Nutter Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Nutter Observatory clear sky chart from 2005-11-06 to 2026-02-16.

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	16% 11% 4 9% 6% 5% 5% 4 5% 7% 30%	131165
	Night	15% 11% 4 10% 6% 5% 4 3 5% 6% 30%	129654
Transparency	Day	52% 7% 12% 19% 10% 1	134696
	Night	53% 6% 12% 18% 10% 1	134210
Seeing	Day	26% 17% 28% 18% 9% 3	16299
	Night	23% 16% 28% 24% 9% 1	25657

By Month

Percentage Cloud Cover NE (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	13% 17% 9% 20% 9% 3 3 3 4 4 16%	4661
	11th to 20th	Day	12% 14% 7% 19% 8% 5% 5% 3 5% 5% 16%	4668
	21st to end	Day	13% 12% 7% 11% 9% 7% 5% 3 5% 5% 23%	5780
February	1st to 10th	Day	15% 14% 6% 14% 8% 5% 5% 5% 4 5% 21%	1890
	11th to 20th	Day	13% 14% 7% 13% 8% 4 6% 4 5% 5% 22%	1863
	21st to end	Day	17% 14% 3 13% 8% 3 4 3 5% 5% 25%	1650
March	1st to 10th	Day	19% 14% 6% 10% 6% 4 3 3 3 4 28%	4563
	11th to 20th	Day	11% 15% 5% 8% 5% 4 4 3 4 5% 36%	4980
	21st to end	Day	15% 11% 4 11% 7% 5% 5% 4 5% 5% 29%	5731
April	1st to 10th	Day	17% 15% 4 9% 4 4 4 3 4 6% 30%	2400
	11th to 20th	Day	16% 12% 3 8% 5% 4 4 4 5% 5% 33%	2374
	21st to end	Day	17% 12% 3 7% 4 4 4 3 4 6% 34%	2400
May	1st to 10th	Day	28% 10% 3 7% 4 3 4 4 5% 6% 25%	6171
	11th to 20th	Day	19% 10% 4 8% 6% 5% 4 3 5% 7% 28%	6318
	21st to end	Day	20% 9% 3 7% 5% 5% 5% 4 7% 8% 29%	6949
June	1st to 10th	Day	17% 7% 2 7% 6% 5% 5% 4 7% 8% 32%	2800
	11th to 20th	Day	17% 7% 2 6% 5% 5% 5% 5% 7% 9% 32%	2800
	21st to end	Day	14% 7% 2 7% 5% 5% 5% 5% 7% 8% 35%	2800
July	1st to 10th	Day	9% 6% 2 8% 5% 6% 6% 5% 7% 10% 36%	6360
	11th to 20th	Day	7% 6% 3 8% 6% 6% 6% 5% 8% 10% 32%	6360
	21st to end	Day	13% 6% 3 6% 5% 4 5% 5% 7% 8% 38%	6893
August	1st to 10th	Day	19% 8% 3 7% 4 4 5% 4 6% 8% 31%	4234
	11th to 20th	Day	16% 7% 3 7% 5% 5% 5% 4 7% 8% 31%	3996
	21st to end	Day	15% 8% 3 6% 4 4 5% 3 6% 7% 39%	4312
September	1st to 10th	Day	16% 7% 2 6% 5% 4 5% 4 5% 8% 40%	2400
	11th to 20th	Day	12% 7% 2 6% 4 4 3 3 5% 8% 45%	2400
	21st to end	Day	19% 8% 3 7% 5% 4 5% 4 5% 7% 33%	2140
October	1st to 10th	Day	16% 9% 3 8% 5% 4 4 4 5% 8% 36%	2000
	11th to 20th	Day	16% 11% 3 6% 6% 5% 4 4 5% 7% 33%	2000
	21st to end	Day	21% 13% 3 8% 5% 5% 3 3 5% 6% 28%	2125
November	1st to 10th	Day	13% 12% 3 8% 4 4 4 3 5% 7% 37%	1851
	11th to 20th	Day	14% 16% 4 9% 6% 4 5% 3 4 5% 31%	1822
	21st to end	Day	17% 16% 6% 9% 5% 5% 4 4 5% 5% 24%	1680
December	1st to 10th	Day	14% 16% 6% 15% 7% 6% 5% 3 4 5% 17%	2680
	11th to 20th	Day	18% 16% 9% 10% 4 3 3 2 4 5% 27%	3066
	21st to end	Day	17% 17% 7% 13% 6% 6% 4 3 5% 5% 18%	4048
January	1st to 10th	Night	11% 15% 7% 22% 9% 4 4 3 4 4 16%	8234
	11th to 20th	Night	10% 13% 7% 18% 8% 6% 5% 4 5% 6% 18%	8198
	21st to end	Night	13% 12% 4 11% 9% 8% 6% 4 5% 5% 23%	8401
February	1st to 10th	Night	14% 13% 5% 12% 6% 6% 7% 3 5% 6% 24%	2730
	11th to 20th	Night	13% 13% 6% 15% 8% 5% 5% 4 5% 5% 21%	2534
	21st to end	Night	17% 13% 4 12% 6% 4 4 3 6% 6% 25%	1980
March	1st to 10th	Night	17% 12% 6% 10% 7% 4 4 3 4 5% 29%	5274
	11th to 20th	Night	13% 13% 5% 9% 5% 4 3 3 4 6% 35%	4979
	21st to end	Night	15% 13% 5% 10% 5% 5% 4 3 4 5% 29%	5200
April	1st to 10th	Night	18% 14% 4 8% 5% 4 5% 3 5% 6% 28%	2000
	11th to 20th	Night	19% 12% 3 7% 4 4 3 3 4 7% 34%	1975
	21st to end	Night	17% 10% 3 7% 4 5% 4 2 4 6% 37%	2000
May	1st to 10th	Night	31% 11% 3 6% 4 4 4 3 4 5% 26%	3808
	11th to 20th	Night	19% 9% 3 7% 4 3 4 4 6% 6% 34%	3575
	21st to end	Night	19% 11% 3 7% 5% 4 4 4 6% 6% 30%	3926
June	1st to 10th	Night	16% 8% 2 6% 5% 4 3 3 6% 8% 38%	1600
	11th to 20th	Night	15% 7% 2 5% 5% 4 5% 3 6% 9% 39%	1600
	21st to end	Night	12% 5% 2 5% 5% 4 5% 4 5% 8% 44%	1600
July	1st to 10th	Night	10% 7% 3 8% 5% 5% 4 4 6% 9% 38%	3600
	11th to 20th	Night	11% 6% 2 8% 5% 4 5% 5% 7% 10% 38%	3600
	21st to end	Night	12% 7% 2 6% 4 4 4 3 5% 7% 45%	3942
August	1st to 10th	Night	19% 7% 3 7% 5% 5% 4 4 5% 7% 35%	2915
	11th to 20th	Night	16% 8% 3 7% 5% 4 4 3 4 6% 41%	3006
	21st to end	Night	13% 8% 2 5% 4 4 4 3 6% 7% 44%	3586
September	1st to 10th	Night	14% 6% 2 6% 4 4 4 3 4 7% 45%	2000
	11th to 20th	Night	13% 6% 2 4 4 3 4 3 5% 7% 51%	2000
	21st to end	Night	18% 7% 3 6% 4 4 4 4 5% 7% 38%	2255
October	1st to 10th	Night	16% 9% 3 7% 5% 4 5% 4 5% 6% 39%	2400
	11th to 20th	Night	19% 8% 3 6% 5% 3 4 4 5% 7% 37%	2400
	21st to end	Night	21% 12% 3 7% 5% 4 4 2 4 6% 30%	2715
November	1st to 10th	Night	12% 12% 3 7% 5% 4 4 3 5% 7% 39%	2667
	11th to 20th	Night	17% 12% 3 8% 4 4 5% 4 5% 5% 32%	2796
	21st to end	Night	14% 13% 4 9% 5% 5% 5% 4 6% 7% 27%	2940
December	1st to 10th	Night	11% 15% 7% 13% 7% 5% 4 3 6% 7% 21%	4720
	11th to 20th	Night	20% 15% 5% 8% 5% 3 4 3 3 4 29%	5348
	21st to end	Night	20% 16% 7% 11% 7% 4 4 3 4 4 21%	7150

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	71% 3 11% 12% 2	4662
	11th to 20th	Day	68% 6% 7% 11% 7% 1	4698
	21st to end	Day	61% 5% 10% 17% 7%	5780
February	1st to 10th	Day	63% 7% 10% 14% 6% 1	1890
	11th to 20th	Day	62% 6% 7% 17% 7%	1872
	21st to end	Day	60% 5% 10% 16% 7% 1	1650
March	1st to 10th	Day	60% 5% 10% 15% 9% 1	4566
	11th to 20th	Day	49% 6% 16% 18% 7% 3	4980
	21st to end	Day	55% 6% 8% 14% 15% 2	5731
April	1st to 10th	Day	55% 6% 7% 21% 9% 2	2400
	11th to 20th	Day	50% 7% 11% 18% 13% 2	2374
	21st to end	Day	51% 5% 10% 20% 12% 3	2400
May	1st to 10th	Day	59% 6% 10% 15% 9% 1	6182
	11th to 20th	Day	55% 7% 9% 19% 9% 1	6322
	21st to end	Day	50% 8% 12% 19% 11% 1	6949
June	1st to 10th	Day	47% 9% 12% 19% 13%	2800
	11th to 20th	Day	46% 8% 14% 21% 10% 1	2800
	21st to end	Day	43% 9% 14% 23% 10% 1	2800
July	1st to 10th	Day	41% 9% 15% 24% 11% 1	6290
	11th to 20th	Day	41% 9% 17% 23% 9%	6285
	21st to end	Day	41% 7% 12% 24% 15%	6695
August	1st to 10th	Day	50% 8% 14% 18% 10%	4221
	11th to 20th	Day	45% 9% 15% 24% 7%	4035
	21st to end	Day	42% 6% 11% 33% 8% 1	4312
September	1st to 10th	Day	43% 7% 12% 25% 13% 1	2400
	11th to 20th	Day	38% 6% 12% 23% 20% 1	2400
	21st to end	Day	48% 6% 12% 19% 12% 1	2140
October	1st to 10th	Day	44% 7% 10% 23% 15% 1	3260
	11th to 20th	Day	44% 6% 10% 23% 15% 3	3240
	21st to end	Day	40% 5% 16% 26% 12% 1	3415
November	1st to 10th	Day	46% 6% 14% 21% 12% 2	1851
	11th to 20th	Day	54% 6% 11% 18% 8% 2	1822
	21st to end	Day	59% 7% 14% 15% 5%	1680
December	1st to 10th	Day	67% 5% 11% 13% 3	2680
	11th to 20th	Day	62% 4 12% 13% 9%	3066
	21st to end	Day	67% 5% 10% 11% 6%	4048
January	1st to 10th	Night	69% 5% 11% 11% 4	8234
	11th to 20th	Night	64% 6% 11% 11% 6% 1	8243
	21st to end	Night	61% 5% 8% 13% 12% 1	8401
February	1st to 10th	Night	59% 6% 12% 15% 7%	2730
	11th to 20th	Night	62% 6% 11% 14% 6% 1	2536
	21st to end	Night	58% 5% 12% 16% 8% 1	1980
March	1st to 10th	Night	58% 4 12% 16% 11%	5284
	11th to 20th	Night	50% 6% 18% 17% 6% 3	4980
	21st to end	Night	53% 7% 9% 17% 12% 2	5200
April	1st to 10th	Night	57% 6% 9% 17% 10% 2	2000
	11th to 20th	Night	50% 6% 11% 18% 15% 1	1975
	21st to end	Night	48% 5% 11% 20% 13% 3	2000
May	1st to 10th	Night	60% 5% 9% 16% 9% 2	3814
	11th to 20th	Night	49% 8% 14% 18% 10% 1	3576
	21st to end	Night	51% 9% 11% 17% 11% 1	3927
June	1st to 10th	Night	43% 7% 13% 22% 15% 1	1600
	11th to 20th	Night	41% 9% 15% 24% 10% 1	1600
	21st to end	Night	37% 8% 15% 27% 13% 1	1600
July	1st to 10th	Night	41% 8% 16% 21% 14% 1	3560
	11th to 20th	Night	39% 9% 16% 22% 13%	3552
	21st to end	Night	37% 6% 12% 26% 18% 1	3825
August	1st to 10th	Night	49% 6% 16% 20% 8%	2904
	11th to 20th	Night	44% 6% 14% 26% 10%	3033
	21st to end	Night	37% 7% 12% 31% 11% 1	3586
September	1st to 10th	Night	39% 7% 13% 25% 15% 1	2000
	11th to 20th	Night	33% 6% 13% 29% 18% 1	2000
	21st to end	Night	45% 6% 12% 21% 15% 1	2255
October	1st to 10th	Night	49% 7% 10% 22% 12% 1	3920
	11th to 20th	Night	39% 5% 13% 22% 15% 5%	3899
	21st to end	Night	41% 5% 16% 25% 11% 1	4375
November	1st to 10th	Night	44% 6% 14% 20% 15% 2	2667
	11th to 20th	Night	51% 6% 13% 18% 10% 2	2796
	21st to end	Night	53% 8% 15% 18% 6%	2940
December	1st to 10th	Night	60% 7% 13% 18% 3	4720
	11th to 20th	Night	59% 5% 12% 15% 10%	5348
	21st to end	Night	66% 6% 9% 12% 7% 1	7150

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	31% 29% 29% 8% 3	220
	11th to 20th	Day	43% 28% 23% 4 1	223
	21st to end	Day	33% 28% 27% 10% 2	363
February	1st to 10th	Day	35% 28% 29% 8%	330
	11th to 20th	Day	38% 28% 29% 5%	380
	21st to end	Day	36% 23% 30% 10% 1	414
March	1st to 10th	Day	35% 20% 31% 11% 4	500
	11th to 20th	Day	37% 20% 27% 12% 3 1	500
	21st to end	Day	34% 26% 25% 10% 3 2	550
April	1st to 10th	Day	37% 22% 25% 11% 4	500
	11th to 20th	Day	32% 25% 25% 10% 6% 2	492
	21st to end	Day	30% 23% 28% 12% 6% 1	500
May	1st to 10th	Day	34% 17% 28% 11% 8% 2	503
	11th to 20th	Day	24% 17% 31% 17% 8% 3	527
	21st to end	Day	23% 11% 31% 20% 11% 4	573
June	1st to 10th	Day	20% 16% 31% 20% 12% 1	530
	11th to 20th	Day	20% 14% 27% 26% 12% 1	530
	21st to end	Day	19% 13% 34% 24% 8% 2	530
July	1st to 10th	Day	13% 8% 32% 29% 13% 5%	525
	11th to 20th	Day	12% 8% 27% 31% 18% 4	524
	21st to end	Day	15% 9% 35% 27% 11% 3	576
August	1st to 10th	Day	14% 10% 27% 23% 20% 7%	526
	11th to 20th	Day	18% 7% 24% 30% 15% 7%	519
	21st to end	Day	13% 12% 27% 26% 13% 9%	583
September	1st to 10th	Day	18% 12% 25% 21% 17% 7%	530
	11th to 20th	Day	14% 9% 25% 22% 20% 11%	530
	21st to end	Day	23% 13% 29% 20% 11% 5%	530
October	1st to 10th	Day	24% 13% 24% 20% 14% 5%	530
	11th to 20th	Day	25% 20% 28% 19% 6% 2	530
	21st to end	Day	32% 17% 27% 17% 6%	542
November	1st to 10th	Day	28% 18% 25% 20% 8% 1	425
	11th to 20th	Day	30% 23% 24% 15% 6% 1	362
	21st to end	Day	40% 22% 30% 6% 1	220
December	1st to 10th	Day	36% 22% 31% 8% 3 1	220
	11th to 20th	Day	38% 24% 26% 11% 1	220
	21st to end	Day	34% 27% 28% 10% 1	242
January	1st to 10th	Night	26% 26% 32% 14% 2	840
	11th to 20th	Night	28% 21% 29% 19% 2	840
	21st to end	Night	21% 26% 35% 17% 2	924
February	1st to 10th	Night	29% 20% 32% 18% 2	840
	11th to 20th	Night	28% 22% 36% 13% 1	804
	21st to end	Night	34% 18% 32% 14% 2	660
March	1st to 10th	Night	28% 24% 28% 18% 3	800
	11th to 20th	Night	28% 22% 31% 16% 3	800
	21st to end	Night	27% 21% 30% 17% 5% 1	880
April	1st to 10th	Night	29% 21% 30% 17% 3	800
	11th to 20th	Night	28% 20% 28% 17% 6%	791
	21st to end	Night	25% 15% 32% 22% 6%	800
May	1st to 10th	Night	29% 12% 24% 28% 8%	497
	11th to 20th	Night	18% 12% 34% 27% 8% 1	394
	21st to end	Night	14% 9% 30% 34% 11% 1	433
June	1st to 10th	Night	19% 8% 24% 36% 13%	400
	11th to 20th	Night	19% 8% 25% 34% 14% 1	400
	21st to end	Night	13% 7% 28% 42% 10%	400
July	1st to 10th	Night	9% 5% 22% 44% 16% 2	390
	11th to 20th	Night	12% 5% 20% 43% 19% 1	388
	21st to end	Night	15% 4 21% 44% 14% 1	442
August	1st to 10th	Night	11% 4 22% 38% 24% 2	597
	11th to 20th	Night	17% 4 15% 33% 28% 3	673
	21st to end	Night	15% 6% 19% 35% 23% 2	880
September	1st to 10th	Night	17% 7% 23% 32% 18% 3	800
	11th to 20th	Night	16% 6% 18% 32% 24% 5%	800
	21st to end	Night	20% 9% 26% 27% 16% 2	800
October	1st to 10th	Night	21% 10% 26% 24% 18% 2	800
	11th to 20th	Night	24% 15% 29% 25% 7% 1	800
	21st to end	Night	30% 15% 28% 21% 6%	880
November	1st to 10th	Night	21% 17% 30% 25% 7%	820
	11th to 20th	Night	27% 18% 31% 17% 5% 1	840
	21st to end	Night	26% 20% 31% 20% 4	840
December	1st to 10th	Night	27% 23% 29% 17% 3	840
	11th to 20th	Night	29% 22% 29% 18% 3	840
	21st to end	Night	30% 20% 31% 16% 3	924

Page generated at 2026-02-13 01:42:16. Version ac07

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