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 2017-05-24.

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%
15617
17%
45843
11%
15575
44%
15575
23%
30816

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
21%
900
14%
2506
4%
895
29%
895
18%
1837
January16th to endNight
22%
960
13%
2649
4%
960
29%
960
16%
2006
February1st to 15thNight
22%
794
13%
2205
4%
794
29%
794
19%
1584
February16th to endNight
26%
631
18%
1893
8%
631
35%
631
18%
1322
March1st to 15thNight
28%
720
13%
2160
9%
720
31%
720
17%
1540
March16th to endNight
30%
621
15%
2010
10%
621
37%
621
18%
1398
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
49%
541
17%
1441
9%
540
54%
540
31%
940
June1st to 15thNight
51%
495
17%
1320
10%
495
55%
495
33%
845
June16th to endNight
52%
495
15%
1320
9%
495
55%
495
31%
873
July1st to 15thNight
61%
465
18%
1240
12%
465
64%
465
39%
756
July16th to endNight
70%
522
20%
1305
16%
489
72%
489
42%
777
August1st to 15thNight
77%
494
23%
1448
22%
491
79%
491
42%
807
August16th to endNight
68%
525
20%
1750
20%
525
72%
525
35%
970
September1st to 15thNight
63%
498
30%
1653
29%
498
68%
498
37%
888
September16th to endNight
58%
657
25%
1815
18%
657
62%
657
35%
1106
October1st to 15thNight
49%
660
27%
1897
20%
660
55%
660
33%
1137
October16th to endNight
30%
704
18%
2236
11%
704
40%
704
22%
1456
November1st to 15thNight
31%
700
23%
2276
16%
700
43%
700
24%
1412
November16th to endNight
26%
756
17%
2376
9%
756
35%
756
20%
1610
December1st to 15thNight
22%
900
13%
2520
5%
900
29%
900
17%
1879
December16th to endNight
17%
960
11%
2688
3%
960
24%
960
11%
2198

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%
46435
Night
15%8%35%424446%39%
46012
Cloud CoverDay
14%8%36%5%3445%7%35%
46444
Night
15%8%35%424446%39%
46017
TransparencyDay
46%6%12%21%11%1
46121
Night
45%5%11%22%13%2
45844
SeeingDay
16%8%28%25%15%6%
5443
Night
18%8%29%29%12%1
15621

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%25%
1432
16th to endDay
13%15%6%9%7%24445%25%
1572
February1st to 15thDay
15%14%6%8%5%36%436%27%
1656
16th to endDay
17%11%38%5%34346%32%
1580
March1st to 15thDay
16%13%37%5%3445%6%29%
1790
16th to endDay
18%9%37%6%35%447%30%
2222
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%25%5%35%46%6%38%
2516
June1st to 15thDay
13%5%24435%5%5%7%41%
2310
16th to endDay
10%436%5%35%7%7%9%35%
2310
July1st to 15thDay
8%315%5%35%5%6%9%44%
2310
16th to endDay
7%425%5%36%6%8%10%39%
2459
August1st to 15thDay
7%5%35%4345%6%9%42%
2170
16th to endDay
9%5%244346%7%9%41%
2100
September1st to 15thDay
13%314425%446%48%
1966
16th to endDay
9%5%245%35%6%5%7%43%
1815
October1st to 15thDay
14%5%25%434337%43%
1730
16th to endDay
19%7%25%5%3345%7%35%
1636
November1st to 15thDay
13%8%36%41445%5%42%
1582
16th to endDay
19%7%47%423346%34%
1583
December1st to 15thDay
18%11%5%8%6%34445%28%
1440
16th to endDay
21%14%6%7%6%24435%23%
1536
January1st to 15thNight
17%11%46%5%23345%34%
2506
16th to endNight
13%12%7%9%5%34445%29%
2649
February1st to 15thNight
15%12%5%6%5%13445%35%
2203
16th to endNight
15%10%47%5%2345%5%34%
1893
March1st to 15thNight
16%11%46%5%3443436%
2147
16th to endNight
17%8%36%5%34447%33%
2010
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%5%5%8%44%
1440
June1st to 15thNight
11%42442445%6%47%
1320
16th to endNight
10%4245%36%6%5%11%40%
1320
July1st to 15thNight
6%3145%1447%6%52%
1320
16th to endNight
6%2143245%8%9%50%
1401
August1st to 15thNight
431433446%10%53%
1457
16th to endNight
9%31342445%8%52%
1750
September1st to 15thNight
11%412313348%55%
1653
16th to endNight
9%414425%45%8%49%
1815
October1st to 15thNight
12%424423247%50%
1897
16th to endNight
20%7%25%323346%41%
2236
November1st to 15thNight
13%7%25%423336%46%
2276
16th to endNight
18%9%36%433246%37%
2376
December1st to 15thNight
21%9%37%5%24436%30%
2520
16th to endNight
24%15%5%8%6%3433421%
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
16%13%48%6%35%45%5%26%
1432
16th to endDay
14%15%6%10%6%34436%25%
1572
February1st to 15thDay
15%14%6%7%5%35%436%27%
1656
16th to endDay
17%11%38%5%34346%32%
1580
March1st to 15thDay
16%13%47%5%3445%6%29%
1800
16th to endDay
18%9%37%6%3445%6%30%
2221
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
14%5%25%5%345%6%6%38%
2516
June1st to 15thDay
13%5%24435%5%5%7%41%
2310
16th to endDay
10%435%5%35%7%7%9%35%
2310
July1st to 15thDay
8%325%5%35%5%6%8%44%
2310
16th to endDay
7%425%6%46%6%7%10%38%
2459
August1st to 15thDay
7%5%35%5%35%5%6%9%42%
2170
16th to endDay
10%424435%5%7%9%41%
2100
September1st to 15thDay
13%42442445%6%48%
1966
16th to endDay
9%5%245%35%6%5%7%44%
1815
October1st to 15thDay
15%5%25%5%34346%44%
1730
16th to endDay
19%7%25%5%3345%7%35%
1636
November1st to 15thDay
13%8%36%42445%5%41%
1582
16th to endDay
19%7%46%5%23346%34%
1583
December1st to 15thDay
18%12%5%8%5%35%335%28%
1440
16th to endDay
22%14%6%7%6%24435%23%
1536
January1st to 15thNight
17%11%46%5%23345%34%
2506
16th to endNight
13%12%7%9%6%24445%29%
2649
February1st to 15thNight
15%12%5%6%5%1344435%
2196
16th to endNight
15%10%47%5%2345%5%34%
1893
March1st to 15thNight
16%11%46%5%3443435%
2160
16th to endNight
17%7%36%5%34448%33%
2010
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
11%4245%35%45%8%45%
1440
June1st to 15thNight
11%425%42446%5%48%
1320
16th to endNight
10%4245%36%6%5%10%40%
1320
July1st to 15thNight
7%3144245%6%7%52%
1320
16th to endNight
5%2133246%8%9%50%
1401
August1st to 15thNight
431333447%9%53%
1457
16th to endNight
9%31442345%8%52%
1750
September1st to 15thNight
11%412313348%55%
1653
16th to endNight
8%414425%45%7%49%
1815
October1st to 15thNight
12%424423247%50%
1897
16th to endNight
20%7%25%323346%40%
2236
November1st to 15thNight
14%7%25%423336%46%
2276
16th to endNight
18%9%46%423246%37%
2376
December1st to 15thNight
21%9%48%5%23336%30%
2520
16th to endNight
24%15%5%8%5%3333421%
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
58%413%13%8%
1433
16th to endDay
61%412%14%6%
1572
February1st to 15thDay
59%5%10%15%8%1
1658
16th to endDay
53%5%12%20%6%1
1580
March1st to 15thDay
56%5%10%19%6%
1800
16th to endDay
52%6%13%19%5%
2222
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%7%12%20%14%2
2520
June1st to 15thDay
40%7%10%24%15%1
2310
16th to endDay
40%8%13%20%14%2
2310
July1st to 15thDay
35%7%12%26%14%2
2161
16th to endDay
37%8%16%22%12%2
2292
August1st to 15thDay
35%8%14%25%14%1
2155
16th to endDay
36%8%13%22%16%3
2100
September1st to 15thDay
35%6%11%24%18%3
1966
16th to endDay
36%7%9%26%17%2
1815
October1st to 15thDay
40%5%11%26%14%1
1730
16th to endDay
47%5%13%22%9%1
1636
November1st to 15thDay
42%5%11%23%14%2
1582
16th to endDay
51%312%23%8%
1583
December1st to 15thDay
59%412%17%6%
1440
16th to endDay
65%49%12%7%
1536
January1st to 15thNight
52%5%11%18%10%
2506
16th to endNight
56%5%13%16%8%
2649
February1st to 15thNight
51%412%20%9%1
2205
16th to endNight
51%412%21%9%1
1893
March1st to 15thNight
52%5%11%17%10%2
2160
16th to endNight
49%6%13%19%9%1
2010
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%18%2
1442
June1st to 15thNight
36%5%12%26%16%3
1320
16th to endNight
37%7%12%21%18%2
1320
July1st to 15thNight
29%6%12%28%19%3
1240
16th to endNight
26%8%16%26%19%3
1305
August1st to 15thNight
25%6%17%29%19%2
1448
16th to endNight
29%5%13%26%18%5%
1750
September1st to 15thNight
28%5%10%27%25%3
1653
16th to endNight
32%6%10%29%18%3
1815
October1st to 15thNight
34%410%27%21%1
1897
16th to endNight
45%411%24%13%1
2236
November1st to 15thNight
41%48%23%18%4
2276
16th to endNight
49%313%22%11%
2376
December1st to 15thNight
56%411%19%8%
2520
16th to endNight
67%48%13%5%
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
28%8%34%24%3
90
16th to endDay
17%11%40%21%8%
96
February1st to 15thDay
21%22%43%12%1
90
16th to endDay
19%12%40%14%9%2
81
March1st to 15thDay
36%13%30%16%3
90
16th to endDay
30%8%29%21%9%1
242
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
16%9%32%23%13%3
481
June1st to 15thDay
15%6%29%30%13%5%
420
16th to endDay
10%5%35%27%14%6%
420
July1st to 15thDay
9%5%31%24%18%11%
400
16th to endDay
8%228%29%20%11%
445
August1st to 15thDay
8%215%34%22%16%
333
16th to endDay
7%414%30%28%13%
268
September1st to 15thDay
11%318%32%26%7%
255
16th to endDay
10%419%29%28%8%
254
October1st to 15thDay
14%5%25%30%19%5%
170
16th to endDay
22%14%34%26%2
96
November1st to 15thDay
11%12%24%32%14%5%
90
16th to endDay
34%11%34%13%6%
90
December1st to 15thDay
21%22%36%14%41
90
16th to endDay
27%22%44%32
96
January1st to 15thNight
21%13%38%20%6%
900
16th to endNight
20%11%33%30%2
960
February1st to 15thNight
21%10%37%25%5%
797
16th to endNight
20%13%34%26%6%
631
March1st to 15thNight
22%11%34%24%6%
720
16th to endNight
20%9%33%25%10%
621
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
12%5%26%37%17%
542
June1st to 15thNight
13%227%43%12%
495
16th to endNight
11%325%37%20%1
495
July1st to 15thNight
9%219%42%25%2
465
16th to endNight
5%15%45%28%3
522
August1st to 15thNight
5%19%50%29%4
494
16th to endNight
10%13%39%31%4
525
September1st to 15thNight
11%316%33%29%5%
498
16th to endNight
11%5%17%34%26%3
657
October1st to 15thNight
14%6%25%32%18%2
660
16th to endNight
23%11%30%24%9%
704
November1st to 15thNight
17%10%36%23%10%1
700
16th to endNight
21%12%32%24%8%
756
December1st to 15thNight
27%10%33%23%5%
900
16th to endNight
31%10%32%21%3
960
Page generated at 2017-05-25 12:27:32. Version ac04

Copyright 2017 A. Danko


Riley Observatory Clear Sky Chart | Charts Home