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 2018-02-14.

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%
15563
18%
48785
8%
15531
39%
15531
20%
31823

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
16%
780
20%
2716
6%
776
34%
776
17%
1696
January16th to endNight
17%
832
20%
2889
5%
832
36%
832
15%
1826
February1st to 15thNight
14%
760
17%
2355
4%
757
29%
757
15%
1589
February16th to endNight
16%
631
24%
1893
7%
631
37%
631
19%
1237
March1st to 15thNight
18%
720
17%
2160
5%
720
32%
720
17%
1463
March16th to endNight
20%
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
24%
540
16%
1620
6%
540
33%
540
17%
1139
May16th to endNight
32%
383
12%
1505
8%
382
39%
382
17%
967
June1st to 15thNight
32%
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
45%
340
20%
853
July16th to endNight
50%
376
14%
1418
15%
354
57%
354
22%
868
August1st to 15thNight
44%
509
16%
1580
10%
507
48%
507
27%
969
August16th to endNight
53%
764
19%
1910
10%
764
55%
764
32%
1261
September1st to 15thNight
39%
720
21%
1830
9%
720
45%
720
30%
1201
September16th to endNight
46%
717
25%
1980
18%
717
51%
717
31%
1253
October1st to 15thNight
35%
720
21%
2047
12%
720
46%
720
25%
1347
October16th to endNight
25%
768
18%
2454
7%
768
39%
768
21%
1580
November1st to 15thNight
28%
760
20%
2472
8%
760
43%
760
21%
1560
November16th to endNight
25%
780
22%
2544
10%
780
40%
780
20%
1619
December1st to 15thNight
22%
780
16%
2730
5%
780
35%
780
15%
1816
December16th to endNight
15%
832
18%
2912
4%
832
33%
832
14%
1946

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%40%
49299
Night
17%6%25%424446%41%
48955
Cloud CoverDay
17%6%25%42445%7%40%
49309
Night
17%6%25%424446%41%
48960
TransparencyDay
43%6%12%21%15%2
48987
Night
43%5%12%21%15%2
48785
SeeingDay
21%15%30%20%9%2
6278
Night
21%14%32%23%7%
15563

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%243245%6%5%42%
1552
16th to endDay
16%7%25%4135%5%6%41%
1685
February1st to 15thDay
17%6%35%5%35%45%6%35%
1806
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%35%5%6%9%36%
2353
16th to endDay
11%5%25%5%245%5%8%43%
2292
September1st to 15thDay
13%5%25%433447%44%
2130
16th to endDay
14%6%14423446%48%
1980
October1st to 15thDay
18%6%25%42235%6%42%
1911
16th to endDay
17%425%434445%42%
1770
November1st to 15thDay
18%7%24423335%43%
1716
16th to endDay
15%6%25%434337%44%
1745
December1st to 15thDay
18%7%25%5%23346%40%
1560
16th to endDay
21%7%35%424347%36%
1664
January1st to 15thNight
17%7%2432345%5%42%
2716
16th to endNight
16%8%35%323336%42%
2889
February1st to 15thNight
15%9%36%5%24346%38%
2354
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%7%7%43%
1590
16th to endNight
12%4245%345%5%7%46%
1910
September1st to 15thNight
13%5%2442445%7%44%
1830
16th to endNight
13%6%2442443449%
1980
October1st to 15thNight
16%6%24424345%44%
2047
16th to endNight
19%6%15%42345%5%41%
2454
November1st to 15thNight
17%6%25%423345%44%
2472
16th to endNight
18%6%24434345%41%
2544
December1st to 15thNight
20%7%25%424436%37%
2730
16th to endNight
21%7%2442445%5%37%
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
16%6%2442445%5%43%
1552
16th to endDay
16%6%36%323445%42%
1685
February1st to 15thDay
17%6%25%5%35%447%36%
1806
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%37%
2353
16th to endDay
11%5%25%5%3446%7%42%
2292
September1st to 15thDay
14%5%14425%346%46%
2130
16th to endDay
15%42442345%6%47%
1980
October1st to 15thDay
18%6%24423435%42%
1911
16th to endDay
17%4245%3345%6%43%
1770
November1st to 15thDay
18%7%23423335%43%
1716
16th to endDay
16%5%24433436%45%
1745
December1st to 15thDay
18%7%25%423446%40%
1560
16th to endDay
21%7%35%423346%36%
1664
January1st to 15thNight
18%6%24423346%42%
2716
16th to endNight
16%8%34323336%42%
2889
February1st to 15thNight
15%8%35%5%33346%39%
2347
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%245%35%6%6%8%41%
1590
16th to endNight
12%4245%44447%45%
1910
September1st to 15thNight
14%5%24424446%45%
1830
16th to endNight
14%5%25%324435%48%
1980
October1st to 15thNight
17%6%25%424435%44%
2047
16th to endNight
19%6%25%32335%6%41%
2454
November1st to 15thNight
17%6%25%423345%44%
2472
16th to endNight
18%6%24424345%42%
2544
December1st to 15thNight
20%7%24424445%37%
2730
16th to endNight
20%8%25%42445%5%37%
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
41%6%13%22%12%4
1553
16th to endDay
42%5%11%24%14%2
1685
February1st to 15thDay
47%5%12%19%13%1
1809
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%8%12%20%15%
2339
16th to endDay
38%6%10%24%18%1
2292
September1st to 15thDay
38%6%12%20%20%2
2130
16th to endDay
37%47%22%25%2
1980
October1st to 15thDay
43%5%11%21%16%2
1911
16th to endDay
41%5%10%24%16%2
1770
November1st to 15thDay
44%412%18%17%2
1716
16th to endDay
40%410%26%15%2
1745
December1st to 15thDay
45%415%17%14%1
1560
16th to endDay
48%5%14%18%12%
1664
January1st to 15thNight
42%412%22%14%2
2716
16th to endNight
43%411%21%16%2
2889
February1st to 15thNight
46%5%12%20%12%2
2355
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
36%7%15%23%14%2
1580
16th to endNight
38%5%10%27%16%
1910
September1st to 15thNight
38%5%11%24%18%
1830
16th to endNight
38%47%22%25%1
1980
October1st to 15thNight
42%5%10%21%17%3
2047
16th to endNight
43%414%22%14%
2454
November1st to 15thNight
41%412%19%18%2
2472
16th to endNight
43%311%25%13%2
2544
December1st to 15thNight
47%5%14%19%12%
2730
16th to endNight
48%412%19%13%1
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
23%30%28%15%2
90
16th to endDay
25%22%41%9%
101
February1st to 15thDay
26%26%31%14%1
242
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
14%6%30%30%13%5%
313
16th to endDay
12%5%30%26%16%8%
332
September1st to 15thDay
16%11%29%20%17%5%
315
16th to endDay
17%8%29%16%16%11%
314
October1st to 15thDay
21%8%26%24%14%4
315
16th to endDay
19%17%30%20%9%1
298
November1st to 15thDay
23%16%26%22%8%1
280
16th to endDay
20%20%30%19%8%1
275
December1st to 15thDay
21%24%35%10%8%
90
16th to endDay
29%32%28%9%1
96
January1st to 15thNight
23%21%36%15%3
780
16th to endNight
23%21%33%19%1
832
February1st to 15thNight
21%19%39%17%1
760
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
13%332%37%12%
509
16th to endNight
14%323%38%17%2
764
September1st to 15thNight
16%6%31%30%13%1
720
16th to endNight
17%5%23%28%20%3
717
October1st to 15thNight
20%9%30%25%12%1
720
16th to endNight
24%14%33%21%6%
768
November1st to 15thNight
21%15%30%22%8%1
760
16th to endNight
22%17%31%21%7%
780
December1st to 15thNight
26%16%30%20%5%
780
16th to endNight
26%20%35%16%1
832
Page generated at 2018-02-15 10:58:44. Version ac04

Copyright 2018 A. Danko


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