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 2024-07-16.

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%	89572
	Night	14% 6% 2 6% 4 4 4 3 5% 7% 45%	89882
Transparency	Day	39% 4 11% 22% 19% 5%	89345
	Night	40% 4 10% 21% 20% 6%	89713
Seeing	Day	17% 11% 25% 24% 16% 6%	14903
	Night	17% 10% 24% 30% 17% 2	25188

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	19% 5% 2 5% 2 3 3 2 3 5% 51%	2836
	11th to 20th	Day	14% 7% 2 5% 3 3 3 2 4 4 52%	2840
	21st to end	Day	19% 10% 3 6% 3 4 4 3 4 5% 41%	3124
February	1st to 10th	Day	14% 7% 2 5% 4 4 3 4 6% 7% 44%	1710
	11th to 20th	Day	21% 6% 1 5% 4 3 3 2 5% 4 45%	1663
	21st to end	Day	19% 7% 2 7% 4 4 3 3 5% 6% 40%	1726
March	1st to 10th	Day	23% 7% 2 6% 4 4 4 3 5% 6% 37%	3435
	11th to 20th	Day	10% 5% 1 4 4 3 4 3 4 7% 55%	3480
	21st to end	Day	20% 6% 2 6% 4 3 3 3 4 5% 43%	3815
April	1st to 10th	Day	11% 4 2 4 3 3 3 3 5% 7% 54%	2199
	11th to 20th	Day	11% 5% 2 6% 3 4 4 4 5% 7% 48%	2254
	21st to end	Day	14% 5% 2 5% 4 4 4 4 6% 7% 46%	2280
May	1st to 10th	Day	11% 7% 3 7% 5% 5% 6% 5% 8% 9% 34%	4076
	11th to 20th	Day	16% 7% 2 7% 5% 5% 5% 5% 6% 9% 33%	4081
	21st to end	Day	8% 4 1 5% 4 4 5% 4 6% 9% 49%	4476
June	1st to 10th	Day	9% 5% 2 6% 5% 5% 5% 6% 8% 10% 39%	2470
	11th to 20th	Day	9% 5% 1 5% 5% 4 5% 4 7% 10% 44%	2470
	21st to end	Day	7% 5% 2 6% 5% 5% 6% 5% 8% 11% 40%	2470
July	1st to 10th	Day	7% 5% 2 8% 6% 6% 6% 6% 8% 9% 36%	2470
	11th to 20th	Day	7% 6% 2 7% 5% 6% 6% 6% 10% 12% 33%	2407
	21st to end	Day	6% 6% 2 7% 6% 6% 7% 6% 8% 11% 34%	2547
August	1st to 10th	Day	10% 6% 3 8% 6% 5% 8% 5% 9% 11% 31%	2327
	11th to 20th	Day	10% 7% 2 9% 6% 6% 6% 6% 7% 8% 32%	2245
	21st to end	Day	11% 6% 3 7% 5% 5% 6% 5% 7% 10% 36%	2376
September	1st to 10th	Day	9% 4 2 6% 4 4 5% 4 7% 8% 46%	2160
	11th to 20th	Day	15% 5% 2 6% 4 4 5% 3 5% 8% 42%	2021
	21st to end	Day	21% 7% 2 6% 4 4 3 3 5% 6% 39%	1949
October	1st to 10th	Day	15% 6% 2 6% 3 3 3 2 4 6% 49%	1800
	11th to 20th	Day	13% 5% 2 4 3 3 4 2 5% 5% 56%	1800
	21st to end	Day	12% 5% 1 5% 3 3 3 2 4 4 56%	1980
November	1st to 10th	Day	13% 4 2 4 4 2 3 2 4 5% 56%	1857
	11th to 20th	Day	15% 5% 2 5% 3 3 2 3 5% 6% 52%	1877
	21st to end	Day	14% 5% 2 5% 4 3 3 3 5% 6% 51%	1630
December	1st to 10th	Day	18% 8% 2 5% 4 3 5% 3 4 7% 41%	1520
	11th to 20th	Day	16% 4 2 4 2 2 3 2 4 6% 55%	2077
	21st to end	Day	17% 6% 2 5% 4 4 4 3 5% 7% 42%	3124
January	1st to 10th	Night	15% 6% 2 6% 4 4 3 3 5% 5% 47%	4109
	11th to 20th	Night	14% 8% 2 6% 3 3 3 3 4 6% 47%	4120
	21st to end	Night	20% 7% 2 5% 3 3 4 3 4 5% 44%	4532
February	1st to 10th	Night	17% 5% 2 6% 4 3 3 4 5% 5% 45%	2470
	11th to 20th	Night	20% 7% 2 6% 3 3 3 3 4 5% 44%	2400
	21st to end	Night	15% 6% 2 5% 4 4 3 4 6% 7% 45%	1727
March	1st to 10th	Night	23% 8% 3 7% 5% 4 4 3 5% 5% 34%	3418
	11th to 20th	Night	14% 6% 2 5% 3 3 4 2 5% 5% 52%	3480
	21st to end	Night	18% 5% 2 5% 3 3 3 3 4 6% 48%	3817
April	1st to 10th	Night	13% 6% 2 4 4 4 4 3 4 7% 49%	1981
	11th to 20th	Night	13% 5% 2 5% 4 3 3 3 4 6% 51%	1875
	21st to end	Night	12% 4 2 6% 4 3 5% 5% 5% 7% 47%	1900
May	1st to 10th	Night	13% 7% 3 7% 5% 4 5% 4 5% 9% 39%	2847
	11th to 20th	Night	19% 7% 3 7% 4 4 3 3 4 6% 38%	2812
	21st to end	Night	8% 5% 2 5% 3 4 5% 4 7% 11% 46%	3079
June	1st to 10th	Night	10% 6% 2 6% 4 5% 6% 4 8% 9% 39%	1710
	11th to 20th	Night	9% 6% 3 7% 5% 4 4 4 8% 10% 41%	1710
	21st to end	Night	7% 6% 2 6% 5% 4 5% 5% 7% 12% 40%	1710
July	1st to 10th	Night	8% 6% 3 7% 6% 5% 6% 5% 8% 11% 37%	1710
	11th to 20th	Night	7% 5% 2 6% 5% 5% 5% 6% 10% 12% 38%	1674
	21st to end	Night	6% 5% 2 8% 6% 5% 7% 6% 7% 12% 37%	1771
August	1st to 10th	Night	11% 6% 3 7% 6% 5% 5% 5% 8% 12% 33%	1611
	11th to 20th	Night	11% 6% 3 6% 6% 5% 6% 5% 8% 9% 35%	1636
	21st to end	Night	9% 5% 2 7% 6% 5% 5% 4 7% 11% 38%	1980
September	1st to 10th	Night	12% 6% 2 5% 4 3 4 4 6% 8% 47%	1800
	11th to 20th	Night	14% 6% 2 5% 4 4 4 3 6% 7% 45%	1934
	21st to end	Night	18% 8% 3 6% 4 4 4 4 6% 6% 37%	2009
October	1st to 10th	Night	15% 6% 3 5% 4 3 3 3 4 5% 50%	2160
	11th to 20th	Night	14% 6% 2 5% 3 3 3 2 4 6% 52%	2160
	21st to end	Night	13% 5% 2 4 3 4 3 3 4 5% 55%	2376
November	1st to 10th	Night	14% 6% 2 5% 4 3 3 3 4 4 53%	2221
	11th to 20th	Night	12% 5% 2 6% 4 4 4 3 4 5% 50%	2301
	21st to end	Night	14% 4 2 4 3 3 3 4 5% 6% 51%	2541
December	1st to 10th	Night	17% 7% 3 6% 5% 4 3 4 4 5% 42%	2660
	11th to 20th	Night	16% 6% 2 5% 3 3 3 3 5% 5% 49%	3109
	21st to end	Night	17% 6% 2 5% 3 3 3 2 5% 7% 46%	4532

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	38% 2 4 27% 23% 6%	2837
	11th to 20th	Day	37% 3 4 18% 32% 6%	2840
	21st to end	Day	47% 3 5% 17% 21% 6%	3124
February	1st to 10th	Day	40% 3 10% 22% 20% 5%	1710
	11th to 20th	Day	44% 2 6% 20% 22% 7%	1672
	21st to end	Day	46% 3 9% 17% 19% 6%	1726
March	1st to 10th	Day	48% 3 7% 20% 15% 8%	3435
	11th to 20th	Day	31% 3 8% 20% 25% 13%	3480
	21st to end	Day	45% 3 7% 21% 20% 4	3815
April	1st to 10th	Day	30% 3 10% 24% 25% 8%	2199
	11th to 20th	Day	35% 3 10% 23% 21% 8%	2254
	21st to end	Day	38% 3 11% 24% 19% 4	2280
May	1st to 10th	Day	46% 4 16% 19% 13% 2	4114
	11th to 20th	Day	47% 4 11% 24% 12% 1	4085
	21st to end	Day	31% 3 17% 24% 20% 4	4489
June	1st to 10th	Day	37% 5% 15% 26% 15% 1	2470
	11th to 20th	Day	34% 5% 17% 25% 16% 2	2470
	21st to end	Day	35% 7% 17% 24% 16% 2	2470
July	1st to 10th	Day	41% 7% 16% 20% 14% 3	2405
	11th to 20th	Day	38% 8% 20% 23% 10% 2	2338
	21st to end	Day	40% 7% 20% 25% 9% 1	2405
August	1st to 10th	Day	44% 6% 18% 22% 9% 1	2314
	11th to 20th	Day	46% 7% 16% 20% 9% 1	2245
	21st to end	Day	42% 5% 17% 22% 11% 3	2376
September	1st to 10th	Day	34% 6% 16% 21% 18% 6%	2160
	11th to 20th	Day	41% 4 11% 19% 22% 4	2021
	21st to end	Day	46% 3 7% 21% 17% 4	1949
October	1st to 10th	Day	38% 2 9% 23% 24% 4	1800
	11th to 20th	Day	32% 2 8% 28% 22% 8%	1800
	21st to end	Day	33% 3 6% 24% 25% 9%	1980
November	1st to 10th	Day	33% 2 7% 22% 28% 8%	1857
	11th to 20th	Day	34% 2 8% 22% 27% 8%	1877
	21st to end	Day	35% 3 7% 24% 25% 6%	1630
December	1st to 10th	Day	44% 2 10% 21% 19% 4	1520
	11th to 20th	Day	33% 1 7% 26% 24% 9%	2074
	21st to end	Day	42% 2 10% 24% 20% 2	3124
January	1st to 10th	Night	39% 2 5% 22% 26% 6%	4109
	11th to 20th	Night	40% 2 7% 19% 27% 6%	4120
	21st to end	Night	44% 2 4 20% 22% 7%	4532
February	1st to 10th	Night	40% 3 8% 23% 21% 5%	2470
	11th to 20th	Night	44% 2 8% 17% 22% 8%	2402
	21st to end	Night	39% 3 9% 21% 20% 8%	1727
March	1st to 10th	Night	54% 3 7% 16% 16% 5%	3418
	11th to 20th	Night	35% 2 6% 16% 23% 17%	3480
	21st to end	Night	39% 3 7% 22% 23% 6%	3817
April	1st to 10th	Night	37% 3 9% 18% 26% 7%	1981
	11th to 20th	Night	35% 2 9% 24% 21% 9%	1875
	21st to end	Night	37% 4 11% 25% 19% 5%	1900
May	1st to 10th	Night	43% 4 16% 22% 13% 2	2874
	11th to 20th	Night	48% 3 9% 22% 16% 1	2813
	21st to end	Night	31% 5% 16% 27% 18% 2	3087
June	1st to 10th	Night	39% 7% 16% 23% 14% 2	1710
	11th to 20th	Night	37% 6% 17% 22% 16% 2	1710
	21st to end	Night	35% 9% 17% 22% 14% 3	1710
July	1st to 10th	Night	39% 7% 18% 20% 13% 3	1665
	11th to 20th	Night	35% 9% 23% 23% 8% 2	1620
	21st to end	Night	37% 7% 21% 22% 12%	1672
August	1st to 10th	Night	42% 8% 20% 20% 9% 1	1600
	11th to 20th	Night	42% 8% 18% 20% 9% 2	1636
	21st to end	Night	39% 6% 19% 24% 9% 2	1980
September	1st to 10th	Night	35% 6% 14% 22% 17% 6%	1800
	11th to 20th	Night	38% 5% 11% 20% 23% 3	1934
	21st to end	Night	47% 5% 11% 18% 16% 4	2009
October	1st to 10th	Night	38% 3 8% 24% 24% 4	2160
	11th to 20th	Night	36% 3 7% 24% 23% 8%	2160
	21st to end	Night	33% 3 6% 21% 26% 11%	2376
November	1st to 10th	Night	35% 3 7% 19% 27% 9%	2221
	11th to 20th	Night	36% 3 7% 18% 25% 12%	2301
	21st to end	Night	34% 3 6% 24% 26% 7%	2541
December	1st to 10th	Night	45% 3 8% 20% 20% 5%	2660
	11th to 20th	Night	39% 2 7% 21% 22% 9%	3111
	21st to end	Night	40% 2 6% 26% 24% 3	4532

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	27% 21% 36% 14% 3	270
	11th to 20th	Day	28% 24% 33% 12% 4	270
	21st to end	Day	22% 22% 42% 11% 3	297
February	1st to 10th	Day	20% 20% 41% 14% 4	270
	11th to 20th	Day	39% 20% 33% 7% 1	270
	21st to end	Day	26% 20% 33% 17% 3	381
March	1st to 10th	Day	24% 18% 33% 17% 7%	460
	11th to 20th	Day	20% 23% 39% 12% 5% 1	460
	21st to end	Day	21% 20% 37% 17% 4 1	506
April	1st to 10th	Day	23% 18% 34% 17% 7% 2	460
	11th to 20th	Day	15% 19% 30% 23% 9% 4	452
	21st to end	Day	18% 11% 31% 26% 11% 2	460
May	1st to 10th	Day	18% 16% 32% 22% 8% 4	463
	11th to 20th	Day	16% 9% 23% 30% 18% 4	487
	21st to end	Day	19% 6% 20% 27% 21% 8%	529
June	1st to 10th	Day	12% 5% 24% 32% 22% 6%	490
	11th to 20th	Day	9% 7% 21% 30% 26% 8%	490
	21st to end	Day	11% 4 20% 33% 26% 6%	490
July	1st to 10th	Day	10% 2 11% 32% 33% 11%	485
	11th to 20th	Day	7% 2 12% 30% 36% 13%	465
	21st to end	Day	9% 2 15% 35% 27% 12%	488
August	1st to 10th	Day	15% 3 17% 28% 26% 11%	446
	11th to 20th	Day	12% 2 13% 29% 27% 16%	439
	21st to end	Day	12% 4 11% 29% 27% 16%	495
September	1st to 10th	Day	12% 4 17% 25% 28% 15%	450
	11th to 20th	Day	17% 4 12% 30% 27% 11%	450
	21st to end	Day	25% 6% 22% 26% 17% 3	450
October	1st to 10th	Day	18% 8% 21% 24% 21% 8%	450
	11th to 20th	Day	16% 8% 25% 28% 18% 5%	450
	21st to end	Day	15% 13% 25% 29% 14% 4	495
November	1st to 10th	Day	19% 10% 30% 24% 14% 3	455
	11th to 20th	Day	19% 15% 28% 26% 11% 1	437
	21st to end	Day	14% 18% 31% 21% 13% 2	232
December	1st to 10th	Day	23% 18% 38% 12% 8% 1	180
	11th to 20th	Day	21% 18% 35% 19% 7% 1	234
	21st to end	Day	26% 17% 30% 17% 9% 1	297
January	1st to 10th	Night	21% 18% 31% 24% 5%	760
	11th to 20th	Night	20% 17% 32% 25% 7%	760
	21st to end	Night	20% 16% 36% 25% 3	836
February	1st to 10th	Night	20% 13% 35% 28% 4	760
	11th to 20th	Night	25% 20% 34% 20% 1	743
	21st to end	Night	19% 17% 36% 24% 4	628
March	1st to 10th	Night	18% 16% 34% 27% 6%	760
	11th to 20th	Night	22% 16% 31% 22% 9% 1	760
	21st to end	Night	18% 14% 38% 25% 5%	836
April	1st to 10th	Night	17% 18% 32% 25% 7%	760
	11th to 20th	Night	15% 14% 33% 25% 12% 1	751
	21st to end	Night	15% 9% 34% 34% 8%	760
May	1st to 10th	Night	15% 10% 32% 33% 9% 1	591
	11th to 20th	Night	17% 6% 24% 34% 17% 1	561
	21st to end	Night	13% 3 14% 40% 27% 2	617
June	1st to 10th	Night	13% 4 17% 40% 25% 2	570
	11th to 20th	Night	11% 4 15% 38% 27% 5%	570
	21st to end	Night	9% 3 14% 39% 31% 3	570
July	1st to 10th	Night	8% 2 13% 33% 40% 4	555
	11th to 20th	Night	9% 2 9% 37% 36% 7%	540
	21st to end	Night	6% 3 13% 40% 34% 5%	591
August	1st to 10th	Night	13% 2 9% 36% 35% 4	537
	11th to 20th	Night	13% 3 9% 33% 34% 7%	572
	21st to end	Night	12% 3 9% 31% 38% 7%	792
September	1st to 10th	Night	15% 2 10% 33% 33% 7%	720
	11th to 20th	Night	17% 3 14% 30% 31% 5%	720
	21st to end	Night	25% 4 16% 33% 22% 1	720
October	1st to 10th	Night	18% 5% 17% 30% 25% 6%	720
	11th to 20th	Night	17% 9% 23% 28% 20% 2	720
	21st to end	Night	16% 10% 26% 29% 18% 2	792
November	1st to 10th	Night	17% 8% 24% 34% 14% 2	740
	11th to 20th	Night	15% 13% 29% 32% 11%	760
	21st to end	Night	16% 13% 27% 30% 13% 1	760
December	1st to 10th	Night	21% 14% 29% 28% 8%	760
	11th to 20th	Night	19% 11% 31% 31% 8%	760
	21st to end	Night	22% 15% 26% 29% 7%	836

Page generated at 2024-07-13 10:19:48. Version ac07

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