University of Connecticut Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the University of Connecticut Observatory clear sky chart from 2005-11-05 to 2017-10-06.

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
25%
15018
18%
47041
7%
14986
38%
14986
19%
30932

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
14%
720
20%
2506
6%
716
32%
716
15%
1599
January16th to endNight
14%
768
20%
2688
4%
768
34%
768
14%
1725
February1st to 15thNight
11%
704
17%
2171
4%
701
28%
701
15%
1475
February16th to endNight
15%
631
23%
1893
6%
631
36%
631
15%
1253
March1st to 15thNight
16%
720
16%
2160
4%
720
30%
720
15%
1474
March16th to endNight
18%
768
23%
2014
6%
768
31%
768
20%
1350
April1st to 15thNight
18%
718
16%
1796
3%
718
26%
718
19%
1301
April16th to endNight
19%
627
17%
1707
3%
627
29%
627
20%
1188
May1st to 15thNight
22%
540
16%
1620
6%
540
31%
540
16%
1134
May16th to endNight
29%
392
11%
1514
7%
391
36%
391
16%
998
June1st to 15thNight
30%
360
12%
1440
7%
360
44%
360
15%
964
June16th to endNight
41%
360
13%
1440
12%
360
50%
360
22%
862
July1st to 15thNight
44%
340
12%
1360
14%
340
49%
340
20%
858
July16th to endNight
44%
376
14%
1418
14%
354
50%
354
19%
896
August1st to 15thNight
42%
497
17%
1568
10%
495
48%
495
27%
970
August16th to endNight
50%
764
20%
1910
11%
764
53%
764
34%
1236
September1st to 15thNight
35%
720
20%
1839
9%
720
42%
720
28%
1231
September16th to endNight
43%
717
25%
1980
16%
717
50%
717
32%
1232
October1st to 15thNight
32%
684
22%
1929
10%
684
45%
684
25%
1268
October16th to endNight
22%
704
16%
2263
5%
704
36%
704
19%
1497
November1st to 15thNight
26%
700
21%
2277
8%
700
42%
700
21%
1444
November16th to endNight
22%
720
20%
2340
7%
720
38%
720
18%
1520
December1st to 15thNight
18%
720
16%
2520
5%
720
33%
720
15%
1653
December16th to endNight
13%
768
19%
2688
4%
768
32%
768
12%
1804

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%5%3445%6%39%
48172
Night
16%6%25%42445%6%40%
47211
Cloud CoverDay
17%6%25%42445%6%39%
48182
Night
16%6%25%42445%6%40%
47216
TransparencyDay
44%6%11%21%14%2
47860
Night
43%5%11%21%15%2
47041
SeeingDay
22%14%30%19%9%3
5943
Night
22%14%32%21%7%
15018

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%5%34425%5%5%6%40%
1432
16th to endDay
15%6%35%42445%7%40%
1536
February1st to 15thDay
15%7%25%6%36%5%46%35%
1690
16th to endDay
16%8%26%5%25%446%37%
1580
March1st to 15thDay
19%7%26%423446%37%
1790
16th to endDay
17%7%35%423445%42%
2208
April1st to 15thDay
19%7%245%24445%39%
2152
16th to endDay
17%425%5%3435%6%42%
2252
May1st to 15thDay
25%6%35%423446%33%
2332
16th to endDay
17%6%25%434447%39%
2610
June1st to 15thDay
23%6%25%4245%5%7%33%
2520
16th to endDay
14%5%25%5%35%5%6%9%34%
2520
July1st to 15thDay
13%6%25%5%45%6%6%8%36%
2520
16th to endDay
13%435%6%35%5%6%7%37%
2654
August1st to 15thDay
13%5%26%6%35%5%6%8%35%
2368
16th to endDay
11%5%25%435%45%8%42%
2292
September1st to 15thDay
13%5%15%5%34446%44%
2121
16th to endDay
14%5%24423336%49%
1980
October1st to 15thDay
17%7%24323445%43%
1829
16th to endDay
15%6%15%42345%6%42%
1609
November1st to 15thDay
19%7%24323335%43%
1581
16th to endDay
17%5%244245%46%42%
1620
December1st to 15thDay
19%5%25%4235%46%40%
1440
16th to endDay
22%7%245%2445%5%35%
1536
January1st to 15thNight
17%7%25%424446%40%
2506
16th to endNight
15%8%35%5%23346%40%
2688
February1st to 15thNight
15%8%36%5%3445%6%38%
2170
16th to endNight
15%7%35%5%1445%6%40%
1893
March1st to 15thNight
18%6%25%5%245%5%7%36%
2147
16th to endNight
16%6%25%423346%42%
2014
April1st to 15thNight
17%6%35%5%3446%6%35%
1796
16th to endNight
22%5%26%423346%39%
1707
May1st to 15thNight
19%7%35%6%33446%35%
1620
16th to endNight
18%6%24335%45%9%35%
1512
June1st to 15thNight
21%6%243245%5%6%36%
1440
16th to endNight
14%415%5%2446%8%42%
1440
July1st to 15thNight
13%425%5%25%6%6%8%38%
1440
16th to endNight
11%6%35%5%36%6%5%7%38%
1514
August1st to 15thNight
9%6%145%35%46%8%42%
1578
16th to endNight
11%42442445%7%46%
1910
September1st to 15thNight
14%5%25%42445%5%43%
1839
16th to endNight
14%5%15%413345%50%
1980
October1st to 15thNight
17%5%15%434336%44%
1929
16th to endNight
17%7%25%5%3345%6%39%
2263
November1st to 15thNight
17%7%24323245%45%
2277
16th to endNight
19%6%25%434445%39%
2340
December1st to 15thNight
20%7%24423445%39%
2520
16th to endNight
19%7%25%4345%46%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
15%6%24425%6%5%5%41%
1432
16th to endDay
15%7%25%5%24447%40%
1536
February1st to 15thDay
16%6%25%5%37%5%5%6%35%
1690
16th to endDay
17%7%26%426%346%38%
1580
March1st to 15thDay
19%7%25%433446%37%
1800
16th to endDay
17%6%35%423345%42%
2208
April1st to 15thDay
19%6%245%24445%39%
2152
16th to endDay
17%415%424446%43%
2252
May1st to 15thDay
25%6%35%423346%33%
2332
16th to endDay
17%6%25%5%24447%39%
2610
June1st to 15thDay
23%5%25%4245%47%33%
2520
16th to endDay
14%5%25%5%35%5%6%9%35%
2520
July1st to 15thDay
14%5%35%5%45%5%6%7%37%
2520
16th to endDay
13%435%6%35%5%6%8%37%
2654
August1st to 15thDay
13%5%26%6%446%5%8%35%
2368
16th to endDay
11%5%26%5%345%5%8%41%
2292
September1st to 15thDay
14%5%25%5%34345%45%
2121
16th to endDay
14%5%24323436%49%
1980
October1st to 15thDay
18%6%24423345%44%
1829
16th to endDay
16%5%244335%5%5%43%
1609
November1st to 15thDay
20%7%23324345%43%
1581
16th to endDay
16%5%143345%46%42%
1620
December1st to 15thDay
19%5%25%42345%6%39%
1440
16th to endDay
21%8%24424446%35%
1536
January1st to 15thNight
17%7%35%423446%40%
2506
16th to endNight
15%8%35%433346%40%
2688
February1st to 15thNight
14%7%35%5%34446%38%
2163
16th to endNight
16%7%25%41445%6%41%
1893
March1st to 15thNight
18%6%245%25%45%6%36%
2160
16th to endNight
17%7%24423346%43%
2013
April1st to 15thNight
18%6%25%5%35%45%6%36%
1796
16th to endNight
22%425%5%23335%40%
1707
May1st to 15thNight
19%7%26%5%33446%36%
1620
16th to endNight
18%5%344245%6%8%35%
1512
June1st to 15thNight
21%5%34315%5%46%37%
1440
16th to endNight
14%4245%2446%9%42%
1440
July1st to 15thNight
12%5%25%435%6%6%8%38%
1440
16th to endNight
11%7%25%5%35%5%7%6%38%
1514
August1st to 15thNight
10%5%25%5%345%6%7%42%
1578
16th to endNight
11%4245%2445%7%46%
1910
September1st to 15thNight
14%5%25%5%245%5%6%43%
1839
16th to endNight
14%424323445%50%
1980
October1st to 15thNight
17%5%25%433336%45%
1929
16th to endNight
18%5%25%43345%6%39%
2263
November1st to 15thNight
17%7%34413346%44%
2277
16th to endNight
19%5%245%34445%39%
2340
December1st to 15thNight
19%7%3442335%5%39%
2520
16th to endNight
19%7%345%245%46%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%13%23%10%2
1433
16th to endDay
42%5%12%24%13%1
1536
February1st to 15thDay
47%6%12%19%12%1
1693
16th to endDay
46%410%22%12%2
1580
March1st to 15thDay
47%5%12%23%9%2
1800
16th to endDay
44%410%20%16%4
2208
April1st to 15thDay
46%5%10%18%15%2
2152
16th to endDay
42%5%9%20%18%3
2252
May1st to 15thDay
51%5%7%19%13%2
2332
16th to endDay
44%7%13%20%13%1
2614
June1st to 15thDay
50%6%11%19%10%1
2520
16th to endDay
42%9%12%21%13%
2520
July1st to 15thDay
45%8%11%21%13%1
2371
16th to endDay
42%8%14%19%13%2
2487
August1st to 15thDay
44%7%12%19%13%
2354
16th to endDay
40%6%9%24%17%1
2292
September1st to 15thDay
40%5%11%20%19%2
2121
16th to endDay
36%47%22%26%2
1980
October1st to 15thDay
42%5%9%23%16%2
1829
16th to endDay
41%5%11%23%15%2
1609
November1st to 15thDay
44%413%18%17%2
1581
16th to endDay
41%6%10%23%15%2
1620
December1st to 15thDay
44%6%14%19%14%
1440
16th to endDay
50%414%17%11%
1536
January1st to 15thNight
44%413%22%13%2
2506
16th to endNight
44%410%21%15%2
2688
February1st to 15thNight
47%5%11%21%11%2
2171
16th to endNight
44%5%9%21%17%2
1893
March1st to 15thNight
46%6%11%19%13%2
2160
16th to endNight
42%49%20%17%5%
2014
April1st to 15thNight
47%6%10%18%14%2
1796
16th to endNight
46%48%19%15%4
1707
May1st to 15thNight
49%5%9%17%14%3
1620
16th to endNight
43%9%18%16%9%1
1514
June1st to 15thNight
47%8%13%17%12%1
1440
16th to endNight
37%8%14%24%14%
1440
July1st to 15thNight
41%8%11%23%14%
1360
16th to endNight
41%8%13%22%11%2
1418
August1st to 15thNight
37%6%13%25%13%2
1568
16th to endNight
36%5%12%28%16%
1910
September1st to 15thNight
39%7%10%23%19%
1839
16th to endNight
36%38%21%26%2
1980
October1st to 15thNight
42%410%21%17%3
1929
16th to endNight
43%6%13%21%13%
2263
November1st to 15thNight
42%411%20%18%3
2277
16th to endNight
45%412%21%13%2
2340
December1st to 15thNight
46%414%20%12%
2520
16th to endNight
48%5%11%21%11%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
26%30%28%13%1
60
16th to endDay
31%18%40%9%
64
February1st to 15thDay
24%29%32%11%2
206
16th to endDay
26%23%33%12%5%
239
March1st to 15thDay
33%18%28%16%2
270
16th to endDay
24%24%29%14%41
288
April1st to 15thDay
27%20%32%13%5%
270
16th to endDay
24%14%32%20%5%2
270
May1st to 15thDay
38%14%23%13%7%2
288
16th to endDay
22%10%35%22%6%3
331
June1st to 15thDay
27%12%25%22%10%1
315
16th to endDay
16%14%35%21%8%1
315
July1st to 15thDay
20%8%28%27%13%1
305
16th to endDay
15%7%31%27%14%3
326
August1st to 15thDay
17%7%27%28%13%4
313
16th to endDay
12%7%31%22%17%8%
332
September1st to 15thDay
17%10%31%19%14%6%
315
16th to endDay
18%8%28%18%11%14%
314
October1st to 15thDay
20%10%25%21%14%6%
278
16th to endDay
18%20%30%21%8%
245
November1st to 15thDay
27%11%30%19%9%2
235
16th to endDay
25%18%28%18%7%1
240
December1st to 15thDay
25%26%35%10%3
60
16th to endDay
39%28%21%10%
64
January1st to 15thNight
24%20%35%15%2
720
16th to endNight
25%20%33%17%1
768
February1st to 15thNight
23%20%40%14%1
704
16th to endNight
26%21%34%16%1
631
March1st to 15thNight
27%22%32%15%2
720
16th to endNight
23%21%34%16%3
768
April1st to 15thNight
23%20%35%18%2
718
16th to endNight
26%15%35%18%4
627
May1st to 15thNight
25%17%32%20%5%
540
16th to endNight
21%10%34%26%6%
392
June1st to 15thNight
24%8%33%26%7%
360
16th to endNight
17%433%36%8%
360
July1st to 15thNight
13%432%35%15%
340
16th to endNight
13%332%35%13%1
376
August1st to 15thNight
12%432%35%13%
497
16th to endNight
14%324%35%20%1
764
September1st to 15thNight
16%8%32%28%13%
720
16th to endNight
17%6%27%27%17%4
717
October1st to 15thNight
22%10%31%22%11%1
684
16th to endNight
25%16%32%19%5%
704
November1st to 15thNight
22%15%31%21%8%1
700
16th to endNight
25%15%31%19%7%
720
December1st to 15thNight
28%16%31%17%5%
720
16th to endNight
27%21%34%14%1
768
Page generated at 2017-10-07 14:07:42. Version ac04

Copyright 2017 A. Danko


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