Three College Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Three College 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
45%
16503
22%
48757
14%
16467
54%
16467
27%
30625

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
28%
776
24%
2522
10%
776
44%
776
22%
1522
January16th to endNight
25%
832
26%
2704
12%
832
43%
832
24%
1629
February1st to 15thNight
26%
757
22%
2468
9%
757
42%
757
24%
1462
February16th to endNight
22%
631
28%
1850
9%
631
41%
631
26%
1118
March1st to 15thNight
30%
720
25%
1980
12%
720
42%
720
28%
1239
March16th to endNight
33%
768
21%
2112
11%
768
41%
768
25%
1387
April1st to 15thNight
34%
718
23%
1859
9%
718
44%
718
31%
1165
April16th to endNight
42%
713
21%
1793
9%
713
48%
713
31%
1171
May1st to 15thNight
46%
540
17%
1620
9%
540
51%
540
25%
1071
May16th to endNight
59%
561
17%
1684
13%
561
61%
561
29%
1065
June1st to 15thNight
61%
540
14%
1620
13%
540
63%
540
27%
1057
June16th to endNight
68%
540
17%
1620
15%
540
70%
540
29%
1043
July1st to 15thNight
67%
510
12%
1530
11%
510
68%
510
27%
1006
July16th to endNight
72%
564
16%
1595
13%
531
74%
531
29%
1020
August1st to 15thNight
68%
539
16%
1609
14%
536
68%
536
25%
1099
August16th to endNight
69%
717
17%
1863
13%
717
69%
717
28%
1280
September1st to 15thNight
66%
720
23%
1852
19%
720
67%
720
34%
1200
September16th to endNight
52%
717
20%
1989
18%
717
55%
717
25%
1364
October1st to 15thNight
50%
720
26%
2160
24%
720
57%
720
29%
1347
October16th to endNight
46%
768
36%
2304
23%
768
64%
768
35%
1279
November1st to 15thNight
45%
760
28%
2281
19%
760
57%
760
33%
1333
November16th to endNight
43%
780
28%
2503
20%
780
54%
780
29%
1438
December1st to 15thNight
36%
780
23%
2535
13%
780
49%
780
24%
1579
December16th to endNight
28%
832
23%
2704
13%
832
42%
832
20%
1751

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
12%5%24435%45%7%43%
49262
Night
13%5%2442445%7%45%
48950
Cloud CoverDay
12%5%24435%45%7%43%
49272
Night
13%5%2442445%7%45%
48954
TransparencyDay
38%6%11%21%17%4
48973
Night
37%5%10%20%19%5%
48757
SeeingDay
16%7%21%23%21%9%
6234
Night
16%7%23%31%18%2
16512

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
16%6%24423345%45%
1746
16th to endDay
16%6%23313345%49%
1872
February1st to 15thDay
16%6%23423447%44%
1695
16th to endDay
14%5%24424336%47%
1620
March1st to 15thDay
13%423324347%49%
1969
16th to endDay
16%6%24424446%44%
2112
April1st to 15thDay
10%31332435%7%53%
2089
16th to endDay
13%4245%3445%6%45%
2178
May1st to 15thDay
13%6%26%5%35%5%6%7%37%
2331
16th to endDay
9%425%5%35%6%6%9%41%
2431
June1st to 15thDay
8%5%35%435%5%7%10%41%
2340
16th to endDay
6%425%5%36%6%6%10%41%
2340
July1st to 15thDay
8%435%7%47%7%8%10%32%
2340
16th to endDay
6%426%7%5%8%7%8%12%31%
2465
August1st to 15thDay
9%6%26%8%37%5%8%10%30%
2340
16th to endDay
10%5%36%6%5%7%6%7%9%31%
2327
September1st to 15thDay
9%414435%5%6%8%43%
2106
16th to endDay
20%6%24424446%38%
1971
October1st to 15thDay
16%5%2422234451%
1800
16th to endDay
11%4135%13235%57%
1920
November1st to 15thDay
15%413323335%52%
1907
16th to endDay
13%42441435%5%50%
1784
December1st to 15thDay
15%5%2432446%5%45%
1707
16th to endDay
19%7%24313446%41%
1872
January1st to 15thNight
15%5%24423346%47%
2522
16th to endNight
16%5%23323445%47%
2704
February1st to 15thNight
16%5%13423346%47%
2466
16th to endNight
13%5%2342345%6%48%
1850
March1st to 15thNight
14%413424445%50%
1968
16th to endNight
16%424434346%44%
2112
April1st to 15thNight
13%5%23424446%49%
1859
16th to endNight
13%5%24433347%46%
1793
May1st to 15thNight
11%436%5%35%5%5%8%40%
1620
16th to endNight
10%425%42447%9%43%
1682
June1st to 15thNight
9%5%345%37%6%6%9%37%
1620
16th to endNight
6%435%5%25%6%7%11%41%
1620
July1st to 15thNight
8%3347%45%6%8%10%37%
1620
16th to endNight
5%435%5%35%7%8%12%38%
1703
August1st to 15thNight
10%5%25%5%35%6%7%9%37%
1620
16th to endNight
8%5%26%5%347%8%11%36%
1863
September1st to 15thNight
10%425%42445%8%46%
1852
16th to endNight
16%7%246%2436%5%39%
1989
October1st to 15thNight
17%6%24322335%48%
2160
16th to endNight
11%31332333458%
2304
November1st to 15thNight
13%413323335%53%
2281
16th to endNight
13%424324346%50%
2503
December1st to 15thNight
15%5%25%424335%45%
2535
16th to endNight
19%6%24423346%40%
2704

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
17%6%24423345%45%
1746
16th to endDay
16%6%23313345%49%
1872
February1st to 15thDay
16%6%23423447%44%
1695
16th to endDay
14%5%24314337%47%
1620
March1st to 15thDay
13%424324347%48%
1980
16th to endDay
16%6%13423445%44%
2111
April1st to 15thDay
10%31332445%8%53%
2089
16th to endDay
13%4245%35%45%7%44%
2178
May1st to 15thDay
12%6%26%6%35%6%6%7%37%
2331
16th to endDay
9%4246%35%5%7%8%41%
2431
June1st to 15thDay
8%435%435%5%7%10%41%
2340
16th to endDay
6%425%436%5%6%10%42%
2340
July1st to 15thDay
8%5%25%6%47%7%8%10%32%
2340
16th to endDay
5%426%6%48%8%8%12%30%
2465
August1st to 15thDay
8%5%36%8%47%6%8%10%31%
2340
16th to endDay
10%5%36%6%5%7%7%8%8%31%
2327
September1st to 15thDay
9%414435%5%6%8%43%
2106
16th to endDay
20%6%25%424446%38%
1971
October1st to 15thDay
15%5%2431234451%
1800
16th to endDay
11%4135%23235%57%
1920
November1st to 15thDay
15%413324335%52%
1907
16th to endDay
12%42442435%5%51%
1784
December1st to 15thDay
15%5%2342446%5%45%
1707
16th to endDay
19%7%24323436%41%
1872
January1st to 15thNight
15%5%24423446%47%
2522
16th to endNight
16%5%24323445%47%
2704
February1st to 15thNight
16%5%13323346%48%
2459
16th to endNight
13%5%2342445%6%48%
1850
March1st to 15thNight
15%323424346%49%
1980
16th to endNight
16%424434446%44%
2112
April1st to 15thNight
13%5%23324346%49%
1859
16th to endNight
13%5%24434346%46%
1793
May1st to 15thNight
11%436%5%35%5%5%8%40%
1619
16th to endNight
10%425%43447%9%43%
1682
June1st to 15thNight
9%5%345%36%6%6%10%37%
1620
16th to endNight
6%425%5%36%5%7%11%41%
1620
July1st to 15thNight
8%425%7%45%6%8%11%37%
1620
16th to endNight
5%425%5%35%8%8%11%38%
1703
August1st to 15thNight
10%6%26%5%45%6%7%8%37%
1620
16th to endNight
8%5%26%5%446%8%11%36%
1863
September1st to 15thNight
10%425%42445%8%46%
1852
16th to endNight
15%8%25%5%3345%6%39%
1989
October1st to 15thNight
16%6%24312335%48%
2160
16th to endNight
11%32332333458%
2304
November1st to 15thNight
13%413324335%53%
2281
16th to endNight
13%324323346%50%
2503
December1st to 15thNight
15%5%25%424335%45%
2535
16th to endNight
19%6%24423346%40%
2704

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%46%20%21%4
1747
16th to endDay
37%37%22%20%8%
1872
February1st to 15thDay
39%47%22%21%4
1697
16th to endDay
37%5%10%22%20%4
1620
March1st to 15thDay
35%410%22%20%5%
1980
16th to endDay
41%5%9%20%18%4
2112
April1st to 15thDay
29%411%27%19%7%
2089
16th to endDay
38%5%10%18%19%7%
2178
May1st to 15thDay
43%7%14%19%12%2
2331
16th to endDay
36%9%15%24%12%1
2435
June1st to 15thDay
35%8%17%22%14%
2340
16th to endDay
35%8%18%19%15%2
2340
July1st to 15thDay
40%12%15%16%11%2
2201
16th to endDay
40%11%17%20%9%
2310
August1st to 15thDay
44%10%15%18%10%1
2327
16th to endDay
45%9%14%18%11%1
2327
September1st to 15thDay
35%9%12%18%18%5%
2106
16th to endDay
46%5%9%17%19%2
1971
October1st to 15thDay
36%49%24%20%5%
1800
16th to endDay
31%25%24%27%8%
1920
November1st to 15thDay
34%38%19%26%7%
1907
16th to endDay
34%37%21%26%5%
1784
December1st to 15thDay
38%5%9%23%18%4
1707
16th to endDay
43%410%20%17%3
1872
January1st to 15thNight
38%48%17%24%6%
2522
16th to endNight
39%36%22%22%6%
2704
February1st to 15thNight
37%38%21%23%5%
2468
16th to endNight
34%410%23%22%5%
1850
March1st to 15thNight
36%5%8%22%20%6%
1980
16th to endNight
40%5%7%22%18%4
2112
April1st to 15thNight
35%48%22%23%6%
1859
16th to endNight
38%5%10%19%17%8%
1793
May1st to 15thNight
40%7%12%21%15%2
1620
16th to endNight
34%9%15%25%12%1
1684
June1st to 15thNight
39%7%17%20%12%1
1620
16th to endNight
34%11%15%20%15%3
1620
July1st to 15thNight
36%10%15%20%13%3
1530
16th to endNight
33%10%18%22%12%1
1595
August1st to 15thNight
40%10%17%17%12%1
1609
16th to endNight
38%10%17%21%9%1
1863
September1st to 15thNight
35%7%13%21%17%5%
1852
16th to endNight
45%5%10%15%19%3
1989
October1st to 15thNight
39%38%20%23%5%
2160
16th to endNight
30%35%23%26%10%
2304
November1st to 15thNight
33%37%18%25%10%
2281
16th to endNight
34%45%20%26%8%
2503
December1st to 15thNight
41%49%20%19%5%
2535
16th to endNight
44%49%18%18%4
2704

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
28%11%37%20%2
135
16th to endDay
22%21%32%13%7%2
144
February1st to 15thDay
26%14%44%12%1
130
16th to endDay
23%14%38%17%41
165
March1st to 15thDay
16%15%37%20%9%
270
16th to endDay
18%15%32%23%7%2
288
April1st to 15thDay
17%11%28%22%15%3
270
16th to endDay
19%421%33%15%6%
270
May1st to 15thDay
17%12%26%23%14%5%
288
16th to endDay
14%318%21%30%10%
331
June1st to 15thDay
12%415%26%30%11%
315
16th to endDay
7%320%28%28%12%
315
July1st to 15thDay
11%12%29%31%14%
305
16th to endDay
7%9%23%36%21%
326
August1st to 15thDay
13%310%24%30%18%
313
16th to endDay
11%27%25%29%23%
332
September1st to 15thDay
14%212%18%33%18%
315
16th to endDay
24%312%24%25%8%
314
October1st to 15thDay
18%419%24%24%9%
315
16th to endDay
13%5%24%28%22%5%
336
November1st to 15thDay
24%6%20%26%19%3
325
16th to endDay
10%11%24%25%23%4
164
December1st to 15thDay
18%14%37%20%8%
124
16th to endDay
27%16%28%21%5%
144
January1st to 15thNight
20%14%31%26%7%
780
16th to endNight
20%13%36%25%3
832
February1st to 15thNight
22%14%33%28%1
760
16th to endNight
17%17%38%23%3
631
March1st to 15thNight
18%12%35%26%6%
720
16th to endNight
19%10%31%29%8%
768
April1st to 15thNight
16%13%32%27%9%
718
16th to endNight
16%6%28%35%12%
713
May1st to 15thNight
13%5%27%38%14%
540
16th to endNight
12%214%36%29%4
563
June1st to 15thNight
12%212%36%31%3
540
16th to endNight
9%110%38%36%3
540
July1st to 15thNight
10%18%35%39%5%
510
16th to endNight
6%18%32%42%9%
564
August1st to 15thNight
12%16%34%39%5%
539
16th to endNight
11%15%35%39%6%
717
September1st to 15thNight
14%29%33%35%6%
720
16th to endNight
21%112%31%29%2
717
October1st to 15thNight
20%318%29%24%3
720
16th to endNight
12%9%27%26%20%3
768
November1st to 15thNight
15%7%26%31%16%2
760
16th to endNight
14%12%26%31%14%1
780
December1st to 15thNight
18%8%31%32%8%
780
16th to endNight
23%11%30%28%5%
832
Page generated at 2018-02-15 10:58:40. Version ac04

Copyright 2018 A. Danko


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