Wheaton College Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Wheaton College Observatory clear sky chart from 2005-11-05 to 2017-12-02.

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%
15241
19%
47750
8%
15209
38%
15209
20%
30989

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%
720
20%
2506
6%
716
34%
716
15%
1594
January16th to endNight
14%
768
19%
2688
3%
768
34%
768
14%
1755
February1st to 15thNight
16%
704
18%
2169
4%
701
32%
701
16%
1451
February16th to endNight
14%
631
25%
1893
5%
631
37%
631
18%
1232
March1st to 15thNight
17%
720
17%
2151
5%
720
32%
720
17%
1451
March16th to endNight
19%
768
23%
2040
6%
768
32%
768
20%
1376
April1st to 15thNight
18%
718
17%
1796
3%
718
27%
718
20%
1281
April16th to endNight
20%
591
19%
1671
4%
591
30%
591
19%
1141
May1st to 15thNight
21%
540
15%
1620
6%
540
29%
540
17%
1123
May16th to endNight
24%
437
11%
1559
6%
436
31%
436
16%
1033
June1st to 15thNight
31%
360
12%
1440
7%
360
41%
360
15%
952
June16th to endNight
35%
360
12%
1440
12%
360
43%
360
22%
862
July1st to 15thNight
35%
340
12%
1360
10%
340
45%
340
22%
810
July16th to endNight
42%
376
16%
1418
14%
354
50%
354
21%
871
August1st to 15thNight
39%
524
17%
1595
11%
522
43%
522
27%
990
August16th to endNight
48%
723
22%
1869
13%
723
51%
723
34%
1189
September1st to 15thNight
39%
720
19%
1869
9%
720
45%
720
31%
1222
September16th to endNight
42%
717
25%
1980
16%
717
49%
717
30%
1249
October1st to 15thNight
35%
720
23%
1996
11%
720
47%
720
26%
1291
October16th to endNight
24%
768
19%
2496
7%
768
40%
768
20%
1583
November1st to 15thNight
27%
760
19%
2472
9%
760
41%
760
21%
1578
November16th to endNight
24%
780
23%
2535
9%
780
41%
780
20%
1558
December1st to 15thNight
20%
728
18%
2499
6%
728
35%
728
16%
1617
December16th to endNight
15%
768
20%
2688
5%
768
33%
768
14%
1780

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
17%6%24424446%40%
48706
Night
17%6%25%424446%40%
47920
Cloud CoverDay
17%6%24424446%40%
48716
Night
17%6%25%424446%40%
47925
TransparencyDay
44%411%22%15%2
48394
Night
44%311%22%15%2
47750
SeeingDay
23%14%31%18%8%2
6196
Night
22%14%32%23%6%
15241

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%9%334135%36%42%
1432
16th to endDay
16%6%35%423346%43%
1536
February1st to 15thDay
19%6%25%335%5%47%36%
1687
16th to endDay
15%8%35%5%24446%41%
1580
March1st to 15thDay
21%8%3342335%7%36%
1799
16th to endDay
17%7%35%423345%42%
2183
April1st to 15thDay
19%7%24424446%38%
2152
16th to endDay
18%4245%33346%42%
2289
May1st to 15thDay
26%7%34423346%33%
2332
16th to endDay
17%5%245%24447%40%
2565
June1st to 15thDay
25%6%24423347%33%
2520
16th to endDay
14%6%245%35%5%5%9%37%
2520
July1st to 15thDay
14%6%25%5%35%6%6%9%34%
2520
16th to endDay
13%425%5%45%6%6%8%36%
2654
August1st to 15thDay
13%5%26%6%35%5%5%8%38%
2341
16th to endDay
12%6%145%245%5%8%42%
2331
September1st to 15thDay
14%5%24424346%46%
2091
16th to endDay
16%5%13223347%48%
1980
October1st to 15thDay
21%5%14323236%44%
1961
16th to endDay
16%6%2442435%6%44%
1728
November1st to 15thDay
22%7%23314345%41%
1716
16th to endDay
16%6%24423345%45%
1755
December1st to 15thDay
19%7%24423347%40%
1498
16th to endDay
23%8%2443434435%
1536
January1st to 15thNight
18%8%25%423345%40%
2506
16th to endNight
18%8%35%42433440%
2688
February1st to 15thNight
15%8%36%4245%45%38%
2168
16th to endNight
16%7%35%324435%42%
1893
March1st to 15thNight
17%7%25%5%2445%6%37%
2138
16th to endNight
16%8%35%5%2333443%
2040
April1st to 15thNight
17%8%25%5%25%445%36%
1796
16th to endNight
21%6%24424335%40%
1671
May1st to 15thNight
22%7%25%433446%34%
1620
16th to endNight
18%5%26%6%245%5%8%33%
1557
June1st to 15thNight
23%5%35%43445%6%34%
1440
16th to endNight
15%5%3442445%8%40%
1440
July1st to 15thNight
12%5%36%426%47%9%38%
1440
16th to endNight
11%5%26%5%36%6%6%9%35%
1514
August1st to 15thNight
8%5%26%345%6%5%8%41%
1605
16th to endNight
12%5%244245%6%6%46%
1869
September1st to 15thNight
13%5%144345%5%6%44%
1869
16th to endNight
15%6%24323345%48%
1980
October1st to 15thNight
18%6%24313445%45%
1996
16th to endNight
18%5%245%33346%42%
2496
November1st to 15thNight
18%5%25%423435%43%
2472
16th to endNight
17%5%25%423446%43%
2535
December1st to 15thNight
21%6%35%423345%39%
2499
16th to endNight
19%9%35%42335%6%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%8%333145%46%41%
1432
16th to endDay
16%6%35%423435%43%
1536
February1st to 15thDay
18%6%344245%47%37%
1687
16th to endDay
15%8%25%42335%5%42%
1580
March1st to 15thDay
21%8%2442435%7%36%
1809
16th to endDay
17%7%35%32434443%
2183
April1st to 15thDay
19%7%2442435%6%38%
2152
16th to endDay
19%425%423346%42%
2289
May1st to 15thDay
26%7%24423445%33%
2332
16th to endDay
17%5%245%24447%40%
2565
June1st to 15thDay
25%5%2442335%7%33%
2520
16th to endDay
14%6%245%36%5%6%9%37%
2520
July1st to 15thDay
14%6%25%5%35%5%6%8%35%
2520
16th to endDay
13%4246%36%5%6%9%35%
2654
August1st to 15thDay
12%6%25%6%35%45%7%38%
2341
16th to endDay
12%6%24425%5%5%8%41%
2331
September1st to 15thDay
14%5%24433445%46%
2091
16th to endDay
16%5%14323346%48%
1980
October1st to 15thDay
21%5%14323236%44%
1961
16th to endDay
16%5%2442345%5%44%
1728
November1st to 15thDay
22%7%24313445%41%
1716
16th to endDay
16%6%24424346%44%
1755
December1st to 15thDay
19%7%24424347%40%
1498
16th to endDay
23%8%235%24335%35%
1536
January1st to 15thNight
18%8%25%5%23335%40%
2506
16th to endNight
18%8%35%43433440%
2688
February1st to 15thNight
15%8%36%43445%6%38%
2161
16th to endNight
16%7%35%424435%42%
1893
March1st to 15thNight
17%8%25%425%446%37%
2151
16th to endNight
16%8%35%42333443%
2039
April1st to 15thNight
18%8%35%5%345%5%5%36%
1796
16th to endNight
21%6%25%42433441%
1671
May1st to 15thNight
23%6%25%424346%35%
1620
16th to endNight
18%5%36%4336%5%7%33%
1557
June1st to 15thNight
22%6%25%4335%46%34%
1440
16th to endNight
15%5%25%425%46%7%41%
1440
July1st to 15thNight
12%5%25%6%25%5%6%8%38%
1440
16th to endNight
12%436%5%36%6%6%8%35%
1514
August1st to 15thNight
8%6%26%445%5%6%7%42%
1605
16th to endNight
12%4245%245%5%7%45%
1869
September1st to 15thNight
14%5%144345%46%44%
1869
16th to endNight
16%6%14323335%47%
1980
October1st to 15thNight
18%6%25%323335%46%
1996
16th to endNight
18%5%24433347%42%
2496
November1st to 15thNight
18%5%25%423345%43%
2472
16th to endNight
17%5%25%323446%43%
2535
December1st to 15thNight
21%7%24423345%39%
2499
16th to endNight
19%9%35%42335%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
43%410%24%12%4
1433
16th to endDay
42%29%27%15%1
1536
February1st to 15thDay
46%412%19%13%2
1690
16th to endDay
44%310%23%14%3
1580
March1st to 15thDay
47%312%22%10%3
1809
16th to endDay
44%28%21%16%5%
2183
April1st to 15thDay
47%29%20%16%3
2152
16th to endDay
42%38%22%18%4
2289
May1st to 15thDay
52%37%17%15%2
2332
16th to endDay
43%414%21%13%2
2569
June1st to 15thDay
50%411%20%10%1
2520
16th to endDay
42%6%12%22%14%1
2520
July1st to 15thDay
44%5%10%21%14%2
2371
16th to endDay
42%6%13%22%11%4
2487
August1st to 15thDay
43%5%12%22%14%1
2327
16th to endDay
39%49%25%19%2
2331
September1st to 15thDay
38%5%10%21%22%1
2091
16th to endDay
37%27%23%26%2
1980
October1st to 15thDay
42%39%24%16%3
1961
16th to endDay
40%311%24%17%2
1728
November1st to 15thDay
45%312%21%14%1
1716
16th to endDay
41%310%25%16%2
1755
December1st to 15thDay
44%314%19%17%
1498
16th to endDay
51%313%19%11%1
1536
January1st to 15thNight
46%212%22%14%1
2506
16th to endNight
48%29%20%15%2
2688
February1st to 15thNight
47%312%21%12%2
2169
16th to endNight
45%29%22%18%2
1893
March1st to 15thNight
47%311%19%14%3
2151
16th to endNight
45%28%19%19%5%
2040
April1st to 15thNight
48%48%19%16%2
1796
16th to endNight
47%28%20%16%4
1671
May1st to 15thNight
50%310%17%13%3
1620
16th to endNight
48%5%16%19%8%2
1559
June1st to 15thNight
49%5%12%18%12%1
1440
16th to endNight
40%5%13%23%15%1
1440
July1st to 15thNight
40%5%13%26%14%
1360
16th to endNight
41%6%13%25%10%2
1418
August1st to 15thNight
39%413%25%14%2
1595
16th to endNight
35%5%11%28%18%
1869
September1st to 15thNight
38%411%24%19%1
1869
16th to endNight
39%26%22%26%1
1980
October1st to 15thNight
41%310%24%16%2
1996
16th to endNight
42%412%23%15%1
2496
November1st to 15thNight
43%311%21%16%3
2472
16th to endNight
41%311%24%15%2
2535
December1st to 15thNight
47%313%21%13%
2499
16th to endNight
48%411%21%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
31%23%36%35%
60
16th to endDay
23%26%39%10%
64
February1st to 15thDay
30%19%34%12%3
241
16th to endDay
25%21%33%14%4
239
March1st to 15thDay
32%18%30%12%5%
270
16th to endDay
23%23%33%9%8%1
288
April1st to 15thDay
26%18%35%13%5%1
270
16th to endDay
23%16%32%20%23
270
May1st to 15thDay
38%16%22%14%5%1
288
16th to endDay
22%11%38%20%6%
331
June1st to 15thDay
28%13%32%16%7%
315
16th to endDay
17%13%38%21%6%2
315
July1st to 15thDay
22%9%31%24%9%1
305
16th to endDay
12%8%34%28%12%2
326
August1st to 15thDay
15%8%31%28%13%2
313
16th to endDay
14%7%27%27%17%5%
332
September1st to 15thDay
17%11%26%26%15%3
315
16th to endDay
21%5%32%16%11%11%
314
October1st to 15thDay
23%11%25%22%10%6%
313
16th to endDay
21%15%32%17%11%1
288
November1st to 15thDay
28%12%29%16%10%2
280
16th to endDay
24%17%28%18%9%1
285
December1st to 15thDay
28%23%28%14%5%
110
16th to endDay
37%25%28%7%1
64
January1st to 15thNight
25%18%35%17%3
720
16th to endNight
26%20%33%17%1
768
February1st to 15thNight
23%20%34%19%1
704
16th to endNight
23%20%34%17%3
631
March1st to 15thNight
24%19%35%17%2
720
16th to endNight
23%21%31%18%4
768
April1st to 15thNight
22%21%34%17%4
718
16th to endNight
25%17%32%19%4
591
May1st to 15thNight
29%13%31%21%3
540
16th to endNight
23%9%36%25%5%
437
June1st to 15thNight
23%8%30%29%8%
360
16th to endNight
16%337%33%7%
360
July1st to 15thNight
13%240%32%9%
340
16th to endNight
13%231%41%10%
376
August1st to 15thNight
11%5%35%37%9%
524
16th to endNight
15%426%37%15%1
723
September1st to 15thNight
16%6%32%33%10%
720
16th to endNight
21%6%24%25%18%3
717
October1st to 15thNight
21%10%27%26%11%2
720
16th to endNight
23%15%33%19%8%
768
November1st to 15thNight
21%14%30%21%10%
760
16th to endNight
21%18%29%21%7%1
780
December1st to 15thNight
27%17%31%18%4
728
16th to endNight
27%20%32%16%2
768
Page generated at 2017-12-03 11:16:36. Version ac04

Copyright 2017 A. Danko


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