Riverwoods Field Laboratory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Riverwoods Field Laboratory clear sky chart from 2005-11-05 to 2017-09-24.

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
63%
16107
20%
47013
16%
16067
67%
16067
30%
29678

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
51%
717
19%
2327
16%
716
57%
716
28%
1425
January16th to endNight
51%
768
25%
2457
21%
768
60%
768
31%
1436
February1st to 15thNight
49%
701
24%
2103
17%
698
58%
698
33%
1262
February16th to endNight
53%
631
29%
1785
18%
631
63%
631
40%
1011
March1st to 15thNight
55%
720
30%
1980
18%
720
66%
720
45%
1083
March16th to endNight
59%
768
27%
2112
21%
768
64%
768
44%
1181
April1st to 15thNight
63%
718
22%
1976
19%
718
67%
718
44%
1100
April16th to endNight
63%
720
27%
1824
17%
720
68%
720
43%
1083
May1st to 15thNight
72%
656
26%
1736
18%
656
75%
656
44%
1012
May16th to endNight
78%
561
21%
1682
17%
561
79%
561
35%
1018
June1st to 15thNight
68%
540
9%
1620
7%
540
68%
540
21%
1160
June16th to endNight
71%
540
10%
1620
8%
540
71%
540
22%
1182
July1st to 15thNight
73%
510
6%
1530
6%
510
73%
510
19%
1167
July16th to endNight
77%
564
7%
1595
7%
531
77%
531
21%
1178
August1st to 15thNight
79%
544
6%
1614
7%
541
79%
541
23%
1116
August16th to endNight
64%
764
6%
2059
5%
764
64%
764
18%
1540
September1st to 15thNight
64%
720
5%
1980
5%
720
64%
720
17%
1485
September16th to endNight
62%
693
9%
1914
10%
693
62%
693
19%
1415
October1st to 15thNight
67%
660
22%
1896
23%
660
69%
660
32%
1187
October16th to endNight
62%
704
24%
2112
23%
704
66%
704
31%
1289
November1st to 15thNight
64%
700
30%
2101
27%
700
70%
700
37%
1195
November16th to endNight
58%
720
30%
2160
26%
720
65%
720
33%
1289
December1st to 15thNight
59%
720
25%
2334
25%
720
64%
720
29%
1417
December16th to endNight
61%
768
23%
2496
23%
768
65%
768
31%
1447

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
7%6%37%7%5%7%6%6%8%33%
47891
Night
7%425%5%35%5%6%9%42%
47222
TransparencyDay
44%8%14%16%12%4
47592
Night
34%8%16%20%14%5%
47025
SeeingDay
8%211%20%26%30%
5938
Night
8%215%32%30%10%
16116

By Month

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
10%6%26%436%5%7%9%36%
1611
16th to endDay
9%4245%35%5%6%8%43%
1764
February1st to 15thDay
9%5%244345%6%7%44%
1750
16th to endDay
6%426%4445%6%9%45%
1685
March1st to 15thDay
43146%45%47%9%49%
1980
16th to endDay
6%425%5%45%6%5%8%45%
2111
April1st to 15thDay
5%4246%46%7%7%9%41%
1972
16th to endDay
6%315%6%46%5%6%8%42%
2133
May1st to 15thDay
5%225%5%47%6%7%8%43%
2222
16th to endDay
7%5%38%8%6%7%7%7%9%29%
2431
June1st to 15thDay
11%8%39%8%5%8%8%7%9%19%
2340
16th to endDay
8%8%37%10%5%10%8%6%8%21%
2340
July1st to 15thDay
9%7%410%10%6%8%8%8%7%18%
2340
16th to endDay
7%7%39%8%7%10%8%8%8%19%
2465
August1st to 15thDay
5%6%38%9%6%9%9%7%10%23%
2322
16th to endDay
8%8%410%9%7%7%7%7%8%20%
2139
September1st to 15thDay
7%6%5%10%9%6%8%8%8%8%19%
1980
16th to endDay
11%9%48%9%5%8%7%7%8%19%
1905
October1st to 15thDay
8%426%8%5%7%7%6%9%32%
1733
16th to endDay
9%6%25%7%46%5%5%8%38%
1760
November1st to 15thDay
5%5%25%7%46%45%7%44%
1757
16th to endDay
8%5%26%446%45%9%41%
1800
December1st to 15thDay
11%7%345%36%5%5%7%38%
1623
16th to endDay
5%425%5%36%5%6%10%43%
1728
January1st to 15thNight
8%6%35%5%345%6%9%40%
2327
16th to endNight
10%424425%45%9%46%
2457
February1st to 15thNight
9%5%25%4235%5%7%47%
2103
16th to endNight
5%3144245%6%8%52%
1785
March1st to 15thNight
431433445%9%56%
1980
16th to endNight
5%31432445%8%56%
2112
April1st to 15thNight
3325%42446%9%54%
1976
16th to endNight
5%4234345%5%7%54%
1824
May1st to 15thNight
3323335%5%49%55%
1736
16th to endNight
5%3245%347%8%11%43%
1682
June1st to 15thNight
8%7%37%6%46%7%8%12%26%
1620
16th to endNight
5%5%36%8%47%7%7%10%32%
1620
July1st to 15thNight
9%6%45%6%46%6%8%10%32%
1620
16th to endNight
45%27%6%48%8%9%11%29%
1703
August1st to 15thNight
45%25%6%45%7%9%13%34%
1625
16th to endNight
10%6%36%7%46%6%9%12%27%
2059
September1st to 15thNight
7%6%27%8%5%7%8%9%11%26%
1980
16th to endNight
8%7%38%7%47%7%6%10%28%
1914
October1st to 15thNight
7%415%6%35%5%7%8%42%
1896
16th to endNight
10%5%244335%5%8%45%
2112
November1st to 15thNight
5%434435%5%6%8%49%
2101
16th to endNight
8%424435%5%5%8%46%
2160
December1st to 15thNight
9%4245%35%6%5%7%44%
2334
16th to endNight
5%4245%36%5%6%9%46%
2496

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
40%6%9%21%17%4
1612
16th to endDay
35%48%22%19%8%
1764
February1st to 15thDay
36%47%18%21%11%
1752
16th to endDay
34%411%20%18%10%
1685
March1st to 15thDay
30%412%26%19%7%
1980
16th to endDay
35%410%21%20%7%
2112
April1st to 15thDay
34%7%12%21%19%5%
1972
16th to endDay
35%5%10%18%22%6%
2133
May1st to 15thDay
33%6%11%21%19%6%
2222
16th to endDay
47%8%12%19%12%1
2435
June1st to 15thDay
56%9%16%13%3
2340
16th to endDay
54%13%18%11%2
2340
July1st to 15thDay
58%13%19%6%1
2201
16th to endDay
55%12%20%10%
2310
August1st to 15thDay
49%12%28%9%
2309
16th to endDay
57%12%21%7%
2139
September1st to 15thDay
54%13%20%9%1
1980
16th to endDay
57%11%16%9%3
1905
October1st to 15thDay
43%7%12%15%15%4
1733
16th to endDay
41%5%8%14%20%9%
1760
November1st to 15thDay
37%48%21%17%9%
1757
16th to endDay
38%5%8%19%16%11%
1800
December1st to 15thDay
41%6%12%18%15%5%
1623
16th to endDay
33%6%12%23%17%7%
1728
January1st to 15thNight
37%5%14%22%14%5%
2327
16th to endNight
34%410%24%18%7%
2457
February1st to 15thNight
34%48%18%21%11%
2112
16th to endNight
28%413%23%21%8%
1785
March1st to 15thNight
25%312%31%21%6%
1980
16th to endNight
24%5%14%26%20%8%
2112
April1st to 15thNight
25%5%17%25%19%6%
1976
16th to endNight
27%5%12%24%23%5%
1824
May1st to 15thNight
24%7%12%29%18%6%
1737
16th to endNight
30%8%18%26%15%
1684
June1st to 15thNight
45%10%23%16%4
1620
16th to endNight
42%14%23%16%3
1620
July1st to 15thNight
44%17%23%12%2
1530
16th to endNight
41%16%27%14%
1595
August1st to 15thNight
35%17%36%10%
1614
16th to endNight
44%16%28%10%
2059
September1st to 15thNight
45%16%24%11%2
1980
16th to endNight
47%12%20%15%3
1914
October1st to 15thNight
34%7%16%18%18%4
1896
16th to endNight
35%5%11%16%21%9%
2112
November1st to 15thNight
29%5%10%23%22%7%
2101
16th to endNight
34%5%8%23%17%10%
2160
December1st to 15thNight
36%6%11%24%14%6%
2334
16th to endNight
32%6%15%21%17%6%
2496

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
16%5%35%24%10%7%
90
16th to endDay
15%11%29%21%18%3
96
February1st to 15thDay
10%8%25%25%21%8%
90
16th to endDay
11%425%36%22%
186
March1st to 15thDay
5%323%37%26%3
270
16th to endDay
7%421%28%31%5%
288
April1st to 15thDay
5%220%30%31%9%
270
16th to endDay
7%112%28%33%17%
270
May1st to 15thDay
7%10%22%37%22%
288
16th to endDay
6%16%17%33%34%
331
June1st to 15thDay
16%25%10%23%41%
315
16th to endDay
7%35%23%59%
315
July1st to 15thDay
42419%68%
305
16th to endDay
10%118%19%58%
326
August1st to 15thDay
5%28%20%63%
313
16th to endDay
13%247%18%53%
332
September1st to 15thDay
7%2410%21%53%
315
16th to endDay
8%2410%27%45%
287
October1st to 15thDay
10%5%28%37%17%
255
16th to endDay
10%118%25%29%13%
272
November1st to 15thDay
10%13%29%32%13%
265
16th to endDay
5%118%40%28%4
270
December1st to 15thDay
12%430%34%16%2
93
16th to endDay
3114%38%34%8%
96
January1st to 15thNight
10%5%23%37%20%1
720
16th to endNight
11%5%25%43%14%
768
February1st to 15thNight
10%6%26%41%14%
704
16th to endNight
9%327%46%13%
631
March1st to 15thNight
6%429%42%16%1
720
16th to endNight
6%424%45%19%
768
April1st to 15thNight
4222%46%23%
718
16th to endNight
8%319%44%23%
720
May1st to 15thNight
5%114%47%28%3
657
16th to endNight
6%5%37%40%8%
563
June1st to 15thNight
11%420%48%13%
540
16th to endNight
7%211%47%31%
540
July1st to 15thNight
9%16%46%35%
510
16th to endNight
4128%44%38%
564
August1st to 15thNight
6%6%49%36%
544
16th to endNight
12%1411%41%27%
764
September1st to 15thNight
10%1413%46%24%
720
16th to endNight
12%1419%44%18%
693
October1st to 15thNight
10%17%33%39%7%
660
16th to endNight
13%114%32%32%4
704
November1st to 15thNight
6%117%42%27%3
700
16th to endNight
8%222%40%23%1
720
December1st to 15thNight
10%218%41%25%1
720
16th to endNight
7%320%38%27%2
768
Page generated at 2017-09-25 09:02:29. Version ac04

Copyright 2017 A. Danko


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