Clay Center Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Clay Center Observatory clear sky chart from 2005-11-06 to 2025-07-21.

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	17% 7% 2 6% 4 4 4 4 5% 7% 39%	119695
	Night	17% 8% 3 6% 4 4 4 3 5% 6% 40%	119410
Transparency	Day	44% 6% 11% 21% 15% 3	123286
	Night	44% 6% 11% 21% 16% 3	123980
Seeing	Day	24% 17% 33% 18% 7% 2	15957
	Night	22% 15% 34% 23% 6%	24827

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	11% 7% 2 8% 4 4 4 4 5% 7% 44%	3597
	11th to 20th	Day	13% 8% 2 6% 3 3 4 3 4 5% 49%	3570
	21st to end	Day	17% 8% 2 5% 3 3 4 3 5% 6% 43%	3960
February	1st to 10th	Day	19% 7% 3 5% 4 5% 5% 4 5% 7% 37%	1876
	11th to 20th	Day	13% 7% 3 6% 4 5% 4 3 5% 7% 41%	1948
	21st to end	Day	17% 9% 2 6% 5% 3 4 4 5% 7% 38%	1650
March	1st to 10th	Day	20% 11% 2 6% 4 4 4 3 5% 5% 36%	4476
	11th to 20th	Day	15% 10% 2 6% 4 4 4 4 6% 7% 39%	4791
	21st to end	Day	26% 8% 2 6% 4 4 4 4 6% 5% 30%	5735
April	1st to 10th	Day	19% 10% 3 6% 5% 4 4 3 3 7% 36%	2400
	11th to 20th	Day	18% 7% 2 5% 5% 4 4 4 5% 8% 38%	2397
	21st to end	Day	19% 6% 3 6% 4 4 4 3 6% 7% 38%	2600
May	1st to 10th	Day	28% 11% 3 7% 4 4 4 3 5% 7% 25%	5886
	11th to 20th	Day	20% 8% 3 7% 5% 4 5% 4 5% 7% 32%	5906
	21st to end	Day	20% 7% 2 6% 4 4 4 4 5% 8% 36%	6949
June	1st to 10th	Day	19% 6% 2 6% 4 5% 5% 4 7% 9% 33%	2800
	11th to 20th	Day	16% 6% 2 6% 4 4 4 4 6% 8% 38%	2800
	21st to end	Day	15% 7% 3 7% 5% 5% 5% 4 6% 9% 36%	2800
July	1st to 10th	Day	13% 7% 2 6% 5% 6% 6% 5% 7% 9% 35%	4580
	11th to 20th	Day	10% 6% 2 6% 5% 5% 5% 5% 6% 10% 39%	4580
	21st to end	Day	16% 7% 3 7% 5% 5% 5% 4 6% 7% 35%	4814
August	1st to 10th	Day	15% 6% 3 7% 5% 5% 6% 5% 7% 8% 32%	4119
	11th to 20th	Day	13% 8% 3 7% 4 4 5% 4 6% 8% 39%	4014
	21st to end	Day	7% 4 2 5% 4 4 5% 4 7% 10% 47%	4180
September	1st to 10th	Day	14% 5% 2 5% 4 4 4 3 5% 7% 47%	2266
	11th to 20th	Day	15% 6% 2 5% 3 3 2 3 4 6% 52%	2090
	21st to end	Day	18% 5% 3 6% 4 3 4 3 5% 7% 43%	2090
October	1st to 10th	Day	24% 5% 2 5% 3 4 3 3 4 6% 41%	2090
	11th to 20th	Day	14% 6% 2 5% 3 3 4 3 5% 6% 48%	1862
	21st to end	Day	19% 7% 3 5% 3 4 4 3 5% 6% 41%	1881
November	1st to 10th	Day	16% 7% 2 4 4 3 3 3 4 5% 50%	1761
	11th to 20th	Day	17% 7% 2 4 3 3 4 3 5% 5% 48%	1800
	21st to end	Day	18% 8% 2 4 4 4 4 3 4 6% 44%	1800
December	1st to 10th	Day	15% 7% 3 4 3 4 4 2 4 5% 49%	2681
	11th to 20th	Day	18% 7% 2 4 2 3 3 2 4 5% 49%	2986
	21st to end	Day	20% 10% 3 6% 4 4 4 2 4 5% 38%	3960
January	1st to 10th	Night	13% 7% 2 4 3 3 3 3 4 6% 51%	6349
	11th to 20th	Night	13% 9% 3 9% 5% 4 4 3 5% 5% 40%	6315
	21st to end	Night	15% 10% 3 7% 5% 4 4 3 4 5% 38%	6996
February	1st to 10th	Night	17% 9% 2 6% 5% 4 4 3 5% 7% 38%	2521
	11th to 20th	Night	15% 9% 3 7% 4 4 4 4 5% 6% 39%	2338
	21st to end	Night	17% 9% 2 6% 4 3 4 3 4 6% 41%	1980
March	1st to 10th	Night	25% 9% 2 5% 3 4 3 3 4 5% 38%	5364
	11th to 20th	Night	16% 11% 3 6% 4 4 4 3 5% 6% 38%	5168
	21st to end	Night	25% 10% 2 6% 4 4 4 4 5% 4 33%	5192
April	1st to 10th	Night	19% 9% 2 6% 5% 4 4 4 5% 7% 36%	2000
	11th to 20th	Night	18% 9% 2 6% 4 4 4 3 5% 6% 39%	1950
	21st to end	Night	20% 7% 3 6% 4 4 4 3 5% 5% 39%	1800
May	1st to 10th	Night	30% 10% 3 6% 5% 3 4 3 5% 6% 26%	4054
	11th to 20th	Night	18% 9% 3 7% 5% 5% 6% 5% 7% 8% 28%	4016
	21st to end	Night	20% 8% 3 8% 5% 4 5% 3 6% 7% 32%	3926
June	1st to 10th	Night	17% 9% 3 7% 4 5% 4 4 5% 6% 37%	1600
	11th to 20th	Night	17% 8% 2 6% 4 4 4 4 6% 8% 37%	1600
	21st to end	Night	16% 5% 2 5% 4 4 5% 5% 5% 9% 40%	1600
July	1st to 10th	Night	11% 6% 3 8% 6% 4 6% 5% 7% 10% 35%	2600
	11th to 20th	Night	11% 7% 2 7% 5% 4 5% 4 6% 10% 39%	2600
	21st to end	Night	16% 9% 3 7% 5% 5% 5% 4 5% 7% 35%	2739
August	1st to 10th	Night	19% 7% 3 7% 5% 6% 5% 5% 6% 8% 31%	2780
	11th to 20th	Night	9% 7% 3 8% 5% 6% 6% 5% 7% 7% 37%	2809
	21st to end	Night	9% 4 1 4 4 4 4 3 5% 8% 55%	3476
September	1st to 10th	Night	12% 5% 2 6% 4 5% 5% 3 6% 6% 46%	1913
	11th to 20th	Night	14% 6% 2 4 3 3 3 3 4 7% 53%	2090
	21st to end	Night	18% 6% 2 6% 4 3 4 4 6% 5% 43%	2090
October	1st to 10th	Night	20% 6% 2 6% 3 5% 3 3 4 6% 42%	2090
	11th to 20th	Night	15% 5% 2 5% 3 4 4 3 5% 6% 47%	2315
	21st to end	Night	20% 7% 3 6% 3 4 3 3 5% 6% 40%	2717
November	1st to 10th	Night	14% 5% 2 6% 4 4 3 3 5% 5% 49%	2537
	11th to 20th	Night	16% 8% 2 5% 4 3 4 3 5% 4 47%	2600
	21st to end	Night	17% 6% 2 5% 4 3 4 3 5% 6% 45%	2600
December	1st to 10th	Night	18% 8% 2 5% 4 3 4 3 4 6% 43%	4481
	11th to 20th	Night	21% 8% 3 6% 3 3 3 3 4 4 42%	5208
	21st to end	Night	17% 10% 4 6% 3 4 3 3 4 5% 40%	6996

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	39% 4 7% 28% 19% 3	3598
	11th to 20th	Day	39% 5% 10% 22% 18% 6%	3600
	21st to end	Day	42% 6% 10% 21% 19% 2	3960
February	1st to 10th	Day	47% 6% 13% 18% 14% 2	1876
	11th to 20th	Day	43% 5% 12% 21% 15% 3	1957
	21st to end	Day	47% 6% 9% 20% 15% 3	1650
March	1st to 10th	Day	50% 4 10% 15% 11% 10%	4479
	11th to 20th	Day	42% 7% 13% 26% 10% 2	4791
	21st to end	Day	54% 5% 6% 15% 13% 7%	5735
April	1st to 10th	Day	48% 4 10% 18% 16% 4	2400
	11th to 20th	Day	43% 7% 11% 20% 16% 3	2397
	21st to end	Day	45% 6% 9% 19% 17% 4	2600
May	1st to 10th	Day	58% 8% 12% 13% 8% 1	5897
	11th to 20th	Day	51% 8% 13% 16% 10% 2	5910
	21st to end	Day	46% 7% 11% 22% 13% 1	6949
June	1st to 10th	Day	46% 7% 13% 22% 12% 1	2800
	11th to 20th	Day	42% 8% 14% 19% 15% 2	2800
	21st to end	Day	44% 7% 14% 20% 14% 1	2800
July	1st to 10th	Day	44% 9% 11% 20% 14% 1	4510
	11th to 20th	Day	39% 8% 15% 24% 12% 2	4505
	21st to end	Day	47% 7% 12% 19% 13% 2	4661
August	1st to 10th	Day	47% 9% 16% 20% 8% 1	4105
	11th to 20th	Day	43% 7% 13% 27% 10%	4053
	21st to end	Day	31% 8% 12% 30% 18% 2	4180
September	1st to 10th	Day	37% 6% 12% 23% 21% 2	2266
	11th to 20th	Day	35% 4 10% 26% 22% 3	2090
	21st to end	Day	42% 6% 8% 20% 22% 2	2090
October	1st to 10th	Day	40% 5% 9% 24% 18% 4	3480
	11th to 20th	Day	31% 4 7% 28% 26% 4	3078
	21st to end	Day	32% 5% 11% 28% 21% 3	3081
November	1st to 10th	Day	38% 4 13% 25% 18% 2	1761
	11th to 20th	Day	39% 4 10% 22% 23% 3	1800
	21st to end	Day	42% 6% 13% 25% 13% 2	1800
December	1st to 10th	Day	39% 4 14% 27% 14% 1	2681
	11th to 20th	Day	40% 4 14% 17% 19% 6%	2986
	21st to end	Day	50% 4 8% 20% 14% 4	3960
January	1st to 10th	Night	34% 4 9% 34% 16% 2	6349
	11th to 20th	Night	46% 4 12% 21% 14% 4	6360
	21st to end	Night	49% 5% 9% 16% 18% 4	6996
February	1st to 10th	Night	46% 5% 12% 21% 14% 2	2521
	11th to 20th	Night	46% 5% 9% 20% 15% 4	2340
	21st to end	Night	45% 4 8% 23% 16% 4	1980
March	1st to 10th	Night	49% 4 8% 15% 16% 7%	5374
	11th to 20th	Night	45% 7% 15% 18% 13% 2	5169
	21st to end	Night	54% 4 5% 12% 19% 6%	5192
April	1st to 10th	Night	48% 5% 10% 19% 15% 3	2000
	11th to 20th	Night	46% 5% 9% 20% 17% 3	1950
	21st to end	Night	45% 6% 10% 18% 17% 4	1800
May	1st to 10th	Night	58% 7% 13% 12% 7% 2	4060
	11th to 20th	Night	51% 10% 13% 15% 8% 2	4017
	21st to end	Night	50% 8% 13% 19% 10% 1	3927
June	1st to 10th	Night	46% 7% 13% 21% 12% 1	1600
	11th to 20th	Night	45% 7% 13% 22% 13% 1	1600
	21st to end	Night	40% 9% 14% 23% 14% 1	1600
July	1st to 10th	Night	43% 8% 13% 21% 15%	2560
	11th to 20th	Night	40% 7% 17% 23% 12% 1	2552
	21st to end	Night	48% 7% 10% 18% 14% 1	2651
August	1st to 10th	Night	51% 8% 14% 18% 9% 1	2770
	11th to 20th	Night	42% 10% 14% 24% 10%	2836
	21st to end	Night	27% 6% 16% 28% 23% 1	3476
September	1st to 10th	Night	38% 7% 11% 23% 19% 1	1913
	11th to 20th	Night	33% 6% 10% 23% 26% 2	2090
	21st to end	Night	42% 7% 9% 21% 19% 2	2090
October	1st to 10th	Night	40% 4 13% 24% 16% 2	3480
	11th to 20th	Night	27% 4 10% 27% 30% 3	3838
	21st to end	Night	34% 4 11% 29% 19% 2	4467
November	1st to 10th	Night	37% 6% 11% 23% 20% 3	2537
	11th to 20th	Night	40% 4 10% 22% 21% 3	2600
	21st to end	Night	40% 5% 14% 26% 13% 2	2600
December	1st to 10th	Night	43% 5% 12% 27% 13% 1	4481
	11th to 20th	Night	47% 3 15% 14% 20% 2	5208
	21st to end	Night	46% 5% 8% 22% 16% 3	6996

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	32% 25% 26% 14% 4	200
	11th to 20th	Day	31% 23% 36% 10% 1 1	200
	21st to end	Day	27% 25% 34% 11% 2	220
February	1st to 10th	Day	31% 21% 35% 12% 2	418
	11th to 20th	Day	29% 28% 33% 8% 1	496
	21st to end	Day	28% 22% 34% 12% 3	414
March	1st to 10th	Day	32% 22% 31% 12% 3	500
	11th to 20th	Day	30% 24% 30% 12% 4 1	500
	21st to end	Day	31% 23% 32% 8% 6% 1	550
April	1st to 10th	Day	35% 23% 24% 12% 5% 1	500
	11th to 20th	Day	29% 26% 29% 12% 3	492
	21st to end	Day	28% 19% 35% 15% 2 1	500
May	1st to 10th	Day	33% 24% 28% 11% 3 1	503
	11th to 20th	Day	24% 20% 37% 13% 6% 1	527
	21st to end	Day	19% 15% 36% 23% 5% 1	573
June	1st to 10th	Day	23% 13% 38% 19% 6% 1	530
	11th to 20th	Day	17% 16% 40% 20% 5% 1	530
	21st to end	Day	16% 14% 39% 22% 8% 1	530
July	1st to 10th	Day	19% 10% 34% 26% 10% 1	525
	11th to 20th	Day	15% 8% 33% 31% 11% 2	524
	21st to end	Day	12% 9% 40% 31% 8% 1	532
August	1st to 10th	Day	12% 11% 35% 31% 10% 1	486
	11th to 20th	Day	15% 10% 33% 26% 14% 2	479
	21st to end	Day	14% 9% 33% 28% 12% 4	539
September	1st to 10th	Day	19% 12% 29% 24% 12% 4	490
	11th to 20th	Day	20% 13% 27% 19% 13% 8%	490
	21st to end	Day	23% 11% 33% 21% 8% 4	490
October	1st to 10th	Day	28% 12% 26% 21% 8% 5%	490
	11th to 20th	Day	19% 16% 35% 19% 9% 3	430
	21st to end	Day	26% 15% 33% 17% 8% 1	429
November	1st to 10th	Day	22% 17% 35% 15% 10% 2	395
	11th to 20th	Day	27% 20% 34% 15% 4 1	400
	21st to end	Day	29% 20% 30% 15% 6% 1	400
December	1st to 10th	Day	27% 25% 30% 14% 4	255
	11th to 20th	Day	35% 23% 33% 8% 2	200
	21st to end	Day	25% 23% 34% 13% 4 2	220
January	1st to 10th	Night	23% 20% 39% 17% 2	800
	11th to 20th	Night	28% 19% 35% 17% 2	800
	21st to end	Night	21% 20% 37% 19% 3	880
February	1st to 10th	Night	25% 18% 36% 19% 1	800
	11th to 20th	Night	26% 20% 39% 14% 1	783
	21st to end	Night	25% 21% 34% 17% 3	660
March	1st to 10th	Night	26% 18% 35% 16% 4	800
	11th to 20th	Night	22% 21% 36% 17% 3	800
	21st to end	Night	26% 19% 34% 18% 4	880
April	1st to 10th	Night	26% 20% 32% 19% 3	800
	11th to 20th	Night	24% 20% 34% 17% 4	766
	21st to end	Night	24% 18% 37% 19% 2	600
May	1st to 10th	Night	29% 17% 32% 19% 4	600
	11th to 20th	Night	20% 18% 39% 20% 3	588
	21st to end	Night	19% 9% 39% 27% 4 1	433
June	1st to 10th	Night	20% 9% 38% 28% 6% 1	400
	11th to 20th	Night	18% 8% 41% 28% 6%	400
	21st to end	Night	17% 5% 40% 32% 6%	400
July	1st to 10th	Night	13% 5% 37% 37% 8%	390
	11th to 20th	Night	14% 5% 32% 40% 9% 1	388
	21st to end	Night	14% 4 34% 39% 9%	416
August	1st to 10th	Night	13% 6% 36% 36% 9%	535
	11th to 20th	Night	13% 5% 32% 40% 10% 1	576
	21st to end	Night	14% 5% 25% 39% 15% 2	836
September	1st to 10th	Night	15% 6% 35% 33% 11%	760
	11th to 20th	Night	17% 8% 29% 27% 17% 3	760
	21st to end	Night	22% 8% 27% 30% 11% 2	760
October	1st to 10th	Night	24% 10% 27% 26% 12% 1	760
	11th to 20th	Night	18% 13% 34% 23% 11% 1	760
	21st to end	Night	25% 14% 33% 21% 7%	836
November	1st to 10th	Night	17% 14% 35% 24% 8% 1	780
	11th to 20th	Night	22% 20% 33% 18% 7%	800
	21st to end	Night	24% 20% 33% 19% 5% 1	800
December	1st to 10th	Night	25% 18% 35% 17% 5%	800
	11th to 20th	Night	26% 18% 33% 20% 2 1	800
	21st to end	Night	25% 18% 35% 20% 3	880

Page generated at 2025-07-18 01:09:59. Version ac07

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