SCSU Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the SCSU Observatory clear sky chart from 2005-11-06 to 2026-07-16.

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	12% 8% 3 8% 5% 4 5% 4 6% 7% 39%	139049
	Night	11% 9% 3 8% 5% 4 5% 4 5% 7% 39%	133908
Transparency	Day	43% 6% 13% 26% 11% 1	142631
	Night	44% 5% 13% 24% 12% 1	138406
Seeing	Day	16% 10% 32% 26% 13% 3	18408
	Night	15% 9% 33% 32% 10% 1	28278

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	8% 12% 5% 15% 10% 6% 6% 4 5% 7% 22%	4274
	11th to 20th	Day	5% 9% 5% 13% 8% 8% 8% 6% 9% 7% 22%	4830
	21st to end	Day	3 10% 6% 10% 5% 5% 7% 4 6% 8% 34%	5812
February	1st to 10th	Day	8% 12% 5% 10% 6% 6% 5% 3 5% 7% 32%	1890
	11th to 20th	Day	7% 10% 3 11% 8% 5% 5% 3 6% 7% 35%	1844
	21st to end	Day	9% 11% 4 11% 6% 6% 5% 3 5% 7% 35%	1569
March	1st to 10th	Day	11% 9% 3 7% 4 4 4 3 5% 7% 43%	5039
	11th to 20th	Day	10% 8% 3 9% 4 5% 4 3 5% 9% 40%	5090
	21st to end	Day	21% 11% 3 9% 5% 4 4 3 5% 6% 27%	5583
April	1st to 10th	Day	17% 10% 4 8% 5% 3 4 3 5% 6% 35%	2509
	11th to 20th	Day	17% 12% 3 8% 5% 4 5% 3 5% 6% 32%	2578
	21st to end	Day	22% 10% 3 6% 4 3 4 3 5% 6% 33%	2730
May	1st to 10th	Day	11% 5% 2 5% 4 4 4 3 5% 6% 51%	7853
	11th to 20th	Day	16% 8% 2 5% 3 3 3 3 4 6% 45%	8222
	21st to end	Day	15% 6% 2 6% 4 5% 5% 4 6% 8% 39%	9047
June	1st to 10th	Day	12% 6% 2 5% 4 4 4 4 6% 9% 45%	2953
	11th to 20th	Day	15% 6% 2 7% 5% 5% 5% 4 6% 8% 37%	3150
	21st to end	Day	11% 4 2 6% 5% 5% 5% 4 7% 10% 40%	3150
July	1st to 10th	Day	8% 4 2 5% 5% 4 5% 4 7% 11% 46%	6950
	11th to 20th	Day	9% 3 2 4 3 3 3 3 5% 8% 56%	6566
	21st to end	Day	9% 4 2 5% 4 4 4 4 6% 9% 48%	6771
August	1st to 10th	Day	12% 6% 2 6% 4 4 4 4 6% 8% 45%	4234
	11th to 20th	Day	12% 5% 2 6% 4 5% 4 4 6% 10% 42%	4171
	21st to end	Day	12% 6% 2 6% 4 3 4 3 5% 9% 47%	4551
September	1st to 10th	Day	10% 5% 2 5% 4 4 4 4 5% 9% 49%	2400
	11th to 20th	Day	12% 5% 2 5% 4 4 5% 4 5% 7% 45%	2315
	21st to end	Day	14% 6% 2 5% 3 3 3 3 5% 8% 46%	2139
October	1st to 10th	Day	18% 7% 2 5% 4 3 3 3 6% 7% 42%	2000
	11th to 20th	Day	15% 10% 3 6% 5% 4 5% 3 5% 7% 39%	2000
	21st to end	Day	18% 10% 3 8% 4 3 4 3 4 6% 37%	2200
November	1st to 10th	Day	13% 12% 3 8% 5% 4 4 4 6% 5% 36%	1663
	11th to 20th	Day	10% 13% 6% 11% 6% 4 4 3 5% 6% 31%	1680
	21st to end	Day	12% 11% 5% 9% 5% 4 4 4 5% 7% 34%	1680
December	1st to 10th	Day	7% 10% 4 11% 9% 6% 7% 5% 7% 7% 25%	2690
	11th to 20th	Day	9% 16% 7% 11% 5% 5% 4 3 6% 6% 29%	3088
	21st to end	Day	20% 18% 6% 19% 5% 3 3 3 3 4 16%	3828
January	1st to 10th	Night	8% 11% 5% 14% 8% 6% 5% 4 5% 6% 29%	8614
	11th to 20th	Night	5% 11% 4 11% 7% 7% 10% 8% 8% 6% 23%	8144
	21st to end	Night	5% 13% 6% 9% 5% 6% 6% 4 7% 6% 35%	8402
February	1st to 10th	Night	7% 12% 5% 9% 6% 5% 5% 3 6% 7% 35%	2730
	11th to 20th	Night	6% 9% 4 9% 5% 4 5% 3 6% 8% 40%	2663
	21st to end	Night	10% 11% 2 8% 6% 5% 4 4 6% 7% 36%	2236
March	1st to 10th	Night	10% 7% 2 6% 4 4 4 3 5% 7% 47%	5079
	11th to 20th	Night	10% 7% 2 7% 4 4 5% 4 7% 9% 40%	5090
	21st to end	Night	23% 11% 3 10% 6% 4 4 3 4 4 28%	5588
April	1st to 10th	Night	15% 10% 3 7% 4 4 5% 3 5% 6% 39%	2110
	11th to 20th	Night	17% 11% 3 7% 4 4 4 3 5% 5% 39%	1989
	21st to end	Night	20% 10% 3 5% 3 4 4 3 5% 6% 37%	1890
May	1st to 10th	Night	10% 5% 2 6% 4 3 3 3 5% 6% 54%	5028
	11th to 20th	Night	16% 6% 2 5% 3 3 4 3 4 5% 50%	4670
	21st to end	Night	16% 8% 2 6% 4 3 4 3 6% 7% 41%	5139
June	1st to 10th	Night	11% 5% 1 5% 5% 3 5% 4 5% 9% 46%	1661
	11th to 20th	Night	10% 5% 2 6% 3 4 5% 5% 6% 8% 46%	1470
	21st to end	Night	10% 4 2 5% 4 4 4 3 6% 7% 51%	1470
July	1st to 10th	Night	9% 3 1 4 4 4 4 4 7% 9% 51%	3248
	11th to 20th	Night	8% 4 1 4 3 4 3 3 6% 8% 58%	3517
	21st to end	Night	9% 5% 2 6% 4 4 4 4 6% 8% 48%	4096
August	1st to 10th	Night	12% 6% 2 5% 3 3 4 3 5% 6% 51%	2926
	11th to 20th	Night	14% 4 2 5% 4 3 4 3 4 7% 49%	2888
	21st to end	Night	10% 5% 3 5% 3 4 4 3 6% 8% 48%	3353
September	1st to 10th	Night	8% 4 1 4 4 4 4 3 5% 8% 54%	2000
	11th to 20th	Night	10% 5% 1 6% 3 3 4 3 5% 7% 53%	2081
	21st to end	Night	12% 5% 2 4 3 3 3 3 6% 6% 53%	2240
October	1st to 10th	Night	14% 6% 2 5% 3 3 3 3 5% 7% 50%	2400
	11th to 20th	Night	14% 9% 3 6% 4 4 4 4 5% 6% 42%	2400
	21st to end	Night	17% 9% 3 7% 4 4 4 3 4 7% 38%	2640
November	1st to 10th	Night	12% 9% 3 7% 4 4 4 4 6% 7% 40%	2849
	11th to 20th	Night	13% 11% 4 10% 5% 4 4 3 5% 6% 35%	2940
	21st to end	Night	12% 8% 4 10% 5% 5% 4 3 5% 6% 38%	2940
December	1st to 10th	Night	6% 7% 4 12% 8% 7% 6% 5% 7% 7% 30%	4720
	11th to 20th	Night	10% 14% 6% 13% 7% 5% 5% 4 5% 5% 25%	5339
	21st to end	Night	20% 18% 6% 16% 6% 4 3 3 3 4 18%	7358

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	61% 5% 11% 14% 10%	4275
	11th to 20th	Day	57% 5% 10% 19% 8%	4830
	21st to end	Day	47% 6% 13% 23% 11%	5812
February	1st to 10th	Day	52% 4 13% 21% 8% 1	1890
	11th to 20th	Day	48% 5% 13% 25% 7% 1	1853
	21st to end	Day	50% 5% 13% 20% 11%	1569
March	1st to 10th	Day	40% 6% 16% 25% 10% 3	5039
	11th to 20th	Day	41% 5% 15% 29% 9% 1	5090
	21st to end	Day	57% 6% 11% 18% 7%	5585
April	1st to 10th	Day	49% 6% 10% 21% 13% 1	2509
	11th to 20th	Day	53% 6% 10% 18% 12% 1	2578
	21st to end	Day	52% 6% 11% 20% 10% 1	2730
May	1st to 10th	Day	33% 5% 12% 29% 20% 1	7891
	11th to 20th	Day	39% 5% 8% 25% 16% 6%	8226
	21st to end	Day	41% 6% 10% 31% 10% 1	9061
June	1st to 10th	Day	35% 8% 15% 30% 11% 1	2953
	11th to 20th	Day	43% 8% 14% 20% 14% 1	3150
	21st to end	Day	36% 8% 18% 27% 10% 2	3150
July	1st to 10th	Day	32% 6% 20% 31% 10%	6876
	11th to 20th	Day	27% 5% 17% 37% 13% 1	6488
	21st to end	Day	31% 6% 18% 30% 13% 1	6578
August	1st to 10th	Day	37% 6% 13% 30% 12% 2	4220
	11th to 20th	Day	37% 6% 14% 32% 9%	4210
	21st to end	Day	35% 5% 18% 28% 12% 1	4550
September	1st to 10th	Day	31% 6% 17% 35% 11%	2400
	11th to 20th	Day	36% 9% 17% 30% 8%	2315
	21st to end	Day	36% 7% 12% 34% 11% 1	2139
October	1st to 10th	Day	31% 4 10% 33% 21% 1	3260
	11th to 20th	Day	39% 5% 13% 24% 19% 1	3240
	21st to end	Day	47% 4 14% 27% 8%	3538
November	1st to 10th	Day	48% 6% 12% 20% 11% 2	1663
	11th to 20th	Day	53% 6% 12% 18% 9%	1680
	21st to end	Day	49% 7% 15% 21% 7% 2	1680
December	1st to 10th	Day	56% 6% 16% 17% 4 1	2690
	11th to 20th	Day	55% 5% 13% 19% 8%	3085
	21st to end	Day	73% 4 8% 10% 5%	3828
January	1st to 10th	Night	56% 5% 14% 16% 9%	8615
	11th to 20th	Night	57% 6% 10% 17% 10% 1	8144
	21st to end	Night	47% 5% 11% 26% 10%	8402
February	1st to 10th	Night	47% 6% 14% 22% 10% 1	2730
	11th to 20th	Night	40% 7% 15% 25% 11% 1	2664
	21st to end	Night	46% 6% 13% 21% 13% 1	2236
March	1st to 10th	Night	35% 5% 15% 27% 16% 2	5079
	11th to 20th	Night	37% 6% 16% 27% 14%	5090
	21st to end	Night	60% 4 9% 17% 9% 1	5588
April	1st to 10th	Night	46% 6% 11% 23% 14% 1	2110
	11th to 20th	Night	46% 5% 9% 22% 15% 2	1989
	21st to end	Night	48% 7% 9% 20% 14% 2	1890
May	1st to 10th	Night	30% 5% 10% 34% 18% 2	5055
	11th to 20th	Night	36% 5% 8% 20% 23% 8%	4671
	21st to end	Night	41% 5% 10% 29% 13% 1	5149
June	1st to 10th	Night	32% 8% 16% 28% 14% 2	1661
	11th to 20th	Night	33% 8% 16% 25% 16% 3	1470
	21st to end	Night	31% 8% 15% 28% 16% 3	1470
July	1st to 10th	Night	29% 6% 21% 28% 16% 1	3215
	11th to 20th	Night	26% 4 16% 38% 15% 2	3476
	21st to end	Night	34% 6% 16% 29% 12% 2	3978
August	1st to 10th	Night	34% 6% 13% 28% 14% 5%	2916
	11th to 20th	Night	36% 6% 14% 32% 13%	2915
	21st to end	Night	34% 6% 18% 30% 11% 2	3353
September	1st to 10th	Night	26% 7% 17% 33% 15% 1	2000
	11th to 20th	Night	30% 6% 18% 30% 16%	2081
	21st to end	Night	31% 5% 13% 34% 16% 1	2240
October	1st to 10th	Night	23% 4 8% 36% 25% 4	3920
	11th to 20th	Night	34% 4 12% 26% 21% 2	3899
	21st to end	Night	42% 4 14% 27% 12%	4252
November	1st to 10th	Night	42% 6% 15% 24% 12% 2	2849
	11th to 20th	Night	50% 6% 12% 20% 11% 1	2940
	21st to end	Night	47% 6% 17% 22% 7% 1	2940
December	1st to 10th	Night	52% 6% 19% 18% 5%	4720
	11th to 20th	Night	59% 6% 10% 18% 5% 1	5341
	21st to end	Night	71% 5% 8% 13% 4	7358

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	12% 13% 31% 31% 12% 2	330
	11th to 20th	Day	15% 11% 36% 32% 5%	330
	21st to end	Day	11% 11% 41% 23% 12% 2	363
February	1st to 10th	Day	14% 18% 42% 22% 4 1	330
	11th to 20th	Day	12% 18% 38% 23% 8% 2	330
	21st to end	Day	14% 13% 42% 27% 4	273
March	1st to 10th	Day	18% 15% 40% 22% 5% 1	330
	11th to 20th	Day	21% 11% 36% 21% 9% 2	330
	21st to end	Day	26% 14% 30% 20% 8% 2	363
April	1st to 10th	Day	26% 14% 32% 22% 6%	529
	11th to 20th	Day	25% 16% 29% 21% 8%	616
	21st to end	Day	25% 12% 33% 21% 8% 2	750
May	1st to 10th	Day	20% 13% 32% 24% 10% 2	753
	11th to 20th	Day	21% 11% 29% 26% 12% 2	775
	21st to end	Day	18% 7% 30% 29% 14% 2	844
June	1st to 10th	Day	11% 8% 29% 30% 18% 4	780
	11th to 20th	Day	16% 10% 35% 28% 10% 1	780
	21st to end	Day	15% 7% 33% 28% 13% 4	780
July	1st to 10th	Day	9% 4 22% 33% 23% 9%	771
	11th to 20th	Day	8% 6% 27% 32% 19% 8%	744
	21st to end	Day	7% 5% 27% 34% 22% 6%	794
August	1st to 10th	Day	10% 5% 25% 32% 24% 5%	726
	11th to 20th	Day	15% 4 23% 29% 23% 5%	715
	21st to end	Day	11% 7% 24% 32% 21% 5%	731
September	1st to 10th	Day	12% 7% 30% 32% 15% 4	530
	11th to 20th	Day	10% 4 30% 33% 18% 4	445
	21st to end	Day	17% 11% 28% 21% 18% 5%	330
October	1st to 10th	Day	18% 11% 32% 23% 11% 5%	330
	11th to 20th	Day	17% 19% 35% 21% 7% 1	330
	21st to end	Day	23% 16% 38% 16% 7% 1	363
November	1st to 10th	Day	21% 15% 33% 23% 6% 2	330
	11th to 20th	Day	17% 20% 42% 19% 2	330
	21st to end	Day	18% 15% 42% 18% 6% 1	330
December	1st to 10th	Day	14% 12% 42% 24% 9%	330
	11th to 20th	Day	20% 15% 39% 23% 3	330
	21st to end	Day	28% 12% 39% 18% 3 1	363
January	1st to 10th	Night	11% 12% 37% 30% 9% 1	1050
	11th to 20th	Night	15% 11% 37% 29% 6% 1	1050
	21st to end	Night	15% 12% 38% 28% 7%	1155
February	1st to 10th	Night	13% 11% 39% 28% 8% 1	1050
	11th to 20th	Night	10% 12% 43% 28% 7% 1	1029
	21st to end	Night	15% 12% 35% 29% 8% 1	852
March	1st to 10th	Night	16% 11% 36% 28% 8% 1	656
	11th to 20th	Night	19% 11% 38% 26% 5%	630
	21st to end	Night	22% 9% 33% 25% 9% 2	693
April	1st to 10th	Night	20% 12% 35% 27% 6%	630
	11th to 20th	Night	21% 14% 30% 28% 7%	623
	21st to end	Night	23% 10% 29% 28% 8% 1	630
May	1st to 10th	Night	16% 10% 36% 28% 10%	630
	11th to 20th	Night	15% 9% 32% 33% 10%	621
	21st to end	Night	15% 8% 27% 37% 13%	683
June	1st to 10th	Night	12% 6% 27% 40% 15%	630
	11th to 20th	Night	11% 5% 33% 42% 8%	630
	21st to end	Night	11% 6% 26% 43% 14% 1	630
July	1st to 10th	Night	5% 2 22% 45% 26% 1	615
	11th to 20th	Night	8% 3 24% 47% 16% 2	597
	21st to end	Night	6% 3 23% 45% 21% 3	657
August	1st to 10th	Night	9% 3 20% 46% 22% 1	597
	11th to 20th	Night	13% 3 19% 40% 24% 2	594
	21st to end	Night	8% 6% 24% 37% 23% 2	660
September	1st to 10th	Night	11% 4 27% 38% 18% 2	600
	11th to 20th	Night	12% 5% 31% 32% 19% 1	600
	21st to end	Night	12% 9% 33% 30% 15% 1	640
October	1st to 10th	Night	16% 10% 31% 31% 12% 1	800
	11th to 20th	Night	20% 10% 35% 28% 6% 1	800
	21st to end	Night	23% 11% 32% 26% 7%	880
November	1st to 10th	Night	16% 11% 34% 31% 7%	1011
	11th to 20th	Night	17% 15% 34% 26% 7% 1	1050
	21st to end	Night	17% 13% 38% 24% 8%	1050
December	1st to 10th	Night	14% 12% 35% 30% 8% 1	1050
	11th to 20th	Night	19% 11% 36% 28% 5%	1050
	21st to end	Night	25% 10% 34% 25% 5% 1	1155

Page generated at 2026-07-13 02:14:50. Version ac07

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