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 2005-11-05 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
34%
14873
19%
46474
10%
14842
44%
14842
23%
29711

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
21%
720
21%
2506
8%
716
38%
716
19%
1550
January16th to endNight
17%
768
20%
2625
5%
768
36%
768
19%
1624
February1st to 15thNight
17%
704
18%
2198
4%
701
33%
701
19%
1446
February16th to endNight
22%
631
25%
1893
9%
631
42%
631
23%
1186
March1st to 15thNight
23%
720
18%
2160
8%
720
37%
720
23%
1421
March16th to endNight
24%
768
24%
1996
8%
768
37%
768
28%
1303
April1st to 15thNight
29%
718
17%
1796
6%
718
35%
718
25%
1255
April16th to endNight
33%
678
18%
1758
5%
678
40%
678
26%
1207
May1st to 15thNight
29%
534
17%
1614
7%
534
38%
534
20%
1115
May16th to endNight
44%
374
15%
1497
13%
374
51%
374
19%
945
June1st to 15thNight
45%
360
15%
1440
14%
360
53%
360
18%
914
June16th to endNight
60%
360
15%
1440
18%
360
66%
360
25%
831
July1st to 15thNight
55%
340
12%
1360
15%
340
57%
340
21%
829
July16th to endNight
60%
376
16%
1418
18%
354
66%
354
24%
820
August1st to 15thNight
60%
533
19%
1582
14%
531
63%
531
32%
945
August16th to endNight
59%
700
17%
1750
9%
700
61%
700
33%
1158
September1st to 15thNight
51%
660
24%
1661
13%
660
55%
660
32%
1098
September16th to endNight
52%
657
24%
1815
18%
657
57%
657
32%
1150
October1st to 15thNight
37%
660
22%
1900
13%
660
46%
660
25%
1241
October16th to endNight
29%
704
20%
2222
9%
704
43%
704
24%
1393
November1st to 15thNight
31%
700
22%
2277
11%
700
46%
700
23%
1415
November16th to endNight
27%
720
20%
2358
10%
720
40%
720
21%
1480
December1st to 15thNight
23%
720
16%
2520
7%
720
36%
720
17%
1622
December16th to endNight
16%
768
19%
2688
4%
768
34%
768
17%
1763

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%5%2442445%7%42%
47503
Night
16%6%2442445%7%42%
46644
Cloud CoverDay
15%6%2442445%7%41%
47513
Night
16%6%25%42445%6%42%
46649
TransparencyDay
41%5%12%22%15%1
47191
Night
42%412%22%15%2
46474
SeeingDay
19%7%27%28%14%3
5689
Night
19%9%30%27%10%1
14874

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%6%25%43445%7%40%
1432
16th to endDay
14%7%26%424447%41%
1598
February1st to 15thDay
16%5%25%5%24347%41%
1662
16th to endDay
14%6%245%25%35%6%44%
1580
March1st to 15thDay
16%7%33434448%39%
1790
16th to endDay
17%6%24323445%44%
2224
April1st to 15thDay
17%6%24433336%42%
2152
16th to endDay
15%5%23324446%46%
2201
May1st to 15thDay
20%6%24324348%38%
2346
16th to endDay
15%42442445%9%40%
2618
June1st to 15thDay
19%5%2442445%8%39%
2520
16th to endDay
10%5%244345%5%9%44%
2520
July1st to 15thDay
9%5%235%25%6%7%9%42%
2520
16th to endDay
11%4245%35%5%6%8%42%
2654
August1st to 15thDay
10%42443447%9%44%
2269
16th to endDay
12%32442345%8%46%
2100
September1st to 15thDay
12%42442445%8%46%
1969
16th to endDay
13%6%135%24346%47%
1815
October1st to 15thDay
18%5%24323346%43%
1727
16th to endDay
17%424423446%44%
1650
November1st to 15thDay
19%5%1442344445%
1581
16th to endDay
14%6%34424446%44%
1599
December1st to 15thDay
16%6%25%32445%8%38%
1440
16th to endDay
21%8%35%323347%37%
1536
January1st to 15thNight
18%5%25%323347%42%
2506
16th to endNight
16%8%34423347%41%
2625
February1st to 15thNight
14%9%35%433446%40%
2197
16th to endNight
17%5%2343435%5%44%
1893
March1st to 15thNight
18%6%24424446%42%
2147
16th to endNight
16%6%24423346%44%
1996
April1st to 15thNight
17%6%344245%46%40%
1796
16th to endNight
18%5%35%5%23336%42%
1758
May1st to 15thNight
19%6%25%5%345%36%37%
1614
16th to endNight
18%6%25%43436%8%37%
1495
June1st to 15thNight
19%6%25%423347%39%
1440
16th to endNight
8%425%435%5%6%8%44%
1440
July1st to 15thNight
11%425%5%25%5%8%9%38%
1440
16th to endNight
9%5%34436%5%7%10%39%
1514
August1st to 15thNight
9%4144345%6%8%46%
1592
16th to endNight
12%43442446%7%43%
1750
September1st to 15thNight
14%41342446%6%46%
1661
16th to endNight
14%425%5%23446%48%
1815
October1st to 15thNight
17%6%25%432335%44%
1900
16th to endNight
18%5%25%423346%42%
2222
November1st to 15thNight
17%5%25%42333446%
2277
16th to endNight
17%6%25%42335%6%41%
2358
December1st to 15thNight
20%5%24424448%37%
2520
16th to endNight
19%7%25%43435%6%37%
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
15%6%25%5%2435%7%41%
1432
16th to endDay
14%7%26%5%34447%41%
1598
February1st to 15thDay
16%5%35%6%24347%41%
1662
16th to endDay
14%6%25%425%35%5%44%
1580
March1st to 15thDay
16%7%25%32445%8%38%
1800
16th to endDay
17%7%24413437%43%
2224
April1st to 15thDay
17%7%34423336%41%
2152
16th to endDay
16%5%1432445%6%45%
2201
May1st to 15thDay
20%7%25%323447%37%
2346
16th to endDay
15%5%244245%5%9%39%
2618
June1st to 15thDay
19%6%245%24448%38%
2520
16th to endDay
10%5%25%5%35%5%5%9%42%
2520
July1st to 15thDay
10%5%245%35%6%7%8%41%
2520
16th to endDay
11%5%25%435%5%7%8%40%
2654
August1st to 15thDay
11%4145%245%6%9%42%
2269
16th to endDay
12%425%43445%9%43%
2100
September1st to 15thDay
13%413435%45%7%45%
1969
16th to endDay
14%6%145%24446%46%
1815
October1st to 15thDay
18%6%2442345%6%42%
1727
16th to endDay
17%5%15%434445%44%
1650
November1st to 15thDay
19%42433334445%
1581
16th to endDay
14%6%24424446%44%
1599
December1st to 15thDay
15%7%26%4235%5%7%39%
1440
16th to endDay
21%8%25%323346%37%
1536
January1st to 15thNight
18%5%25%423336%44%
2506
16th to endNight
16%8%34423346%41%
2625
February1st to 15thNight
13%9%26%424445%41%
2190
16th to endNight
17%5%24334345%45%
1893
March1st to 15thNight
18%6%24324446%42%
2160
16th to endNight
16%6%24423446%44%
1995
April1st to 15thNight
17%6%3442445%6%39%
1796
16th to endNight
18%5%35%423336%41%
1758
May1st to 15thNight
18%7%25%5%2445%6%36%
1614
16th to endNight
19%6%2442445%7%37%
1495
June1st to 15thNight
19%6%25%424448%38%
1440
16th to endNight
9%4245%35%5%6%8%44%
1440
July1st to 15thNight
11%5%25%425%5%8%9%38%
1440
16th to endNight
8%6%245%36%6%7%10%39%
1514
August1st to 15thNight
9%414435%5%7%8%45%
1592
16th to endNight
12%4245%245%6%8%43%
1750
September1st to 15thNight
14%41342445%6%45%
1661
16th to endNight
14%4245%23336%48%
1815
October1st to 15thNight
16%6%25%5%32336%44%
1900
16th to endNight
18%5%25%423346%42%
2222
November1st to 15thNight
17%5%2442334446%
2277
16th to endNight
17%6%35%43345%6%41%
2358
December1st to 15thNight
19%6%25%424347%38%
2520
16th to endNight
19%7%25%42445%5%38%
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
42%413%24%11%2
1433
16th to endDay
42%312%25%13%2
1598
February1st to 15thDay
43%412%22%14%1
1665
16th to endDay
41%410%27%14%1
1580
March1st to 15thDay
43%5%15%22%11%2
1800
16th to endDay
43%310%21%17%3
2224
April1st to 15thDay
44%48%22%16%3
2152
16th to endDay
39%5%9%22%19%3
2201
May1st to 15thDay
46%5%9%21%15%1
2346
16th to endDay
40%7%17%19%12%2
2622
June1st to 15thDay
45%5%12%22%12%1
2520
16th to endDay
37%7%14%25%14%1
2520
July1st to 15thDay
38%7%12%22%17%1
2371
16th to endDay
37%7%14%24%12%3
2487
August1st to 15thDay
35%7%15%23%16%1
2255
16th to endDay
37%5%10%24%19%2
2100
September1st to 15thDay
36%5%13%20%21%1
1969
16th to endDay
39%48%23%23%2
1815
October1st to 15thDay
42%411%22%18%1
1727
16th to endDay
41%410%25%16%2
1650
November1st to 15thDay
42%312%20%19%1
1581
16th to endDay
40%313%25%14%2
1599
December1st to 15thDay
43%5%17%19%14%
1440
16th to endDay
47%5%13%20%12%
1536
January1st to 15thNight
42%313%23%13%2
2506
16th to endNight
42%311%24%15%2
2625
February1st to 15thNight
44%5%11%22%13%2
2198
16th to endNight
40%38%25%18%3
1893
March1st to 15thNight
43%313%20%14%4
2160
16th to endNight
41%29%23%17%4
1996
April1st to 15thNight
45%410%19%16%2
1796
16th to endNight
44%48%17%20%3
1758
May1st to 15thNight
48%5%10%18%14%2
1614
16th to endNight
45%5%15%19%13%1
1497
June1st to 15thNight
45%5%12%20%15%1
1440
16th to endNight
36%6%13%26%15%1
1440
July1st to 15thNight
39%8%13%22%14%1
1360
16th to endNight
36%7%16%25%11%2
1418
August1st to 15thNight
33%6%14%27%16%1
1582
16th to endNight
36%7%12%25%16%1
1750
September1st to 15thNight
38%5%11%25%18%
1661
16th to endNight
38%47%22%25%1
1815
October1st to 15thNight
43%410%23%16%3
1900
16th to endNight
41%314%24%15%1
2222
November1st to 15thNight
41%410%21%19%3
2277
16th to endNight
42%412%26%11%2
2358
December1st to 15thNight
46%415%20%12%
2520
16th to endNight
46%5%13%21%12%1
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
20%16%36%25%1
60
16th to endDay
22%9%42%22%2
75
February1st to 15thDay
20%15%38%24%1
178
16th to endDay
17%15%36%23%7%
239
March1st to 15thDay
20%13%38%21%5%
270
16th to endDay
20%12%39%18%5%2
288
April1st to 15thDay
22%8%34%21%12%
270
16th to endDay
23%6%30%27%10%1
270
May1st to 15thDay
30%10%26%21%9%2
288
16th to endDay
19%321%33%19%1
331
June1st to 15thDay
22%6%23%27%18%2
315
16th to endDay
14%129%36%15%1
315
July1st to 15thDay
14%20%40%21%2
305
16th to endDay
12%219%36%22%6%
326
August1st to 15thDay
13%120%33%25%6%
297
16th to endDay
13%315%35%23%9%
268
September1st to 15thDay
14%323%25%23%9%
255
16th to endDay
20%318%26%20%10%
254
October1st to 15thDay
23%5%20%27%16%5%
255
16th to endDay
13%12%34%28%10%
252
November1st to 15thDay
20%11%25%26%12%2
235
16th to endDay
14%17%29%26%10%1
219
December1st to 15thDay
18%18%35%21%6%
60
16th to endDay
35%7%43%9%3
64
January1st to 15thNight
20%17%36%20%4
720
16th to endNight
22%15%39%21%1
768
February1st to 15thNight
19%15%42%21%1
704
16th to endNight
20%16%36%22%3
631
March1st to 15thNight
21%13%37%21%5%
720
16th to endNight
20%12%38%22%6%1
768
April1st to 15thNight
21%10%34%27%5%
718
16th to endNight
20%7%33%30%6%
678
May1st to 15thNight
20%7%35%28%7%
534
16th to endNight
22%421%35%14%1
375
June1st to 15thNight
23%319%36%17%
360
16th to endNight
12%120%48%15%1
360
July1st to 15thNight
12%322%45%15%1
340
16th to endNight
11%117%47%19%1
376
August1st to 15thNight
11%120%44%20%1
533
16th to endNight
14%313%35%29%3
700
September1st to 15thNight
17%321%35%17%3
660
16th to endNight
17%220%31%22%5%
657
October1st to 15thNight
20%7%28%26%15%1
660
16th to endNight
20%12%32%25%8%
704
November1st to 15thNight
19%11%32%21%12%2
700
16th to endNight
20%13%32%23%9%1
720
December1st to 15thNight
23%15%33%20%6%1
720
16th to endNight
22%15%40%17%2
768
Page generated at 2017-08-13 14:16:53. Version ac04

Copyright 2017 A. Danko


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