Raleigh
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Raleigh clear sky chart from 2005-11-06 to 2026-03-18.

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	13% 6% 2 6% 4 4 4 4 6% 8% 43%	130625
	Night	14% 6% 2 6% 4 4 4 3 5% 7% 45%	130904
Transparency	Day	38% 6% 12% 20% 18% 5%	134229
	Night	38% 6% 10% 19% 20% 6%	135371
Seeing	Day	16% 10% 24% 26% 18% 7%	17112
	Night	16% 8% 26% 32% 17% 2	27561

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
Month	Days in Month		Occurrence of each Forecast	Data Points
January	1st to 10th	Day	14% 6% 2 4 4 3 3 2 3 6% 54%	5258
	11th to 20th	Day	12% 5% 2 4 3 2 3 2 3 4 60%	5280
	21st to end	Day	16% 8% 3 7% 4 4 4 3 4 6% 43%	5780
February	1st to 10th	Day	14% 5% 2 6% 3 4 4 3 5% 6% 45%	1890
	11th to 20th	Day	21% 7% 2 4 4 3 3 3 5% 5% 45%	1874
	21st to end	Day	19% 7% 3 5% 4 3 4 3 5% 5% 42%	1870
March	1st to 10th	Day	20% 7% 2 6% 3 4 4 4 5% 6% 40%	5045
	11th to 20th	Day	11% 5% 2 5% 4 4 3 3 4 6% 54%	5057
	21st to end	Day	14% 6% 2 5% 4 4 4 4 5% 6% 47%	5465
April	1st to 10th	Day	10% 4 2 5% 4 3 3 4 5% 8% 53%	2336
	11th to 20th	Day	11% 4 2 5% 4 5% 4 4 5% 7% 49%	2374
	21st to end	Day	13% 5% 2 6% 4 4 4 4 6% 7% 45%	2400
May	1st to 10th	Day	9% 6% 3 8% 6% 5% 6% 5% 7% 10% 36%	5759
	11th to 20th	Day	18% 7% 3 8% 5% 5% 5% 5% 7% 9% 29%	5861
	21st to end	Day	12% 5% 2 6% 4 5% 5% 4 7% 8% 43%	6433
June	1st to 10th	Day	9% 6% 2 6% 5% 5% 6% 5% 7% 10% 39%	2600
	11th to 20th	Day	9% 5% 2 5% 4 5% 7% 5% 7% 10% 42%	2600
	21st to end	Day	7% 4 2 6% 6% 6% 5% 5% 8% 11% 39%	2600
July	1st to 10th	Day	9% 5% 2 7% 6% 6% 7% 5% 10% 11% 32%	5900
	11th to 20th	Day	8% 7% 2 8% 6% 7% 6% 6% 9% 13% 27%	5900
	21st to end	Day	8% 6% 3 7% 6% 6% 7% 5% 8% 11% 33%	6424
August	1st to 10th	Day	20% 8% 4 9% 5% 5% 6% 5% 8% 9% 21%	4234
	11th to 20th	Day	9% 6% 3 8% 6% 6% 6% 5% 8% 10% 35%	4014
	21st to end	Day	6% 5% 2 6% 4 5% 4 4 6% 10% 49%	4312
September	1st to 10th	Day	9% 4 2 5% 5% 4 5% 5% 7% 9% 47%	2400
	11th to 20th	Day	14% 6% 2 6% 4 5% 4 3 5% 8% 41%	2288
	21st to end	Day	20% 6% 3 7% 4 3 4 4 5% 8% 37%	2144
October	1st to 10th	Day	13% 7% 2 6% 3 3 3 3 5% 6% 49%	2000
	11th to 20th	Day	12% 4 2 4 3 3 3 3 4 6% 57%	2000
	21st to end	Day	12% 4 2 5% 3 4 3 2 4 5% 57%	2200
November	1st to 10th	Day	13% 5% 2 5% 3 3 3 3 4 5% 55%	2057
	11th to 20th	Day	13% 5% 2 4 3 3 3 3 5% 7% 52%	2100
	21st to end	Day	13% 5% 1 6% 3 3 4 3 5% 6% 52%	1739
December	1st to 10th	Day	12% 6% 2 6% 4 3 4 3 4 5% 52%	2680
	11th to 20th	Day	16% 6% 2 6% 4 4 4 3 4 5% 47%	3186
	21st to end	Day	15% 6% 3 6% 4 3 4 3 5% 6% 46%	4565
January	1st to 10th	Night	10% 6% 2 5% 3 3 4 3 4 6% 53%	7635
	11th to 20th	Night	13% 6% 2 5% 3 3 2 2 3 4 57%	7680
	21st to end	Night	17% 8% 2 6% 4 3 4 3 4 5% 45%	8401
February	1st to 10th	Night	17% 6% 2 5% 4 4 3 4 5% 6% 45%	2730
	11th to 20th	Night	20% 7% 2 6% 3 3 4 3 4 5% 42%	2631
	21st to end	Night	16% 6% 1 5% 3 3 4 3 6% 7% 46%	1933
March	1st to 10th	Night	18% 7% 2 7% 4 4 3 3 4 4 42%	5028
	11th to 20th	Night	12% 6% 2 4 3 3 3 3 4 7% 53%	5058
	21st to end	Night	13% 5% 2 5% 4 4 4 3 5% 6% 50%	5467
April	1st to 10th	Night	15% 4 2 5% 4 3 3 3 5% 7% 48%	2062
	11th to 20th	Night	12% 5% 2 5% 3 4 3 4 5% 6% 50%	1975
	21st to end	Night	10% 3 2 6% 5% 4 5% 5% 5% 7% 48%	2000
May	1st to 10th	Night	11% 8% 3 8% 5% 4 5% 4 5% 9% 37%	4177
	11th to 20th	Night	19% 8% 3 8% 4 5% 3 4 5% 8% 33%	4032
	21st to end	Night	15% 5% 2 5% 4 3 4 4 6% 9% 43%	4421
June	1st to 10th	Night	11% 5% 3 6% 5% 5% 5% 6% 7% 9% 39%	1800
	11th to 20th	Night	10% 6% 2 6% 4 4 5% 4 8% 8% 41%	1800
	21st to end	Night	7% 6% 3 8% 4 5% 4 5% 9% 11% 38%	1800
July	1st to 10th	Night	11% 7% 3 8% 5% 5% 6% 5% 8% 11% 31%	4058
	11th to 20th	Night	12% 7% 3 8% 6% 7% 7% 5% 8% 12% 26%	4057
	21st to end	Night	12% 8% 3 8% 5% 5% 6% 5% 7% 10% 31%	4427
August	1st to 10th	Night	22% 8% 3 7% 5% 5% 6% 5% 7% 9% 23%	2915
	11th to 20th	Night	8% 6% 2 7% 6% 6% 6% 5% 7% 11% 34%	3001
	21st to end	Night	6% 4 2 6% 4 4 4 4 7% 10% 47%	3586
September	1st to 10th	Night	11% 5% 2 6% 3 4 4 4 6% 8% 46%	2000
	11th to 20th	Night	13% 6% 2 7% 4 4 4 4 5% 7% 43%	2108
	21st to end	Night	20% 8% 3 7% 5% 4 4 3 5% 6% 37%	2255
October	1st to 10th	Night	13% 6% 2 6% 4 3 3 3 5% 6% 50%	2400
	11th to 20th	Night	13% 5% 2 5% 3 3 3 3 3 6% 54%	2400
	21st to end	Night	12% 5% 2 5% 3 3 3 3 5% 5% 55%	2640
November	1st to 10th	Night	13% 6% 2 5% 3 2 3 4 4 5% 53%	2461
	11th to 20th	Night	12% 5% 2 5% 3 3 4 4 4 6% 51%	2520
	21st to end	Night	12% 5% 2 5% 4 4 3 3 5% 6% 52%	2871
December	1st to 10th	Night	12% 6% 2 6% 4 4 3 3 4 4 52%	4720
	11th to 20th	Night	16% 8% 3 8% 5% 4 3 3 4 5% 40%	5222
	21st to end	Night	14% 5% 2 6% 3 3 3 3 4 7% 48%	6633

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	35% 2 10% 21% 27% 5%	5259
	11th to 20th	Day	31% 3 3 14% 38% 11%	5280
	21st to end	Day	44% 3 5% 21% 19% 9%	5780
February	1st to 10th	Day	39% 4 11% 21% 20% 5%	1890
	11th to 20th	Day	43% 4 7% 19% 21% 6%	1883
	21st to end	Day	44% 5% 9% 15% 21% 6%	1870
March	1st to 10th	Day	45% 4 7% 16% 18% 11%	5045
	11th to 20th	Day	32% 5% 7% 16% 25% 16%	5057
	21st to end	Day	39% 4 9% 21% 21% 6%	5465
April	1st to 10th	Day	29% 6% 10% 24% 23% 8%	2336
	11th to 20th	Day	33% 6% 10% 23% 21% 7%	2374
	21st to end	Day	37% 7% 12% 21% 18% 4	2400
May	1st to 10th	Day	40% 9% 17% 21% 10% 1	5793
	11th to 20th	Day	50% 8% 12% 20% 10% 1	5865
	21st to end	Day	36% 7% 13% 23% 18% 3	6447
June	1st to 10th	Day	37% 10% 15% 22% 16% 1	2600
	11th to 20th	Day	33% 11% 16% 23% 15% 2	2600
	21st to end	Day	35% 11% 16% 21% 15% 1	2600
July	1st to 10th	Day	41% 10% 18% 20% 10% 2	5835
	11th to 20th	Day	44% 13% 27% 12% 3 1	5831
	21st to end	Day	41% 9% 19% 21% 9%	6240
August	1st to 10th	Day	56% 8% 17% 13% 5%	4221
	11th to 20th	Day	40% 10% 18% 23% 8% 1	4053
	21st to end	Day	30% 7% 14% 31% 16% 2	4312
September	1st to 10th	Day	33% 10% 14% 19% 20% 4	2400
	11th to 20th	Day	41% 7% 10% 19% 20% 3	2288
	21st to end	Day	45% 6% 10% 20% 14% 4	2144
October	1st to 10th	Day	32% 5% 9% 21% 24% 8%	3260
	11th to 20th	Day	20% 3 5% 21% 27% 24%	3240
	21st to end	Day	24% 3 6% 24% 25% 16%	3538
November	1st to 10th	Day	33% 4 8% 22% 27% 7%	2057
	11th to 20th	Day	33% 4 8% 22% 26% 7%	2100
	21st to end	Day	34% 4 8% 24% 25% 6%	1739
December	1st to 10th	Day	36% 3 8% 21% 25% 7%	2680
	11th to 20th	Day	41% 3 11% 18% 21% 6%	3182
	21st to end	Day	39% 5% 12% 24% 19% 2	4565
January	1st to 10th	Night	33% 3 9% 21% 26% 8%	7636
	11th to 20th	Night	33% 3 5% 15% 34% 10%	7680
	21st to end	Night	43% 3 5% 17% 24% 8%	8401
February	1st to 10th	Night	40% 5% 9% 20% 21% 5%	2730
	11th to 20th	Night	45% 4 8% 16% 21% 7%	2633
	21st to end	Night	37% 5% 8% 19% 23% 8%	1933
March	1st to 10th	Night	45% 4 7% 13% 21% 10%	5028
	11th to 20th	Night	32% 4 7% 15% 24% 18%	5058
	21st to end	Night	36% 4 8% 23% 22% 6%	5467
April	1st to 10th	Night	35% 5% 10% 17% 24% 9%	2062
	11th to 20th	Night	34% 5% 10% 22% 21% 8%	1975
	21st to end	Night	35% 7% 11% 25% 18% 4	2000
May	1st to 10th	Night	42% 8% 17% 18% 13% 2	4206
	11th to 20th	Night	48% 7% 12% 18% 13% 1	4033
	21st to end	Night	36% 7% 14% 23% 17% 2	4429
June	1st to 10th	Night	38% 10% 14% 19% 16% 1	1800
	11th to 20th	Night	38% 9% 17% 19% 16% 2	1800
	21st to end	Night	35% 12% 17% 20% 13% 3	1800
July	1st to 10th	Night	43% 11% 20% 17% 8% 1	4015
	11th to 20th	Night	48% 15% 21% 12% 3 1	4006
	21st to end	Night	46% 9% 16% 20% 9%	4297
August	1st to 10th	Night	54% 10% 17% 14% 4 1	2904
	11th to 20th	Night	41% 10% 20% 22% 6% 1	3028
	21st to end	Night	30% 7% 12% 28% 20% 2	3586
September	1st to 10th	Night	35% 9% 13% 22% 16% 5%	2000
	11th to 20th	Night	40% 6% 12% 19% 20% 3	2108
	21st to end	Night	48% 6% 12% 15% 15% 4	2255
October	1st to 10th	Night	37% 4 8% 21% 24% 6%	3920
	11th to 20th	Night	24% 3 6% 18% 27% 21%	3899
	21st to end	Night	23% 3 7% 27% 24% 16%	4252
November	1st to 10th	Night	34% 4 9% 18% 28% 8%	2461
	11th to 20th	Night	35% 5% 8% 16% 26% 11%	2520
	21st to end	Night	33% 4 7% 22% 25% 8%	2871
December	1st to 10th	Night	36% 4 6% 19% 27% 8%	4720
	11th to 20th	Night	47% 4 9% 17% 17% 8%	5225
	21st to end	Night	37% 4 12% 25% 21% 2	6633

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	21% 22% 37% 15% 4	330
	11th to 20th	Day	23% 25% 33% 14% 5%	330
	21st to end	Day	19% 20% 44% 15% 2	363
February	1st to 10th	Day	19% 16% 39% 21% 4	330
	11th to 20th	Day	35% 16% 35% 12% 2	361
	21st to end	Day	25% 17% 32% 22% 4	446
March	1st to 10th	Day	21% 14% 34% 24% 6%	540
	11th to 20th	Day	21% 18% 37% 18% 5% 1	528
	21st to end	Day	21% 16% 36% 20% 6% 1	550
April	1st to 10th	Day	17% 18% 34% 22% 8% 1	500
	11th to 20th	Day	13% 17% 29% 24% 15% 3	492
	21st to end	Day	17% 13% 26% 25% 18% 2	500
May	1st to 10th	Day	13% 16% 31% 24% 11% 5%	503
	11th to 20th	Day	17% 6% 22% 31% 19% 6%	527
	21st to end	Day	18% 7% 18% 27% 19% 10%	573
June	1st to 10th	Day	12% 5% 22% 32% 22% 8%	530
	11th to 20th	Day	9% 3 22% 28% 26% 12%	530
	21st to end	Day	8% 3 15% 31% 32% 11%	530
July	1st to 10th	Day	9% 3 9% 30% 34% 16%	525
	11th to 20th	Day	7% 2 8% 28% 37% 19%	524
	21st to end	Day	9% 2 13% 31% 32% 14%	576
August	1st to 10th	Day	16% 2 13% 31% 26% 11%	526
	11th to 20th	Day	10% 3 13% 27% 29% 18%	519
	21st to end	Day	11% 3 10% 31% 25% 20%	583
September	1st to 10th	Day	10% 3 18% 27% 28% 15%	530
	11th to 20th	Day	16% 3 14% 28% 27% 12%	530
	21st to end	Day	23% 5% 19% 28% 18% 6%	530
October	1st to 10th	Day	15% 6% 22% 22% 24% 11%	530
	11th to 20th	Day	13% 8% 24% 31% 19% 5%	530
	21st to end	Day	14% 10% 25% 30% 16% 5%	583
November	1st to 10th	Day	17% 9% 28% 29% 14% 3	535
	11th to 20th	Day	16% 12% 31% 29% 9% 2	540
	21st to end	Day	14% 18% 28% 25% 14% 2	257
December	1st to 10th	Day	20% 15% 36% 21% 6% 1	220
	11th to 20th	Day	19% 13% 35% 24% 9%	248
	21st to end	Day	22% 14% 35% 20% 8% 1	363
January	1st to 10th	Night	19% 14% 36% 26% 5%	840
	11th to 20th	Night	18% 14% 36% 26% 5%	840
	21st to end	Night	19% 9% 41% 27% 4	924
February	1st to 10th	Night	20% 9% 41% 27% 3	840
	11th to 20th	Night	24% 15% 39% 21% 1	823
	21st to end	Night	19% 15% 36% 26% 3	692
March	1st to 10th	Night	15% 15% 34% 32% 5%	840
	11th to 20th	Night	20% 12% 36% 25% 7%	829
	21st to end	Night	18% 11% 40% 27% 4	880
April	1st to 10th	Night	17% 15% 34% 27% 7%	800
	11th to 20th	Night	15% 12% 34% 28% 12%	791
	21st to end	Night	13% 5% 37% 36% 10%	800
May	1st to 10th	Night	11% 10% 33% 37% 9%	668
	11th to 20th	Night	18% 5% 24% 36% 17% 1	591
	21st to end	Night	13% 3 15% 41% 26% 2	650
June	1st to 10th	Night	12% 4 18% 43% 22% 1	600
	11th to 20th	Night	13% 3 17% 38% 26% 4	600
	21st to end	Night	7% 3 14% 41% 33% 3	600
July	1st to 10th	Night	9% 2 10% 38% 38% 3	585
	11th to 20th	Night	9% 2 8% 38% 38% 6%	582
	21st to end	Night	7% 1 13% 39% 35% 5%	657
August	1st to 10th	Night	14% 2 8% 41% 32% 3	597
	11th to 20th	Night	12% 3 11% 36% 34% 5%	668
	21st to end	Night	11% 2 11% 32% 39% 5%	880
September	1st to 10th	Night	13% 2 11% 33% 36% 6%	800
	11th to 20th	Night	15% 3 13% 34% 33% 3	800
	21st to end	Night	23% 4 16% 33% 23% 2	800
October	1st to 10th	Night	16% 4 18% 31% 27% 5%	800
	11th to 20th	Night	14% 8% 25% 31% 20% 2	800
	21st to end	Night	14% 7% 26% 31% 20% 2	880
November	1st to 10th	Night	16% 7% 25% 35% 14% 3	820
	11th to 20th	Night	15% 10% 30% 34% 12%	840
	21st to end	Night	14% 9% 32% 32% 13% 1	840
December	1st to 10th	Night	20% 10% 32% 32% 6%	840
	11th to 20th	Night	20% 7% 32% 35% 7%	840
	21st to end	Night	19% 11% 30% 32% 9%	924

Page generated at 2026-03-15 02:23:55. Version ac07

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