8%41%
2158
21st to endDay
13%5%26%5%346%6%8%42%
2376
September1st to 10thDay
14%6%25%5%345%5%7%43%
2160
11th to 20thDay
13%6%25%4235%47%50%
2026
21st to endDay
17%6%26%43345%6%44%
1980
October1st to 10thDay
22%7%25%42435%5%40%
1967
11th to 20thDay
14%6%25%5%235%6%7%45%
1774
21st to endDay
21%6%35%43445%6%39%
1782
November1st to 10thDay
14%7%25%5%34436%48%
1671
11th to 20thDay
19%6%24423437%46%
1710
21st to endDay
15%7%26%434446%44%
1695
December1st to 10thDay
19%7%25%6%4445%6%37%
1520
11th to 20thDay
22%6%24323336%44%
1906
21st to endDay
20%11%46%6%34346%32%
2772
January1st to 10thNight
17%6%3442345%6%45%
4429
11th to 20thNight
16%10%36%434335%43%
4440
21st to endNight
24%12%47%5%23335%32%
4838
February1st to 10thNight
16%9%36%5%2435%6%40%
2392
11th to 20thNight
16%8%36%43445%6%40%
2218
21st to endNight
20%8%35%5%2445%5%40%
1884
March1st to 10thNight
18%7%25%43446%6%41%
2270
11th to 20thNight
16%8%35%5%345%6%7%39%
2230
21st to endNight
18%7%35%33445%6%42%
2001
April1st to 10thNight
19%7%35%6%35%45%6%37%
1800
11th to 20thNight
18%6%35%434447%41%
1775
21st to endNight
22%6%37%5%23436%39%
1688
May1st to 10thNight
23%7%26%5%45%5%5%8%31%
1620
11th to 20thNight
18%7%35%5%345%5%8%37%
1519
21st to endNight
17%7%24425%5%6%8%40%
1550
June1st to 10thNight
18%7%26%5%345%5%7%38%
1440
11th to 20thNight
17%5%36%5%35%5%5%9%38%
1440
21st to endNight
12%5%35%5%35%46%9%42%
1440
July1st to 10thNight
13%6%35%5%35%6%7%9%39%
1440
11th to 20thNight
14%5%35%6%35%6%6%10%37%
1440
21st to endNight
11%5%26%5%45%5%7%9%40%
1574
August1st to 10thNight
8%5%26%6%35%6%8%10%42%
1512
11th to 20thNight
11%6%36%7%5%46%6%8%38%
1739
21st to endNight
14%5%345%345%5%8%47%
1980
September1st to 10thNight
14%6%36%6%3445%8%42%
1800
11th to 20thNight
14%5%24423346%53%
1933
21st to endNight
17%7%24435%5%46%43%
1980
October1st to 10thNight
20%7%35%5%34445%41%
1989
11th to 20thNight
16%6%2442445%6%46%
2186
21st to endNight
23%7%25%42445%5%39%
2574
November1st to 10thNight
14%5%25%5%3435%6%49%
2407
11th to 20thNight
19%7%2342435%6%44%
2470
21st to endNight
16%6%2443445%7%44%
2483
December1st to 10thNight
21%7%25%425%5%35%39%
2660
11th to 20thNight
20%7%245%34446%41%
3288
21st to endNight
22%10%36%5%3344436%
4884

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 10thDay
41%410%31%11%3
2518
11th to 20thDay
48%6%10%24%11%1
2520
21st to endDay
54%5%8%21%10%2
2816
February1st to 10thDay
46%6%13%20%14%2
1786
11th to 20thDay
43%5%11%23%16%3
1857
21st to endDay
48%5%11%20%12%3
1570
March1st to 10thDay
45%414%21%13%4
1900
11th to 20thDay
43%5%14%23%14%2
1945
21st to endDay
48%48%18%18%5%
2400
April1st to 10thDay
44%6%11%20%16%3
2160
11th to 20thDay
42%5%11%21%19%3
2134
21st to endDay
46%6%10%20%15%4
2272
May1st to 10thDay
50%6%10%19%13%2
2340
11th to 20thDay
43%6%11%20%17%3
2393
21st to endDay
40%7%15%21%14%2
2725
June1st to 10thDay
46%8%13%23%10%1
2520
11th to 20thDay
44%9%12%21%13%1
2520
21st to endDay
42%8%14%22%14%1
2520
July1st to 10thDay
42%7%11%21%17%2
2450
11th to 20thDay
38%8%14%25%13%2
2445
21st to endDay
40%8%13%20%17%3
2591
August1st to 10thDay
40%10%13%23%13%
2392
11th to 20thDay
40%7%15%22%15%1
2158
21st to endDay
39%7%11%22%18%3
2376
September1st to 10thDay
39%7%12%21%18%2
2160
11th to 20thDay
34%5%12%23%23%3
2026
21st to endDay
42%6%8%22%21%3
1980
October1st to 10thDay
46%5%9%21%15%3
1967
11th to 20thDay
37%413%28%17%1
1774
21st to endDay
46%6%10%20%16%3
1782
November1st to 10thDay
40%5%12%21%20%2
1671
11th to 20thDay
40%5%11%23%19%3
1710
21st to endDay
41%5%13%26%13%3
1695
December1st to 10thDay
48%5%14%17%15%1
1520
11th to 20thDay
44%315%19%17%2
1906
21st to endDay
53%411%21%10%1
2772
January1st to 10thNight
39%5%11%31%11%3
4429
11th to 20thNight
46%412%23%12%3
4440
21st to endNight
56%39%19%11%2
4838
February1st to 10thNight
46%5%12%24%12%2
2392
11th to 20thNight
44%5%11%20%17%4
2220
21st to endNight
46%49%22%14%4
1884
March1st to 10thNight
43%5%13%21%15%3
2280
11th to 20thNight
44%5%12%21%15%3
2231
21st to endNight
44%410%18%19%5%
2001
April1st to 10thNight
47%5%11%19%15%3
1800
11th to 20thNight
44%5%10%20%19%2
1775
21st to endNight
47%5%9%21%13%5%
1688
May1st to 10thNight
51%7%10%16%13%2
1620
11th to 20thNight
45%6%12%18%15%3
1520
21st to endNight
43%8%16%20%13%1
1551
June1st to 10thNight
44%7%12%20%15%1
1440
11th to 20thNight
44%7%13%25%11%1
1440
21st to endNight
38%9%14%25%14%
1440
July1st to 10thNight
40%8%15%19%15%2
1400
11th to 20thNight
41%8%15%26%9%1
1392
21st to endNight
38%7%16%22%14%2
1486
August1st to 10thNight
36%9%17%21%15%2
1502
11th to 20thNight
44%8%13%24%11%1
1739
21st to endNight
38%6%12%25%18%1
1980
September1st to 10thNight
42%6%9%25%16%1
1800
11th to 20thNight
34%412%21%26%3
1933
21st to endNight
43%5%8%21%21%3
1980
October1st to 10thNight
46%5%10%22%14%3
1989
11th to 20thNight
39%5%12%27%16%1
2186
21st to endNight
47%413%20%15%1
2574
November1st to 10thNight
37%5%12%22%20%4
2407
11th to 20thNight
42%410%22%18%3
2470
21st to endNight
41%413%29%12%2
2483
December1st to 10thNight
47%5%13%21%14%1
2660
11th to 20thNight
45%5%14%18%17%2
3288
21st to endNight
52%410%22%11%
4884

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 10thDay
29%23%29%17%2
180
11th to 20thDay
29%29%28%11%3
180
21st to endDay
24%30%32%8%5%
213
February1st to 10thDay
32%25%28%15%
346
11th to 20thDay
28%29%33%9%1
456
21st to endDay
31%25%29%11%4
382
March1st to 10thDay
26%24%35%13%2
460
11th to 20thDay
27%28%31%12%21
460
21st to endDay
31%22%31%10%41
470
April1st to 10thDay
28%29%27%11%4
420
11th to 20thDay
27%23%30%15%5%
412
21st to endDay
30%21%32%12%41
420
May1st to 10thDay
33%20%29%11%6%1
423
11th to 20thDay
24%19%38%13%6%1
447
21st to endDay
21%16%28%24%9%2
485
June1st to 10thDay
19%15%32%23%10%2
450
11th to 20thDay
18%16%37%21%8%1
450
21st to endDay
15%16%31%26%10%2
450
July1st to 10thDay
18%8%29%31%11%3
445
11th to 20thDay
14%7%33%30%14%2
444
21st to endDay
14%9%39%30%7%2
488
August1st to 10thDay
13%8%31%34%10%4
446
11th to 20thDay
15%8%32%27%15%4
439
21st to endDay
13%10%33%25%13%6%
495
September1st to 10thDay
21%10%25%23%17%3
450
11th to 20thDay
15%13%29%24%11%8%
450
21st to endDay
21%13%35%16%11%4
450
October1st to 10thDay
28%12%26%19%11%4
450
11th to 20thDay
18%19%31%22%9%1
437
21st to endDay
26%16%32%18%7%2
396
November1st to 10thDay
21%16%28%24%8%2
365
11th to 20thDay
26%22%27%19%5%
370
21st to endDay
24%21%31%17%7%1
355
December1st to 10thDay
23%25%32%15%6%
180
11th to 20thDay
32%26%31%9%2
180
21st to endDay
27%25%27%16%42
198
January1st to 10thNight
22%21%38%16%3
760
11th to 20thNight
29%19%32%17%2
760
21st to endNight
23%23%32%19%3
836
February1st to 10thNight
23%19%35%20%2
760
11th to 20thNight
24%24%37%14%1
743
21st to endNight
27%22%33%16%3
628
March1st to 10thNight
26%19%35%16%3
760
11th to 20thNight
23%21%34%18%3
760
21st to endNight
24%22%34%17%3
801
April1st to 10thNight
24%22%33%19%3
720
11th to 20thNight
22%20%33%21%4
711
21st to endNight
23%18%35%18%5%
608
May1st to 10thNight
26%18%30%20%6%
540
11th to 20thNight
21%17%38%20%4
458
21st to endNight
20%7%30%34%8%1
389
June1st to 10thNight
22%5%33%30%9%
360
11th to 20thNight
21%6%31%34%9%
360
21st to endNight
14%5%34%39%8%
360
July1st to 10thNight
14%431%37%14%
350
11th to 20thNight
14%226%43%14%1
348
21st to endNight
15%7%28%40%9%1
394
August1st to 10thNight
10%433%41%11%1
438
11th to 20thNight
13%5%25%39%16%1
675
21st to endNight
15%5%26%36%16%2
792
September1st to 10thNight
17%5%31%33%12%2
720
11th to 20thNight
18%7%27%30%16%2
720
21st to endNight
20%7%29%27%15%1
720
October1st to 10thNight
25%9%27%25%11%2
720
11th to 20thNight
20%14%31%26%9%
720
21st to endNight
30%11%32%21%6%
792
November1st to 10thNight
17%14%34%24%9%1
740
11th to 20thNight
24%18%33%19%6%1
760
21st to endNight
22%19%32%21%5%1
760
December1st to 10thNight
28%17%30%20%5%1
760
11th to 20thNight
24%22%31%20%3
760
21st to endNight
27%18%36%16%2
836
Page generated at 2024-03-19 23:31:07. Version ac07

Copyright © 2024 Attilla Danko.


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