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 2005-11-05 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
27%
14817
18%
46465
8%
14785
39%
14785
20%
30378

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
17%
720
20%
2506
7%
716
34%
716
16%
1583
January16th to endNight
17%
768
21%
2667
5%
768
37%
768
15%
1688
February1st to 15thNight
13%
704
18%
2182
4%
701
29%
701
16%
1456
February16th to endNight
16%
631
24%
1893
7%
631
36%
631
19%
1237
March1st to 15thNight
17%
720
17%
2160
5%
720
31%
720
17%
1463
March16th to endNight
19%
768
23%
2009
6%
768
33%
768
22%
1338
April1st to 15thNight
19%
718
16%
1796
4%
718
26%
718
20%
1296
April16th to endNight
22%
645
17%
1725
3%
645
31%
645
21%
1189
May1st to 15thNight
23%
540
16%
1620
6%
540
32%
540
17%
1139
May16th to endNight
32%
383
12%
1505
8%
382
39%
382
17%
967
June1st to 15thNight
31%
360
13%
1440
10%
360
44%
360
16%
960
June16th to endNight
45%
360
14%
1440
13%
360
54%
360
24%
850
July1st to 15thNight
40%
340
11%
1360
13%
340
44%
340
20%
853
July16th to endNight
49%
376
14%
1418
15%
354
55%
354
22%
868
August1st to 15thNight
44%
495
16%
1544
10%
493
49%
493
28%
941
August16th to endNight
51%
700
17%
1750
9%
700
54%
700
31%
1165
September1st to 15thNight
40%
660
22%
1677
10%
660
46%
660
30%
1110
September16th to endNight
46%
657
24%
1815
18%
657
51%
657
31%
1152
October1st to 15thNight
35%
660
22%
1876
11%
660
46%
660
25%
1232
October16th to endNight
25%
704
17%
2249
6%
704
38%
704
21%
1462
November1st to 15thNight
27%
700
20%
2277
9%
700
43%
700
22%
1432
November16th to endNight
24%
720
20%
2348
9%
720
38%
720
19%
1523
December1st to 15thNight
22%
720
16%
2520
6%
720
35%
720
15%
1662
December16th to endNight
15%
768
18%
2688
4%
768
33%
768
14%
1812

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
16%6%25%42445%7%39%
47515
Night
17%6%25%424446%41%
46635
Cloud CoverDay
17%6%25%42445%7%40%
47525
Night
17%6%25%424446%40%
46640
TransparencyDay
43%6%12%21%14%2
47203
Night
43%5%12%21%15%2
46465
SeeingDay
20%15%30%20%9%2
5704
Night
21%14%32%23%7%
14817

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%244245%6%5%40%
1432
16th to endDay
15%7%15%4245%5%6%41%
1555
February1st to 15thDay
16%6%35%5%35%45%6%35%
1678
16th to endDay
17%6%25%444446%38%
1580
March1st to 15thDay
19%5%35%5%2445%6%37%
1790
16th to endDay
17%6%35%423345%42%
2213
April1st to 15thDay
19%7%33423446%39%
2152
16th to endDay
18%414424345%44%
2235
May1st to 15thDay
24%6%25%324445%35%
2333
16th to endDay
17%6%24424447%40%
2618
June1st to 15thDay
22%5%35%42445%7%33%
2520
16th to endDay
12%6%25%435%5%5%10%36%
2520
July1st to 15thDay
12%5%25%6%35%6%5%8%37%
2520
16th to endDay
13%425%5%35%6%6%8%37%
2654
August1st to 15thDay
12%6%246%36%5%6%9%36%
2304
16th to endDay
11%5%26%6%245%5%8%41%
2100
September1st to 15thDay
13%5%25%424447%43%
1953
16th to endDay
14%5%14413446%48%
1815
October1st to 15thDay
18%6%25%412346%42%
1752
16th to endDay
16%425%43445%6%41%
1623
November1st to 15thDay
18%7%2442333444%
1581
16th to endDay
16%6%25%434337%43%
1611
December1st to 15thDay
18%6%25%5%2335%6%40%
1440
16th to endDay
22%8%25%324347%35%
1536
January1st to 15thNight
18%7%2432345%5%41%
2506
16th to endNight
16%8%35%323436%41%
2667
February1st to 15thNight
14%9%35%5%24346%39%
2181
16th to endNight
17%7%24434445%42%
1893
March1st to 15thNight
19%6%25%33435%6%38%
2147
16th to endNight
17%7%24323345%44%
2009
April1st to 15thNight
18%7%35%5%35%435%38%
1796
16th to endNight
20%5%26%5%22436%40%
1725
May1st to 15thNight
20%7%25%43445%6%35%
1620
16th to endNight
17%6%2432445%8%37%
1503
June1st to 15thNight
21%6%25%42445%6%36%
1440
16th to endNight
12%4245%2445%8%45%
1440
July1st to 15thNight
13%5%246%25%5%7%9%37%
1440
16th to endNight
10%425%5%45%5%7%9%37%
1514
August1st to 15thNight
9%5%25%5%246%6%7%44%
1554
16th to endNight
12%4245%345%47%45%
1750
September1st to 15thNight
14%6%2442445%7%43%
1677
16th to endNight
14%5%2442443449%
1815
October1st to 15thNight
17%6%24424345%44%
1876
16th to endNight
19%6%25%42345%5%40%
2249
November1st to 15thNight
18%6%24423345%45%
2277
16th to endNight
19%6%24434346%40%
2348
December1st to 15thNight
20%7%25%424435%38%
2520
16th to endNight
21%7%2442445%5%36%
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
16%6%34425%5%5%5%41%
1432
16th to endDay
16%6%35%32345%6%41%
1555
February1st to 15thDay
16%6%25%5%35%45%7%36%
1678
16th to endDay
17%6%25%5%35%45%5%38%
1580
March1st to 15thDay
20%6%25%5%34347%37%
1800
16th to endDay
18%6%25%5%23345%42%
2213
April1st to 15thDay
20%7%2442335%6%40%
2152
16th to endDay
19%414424446%43%
2235
May1st to 15thDay
24%7%25%413346%35%
2333
16th to endDay
18%5%25%42345%7%39%
2618
June1st to 15thDay
23%6%3443445%7%33%
2520
16th to endDay
13%6%25%5%345%6%9%37%
2520
July1st to 15thDay
13%5%25%5%35%45%8%37%
2520
16th to endDay
12%5%25%425%6%6%9%37%
2654
August1st to 15thDay
13%5%15%5%35%5%6%8%38%
2304
16th to endDay
11%5%26%5%345%6%7%41%
2100
September1st to 15thDay
14%5%14424346%45%
1953
16th to endDay
16%42332445%6%47%
1815
October1st to 15thDay
19%6%24423435%42%
1752
16th to endDay
16%4245%3345%6%42%
1623
November1st to 15thDay
18%7%23423335%44%
1581
16th to endDay
16%5%24433436%44%
1611
December1st to 15thDay
18%6%25%5%23446%40%
1440
16th to endDay
22%7%35%423345%36%
1536
January1st to 15thNight
18%7%24423346%41%
2506
16th to endNight
17%8%35%323336%42%
2667
February1st to 15thNight
14%9%35%6%33346%39%
2174
16th to endNight
18%6%24333445%42%
1893
March1st to 15thNight
19%6%25%42345%6%38%
2160
16th to endNight
17%7%24423336%44%
2008
April1st to 15thNight
18%7%35%5%24446%38%
1796
16th to endNight
20%5%26%422346%40%
1725
May1st to 15thNight
20%7%25%334447%34%
1620
16th to endNight
18%6%25%324449%37%
1503
June1st to 15thNight
22%5%35%424436%37%
1440
16th to endNight
12%5%2442346%9%44%
1440
July1st to 15thNight
13%5%245%35%5%6%8%37%
1440
16th to endNight
11%5%35%5%36%48%8%36%
1514
August1st to 15thNight
10%5%145%35%6%6%8%42%
1554
16th to endNight
12%4245%45%447%44%
1750
September1st to 15thNight
14%5%24424446%44%
1677
16th to endNight
14%5%2432442449%
1815
October1st to 15thNight
17%5%25%424335%44%
1876
16th to endNight
19%6%25%424346%40%
2249
November1st to 15thNight
18%6%25%323335%44%
2277
16th to endNight
20%6%24424345%40%
2348
December1st to 15thNight
21%7%24423445%38%
2520
16th to endNight
21%8%25%42445%5%36%
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%6%14%22%11%3
1433
16th to endDay
41%5%10%25%13%2
1555
February1st to 15thDay
46%6%12%19%13%1
1681
16th to endDay
44%5%11%23%12%2
1580
March1st to 15thDay
47%413%22%9%2
1800
16th to endDay
44%410%19%17%4
2213
April1st to 15thDay
45%5%10%20%15%2
2152
16th to endDay
40%5%9%21%18%3
2235
May1st to 15thDay
49%5%9%19%13%2
2333
16th to endDay
42%7%14%19%13%2
2622
June1st to 15thDay
49%7%12%19%10%1
2520
16th to endDay
41%8%14%21%13%
2520
July1st to 15thDay
44%6%11%20%15%1
2371
16th to endDay
39%8%14%20%13%2
2487
August1st to 15thDay
41%9%13%20%15%
2290
16th to endDay
39%6%10%24%17%1
2100
September1st to 15thDay
39%6%13%19%20%1
1953
16th to endDay
36%5%7%23%25%2
1815
October1st to 15thDay
43%5%11%21%16%2
1752
16th to endDay
41%5%11%24%15%2
1623
November1st to 15thDay
44%412%18%17%2
1581
16th to endDay
41%411%25%14%2
1611
December1st to 15thDay
45%415%18%14%1
1440
16th to endDay
49%5%14%18%11%
1536
January1st to 15thNight
43%413%22%13%2
2506
16th to endNight
43%410%22%16%2
2667
February1st to 15thNight
45%5%12%21%11%2
2182
16th to endNight
43%410%22%16%2
1893
March1st to 15thNight
45%5%13%18%13%3
2160
16th to endNight
42%39%21%18%4
2009
April1st to 15thNight
47%5%10%19%14%2
1796
16th to endNight
45%48%20%16%4
1725
May1st to 15thNight
48%6%10%17%14%2
1620
16th to endNight
44%8%17%17%11%1
1505
June1st to 15thNight
48%6%12%18%13%1
1440
16th to endNight
36%7%14%25%14%
1440
July1st to 15thNight
41%7%13%21%14%
1360
16th to endNight
39%7%16%22%11%2
1418
August1st to 15thNight
35%7%15%23%15%2
1544
16th to endNight
39%5%10%27%15%
1750
September1st to 15thNight
39%5%11%24%18%
1677
16th to endNight
38%46%22%26%1
1815
October1st to 15thNight
42%5%10%21%17%3
1876
16th to endNight
44%414%21%14%
2249
November1st to 15thNight
41%412%19%18%3
2277
16th to endNight
45%311%24%12%2
2348
December1st to 15thNight
47%414%19%12%
2520
16th to endNight
49%412%19%12%
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
21%28%31%16%1
60
16th to endDay
26%23%40%8%
67
February1st to 15thDay
24%24%35%14%1
194
16th to endDay
18%25%36%14%4
239
March1st to 15thDay
25%23%31%16%2
270
16th to endDay
22%25%33%12%41
288
April1st to 15thDay
27%19%31%16%5%
270
16th to endDay
21%15%35%16%8%2
270
May1st to 15thDay
35%16%24%13%7%1
288
16th to endDay
21%14%30%22%9%1
331
June1st to 15thDay
24%13%26%22%10%2
315
16th to endDay
16%15%32%25%8%1
315
July1st to 15thDay
19%9%29%28%11%1
305
16th to endDay
14%5%34%27%13%3
326
August1st to 15thDay
13%6%29%31%13%5%
297
16th to endDay
12%5%26%27%18%9%
268
September1st to 15thDay
17%7%27%22%19%5%
255
16th to endDay
17%7%30%16%16%11%
254
October1st to 15thDay
22%9%25%24%14%4
255
16th to endDay
17%19%32%20%8%
247
November1st to 15thDay
22%16%26%22%9%2
235
16th to endDay
20%18%32%19%8%1
231
December1st to 15thDay
18%28%36%10%6%
60
16th to endDay
37%23%26%10%1
64
January1st to 15thNight
24%20%36%15%3
720
16th to endNight
23%21%33%19%1
768
February1st to 15thNight
20%19%40%17%1
704
16th to endNight
22%20%35%18%2
631
March1st to 15thNight
25%20%34%17%2
720
16th to endNight
20%21%34%19%3
768
April1st to 15thNight
22%20%35%19%2
718
16th to endNight
22%16%34%21%4
645
May1st to 15thNight
22%18%31%21%5%
540
16th to endNight
20%7%34%29%7%
383
June1st to 15thNight
25%5%31%30%7%
360
16th to endNight
15%331%40%8%
360
July1st to 15thNight
15%235%33%13%
340
16th to endNight
14%324%41%14%1
376
August1st to 15thNight
12%432%37%12%
495
16th to endNight
14%323%37%18%2
700
September1st to 15thNight
17%6%30%31%13%1
660
16th to endNight
18%5%23%29%20%3
657
October1st to 15thNight
20%10%30%25%11%1
660
16th to endNight
24%13%34%21%5%
704
November1st to 15thNight
22%15%30%21%8%1
700
16th to endNight
23%16%30%20%7%
720
December1st to 15thNight
26%16%30%20%5%
720
16th to endNight
26%19%35%16%1
768
Page generated at 2017-08-13 14:16:34. Version ac04

Copyright 2017 A. Danko


Van Vleck Observatory Clear Sky Chart | Charts Home