This page made possible by the sponsorship of: Naperville Astronomical Association.

Riley Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Riley Observatory clear sky chart from 2005-11-05 to 2018-03-21.

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
37%
16772
18%
49205
11%
16730
44%
16730
24%
32929

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
22%
975
14%
2716
4%
970
30%
970
19%
1975
January16th to endNight
22%
1040
13%
2870
4%
1040
29%
1040
16%
2161
February1st to 15thNight
22%
862
14%
2393
4%
862
29%
862
18%
1726
February16th to endNight
25%
683
18%
2049
8%
683
34%
683
18%
1441
March1st to 15thNight
28%
780
14%
2340
9%
780
33%
780
18%
1635
March16th to endNight
30%
643
16%
2078
11%
643
37%
643
18%
1441
April1st to 15thNight
30%
539
15%
1797
11%
539
40%
539
20%
1197
April16th to endNight
33%
540
18%
1739
11%
540
43%
540
21%
1177
May1st to 15thNight
37%
540
16%
1599
9%
540
44%
540
23%
1101
May16th to endNight
50%
562
17%
1497
9%
561
54%
561
32%
971
June1st to 15thNight
52%
540
16%
1440
10%
540
56%
540
36%
899
June16th to endNight
50%
540
17%
1440
9%
540
54%
540
31%
951
July1st to 15thNight
61%
510
17%
1360
11%
510
64%
510
39%
829
July16th to endNight
70%
564
23%
1417
17%
531
73%
531
43%
833
August1st to 15thNight
77%
539
22%
1581
21%
536
79%
536
42%
882
August16th to endNight
68%
573
21%
1910
22%
573
72%
573
35%
1062
September1st to 15thNight
64%
543
29%
1803
29%
543
69%
543
37%
970
September16th to endNight
59%
717
27%
1980
20%
717
63%
717
36%
1195
October1st to 15thNight
47%
720
25%
2070
19%
720
53%
720
32%
1256
October16th to endNight
30%
768
20%
2439
12%
768
40%
768
22%
1586
November1st to 15thNight
31%
760
23%
2471
15%
760
41%
760
24%
1551
November16th to endNight
26%
819
18%
2574
9%
819
36%
819
20%
1717
December1st to 15thNight
23%
975
13%
2730
4%
975
30%
975
17%
2009
December16th to endNight
17%
1040
11%
2912
3%
1040
24%
1040
11%
2364

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
14%7%36%5%3445%7%36%
49648
Night
14%7%35%424446%40%
49374
Cloud CoverDay
14%7%36%5%3445%7%36%
49657
Night
14%7%35%424446%40%
49379
TransparencyDay
46%6%12%21%12%1
49334
Night
44%5%11%22%14%2
49206
SeeingDay
15%8%29%25%15%5%
6549
Night
18%8%29%29%12%1
16776

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%13%48%6%35%35%6%27%
1552
16th to endDay
13%15%6%9%6%24446%26%
1703
February1st to 15thDay
15%14%6%8%5%35%436%27%
1798
16th to endDay
18%11%48%5%33346%31%
1710
March1st to 15thDay
15%13%37%5%3445%6%30%
1940
16th to endDay
18%9%37%6%35%447%30%
2282
April1st to 15thDay
18%9%46%5%3445%5%31%
2151
16th to endDay
20%7%25%5%3445%7%34%
2220
May1st to 15thDay
19%8%35%5%35%5%5%7%29%
2349
16th to endDay
14%5%26%5%35%5%6%6%39%
2617
June1st to 15thDay
12%5%24435%5%5%6%44%
2520
16th to endDay
10%435%6%35%7%7%9%35%
2520
July1st to 15thDay
7%325%5%35%5%6%9%44%
2520
16th to endDay
7%425%5%36%6%7%10%40%
2655
August1st to 15thDay
7%5%25%5%35%6%6%10%41%
2367
16th to endDay
9%5%245%346%6%9%42%
2292
September1st to 15thDay
12%314425%446%49%
2145
16th to endDay
8%5%245%35%6%5%7%45%
1980
October1st to 15thDay
16%6%25%434336%42%
1887
16th to endDay
19%7%25%5%3345%7%36%
1784
November1st to 15thDay
15%8%36%414445%40%
1717
16th to endDay
18%7%46%423346%37%
1715
December1st to 15thDay
17%11%48%6%34445%29%
1560
16th to endDay
20%14%6%7%6%34435%23%
1664
January1st to 15thNight
17%11%46%5%24345%35%
2716
16th to endNight
13%11%7%9%6%24345%30%
2870
February1st to 15thNight
16%12%5%6%5%13445%34%
2391
16th to endNight
16%10%47%5%23445%34%
2049
March1st to 15thNight
15%10%45%5%2443437%
2327
16th to endNight
16%8%36%5%35%447%33%
2078
April1st to 15thNight
17%8%35%424447%36%
1797
16th to endNight
19%7%25%423346%38%
1739
May1st to 15thNight
18%6%15%5%345%6%7%35%
1599
16th to endNight
10%414435%45%9%44%
1496
June1st to 15thNight
10%42442435%6%50%
1440
16th to endNight
9%4245%36%6%5%10%40%
1440
July1st to 15thNight
6%3146%15%47%7%51%
1440
16th to endNight
5%2143245%8%9%51%
1513
August1st to 15thNight
431433446%10%53%
1590
16th to endNight
9%31342445%8%52%
1910
September1st to 15thNight
10%312323347%55%
1803
16th to endNight
8%4144245%5%7%50%
1980
October1st to 15thNight
13%424423247%49%
2070
16th to endNight
19%7%25%423336%41%
2439
November1st to 15thNight
14%7%25%424345%45%
2471
16th to endNight
17%8%36%423246%38%
2574
December1st to 15thNight
20%9%37%5%24436%31%
2730
16th to endNight
23%15%5%8%6%3433422%
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%13%48%6%35%35%5%27%
1552
16th to endDay
13%15%6%10%6%23446%26%
1703
February1st to 15thDay
15%14%6%8%5%35%435%27%
1798
16th to endDay
18%11%48%5%33346%31%
1710
March1st to 15thDay
16%13%47%5%34446%30%
1950
16th to endDay
18%8%37%6%35%45%6%30%
2281
April1st to 15thDay
18%9%5%7%5%2445%5%31%
2151
16th to endDay
20%8%25%5%2345%7%34%
2220
May1st to 15thDay
19%8%36%5%25%5%5%7%29%
2349
16th to endDay
13%5%25%5%345%6%6%39%
2617
June1st to 15thDay
12%5%24435%5%5%6%44%
2520
16th to endDay
10%435%6%35%7%7%9%35%
2520
July1st to 15thDay
7%325%5%35%5%6%9%44%
2520
16th to endDay
7%425%6%46%6%7%10%40%
2655
August1st to 15thDay
7%5%25%5%35%6%7%9%41%
2367
16th to endDay
9%4245%35%5%7%9%42%
2292
September1st to 15thDay
12%32442445%6%49%
2145
16th to endDay
8%5%245%35%5%5%7%46%
1980
October1st to 15thDay
16%5%25%434336%42%
1887
16th to endDay
19%7%25%43335%7%36%
1784
November1st to 15thDay
15%8%36%41445%5%40%
1717
16th to endDay
18%7%46%423346%37%
1715
December1st to 15thDay
16%11%5%8%5%35%345%29%
1560
16th to endDay
21%14%6%7%6%34425%23%
1664
January1st to 15thNight
17%11%46%5%23335%35%
2716
16th to endNight
13%11%7%9%6%24445%30%
2870
February1st to 15thNight
15%12%5%6%5%1344434%
2384
16th to endNight
16%10%47%5%23445%34%
2049
March1st to 15thNight
16%10%45%5%2443437%
2340
16th to endNight
17%8%36%5%35%448%33%
2078
April1st to 15thNight
17%8%36%42335%7%36%
1797
16th to endNight
19%7%26%42335%5%39%
1739
May1st to 15thNight
18%6%15%4345%6%7%35%
1598
16th to endNight
10%424435%45%9%45%
1496
June1st to 15thNight
10%42432436%5%50%
1440
16th to endNight
9%4245%36%6%5%10%40%
1440
July1st to 15thNight
6%3145%1446%7%51%
1440
16th to endNight
5%2133245%8%9%51%
1513
August1st to 15thNight
331343447%10%53%
1590
16th to endNight
9%31342345%8%52%
1910
September1st to 15thNight
10%312413347%55%
1803
16th to endNight
8%414425%5%5%7%50%
1980
October1st to 15thNight
14%424423247%49%
2070
16th to endNight
19%7%25%423345%41%
2439
November1st to 15thNight
14%7%25%424335%45%
2471
16th to endNight
17%8%46%423246%38%
2574
December1st to 15thNight
20%9%48%5%23436%31%
2730
16th to endNight
23%15%5%8%6%3433422%
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
57%412%14%9%
1553
16th to endDay
59%413%15%7%
1703
February1st to 15thDay
59%5%10%15%8%1
1800
16th to endDay
54%411%20%6%1
1710
March1st to 15thDay
55%5%9%20%7%
1950
16th to endDay
53%6%13%20%5%
2282
April1st to 15thDay
53%5%9%19%11%
2151
16th to endDay
49%6%12%16%11%3
2220
May1st to 15thDay
52%7%12%21%6%
2349
16th to endDay
43%6%12%20%15%2
2621
June1st to 15thDay
38%7%10%24%17%1
2520
16th to endDay
40%8%13%20%14%2
2520
July1st to 15thDay
35%7%13%25%14%2
2371
16th to endDay
36%8%16%22%13%2
2488
August1st to 15thDay
34%8%14%25%14%1
2352
16th to endDay
35%8%13%23%16%2
2292
September1st to 15thDay
34%6%11%24%19%4
2145
16th to endDay
34%7%8%27%19%2
1980
October1st to 15thDay
42%5%11%24%14%2
1887
16th to endDay
47%5%12%22%11%1
1784
November1st to 15thDay
44%5%11%22%14%2
1717
16th to endDay
49%412%25%8%
1715
December1st to 15thDay
57%313%18%7%
1560
16th to endDay
65%410%12%6%
1664
January1st to 15thNight
52%411%18%12%
2716
16th to endNight
56%5%12%16%8%
2870
February1st to 15thNight
52%411%20%10%1
2393
16th to endNight
52%311%20%9%1
2049
March1st to 15thNight
50%5%10%20%10%2
2340
16th to endNight
49%6%13%19%9%1
2078
April1st to 15thNight
47%410%21%14%1
1797
16th to endNight
46%6%8%21%12%4
1739
May1st to 15thNight
46%5%12%23%11%
1599
16th to endNight
35%8%14%19%19%2
1498
June1st to 15thNight
34%5%11%26%18%3
1440
16th to endNight
37%7%12%21%18%2
1440
July1st to 15thNight
30%6%12%27%19%2
1360
16th to endNight
25%8%15%27%19%4
1417
August1st to 15thNight
24%7%16%30%18%2
1581
16th to endNight
29%6%13%26%19%4
1910
September1st to 15thNight
28%5%10%27%25%3
1803
16th to endNight
31%6%10%29%19%2
1980
October1st to 15thNight
35%49%27%20%2
2070
16th to endNight
45%311%23%14%1
2439
November1st to 15thNight
42%49%23%17%3
2471
16th to endNight
47%313%23%11%
2574
December1st to 15thNight
54%410%19%9%1
2730
16th to endNight
66%48%13%6%
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
26%8%31%28%2
135
16th to endDay
18%12%43%19%6%
144
February1st to 15thDay
26%18%42%11%1
135
16th to endDay
25%15%38%12%6%1
120
March1st to 15thDay
30%12%33%20%3
135
16th to endDay
28%8%28%23%9%1
261
April1st to 15thDay
23%14%28%21%11%1
270
16th to endDay
20%11%28%24%10%4
270
May1st to 15thDay
22%10%31%20%10%4
306
16th to endDay
15%10%33%22%14%3
517
June1st to 15thDay
13%6%29%32%14%4
495
16th to endDay
11%7%37%25%12%5%
495
July1st to 15thDay
9%431%26%17%10%
475
16th to endDay
7%228%28%21%11%
515
August1st to 15thDay
7%316%33%23%15%
400
16th to endDay
7%416%29%28%13%
332
September1st to 15thDay
10%317%33%26%8%
315
16th to endDay
8%419%28%30%8%
314
October1st to 15thDay
19%5%24%30%15%4
222
16th to endDay
25%13%34%22%4
144
November1st to 15thDay
20%11%26%25%11%5%
135
16th to endDay
26%14%37%15%6%
135
December1st to 15thDay
17%25%38%11%41
135
16th to endDay
26%22%45%41
144
January1st to 15thNight
21%12%37%20%6%
975
16th to endNight
19%12%34%30%2
1040
February1st to 15thNight
22%10%36%25%4
865
16th to endNight
21%13%33%25%5%
683
March1st to 15thNight
21%11%34%24%6%
780
16th to endNight
20%9%33%25%10%
643
April1st to 15thNight
20%11%31%27%7%
539
16th to endNight
22%7%33%25%10%
540
May1st to 15thNight
21%430%30%12%
540
16th to endNight
11%5%27%38%17%
563
June1st to 15thNight
12%227%43%13%
540
16th to endNight
10%328%36%19%
540
July1st to 15thNight
8%119%43%25%1
510
16th to endNight
5%15%45%29%3
564
August1st to 15thNight
419%49%30%4
539
16th to endNight
10%14%38%30%4
573
September1st to 15thNight
11%316%34%29%5%
543
16th to endNight
10%5%18%34%27%3
717
October1st to 15thNight
16%6%24%32%17%1
720
16th to endNight
22%12%30%24%9%
768
November1st to 15thNight
18%10%36%23%10%1
760
16th to endNight
21%12%34%24%7%
819
December1st to 15thNight
26%10%33%23%5%
975
16th to endNight
30%11%33%21%3
1040
Page generated at 2018-03-22 10:34:24. Version ac04

Copyright 2018 A. Danko


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