ScottNC Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the ScottNC Observatory clear sky chart from 2005-11-06 to 2025-05-26.

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% 44%	114613
	Night	14% 6% 2 5% 4 4 4 3 5% 7% 46%	114682
Transparency	Day	37% 6% 11% 22% 18% 6%	118289
	Night	37% 6% 10% 20% 21% 6%	119145
Seeing	Day	17% 7% 22% 25% 19% 10%	16013
	Night	16% 6% 23% 33% 19% 2	26155

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	16% 6% 2 4 3 2 2 2 3 4 57%	4056
	11th to 20th	Day	13% 6% 2 4 3 3 3 3 3 4 58%	4060
	21st to end	Day	18% 5% 2 6% 4 4 3 2 4 4 48%	4466
February	1st to 10th	Day	15% 5% 2 6% 4 4 4 4 6% 7% 44%	1800
	11th to 20th	Day	20% 7% 2 4 3 3 4 3 4 6% 44%	1862
	21st to end	Day	19% 6% 3 7% 3 4 4 3 5% 6% 41%	1719
March	1st to 10th	Day	20% 7% 2 5% 4 4 3 3 5% 6% 41%	4935
	11th to 20th	Day	10% 5% 2 5% 4 3 3 3 4 6% 54%	4980
	21st to end	Day	17% 7% 2 6% 4 3 3 3 4 5% 46%	5465
April	1st to 10th	Day	12% 4 1 4 3 4 3 3 6% 8% 52%	2235
	11th to 20th	Day	14% 5% 1 5% 3 3 4 3 5% 7% 49%	2374
	21st to end	Day	13% 6% 2 5% 4 4 4 4 5% 8% 45%	2535
May	1st to 10th	Day	12% 7% 2 7% 5% 5% 5% 5% 8% 9% 34%	4212
	11th to 20th	Day	18% 7% 3 6% 4 5% 5% 4 6% 8% 34%	4211
	21st to end	Day	8% 4 2 5% 4 4 5% 4 7% 10% 46%	4541
June	1st to 10th	Day	9% 5% 2 6% 5% 5% 5% 4 7% 11% 40%	2470
	11th to 20th	Day	8% 5% 2 6% 4 5% 6% 6% 8% 11% 39%	2470
	21st to end	Day	5% 6% 2 6% 5% 5% 6% 6% 7% 11% 40%	2470
July	1st to 10th	Day	7% 5% 2 7% 6% 5% 6% 5% 8% 11% 38%	4120
	11th to 20th	Day	8% 6% 2 8% 6% 6% 6% 5% 8% 11% 35%	4120
	21st to end	Day	13% 7% 4 9% 6% 6% 6% 6% 7% 9% 26%	4466
August	1st to 10th	Day	16% 8% 3 7% 5% 5% 6% 5% 8% 11% 27%	4104
	11th to 20th	Day	8% 6% 2 7% 6% 6% 7% 6% 7% 11% 35%	4044
	21st to end	Day	6% 3 2 4 4 4 4 4 7% 11% 51%	4222
September	1st to 10th	Day	10% 4 1 5% 5% 4 4 4 6% 9% 47%	2183
	11th to 20th	Day	13% 6% 2 5% 3 4 4 4 7% 8% 42%	2090
	21st to end	Day	18% 7% 3 7% 4 4 6% 4 4 6% 37%	2090
October	1st to 10th	Day	12% 6% 3 5% 3 3 3 3 4 5% 54%	1942
	11th to 20th	Day	12% 5% 2 5% 3 2 3 3 4 5% 56%	1900
	21st to end	Day	14% 5% 2 5% 3 3 2 2 3 5% 56%	2090
November	1st to 10th	Day	13% 6% 2 5% 4 3 3 2 4 5% 55%	1957
	11th to 20th	Day	13% 5% 2 4 3 3 3 3 5% 6% 54%	1855
	21st to end	Day	14% 6% 2 4 3 3 3 3 5% 6% 51%	1800
December	1st to 10th	Day	15% 5% 1 4 3 4 4 2 4 4 54%	2930
	11th to 20th	Day	18% 7% 2 6% 3 3 3 2 5% 7% 44%	3373
	21st to end	Day	22% 6% 2 4 3 3 4 3 5% 7% 42%	4466
January	1st to 10th	Night	13% 5% 2 4 3 3 3 2 4 5% 57%	5889
	11th to 20th	Night	14% 5% 2 4 3 2 2 2 3 4 58%	5900
	21st to end	Night	18% 7% 2 5% 3 3 3 3 4 5% 48%	6490
February	1st to 10th	Night	16% 6% 2 6% 4 4 4 4 5% 7% 44%	2600
	11th to 20th	Night	20% 7% 2 5% 3 3 4 2 4 5% 45%	2420
	21st to end	Night	16% 7% 3 5% 4 4 3 3 5% 5% 45%	1910
March	1st to 10th	Night	17% 7% 2 6% 3 4 3 3 4 5% 46%	4918
	11th to 20th	Night	10% 4 2 4 3 3 3 3 5% 6% 56%	4980
	21st to end	Night	16% 5% 1 5% 4 4 4 3 5% 7% 47%	5467
April	1st to 10th	Night	14% 5% 2 5% 3 4 4 3 5% 8% 47%	2165
	11th to 20th	Night	13% 5% 2 5% 4 3 4 3 5% 8% 49%	1975
	21st to end	Night	14% 5% 2 6% 3 4 4 4 5% 7% 47%	1880
May	1st to 10th	Night	14% 7% 3 7% 5% 4 4 3 5% 7% 41%	2903
	11th to 20th	Night	17% 8% 4 7% 4 4 3 3 6% 7% 36%	2902
	21st to end	Night	9% 5% 2 6% 4 4 5% 4 8% 10% 44%	3130
June	1st to 10th	Night	9% 6% 2 6% 5% 5% 5% 5% 9% 10% 37%	1710
	11th to 20th	Night	9% 5% 2 6% 5% 4 5% 4 8% 11% 40%	1710
	21st to end	Night	7% 5% 2 6% 5% 6% 6% 6% 7% 9% 41%	1710
July	1st to 10th	Night	9% 7% 4 8% 6% 6% 6% 5% 7% 9% 33%	2840
	11th to 20th	Night	9% 6% 3 7% 6% 6% 6% 6% 8% 11% 32%	2840
	21st to end	Night	14% 8% 3 9% 6% 6% 5% 6% 8% 9% 25%	3087
August	1st to 10th	Night	19% 8% 3 7% 6% 5% 6% 5% 7% 10% 25%	2825
	11th to 20th	Night	10% 6% 3 7% 5% 6% 6% 6% 9% 10% 32%	2785
	21st to end	Night	5% 4 2 6% 4 4 4 5% 6% 9% 51%	3402
September	1st to 10th	Night	9% 6% 2 6% 5% 5% 5% 4 6% 7% 45%	1992
	11th to 20th	Night	14% 6% 3 8% 5% 5% 5% 3 5% 8% 38%	2090
	21st to end	Night	18% 8% 2 6% 4 4 5% 4 6% 6% 36%	2090
October	1st to 10th	Night	13% 6% 2 5% 4 3 4 3 4 6% 48%	2237
	11th to 20th	Night	14% 5% 2 4 3 3 3 2 4 5% 55%	2280
	21st to end	Night	14% 4 2 4 2 3 3 2 3 5% 57%	2508
November	1st to 10th	Night	16% 5% 2 4 3 3 3 2 3 5% 54%	2341
	11th to 20th	Night	14% 5% 2 4 4 4 3 3 4 5% 53%	2544
	21st to end	Night	14% 4 1 3 3 3 3 4 5% 6% 52%	2600
December	1st to 10th	Night	14% 6% 2 5% 3 3 3 3 5% 6% 51%	4250
	11th to 20th	Night	22% 8% 3 6% 4 3 3 3 4 4 41%	4822
	21st to end	Night	21% 4 2 4 3 3 4 3 5% 7% 45%	6490

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	33% 3 5% 21% 30% 8%	4057
	11th to 20th	Day	31% 3 4 17% 36% 8%	4060
	21st to end	Day	41% 3 5% 23% 23% 5%	4466
February	1st to 10th	Day	40% 5% 11% 21% 19% 5%	1800
	11th to 20th	Day	42% 4 8% 22% 19% 6%	1871
	21st to end	Day	44% 5% 10% 15% 19% 7%	1719
March	1st to 10th	Day	43% 4 6% 17% 17% 12%	4935
	11th to 20th	Day	30% 5% 6% 19% 23% 17%	4980
	21st to end	Day	42% 4 7% 23% 19% 6%	5465
April	1st to 10th	Day	31% 5% 11% 23% 24% 6%	2235
	11th to 20th	Day	34% 6% 10% 22% 20% 9%	2374
	21st to end	Day	38% 7% 12% 24% 16% 3	2535
May	1st to 10th	Day	42% 11% 15% 19% 11% 2	4250
	11th to 20th	Day	46% 9% 11% 22% 10% 1	4215
	21st to end	Day	30% 9% 17% 23% 17% 4	4554
June	1st to 10th	Day	35% 9% 16% 26% 13% 1	2470
	11th to 20th	Day	35% 10% 18% 21% 14% 2	2470
	21st to end	Day	33% 11% 17% 23% 14% 1	2470
July	1st to 10th	Day	38% 12% 17% 18% 12% 3	4055
	11th to 20th	Day	40% 10% 21% 23% 6% 1	4051
	21st to end	Day	52% 11% 13% 17% 7% 1	4324
August	1st to 10th	Day	48% 9% 17% 20% 5%	4091
	11th to 20th	Day	39% 10% 20% 21% 8% 2	4083
	21st to end	Day	26% 7% 17% 34% 14% 2	4222
September	1st to 10th	Day	34% 8% 14% 22% 18% 4	2183
	11th to 20th	Day	37% 9% 13% 19% 20% 3	2090
	21st to end	Day	47% 7% 9% 18% 17% 3	2090
October	1st to 10th	Day	29% 4 7% 22% 28% 11%	3228
	11th to 20th	Day	21% 3 5% 22% 25% 24%	3140
	21st to end	Day	26% 3 8% 25% 23% 15%	3428
November	1st to 10th	Day	34% 4 7% 22% 26% 8%	1957
	11th to 20th	Day	31% 4 8% 23% 27% 7%	1855
	21st to end	Day	35% 4 9% 22% 25% 5%	1800
December	1st to 10th	Day	34% 4 6% 20% 28% 7%	2930
	11th to 20th	Day	42% 3 10% 17% 22% 6%	3370
	21st to end	Day	41% 4 11% 27% 16% 2	4466
January	1st to 10th	Night	31% 3 5% 20% 31% 10%	5889
	11th to 20th	Night	32% 3 6% 18% 34% 8%	5900
	21st to end	Night	38% 3 6% 21% 26% 6%	6490
February	1st to 10th	Night	40% 5% 10% 21% 19% 4	2600
	11th to 20th	Night	43% 4 9% 18% 20% 7%	2422
	21st to end	Night	41% 5% 9% 18% 21% 7%	1910
March	1st to 10th	Night	41% 4 5% 16% 25% 9%	4918
	11th to 20th	Night	28% 4 6% 18% 26% 17%	4980
	21st to end	Night	38% 4 6% 26% 21% 5%	5467
April	1st to 10th	Night	36% 5% 11% 20% 22% 6%	2165
	11th to 20th	Night	35% 5% 9% 21% 22% 8%	1975
	21st to end	Night	37% 6% 11% 24% 18% 4	1880
May	1st to 10th	Night	42% 7% 14% 23% 13% 1	2930
	11th to 20th	Night	45% 9% 10% 20% 14% 1	2903
	21st to end	Night	31% 9% 16% 27% 15% 2	3138
June	1st to 10th	Night	37% 10% 16% 22% 14% 2	1710
	11th to 20th	Night	35% 10% 17% 22% 15% 2	1710
	21st to end	Night	35% 10% 19% 19% 14% 4	1710
July	1st to 10th	Night	45% 12% 13% 16% 12% 2	2795
	11th to 20th	Night	41% 12% 23% 18% 5% 1	2786
	21st to end	Night	52% 11% 14% 18% 5%	2988
August	1st to 10th	Night	53% 8% 18% 15% 5%	2814
	11th to 20th	Night	42% 10% 20% 18% 9% 1	2812
	21st to end	Night	28% 8% 13% 32% 16% 3	3402
September	1st to 10th	Night	35% 8% 15% 20% 16% 6%	1992
	11th to 20th	Night	43% 9% 11% 17% 19% 2	2090
	21st to end	Night	45% 7% 11% 17% 16% 4	2090
October	1st to 10th	Night	37% 5% 8% 18% 24% 9%	3731
	11th to 20th	Night	22% 3 6% 23% 28% 18%	3779
	21st to end	Night	26% 3 6% 24% 26% 15%	4120
November	1st to 10th	Night	35% 4 8% 20% 24% 9%	2341
	11th to 20th	Night	35% 3 7% 19% 26% 10%	2544
	21st to end	Night	33% 4 7% 25% 26% 6%	2600
December	1st to 10th	Night	35% 4 6% 21% 29% 5%	4250
	11th to 20th	Night	48% 3 7% 17% 16% 9%	4824
	21st to end	Night	38% 4 14% 22% 20% 2	6490

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% 17% 31% 20% 6%	300
	11th to 20th	Day	25% 17% 33% 17% 8%	300
	21st to end	Day	22% 13% 41% 18% 5%	330
February	1st to 10th	Day	20% 15% 35% 21% 8%	300
	11th to 20th	Day	37% 14% 33% 12% 4	300
	21st to end	Day	25% 16% 34% 20% 5%	318
March	1st to 10th	Day	22% 12% 30% 25% 10% 1	500
	11th to 20th	Day	20% 14% 37% 22% 5% 1	500
	21st to end	Day	23% 13% 32% 26% 7% 1	550
April	1st to 10th	Day	19% 16% 28% 23% 11% 2	500
	11th to 20th	Day	17% 14% 25% 24% 16% 4	492
	21st to end	Day	16% 9% 26% 26% 18% 4	500
May	1st to 10th	Day	14% 12% 28% 26% 14% 5%	503
	11th to 20th	Day	17% 4 21% 27% 19% 11%	527
	21st to end	Day	17% 3 16% 23% 26% 15%	549
June	1st to 10th	Day	11% 3 16% 29% 25% 16%	490
	11th to 20th	Day	12% 3 16% 23% 23% 22%	490
	21st to end	Day	10% 2 11% 31% 31% 16%	490
July	1st to 10th	Day	9% 2 8% 24% 34% 24%	485
	11th to 20th	Day	6% 10% 24% 30% 29%	484
	21st to end	Day	9% 2 10% 26% 30% 24%	532
August	1st to 10th	Day	14% 1 12% 24% 29% 20%	486
	11th to 20th	Day	11% 2 10% 27% 25% 24%	479
	21st to end	Day	10% 1 8% 23% 34% 24%	539
September	1st to 10th	Day	12% 2 9% 24% 30% 23%	490
	11th to 20th	Day	18% 3 11% 20% 29% 20%	490
	21st to end	Day	23% 4 18% 28% 20% 7%	490
October	1st to 10th	Day	17% 3 17% 29% 21% 12%	490
	11th to 20th	Day	14% 5% 22% 29% 22% 7%	490
	21st to end	Day	16% 6% 21% 32% 18% 7%	539
November	1st to 10th	Day	20% 5% 24% 31% 15% 4	495
	11th to 20th	Day	17% 10% 33% 23% 14% 3	355
	21st to end	Day	14% 13% 31% 25% 13% 4	300
December	1st to 10th	Day	26% 9% 33% 24% 7% 1	300
	11th to 20th	Day	21% 11% 26% 28% 13% 1	300
	21st to end	Day	26% 13% 27% 24% 8% 2	330
January	1st to 10th	Night	19% 11% 36% 29% 6%	800
	11th to 20th	Night	20% 11% 34% 31% 4	800
	21st to end	Night	18% 8% 39% 33% 2	880
February	1st to 10th	Night	19% 10% 34% 34% 4	800
	11th to 20th	Night	23% 15% 34% 26% 2	783
	21st to end	Night	18% 12% 37% 31% 2	660
March	1st to 10th	Night	17% 11% 32% 34% 6%	800
	11th to 20th	Night	18% 12% 34% 29% 8% 1	800
	21st to end	Night	18% 10% 33% 33% 6%	880
April	1st to 10th	Night	17% 12% 32% 30% 9%	800
	11th to 20th	Night	15% 10% 31% 32% 13%	791
	21st to end	Night	17% 6% 28% 35% 14%	680
May	1st to 10th	Night	12% 7% 27% 37% 17% 1	600
	11th to 20th	Night	17% 4 22% 31% 24% 2	591
	21st to end	Night	14% 2 11% 36% 32% 4	634
June	1st to 10th	Night	11% 2 12% 39% 33% 4	570
	11th to 20th	Night	11% 2 12% 31% 38% 6%	570
	21st to end	Night	9% 1 11% 32% 44% 3	570
July	1st to 10th	Night	10% 2 10% 29% 43% 6%	555
	11th to 20th	Night	7% 7% 30% 48% 8%	552
	21st to end	Night	9% 1 8% 32% 42% 7%	624
August	1st to 10th	Night	16% 2 8% 28% 40% 8%	567
	11th to 20th	Night	12% 1 10% 32% 39% 7%	564
	21st to end	Night	10% 1 8% 27% 46% 8%	788
September	1st to 10th	Night	12% 2 9% 31% 39% 7%	760
	11th to 20th	Night	16% 2 11% 30% 35% 6%	760
	21st to end	Night	22% 3 16% 30% 27% 2	760
October	1st to 10th	Night	16% 5% 17% 28% 29% 5%	760
	11th to 20th	Night	16% 6% 22% 37% 18% 2	760
	21st to end	Night	16% 5% 22% 38% 18% 2	836
November	1st to 10th	Night	18% 5% 21% 39% 16% 2	780
	11th to 20th	Night	17% 7% 26% 38% 12%	800
	21st to end	Night	16% 6% 29% 36% 13%	800
December	1st to 10th	Night	22% 8% 32% 32% 8%	800
	11th to 20th	Night	22% 4 30% 36% 8%	800
	21st to end	Night	25% 6% 27% 35% 8%	880

Page generated at 2025-05-23 09:20:59. Version ac07

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