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 2024-11-02.

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%	101174
	Night	15% 6% 2 6% 4 4 4 4 5% 7% 43%	97762
Transparency	Day	38% 7% 12% 22% 17% 4	100986
	Night	39% 6% 11% 20% 18% 5%	97620
Seeing	Day	17% 7% 21% 25% 19% 11%	15298
	Night	16% 6% 23% 33% 20% 2	25368

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% 4 1 3 3 2 3 3 4 6% 52%	2836
	11th to 20th	Day	15% 6% 2 5% 3 3 3 3 4 5% 51%	2840
	21st to end	Day	24% 6% 2 6% 3 3 3 3 4 4 42%	3124
February	1st to 10th	Day	15% 5% 2 6% 4 4 4 3 5% 7% 44%	1710
	11th to 20th	Day	19% 7% 2 4 3 3 4 3 4 6% 44%	1766
	21st to end	Day	20% 6% 2 7% 4 4 4 3 5% 6% 39%	1635
March	1st to 10th	Day	24% 6% 2 4 3 4 3 2 6% 6% 39%	3435
	11th to 20th	Day	11% 5% 2 4 4 3 4 3 5% 6% 54%	3480
	21st to end	Day	20% 8% 2 5% 3 3 3 3 4 6% 43%	3815
April	1st to 10th	Day	11% 4 1 4 3 4 4 3 6% 8% 52%	2123
	11th to 20th	Day	14% 5% 1 5% 3 3 4 4 5% 7% 49%	2254
	21st to end	Day	13% 6% 2 5% 4 4 4 4 5% 7% 46%	2408
May	1st to 10th	Day	12% 7% 2 7% 5% 5% 6% 5% 8% 9% 34%	4082
	11th to 20th	Day	18% 7% 3 6% 4 5% 4 4 6% 8% 35%	4081
	21st to end	Day	8% 4 2 5% 4 4 5% 4 7% 10% 46%	4476
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 3 2 3 5% 56%	2090
November	1st to 10th	Day	12% 6% 1 4 4 3 3 2 4 4 56%	1868
	11th to 20th	Day	13% 5% 2 4 3 3 3 3 5% 6% 54%	1763
	21st to end	Day	15% 6% 1 4 4 3 3 3 5% 6% 51%	1710
December	1st to 10th	Day	19% 7% 2 5% 4 5% 4 3 5% 5% 41%	1710
	11th to 20th	Day	15% 5% 1 4 2 2 2 2 4 7% 53%	2153
	21st to end	Day	19% 6% 2 5% 3 4 4 4 5% 6% 41%	3124
January	1st to 10th	Night	15% 6% 2 5% 4 4 3 3 4 5% 49%	4109
	11th to 20th	Night	15% 6% 2 5% 3 3 3 3 4 5% 50%	4120
	21st to end	Night	24% 7% 2 5% 3 3 3 3 4 5% 42%	4532
February	1st to 10th	Night	15% 6% 2 6% 4 4 4 4 5% 6% 44%	2470
	11th to 20th	Night	20% 7% 2 5% 3 3 4 2 4 5% 45%	2296
	21st to end	Night	16% 7% 3 6% 4 4 3 3 5% 6% 43%	1818
March	1st to 10th	Night	21% 9% 2 7% 4 4 4 3 4 5% 38%	3418
	11th to 20th	Night	12% 5% 2 4 3 3 4 3 4 6% 54%	3480
	21st to end	Night	19% 5% 1 4 3 3 4 3 4 6% 46%	3817
April	1st to 10th	Night	13% 6% 2 5% 3 3 4 3 5% 7% 48%	2057
	11th to 20th	Night	13% 5% 2 5% 4 4 4 3 5% 8% 48%	1875
	21st to end	Night	14% 5% 2 6% 3 4 4 4 4 6% 48%	1786
May	1st to 10th	Night	14% 7% 3 7% 5% 4 4 3 5% 7% 41%	2813
	11th to 20th	Night	17% 8% 4 7% 5% 4 4 4 6% 7% 37%	2812
	21st to end	Night	9% 5% 2 6% 4 4 5% 4 8% 10% 44%	3079
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 2 2 3 4 54%	2244
	11th to 20th	Night	13% 5% 2 4 4 4 3 3 4 5% 53%	2416
	21st to end	Night	15% 5% 1 3 3 3 3 4 5% 6% 51%	2470
December	1st to 10th	Night	19% 7% 2 6% 3 3 3 3 5% 6% 42%	2470
	11th to 20th	Night	17% 6% 1 5% 3 3 4 3 4 5% 49%	3042
	21st to end	Night	21% 4 2 5% 3 3 4 3 6% 7% 41%	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	35% 3 6% 28% 21% 7%	2837
	11th to 20th	Day	37% 4 5% 18% 31% 5%	2840
	21st to end	Day	46% 3 6% 18% 22% 5%	3124
February	1st to 10th	Day	40% 5% 11% 21% 19% 5%	1710
	11th to 20th	Day	41% 4 8% 22% 19% 6%	1775
	21st to end	Day	45% 5% 11% 15% 18% 7%	1635
March	1st to 10th	Day	45% 5% 8% 19% 15% 8%	3435
	11th to 20th	Day	29% 6% 8% 21% 23% 12%	3480
	21st to end	Day	44% 4 8% 21% 20% 3	3815
April	1st to 10th	Day	29% 5% 11% 23% 25% 7%	2123
	11th to 20th	Day	35% 6% 10% 21% 20% 9%	2254
	21st to end	Day	37% 7% 12% 24% 17% 4	2408
May	1st to 10th	Day	42% 11% 15% 19% 11% 3	4120
	11th to 20th	Day	46% 9% 11% 23% 11% 1	4085
	21st to end	Day	30% 9% 17% 23% 17% 4	4489
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	33% 4 10% 23% 25% 5%	1942
	11th to 20th	Day	30% 5% 8% 26% 22% 9%	1900
	21st to end	Day	32% 4 7% 25% 23% 9%	2090
November	1st to 10th	Day	33% 4 7% 21% 27% 8%	1868
	11th to 20th	Day	31% 4 8% 23% 27% 7%	1763
	21st to end	Day	35% 4 9% 22% 25% 5%	1710
December	1st to 10th	Day	44% 6% 7% 20% 19% 4	1710
	11th to 20th	Day	32% 3 9% 23% 25% 9%	2150
	21st to end	Day	41% 5% 10% 24% 18% 2	3124
January	1st to 10th	Night	37% 4 7% 21% 25% 6%	4109
	11th to 20th	Night	37% 4 8% 18% 27% 6%	4120
	21st to end	Night	45% 4 6% 19% 20% 7%	4532
February	1st to 10th	Night	40% 5% 10% 21% 20% 4	2470
	11th to 20th	Night	42% 4 8% 18% 20% 7%	2298
	21st to end	Night	43% 5% 9% 18% 20% 6%	1818
March	1st to 10th	Night	48% 5% 8% 16% 17% 5%	3418
	11th to 20th	Night	31% 5% 8% 19% 23% 14%	3480
	21st to end	Night	39% 5% 7% 24% 19% 6%	3817
April	1st to 10th	Night	35% 5% 11% 20% 23% 6%	2057
	11th to 20th	Night	35% 5% 9% 21% 22% 8%	1875
	21st to end	Night	37% 6% 11% 24% 18% 4	1786
May	1st to 10th	Night	42% 7% 14% 23% 12% 1	2840
	11th to 20th	Night	45% 9% 10% 21% 14% 1	2813
	21st to end	Night	30% 10% 16% 27% 15% 2	3087
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% 9% 22% 23% 4	2237
	11th to 20th	Night	32% 5% 7% 25% 23% 8%	2280
	21st to end	Night	32% 4 7% 21% 26% 11%	2508
November	1st to 10th	Night	34% 4 8% 19% 25% 10%	2244
	11th to 20th	Night	34% 3 7% 19% 26% 10%	2416
	21st to end	Night	33% 4 7% 24% 26% 6%	2470
December	1st to 10th	Night	44% 5% 8% 20% 19% 5%	2470
	11th to 20th	Night	38% 4 7% 19% 23% 9%	3044
	21st to end	Night	40% 5% 10% 22% 20% 2	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	25% 15% 31% 21% 7%	270
	11th to 20th	Day	27% 15% 33% 17% 8%	270
	21st to end	Day	24% 13% 40% 18% 5%	297
February	1st to 10th	Day	20% 15% 34% 22% 9%	270
	11th to 20th	Day	34% 14% 35% 12% 5%	270
	21st to end	Day	27% 16% 34% 19% 5%	290
March	1st to 10th	Day	21% 12% 30% 25% 10% 1	460
	11th to 20th	Day	19% 14% 38% 23% 5% 1	460
	21st to end	Day	23% 12% 31% 26% 8% 1	506
April	1st to 10th	Day	17% 16% 29% 24% 12% 3	460
	11th to 20th	Day	17% 14% 23% 25% 17% 4	452
	21st to end	Day	16% 9% 27% 26% 18% 4	460
May	1st to 10th	Day	15% 12% 27% 26% 14% 6%	463
	11th to 20th	Day	16% 4 19% 28% 21% 12%	487
	21st to end	Day	17% 3 14% 23% 27% 16%	529
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	17% 6% 21% 32% 18% 7%	539
November	1st to 10th	Day	20% 5% 24% 30% 16% 4	460
	11th to 20th	Day	17% 11% 32% 23% 14% 3	323
	21st to end	Day	14% 12% 32% 24% 14% 5%	270
December	1st to 10th	Day	26% 7% 33% 25% 8% 1	270
	11th to 20th	Day	20% 11% 26% 29% 13% 1	270
	21st to end	Day	25% 13% 27% 24% 8% 2	297
January	1st to 10th	Night	19% 9% 35% 30% 6%	760
	11th to 20th	Night	20% 11% 34% 31% 5%	760
	21st to end	Night	19% 8% 39% 32% 3	836
February	1st to 10th	Night	19% 9% 34% 35% 4	760
	11th to 20th	Night	23% 15% 34% 27% 2	743
	21st to end	Night	19% 12% 37% 30% 2	628
March	1st to 10th	Night	17% 11% 31% 35% 6%	760
	11th to 20th	Night	19% 11% 33% 29% 8% 1	760
	21st to end	Night	19% 9% 32% 34% 6%	836
April	1st to 10th	Night	17% 11% 32% 31% 9%	760
	11th to 20th	Night	15% 10% 29% 32% 13%	751
	21st to end	Night	17% 6% 29% 35% 13%	646
May	1st to 10th	Night	12% 7% 27% 37% 17% 1	570
	11th to 20th	Night	17% 4 21% 31% 25% 2	561
	21st to end	Night	15% 2 10% 37% 33% 4	617
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	17% 5% 21% 39% 17% 2	748
	11th to 20th	Night	16% 7% 26% 38% 12%	760
	21st to end	Night	16% 5% 28% 36% 14%	760
December	1st to 10th	Night	22% 7% 31% 33% 8%	760
	11th to 20th	Night	20% 4 31% 36% 8%	760
	21st to end	Night	26% 5% 27% 34% 8%	836

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