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 2005-11-06 to 2025-11-03.

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 Type	Minimum conditions for hour to be usable				Out of hours
	Clear of Clouds	Transparency	Seeing	Sky Brightness
Planetary:	50%	(no limit)	Average	(no limit)	Nighttime Hours
Deep Sky:	70%	Average	(no limit)	5.9	Nighttime Hours
CCD Imaging:	70%	Below Average	Poor	(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

By Month

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 Forecast	Data Points
Cloud Cover	Day	5% 5% 2 5% 4 4 5% 4 6% 8% 53%	135492
	Night	5% 5% 2 5% 3 3 3 3 5% 7% 59%	131991
Transparency	Day	26% 7% 11% 22% 23% 10%	130443
	Night	24% 5% 11% 24% 26% 11%	128045
Seeing	Day	8% 9% 30% 32% 16% 4	16954
	Night	6% 6% 32% 40% 13% 2	26930

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
Month	Days in Month		Occurrence of each Forecast	Data Points
January	1st to 10th	Day	1 7% 2 5% 3 3 3 3 5% 7% 62%	3638
	11th to 20th	Day	1 3 2 7% 5% 5% 4 3 5% 7% 58%	3774
	21st to end	Day	1 4 2 8% 3 2 3 3 4 7% 62%	4510
February	1st to 10th	Day	2 7% 3 7% 3 4 4 3 6% 6% 56%	1800
	11th to 20th	Day	1 5% 2 5% 5% 3 5% 3 6% 9% 56%	1917
	21st to end	Day	2 6% 2 6% 3 3 3 4 5% 5% 59%	1650
March	1st to 10th	Day	3 10% 3 4 3 2 3 2 5% 5% 59%	4500
	11th to 20th	Day	4 8% 2 6% 4 4 4 4 6% 8% 49%	4953
	21st to end	Day	4 7% 2 5% 4 4 5% 4 6% 8% 52%	5810
April	1st to 10th	Day	5% 7% 2 5% 3 3 4 3 7% 7% 55%	2400
	11th to 20th	Day	8% 9% 2 6% 4 3 4 4 5% 7% 48%	2371
	21st to end	Day	9% 8% 1 5% 4 4 5% 4 6% 8% 46%	2400
May	1st to 10th	Day	11% 6% 2 6% 4 4 5% 4 7% 8% 43%	5713
	11th to 20th	Day	8% 6% 2 5% 4 4 6% 5% 7% 8% 44%	6298
	21st to end	Day	7% 4 2 5% 4 5% 5% 5% 8% 9% 47%	6925
June	1st to 10th	Day	5% 3 1 4 4 5% 5% 5% 8% 10% 48%	2800
	11th to 20th	Day	6% 3 1 4 4 4 6% 5% 7% 9% 51%	2800
	21st to end	Day	5% 3 1 4 4 5% 7% 5% 8% 9% 49%	2800
July	1st to 10th	Day	4 3 1 4 4 4 5% 5% 7% 9% 56%	6339
	11th to 20th	Day	3 3 1 4 4 6% 7% 6% 10% 12% 45%	6340
	21st to end	Day	6% 3 2 6% 5% 5% 6% 5% 7% 8% 47%	6754
August	1st to 10th	Day	7% 4 2 5% 5% 5% 7% 6% 9% 10% 40%	5779
	11th to 20th	Day	3 3 1 4 4 4 6% 6% 7% 9% 53%	5374
	21st to end	Day	8% 4 1 6% 4 5% 6% 5% 7% 8% 45%	5984
September	1st to 10th	Day	5% 3 1 4 4 3 4 4 5% 7% 60%	2400
	11th to 20th	Day	6% 3 1 4 3 4 4 4 5% 8% 58%	2382
	21st to end	Day	5% 4 1 4 2 3 3 3 4 6% 64%	2200
October	1st to 10th	Day	3 3 1 2 2 2 3 2 3 5% 73%	3279
	11th to 20th	Day	5% 4 2 5% 3 4 4 3 5% 7% 59%	3250
	21st to end	Day	5% 5% 1 3 2 2 2 2 4 7% 66%	3431
November	1st to 10th	Day	3 5% 1 3 2 3 3 3 6% 8% 65%	1781
	11th to 20th	Day	2 5% 1 4 3 2 3 3 5% 7% 65%	1800
	21st to end	Day	3 5% 3 5% 3 3 3 3 5% 8% 60%	1690
December	1st to 10th	Day	1 4 1 5% 3 2 3 2 4 6% 70%	2620
	11th to 20th	Day	1 3 1 4 2 2 2 2 5% 7% 69%	3026
	21st to end	Day	2 5% 2 6% 4 4 4 4 6% 8% 54%	4004
January	1st to 10th	Night	1 8% 3 7% 3 3 3 2 4 7% 58%	6327
	11th to 20th	Night	1 4 2 6% 4 4 3 2 4 7% 62%	6248
	21st to end	Night	2 6% 2 7% 3 3 3 3 4 5% 61%	6468
February	1st to 10th	Night	2 8% 2 6% 3 4 4 3 4 8% 57%	2600
	11th to 20th	Night	2 7% 3 7% 4 4 4 3 5% 6% 55%	2368
	21st to end	Night	2 7% 2 6% 3 3 3 3 4 5% 61%	1980
March	1st to 10th	Night	3 10% 3 5% 3 3 3 3 4 6% 56%	5374
	11th to 20th	Night	6% 11% 2 4 3 3 3 3 4 7% 55%	5030
	21st to end	Night	7% 10% 3 4 3 3 3 3 4 6% 53%	5150
April	1st to 10th	Night	6% 7% 1 4 3 3 3 3 5% 7% 57%	2000
	11th to 20th	Night	10% 8% 2 4 2 3 4 3 5% 6% 52%	1977
	21st to end	Night	12% 7% 2 5% 3 3 3 2 5% 6% 50%	2000
May	1st to 10th	Night	14% 6% 2 5% 3 4 4 3 5% 8% 45%	4248
	11th to 20th	Night	9% 6% 2 6% 4 3 4 3 5% 7% 50%	3617
	21st to end	Night	10% 5% 2 4 3 3 3 3 4 6% 58%	3972
June	1st to 10th	Night	6% 3 1 3 3 3 3 4 6% 9% 61%	1600
	11th to 20th	Night	5% 3 1 2 2 3 3 3 6% 8% 64%	1600
	21st to end	Night	5% 2 1 4 2 3 4 4 7% 9% 60%	1600
July	1st to 10th	Night	3 2 1 3 3 3 4 4 6% 7% 64%	3640
	11th to 20th	Night	4 2 1 3 3 3 4 5% 8% 11% 55%	3640
	21st to end	Night	5% 3 1 4 3 3 5% 5% 8% 9% 54%	4101
August	1st to 10th	Night	6% 4 1 5% 3 4 4 4 8% 9% 52%	4033
	11th to 20th	Night	3 2 1 3 3 4 4 5% 7% 11% 58%	4474
	21st to end	Night	7% 4 2 6% 4 5% 4 4 7% 9% 48%	4993
September	1st to 10th	Night	6% 3 1 4 3 4 3 3 6% 6% 61%	2000
	11th to 20th	Night	7% 2 1 3 2 2 3 3 4 8% 65%	2017
	21st to end	Night	8% 3 1 4 3 3 3 2 4 5% 64%	2200
October	1st to 10th	Night	5% 3 1 3 1 2 2 2 3 4 74%	3909
	11th to 20th	Night	5% 4 2 4 2 2 3 2 4 6% 66%	3898
	21st to end	Night	5% 6% 1 3 2 2 2 2 3 5% 67%	4335
November	1st to 10th	Night	3 4 1 3 3 2 4 3 4 6% 67%	2562
	11th to 20th	Night	3 6% 1 5% 3 2 3 3 3 5% 67%	2600
	21st to end	Night	3 6% 3 4 2 3 3 3 5% 7% 62%	2705
December	1st to 10th	Night	1 4 2 4 2 2 2 1 3 5% 73%	4570
	11th to 20th	Night	1 4 2 4 2 3 2 2 4 8% 67%	5181
	21st to end	Night	3 7% 3 5% 4 4 4 4 5% 7% 53%	6974

Transparency (when cloud cover <= 30%)
Color Meanings: Too cloudy to forecast Poor Below Average Average Above average Transparent
Month	Days in Month		Occurrence of each Forecast	Data Points
January	1st to 10th	Day	21% 6% 10% 32% 22% 9%	3639
	11th to 20th	Day	25% 6% 10% 22% 27% 10%	3774
	21st to end	Day	22% 6% 8% 27% 25% 12%	4510
February	1st to 10th	Day	28% 6% 12% 25% 21% 9%	1800
	11th to 20th	Day	23% 6% 10% 27% 26% 7%	1927
	21st to end	Day	23% 6% 10% 21% 27% 13%	1650
March	1st to 10th	Day	26% 6% 6% 18% 29% 15%	4500
	11th to 20th	Day	31% 10% 10% 19% 26% 5%	4953
	21st to end	Day	26% 8% 12% 34% 16% 2	5810
April	1st to 10th	Day	26% 7% 13% 25% 21% 9%	2400
	11th to 20th	Day	34% 9% 12% 18% 19% 8%	2374
	21st to end	Day	32% 10% 14% 23% 19% 2	2400
May	1st to 10th	Day	34% 9% 13% 24% 15% 5%	5713
	11th to 20th	Day	32% 9% 12% 19% 18% 9%	6302
	21st to end	Day	29% 9% 15% 22% 19% 5%	6926
June	1st to 10th	Day	26% 12% 16% 22% 18% 7%	2800
	11th to 20th	Day	26% 10% 11% 18% 21% 14%	2800
	21st to end	Day	27% 9% 13% 19% 21% 12%	2800
July	1st to 10th	Day	24% 5% 6% 16% 28% 20%	6271
	11th to 20th	Day	29% 5% 14% 23% 19% 10%	6265
	21st to end	Day	33% 6% 10% 18% 18% 15%	6559
August	1st to 10th	Day	38% 8% 14% 20% 15% 5%	4208
	11th to 20th	Day	29% 6% 15% 16% 21% 13%	3890
	21st to end	Day	29% 6% 11% 19% 24% 11%	4311
September	1st to 10th	Day	23% 6% 9% 21% 25% 16%	2400
	11th to 20th	Day	22% 7% 10% 20% 27% 14%	2382
	21st to end	Day	21% 6% 7% 20% 33% 13%	2200
October	1st to 10th	Day	15% 4 6% 15% 35% 24%	3279
	11th to 20th	Day	26% 3 8% 26% 30% 8%	3250
	21st to end	Day	21% 3 5% 24% 36% 11%	3431
November	1st to 10th	Day	18% 4 10% 23% 29% 15%	1781
	11th to 20th	Day	18% 4 10% 25% 32% 10%	1800
	21st to end	Day	23% 4 10% 22% 29% 13%	1690
December	1st to 10th	Day	17% 4 8% 34% 28% 10%	2620
	11th to 20th	Day	15% 4 9% 30% 33% 11%	3024
	21st to end	Day	25% 6% 9% 24% 27% 9%	4004
January	1st to 10th	Night	26% 4 8% 28% 25% 8%	6327
	11th to 20th	Night	20% 5% 10% 25% 23% 17%	6248
	21st to end	Night	25% 6% 9% 22% 25% 13%	6468
February	1st to 10th	Night	26% 5% 11% 27% 24% 8%	2600
	11th to 20th	Night	29% 4 10% 26% 23% 7%	2368
	21st to end	Night	24% 4 7% 22% 27% 15%	1980
March	1st to 10th	Night	27% 5% 7% 16% 34% 11%	5374
	11th to 20th	Night	29% 6% 10% 15% 26% 14%	5030
	21st to end	Night	30% 5% 10% 33% 19% 3	5150
April	1st to 10th	Night	25% 6% 12% 23% 23% 10%	2000
	11th to 20th	Night	31% 7% 11% 22% 20% 8%	1977
	21st to end	Night	35% 6% 11% 24% 20% 4	2000
May	1st to 10th	Night	34% 8% 15% 26% 12% 7%	4245
	11th to 20th	Night	31% 8% 14% 21% 19% 7%	3618
	21st to end	Night	25% 7% 19% 25% 19% 5%	3972
June	1st to 10th	Night	19% 9% 19% 30% 15% 9%	1600
	11th to 20th	Night	18% 6% 14% 20% 24% 18%	1600
	21st to end	Night	18% 7% 12% 25% 27% 12%	1600
July	1st to 10th	Night	19% 4 10% 16% 35% 15%	3600
	11th to 20th	Night	22% 5% 17% 28% 20% 8%	3592
	21st to end	Night	24% 5% 17% 22% 19% 13%	3985
August	1st to 10th	Night	32% 6% 16% 22% 18% 5%	2928
	11th to 20th	Night	19% 6% 21% 22% 21% 10%	3234
	21st to end	Night	25% 6% 14% 22% 26% 6%	3597
September	1st to 10th	Night	23% 5% 11% 22% 24% 16%	2000
	11th to 20th	Night	18% 5% 11% 23% 27% 15%	2017
	21st to end	Night	23% 4 8% 21% 31% 13%	2200
October	1st to 10th	Night	15% 4 6% 19% 30% 25%	3909
	11th to 20th	Night	21% 2 8% 26% 32% 10%	3898
	21st to end	Night	21% 3 5% 22% 35% 14%	4335
November	1st to 10th	Night	17% 5% 10% 23% 29% 16%	2562
	11th to 20th	Night	19% 4 8% 25% 33% 10%	2600
	21st to end	Night	23% 5% 9% 21% 31% 12%	2705
December	1st to 10th	Night	15% 3 8% 30% 35% 9%	4570
	11th to 20th	Night	17% 4 7% 29% 36% 7%	5182
	21st to end	Night	28% 6% 9% 22% 25% 10%	6974

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
Month	Days in Month		Occurrence of each Forecast	Data Points
January	1st to 10th	Day	5% 10% 31% 40% 12% 2	300
	11th to 20th	Day	7% 12% 41% 33% 7%	300
	21st to end	Day	4 12% 45% 29% 8% 1	330
February	1st to 10th	Day	5% 19% 49% 22% 5% 1	300
	11th to 20th	Day	4 11% 47% 33% 4 1	300
	21st to end	Day	4 15% 48% 29% 3	249
March	1st to 10th	Day	3 14% 43% 29% 10% 1	300
	11th to 20th	Day	11% 18% 34% 28% 7% 1	479
	21st to end	Day	12% 22% 35% 24% 5% 1	550
April	1st to 10th	Day	11% 20% 36% 25% 7% 1	500
	11th to 20th	Day	15% 24% 35% 20% 5% 1	492
	21st to end	Day	14% 16% 34% 26% 10% 1	500
May	1st to 10th	Day	15% 14% 31% 29% 10% 2	620
	11th to 20th	Day	17% 7% 28% 30% 15% 3	725
	21st to end	Day	9% 7% 30% 32% 17% 4	789
June	1st to 10th	Day	6% 5% 25% 35% 24% 6%	730
	11th to 20th	Day	6% 7% 29% 36% 18% 5%	730
	21st to end	Day	6% 4 28% 36% 20% 6%	730
July	1st to 10th	Day	6% 3 20% 33% 30% 8%	721
	11th to 20th	Day	5% 2 18% 34% 33% 9%	718
	21st to end	Day	6% 2 16% 33% 35% 8%	733
August	1st to 10th	Day	6% 5% 25% 33% 24% 7%	527
	11th to 20th	Day	4 3 23% 37% 25% 8%	517
	21st to end	Day	7% 3 23% 39% 21% 7%	583
September	1st to 10th	Day	6% 4 22% 38% 23% 7%	530
	11th to 20th	Day	6% 4 29% 34% 23% 4	512
	21st to end	Day	8% 5% 28% 30% 23% 6%	330
October	1st to 10th	Day	10% 5% 27% 30% 23% 5%	330
	11th to 20th	Day	5% 11% 31% 41% 9% 4	330
	21st to end	Day	8% 12% 35% 31% 12% 2	363
November	1st to 10th	Day	6% 8% 38% 31% 15% 2	306
	11th to 20th	Day	8% 12% 28% 42% 8% 3	300
	21st to end	Day	6% 11% 39% 34% 9% 2	300
December	1st to 10th	Day	5% 13% 37% 31% 11% 3	300
	11th to 20th	Day	5% 11% 42% 36% 5%	300
	21st to end	Day	5% 11% 46% 30% 6% 1	330
January	1st to 10th	Night	3 11% 40% 35% 9% 2	1000
	11th to 20th	Night	4 13% 46% 28% 8% 1	960
	21st to end	Night	4 10% 42% 36% 7%	880
February	1st to 10th	Night	5% 11% 49% 31% 4 1	800
	11th to 20th	Night	3 10% 47% 35% 5%	784
	21st to end	Night	6% 9% 46% 36% 2	660
March	1st to 10th	Night	4 8% 44% 37% 7%	800
	11th to 20th	Night	9% 6% 37% 38% 9% 1	800
	21st to end	Night	10% 8% 42% 35% 4 1	735
April	1st to 10th	Night	11% 9% 41% 36% 5%	600
	11th to 20th	Night	13% 10% 35% 33% 8%	593
	21st to end	Night	15% 7% 35% 33% 9% 1	600
May	1st to 10th	Night	14% 5% 29% 39% 12% 2	600
	11th to 20th	Night	15% 4 22% 46% 13% 2	591
	21st to end	Night	10% 5% 25% 39% 20% 2	650
June	1st to 10th	Night	6% 3 17% 49% 23% 3	600
	11th to 20th	Night	7% 2 24% 51% 15% 2	600
	21st to end	Night	4 2 18% 48% 27% 1	600
July	1st to 10th	Night	5% 2 13% 42% 35% 3	585
	11th to 20th	Night	2 1 11% 46% 36% 3	582
	21st to end	Night	6% 1 7% 45% 36% 5%	657
August	1st to 10th	Night	5% 2 12% 45% 34% 2	597
	11th to 20th	Night	5% 1 12% 52% 28% 3	594
	21st to end	Night	5% 2 11% 49% 30% 4	660
September	1st to 10th	Night	6% 2 15% 49% 24% 6%	600
	11th to 20th	Night	8% 2 19% 47% 20% 5%	600
	21st to end	Night	7% 3 22% 46% 18% 4	600
October	1st to 10th	Night	11% 3 26% 42% 15% 3	789
	11th to 20th	Night	5% 4 31% 46% 12% 2	800
	21st to end	Night	9% 6% 34% 38% 11% 2	927
November	1st to 10th	Night	4 5% 38% 41% 11% 1	986
	11th to 20th	Night	5% 10% 38% 40% 5% 1	1000
	21st to end	Night	6% 6% 38% 42% 8%	1000
December	1st to 10th	Night	3 10% 39% 41% 6%	1000
	11th to 20th	Night	4 8% 48% 33% 6% 1	1000
	21st to end	Night	5% 11% 42% 35% 6% 1	1100

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