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 2025-03-12.

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% 38%	109344
	Night	11% 9% 3 9% 5% 4 4 4 5% 7% 38%	109805
Transparency	Day	44% 7% 14% 24% 11% 1	112968
	Night	44% 6% 13% 24% 12% 1	114322
Seeing	Day	16% 10% 31% 27% 13% 3	16437
	Night	15% 9% 33% 32% 11% 1	26534

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% 11% 4 13% 10% 6% 6% 4 5% 7% 26%	3289
	11th to 20th	Day	6% 8% 5% 13% 8% 7% 8% 7% 9% 7% 21%	3712
	21st to end	Day	4 13% 8% 12% 6% 5% 5% 3 5% 6% 31%	4488
February	1st to 10th	Day	8% 12% 5% 10% 6% 6% 5% 3 5% 7% 33%	1800
	11th to 20th	Day	7% 9% 3 11% 8% 5% 5% 3 6% 7% 35%	1754
	21st to end	Day	9% 11% 4 11% 6% 5% 5% 3 5% 6% 34%	1496
March	1st to 10th	Day	11% 11% 4 8% 5% 5% 5% 3 5% 6% 36%	3539
	11th to 20th	Day	9% 9% 2 8% 4 5% 4 4 5% 8% 40%	3491
	21st to end	Day	22% 11% 4 10% 7% 4 4 4 5% 6% 24%	3814
April	1st to 10th	Day	17% 10% 4 8% 5% 3 4 3 6% 6% 35%	2270
	11th to 20th	Day	18% 12% 3 8% 5% 4 4 3 4 6% 32%	2330
	21st to end	Day	22% 11% 3 7% 4 3 4 3 5% 6% 32%	2470
May	1st to 10th	Day	18% 6% 2 6% 5% 4 5% 4 5% 6% 39%	4208
	11th to 20th	Day	14% 6% 2 6% 4 4 4 4 5% 7% 44%	4399
	21st to end	Day	18% 6% 2 7% 4 5% 5% 4 6% 8% 35%	4824
June	1st to 10th	Day	12% 6% 2 5% 4 4 4 4 6% 8% 47%	2671
	11th to 20th	Day	14% 6% 2 7% 5% 4 5% 4 6% 8% 38%	2850
	21st to end	Day	10% 4 2 6% 5% 5% 5% 4 7% 10% 40%	2850
July	1st to 10th	Day	10% 5% 2 6% 5% 5% 5% 4 8% 11% 39%	4750
	11th to 20th	Day	6% 3 1 4 3 3 4 3 5% 8% 60%	4533
	21st to end	Day	7% 4 2 5% 4 5% 5% 4 7% 9% 49%	4702
August	1st to 10th	Day	12% 6% 2 5% 4 4 4 4 6% 8% 45%	4104
	11th to 20th	Day	12% 5% 2 6% 5% 5% 4 4 6% 10% 42%	4041
	21st to end	Day	12% 6% 2 6% 4 3 4 3 5% 8% 47%	4412
September	1st to 10th	Day	9% 4 1 5% 4 4 4 4 5% 9% 50%	2280
	11th to 20th	Day	12% 5% 2 5% 5% 4 5% 4 5% 7% 46%	2199
	21st to end	Day	14% 6% 2 6% 4 3 3 3 5% 8% 46%	2032
October	1st to 10th	Day	18% 7% 2 5% 4 3 3 3 6% 7% 42%	1900
	11th to 20th	Day	14% 10% 3 6% 5% 4 5% 3 5% 7% 39%	1900
	21st to end	Day	17% 10% 3 8% 4 3 4 3 4 6% 38%	2090
November	1st to 10th	Day	13% 12% 3 8% 5% 4 5% 3 6% 5% 37%	1583
	11th to 20th	Day	10% 13% 6% 12% 7% 4 4 3 5% 6% 30%	1600
	21st to end	Day	12% 11% 4 8% 5% 4 4 4 5% 7% 35%	1600
December	1st to 10th	Day	8% 10% 4 11% 9% 6% 7% 5% 7% 7% 25%	2610
	11th to 20th	Day	9% 16% 7% 11% 5% 5% 4 3 6% 6% 29%	3008
	21st to end	Day	20% 19% 6% 19% 5% 3 3 3 3 4 16%	3745
January	1st to 10th	Night	7% 10% 4 15% 8% 6% 4 3 5% 6% 31%	6641
	11th to 20th	Night	6% 11% 4 10% 7% 7% 10% 8% 8% 7% 23%	6252
	21st to end	Night	6% 16% 7% 9% 5% 5% 5% 4 6% 5% 31%	6490
February	1st to 10th	Night	7% 12% 5% 9% 6% 5% 5% 4 6% 7% 35%	2600
	11th to 20th	Night	5% 9% 5% 9% 5% 4 5% 3 6% 8% 40%	2533
	21st to end	Night	11% 11% 3 8% 6% 5% 4 4 6% 7% 36%	2133
March	1st to 10th	Night	8% 8% 3 7% 4 4 4 3 4 7% 47%	3565
	11th to 20th	Night	9% 7% 2 7% 5% 4 5% 3 6% 7% 43%	3497
	21st to end	Night	24% 12% 3 10% 6% 3 3 3 4 4 28%	3817
April	1st to 10th	Night	15% 10% 3 7% 4 4 5% 3 5% 6% 39%	1910
	11th to 20th	Night	18% 11% 3 7% 4 4 4 3 4 5% 38%	1797
	21st to end	Night	20% 10% 3 5% 3 4 4 3 5% 6% 37%	1710
May	1st to 10th	Night	15% 7% 3 7% 4 3 3 3 5% 6% 43%	2695
	11th to 20th	Night	15% 5% 2 4 3 4 4 4 5% 7% 47%	2506
	21st to end	Night	17% 7% 2 5% 4 3 4 3 5% 8% 41%	2743
June	1st to 10th	Night	10% 5% 1 5% 5% 3 5% 4 6% 9% 47%	1503
	11th to 20th	Night	9% 5% 2 6% 3 4 5% 5% 5% 9% 47%	1330
	21st to end	Night	8% 4 1 5% 4 4 4 3 6% 7% 53%	1330
July	1st to 10th	Night	11% 3 1 4 4 4 4 5% 7% 10% 45%	2220
	11th to 20th	Night	4 3 1 3 2 4 3 3 6% 7% 63%	2432
	21st to end	Night	5% 4 1 5% 3 3 3 3 6% 8% 57%	2857
August	1st to 10th	Night	12% 6% 2 5% 3 3 4 3 5% 6% 51%	2836
	11th to 20th	Night	13% 4 2 5% 4 3 4 4 4 8% 49%	2798
	21st to end	Night	11% 6% 3 6% 3 3 4 3 6% 8% 48%	3250
September	1st to 10th	Night	8% 4 1 4 4 4 3 3 5% 8% 55%	1900
	11th to 20th	Night	9% 5% 1 6% 3 3 4 3 5% 7% 54%	1977
	21st to end	Night	13% 5% 2 4 3 3 3 3 6% 6% 52%	2128
October	1st to 10th	Night	14% 5% 2 5% 3 3 3 3 6% 7% 49%	2280
	11th to 20th	Night	14% 9% 3 6% 4 4 4 4 5% 6% 43%	2280
	21st to end	Night	17% 9% 3 7% 4 4 4 4 4 7% 38%	2508
November	1st to 10th	Night	12% 9% 3 6% 4 4 4 4 6% 7% 40%	2709
	11th to 20th	Night	13% 11% 4 10% 5% 5% 4 3 5% 6% 34%	2800
	21st to end	Night	12% 8% 4 9% 5% 5% 4 3 5% 6% 38%	2800
December	1st to 10th	Night	6% 7% 4 12% 8% 8% 6% 5% 7% 8% 30%	4580
	11th to 20th	Night	10% 15% 6% 13% 7% 5% 4 4 5% 5% 25%	5199
	21st to end	Night	20% 18% 5% 17% 6% 4 3 3 3 4 17%	7199

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	56% 6% 10% 16% 11%	3290
	11th to 20th	Day	56% 6% 10% 20% 8%	3712
	21st to end	Day	53% 6% 15% 16% 9%	4488
February	1st to 10th	Day	52% 4 13% 21% 9% 1	1800
	11th to 20th	Day	48% 6% 13% 25% 8% 1	1763
	21st to end	Day	51% 5% 13% 19% 11%	1496
March	1st to 10th	Day	46% 8% 13% 23% 9% 1	3539
	11th to 20th	Day	40% 7% 13% 31% 9% 1	3491
	21st to end	Day	60% 8% 10% 17% 5%	3814
April	1st to 10th	Day	49% 6% 11% 20% 13% 1	2270
	11th to 20th	Day	54% 6% 9% 18% 12% 1	2330
	21st to end	Day	52% 7% 12% 19% 9% 1	2470
May	1st to 10th	Day	41% 8% 13% 24% 13% 1	4246
	11th to 20th	Day	37% 7% 12% 28% 13% 2	4403
	21st to end	Day	43% 10% 14% 25% 8% 1	4837
June	1st to 10th	Day	35% 8% 15% 29% 12% 1	2671
	11th to 20th	Day	42% 8% 15% 19% 14% 1	2850
	21st to end	Day	35% 9% 18% 27% 10% 1	2850
July	1st to 10th	Day	37% 8% 21% 21% 12%	4676
	11th to 20th	Day	22% 6% 18% 38% 15% 2	4455
	21st to end	Day	29% 7% 18% 31% 14% 1	4554
August	1st to 10th	Day	36% 6% 13% 31% 11% 2	4090
	11th to 20th	Day	37% 7% 15% 32% 9%	4080
	21st to end	Day	36% 5% 18% 27% 13% 1	4411
September	1st to 10th	Day	30% 6% 16% 35% 12%	2280
	11th to 20th	Day	35% 9% 16% 31% 8% 1	2199
	21st to end	Day	36% 7% 12% 34% 10% 1	2032
October	1st to 10th	Day	31% 4 10% 33% 21% 1	3160
	11th to 20th	Day	38% 5% 13% 24% 19% 1	3140
	21st to end	Day	46% 4 14% 28% 8%	3428
November	1st to 10th	Day	48% 6% 12% 20% 11% 2	1583
	11th to 20th	Day	54% 7% 12% 17% 10%	1600
	21st to end	Day	48% 7% 15% 21% 7% 2	1600
December	1st to 10th	Day	55% 6% 17% 17% 4 1	2610
	11th to 20th	Day	55% 5% 13% 19% 7%	3005
	21st to end	Day	73% 4 8% 10% 5%	3745
January	1st to 10th	Night	53% 6% 11% 20% 10%	6641
	11th to 20th	Night	55% 7% 11% 15% 11% 1	6252
	21st to end	Night	53% 6% 13% 23% 5%	6490
February	1st to 10th	Night	46% 6% 14% 23% 11% 1	2600
	11th to 20th	Night	40% 7% 15% 25% 11% 1	2534
	21st to end	Night	46% 6% 13% 21% 13% 2	2133
March	1st to 10th	Night	36% 6% 15% 26% 16% 2	3565
	11th to 20th	Night	35% 8% 13% 28% 15% 1	3497
	21st to end	Night	60% 5% 8% 20% 6% 1	3817
April	1st to 10th	Night	45% 6% 11% 23% 13% 1	1910
	11th to 20th	Night	48% 5% 7% 21% 16% 2	1797
	21st to end	Night	47% 8% 10% 20% 13% 2	1710
May	1st to 10th	Night	40% 7% 12% 24% 15% 2	2722
	11th to 20th	Night	35% 8% 11% 24% 19% 3	2507
	21st to end	Night	39% 8% 14% 24% 13% 1	2751
June	1st to 10th	Night	31% 9% 17% 28% 14% 2	1503
	11th to 20th	Night	32% 9% 16% 25% 16% 3	1330
	21st to end	Night	29% 8% 15% 28% 17% 3	1330
July	1st to 10th	Night	33% 7% 23% 22% 15% 1	2185
	11th to 20th	Night	20% 5% 14% 40% 18% 2	2388
	21st to end	Night	25% 6% 17% 34% 15% 3	2766
August	1st to 10th	Night	34% 6% 13% 29% 14% 5%	2826
	11th to 20th	Night	35% 6% 14% 32% 13%	2825
	21st to end	Night	34% 6% 17% 30% 11% 2	3250
September	1st to 10th	Night	25% 7% 17% 33% 15% 1	1900
	11th to 20th	Night	29% 6% 18% 30% 16%	1977
	21st to end	Night	32% 6% 13% 33% 16% 1	2128
October	1st to 10th	Night	23% 4 7% 36% 25% 4	3800
	11th to 20th	Night	34% 5% 12% 26% 21% 2	3779
	21st to end	Night	42% 4 14% 27% 12%	4120
November	1st to 10th	Night	42% 6% 15% 24% 12% 2	2709
	11th to 20th	Night	51% 7% 12% 19% 11%	2800
	21st to end	Night	46% 7% 17% 22% 7% 1	2800
December	1st to 10th	Night	51% 6% 19% 19% 5%	4580
	11th to 20th	Night	59% 6% 10% 18% 5% 1	5201
	21st to end	Night	71% 5% 7% 13% 4	7199

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% 30% 13% 2	300
	11th to 20th	Day	15% 11% 36% 32% 5%	300
	21st to end	Day	12% 10% 40% 24% 13% 2	330
February	1st to 10th	Day	14% 17% 41% 23% 4	300
	11th to 20th	Day	12% 19% 39% 22% 7% 1	300
	21st to end	Day	15% 12% 42% 28% 3	249
March	1st to 10th	Day	18% 13% 39% 23% 5% 1	300
	11th to 20th	Day	19% 9% 37% 22% 10% 2	273
	21st to end	Day	25% 13% 28% 23% 9% 2	297
April	1st to 10th	Day	25% 14% 32% 21% 8%	450
	11th to 20th	Day	27% 15% 27% 22% 9% 1	528
	21st to end	Day	24% 12% 32% 21% 9% 2	650
May	1st to 10th	Day	21% 13% 32% 24% 8% 2	653
	11th to 20th	Day	19% 10% 30% 26% 12% 2	675
	21st to end	Day	19% 7% 29% 28% 14% 3	734
June	1st to 10th	Day	11% 9% 28% 30% 18% 4	680
	11th to 20th	Day	14% 9% 36% 29% 11% 1	680
	21st to end	Day	14% 6% 32% 28% 15% 4	680
July	1st to 10th	Day	8% 4 23% 32% 23% 9%	671
	11th to 20th	Day	8% 6% 27% 30% 20% 8%	668
	21st to end	Day	6% 5% 25% 35% 23% 6%	739
August	1st to 10th	Day	10% 5% 24% 32% 24% 5%	676
	11th to 20th	Day	14% 5% 23% 29% 23% 6%	665
	21st to end	Day	12% 6% 25% 32% 20% 5%	680
September	1st to 10th	Day	11% 6% 30% 32% 16% 4	490
	11th to 20th	Day	11% 4 30% 33% 19% 4	409
	21st to end	Day	18% 12% 31% 21% 15% 4	300
October	1st to 10th	Day	20% 10% 32% 22% 11% 6%	300
	11th to 20th	Day	15% 19% 35% 22% 8% 1	300
	21st to end	Day	22% 16% 38% 16% 7% 1	330
November	1st to 10th	Day	22% 13% 32% 25% 7% 2	300
	11th to 20th	Day	17% 19% 42% 20% 2	300
	21st to end	Day	19% 14% 43% 18% 6%	300
December	1st to 10th	Day	14% 11% 42% 24% 9%	300
	11th to 20th	Day	19% 15% 40% 23% 3	300
	21st to end	Day	29% 12% 37% 18% 4 1	330
January	1st to 10th	Night	10% 12% 37% 30% 10% 1	1000
	11th to 20th	Night	15% 11% 37% 30% 7% 1	1000
	21st to end	Night	15% 11% 37% 28% 7%	1100
February	1st to 10th	Night	13% 10% 39% 29% 8% 1	1000
	11th to 20th	Night	10% 12% 42% 28% 7% 1	979
	21st to end	Night	16% 11% 34% 30% 8% 1	813
March	1st to 10th	Night	16% 10% 37% 28% 8% 1	625
	11th to 20th	Night	17% 10% 39% 27% 6% 1	575
	21st to end	Night	22% 9% 32% 26% 10% 2	627
April	1st to 10th	Night	19% 12% 36% 28% 5%	570
	11th to 20th	Night	22% 12% 30% 28% 7%	563
	21st to end	Night	22% 9% 30% 28% 9% 2	570
May	1st to 10th	Night	17% 11% 36% 29% 8% 1	570
	11th to 20th	Night	15% 9% 32% 33% 11%	561
	21st to end	Night	14% 9% 27% 37% 13%	617
June	1st to 10th	Night	11% 6% 28% 39% 15%	570
	11th to 20th	Night	11% 5% 33% 42% 8%	570
	21st to end	Night	10% 6% 24% 44% 15% 1	570
July	1st to 10th	Night	4 2 23% 43% 26% 2	555
	11th to 20th	Night	7% 3 24% 47% 17% 1	552
	21st to end	Night	5% 3 23% 44% 22% 3	624
August	1st to 10th	Night	9% 2 20% 46% 21% 1	567
	11th to 20th	Night	12% 3 19% 41% 24% 2	564
	21st to end	Night	9% 5% 25% 36% 24% 2	627
September	1st to 10th	Night	11% 4 27% 39% 18% 2	570
	11th to 20th	Night	12% 5% 31% 32% 19% 1	570
	21st to end	Night	13% 9% 34% 29% 14% 1	608
October	1st to 10th	Night	17% 10% 31% 30% 12% 1	760
	11th to 20th	Night	19% 10% 36% 29% 6% 1	760
	21st to end	Night	23% 11% 32% 27% 7%	836
November	1st to 10th	Night	17% 10% 33% 32% 8% 1	961
	11th to 20th	Night	17% 15% 35% 26% 7% 1	1000
	21st to end	Night	17% 14% 38% 24% 8%	1000
December	1st to 10th	Night	14% 12% 35% 30% 8% 1	1000
	11th to 20th	Night	19% 10% 37% 28% 6% 1	1000
	21st to end	Night	25% 10% 33% 26% 5% 1	1100

Page generated at 2025-03-09 07:08:47. Version ac07

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