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 2019-07-16.

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%
18565
20%
54229
17%
18525
67%
18525
31%
33890

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%
837
23%
2717
19%
836
59%
836
29%
1625
January16th to endNight
50%
896
25%
2867
20%
896
59%
896
31%
1686
February1st to 15thNight
52%
821
25%
2463
18%
818
60%
818
34%
1467
February16th to endNight
57%
735
30%
2079
20%
735
66%
735
42%
1146
March1st to 15thNight
56%
840
30%
2310
19%
840
66%
840
46%
1250
March16th to endNight
58%
896
28%
2464
21%
896
64%
896
45%
1361
April1st to 15thNight
63%
838
23%
2306
19%
838
67%
838
43%
1291
April16th to endNight
64%
831
27%
2105
17%
831
68%
831
45%
1228
May1st to 15thNight
71%
766
24%
2026
17%
766
73%
766
42%
1202
May16th to endNight
76%
657
21%
1970
17%
657
77%
657
34%
1206
June1st to 15thNight
70%
630
10%
1890
8%
630
70%
630
22%
1340
June16th to endNight
72%
630
10%
1890
9%
630
72%
630
22%
1382
July1st to 15thNight
73%
600
6%
1800
5%
600
73%
600
19%
1362
July16th to endNight
77%
615
6%
1748
7%
582
77%
582
21%
1287
August1st to 15thNight
80%
580
7%
1720
7%
577
79%
577
24%
1185
August16th to endNight
64%
828
5%
2231
5%
828
64%
828
18%
1669
September1st to 15thNight
66%
780
6%
2145
5%
780
66%
780
17%
1606
September16th to endNight
64%
777
9%
2145
9%
777
64%
777
20%
1556
October1st to 15thNight
67%
780
22%
2241
23%
780
70%
780
31%
1416
October16th to endNight
63%
832
25%
2496
24%
832
67%
832
33%
1486
November1st to 15thNight
66%
820
28%
2461
25%
820
71%
820
37%
1392
November16th to endNight
59%
840
30%
2520
26%
840
65%
840
33%
1502
December1st to 15thNight
59%
840
27%
2723
25%
840
65%
840
31%
1606
December16th to endNight
63%
896
23%
2912
24%
896
67%
896
33%
1639

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%5%27%7%5%7%6%7%8%34%
55060
Night
6%425%5%35%5%6%9%43%
54438
TransparencyDay
43%8%14%16%12%4
54761
Night
33%8%17%21%14%4
54241
SeeingDay
8%213%22%26%27%
8390
Night
8%215%33%30%9%
18574

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
9%5%26%435%5%6%9%38%
1881
16th to endDay
9%5%245%35%5%6%9%42%
2058
February1st to 15thDay
9%5%244445%6%8%44%
2050
16th to endDay
5%425%5%445%6%9%46%
1963
March1st to 15thDay
33145%35%46%8%51%
2310
16th to endDay
6%425%5%45%6%5%8%47%
2463
April1st to 15thDay
5%3246%46%7%7%8%42%
2302
16th to endDay
5%315%6%46%5%7%8%44%
2456
May1st to 15thDay
6%325%6%46%6%7%9%41%
2594
16th to endDay
8%5%38%7%5%7%6%7%8%31%
2847
June1st to 15thDay
9%7%39%8%6%8%8%8%9%19%
2730
16th to endDay
7%7%37%10%5%10%8%6%8%22%
2730
July1st to 15thDay
8%7%49%10%6%8%7%7%7%19%
2730
16th to endDay
7%6%39%9%7%10%8%8%8%20%
2676
August1st to 15thDay
5%5%38%8%6%9%9%8%10%23%
2468
16th to endDay
7%8%410%9%7%8%8%7%8%20%
2318
September1st to 15thDay
7%6%5%10%10%6%8%8%8%8%19%
2145
16th to endDay
10%8%38%9%5%8%8%7%8%20%
2145
October1st to 15thDay
8%426%9%5%6%7%6%9%32%
2048
16th to endDay
8%5%25%7%46%5%6%8%40%
2080
November1st to 15thDay
5%5%25%7%47%5%5%8%43%
2057
16th to endDay
7%5%26%5%46%5%6%8%41%
2100
December1st to 15thDay
10%6%245%35%5%5%8%41%
1893
16th to endDay
5%425%5%36%5%6%11%44%
2016
January1st to 15thNight
8%5%25%5%345%6%9%43%
2717
16th to endNight
10%5%24425%45%9%45%
2867
February1st to 15thNight
8%5%25%4235%5%8%48%
2463
16th to endNight
5%31332446%8%54%
2079
March1st to 15thNight
331333445%9%57%
2310
16th to endNight
5%313423347%58%
2464
April1st to 15thNight
332442446%9%54%
2306
16th to endNight
43133245%5%8%56%
2105
May1st to 15thNight
4323335%5%49%53%
2026
16th to endNight
6%4245%346%7%10%44%
1970
June1st to 15thNight
7%6%37%6%46%6%8%12%28%
1890
16th to endNight
5%5%36%8%47%7%7%10%33%
1890
July1st to 15thNight
9%5%36%5%45%7%8%10%31%
1890
16th to endNight
4428%7%48%8%9%10%30%
1856
August1st to 15thNight
45%25%6%45%8%9%13%34%
1731
16th to endNight
9%6%36%7%46%6%9%11%28%
2231
September1st to 15thNight
6%5%27%8%5%7%8%8%11%28%
2145
16th to endNight
8%6%28%7%46%6%6%10%30%
2145
October1st to 15thNight
7%5%15%6%35%6%7%9%41%
2241
16th to endNight
9%5%244335%5%8%47%
2496
November1st to 15thNight
5%434435%5%6%9%49%
2461
16th to endNight
7%4245%35%5%6%8%45%
2520
December1st to 15thNight
8%3245%345%5%7%46%
2723
16th to endNight
5%4245%25%5%6%10%46%
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
38%6%8%21%20%4
1882
16th to endDay
36%48%22%19%7%
2058
February1st to 15thDay
35%5%8%21%19%9%
2052
16th to endDay
33%411%20%20%9%
1963
March1st to 15thDay
28%413%25%18%8%
2310
16th to endDay
33%49%22%21%7%
2464
April1st to 15thDay
33%8%13%21%19%4
2302
16th to endDay
33%6%9%19%23%7%
2456
May1st to 15thDay
35%7%12%19%18%6%
2594
16th to endDay
46%7%11%17%15%1
2851
June1st to 15thDay
54%10%18%13%2
2730
16th to endDay
52%13%19%11%2
2730
July1st to 15thDay
57%13%22%5%1
2591
16th to endDay
54%12%21%10%
2521
August1st to 15thDay
48%12%29%9%
2455
16th to endDay
57%12%21%7%
2318
September1st to 15thDay
55%14%19%9%1
2145
16th to endDay
55%12%17%10%3
2145
October1st to 15thDay
44%7%12%17%14%3
2048
16th to endDay
39%5%9%14%20%9%
2080
November1st to 15thDay
37%411%21%16%8%
2057
16th to endDay
37%5%9%20%17%9%
2100
December1st to 15thDay
39%5%13%20%14%6%
1893
16th to endDay
31%6%13%26%16%6%
2016
January1st to 15thNight
35%5%13%23%16%5%
2717
16th to endNight
34%410%25%18%6%
2867
February1st to 15thNight
33%5%10%20%19%10%
2472
16th to endNight
26%413%24%23%7%
2079
March1st to 15thNight
23%414%29%21%6%
2310
16th to endNight
24%5%13%26%21%8%
2464
April1st to 15thNight
25%5%18%26%18%5%
2306
16th to endNight
25%6%12%25%23%6%
2105
May1st to 15thNight
26%7%14%27%18%6%
2027
16th to endNight
31%8%17%24%17%
1972
June1st to 15thNight
44%10%25%16%3
1890
16th to endNight
41%15%23%16%2
1890
July1st to 15thNight
43%17%26%11%1
1800
16th to endNight
41%16%27%13%
1748
August1st to 15thNight
34%17%36%10%
1720
16th to endNight
44%16%28%9%
2231
September1st to 15thNight
44%17%24%11%2
2145
16th to endNight
45%12%21%16%3
2145
October1st to 15thNight
34%8%17%18%17%3
2241
16th to endNight
33%5%12%16%21%9%
2496
November1st to 15thNight
29%6%12%24%21%6%
2461
16th to endNight
33%5%9%23%18%9%
2520
December1st to 15thNight
34%5%11%25%15%7%
2723
16th to endNight
30%6%16%24%16%5%
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
10%6%37%26%13%6%
180
16th to endDay
14%9%36%23%14%1
192
February1st to 15thDay
8%5%21%33%23%7%
180
16th to endDay
8%221%33%29%3
282
March1st to 15thDay
4327%37%23%3
390
16th to endDay
7%423%30%28%5%
416
April1st to 15thDay
5%223%31%29%8%
390
16th to endDay
6%112%31%32%14%
379
May1st to 15thDay
8%13%26%33%18%
408
16th to endDay
10%16%18%31%32%
459
June1st to 15thDay
13%2412%28%39%
435
16th to endDay
7%26%23%58%
435
July1st to 15thDay
6%25%23%62%
425
16th to endDay
9%119%20%56%
391
August1st to 15thDay
429%22%60%
358
16th to endDay
11%148%19%54%
396
September1st to 15thDay
6%2411%22%52%
375
16th to endDay
8%1412%28%45%
374
October1st to 15thDay
10%8%27%36%17%
375
16th to endDay
8%218%30%27%13%
400
November1st to 15thDay
9%14%31%31%11%
385
16th to endDay
6%121%43%24%3
390
December1st to 15thDay
8%7%31%34%14%3
183
16th to endDay
3216%36%30%10%
192
January1st to 15thNight
10%5%24%38%20%1
840
16th to endNight
11%427%42%13%
896
February1st to 15thNight
10%5%25%42%16%
824
16th to endNight
8%224%46%16%
735
March1st to 15thNight
5%330%43%15%
840
16th to endNight
6%324%45%19%
896
April1st to 15thNight
5%222%45%23%
838
16th to endNight
7%320%45%23%
831
May1st to 15thNight
6%113%46%29%2
767
16th to endNight
7%5%36%41%8%
659
June1st to 15thNight
10%319%51%14%
630
16th to endNight
7%19%46%33%
630
July1st to 15thNight
9%16%45%36%
600
16th to endNight
429%43%39%
615
August1st to 15thNight
6%6%50%35%
580
16th to endNight
12%1411%41%28%
828
September1st to 15thNight
9%1413%47%24%
780
16th to endNight
11%418%45%18%
777
October1st to 15thNight
10%18%33%39%6%
780
16th to endNight
12%114%32%32%5%
832
November1st to 15thNight
6%116%42%29%3
820
16th to endNight
7%221%43%22%1
840
December1st to 15thNight
10%218%42%24%1
840
16th to endNight
6%319%38%29%2
896
Page generated at 2019-07-17 08:53:06. Version ac04

Copyright 2019 A. Danko


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