University of Oklahoma Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the University of Oklahoma 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	10% 5% 2 5% 3 3 4 3 5% 6% 53%	109443
	Night	9% 5% 2 4 3 3 3 3 5% 6% 57%	109796
Transparency	Day	30% 5% 10% 25% 24% 7%	113199
	Night	27% 4 9% 25% 26% 9%	114167
Seeing	Day	12% 9% 27% 29% 18% 5%	15745
	Night	11% 9% 32% 33% 13% 1	25799

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% 7% 4 7% 3 3 4 3 5% 5% 48%	4077
	11th to 20th	Day	6% 6% 2 4 2 2 2 3 4 4 66%	4080
	21st to end	Day	15% 9% 3 5% 2 3 2 2 3 4 53%	4488
February	1st to 10th	Day	13% 8% 3 6% 3 3 3 3 4 6% 47%	1890
	11th to 20th	Day	13% 7% 2 5% 3 4 3 3 5% 6% 49%	1949
	21st to end	Day	13% 8% 2 5% 3 3 3 2 3 5% 52%	1650
March	1st to 10th	Day	11% 6% 2 5% 3 2 2 2 4 5% 57%	3485
	11th to 20th	Day	10% 7% 2 6% 4 3 3 3 4 6% 53%	3491
	21st to end	Day	15% 11% 3 6% 4 3 4 3 4 7% 40%	3814
April	1st to 10th	Day	9% 5% 2 5% 4 3 4 3 5% 6% 54%	2090
	11th to 20th	Day	12% 5% 2 5% 4 5% 3 3 5% 6% 49%	2237
	21st to end	Day	14% 7% 2 5% 4 3 3 3 4 6% 48%	2470
May	1st to 10th	Day	17% 8% 3 6% 4 3 3 3 4 5% 43%	4082
	11th to 20th	Day	11% 6% 2 6% 4 4 4 4 5% 6% 48%	4082
	21st to end	Day	14% 7% 2 6% 5% 4 5% 4 5% 6% 41%	4477
June	1st to 10th	Day	6% 5% 2 5% 4 4 5% 4 7% 8% 50%	2470
	11th to 20th	Day	6% 3 1 4 4 4 4 4 5% 9% 54%	2470
	21st to end	Day	5% 3 1 4 3 3 4 4 6% 8% 60%	2470
July	1st to 10th	Day	7% 4 1 5% 4 4 5% 5% 7% 9% 48%	4120
	11th to 20th	Day	5% 3 1 4 3 4 4 4 5% 9% 57%	4120
	21st to end	Day	4 3 1 4 4 3 4 4 6% 9% 57%	4465
August	1st to 10th	Day	5% 2 1 5% 4 4 4 3 5% 8% 57%	4104
	11th to 20th	Day	6% 4 1 4 3 3 4 4 6% 9% 56%	4041
	21st to end	Day	3 2 1 3 3 3 4 4 6% 7% 62%	4201
September	1st to 10th	Day	10% 4 1 4 3 3 3 3 5% 6% 57%	2090
	11th to 20th	Day	12% 5% 2 6% 4 4 3 3 5% 7% 51%	2090
	21st to end	Day	7% 4 2 5% 2 3 3 3 5% 6% 60%	2090
October	1st to 10th	Day	9% 4 1 4 3 3 3 2 4 5% 61%	2053
	11th to 20th	Day	10% 3 1 3 2 2 2 2 3 4 67%	1900
	21st to end	Day	12% 5% 1 4 3 3 2 3 4 4 59%	2090
November	1st to 10th	Day	9% 5% 2 3 3 3 3 3 4 7% 58%	1888
	11th to 20th	Day	7% 4 1 3 2 3 3 3 3 5% 65%	1800
	21st to end	Day	12% 6% 2 3 2 3 2 2 4 5% 60%	1800
December	1st to 10th	Day	6% 6% 2 5% 3 3 4 3 5% 8% 56%	2940
	11th to 20th	Day	10% 5% 1 4 3 3 3 3 4 5% 58%	3391
	21st to end	Day	17% 8% 2 6% 4 3 3 3 4 5% 44%	4488
January	1st to 10th	Night	13% 6% 2 7% 4 3 3 3 4 6% 49%	5888
	11th to 20th	Night	5% 3 1 3 3 2 3 2 4 6% 67%	5900
	21st to end	Night	14% 8% 2 5% 2 3 3 2 3 5% 52%	6490
February	1st to 10th	Night	12% 8% 3 5% 3 3 2 3 4 5% 53%	2525
	11th to 20th	Night	12% 8% 2 5% 3 3 3 3 4 6% 52%	2338
	21st to end	Night	10% 7% 2 5% 4 4 3 2 4 5% 53%	1980
March	1st to 10th	Night	10% 6% 3 5% 3 3 3 2 4 6% 53%	3591
	11th to 20th	Night	10% 5% 2 3 2 2 2 3 4 7% 60%	3497
	21st to end	Night	11% 7% 2 5% 4 3 4 3 6% 7% 48%	3817
April	1st to 10th	Night	9% 5% 1 5% 3 3 3 3 4 6% 57%	2090
	11th to 20th	Night	13% 4 2 4 3 2 4 3 5% 7% 54%	1889
	21st to end	Night	13% 5% 2 5% 3 2 4 3 4 6% 53%	1710
May	1st to 10th	Night	16% 8% 2 5% 3 3 3 3 4 5% 47%	2823
	11th to 20th	Night	12% 5% 3 6% 3 4 4 4 6% 6% 49%	2823
	21st to end	Night	10% 5% 2 5% 4 4 3 3 6% 7% 52%	3090
June	1st to 10th	Night	5% 4 1 4 2 3 3 3 6% 12% 57%	1710
	11th to 20th	Night	5% 5% 1 3 3 3 3 4 6% 9% 59%	1710
	21st to end	Night	3 3 1 2 2 2 3 3 6% 8% 67%	1710
July	1st to 10th	Night	7% 4 2 5% 4 4 4 4 6% 9% 53%	2850
	11th to 20th	Night	2 2 3 2 3 3 3 6% 9% 67%	2850
	21st to end	Night	4 2 1 3 3 2 3 3 6% 9% 65%	3098
August	1st to 10th	Night	2 2 3 3 3 4 3 6% 9% 65%	2836
	11th to 20th	Night	5% 2 1 4 4 3 5% 4 6% 9% 57%	2798
	21st to end	Night	2 1 1 3 2 3 3 3 5% 9% 66%	3457
September	1st to 10th	Night	7% 4 2 4 3 3 4 3 4 5% 60%	2090
	11th to 20th	Night	10% 4 2 6% 4 3 4 3 4 6% 53%	2090
	21st to end	Night	6% 3 2 4 3 3 3 3 5% 8% 61%	2090
October	1st to 10th	Night	7% 2 1 4 3 3 3 3 5% 7% 63%	2106
	11th to 20th	Night	9% 4 1 4 2 3 2 2 3 4 65%	2280
	21st to end	Night	12% 4 2 3 3 1 2 2 4 5% 62%	2508
November	1st to 10th	Night	9% 4 1 4 2 2 2 2 4 5% 65%	2406
	11th to 20th	Night	7% 4 2 4 3 3 3 2 4 5% 63%	2600
	21st to end	Night	11% 5% 2 4 3 3 3 3 4 5% 58%	2600
December	1st to 10th	Night	6% 5% 1 4 2 2 3 2 5% 6% 63%	4250
	11th to 20th	Night	11% 5% 2 5% 4 3 3 2 3 4 59%	4816
	21st to end	Night	17% 8% 2 4 3 2 3 2 4 4 51%	6490

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	37% 4 10% 25% 21% 3	4078
	11th to 20th	Day	24% 3 9% 21% 36% 7%	4080
	21st to end	Day	38% 2 4 20% 29% 7%	4488
February	1st to 10th	Day	39% 4 8% 22% 23% 5%	1890
	11th to 20th	Day	36% 4 7% 27% 21% 6%	1959
	21st to end	Day	38% 3 5% 17% 27% 11%	1650
March	1st to 10th	Day	31% 4 6% 19% 24% 16%	3485
	11th to 20th	Day	34% 5% 10% 25% 21% 5%	3491
	21st to end	Day	44% 5% 9% 22% 16% 4	3814
April	1st to 10th	Day	31% 6% 10% 24% 22% 8%	2090
	11th to 20th	Day	36% 5% 9% 21% 21% 8%	2237
	21st to end	Day	38% 6% 9% 25% 19% 4	2470
May	1st to 10th	Day	43% 7% 10% 18% 17% 5%	4120
	11th to 20th	Day	35% 8% 12% 26% 17% 2	4086
	21st to end	Day	41% 8% 11% 19% 15% 6%	4489
June	1st to 10th	Day	31% 8% 18% 26% 15% 3	2470
	11th to 20th	Day	27% 7% 13% 28% 22% 4	2470
	21st to end	Day	22% 6% 14% 29% 23% 6%	2470
July	1st to 10th	Day	30% 7% 15% 29% 16% 3	4056
	11th to 20th	Day	23% 5% 9% 33% 26% 3	4051
	21st to end	Day	22% 7% 15% 31% 24% 1	4325
August	1st to 10th	Day	24% 5% 10% 26% 25% 10%	4090
	11th to 20th	Day	24% 6% 15% 32% 19% 4	4080
	21st to end	Day	20% 7% 12% 42% 16% 3	4200
September	1st to 10th	Day	27% 5% 9% 24% 24% 11%	2090
	11th to 20th	Day	33% 6% 10% 21% 24% 6%	2090
	21st to end	Day	25% 4 8% 25% 31% 7%	2090
October	1st to 10th	Day	17% 3 6% 13% 45% 17%	3417
	11th to 20th	Day	17% 3 6% 23% 34% 17%	3140
	21st to end	Day	24% 3 6% 29% 29% 9%	3428
November	1st to 10th	Day	27% 4 7% 23% 29% 11%	1888
	11th to 20th	Day	23% 3 8% 26% 29% 11%	1800
	21st to end	Day	29% 2 6% 25% 28% 10%	1800
December	1st to 10th	Day	28% 4 9% 34% 22% 3	2940
	11th to 20th	Day	29% 3 6% 22% 31% 9%	3389
	21st to end	Day	42% 4 6% 24% 20% 4	4488
January	1st to 10th	Night	37% 4 8% 27% 21% 4	5888
	11th to 20th	Night	20% 4 11% 20% 33% 12%	5900
	21st to end	Night	36% 3 6% 17% 30% 8%	6490
February	1st to 10th	Night	36% 3 7% 21% 25% 7%	2525
	11th to 20th	Night	34% 4 7% 24% 23% 7%	2338
	21st to end	Night	34% 3 5% 18% 25% 14%	1980
March	1st to 10th	Night	32% 5% 6% 18% 24% 16%	3591
	11th to 20th	Night	26% 4 8% 32% 22% 8%	3497
	21st to end	Night	34% 6% 10% 26% 20% 3	3817
April	1st to 10th	Night	28% 5% 10% 22% 22% 13%	2090
	11th to 20th	Night	30% 5% 11% 24% 19% 12%	1889
	21st to end	Night	33% 5% 11% 25% 22% 4	1710
May	1st to 10th	Night	39% 7% 9% 19% 20% 7%	2850
	11th to 20th	Night	34% 8% 13% 26% 17% 2	2824
	21st to end	Night	30% 7% 13% 23% 20% 8%	3099
June	1st to 10th	Night	20% 7% 19% 31% 19% 4	1710
	11th to 20th	Night	22% 6% 16% 28% 23% 5%	1710
	21st to end	Night	16% 5% 13% 29% 29% 8%	1710
July	1st to 10th	Night	28% 6% 14% 32% 18% 3	2805
	11th to 20th	Night	13% 5% 11% 41% 27% 4	2796
	21st to end	Night	17% 5% 14% 33% 28% 2	2999
August	1st to 10th	Night	15% 4 13% 25% 35% 8%	2826
	11th to 20th	Night	22% 5% 12% 34% 24% 4	2825
	21st to end	Night	15% 5% 14% 41% 20% 4	3457
September	1st to 10th	Night	26% 5% 8% 23% 26% 12%	2090
	11th to 20th	Night	32% 5% 9% 19% 26% 9%	2090
	21st to end	Night	22% 4 8% 25% 31% 10%	2090
October	1st to 10th	Night	15% 4 7% 14% 38% 23%	3509
	11th to 20th	Night	17% 3 5% 18% 38% 19%	3779
	21st to end	Night	21% 3 6% 28% 33% 10%	4120
November	1st to 10th	Night	24% 3 8% 21% 31% 13%	2406
	11th to 20th	Night	26% 3 7% 23% 29% 12%	2600
	21st to end	Night	30% 3 7% 21% 29% 11%	2600
December	1st to 10th	Night	24% 3 6% 35% 27% 5%	4250
	11th to 20th	Night	32% 2 4 21% 31% 10%	4817
	21st to end	Night	36% 4 7% 25% 22% 7%	6490

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	19% 14% 38% 24% 5%	300
	11th to 20th	Day	18% 20% 36% 22% 3 1	300
	21st to end	Day	15% 15% 40% 22% 7% 1	330
February	1st to 10th	Day	22% 15% 41% 17% 4 1	300
	11th to 20th	Day	19% 22% 42% 16% 1	300
	21st to end	Day	25% 14% 42% 16% 4	249
March	1st to 10th	Day	19% 20% 37% 19% 5%	300
	11th to 20th	Day	20% 18% 39% 16% 5% 1	273
	21st to end	Day	21% 19% 38% 18% 5%	297
April	1st to 10th	Day	13% 23% 32% 26% 5%	270
	11th to 20th	Day	15% 20% 43% 19% 3	435
	21st to end	Day	19% 15% 34% 23% 7% 2	650
May	1st to 10th	Day	14% 14% 37% 29% 6% 1	653
	11th to 20th	Day	16% 10% 32% 26% 14% 3	675
	21st to end	Day	15% 7% 32% 26% 15% 5%	734
June	1st to 10th	Day	7% 4 20% 36% 26% 7%	680
	11th to 20th	Day	7% 3 26% 40% 20% 4	680
	21st to end	Day	4 2 20% 42% 25% 8%	680
July	1st to 10th	Day	7% 2 12% 32% 34% 14%	671
	11th to 20th	Day	5% 1 10% 38% 36% 10%	668
	21st to end	Day	5% 1 7% 32% 41% 15%	739
August	1st to 10th	Day	5% 1 11% 40% 36% 7%	676
	11th to 20th	Day	8% 1 12% 36% 35% 8%	665
	21st to end	Day	5% 1 7% 38% 33% 16%	560
September	1st to 10th	Day	9% 1 8% 30% 33% 18%	300
	11th to 20th	Day	8% 2 13% 36% 30% 12%	300
	21st to end	Day	13% 4 18% 34% 24% 7%	300
October	1st to 10th	Day	9% 6% 28% 33% 17% 8%	300
	11th to 20th	Day	9% 12% 35% 33% 10% 1	300
	21st to end	Day	22% 13% 30% 23% 11% 2	330
November	1st to 10th	Day	14% 10% 40% 24% 10% 2	300
	11th to 20th	Day	15% 12% 37% 23% 11% 2	300
	21st to end	Day	19% 15% 37% 19% 9%	300
December	1st to 10th	Day	12% 17% 37% 24% 7% 2	300
	11th to 20th	Day	18% 14% 39% 22% 8%	300
	21st to end	Day	21% 15% 39% 20% 5%	330
January	1st to 10th	Night	12% 13% 40% 29% 6% 1	1000
	11th to 20th	Night	13% 15% 36% 31% 5%	1000
	21st to end	Night	12% 12% 40% 29% 6%	1100
February	1st to 10th	Night	17% 13% 42% 24% 4	925
	11th to 20th	Night	17% 17% 38% 24% 4	784
	21st to end	Night	16% 17% 40% 25% 2	660
March	1st to 10th	Night	15% 14% 34% 33% 3	640
	11th to 20th	Night	19% 13% 37% 26% 5%	575
	21st to end	Night	15% 15% 42% 22% 6%	627
April	1st to 10th	Night	13% 15% 44% 22% 5%	570
	11th to 20th	Night	15% 15% 41% 25% 5%	563
	21st to end	Night	15% 11% 39% 26% 7% 1	570
May	1st to 10th	Night	11% 11% 42% 28% 8%	570
	11th to 20th	Night	12% 7% 36% 29% 13% 2	561
	21st to end	Night	8% 7% 34% 33% 16% 2	617
June	1st to 10th	Night	5% 4 22% 44% 21% 3	570
	11th to 20th	Night	6% 3 28% 49% 13%	570
	21st to end	Night	3 1 25% 46% 22% 2	570
July	1st to 10th	Night	6% 12% 45% 33% 3	555
	11th to 20th	Night	2 1 11% 52% 31% 2	552
	21st to end	Night	3 1 9% 49% 36% 2	624
August	1st to 10th	Night	2 13% 51% 31% 2	567
	11th to 20th	Night	4 1 17% 42% 34% 2	564
	21st to end	Night	3 10% 44% 39% 5%	627
September	1st to 10th	Night	10% 2 13% 40% 33% 2	570
	11th to 20th	Night	12% 1 20% 36% 27% 4	570
	21st to end	Night	9% 3 19% 44% 21% 5%	570
October	1st to 10th	Night	8% 4 30% 35% 21% 2	586
	11th to 20th	Night	11% 8% 35% 35% 11% 1	760
	21st to end	Night	16% 9% 32% 31% 12% 1	836
November	1st to 10th	Night	10% 7% 37% 33% 11% 1	846
	11th to 20th	Night	10% 14% 34% 33% 9% 1	1000
	21st to end	Night	15% 12% 37% 27% 9%	1000
December	1st to 10th	Night	12% 13% 36% 32% 7%	1000
	11th to 20th	Night	18% 11% 37% 28% 6%	1000
	21st to end	Night	16% 12% 36% 30% 5%	1100

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