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 2024-11-06.

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 4 4 6% 8% 53%	105174
	Night	4 5% 2 5% 3 3 3 3 5% 7% 59%	102578
Transparency	Day	26% 8% 12% 22% 23% 10%	104919
	Night	23% 6% 12% 24% 25% 10%	102424
Seeing	Day	8% 9% 30% 32% 17% 4	15591
	Night	6% 6% 32% 40% 14% 2	25597

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 5% 2 5% 3 3 3 3 6% 8% 60%	2538
	11th to 20th	Day	2 4 3 6% 4 4 4 3 5% 8% 56%	2632
	21st to end	Day	2 6% 2 6% 3 3 3 3 5% 8% 59%	3146
February	1st to 10th	Day	2 8% 3 7% 4 4 4 3 5% 6% 55%	1710
	11th to 20th	Day	1 5% 2 5% 4 3 5% 3 6% 9% 58%	1818
	21st to end	Day	2 7% 2 6% 3 3 3 4 6% 5% 58%	1570
March	1st to 10th	Day	3 9% 3 5% 3 2 3 3 5% 6% 56%	3130
	11th to 20th	Day	5% 10% 2 6% 4 4 4 4 5% 7% 49%	3457
	21st to end	Day	4 7% 2 5% 4 4 5% 4 7% 9% 50%	4058
April	1st to 10th	Day	5% 7% 1 5% 3 3 4 3 7% 8% 55%	2280
	11th to 20th	Day	9% 9% 2 6% 4 3 4 4 5% 7% 47%	2251
	21st to end	Day	9% 8% 1 5% 4 4 5% 4 6% 8% 46%	2280
May	1st to 10th	Day	9% 7% 3 7% 5% 5% 5% 5% 8% 7% 39%	3989
	11th to 20th	Day	10% 7% 2 6% 4 4 6% 5% 8% 9% 40%	4388
	21st to end	Day	5% 4 1 4 4 4 5% 5% 8% 9% 50%	4825
June	1st to 10th	Day	5% 3 1 4 4 5% 5% 5% 8% 10% 49%	2660
	11th to 20th	Day	6% 3 1 4 4 4 6% 5% 8% 9% 51%	2660
	21st to end	Day	5% 3 1 4 4 5% 7% 5% 8% 9% 48%	2660
July	1st to 10th	Day	5% 3 1 4 4 4 4 4 7% 8% 56%	4430
	11th to 20th	Day	3 3 1 4 4 5% 6% 5% 10% 11% 47%	4430
	21st to end	Day	5% 3 2 5% 4 4 5% 5% 6% 8% 53%	4695
August	1st to 10th	Day	9% 4 2 6% 5% 5% 7% 6% 9% 10% 37%	4092
	11th to 20th	Day	3 3 1 4 5% 5% 6% 6% 8% 9% 49%	3734
	21st to end	Day	5% 3 1 5% 4 5% 6% 5% 8% 9% 50%	4180
September	1st to 10th	Day	5% 3 1 5% 4 3 4 4 5% 7% 60%	2280
	11th to 20th	Day	6% 3 1 4 3 3 4 4 5% 7% 59%	2263
	21st to end	Day	5% 4 1 4 2 3 3 2 4 6% 65%	2090
October	1st to 10th	Day	3 3 1 2 2 2 3 2 3 5% 73%	3179
	11th to 20th	Day	5% 4 2 5% 3 4 4 3 5% 7% 59%	3150
	21st to end	Day	6% 6% 1 3 2 2 2 2 4 7% 66%	3325
November	1st to 10th	Day	2 5% 1 3 2 3 3 3 6% 8% 66%	1718
	11th to 20th	Day	2 5% 1 4 3 2 3 3 5% 7% 64%	1710
	21st to end	Day	3 5% 3 5% 3 3 3 3 5% 7% 60%	1606
December	1st to 10th	Day	1 6% 1 5% 3 3 4 3 5% 10% 59%	1520
	11th to 20th	Day	2 5% 2 5% 3 3 3 3 6% 9% 60%	1926
	21st to end	Day	2 6% 3 8% 4 5% 4 4 5% 7% 53%	2794
January	1st to 10th	Night	1 7% 3 7% 4 3 4 3 5% 8% 56%	4417
	11th to 20th	Night	1 4 2 5% 3 4 4 3 5% 8% 62%	4366
	21st to end	Night	3 7% 2 7% 4 4 4 3 4 6% 57%	4521
February	1st to 10th	Night	2 8% 2 6% 4 4 4 3 4 7% 56%	2470
	11th to 20th	Night	2 7% 3 7% 4 4 4 3 5% 6% 56%	2247
	21st to end	Night	2 7% 2 7% 3 3 3 3 4 5% 60%	1884
March	1st to 10th	Night	4 8% 3 5% 3 3 3 3 5% 6% 58%	3734
	11th to 20th	Night	8% 12% 2 5% 3 3 3 2 3 6% 52%	3507
	21st to end	Night	5% 9% 2 5% 3 3 3 3 5% 7% 54%	3587
April	1st to 10th	Night	6% 7% 1 4 3 3 3 3 5% 7% 58%	1900
	11th to 20th	Night	10% 8% 3 4 3 3 4 3 5% 6% 52%	1877
	21st to end	Night	12% 7% 2 5% 3 3 3 2 5% 6% 49%	1900
May	1st to 10th	Night	11% 6% 2 5% 4 4 4 3 5% 7% 47%	2962
	11th to 20th	Night	11% 5% 2 6% 4 3 4 3 6% 8% 49%	2517
	21st to end	Night	7% 4 2 4 3 3 3 3 5% 7% 59%	2762
June	1st to 10th	Night	5% 3 1 3 3 3 3 3 6% 9% 61%	1520
	11th to 20th	Night	6% 3 1 2 2 3 2 3 6% 8% 63%	1520
	21st to end	Night	5% 2 1 3 2 3 4 4 7% 10% 59%	1520
July	1st to 10th	Night	3 3 1 3 3 3 4 4 6% 7% 63%	2540
	11th to 20th	Night	2 2 1 3 3 4 4 4 8% 10% 58%	2540
	21st to end	Night	4 2 1 3 2 3 4 5% 8% 9% 59%	2850
August	1st to 10th	Night	7% 5% 1 6% 3 5% 5% 4 9% 10% 45%	2847
	11th to 20th	Night	3 2 1 3 3 3 4 5% 8% 12% 56%	3104
	21st to end	Night	3 2 1 5% 4 5% 5% 4 7% 10% 53%	3487
September	1st to 10th	Night	6% 3 1 4 3 4 3 3 6% 6% 60%	1900
	11th to 20th	Night	7% 2 1 2 2 2 3 3 4 8% 65%	1916
	21st to end	Night	7% 3 1 4 3 3 3 2 4 5% 65%	2090
October	1st to 10th	Night	4 3 1 3 1 2 2 2 3 4 75%	3789
	11th to 20th	Night	5% 4 2 4 2 2 3 2 4 7% 65%	3778
	21st to end	Night	5% 6% 1 3 2 2 2 2 3 5% 67%	4200
November	1st to 10th	Night	3 4 1 3 2 2 3 3 4 7% 68%	2483
	11th to 20th	Night	3 6% 1 5% 3 2 3 3 4 5% 66%	2470
	21st to end	Night	3 6% 3 5% 2 3 3 3 5% 6% 62%	2569
December	1st to 10th	Night	1 5% 2 6% 4 3 3 2 4 8% 61%	2660
	11th to 20th	Night	2 6% 2 5% 3 3 3 3 5% 9% 59%	3271
	21st to end	Night	3 6% 2 5% 4 4 5% 4 6% 7% 53%	4873

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	19% 8% 14% 27% 23% 9%	2539
	11th to 20th	Day	24% 8% 15% 29% 18% 7%	2632
	21st to end	Day	23% 8% 12% 28% 23% 5%	3146
February	1st to 10th	Day	28% 6% 12% 25% 20% 8%	1710
	11th to 20th	Day	22% 7% 10% 27% 27% 7%	1828
	21st to end	Day	24% 6% 10% 21% 26% 12%	1570
March	1st to 10th	Day	26% 8% 9% 20% 28% 9%	3130
	11th to 20th	Day	30% 13% 14% 25% 15% 3	3457
	21st to end	Day	25% 12% 16% 27% 16% 4	4058
April	1st to 10th	Day	26% 7% 13% 24% 21% 9%	2280
	11th to 20th	Day	34% 9% 13% 18% 18% 8%	2254
	21st to end	Day	32% 10% 14% 23% 19% 2	2280
May	1st to 10th	Day	33% 13% 15% 20% 14% 6%	3989
	11th to 20th	Day	32% 13% 16% 22% 15% 2	4392
	21st to end	Day	22% 13% 16% 22% 20% 6%	4825
June	1st to 10th	Day	25% 12% 16% 22% 17% 8%	2660
	11th to 20th	Day	25% 10% 12% 18% 21% 14%	2660
	21st to end	Day	26% 9% 13% 19% 21% 11%	2660
July	1st to 10th	Day	25% 7% 9% 15% 30% 15%	4361
	11th to 20th	Day	26% 7% 11% 22% 22% 11%	4355
	21st to end	Day	27% 8% 10% 18% 20% 17%	4547
August	1st to 10th	Day	38% 9% 14% 20% 14% 5%	4078
	11th to 20th	Day	29% 6% 15% 16% 20% 14%	3770
	21st to end	Day	28% 6% 11% 19% 24% 12%	4179
September	1st to 10th	Day	24% 6% 9% 20% 25% 17%	2280
	11th to 20th	Day	22% 7% 11% 19% 27% 14%	2263
	21st to end	Day	20% 6% 7% 20% 34% 13%	2090
October	1st to 10th	Day	15% 4 7% 15% 35% 25%	3179
	11th to 20th	Day	26% 3 8% 26% 30% 7%	3150
	21st to end	Day	21% 3 5% 24% 35% 12%	3325
November	1st to 10th	Day	17% 4 11% 24% 29% 15%	1718
	11th to 20th	Day	19% 5% 10% 26% 32% 8%	1710
	21st to end	Day	23% 4 10% 21% 28% 13%	1606
December	1st to 10th	Day	21% 6% 13% 29% 22% 9%	1520
	11th to 20th	Day	19% 5% 13% 26% 29% 7%	1924
	21st to end	Day	26% 8% 12% 21% 25% 8%	2794
January	1st to 10th	Night	25% 6% 11% 26% 27% 5%	4417
	11th to 20th	Night	16% 7% 14% 29% 23% 11%	4366
	21st to end	Night	26% 8% 12% 28% 20% 6%	4521
February	1st to 10th	Night	26% 5% 12% 27% 23% 7%	2470
	11th to 20th	Night	28% 4 10% 26% 25% 8%	2247
	21st to end	Night	25% 4 8% 23% 27% 14%	1884
March	1st to 10th	Night	22% 7% 10% 18% 34% 8%	3734
	11th to 20th	Night	31% 8% 14% 21% 22% 4	3507
	21st to end	Night	26% 8% 13% 32% 18% 3	3587
April	1st to 10th	Night	25% 6% 13% 22% 23% 10%	1900
	11th to 20th	Night	31% 7% 12% 23% 19% 8%	1877
	21st to end	Night	34% 6% 11% 24% 20% 4	1900
May	1st to 10th	Night	30% 11% 14% 22% 14% 10%	2962
	11th to 20th	Night	30% 12% 17% 22% 17% 2	2518
	21st to end	Night	21% 9% 21% 22% 20% 7%	2762
June	1st to 10th	Night	18% 9% 19% 30% 15% 9%	1520
	11th to 20th	Night	18% 6% 15% 19% 24% 18%	1520
	21st to end	Night	18% 8% 13% 24% 27% 11%	1520
July	1st to 10th	Night	19% 6% 12% 18% 32% 12%	2500
	11th to 20th	Night	19% 7% 14% 27% 23% 10%	2492
	21st to end	Night	19% 6% 15% 24% 19% 16%	2761
August	1st to 10th	Night	32% 7% 16% 23% 17% 5%	2838
	11th to 20th	Night	19% 7% 22% 21% 21% 10%	3134
	21st to end	Night	25% 6% 14% 22% 27% 6%	3487
September	1st to 10th	Night	23% 5% 11% 21% 23% 16%	1900
	11th to 20th	Night	18% 5% 12% 23% 27% 16%	1916
	21st to end	Night	22% 4 8% 21% 32% 13%	2090
October	1st to 10th	Night	15% 4 6% 19% 30% 25%	3789
	11th to 20th	Night	21% 2 8% 27% 32% 10%	3778
	21st to end	Night	22% 3 5% 22% 34% 14%	4200
November	1st to 10th	Night	17% 5% 10% 23% 29% 16%	2483
	11th to 20th	Night	20% 4 9% 26% 32% 9%	2470
	21st to end	Night	23% 5% 9% 20% 31% 12%	2569
December	1st to 10th	Night	22% 5% 13% 24% 26% 10%	2660
	11th to 20th	Night	21% 6% 11% 25% 30% 6%	3272
	21st to end	Night	26% 8% 12% 22% 23% 8%	4873

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	4 10% 33% 39% 12% 2	270
	11th to 20th	Day	7% 13% 41% 33% 7%	270
	21st to end	Day	4 11% 45% 30% 8% 1	297
February	1st to 10th	Day	5% 19% 48% 21% 5% 1	270
	11th to 20th	Day	3 12% 47% 32% 4 1	270
	21st to end	Day	4 15% 47% 31% 3	225
March	1st to 10th	Day	3 15% 43% 28% 10% 1	270
	11th to 20th	Day	11% 18% 34% 29% 8% 1	440
	21st to end	Day	12% 22% 36% 24% 6%	506
April	1st to 10th	Day	11% 21% 36% 24% 7% 1	460
	11th to 20th	Day	15% 25% 34% 20% 5% 1	452
	21st to end	Day	14% 16% 34% 25% 10% 1	460
May	1st to 10th	Day	15% 14% 32% 29% 9% 1	574
	11th to 20th	Day	17% 7% 27% 31% 16% 3	675
	21st to end	Day	8% 7% 30% 32% 18% 4	734
June	1st to 10th	Day	6% 5% 24% 34% 24% 7%	680
	11th to 20th	Day	6% 7% 29% 36% 17% 5%	680
	21st to end	Day	5% 4 28% 36% 20% 6%	680
July	1st to 10th	Day	7% 3 20% 32% 31% 8%	671
	11th to 20th	Day	5% 1 17% 34% 33% 9%	668
	21st to end	Day	6% 2 16% 33% 35% 8%	681
August	1st to 10th	Day	7% 5% 23% 33% 25% 8%	487
	11th to 20th	Day	5% 3 24% 37% 24% 7%	477
	21st to end	Day	7% 3 24% 39% 20% 7%	539
September	1st to 10th	Day	6% 4 21% 38% 24% 7%	490
	11th to 20th	Day	6% 4 27% 35% 23% 4	473
	21st to end	Day	7% 6% 29% 30% 22% 6%	300
October	1st to 10th	Day	9% 5% 27% 30% 24% 6%	300
	11th to 20th	Day	3 10% 30% 43% 10% 4	300
	21st to end	Day	9% 12% 34% 30% 14% 2	330
November	1st to 10th	Day	5% 8% 38% 31% 16% 2	285
	11th to 20th	Day	8% 13% 27% 42% 7% 3	270
	21st to end	Day	6% 12% 40% 31% 9% 2	270
December	1st to 10th	Day	6% 14% 39% 31% 9% 1	270
	11th to 20th	Day	5% 11% 42% 35% 6%	270
	21st to end	Day	5% 12% 47% 28% 7% 1	297
January	1st to 10th	Night	3 12% 39% 35% 9% 2	950
	11th to 20th	Night	4 13% 46% 27% 8% 1	912
	21st to end	Night	4 10% 41% 37% 7%	836
February	1st to 10th	Night	5% 11% 48% 31% 5% 1	760
	11th to 20th	Night	3 10% 45% 36% 5%	744
	21st to end	Night	6% 9% 46% 37% 3	628
March	1st to 10th	Night	3 9% 44% 37% 7%	760
	11th to 20th	Night	9% 6% 35% 40% 10% 1	760
	21st to end	Night	10% 8% 42% 35% 5% 1	698
April	1st to 10th	Night	11% 9% 39% 36% 5%	570
	11th to 20th	Night	13% 10% 35% 33% 9%	563
	21st to end	Night	15% 6% 36% 33% 9% 1	570
May	1st to 10th	Night	14% 5% 29% 39% 12% 2	570
	11th to 20th	Night	15% 4 20% 46% 13% 2	561
	21st to end	Night	9% 5% 24% 39% 21% 2	617
June	1st to 10th	Night	5% 3 17% 49% 24% 3	570
	11th to 20th	Night	7% 2 24% 50% 15% 2	570
	21st to end	Night	4 2 16% 48% 28% 1	570
July	1st to 10th	Night	5% 2 12% 41% 36% 3	555
	11th to 20th	Night	2 1 11% 45% 38% 4	552
	21st to end	Night	6% 1 8% 43% 37% 5%	624
August	1st to 10th	Night	5% 2 10% 45% 36% 2	567
	11th to 20th	Night	5% 1 12% 51% 28% 3	564
	21st to end	Night	4 2 11% 48% 31% 4	627
September	1st to 10th	Night	6% 2 13% 48% 25% 6%	570
	11th to 20th	Night	7% 2 18% 47% 21% 5%	570
	21st to end	Night	6% 3 22% 46% 18% 4	570
October	1st to 10th	Night	11% 3 26% 42% 15% 4	749
	11th to 20th	Night	4 4 30% 47% 12% 2	760
	21st to end	Night	9% 6% 33% 39% 11% 2	880
November	1st to 10th	Night	4 5% 37% 41% 12% 1	955
	11th to 20th	Night	5% 11% 37% 41% 6% 1	950
	21st to end	Night	6% 6% 37% 42% 9%	950
December	1st to 10th	Night	3 11% 39% 41% 6%	950
	11th to 20th	Night	4 8% 47% 34% 7% 1	950
	21st to end	Night	5% 11% 42% 35% 6% 1	1045

Page generated at 2024-11-03 11:18:59. Version ac07

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