OKAstronomy Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the OKAstronomy 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% 6% 2 6% 4 4 4 3 5% 7% 47%	138212
	Night	12% 6% 2 5% 3 3 3 3 5% 6% 51%	134809
Transparency	Day	36% 4 11% 25% 19% 5%	141824
	Night	33% 4 10% 25% 22% 6%	139264
Seeing	Day	17% 13% 29% 26% 14% 2	17740
	Night	16% 12% 31% 29% 12% 1	27541

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	12% 10% 4 6% 3 3 4 3 3 5% 48%	5289
	11th to 20th	Day	6% 6% 2 6% 4 3 3 2 3 5% 59%	5310
	21st to end	Day	17% 9% 3 5% 2 2 2 2 3 4 49%	5812
February	1st to 10th	Day	17% 10% 3 6% 4 3 3 4 4 4 42%	1890
	11th to 20th	Day	17% 8% 3 6% 4 3 3 3 5% 6% 44%	1923
	21st to end	Day	15% 8% 2 5% 3 3 4 3 6% 5% 44%	1840
March	1st to 10th	Day	15% 7% 2 5% 3 3 3 3 4 5% 49%	5046
	11th to 20th	Day	9% 5% 2 4 3 4 3 3 5% 6% 54%	5090
	21st to end	Day	14% 8% 3 7% 5% 4 5% 3 6% 7% 38%	5583
April	1st to 10th	Day	15% 6% 2 6% 4 3 4 3 6% 6% 45%	2374
	11th to 20th	Day	17% 6% 3 7% 4 4 3 3 5% 6% 43%	2494
	21st to end	Day	21% 9% 2 7% 4 4 3 3 4 5% 38%	2584
May	1st to 10th	Day	16% 8% 3 7% 4 4 4 3 5% 6% 41%	7617
	11th to 20th	Day	12% 6% 3 7% 5% 5% 5% 4 6% 7% 41%	7627
	21st to end	Day	19% 7% 3 8% 5% 5% 5% 4 6% 7% 31%	8392
June	1st to 10th	Day	9% 5% 2 7% 4 4 4 4 7% 9% 44%	2730
	11th to 20th	Day	8% 5% 2 5% 4 4 5% 5% 6% 9% 47%	2730
	21st to end	Day	5% 4 1 4 4 5% 4 4 6% 8% 56%	2730
July	1st to 10th	Day	7% 4 1 5% 4 5% 6% 5% 8% 12% 42%	6030
	11th to 20th	Day	5% 5% 2 5% 4 4 4 4 6% 9% 52%	5966
	21st to end	Day	4 3 1 4 3 4 4 4 8% 11% 53%	6422
August	1st to 10th	Day	4 3 1 4 3 3 4 4 6% 9% 59%	4234
	11th to 20th	Day	8% 4 2 5% 4 4 4 5% 6% 9% 50%	4171
	21st to end	Day	4 4 2 5% 3 4 4 4 6% 9% 55%	4326
September	1st to 10th	Day	11% 4 2 4 3 3 3 3 5% 7% 54%	2346
	11th to 20th	Day	11% 4 2 5% 4 5% 4 4 6% 8% 47%	2200
	21st to end	Day	7% 4 2 5% 4 3 4 4 5% 5% 56%	2200
October	1st to 10th	Day	9% 4 2 4 4 3 4 3 4 5% 59%	2015
	11th to 20th	Day	12% 5% 1 4 2 2 3 2 4 5% 61%	2000
	21st to end	Day	20% 6% 2 4 3 3 3 2 3 5% 51%	2200
November	1st to 10th	Day	11% 6% 2 4 3 2 2 2 3 6% 58%	2058
	11th to 20th	Day	12% 7% 2 4 3 3 2 3 4 6% 55%	1995
	21st to end	Day	17% 7% 2 4 3 2 2 3 5% 6% 49%	1890
December	1st to 10th	Day	14% 9% 2 4 3 3 3 2 3 5% 52%	3030
	11th to 20th	Day	16% 8% 3 6% 3 2 3 2 4 6% 48%	3481
	21st to end	Day	20% 15% 5% 7% 4 3 3 2 4 5% 32%	4587
January	1st to 10th	Night	12% 8% 3 6% 4 3 3 3 4 6% 47%	7634
	11th to 20th	Night	7% 5% 2 5% 3 3 3 3 4 5% 61%	7680
	21st to end	Night	19% 8% 3 6% 3 3 3 3 4 5% 44%	8402
February	1st to 10th	Night	15% 10% 3 6% 3 3 3 3 4 6% 46%	2730
	11th to 20th	Night	15% 8% 3 6% 4 3 3 2 4 6% 48%	2600
	21st to end	Night	14% 8% 2 5% 3 3 4 3 4 5% 49%	1966
March	1st to 10th	Night	16% 9% 3 5% 3 3 3 3 4 5% 47%	5029
	11th to 20th	Night	9% 6% 2 4 3 3 4 2 5% 5% 56%	5090
	21st to end	Night	12% 6% 2 6% 4 4 4 3 5% 8% 46%	5588
April	1st to 10th	Night	15% 6% 2 4 3 3 4 3 5% 5% 50%	2244
	11th to 20th	Night	14% 7% 1 5% 4 3 3 3 4 6% 50%	2075
	21st to end	Night	15% 7% 3 5% 4 3 3 3 5% 7% 45%	2031
May	1st to 10th	Night	14% 6% 2 6% 4 5% 4 3 5% 7% 44%	5266
	11th to 20th	Night	12% 6% 2 7% 4 4 4 4 6% 8% 44%	5265
	21st to end	Night	17% 7% 2 6% 5% 4 4 4 6% 8% 38%	5793
June	1st to 10th	Night	7% 4 2 6% 4 3 3 3 6% 9% 53%	1890
	11th to 20th	Night	6% 4 1 4 4 3 4 4 7% 9% 55%	1890
	21st to end	Night	4 2 1 3 3 3 4 3 6% 8% 63%	1890
July	1st to 10th	Night	7% 4 1 5% 4 4 5% 5% 7% 10% 49%	4168
	11th to 20th	Night	4 2 1 4 3 3 4 3 5% 8% 63%	4122
	21st to end	Night	3 3 1 4 3 3 3 3 6% 10% 60%	4449
August	1st to 10th	Night	2 2 1 4 3 4 4 4 6% 9% 61%	2926
	11th to 20th	Night	5% 4 1 5% 3 4 4 5% 7% 7% 55%	2888
	21st to end	Night	3 3 1 3 3 3 3 3 5% 8% 63%	3571
September	1st to 10th	Night	9% 4 1 4 3 3 4 3 5% 6% 59%	2050
	11th to 20th	Night	8% 5% 2 6% 3 3 4 3 6% 7% 52%	2200
	21st to end	Night	7% 4 2 6% 3 3 4 3 5% 6% 57%	2200
October	1st to 10th	Night	7% 3 2 3 3 3 3 3 4 6% 64%	2363
	11th to 20th	Night	12% 3 1 3 3 2 3 3 4 5% 59%	2400
	21st to end	Night	17% 6% 2 3 2 2 3 3 3 5% 53%	2640
November	1st to 10th	Night	12% 5% 1 3 2 2 2 2 3 5% 62%	2460
	11th to 20th	Night	10% 6% 2 5% 3 2 3 3 4 5% 58%	2620
	21st to end	Night	16% 7% 2 4 3 2 2 2 4 5% 53%	2730
December	1st to 10th	Night	11% 8% 2 5% 4 3 3 2 3 5% 54%	4380
	11th to 20th	Night	17% 7% 2 5% 2 2 3 2 3 4 52%	4946
	21st to end	Night	20% 10% 5% 7% 3 3 3 2 3 5% 39%	6633

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	40% 3 7% 28% 19% 2	5290
	11th to 20th	Day	30% 2 9% 27% 26% 5%	5310
	21st to end	Day	40% 2 5% 24% 22% 6%	5812
February	1st to 10th	Day	47% 4 8% 23% 15% 3	1890
	11th to 20th	Day	42% 3 9% 24% 18% 4	1932
	21st to end	Day	42% 3 7% 21% 20% 8%	1840
March	1st to 10th	Day	39% 2 7% 19% 17% 16%	5046
	11th to 20th	Day	32% 3 9% 20% 22% 14%	5090
	21st to end	Day	44% 4 10% 30% 12% 1	5585
April	1st to 10th	Day	40% 4 10% 21% 18% 6%	2374
	11th to 20th	Day	43% 4 12% 20% 16% 5%	2494
	21st to end	Day	49% 5% 9% 22% 14% 2	2584
May	1st to 10th	Day	44% 4 9% 22% 19% 2	7655
	11th to 20th	Day	41% 5% 12% 21% 19% 3	7631
	21st to end	Day	51% 6% 12% 15% 14% 1	8405
June	1st to 10th	Day	36% 6% 15% 27% 16% 2	2730
	11th to 20th	Day	33% 7% 15% 27% 17% 2	2730
	21st to end	Day	26% 6% 16% 30% 19% 4	2730
July	1st to 10th	Day	31% 7% 18% 31% 11% 1	5966
	11th to 20th	Day	28% 6% 14% 35% 17% 1	5897
	21st to end	Day	22% 7% 20% 36% 14%	6240
August	1st to 10th	Day	21% 5% 15% 29% 24% 5%	4220
	11th to 20th	Day	30% 7% 17% 29% 15% 2	4210
	21st to end	Day	25% 6% 13% 40% 12% 3	4325
September	1st to 10th	Day	30% 5% 10% 25% 21% 9%	2346
	11th to 20th	Day	33% 6% 10% 24% 21% 5%	2200
	21st to end	Day	29% 4 10% 22% 28% 7%	2200
October	1st to 10th	Day	17% 3 7% 15% 41% 19%	3275
	11th to 20th	Day	18% 2 4 19% 42% 15%	3240
	21st to end	Day	29% 3 8% 29% 26% 6%	3538
November	1st to 10th	Day	28% 3 9% 26% 26% 8%	2058
	11th to 20th	Day	31% 3 8% 27% 22% 9%	1995
	21st to end	Day	38% 3 8% 24% 23% 4	1890
December	1st to 10th	Day	37% 2 7% 32% 21% 1	3030
	11th to 20th	Day	39% 3 5% 26% 22% 4	3479
	21st to end	Day	55% 3 6% 21% 12% 2	4587
January	1st to 10th	Night	39% 2 9% 29% 18% 2	7635
	11th to 20th	Night	26% 2 11% 18% 37% 6%	7680
	21st to end	Night	44% 2 7% 17% 25% 4	8402
February	1st to 10th	Night	41% 3 10% 25% 17% 4	2730
	11th to 20th	Night	41% 3 8% 24% 20% 4	2601
	21st to end	Night	39% 2 7% 19% 24% 9%	1966
March	1st to 10th	Night	42% 3 6% 18% 19% 12%	5029
	11th to 20th	Night	31% 3 8% 23% 22% 14%	5090
	21st to end	Night	36% 4 12% 30% 15% 3	5588
April	1st to 10th	Night	36% 4 10% 23% 19% 8%	2244
	11th to 20th	Night	36% 4 10% 22% 20% 8%	2075
	21st to end	Night	39% 5% 10% 24% 20% 2	2031
May	1st to 10th	Night	40% 4 12% 21% 18% 4	5293
	11th to 20th	Night	37% 5% 13% 22% 18% 4	5266
	21st to end	Night	44% 6% 17% 19% 12% 2	5805
June	1st to 10th	Night	27% 6% 16% 30% 19% 3	1890
	11th to 20th	Night	24% 6% 13% 31% 22% 3	1890
	21st to end	Night	19% 5% 15% 32% 23% 6%	1890
July	1st to 10th	Night	29% 7% 18% 28% 16% 3	4125
	11th to 20th	Night	20% 5% 14% 38% 21% 2	4071
	21st to end	Night	19% 5% 17% 38% 20% 1	4320
August	1st to 10th	Night	20% 5% 14% 31% 26% 4	2916
	11th to 20th	Night	26% 5% 18% 31% 18% 3	2915
	21st to end	Night	20% 4 11% 40% 22% 3	3571
September	1st to 10th	Night	26% 4 9% 27% 24% 10%	2050
	11th to 20th	Night	30% 6% 10% 25% 23% 6%	2200
	21st to end	Night	28% 5% 7% 24% 30% 7%	2200
October	1st to 10th	Night	14% 2 7% 14% 36% 28%	3870
	11th to 20th	Night	20% 1 5% 17% 37% 19%	3899
	21st to end	Night	30% 3 6% 27% 26% 8%	4252
November	1st to 10th	Night	28% 2 8% 24% 27% 10%	2460
	11th to 20th	Night	29% 3 7% 27% 25% 9%	2620
	21st to end	Night	36% 3 8% 23% 26% 6%	2730
December	1st to 10th	Night	35% 3 8% 33% 19% 3	4380
	11th to 20th	Night	38% 3 5% 23% 23% 8%	4947
	21st to end	Night	49% 3 8% 22% 14% 4	6633

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	26% 20% 40% 11% 3	330
	11th to 20th	Day	27% 24% 37% 10% 2	330
	21st to end	Day	24% 24% 36% 14% 3	363
February	1st to 10th	Day	32% 20% 37% 10% 2	330
	11th to 20th	Day	31% 29% 29% 10% 1	330
	21st to end	Day	31% 24% 34% 8% 3	273
March	1st to 10th	Day	29% 25% 29% 15% 2	330
	11th to 20th	Day	34% 22% 34% 10% 1	330
	21st to end	Day	32% 22% 35% 8% 2	363
April	1st to 10th	Day	26% 24% 31% 14% 5% 1	394
	11th to 20th	Day	29% 21% 35% 13% 3	532
	21st to end	Day	27% 22% 32% 14% 5%	604
May	1st to 10th	Day	18% 18% 39% 20% 5%	753
	11th to 20th	Day	18% 17% 34% 22% 8% 1	775
	21st to end	Day	16% 12% 35% 21% 12% 3	844
June	1st to 10th	Day	11% 8% 27% 33% 19% 2	780
	11th to 20th	Day	9% 6% 29% 35% 19% 2	780
	21st to end	Day	6% 4 33% 37% 17% 3	780
July	1st to 10th	Day	7% 4 19% 36% 27% 7%	771
	11th to 20th	Day	4 2 19% 43% 26% 6%	744
	21st to end	Day	7% 2 14% 40% 29% 7%	794
August	1st to 10th	Day	8% 2 17% 45% 25% 4	726
	11th to 20th	Day	10% 3 15% 41% 27% 4	715
	21st to end	Day	8% 2 14% 38% 31% 7%	597
September	1st to 10th	Day	8% 4 13% 42% 27% 6%	476
	11th to 20th	Day	6% 3 23% 39% 25% 4	330
	21st to end	Day	12% 5% 24% 33% 22% 4	330
October	1st to 10th	Day	8% 10% 32% 31% 16% 3	330
	11th to 20th	Day	15% 14% 39% 25% 7%	330
	21st to end	Day	29% 17% 31% 17% 5% 1	363
November	1st to 10th	Day	16% 15% 41% 22% 5% 1	330
	11th to 20th	Day	18% 19% 37% 19% 6% 1	330
	21st to end	Day	28% 17% 35% 17% 3	330
December	1st to 10th	Day	22% 21% 37% 15% 4 2	330
	11th to 20th	Day	23% 23% 41% 9% 4 1	330
	21st to end	Day	25% 25% 37% 11% 3	363
January	1st to 10th	Night	19% 19% 35% 23% 4	1050
	11th to 20th	Night	21% 20% 37% 20% 3	1050
	21st to end	Night	19% 19% 34% 25% 3	1155
February	1st to 10th	Night	23% 19% 36% 19% 3	1050
	11th to 20th	Night	22% 23% 34% 20% 2	966
	21st to end	Night	22% 20% 35% 21% 2	582
March	1st to 10th	Night	21% 18% 33% 25% 3	630
	11th to 20th	Night	22% 18% 35% 23% 3	630
	21st to end	Night	19% 18% 41% 18% 4	693
April	1st to 10th	Night	20% 19% 34% 23% 4	630
	11th to 20th	Night	18% 16% 38% 21% 6%	623
	21st to end	Night	20% 14% 41% 17% 7%	630
May	1st to 10th	Night	14% 12% 38% 30% 6%	630
	11th to 20th	Night	14% 10% 31% 31% 13%	621
	21st to end	Night	12% 8% 35% 27% 17% 2	683
June	1st to 10th	Night	8% 6% 21% 41% 23% 1	630
	11th to 20th	Night	7% 3 32% 41% 16% 1	630
	21st to end	Night	3 3 25% 47% 20% 2	630
July	1st to 10th	Night	6% 1 17% 41% 29% 5%	615
	11th to 20th	Night	5% 1 16% 44% 30% 4	597
	21st to end	Night	4 1 11% 47% 34% 4	657
August	1st to 10th	Night	3 1 17% 47% 28% 3	597
	11th to 20th	Night	8% 2 15% 45% 26% 4	594
	21st to end	Night	5% 2 15% 43% 32% 3	660
September	1st to 10th	Night	11% 2 16% 41% 30% 1	600
	11th to 20th	Night	10% 2 18% 43% 24% 4	600
	21st to end	Night	9% 5% 26% 34% 24% 3	600
October	1st to 10th	Night	8% 5% 29% 33% 23% 2	763
	11th to 20th	Night	14% 9% 35% 32% 10%	800
	21st to end	Night	20% 14% 29% 27% 9% 1	880
November	1st to 10th	Night	15% 12% 38% 26% 8%	820
	11th to 20th	Night	16% 15% 36% 26% 7%	940
	21st to end	Night	24% 14% 33% 22% 6%	1050
December	1st to 10th	Night	20% 15% 36% 24% 5%	1050
	11th to 20th	Night	23% 15% 33% 24% 4	1050
	21st to end	Night	20% 21% 34% 21% 4	1155

Page generated at 2026-07-13 02:14:49. Version ac07

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