This page made possible by the sponsorship of: Astronomical Society of Long Island.

Vanderbilt Planetarium & Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Vanderbilt Planetarium & Observatory clear sky chart from 2005-11-06 to 2024-01-16.

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
33%
23125
19%
74610
10%
23082
44%
23082
22%
47620

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
19%
760
23%
2649
7%
757
38%
757
20%
1576
January11th to 20thNight
22%
741
19%
2591
6%
741
39%
741
16%
1683
January21st to endNight
18%
792
23%
2678
8%
792
38%
792
22%
1604
February1st to 10thNight
18%
720
17%
2290
5%
720
33%
720
19%
1504
February11th to 20thNight
17%
703
20%
2100
6%
700
34%
700
19%
1399
February21st to endNight
19%
592
25%
1776
8%
592
40%
592
20%
1155
March1st to 10thNight
22%
720
22%
2160
9%
720
40%
720
24%
1368
March11th to 20thNight
25%
720
21%
2096
9%
720
38%
720
24%
1369
March21st to endNight
22%
792
23%
1980
6%
792
34%
792
25%
1368
April1st to 10thNight
25%
720
19%
1800
6%
720
33%
720
23%
1269
April11th to 20thNight
31%
711
18%
1775
5%
711
37%
711
26%
1234
April21st to endNight
29%
658
20%
1738
6%
658
37%
658
25%
1196
May1st to 10thNight
29%
540
14%
1620
6%
540
36%
540
18%
1158
May11th to 20thNight
29%
436
18%
1498
9%
436
39%
436
17%
1005
May21st to endNight
49%
388
14%
1551
14%
388
54%
388
19%
982
June1st to 10thNight
42%
360
17%
1440
16%
360
52%
360
19%
896
June11th to 20thNight
52%
360
15%
1440
17%
360
57%
360
21%
871
June21st to endNight
58%
360
15%
1440
18%
360
64%
360
22%
854
July1st to 10thNight
55%
350
12%
1400
16%
350
58%
350
20%
865
July11th to 20thNight
59%
348
15%
1392
18%
348
62%
348
22%
845
July21st to endNight
56%
394
18%
1486
20%
372
62%
372
24%
855
August1st to 10thNight
59%
457
17%
1521
14%
455
62%
455
29%
886
August11th to 20thNight
56%
710
16%
1774
9%
710
57%
710
29%
1230
August21st to endNight
58%
792
19%
1980
9%
792
61%
792
34%
1309
September1st to 10thNight
49%
720
21%
1800
11%
720
53%
720
31%
1222
September11th to 20thNight
53%
720
26%
1906
17%
720
58%
720
35%
1181
September21st to endNight
45%
720
24%
1980
16%
720
51%
720
27%
1325
October1st to 10thNight
36%
720
17%
2029
11%
720
42%
720
23%
1393
October11th to 20thNight
40%
720
24%
2160
14%
720
52%
720
29%
1311
October21st to endNight
26%
792
19%
2556
9%
792
39%
792
21%
1667
November1st to 10thNight
36%
740
23%
2407
13%
740
51%
740
27%
1426
November11th to 20thNight
27%
760
25%
2470
13%
760
44%
760
24%
1500
November21st to endNight
28%
760
23%
2498
10%
760
44%
760
24%
1477
December1st to 10thNight
22%
760
19%
2660
7%
760
37%
760
17%
1714
December11th to 20thNight
20%
760
19%
3179
6%
759
35%
759
15%
1933
December21st to endNight
19%
829
13%
4790
6%
817
36%
817
11%
2990

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
15%6%25%5%3445%8%43%
74796
Night
17%7%35%5%3445%7%42%
74984
TransparencyDay
41%6%12%23%16%2
74501
Night
42%5%11%23%16%2
74813
SeeingDay
18%10%28%28%13%3
13920
Night
20%10%31%28%10%1
23133

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
14%5%26%43446%8%43%
1517
11th to 20thDay
16%8%35%5%345%7%7%37%
1480
21st to endDay
15%7%26%5%3445%8%41%
1677
February1st to 10thDay
19%6%36%5%24448%39%
1669
11th to 20thDay
15%6%16%5%3435%6%45%
1748
21st to endDay
19%8%35%5%34446%40%
1480
March1st to 10thDay
14%7%35%43445%7%45%
1797
11th to 20thDay
15%6%245%35%5%5%9%41%
1858
21st to endDay
19%7%35%5%24445%42%
2376
April1st to 10thDay
17%7%245%3445%7%42%
2160
11th to 20thDay
16%6%245%34447%44%
2134
21st to endDay
19%5%24325%5%5%6%43%
2219
May1st to 10thDay
21%7%2443445%8%38%
2340
11th to 20thDay
18%5%245%245%6%8%41%
2410
21st to endDay
13%5%245%245%6%9%44%
2725
June1st to 10thDay
17%5%25%5%3346%9%41%
2520
11th to 20thDay
13%6%25%5%45%5%6%9%40%
2520
21st to endDay
10%6%35%5%345%5%9%45%
2520
July1st to 10thDay
10%5%25%5%35%6%7%9%44%
2520
11th to 20thDay
9%5%25%425%5%6%10%45%
2520
21st to endDay
12%5%25%5%45%5%6%8%42%
2744
August1st to 10thDay
9%5%25%5%45%5%8%10%42%
2386
11th to 20thDay
12%5%25%435%46%9%45%
2126
21st to endDay
12%425%5%345%6%9%46%
2376
September1st to 10thDay
14%5%24435%5%5%7%46%
2160
11th to 20thDay
12%5%2342435%7%52%
2052
21st to endDay
17%6%246%245%46%43%
1980
October1st to 10thDay
22%6%24323447%43%
1926
11th to 20thDay
15%5%25%5%3445%7%46%
1800
21st to endDay
21%5%25%43445%7%41%
1800
November1st to 10thDay
15%5%245%33446%50%
1671
11th to 20thDay
17%6%34423346%47%
1710
21st to endDay
14%7%3442445%7%45%
1677
December1st to 10thDay
17%7%36%5%35%47%8%36%
1520
11th to 20thDay
21%6%34223347%45%
1906
21st to endDay
18%10%38%5%3445%6%33%
2772
January1st to 10thNight
16%6%25%423449%44%
2649
11th to 20thNight
19%8%36%42445%7%39%
2591
21st to endNight
14%8%35%42345%8%43%
2678
February1st to 10thNight
17%9%35%5%3445%6%40%
2290
11th to 20thNight
19%8%345%3445%7%39%
2098
21st to endNight
21%6%245%3436%5%42%
1776
March1st to 10thNight
17%6%24424446%46%
2150
11th to 20thNight
17%6%25%5%3445%7%41%
2095
21st to endNight
18%8%35%43445%6%41%
1980
April1st to 10thNight
19%7%35%435%5%47%39%
1800
11th to 20thNight
19%6%35%5%24446%41%
1775
21st to endNight
20%6%35%5%23445%42%
1738
May1st to 10thNight
22%7%36%5%445%48%34%
1620
11th to 20thNight
19%8%36%435%5%5%6%36%
1497
21st to endNight
17%6%35%5%35%46%7%39%
1550
June1st to 10thNight
17%6%35%5%34448%40%
1440
11th to 20thNight
15%6%25%425%46%10%40%
1440
21st to endNight
9%5%26%5%36%6%7%10%42%
1440
July1st to 10thNight
10%6%26%5%36%5%7%9%39%
1440
11th to 20thNight
11%6%36%5%36%5%8%9%39%
1440
21st to endNight
11%5%345%36%47%11%41%
1574
August1st to 10thNight
8%5%25%5%46%6%8%8%45%
1531
11th to 20thNight
12%5%36%6%45%6%8%9%37%
1774
21st to endNight
13%5%25%435%5%6%8%44%
1980
September1st to 10thNight
13%5%25%5%345%6%7%44%
1800
11th to 20thNight
14%425%32435%6%51%
1906
21st to endNight
17%5%25%6%24447%44%
1980
October1st to 10thNight
20%7%36%5%33346%40%
2029
11th to 20thNight
16%5%25%43345%7%48%
2160
21st to endNight
22%6%35%434346%40%
2556
November1st to 10thNight
14%5%25%434446%49%
2407
11th to 20thNight
19%7%24423436%47%
2470
21st to endNight
15%5%3443345%7%47%
2498
December1st to 10thNight
21%6%34434447%38%
2660
11th to 20thNight
20%7%25%43345%7%42%
3288
21st to endNight
20%9%35%5%34446%37%
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%411%28%13%3
1518
11th to 20thDay
45%5%13%22%12%2
1480
21st to endDay
43%5%10%26%14%2
1677
February1st to 10thDay
46%5%12%22%13%1
1669
11th to 20thDay
42%410%26%16%2
1757
21st to endDay
48%49%21%15%3
1480
March1st to 10thDay
39%6%14%23%16%3
1800
11th to 20thDay
40%6%13%24%15%2
1858
21st to endDay
47%48%19%17%5%
2376
April1st to 10thDay
44%6%9%21%17%4
2160
11th to 20thDay
41%6%10%22%18%3
2134
21st to endDay
43%6%10%21%17%4
2219
May1st to 10thDay
47%5%10%20%15%2
2340
11th to 20thDay
43%6%12%21%16%3
2414
21st to endDay
38%7%19%21%13%2
2725
June1st to 10thDay
41%7%13%23%14%2
2520
11th to 20thDay
42%7%12%24%13%2
2520
21st to endDay
37%7%15%24%15%1
2520
July1st to 10thDay
37%9%14%22%17%1
2450
11th to 20thDay
35%7%16%27%14%2
2445
21st to endDay
38%8%13%22%16%3
2591
August1st to 10thDay
36%8%16%25%15%1
2372
11th to 20thDay
38%7%14%25%14%1
2126
21st to endDay
37%6%13%22%19%2
2376
September1st to 10thDay
39%6%14%21%19%2
2160
11th to 20thDay
34%412%24%25%2
2052
21st to endDay
42%5%10%20%20%3
1980
October1st to 10thDay
43%5%10%22%18%2
1926
11th to 20thDay
38%5%12%26%18%2
1800
21st to endDay
44%6%9%22%17%2
1800
November1st to 10thDay
37%413%24%21%2
1671
11th to 20thDay
39%310%23%21%3
1710
21st to endDay
39%414%28%12%2
1677
December1st to 10thDay
45%5%16%19%14%1
1520
11th to 20thDay
42%414%22%16%2
1906
21st to endDay
50%6%11%23%9%
2772
January1st to 10thNight
41%312%25%17%4
2649
11th to 20thNight
46%5%13%23%11%2
2591
21st to endNight
40%310%25%19%2
2678
February1st to 10thNight
45%5%13%23%13%2
2290
11th to 20thNight
45%5%10%21%16%4
2100
21st to endNight
44%47%25%17%4
1776
March1st to 10thNight
40%412%24%16%4
2160
11th to 20thNight
43%49%23%17%4
2096
21st to endNight
45%38%19%18%6%
1980
April1st to 10thNight
47%49%19%18%3
1800
11th to 20thNight
44%5%10%19%21%1
1775
21st to endNight
46%48%20%16%5%
1738
May1st to 10thNight
51%5%10%17%14%2
1620
11th to 20thNight
48%6%9%18%17%2
1498
21st to endNight
44%6%15%21%11%2
1551
June1st to 10thNight
44%5%10%21%18%1
1440
11th to 20thNight
41%6%15%25%12%1
1440
21st to endNight
37%8%13%26%14%1
1440
July1st to 10thNight
40%9%15%20%15%2
1400
11th to 20thNight
39%8%13%27%12%1
1392
21st to endNight
36%7%17%23%14%2
1486
August1st to 10thNight
33%8%18%24%15%1
1521
11th to 20thNight
41%7%14%26%11%1
1774
21st to endNight
36%7%14%23%18%2
1980
September1st to 10thNight
40%6%10%25%17%1
1800
11th to 20thNight
35%410%24%23%3
1906
21st to endNight
42%6%7%21%21%3
1980
October1st to 10thNight
47%410%22%14%3
2029
11th to 20thNight
37%413%29%16%1
2160
21st to endNight
45%412%22%16%1
2556
November1st to 10thNight
37%411%23%20%4
2407
11th to 20thNight
40%39%25%19%4
2470
21st to endNight
37%413%30%14%2
2498
December1st to 10thNight
46%5%13%22%14%1
2660
11th to 20thNight
43%412%20%18%3
3288
21st to endNight
48%5%10%25%12%1
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
23%18%34%20%4
180
11th to 20thDay
18%25%36%17%4
170
21st to endDay
18%15%43%18%5%1
218
February1st to 10thDay
27%13%41%18%2
289
11th to 20thDay
20%18%40%20%2
416
21st to endDay
25%18%34%19%5%
346
March1st to 10thDay
14%18%39%25%4
420
11th to 20thDay
18%12%40%25%41
420
21st to endDay
23%15%36%19%7%1
462
April1st to 10thDay
21%15%37%18%9%1
420
11th to 20thDay
19%12%33%26%10%
412
21st to endDay
26%11%32%24%5%
420
May1st to 10thDay
28%10%30%24%7%1
423
11th to 20thDay
20%8%29%32%10%1
447
21st to endDay
17%420%34%22%3
485
June1st to 10thDay
19%5%22%33%18%3
450
11th to 20thDay
15%7%26%36%14%2
450
21st to endDay
12%426%35%20%3
450
July1st to 10thDay
13%217%38%25%4
445
11th to 20thDay
9%317%39%26%5%
444
21st to endDay
13%418%43%20%3
488
August1st to 10thDay
12%216%39%24%6%
446
11th to 20thDay
14%316%37%26%4
439
21st to endDay
12%420%38%19%7%
495
September1st to 10thDay
18%421%28%22%7%
450
11th to 20thDay
15%421%30%21%9%
450
21st to endDay
21%5%26%28%16%4
450
October1st to 10thDay
26%7%22%24%16%5%
450
11th to 20thDay
15%12%28%29%14%2
450
21st to endDay
22%12%32%24%10%1
405
November1st to 10thDay
15%12%25%34%11%2
365
11th to 20thDay
19%19%31%24%7%1
370
21st to endDay
18%16%32%26%8%1
337
December1st to 10thDay
19%18%37%19%7%1
180
11th to 20thDay
29%22%27%17%5%
180
21st to endDay
23%18%38%14%7%1
198
January1st to 10thNight
20%18%39%19%4
760
11th to 20thNight
25%16%32%25%2
741
21st to endNight
19%16%42%21%2
792
February1st to 10thNight
21%14%39%23%3
720
11th to 20thNight
24%17%39%19%2
703
21st to endNight
24%17%36%19%4
592
March1st to 10thNight
21%16%38%21%5%
720
11th to 20thNight
20%13%39%22%5%1
720
21st to endNight
21%12%39%22%5%
792
April1st to 10thNight
22%13%36%24%5%
720
11th to 20thNight
21%11%33%29%6%
711
21st to endNight
22%9%34%29%6%1
658
May1st to 10thNight
24%8%32%28%8%1
540
11th to 20thNight
22%9%35%28%6%
436
21st to endNight
21%418%38%17%2
389
June1st to 10thNight
20%425%33%17%
360
11th to 20thNight
18%419%44%16%
360
21st to endNight
11%222%50%14%1
360
July1st to 10thNight
12%322%42%19%2
350
11th to 20thNight
12%417%44%20%3
348
21st to endNight
14%218%47%18%1
394
August1st to 10thNight
9%220%47%21%1
457
11th to 20thNight
15%316%38%26%2
710
21st to endNight
14%416%38%25%4
792
September1st to 10thNight
16%323%37%17%4
720
11th to 20thNight
17%5%19%34%20%6%
720
21st to endNight
21%323%33%18%2
720
October1st to 10thNight
24%7%25%26%16%3
720
11th to 20thNight
17%10%29%31%12%1
720
21st to endNight
24%10%34%24%8%
792
November1st to 10thNight
15%11%33%28%11%2
740
11th to 20thNight
21%14%33%23%8%
760
21st to endNight
17%15%34%25%7%1
760
December1st to 10thNight
25%16%33%19%5%1
760
11th to 20thNight
22%15%36%22%4
760
21st to endNight
23%14%39%21%3
836
Page generated at 2024-01-13 01:38:11. Version ac07

Copyright 2024 Attilla Danko


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