Chapel Hill
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Chapel Hill clear sky chart from 2005-11-06 to 2026-04-17.

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%	131241
	Night	14% 6% 2 6% 4 4 4 3 5% 7% 45%	130927
Transparency	Day	38% 5% 12% 21% 18% 6%	134847
	Night	38% 4 10% 20% 21% 6%	135391
Seeing	Day	17% 12% 26% 24% 16% 5%	17237
	Night	17% 10% 25% 30% 16% 2	27604

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 5% 3 3 3 2 3 5% 53%	5258
	11th to 20th	Day	12% 6% 2 4 3 2 3 2 3 3 60%	5280
	21st to end	Day	16% 8% 3 6% 3 4 4 3 4 5% 44%	5780
February	1st to 10th	Day	13% 7% 2 6% 5% 4 3 4 6% 7% 44%	1890
	11th to 20th	Day	22% 7% 2 5% 4 3 3 2 5% 4 44%	1843
	21st to end	Day	19% 7% 2 6% 4 4 3 3 5% 6% 41%	1902
March	1st to 10th	Day	19% 6% 2 6% 4 4 4 3 5% 6% 40%	5045
	11th to 20th	Day	10% 5% 2 5% 3 3 4 3 4 6% 53%	5090
	21st to end	Day	15% 5% 2 5% 4 4 4 3 4 6% 47%	5586
April	1st to 10th	Day	11% 4 2 4 3 3 4 3 6% 8% 53%	2431
	11th to 20th	Day	10% 5% 2 5% 3 4 4 4 5% 8% 49%	2446
	21st to end	Day	14% 5% 2 5% 4 4 5% 4 6% 7% 45%	2400
May	1st to 10th	Day	9% 6% 3 7% 5% 5% 6% 5% 8% 10% 35%	5856
	11th to 20th	Day	18% 7% 2 8% 5% 5% 5% 5% 7% 9% 28%	5861
	21st to end	Day	12% 5% 1 6% 4 5% 5% 4 6% 9% 43%	6433
June	1st to 10th	Day	9% 5% 2 6% 5% 5% 5% 6% 8% 10% 39%	2600
	11th to 20th	Day	8% 5% 1 6% 5% 4 5% 5% 7% 10% 44%	2600
	21st to end	Day	7% 5% 2 5% 4 5% 6% 5% 8% 12% 41%	2600
July	1st to 10th	Day	9% 5% 2 8% 5% 5% 6% 6% 8% 11% 35%	5900
	11th to 20th	Day	8% 6% 2 8% 5% 6% 7% 6% 10% 14% 28%	5900
	21st to end	Day	9% 6% 3 7% 5% 6% 6% 5% 7% 11% 34%	6424
August	1st to 10th	Day	20% 8% 3 8% 6% 4 7% 5% 8% 10% 22%	4234
	11th to 20th	Day	8% 6% 2 8% 5% 6% 6% 5% 7% 10% 36%	4057
	21st to end	Day	7% 4 2 5% 4 4 4 4 6% 11% 50%	4312
September	1st to 10th	Day	9% 4 2 6% 4 4 5% 4 7% 8% 47%	2400
	11th to 20th	Day	15% 6% 2 6% 4 4 4 3 5% 8% 41%	2245
	21st to end	Day	21% 7% 2 7% 4 4 4 3 5% 6% 38%	2165
October	1st to 10th	Day	14% 6% 3 6% 3 3 3 2 4 6% 50%	2000
	11th to 20th	Day	13% 4 1 4 3 3 4 2 4 5% 58%	2000
	21st to end	Day	12% 5% 1 5% 3 3 3 2 4 4 57%	2200
November	1st to 10th	Day	13% 5% 2 5% 4 2 3 2 4 5% 55%	2057
	11th to 20th	Day	14% 5% 1 4 3 3 2 3 5% 6% 53%	2073
	21st to end	Day	14% 5% 2 5% 4 3 3 3 4 6% 51%	1801
December	1st to 10th	Day	13% 6% 2 5% 4 3 4 3 3 5% 52%	2680
	11th to 20th	Day	17% 6% 2 6% 4 4 3 3 4 6% 45%	3327
	21st to end	Day	16% 6% 2 5% 3 3 3 3 5% 7% 45%	4565
January	1st to 10th	Night	11% 6% 2 6% 3 3 3 2 4 5% 54%	7635
	11th to 20th	Night	12% 6% 2 5% 3 3 2 2 3 4 58%	7680
	21st to end	Night	17% 7% 2 5% 3 3 4 3 4 5% 46%	8401
February	1st to 10th	Night	17% 6% 2 6% 4 3 3 3 5% 5% 46%	2730
	11th to 20th	Night	20% 7% 2 5% 3 3 3 3 4 5% 43%	2659
	21st to end	Night	15% 6% 2 5% 4 4 3 4 5% 7% 46%	1903
March	1st to 10th	Night	19% 7% 2 6% 4 4 4 3 4 5% 43%	5028
	11th to 20th	Night	12% 5% 2 4 3 3 3 2 5% 7% 55%	5090
	21st to end	Night	14% 4 2 5% 4 4 4 3 5% 7% 49%	5588
April	1st to 10th	Night	14% 5% 2 4 4 4 3 3 5% 7% 49%	2189
	11th to 20th	Night	12% 5% 2 5% 4 3 3 3 4 6% 52%	2036
	21st to end	Night	11% 5% 3 6% 4 3 6% 5% 5% 7% 47%	2000
May	1st to 10th	Night	12% 7% 3 7% 5% 5% 5% 4 5% 10% 37%	4081
	11th to 20th	Night	20% 7% 3 7% 4 4 4 3 5% 8% 35%	4032
	21st to end	Night	16% 5% 2 5% 3 3 4 3 5% 10% 43%	4421
June	1st to 10th	Night	10% 7% 2 6% 4 5% 6% 4 8% 9% 39%	1800
	11th to 20th	Night	9% 6% 3 7% 5% 4 4 4 8% 10% 41%	1800
	21st to end	Night	7% 5% 2 6% 5% 4 5% 5% 8% 12% 40%	1800
July	1st to 10th	Night	12% 6% 3 8% 5% 5% 7% 5% 8% 10% 32%	4058
	11th to 20th	Night	13% 7% 3 8% 6% 5% 6% 6% 9% 11% 27%	4057
	21st to end	Night	11% 7% 2 8% 5% 5% 6% 5% 7% 11% 31%	4427
August	1st to 10th	Night	23% 6% 3 7% 6% 5% 5% 5% 8% 10% 23%	2915
	11th to 20th	Night	8% 6% 2 7% 5% 5% 6% 6% 8% 10% 37%	2957
	21st to end	Night	6% 4 2 6% 5% 4 5% 3 6% 9% 50%	3586
September	1st to 10th	Night	11% 6% 2 6% 4 3 4 3 6% 8% 47%	2000
	11th to 20th	Night	15% 6% 2 6% 4 4 4 3 6% 7% 43%	2150
	21st to end	Night	19% 8% 3 6% 4 4 4 3 6% 6% 37%	2233
October	1st to 10th	Night	15% 6% 3 5% 4 3 3 3 4 5% 50%	2400
	11th to 20th	Night	14% 6% 2 5% 3 3 3 2 4 5% 54%	2400
	21st to end	Night	13% 5% 2 4 3 3 3 3 4 5% 55%	2640
November	1st to 10th	Night	13% 6% 2 5% 4 3 3 3 4 5% 53%	2461
	11th to 20th	Night	12% 5% 2 6% 4 4 4 3 5% 5% 51%	2544
	21st to end	Night	13% 4 2 4 3 3 4 4 5% 6% 51%	2809
December	1st to 10th	Night	12% 6% 2 6% 4 3 3 3 4 5% 52%	4720
	11th to 20th	Night	17% 8% 3 7% 4 4 4 3 5% 5% 41%	5064
	21st to end	Night	16% 6% 2 5% 3 3 3 2 5% 6% 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	36% 2 9% 21% 27% 5%	5259
	11th to 20th	Day	31% 2 3 14% 38% 12%	5280
	21st to end	Day	44% 3 4 21% 20% 8%	5780
February	1st to 10th	Day	40% 3 10% 22% 20% 5%	1890
	11th to 20th	Day	44% 2 6% 21% 20% 6%	1852
	21st to end	Day	45% 2 9% 17% 20% 6%	1902
March	1st to 10th	Day	45% 3 6% 17% 18% 11%	5045
	11th to 20th	Day	33% 3 7% 17% 24% 16%	5090
	21st to end	Day	40% 4 8% 21% 21% 6%	5586
April	1st to 10th	Day	30% 3 10% 25% 24% 8%	2431
	11th to 20th	Day	34% 3 11% 25% 21% 7%	2446
	21st to end	Day	38% 4 11% 24% 19% 4	2400
May	1st to 10th	Day	42% 5% 18% 22% 11% 1	5891
	11th to 20th	Day	50% 6% 12% 21% 9% 1	5865
	21st to end	Day	37% 4 14% 23% 18% 3	6447
June	1st to 10th	Day	37% 6% 16% 25% 15% 1	2600
	11th to 20th	Day	34% 6% 18% 25% 16% 2	2600
	21st to end	Day	33% 7% 18% 24% 16% 2	2600
July	1st to 10th	Day	39% 9% 18% 20% 12% 2	5835
	11th to 20th	Day	41% 14% 26% 15% 4 1	5831
	21st to end	Day	41% 9% 17% 23% 9%	6240
August	1st to 10th	Day	54% 7% 18% 15% 5%	4221
	11th to 20th	Day	39% 9% 21% 22% 8% 1	4096
	21st to end	Day	29% 5% 14% 35% 15% 2	4312
September	1st to 10th	Day	33% 6% 15% 21% 19% 5%	2400
	11th to 20th	Day	41% 4 11% 19% 21% 3	2245
	21st to end	Day	47% 4 8% 21% 16% 4	2165
October	1st to 10th	Day	33% 4 7% 20% 27% 9%	3260
	11th to 20th	Day	21% 2 5% 21% 27% 25%	3240
	21st to end	Day	26% 3 7% 25% 24% 16%	3538
November	1st to 10th	Day	33% 2 8% 23% 27% 8%	2057
	11th to 20th	Day	33% 2 8% 23% 27% 7%	2073
	21st to end	Day	35% 3 7% 24% 25% 6%	1801
December	1st to 10th	Day	36% 3 7% 20% 26% 7%	2680
	11th to 20th	Day	42% 4 9% 18% 21% 6%	3324
	21st to end	Day	40% 2 11% 27% 18% 2	4565
January	1st to 10th	Night	34% 2 9% 21% 26% 8%	7636
	11th to 20th	Night	33% 2 4 16% 35% 10%	7680
	21st to end	Night	43% 2 4 19% 24% 7%	8401
February	1st to 10th	Night	40% 3 9% 22% 21% 5%	2730
	11th to 20th	Night	44% 3 8% 17% 21% 7%	2661
	21st to end	Night	39% 3 8% 20% 22% 9%	1903
March	1st to 10th	Night	45% 3 6% 15% 22% 10%	5028
	11th to 20th	Night	31% 2 6% 17% 25% 19%	5090
	21st to end	Night	36% 3 8% 24% 23% 6%	5588
April	1st to 10th	Night	36% 3 10% 19% 25% 7%	2189
	11th to 20th	Night	34% 2 9% 25% 21% 8%	2036
	21st to end	Night	37% 4 12% 25% 18% 4	2000
May	1st to 10th	Night	43% 6% 17% 20% 14% 1	4108
	11th to 20th	Night	49% 5% 12% 20% 14% 1	4033
	21st to end	Night	37% 5% 14% 25% 17% 3	4429
June	1st to 10th	Night	39% 7% 16% 22% 15% 2	1800
	11th to 20th	Night	38% 7% 17% 22% 15% 2	1800
	21st to end	Night	34% 9% 17% 22% 14% 3	1800
July	1st to 10th	Night	44% 11% 16% 16% 11% 1	4015
	11th to 20th	Night	48% 13% 22% 13% 3 1	4006
	21st to end	Night	45% 8% 17% 20% 10%	4297
August	1st to 10th	Night	53% 8% 18% 15% 5% 1	2904
	11th to 20th	Night	37% 10% 20% 25% 7% 1	2984
	21st to end	Night	30% 5% 12% 30% 21% 2	3586
September	1st to 10th	Night	35% 6% 14% 23% 17% 5%	2000
	11th to 20th	Night	40% 5% 11% 19% 22% 3	2150
	21st to end	Night	47% 5% 11% 18% 14% 4	2233
October	1st to 10th	Night	38% 3 7% 21% 25% 6%	3920
	11th to 20th	Night	25% 2 5% 20% 27% 20%	3899
	21st to end	Night	25% 3 6% 25% 24% 16%	4252
November	1st to 10th	Night	35% 3 6% 21% 26% 8%	2461
	11th to 20th	Night	36% 2 7% 17% 26% 11%	2544
	21st to end	Night	33% 3 7% 24% 25% 7%	2809
December	1st to 10th	Night	36% 3 6% 20% 28% 7%	4720
	11th to 20th	Night	46% 3 8% 18% 16% 8%	5066
	21st to end	Night	38% 2 11% 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	26% 23% 34% 14% 3	330
	11th to 20th	Day	29% 25% 32% 11% 3	330
	21st to end	Day	23% 22% 41% 12% 3	363
February	1st to 10th	Day	19% 21% 43% 13% 3	330
	11th to 20th	Day	39% 22% 31% 7% 1	330
	21st to end	Day	27% 20% 32% 18% 4	445
March	1st to 10th	Day	24% 18% 33% 18% 7%	540
	11th to 20th	Day	21% 22% 37% 14% 5% 1	540
	21st to end	Day	20% 19% 39% 17% 4 1	594
April	1st to 10th	Day	21% 18% 34% 19% 7% 1	540
	11th to 20th	Day	14% 18% 32% 24% 9% 3	516
	21st to end	Day	17% 11% 31% 28% 12% 2	500
May	1st to 10th	Day	17% 17% 32% 23% 9% 4	503
	11th to 20th	Day	17% 10% 24% 28% 17% 4	527
	21st to end	Day	21% 7% 21% 25% 19% 7%	573
June	1st to 10th	Day	12% 5% 25% 32% 21% 6%	530
	11th to 20th	Day	8% 6% 22% 31% 24% 8%	530
	21st to end	Day	11% 3 19% 33% 27% 7%	530
July	1st to 10th	Day	10% 3 11% 33% 33% 10%	525
	11th to 20th	Day	6% 2 11% 31% 35% 14%	524
	21st to end	Day	9% 2 15% 34% 27% 13%	576
August	1st to 10th	Day	17% 4 17% 28% 25% 10%	526
	11th to 20th	Day	12% 3 14% 31% 26% 14%	519
	21st to end	Day	10% 4 13% 31% 26% 15%	583
September	1st to 10th	Day	12% 4 19% 25% 26% 13%	530
	11th to 20th	Day	17% 4 14% 28% 26% 11%	530
	21st to end	Day	25% 7% 22% 27% 15% 3	530
October	1st to 10th	Day	16% 8% 20% 25% 21% 9%	530
	11th to 20th	Day	15% 10% 25% 29% 16% 4	530
	21st to end	Day	15% 14% 24% 28% 14% 5%	583
November	1st to 10th	Day	17% 10% 33% 24% 13% 3	535
	11th to 20th	Day	18% 16% 29% 25% 10% 1	513
	21st to end	Day	16% 19% 31% 20% 12% 2	283
December	1st to 10th	Day	22% 20% 35% 15% 7%	220
	11th to 20th	Day	21% 17% 36% 19% 6% 1	286
	21st to end	Day	25% 17% 33% 16% 8% 1	363
January	1st to 10th	Night	20% 19% 32% 24% 5%	840
	11th to 20th	Night	19% 18% 32% 24% 6%	840
	21st to end	Night	20% 15% 37% 24% 3	924
February	1st to 10th	Night	21% 13% 35% 27% 3	840
	11th to 20th	Night	25% 20% 33% 21% 1	823
	21st to end	Night	19% 18% 35% 24% 4	692
March	1st to 10th	Night	17% 16% 33% 28% 5%	840
	11th to 20th	Night	21% 17% 31% 22% 8%	840
	21st to end	Night	18% 14% 40% 24% 4	924
April	1st to 10th	Night	17% 17% 34% 25% 7%	840
	11th to 20th	Night	15% 14% 34% 25% 11%	816
	21st to end	Night	14% 9% 34% 35% 8%	800
May	1st to 10th	Night	14% 10% 33% 33% 9%	622
	11th to 20th	Night	17% 6% 25% 33% 18% 1	591
	21st to end	Night	15% 4 14% 39% 26% 2	650
June	1st to 10th	Night	13% 4 17% 40% 24% 2	600
	11th to 20th	Night	12% 4 15% 38% 27% 5%	600
	21st to end	Night	9% 3 14% 39% 32% 3	600
July	1st to 10th	Night	9% 2 12% 34% 39% 3	585
	11th to 20th	Night	9% 2 9% 37% 36% 7%	582
	21st to end	Night	6% 3 12% 39% 35% 5%	657
August	1st to 10th	Night	16% 2 9% 35% 34% 4	597
	11th to 20th	Night	12% 3 10% 34% 34% 7%	637
	21st to end	Night	11% 4 9% 32% 37% 6%	880
September	1st to 10th	Night	15% 2 12% 34% 32% 6%	800
	11th to 20th	Night	18% 3 14% 31% 30% 5%	800
	21st to end	Night	25% 4 16% 33% 21% 1	800
October	1st to 10th	Night	17% 5% 17% 30% 26% 6%	800
	11th to 20th	Night	17% 9% 24% 29% 19% 2	800
	21st to end	Night	16% 9% 25% 29% 20% 1	880
November	1st to 10th	Night	17% 8% 24% 36% 13% 2	820
	11th to 20th	Night	15% 13% 30% 32% 10%	840
	21st to end	Night	16% 13% 28% 30% 12% 1	840
December	1st to 10th	Night	22% 13% 30% 28% 7%	840
	11th to 20th	Night	19% 11% 31% 32% 8%	840
	21st to end	Night	22% 15% 26% 30% 7%	924

Page generated at 2026-04-14 02:44:23. Version ac07

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