Cedar Springs Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Cedar Springs 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	13% 9% 4 9% 6% 5% 5% 4 6% 7% 33%	119303
	Night	13% 11% 4 9% 5% 5% 5% 4 6% 7% 33%	119930
Transparency	Day	49% 6% 12% 20% 12% 1	122874
	Night	49% 6% 12% 21% 11% 1	124498
Seeing	Day	17% 15% 32% 22% 9% 3	16472
	Night	18% 12% 32% 27% 10% 1	26070

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	10% 13% 6% 16% 7% 5% 6% 4 5% 6% 22%	3617
	11th to 20th	Day	12% 12% 6% 17% 10% 10% 7% 4 5% 5% 12%	3620
	21st to end	Day	15% 16% 6% 12% 6% 4 5% 4 5% 6% 20%	4053
February	1st to 10th	Day	13% 13% 5% 11% 7% 7% 6% 4 4 5% 24%	1800
	11th to 20th	Day	12% 12% 6% 13% 9% 6% 5% 4 5% 5% 24%	1884
	21st to end	Day	12% 14% 5% 10% 7% 7% 6% 3 6% 7% 25%	1650
March	1st to 10th	Day	12% 10% 3 8% 6% 5% 5% 4 5% 7% 34%	4498
	11th to 20th	Day	12% 11% 5% 8% 6% 4 5% 4 6% 9% 30%	4980
	21st to end	Day	20% 11% 4 9% 6% 5% 5% 3 4 5% 29%	5465
April	1st to 10th	Day	17% 12% 5% 8% 5% 5% 4 4 5% 6% 30%	2390
	11th to 20th	Day	17% 10% 3 7% 5% 4 4 3 5% 6% 37%	2532
	21st to end	Day	14% 9% 3 9% 6% 5% 5% 4 5% 6% 34%	2600
May	1st to 10th	Day	17% 8% 3 8% 5% 5% 4 4 5% 6% 33%	5862
	11th to 20th	Day	15% 7% 3 7% 4 4 4 4 6% 8% 38%	5861
	21st to end	Day	12% 7% 3 6% 4 4 4 4 5% 7% 43%	6856
June	1st to 10th	Day	12% 6% 3 6% 4 4 5% 5% 6% 9% 40%	3000
	11th to 20th	Day	11% 5% 3 6% 5% 4 5% 5% 6% 9% 41%	3000
	21st to end	Day	11% 5% 3 7% 6% 4 6% 5% 7% 9% 38%	2997
July	1st to 10th	Day	9% 6% 3 9% 6% 5% 6% 5% 8% 8% 35%	4580
	11th to 20th	Day	6% 5% 2 6% 5% 5% 5% 4 7% 9% 46%	4580
	21st to end	Day	6% 4 2 7% 5% 6% 7% 5% 7% 9% 41%	4764
August	1st to 10th	Day	9% 5% 2 6% 5% 6% 6% 5% 8% 9% 40%	4104
	11th to 20th	Day	11% 5% 2 6% 4 4 5% 5% 7% 8% 44%	4044
	21st to end	Day	9% 7% 2 7% 5% 5% 5% 4 7% 7% 42%	4474
September	1st to 10th	Day	9% 6% 2 7% 5% 6% 5% 4 6% 9% 41%	2190
	11th to 20th	Day	11% 5% 2 5% 4 4 4 4 6% 9% 46%	2090
	21st to end	Day	12% 7% 3 8% 5% 5% 5% 4 5% 9% 37%	2090
October	1st to 10th	Day	15% 7% 2 7% 5% 4 4 4 5% 6% 41%	2050
	11th to 20th	Day	18% 11% 3 9% 5% 5% 5% 3 5% 7% 28%	1881
	21st to end	Day	19% 10% 4 9% 6% 6% 6% 4 6% 7% 23%	1881
November	1st to 10th	Day	12% 10% 3 9% 5% 6% 6% 4 7% 6% 31%	1762
	11th to 20th	Day	14% 13% 4 11% 7% 5% 6% 4 5% 7% 25%	1800
	21st to end	Day	16% 15% 7% 10% 7% 5% 4 4 5% 5% 22%	1618
December	1st to 10th	Day	13% 16% 5% 15% 7% 7% 6% 5% 6% 6% 14%	2425
	11th to 20th	Day	16% 19% 6% 11% 7% 5% 4 4 5% 4 19%	2625
	21st to end	Day	26% 24% 7% 12% 5% 4 3 2 3 3 11%	3680
January	1st to 10th	Night	11% 11% 5% 15% 7% 5% 5% 4 5% 6% 26%	6348
	11th to 20th	Night	12% 14% 6% 14% 11% 8% 7% 4 5% 6% 14%	6360
	21st to end	Night	12% 17% 7% 11% 7% 5% 5% 4 7% 6% 20%	6907
February	1st to 10th	Night	12% 14% 5% 11% 7% 5% 5% 5% 6% 6% 25%	2600
	11th to 20th	Night	13% 13% 5% 9% 6% 5% 5% 4 7% 6% 27%	2399
	21st to end	Night	14% 15% 4 11% 6% 5% 4 4 5% 7% 26%	1980
March	1st to 10th	Night	14% 8% 3 7% 5% 5% 4 4 5% 7% 39%	5355
	11th to 20th	Night	10% 9% 3 8% 5% 4 4 4 7% 8% 37%	4980
	21st to end	Night	16% 11% 4 8% 5% 4 5% 3 4 5% 34%	5467
April	1st to 10th	Night	16% 10% 3 9% 5% 4 4 4 4 7% 34%	2029
	11th to 20th	Night	18% 8% 3 6% 5% 5% 4 3 5% 6% 37%	1817
	21st to end	Night	16% 10% 3 7% 5% 5% 4 3 5% 6% 37%	1800
May	1st to 10th	Night	13% 9% 3 8% 5% 5% 5% 4 5% 7% 37%	4033
	11th to 20th	Night	16% 8% 2 6% 5% 4 5% 4 6% 8% 36%	4032
	21st to end	Night	17% 5% 2 5% 4 3 3 3 5% 7% 46%	3995
June	1st to 10th	Night	12% 7% 2 5% 4 5% 5% 4 5% 8% 44%	1400
	11th to 20th	Night	10% 4 3 7% 4 4 5% 4 7% 9% 44%	1400
	21st to end	Night	8% 5% 2 6% 6% 5% 5% 3 8% 8% 44%	1400
July	1st to 10th	Night	7% 7% 2 7% 5% 4 5% 4 7% 9% 44%	2606
	11th to 20th	Night	6% 4 2 5% 5% 4 5% 4 6% 7% 54%	2610
	21st to end	Night	6% 5% 2 7% 4 5% 5% 4 6% 9% 48%	2796
August	1st to 10th	Night	9% 5% 2 5% 4 3 4 4 7% 8% 49%	2825
	11th to 20th	Night	11% 5% 2 5% 3 4 4 3 6% 8% 49%	2785
	21st to end	Night	8% 5% 2 5% 4 4 5% 3 6% 8% 50%	3178
September	1st to 10th	Night	8% 5% 2 5% 4 4 4 4 6% 8% 50%	1970
	11th to 20th	Night	9% 4 2 5% 3 4 3 3 5% 7% 55%	2090
	21st to end	Night	13% 7% 3 7% 4 4 4 4 6% 8% 41%	2090
October	1st to 10th	Night	14% 6% 2 5% 3 3 4 4 5% 8% 45%	2129
	11th to 20th	Night	17% 10% 3 8% 5% 4 4 3 5% 7% 34%	2297
	21st to end	Night	20% 10% 3 8% 4 4 5% 3 5% 7% 31%	2717
November	1st to 10th	Night	11% 9% 3 8% 5% 4 4 4 6% 7% 38%	2536
	11th to 20th	Night	13% 12% 4 9% 6% 5% 6% 5% 7% 7% 27%	2600
	21st to end	Night	17% 13% 4 10% 6% 5% 4 4 5% 6% 25%	2779
December	1st to 10th	Night	12% 13% 4 11% 7% 5% 6% 5% 6% 7% 24%	4745
	11th to 20th	Night	13% 18% 6% 11% 6% 5% 4 4 5% 5% 22%	5582
	21st to end	Night	24% 22% 6% 9% 4 4 4 3 4 4 17%	7293

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	62% 6% 11% 15% 4 1	3618
	11th to 20th	Day	72% 8% 6% 8% 6%	3620
	21st to end	Day	65% 5% 13% 10% 7% 1	4053
February	1st to 10th	Day	60% 7% 10% 17% 5%	1800
	11th to 20th	Day	61% 6% 11% 14% 7% 1	1894
	21st to end	Day	58% 7% 12% 17% 6% 1	1650
March	1st to 10th	Day	48% 5% 15% 18% 11% 3	4498
	11th to 20th	Day	49% 6% 17% 17% 10% 1	4980
	21st to end	Day	57% 5% 10% 16% 11% 1	5465
April	1st to 10th	Day	55% 6% 10% 17% 11% 1	2390
	11th to 20th	Day	47% 6% 10% 20% 15% 1	2532
	21st to end	Day	49% 6% 12% 16% 14% 2	2600
May	1st to 10th	Day	47% 7% 10% 17% 17% 2	5897
	11th to 20th	Day	42% 7% 11% 22% 15% 2	5865
	21st to end	Day	41% 5% 11% 20% 20% 4	6871
June	1st to 10th	Day	39% 7% 14% 25% 13% 2	3000
	11th to 20th	Day	38% 7% 13% 23% 17% 2	3000
	21st to end	Day	40% 8% 14% 24% 14% 1	2997
July	1st to 10th	Day	44% 5% 14% 20% 15% 2	4511
	11th to 20th	Day	34% 5% 14% 30% 17% 1	4505
	21st to end	Day	38% 6% 15% 20% 20% 1	4614
August	1st to 10th	Day	39% 7% 12% 27% 14% 1	4091
	11th to 20th	Day	38% 5% 14% 30% 12% 1	4083
	21st to end	Day	40% 6% 14% 27% 12% 1	4473
September	1st to 10th	Day	38% 7% 14% 27% 12% 2	2190
	11th to 20th	Day	34% 7% 16% 29% 13% 1	2090
	21st to end	Day	43% 8% 13% 25% 11%	2090
October	1st to 10th	Day	33% 4 9% 33% 18% 3	3401
	11th to 20th	Day	46% 4 12% 25% 13%	3109
	21st to end	Day	55% 5% 12% 19% 9%	3081
November	1st to 10th	Day	50% 9% 11% 21% 9%	1762
	11th to 20th	Day	56% 9% 15% 13% 7%	1800
	21st to end	Day	63% 8% 12% 13% 4 1	1618
December	1st to 10th	Day	69% 6% 13% 10% 1	2425
	11th to 20th	Day	68% 6% 10% 11% 5%	2621
	21st to end	Day	79% 4 5% 7% 5%	3680
January	1st to 10th	Night	58% 6% 9% 16% 7% 2	6348
	11th to 20th	Night	70% 7% 8% 9% 5% 1	6360
	21st to end	Night	62% 5% 10% 15% 7%	6907
February	1st to 10th	Night	57% 8% 10% 17% 8% 1	2600
	11th to 20th	Night	55% 7% 12% 18% 7% 1	2400
	21st to end	Night	57% 7% 11% 17% 7% 1	1980
March	1st to 10th	Night	44% 5% 17% 21% 12% 1	5355
	11th to 20th	Night	42% 5% 21% 20% 10% 1	4980
	21st to end	Night	51% 7% 12% 18% 12% 1	5467
April	1st to 10th	Night	49% 6% 9% 21% 13% 2	2029
	11th to 20th	Night	46% 6% 9% 21% 16% 2	1817
	21st to end	Night	48% 5% 10% 19% 15% 4	1800
May	1st to 10th	Night	44% 6% 11% 17% 19% 3	4060
	11th to 20th	Night	44% 6% 10% 24% 14% 2	4033
	21st to end	Night	39% 5% 10% 26% 16% 5%	4002
June	1st to 10th	Night	36% 7% 12% 25% 18% 2	1400
	11th to 20th	Night	35% 7% 16% 23% 18% 2	1400
	21st to end	Night	35% 9% 12% 26% 16% 2	1400
July	1st to 10th	Night	37% 4 14% 26% 15% 5%	2566
	11th to 20th	Night	29% 4 12% 38% 15% 1	2562
	21st to end	Night	34% 5% 18% 23% 18% 1	2707
August	1st to 10th	Night	33% 6% 15% 31% 15% 1	2814
	11th to 20th	Night	33% 5% 16% 31% 14% 1	2812
	21st to end	Night	31% 5% 15% 32% 16% 1	3178
September	1st to 10th	Night	32% 7% 13% 29% 18% 1	1970
	11th to 20th	Night	29% 5% 14% 37% 16%	2090
	21st to end	Night	40% 6% 15% 28% 10% 1	2090
October	1st to 10th	Night	29% 4 12% 35% 19% 1	3558
	11th to 20th	Night	43% 4 12% 26% 15%	3808
	21st to end	Night	47% 4 12% 27% 9%	4467
November	1st to 10th	Night	42% 8% 15% 24% 10% 1	2536
	11th to 20th	Night	54% 9% 15% 16% 6% 1	2600
	21st to end	Night	59% 8% 15% 14% 4	2779
December	1st to 10th	Night	58% 5% 19% 13% 4	4745
	11th to 20th	Night	63% 6% 10% 12% 9% 1	5585
	21st to end	Night	72% 5% 7% 8% 8%	7293

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	20% 20% 39% 16% 4 1	300
	11th to 20th	Day	23% 19% 40% 14% 3	300
	21st to end	Day	24% 18% 35% 18% 4 1	330
February	1st to 10th	Day	21% 30% 33% 13% 2 1	300
	11th to 20th	Day	22% 26% 40% 10% 2	300
	21st to end	Day	27% 30% 34% 8% 1	249
March	1st to 10th	Day	25% 27% 37% 10% 1	305
	11th to 20th	Day	23% 17% 34% 19% 6% 1	500
	21st to end	Day	27% 17% 33% 18% 5% 1	550
April	1st to 10th	Day	25% 21% 31% 18% 5%	500
	11th to 20th	Day	27% 23% 28% 14% 7% 2	492
	21st to end	Day	23% 17% 30% 21% 7% 2	500
May	1st to 10th	Day	20% 19% 36% 16% 7% 3	503
	11th to 20th	Day	17% 14% 35% 21% 9% 4	527
	21st to end	Day	14% 14% 31% 27% 10% 3	735
June	1st to 10th	Day	14% 14% 37% 24% 8% 2	730
	11th to 20th	Day	13% 14% 37% 24% 9% 3	730
	21st to end	Day	13% 10% 37% 27% 9% 3	730
July	1st to 10th	Day	6% 8% 34% 31% 17% 5%	721
	11th to 20th	Day	10% 10% 33% 30% 12% 5%	718
	21st to end	Day	9% 6% 33% 33% 14% 5%	714
August	1st to 10th	Day	10% 7% 27% 29% 19% 8%	486
	11th to 20th	Day	9% 6% 22% 34% 19% 10%	479
	21st to end	Day	12% 8% 24% 33% 16% 6%	539
September	1st to 10th	Day	13% 9% 28% 26% 16% 8%	490
	11th to 20th	Day	10% 8% 23% 28% 22% 10%	490
	21st to end	Day	19% 10% 24% 26% 14% 6%	490
October	1st to 10th	Day	19% 16% 29% 21% 13% 2	490
	11th to 20th	Day	21% 21% 32% 20% 5% 1	471
	21st to end	Day	27% 22% 35% 13% 3 1	330
November	1st to 10th	Day	22% 20% 37% 15% 5% 1	300
	11th to 20th	Day	22% 25% 34% 13% 6%	300
	21st to end	Day	22% 24% 36% 14% 4	208
December	1st to 10th	Day	25% 21% 38% 12% 5% 1	200
	11th to 20th	Day	24% 28% 35% 14% 1	200
	21st to end	Day	31% 25% 29% 12% 3	265
January	1st to 10th	Night	19% 18% 37% 22% 5% 1	1000
	11th to 20th	Night	21% 16% 36% 21% 6%	1000
	21st to end	Night	18% 12% 35% 29% 5%	1063
February	1st to 10th	Night	18% 16% 39% 23% 3	800
	11th to 20th	Night	21% 17% 37% 21% 4	783
	21st to end	Night	24% 16% 37% 19% 3	660
March	1st to 10th	Night	17% 14% 39% 23% 7%	800
	11th to 20th	Night	18% 17% 36% 24% 5% 1	800
	21st to end	Night	23% 14% 33% 25% 4 1	880
April	1st to 10th	Night	21% 14% 36% 23% 6%	629
	11th to 20th	Night	23% 13% 32% 23% 7% 2	593
	21st to end	Night	21% 10% 30% 31% 8%	600
May	1st to 10th	Night	12% 9% 36% 32% 9% 1	600
	11th to 20th	Night	17% 8% 28% 35% 12% 1	591
	21st to end	Night	16% 6% 28% 31% 18% 1	630
June	1st to 10th	Night	13% 7% 32% 38% 11%	400
	11th to 20th	Night	12% 5% 32% 36% 15%	400
	21st to end	Night	10% 5% 33% 34% 17% 2	400
July	1st to 10th	Night	6% 3 24% 45% 21% 2	581
	11th to 20th	Night	8% 4 23% 41% 21% 3	582
	21st to end	Night	10% 2 24% 41% 21% 1	624
August	1st to 10th	Night	8% 2 24% 41% 23% 2	567
	11th to 20th	Night	9% 3 20% 36% 28% 3	564
	21st to end	Night	10% 5% 22% 39% 23% 1	627
September	1st to 10th	Night	10% 6% 26% 33% 22% 4	640
	11th to 20th	Night	11% 6% 21% 35% 22% 4	760
	21st to end	Night	17% 8% 24% 29% 18% 4	760
October	1st to 10th	Night	17% 11% 29% 27% 16% 1	760
	11th to 20th	Night	21% 14% 33% 24% 7%	760
	21st to end	Night	25% 15% 32% 23% 5% 1	836
November	1st to 10th	Night	17% 14% 37% 22% 10% 1	780
	11th to 20th	Night	19% 18% 38% 20% 6%	800
	21st to end	Night	24% 15% 39% 19% 4	800
December	1st to 10th	Night	23% 16% 35% 21% 4	900
	11th to 20th	Night	24% 16% 38% 18% 4	1000
	21st to end	Night	28% 15% 33% 20% 3	1100

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

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