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 2026-09-29.

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%	149026
	Night	15% 6% 2 6% 4 4 4 3 5% 7% 44%	142397
Transparency	Day	38% 6% 13% 21% 17% 5%	152658
	Night	39% 6% 11% 21% 19% 6%	146837
Seeing	Day	17% 8% 22% 25% 19% 10%	17872
	Night	16% 6% 23% 33% 19% 2	27908

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	17% 7% 2 4 3 2 3 3 4 5% 51%	5258
	11th to 20th	Day	13% 6% 1 4 2 2 2 2 3 4 60%	5280
	21st to end	Day	19% 7% 2 6% 3 4 3 3 4 5% 45%	5780
February	1st to 10th	Day	15% 5% 2 6% 4 4 4 4 6% 7% 44%	1890
	11th to 20th	Day	20% 7% 2 4 3 3 4 3 4 6% 44%	1958
	21st to end	Day	19% 6% 3 7% 3 4 4 3 5% 7% 40%	1802
March	1st to 10th	Day	20% 7% 2 5% 4 4 3 3 5% 6% 41%	5045
	11th to 20th	Day	10% 5% 2 5% 4 3 3 3 4 6% 54%	5090
	21st to end	Day	17% 7% 2 6% 4 3 3 3 4 6% 47%	5586
April	1st to 10th	Day	12% 4 1 4 3 4 4 3 6% 8% 52%	2347
	11th to 20th	Day	13% 5% 1 5% 3 3 4 3 5% 7% 50%	2494
	21st to end	Day	13% 6% 2 5% 4 4 4 4 5% 8% 45%	2662
May	1st to 10th	Day	12% 6% 2 7% 4 5% 5% 4 7% 9% 38%	7616
	11th to 20th	Day	17% 6% 2 6% 4 4 4 4 6% 8% 38%	7626
	21st to end	Day	17% 6% 3 7% 4 4 5% 4 6% 9% 35%	8391
June	1st to 10th	Day	9% 5% 2 6% 4 5% 5% 4 7% 11% 40%	2730
	11th to 20th	Day	8% 5% 2 6% 4 5% 6% 6% 8% 11% 39%	2730
	21st to end	Day	5% 5% 2 6% 5% 5% 6% 6% 7% 12% 41%	2730
July	1st to 10th	Day	6% 5% 2 6% 5% 5% 5% 5% 7% 12% 42%	7680
	11th to 20th	Day	9% 6% 2 7% 6% 6% 7% 5% 8% 12% 32%	7680
	21st to end	Day	13% 6% 3 8% 5% 5% 6% 5% 8% 10% 31%	8340
August	1st to 10th	Day	16% 8% 3 8% 5% 5% 7% 6% 9% 11% 23%	6014
	11th to 20th	Day	6% 5% 2 6% 5% 6% 6% 5% 8% 13% 38%	5951
	21st to end	Day	7% 4 2 5% 4 4 5% 4 6% 11% 49%	6149
September	1st to 10th	Day	10% 4 1 5% 5% 4 4 4 6% 9% 48%	2413
	11th to 20th	Day	12% 6% 2 5% 4 4 5% 4 7% 8% 43%	2310
	21st to end	Day	19% 7% 3 7% 4 4 6% 4 4 6% 37%	2288
October	1st to 10th	Day	11% 5% 3 5% 3 3 3 3 4 6% 55%	2044
	11th to 20th	Day	12% 5% 2 5% 3 2 3 3 4 5% 57%	2000
	21st to end	Day	14% 5% 2 5% 3 3 3 2 4 5% 56%	2200
November	1st to 10th	Day	12% 6% 2 5% 4 3 3 2 4 5% 55%	2057
	11th to 20th	Day	12% 5% 1 3 3 3 3 3 5% 7% 55%	1947
	21st to end	Day	14% 6% 1 4 3 3 3 3 4 6% 51%	1890
December	1st to 10th	Day	16% 5% 1 4 3 4 4 2 4 4 53%	3020
	11th to 20th	Day	18% 7% 2 6% 3 3 3 2 5% 6% 44%	3463
	21st to end	Day	22% 6% 2 4 3 3 4 3 5% 7% 42%	4565
January	1st to 10th	Night	14% 6% 2 4 3 3 3 2 3 5% 55%	7635
	11th to 20th	Night	14% 5% 2 4 3 2 2 2 3 4 59%	7680
	21st to end	Night	18% 7% 2 5% 3 3 3 3 4 5% 47%	8401
February	1st to 10th	Night	16% 6% 2 6% 4 4 4 4 5% 6% 45%	2730
	11th to 20th	Night	20% 7% 2 5% 3 3 4 2 4 5% 45%	2544
	21st to end	Night	16% 7% 3 6% 4 4 3 3 5% 5% 44%	2003
March	1st to 10th	Night	17% 7% 2 6% 4 4 4 3 4 5% 45%	5028
	11th to 20th	Night	10% 4 2 4 3 3 3 3 5% 6% 56%	5090
	21st to end	Night	15% 5% 1 5% 4 4 4 3 5% 7% 47%	5588
April	1st to 10th	Night	13% 5% 2 5% 3 4 4 3 5% 8% 48%	2273
	11th to 20th	Night	12% 5% 2 5% 4 4 4 3 5% 8% 48%	2075
	21st to end	Night	13% 5% 2 6% 3 4 4 4 4 7% 47%	1974
May	1st to 10th	Night	14% 7% 2 7% 5% 4 5% 3 6% 8% 40%	5237
	11th to 20th	Night	15% 7% 3 6% 4 4 4 3 5% 8% 41%	5234
	21st to end	Night	21% 7% 2 6% 4 3 4 3 6% 8% 35%	5761
June	1st to 10th	Night	9% 6% 2 7% 5% 5% 5% 5% 9% 10% 38%	1890
	11th to 20th	Night	10% 5% 2 6% 5% 4 5% 4 8% 11% 40%	1890
	21st to end	Night	7% 5% 2 6% 5% 5% 6% 6% 7% 10% 42%	1890
July	1st to 10th	Night	9% 7% 3 7% 6% 5% 6% 5% 7% 11% 33%	5278
	11th to 20th	Night	12% 7% 3 8% 6% 6% 6% 6% 8% 10% 27%	5277
	21st to end	Night	15% 7% 3 8% 6% 5% 5% 5% 7% 10% 29%	5736
August	1st to 10th	Night	21% 9% 3 8% 6% 5% 6% 5% 7% 10% 20%	4135
	11th to 20th	Night	8% 5% 2 7% 5% 6% 6% 5% 9% 12% 34%	4095
	21st to end	Night	8% 5% 2 7% 4 5% 5% 5% 6% 8% 46%	4930
September	1st to 10th	Night	9% 6% 2 5% 4 5% 5% 4 6% 7% 48%	2202
	11th to 20th	Night	13% 6% 3 8% 5% 5% 5% 3 5% 8% 39%	2310
	21st to end	Night	19% 8% 2 6% 4 4 5% 4 6% 7% 36%	2289
October	1st to 10th	Night	13% 6% 2 5% 4 3 4 3 4 6% 49%	2355
	11th to 20th	Night	14% 5% 2 4 3 3 3 2 4 5% 55%	2400
	21st to end	Night	15% 4 2 4 2 3 2 2 3 5% 57%	2640
November	1st to 10th	Night	16% 5% 2 4 3 3 3 2 3 5% 54%	2461
	11th to 20th	Night	13% 5% 2 4 4 4 3 3 4 5% 53%	2671
	21st to end	Night	14% 4 2 3 3 3 3 4 5% 6% 52%	2730
December	1st to 10th	Night	14% 6% 2 5% 3 3 3 3 5% 6% 51%	4380
	11th to 20th	Night	21% 8% 3 6% 4 3 3 3 4 4 41%	4952
	21st to end	Night	21% 4 2 4 3 3 4 3 5% 7% 45%	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	38% 2 8% 20% 26% 6%	5259
	11th to 20th	Day	30% 3 3 14% 36% 14%	5280
	21st to end	Day	43% 3 5% 23% 20% 7%	5780
February	1st to 10th	Day	40% 5% 11% 21% 19% 5%	1890
	11th to 20th	Day	43% 4 8% 22% 18% 5%	1967
	21st to end	Day	44% 5% 10% 15% 19% 7%	1802
March	1st to 10th	Day	43% 4 7% 17% 17% 11%	5045
	11th to 20th	Day	30% 5% 6% 19% 23% 16%	5090
	21st to end	Day	41% 4 7% 24% 19% 6%	5586
April	1st to 10th	Day	30% 5% 11% 24% 24% 6%	2347
	11th to 20th	Day	33% 6% 10% 22% 20% 8%	2494
	21st to end	Day	37% 6% 12% 25% 16% 3	2662
May	1st to 10th	Day	40% 8% 15% 22% 14% 2	7651
	11th to 20th	Day	44% 6% 12% 24% 13% 2	7630
	21st to end	Day	45% 8% 13% 18% 14% 3	8405
June	1st to 10th	Day	35% 9% 15% 25% 14% 1	2730
	11th to 20th	Day	34% 10% 18% 22% 14% 2	2730
	21st to end	Day	32% 11% 17% 24% 14% 1	2730
July	1st to 10th	Day	34% 9% 20% 25% 10% 2	7615
	11th to 20th	Day	42% 10% 23% 19% 6% 1	7611
	21st to end	Day	47% 8% 15% 19% 11% 1	8156
August	1st to 10th	Day	51% 11% 20% 14% 4	6001
	11th to 20th	Day	34% 9% 26% 23% 7% 1	5990
	21st to end	Day	29% 6% 18% 32% 13% 1	6149
September	1st to 10th	Day	33% 8% 14% 23% 19% 4	2413
	11th to 20th	Day	36% 8% 13% 20% 19% 3	2310
	21st to end	Day	47% 6% 10% 17% 15% 4	2288
October	1st to 10th	Day	29% 4 8% 22% 28% 10%	3330
	11th to 20th	Day	21% 3 5% 22% 25% 24%	3240
	21st to end	Day	26% 3 8% 25% 23% 15%	3538
November	1st to 10th	Day	33% 4 8% 22% 26% 8%	2057
	11th to 20th	Day	30% 4 8% 24% 27% 6%	1947
	21st to end	Day	35% 4 9% 23% 25% 5%	1890
December	1st to 10th	Day	35% 4 6% 20% 28% 7%	3020
	11th to 20th	Day	42% 3 10% 17% 23% 6%	3460
	21st to end	Day	41% 4 11% 27% 16% 2	4565
January	1st to 10th	Night	33% 2 10% 22% 25% 8%	7636
	11th to 20th	Night	32% 3 5% 16% 35% 10%	7680
	21st to end	Night	40% 3 5% 22% 23% 7%	8401
February	1st to 10th	Night	39% 5% 10% 22% 20% 4	2730
	11th to 20th	Night	43% 4 9% 18% 20% 7%	2546
	21st to end	Night	42% 5% 8% 17% 21% 6%	2003
March	1st to 10th	Night	41% 4 6% 16% 24% 9%	5028
	11th to 20th	Night	28% 4 6% 19% 25% 17%	5090
	21st to end	Night	37% 4 7% 26% 21% 5%	5588
April	1st to 10th	Night	35% 5% 12% 20% 22% 5%	2273
	11th to 20th	Night	34% 5% 9% 22% 22% 8%	2075
	21st to end	Night	37% 6% 11% 24% 19% 4	1974
May	1st to 10th	Night	42% 6% 15% 22% 14% 2	5264
	11th to 20th	Night	41% 7% 13% 24% 15% 1	5235
	21st to end	Night	46% 7% 12% 22% 12% 2	5771
June	1st to 10th	Night	37% 10% 15% 22% 15% 2	1890
	11th to 20th	Night	35% 10% 17% 22% 14% 1	1890
	21st to end	Night	34% 10% 19% 20% 14% 3	1890
July	1st to 10th	Night	43% 10% 16% 20% 9% 1	5235
	11th to 20th	Night	49% 12% 20% 13% 6% 1	5226
	21st to end	Night	49% 8% 16% 17% 9% 1	5606
August	1st to 10th	Night	57% 11% 14% 13% 4	4124
	11th to 20th	Night	39% 9% 23% 22% 7% 1	4122
	21st to end	Night	34% 7% 13% 31% 13% 2	4930
September	1st to 10th	Night	33% 8% 15% 22% 17% 6%	2202
	11th to 20th	Night	42% 8% 11% 18% 19% 2	2310
	21st to end	Night	46% 7% 12% 16% 15% 4	2289
October	1st to 10th	Night	37% 5% 7% 18% 25% 8%	3849
	11th to 20th	Night	23% 3 6% 23% 28% 17%	3899
	21st to end	Night	27% 3 6% 24% 27% 15%	4252
November	1st to 10th	Night	35% 4 8% 20% 24% 9%	2461
	11th to 20th	Night	34% 3 7% 19% 26% 10%	2671
	21st to end	Night	33% 4 7% 24% 26% 6%	2730
December	1st to 10th	Night	35% 4 7% 21% 29% 4	4380
	11th to 20th	Night	48% 3 7% 17% 16% 9%	4954
	21st to end	Night	38% 4 14% 22% 20% 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% 16% 31% 20% 6%	330
	11th to 20th	Day	26% 18% 32% 16% 7%	330
	21st to end	Day	24% 13% 40% 18% 4	363
February	1st to 10th	Day	20% 16% 35% 21% 8%	330
	11th to 20th	Day	35% 15% 33% 12% 4	330
	21st to end	Day	26% 17% 33% 19% 6%	345
March	1st to 10th	Day	21% 13% 30% 26% 10% 1	540
	11th to 20th	Day	20% 14% 37% 22% 6% 1	540
	21st to end	Day	21% 13% 32% 25% 7% 1	594
April	1st to 10th	Day	18% 15% 28% 24% 11% 3	540
	11th to 20th	Day	16% 13% 26% 25% 16% 4	532
	21st to end	Day	16% 9% 26% 26% 18% 4	540
May	1st to 10th	Day	15% 13% 29% 25% 14% 5%	543
	11th to 20th	Day	16% 5% 21% 28% 19% 11%	567
	21st to end	Day	20% 4 16% 22% 25% 14%	617
June	1st to 10th	Day	10% 4 17% 30% 25% 15%	570
	11th to 20th	Day	11% 4 18% 25% 24% 19%	570
	21st to end	Day	9% 2 10% 30% 33% 16%	570
July	1st to 10th	Day	9% 1 8% 23% 34% 24%	565
	11th to 20th	Day	7% 10% 24% 30% 29%	564
	21st to end	Day	10% 2 10% 25% 29% 23%	620
August	1st to 10th	Day	16% 2 12% 23% 29% 17%	566
	11th to 20th	Day	11% 2 10% 27% 26% 24%	559
	21st to end	Day	10% 2 9% 25% 32% 22%	627
September	1st to 10th	Day	11% 2 9% 25% 29% 23%	570
	11th to 20th	Day	16% 3 11% 20% 30% 20%	570
	21st to end	Day	23% 5% 19% 27% 19% 7%	562
October	1st to 10th	Day	16% 3 17% 29% 22% 13%	530
	11th to 20th	Day	14% 5% 23% 29% 22% 6%	530
	21st to end	Day	17% 8% 22% 30% 17% 6%	583
November	1st to 10th	Day	19% 6% 25% 31% 15% 4	535
	11th to 20th	Day	17% 11% 33% 23% 13% 3	387
	21st to end	Day	15% 15% 31% 24% 12% 4	330
December	1st to 10th	Day	27% 9% 34% 24% 6% 1	330
	11th to 20th	Day	22% 12% 26% 27% 13% 1	330
	21st to end	Day	26% 13% 28% 23% 8% 2	363
January	1st to 10th	Night	19% 11% 36% 29% 6%	840
	11th to 20th	Night	20% 11% 35% 30% 4	840
	21st to end	Night	18% 8% 40% 32% 2	924
February	1st to 10th	Night	19% 10% 34% 34% 3	840
	11th to 20th	Night	23% 15% 34% 26% 2	823
	21st to end	Night	18% 13% 37% 29% 2	692
March	1st to 10th	Night	17% 11% 31% 35% 6%	840
	11th to 20th	Night	18% 12% 33% 29% 8%	840
	21st to end	Night	17% 10% 34% 33% 6%	924
April	1st to 10th	Night	17% 12% 32% 30% 9%	840
	11th to 20th	Night	14% 10% 31% 33% 12%	831
	21st to end	Night	16% 6% 29% 35% 14%	714
May	1st to 10th	Night	12% 7% 28% 37% 16% 1	630
	11th to 20th	Night	16% 4 22% 32% 24% 2	621
	21st to end	Night	16% 2 12% 36% 30% 4	683
June	1st to 10th	Night	10% 2 13% 39% 32% 3	630
	11th to 20th	Night	11% 2 13% 32% 36% 6%	630
	21st to end	Night	9% 1 10% 32% 44% 3	630
July	1st to 10th	Night	10% 2 9% 29% 44% 6%	615
	11th to 20th	Night	7% 1 7% 29% 47% 9%	612
	21st to end	Night	10% 1 8% 31% 42% 7%	690
August	1st to 10th	Night	17% 2 8% 27% 39% 7%	627
	11th to 20th	Night	11% 1 9% 31% 40% 8%	624
	21st to end	Night	10% 2 8% 29% 44% 7%	871
September	1st to 10th	Night	11% 1 8% 32% 40% 7%	840
	11th to 20th	Night	16% 2 11% 30% 36% 7%	840
	21st to end	Night	22% 3 16% 30% 26% 2	833
October	1st to 10th	Night	15% 5% 17% 29% 29% 5%	800
	11th to 20th	Night	16% 6% 22% 37% 17% 2	800
	21st to end	Night	17% 6% 22% 37% 17% 2	880
November	1st to 10th	Night	17% 6% 21% 39% 15% 1	820
	11th to 20th	Night	16% 8% 27% 38% 11%	840
	21st to end	Night	15% 6% 29% 36% 13%	840
December	1st to 10th	Night	22% 7% 32% 32% 7%	840
	11th to 20th	Night	21% 4 31% 36% 8%	840
	21st to end	Night	25% 6% 27% 34% 7%	924

Page generated at 2026-09-26 04:18:39. Version ac07

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