NREL- Solar Radiation Research Laboratory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the NREL- Solar Radiation Research Laboratory clear sky chart from 2005-11-05 to 2017-09-21.

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
53%
15950
27%
46909
17%
15908
63%
15908
41%
25342

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
38%
898
32%
2506
14%
895
55%
895
45%
1292
January16th to endNight
36%
804
33%
2532
16%
804
56%
804
38%
1297
February1st to 15thNight
31%
699
25%
2250
9%
698
51%
698
34%
1201
February16th to endNight
36%
632
31%
1894
13%
632
54%
632
39%
1018
March1st to 15thNight
42%
720
28%
2111
13%
720
56%
720
41%
1139
March16th to endNight
43%
681
27%
2025
14%
681
56%
681
37%
1108
April1st to 15thNight
43%
539
24%
1798
15%
539
56%
539
34%
943
April16th to endNight
42%
540
22%
1800
15%
540
53%
540
28%
1060
May1st to 15thNight
52%
540
19%
1605
16%
540
58%
540
28%
1028
May16th to endNight
60%
563
18%
1498
12%
561
65%
561
37%
893
June1st to 15thNight
72%
540
22%
1440
17%
540
75%
540
51%
761
June16th to endNight
80%
540
28%
1440
23%
540
82%
540
58%
713
July1st to 15thNight
76%
510
18%
1360
15%
510
78%
510
36%
824
July16th to endNight
81%
564
24%
1447
23%
531
82%
531
43%
812
August1st to 15thNight
79%
539
19%
1665
20%
536
80%
536
39%
903
August16th to endNight
79%
573
28%
1910
32%
573
81%
573
39%
997
September1st to 15thNight
74%
540
27%
1800
27%
540
78%
540
40%
944
September16th to endNight
71%
510
36%
1831
33%
510
80%
510
48%
817
October1st to 15thNight
57%
654
29%
1974
20%
654
63%
654
41%
1067
October16th to endNight
55%
729
33%
2137
17%
729
65%
729
46%
1117
November1st to 15thNight
53%
875
33%
2275
17%
875
61%
875
50%
1204
November16th to endNight
49%
900
34%
2403
16%
900
62%
900
51%
1258
December1st to 15thNight
38%
900
25%
2520
9%
900
52%
900
41%
1378
December16th to endNight
39%
960
26%
2688
10%
960
53%
960
35%
1568

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
5%4234245%6%8%52%
47966
Night
442332335%7%59%
47084
Cloud CoverDay
5%4244245%6%8%52%
47964
Night
442332335%7%58%
47084
TransparencyDay
28%8%12%23%20%7%
47644
Night
24%5%13%25%23%8%
46909
SeeingDay
9%8%28%30%17%5%
5870
Night
6%6%28%39%16%2
15957

By Month

Percentage Cloud Cover SW (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%24322348%61%
1432
16th to endDay
1433312348%63%
1701
February1st to 15thDay
16%25%325%45%8%54%
1610
16th to endDay
15%25%42335%7%58%
1580
March1st to 15thDay
442432336%7%56%
1848
16th to endDay
6%6%2332446%8%50%
2199
April1st to 15thDay
7%6%13423348%54%
2150
16th to endDay
11%7%2342445%7%44%
2160
May1st to 15thDay
11%7%245%346%6%7%40%
2342
16th to endDay
9%424425%6%7%9%42%
2616
June1st to 15thDay
5%212436%5%7%9%50%
2520
16th to endDay
3212426%6%8%10%52%
2520
July1st to 15thDay
7%324435%6%8%10%43%
2520
16th to endDay
5%324435%6%7%11%45%
2620
August1st to 15thDay
5%3245%37%7%8%9%41%
2285
16th to endDay
5%313435%6%8%8%49%
2292
September1st to 15thDay
6%313433447%55%
2160
16th to endDay
3213323446%65%
1920
October1st to 15thDay
5%312414447%59%
1656
16th to endDay
6%412212337%65%
1722
November1st to 15thDay
231332335%9%61%
1583
16th to endDay
232331235%8%62%
1554
December1st to 15thDay
1425%32435%9%56%
1440
16th to endDay
35%25%323348%57%
1536
January1st to 15thNight
324313347%65%
2506
16th to endNight
26%24314335%62%
2532
February1st to 15thNight
17%25%423347%57%
2240
16th to endNight
26%25%32335%5%58%
1894
March1st to 15thNight
242432446%7%57%
2111
16th to endNight
6%5%23313247%58%
2025
April1st to 15thNight
7%5%2221345%7%57%
1798
16th to endNight
13%6%24323247%50%
1800
May1st to 15thNight
14%5%2432445%8%45%
1604
16th to endNight
7%41441345%7%54%
1497
June1st to 15thNight
42221345%8%62%
1440
16th to endNight
21211247%9%65%
1440
July1st to 15thNight
5%223425%5%7%9%50%
1440
16th to endNight
4113325%6%7%9%54%
1545
August1st to 15thNight
32133345%7%9%55%
1674
16th to endNight
421432446%8%56%
1910
September1st to 15thNight
8%21223335%7%58%
1800
16th to endNight
422313346%68%
1831
October1st to 15thNight
7%413212346%61%
1974
16th to endNight
5%413212237%62%
2137
November1st to 15thNight
2313312345%67%
2275
16th to endNight
2422213346%65%
2403
December1st to 15thNight
25%35%322249%58%
2520
16th to endNight
27%2432445%7%55%
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%2332235%8%61%
1432
16th to endDay
15%23312348%63%
1701
February1st to 15thDay
16%25%415%449%54%
1610
16th to endDay
15%25%333448%57%
1580
March1st to 15thDay
4425%32345%8%55%
1848
16th to endDay
5%6%233245%6%9%50%
2197
April1st to 15thDay
6%6%13424348%53%
2150
16th to endDay
11%7%23425%5%5%7%44%
2160
May1st to 15thDay
10%7%25%425%5%7%7%40%
2342
16th to endDay
9%424426%6%7%9%41%
2616
June1st to 15thDay
5%213435%6%7%10%49%
2520
16th to endDay
3212325%7%7%11%52%
2520
July1st to 15thDay
6%3245%36%6%8%10%42%
2520
16th to endDay
43145%36%6%7%11%45%
2620
August1st to 15thDay
5%3245%46%7%9%10%40%
2285
16th to endDay
5%314435%6%8%8%49%
2292
September1st to 15thDay
6%31442345%7%55%
2160
16th to endDay
321232335%6%64%
1920
October1st to 15thDay
5%3134135%48%59%
1656
16th to endDay
5%42212337%65%
1722
November1st to 15thDay
241331336%9%61%
1583
16th to endDay
232331335%8%62%
1554
December1st to 15thDay
1425%32435%9%56%
1440
16th to endDay
35%25%41335%8%56%
1536
January1st to 15thNight
324312348%64%
2506
16th to endNight
16%25%323335%62%
2532
February1st to 15thNight
17%25%42335%7%57%
2240
16th to endNight
17%2442345%6%58%
1894
March1st to 15thNight
242332436%8%57%
2111
16th to endNight
6%5%23313247%58%
2025
April1st to 15thNight
6%5%22223448%57%
1798
16th to endNight
12%7%23323337%50%
1800
May1st to 15thNight
14%5%14324448%45%
1604
16th to endNight
7%42342345%8%54%
1497
June1st to 15thNight
5%11221345%9%61%
1440
16th to endNight
21211337%10%64%
1440
July1st to 15thNight
5%313425%5%7%10%49%
1440
16th to endNight
3113225%6%7%10%53%
1545
August1st to 15thNight
32133345%7%9%55%
1674
16th to endNight
421332446%8%56%
1910
September1st to 15thNight
8%21222345%7%57%
1800
16th to endNight
422322336%68%
1831
October1st to 15thNight
6%413313346%61%
1974
16th to endNight
5%422322247%62%
2137
November1st to 15thNight
2313312346%67%
2275
16th to endNight
2423212247%65%
2403
December1st to 15thNight
25%35%322249%58%
2520
16th to endNight
27%34323447%54%
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
21%6%13%32%21%4
1433
16th to endDay
21%5%10%33%20%7%
1701
February1st to 15thDay
27%6%14%30%15%4
1611
16th to endDay
25%410%26%27%5%
1580
March1st to 15thDay
26%6%11%26%22%6%
1848
16th to endDay
30%7%13%23%20%5%
2199
April1st to 15thDay
29%7%13%23%18%7%
2150
16th to endDay
37%9%12%20%16%3
2160
May1st to 15thDay
41%10%13%18%13%2
2342
16th to endDay
35%11%14%20%11%5%
2620
June1st to 15thDay
27%10%15%19%17%10%
2520
16th to endDay
22%8%12%20%25%10%
2520
July1st to 15thDay
32%11%12%18%15%9%
2371
16th to endDay
30%11%15%22%13%6%
2456
August1st to 15thDay
34%11%13%17%14%8%
2270
16th to endDay
28%10%12%21%21%6%
2292
September1st to 15thDay
27%6%11%20%20%12%
2160
16th to endDay
19%6%9%23%33%8%
1920
October1st to 15thDay
24%6%13%19%26%8%
1656
16th to endDay
21%49%26%29%9%
1722
November1st to 15thDay
20%412%24%26%11%
1583
16th to endDay
19%310%24%31%9%
1554
December1st to 15thDay
25%5%14%31%20%3
1440
16th to endDay
26%6%14%26%19%6%
1536
January1st to 15thNight
19%413%30%25%5%
2506
16th to endNight
24%311%27%24%9%
2532
February1st to 15thNight
27%413%30%19%4
2250
16th to endNight
25%49%26%24%9%
1894
March1st to 15thNight
23%5%10%25%27%7%
2111
16th to endNight
27%412%25%24%5%
2025
April1st to 15thNight
25%6%13%24%21%8%
1798
16th to endNight
35%5%11%24%18%4
1800
May1st to 15thNight
37%8%14%20%14%4
1605
16th to endNight
29%7%20%22%13%5%
1498
June1st to 15thNight
18%7%17%27%15%12%
1440
16th to endNight
13%6%12%24%29%12%
1440
July1st to 15thNight
26%11%15%20%15%9%
1360
16th to endNight
22%9%16%29%16%6%
1447
August1st to 15thNight
24%8%17%24%17%7%
1665
16th to endNight
23%7%12%22%25%7%
1910
September1st to 15thNight
25%5%13%21%19%13%
1800
16th to endNight
19%39%26%33%7%
1831
October1st to 15thNight
24%6%12%20%26%10%
1974
16th to endNight
22%410%23%29%9%
2137
November1st to 15thNight
19%310%24%27%13%
2275
16th to endNight
20%49%24%31%10%
2403
December1st to 15thNight
24%415%29%20%5%
2520
16th to endNight
28%6%14%25%20%3
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
6%12%26%32%17%4
90
16th to endDay
9%440%34%10%1
96
February1st to 15thDay
5%20%44%27%2
90
16th to endDay
311%41%30%11%1
81
March1st to 15thDay
323%36%28%7%
138
16th to endDay
13%28%32%21%2
288
April1st to 15thDay
11%22%37%18%6%2
269
16th to endDay
21%20%37%16%5%
270
May1st to 15thDay
16%8%30%31%10%2
419
16th to endDay
13%7%27%29%16%5%
517
June1st to 15thDay
5%327%33%23%7%
495
16th to endDay
3227%35%22%8%
495
July1st to 15thDay
10%220%28%28%9%
475
16th to endDay
5%217%32%32%9%
479
August1st to 15thDay
8%6%27%34%19%4
315
16th to endDay
9%423%38%18%5%
332
September1st to 15thDay
7%431%34%16%5%
315
16th to endDay
3229%37%20%5%
154
October1st to 15thDay
3327%32%27%5%
90
16th to endDay
10%429%32%18%5%
96
November1st to 15thDay
3425%30%30%6%
90
16th to endDay
7%16%36%27%11%
90
December1st to 15thDay
7%20%38%27%5%
90
16th to endDay
45%41%42%5%1
96
January1st to 15thNight
312%41%30%10%2
900
16th to endNight
413%40%32%8%1
804
February1st to 15thNight
5%11%43%34%5%
704
16th to endNight
5%9%43%36%4
632
March1st to 15thNight
47%40%37%9%
720
16th to endNight
9%8%35%38%7%
681
April1st to 15thNight
11%10%32%38%7%
539
16th to endNight
16%6%31%33%11%
540
May1st to 15thNight
16%320%42%14%2
540
16th to endNight
9%222%41%21%2
563
June1st to 15thNight
5%116%51%21%3
540
16th to endNight
3112%47%33%1
540
July1st to 15thNight
5%19%35%42%5%
510
16th to endNight
46%35%46%6%
564
August1st to 15thNight
417%43%39%3
539
16th to endNight
5%16%42%36%5%
573
September1st to 15thNight
8%19%44%27%8%
540
16th to endNight
4116%43%27%6%
510
October1st to 15thNight
10%326%39%16%4
654
16th to endNight
7%5%26%41%15%3
729
November1st to 15thNight
48%29%41%14%2
875
16th to endNight
45%35%43%9%1
900
December1st to 15thNight
310%41%37%6%
900
16th to endNight
5%9%38%36%8%1
960
Page generated at 2017-09-22 05:35:32. Version ac04

Copyright 2017 A. Danko


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