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 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% 7% 2 5% 4 3 4 3 5% 7% 47%	109505
	Night	12% 6% 2 5% 3 3 3 3 5% 6% 52%	109723
Transparency	Day	36% 4 11% 25% 19% 4	113156
	Night	33% 4 10% 25% 22% 6%	114199
Seeing	Day	17% 12% 29% 26% 14% 2	15820
	Night	16% 12% 31% 29% 12% 1	25832

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	14% 12% 4 6% 3 3 3 3 3 5% 45%	4077
	11th to 20th	Day	7% 5% 2 8% 4 3 4 2 4 5% 55%	4080
	21st to end	Day	20% 9% 2 4 3 2 2 2 3 4 47%	4488
February	1st to 10th	Day	18% 11% 3 6% 4 3 3 4 4 4 41%	1800
	11th to 20th	Day	17% 8% 3 6% 4 3 3 3 5% 5% 44%	1829
	21st to end	Day	15% 9% 3 6% 4 4 4 3 6% 5% 43%	1755
March	1st to 10th	Day	17% 7% 2 5% 3 3 3 3 4 5% 49%	3546
	11th to 20th	Day	13% 6% 2 5% 3 4 4 3 5% 6% 49%	3491
	21st to end	Day	17% 10% 3 8% 5% 4 5% 3 6% 6% 35%	3814
April	1st to 10th	Day	14% 6% 2 6% 4 3 4 3 6% 6% 45%	2148
	11th to 20th	Day	17% 7% 3 6% 4 4 3 3 5% 6% 42%	2254
	21st to end	Day	21% 9% 3 6% 4 4 3 3 4 5% 40%	2338
May	1st to 10th	Day	20% 9% 3 6% 4 4 4 3 5% 5% 37%	4082
	11th to 20th	Day	13% 6% 3 6% 5% 4 5% 3 6% 7% 41%	4082
	21st to end	Day	15% 6% 2 8% 5% 5% 5% 4 6% 7% 36%	4477
June	1st to 10th	Day	9% 5% 2 7% 4 4 4 4 7% 9% 45%	2470
	11th to 20th	Day	7% 5% 2 4 4 4 5% 5% 6% 9% 49%	2470
	21st to end	Day	5% 4 1 4 4 5% 4 4 6% 8% 57%	2470
July	1st to 10th	Day	9% 4 1 5% 4 4 6% 5% 8% 10% 45%	4120
	11th to 20th	Day	4 4 2 5% 4 4 4 4 6% 9% 54%	4120
	21st to end	Day	6% 4 2 5% 4 4 5% 5% 9% 11% 45%	4465
August	1st to 10th	Day	4 3 1 4 3 3 4 4 6% 9% 59%	4104
	11th to 20th	Day	8% 4 2 5% 4 4 4 5% 6% 9% 49%	4041
	21st to end	Day	4 3 1 5% 3 4 4 4 6% 9% 56%	4194
September	1st to 10th	Day	11% 4 2 4 3 3 3 3 5% 8% 54%	2229
	11th to 20th	Day	11% 4 2 5% 4 4 4 4 6% 7% 47%	2090
	21st to end	Day	7% 4 2 5% 4 3 4 4 5% 5% 57%	2090
October	1st to 10th	Day	10% 4 2 4 4 3 4 3 4 5% 58%	1914
	11th to 20th	Day	12% 5% 1 4 2 2 3 2 3 5% 61%	1900
	21st to end	Day	19% 6% 2 4 3 3 3 2 3 4 52%	2090
November	1st to 10th	Day	11% 6% 2 4 3 2 3 3 3 6% 57%	1958
	11th to 20th	Day	12% 7% 2 4 3 3 2 3 4 6% 55%	1900
	21st to end	Day	17% 7% 2 4 3 2 2 3 5% 6% 49%	1800
December	1st to 10th	Day	14% 9% 2 4 3 3 3 2 3 6% 52%	2940
	11th to 20th	Day	16% 8% 3 6% 3 3 3 2 4 5% 48%	3391
	21st to end	Day	21% 15% 5% 7% 4 3 3 2 4 5% 32%	4488
January	1st to 10th	Night	14% 8% 3 7% 5% 3 3 3 4 6% 45%	5888
	11th to 20th	Night	8% 5% 2 5% 3 3 3 3 4 6% 56%	5900
	21st to end	Night	19% 9% 3 6% 3 2 2 2 3 5% 46%	6490
February	1st to 10th	Night	16% 10% 3 6% 3 3 3 3 4 6% 45%	2600
	11th to 20th	Night	15% 8% 3 6% 4 3 3 2 3 5% 48%	2473
	21st to end	Night	14% 8% 2 5% 3 3 4 3 5% 5% 48%	1875
March	1st to 10th	Night	17% 8% 3 5% 3 3 4 3 5% 5% 45%	3529
	11th to 20th	Night	13% 6% 2 5% 3 3 3 2 5% 5% 52%	3497
	21st to end	Night	14% 7% 2 6% 4 4 4 4 4 6% 45%	3817
April	1st to 10th	Night	14% 6% 2 4 3 3 4 3 5% 6% 50%	2030
	11th to 20th	Night	13% 7% 1 5% 3 3 3 3 4 6% 50%	1875
	21st to end	Night	16% 6% 3 5% 4 4 3 3 5% 6% 45%	1838
May	1st to 10th	Night	15% 6% 3 6% 4 4 3 3 4 7% 45%	2823
	11th to 20th	Night	13% 6% 2 7% 4 4 4 4 5% 7% 44%	2823
	21st to end	Night	15% 6% 2 5% 5% 4 4 4 6% 8% 42%	3090
June	1st to 10th	Night	5% 4 2 5% 4 3 3 3 6% 10% 55%	1710
	11th to 20th	Night	4 3 1 4 3 3 4 4 7% 9% 57%	1710
	21st to end	Night	4 2 1 3 3 3 3 4 5% 8% 64%	1710
July	1st to 10th	Night	9% 4 2 4 3 3 3 4 6% 10% 52%	2850
	11th to 20th	Night	3 2 1 3 3 3 4 3 5% 8% 65%	2850
	21st to end	Night	5% 4 1 4 4 3 4 4 7% 10% 54%	3098
August	1st to 10th	Night	2 2 1 4 3 4 4 4 6% 9% 61%	2836
	11th to 20th	Night	6% 4 1 5% 3 4 4 5% 7% 7% 55%	2798
	21st to end	Night	3 3 1 3 3 3 3 3 5% 8% 64%	3462
September	1st to 10th	Night	9% 4 1 4 3 3 4 3 5% 6% 59%	1947
	11th to 20th	Night	9% 5% 2 6% 3 3 4 3 6% 7% 52%	2090
	21st to end	Night	7% 4 2 6% 3 3 3 3 5% 6% 58%	2090
October	1st to 10th	Night	7% 3 2 3 3 3 3 4 4 6% 63%	2245
	11th to 20th	Night	13% 3 1 4 2 2 3 3 4 5% 60%	2280
	21st to end	Night	16% 6% 2 3 2 2 3 3 3 5% 54%	2508
November	1st to 10th	Night	12% 6% 1 3 2 2 2 2 3 5% 61%	2340
	11th to 20th	Night	10% 6% 2 5% 3 2 3 3 4 5% 58%	2495
	21st to end	Night	16% 7% 2 3 3 2 2 2 4 5% 54%	2600
December	1st to 10th	Night	11% 8% 2 5% 4 3 3 2 3 5% 54%	4250
	11th to 20th	Night	17% 7% 3 5% 2 3 3 2 3 4 51%	4816
	21st to end	Night	20% 11% 5% 7% 3 3 3 2 3 5% 38%	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	43% 3 8% 26% 18% 2	4078
	11th to 20th	Day	32% 2 12% 26% 23% 4	4080
	21st to end	Day	42% 2 6% 22% 22% 6%	4488
February	1st to 10th	Day	48% 4 8% 21% 15% 3	1800
	11th to 20th	Day	43% 3 9% 24% 18% 3	1838
	21st to end	Day	43% 3 7% 21% 19% 8%	1755
March	1st to 10th	Day	39% 3 8% 20% 19% 11%	3546
	11th to 20th	Day	37% 4 12% 24% 20% 4	3491
	21st to end	Day	49% 5% 9% 25% 11% 1	3814
April	1st to 10th	Day	39% 5% 10% 22% 18% 6%	2148
	11th to 20th	Day	43% 4 11% 20% 16% 5%	2254
	21st to end	Day	48% 5% 8% 22% 15% 2	2338
May	1st to 10th	Day	48% 5% 8% 22% 16% 2	4120
	11th to 20th	Day	41% 5% 12% 24% 17%	4086
	21st to end	Day	46% 6% 14% 19% 14% 2	4489
June	1st to 10th	Day	35% 6% 15% 28% 14% 2	2470
	11th to 20th	Day	31% 7% 16% 28% 17% 2	2470
	21st to end	Day	25% 6% 16% 30% 19% 4	2470
July	1st to 10th	Day	31% 7% 18% 28% 13% 2	4056
	11th to 20th	Day	24% 6% 12% 36% 19% 2	4051
	21st to end	Day	28% 8% 20% 31% 13% 1	4325
August	1st to 10th	Day	22% 5% 15% 28% 25% 5%	4090
	11th to 20th	Day	31% 7% 17% 28% 15% 2	4080
	21st to end	Day	24% 6% 13% 41% 12% 3	4193
September	1st to 10th	Day	29% 6% 10% 24% 21% 9%	2229
	11th to 20th	Day	34% 6% 10% 23% 22% 5%	2090
	21st to end	Day	28% 4 10% 22% 28% 7%	2090
October	1st to 10th	Day	17% 3 7% 15% 40% 19%	3174
	11th to 20th	Day	18% 2 4 19% 43% 15%	3140
	21st to end	Day	28% 3 8% 29% 26% 6%	3428
November	1st to 10th	Day	29% 3 9% 25% 25% 8%	1958
	11th to 20th	Day	32% 3 8% 26% 23% 9%	1900
	21st to end	Day	37% 3 8% 24% 24% 5%	1800
December	1st to 10th	Day	36% 2 7% 32% 21% 1	2940
	11th to 20th	Day	40% 3 5% 26% 23% 4	3389
	21st to end	Day	56% 3 6% 20% 12% 2	4488
January	1st to 10th	Night	42% 3 10% 27% 16% 3	5888
	11th to 20th	Night	30% 3 13% 19% 32% 4	5900
	21st to end	Night	42% 2 8% 17% 27% 4	6490
February	1st to 10th	Night	42% 3 10% 24% 17% 4	2600
	11th to 20th	Night	41% 3 8% 24% 20% 4	2474
	21st to end	Night	40% 2 6% 19% 24% 8%	1875
March	1st to 10th	Night	42% 4 8% 18% 17% 11%	3529
	11th to 20th	Night	35% 4 10% 28% 18% 6%	3497
	21st to end	Night	40% 4 11% 29% 13% 2	3817
April	1st to 10th	Night	36% 4 10% 22% 20% 9%	2030
	11th to 20th	Night	35% 4 10% 22% 20% 8%	1875
	21st to end	Night	40% 5% 9% 24% 21% 2	1838
May	1st to 10th	Night	40% 5% 13% 22% 17% 5%	2850
	11th to 20th	Night	39% 5% 14% 23% 16% 2	2824
	21st to end	Night	39% 6% 15% 23% 15% 2	3099
June	1st to 10th	Night	25% 6% 17% 31% 19% 3	1710
	11th to 20th	Night	22% 6% 14% 33% 23% 3	1710
	21st to end	Night	18% 5% 14% 32% 24% 7%	1710
July	1st to 10th	Night	28% 6% 18% 26% 17% 5%	2805
	11th to 20th	Night	17% 4 10% 41% 24% 3	2796
	21st to end	Night	24% 6% 20% 33% 17% 1	2999
August	1st to 10th	Night	19% 5% 14% 31% 27% 4	2826
	11th to 20th	Night	26% 5% 18% 30% 18% 3	2825
	21st to end	Night	19% 4 11% 41% 22% 3	3462
September	1st to 10th	Night	26% 4 9% 26% 24% 11%	1947
	11th to 20th	Night	30% 6% 10% 24% 24% 6%	2090
	21st to end	Night	27% 5% 7% 24% 30% 7%	2090
October	1st to 10th	Night	14% 2 7% 14% 35% 28%	3752
	11th to 20th	Night	20% 1 5% 17% 37% 20%	3779
	21st to end	Night	29% 3 6% 28% 26% 8%	4120
November	1st to 10th	Night	29% 2 8% 24% 26% 10%	2340
	11th to 20th	Night	29% 3 7% 25% 26% 10%	2495
	21st to end	Night	35% 3 8% 22% 27% 6%	2600
December	1st to 10th	Night	35% 3 8% 33% 19% 3	4250
	11th to 20th	Night	39% 3 5% 23% 23% 8%	4817
	21st to end	Night	50% 3 8% 22% 13% 4	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	26% 20% 40% 11% 3	300
	11th to 20th	Day	28% 24% 37% 9% 2	300
	21st to end	Day	25% 24% 34% 14% 3	330
February	1st to 10th	Day	34% 20% 36% 9% 1	300
	11th to 20th	Day	31% 30% 27% 10% 1	300
	21st to end	Day	31% 24% 33% 8% 2	249
March	1st to 10th	Day	27% 25% 30% 15% 2	300
	11th to 20th	Day	34% 20% 35% 11% 1	273
	21st to end	Day	34% 24% 33% 7% 2	297
April	1st to 10th	Day	24% 25% 34% 13% 5% 1	328
	11th to 20th	Day	27% 20% 36% 13% 4	452
	21st to end	Day	27% 21% 32% 14% 5%	518
May	1st to 10th	Day	19% 18% 38% 20% 5%	653
	11th to 20th	Day	18% 15% 34% 23% 9% 1	675
	21st to end	Day	15% 12% 36% 21% 13% 3	734
June	1st to 10th	Day	10% 7% 26% 34% 21% 2	680
	11th to 20th	Day	8% 5% 29% 36% 20% 2	680
	21st to end	Day	6% 4 32% 38% 17% 3	680
July	1st to 10th	Day	8% 4 19% 35% 27% 7%	671
	11th to 20th	Day	4 2 19% 44% 26% 6%	668
	21st to end	Day	8% 2 14% 40% 28% 7%	739
August	1st to 10th	Day	8% 2 17% 44% 26% 4	676
	11th to 20th	Day	11% 3 16% 41% 26% 3	665
	21st to end	Day	7% 1 13% 39% 32% 8%	553
September	1st to 10th	Day	8% 3 14% 41% 27% 6%	439
	11th to 20th	Day	6% 3 25% 39% 25% 3	300
	21st to end	Day	12% 5% 24% 35% 20% 4	300
October	1st to 10th	Day	9% 10% 34% 29% 15% 3	300
	11th to 20th	Day	16% 14% 40% 25% 5%	300
	21st to end	Day	28% 17% 33% 16% 5%	330
November	1st to 10th	Day	17% 14% 41% 22% 5% 1	300
	11th to 20th	Day	18% 20% 36% 20% 6% 1	300
	21st to end	Day	27% 17% 36% 17% 3	300
December	1st to 10th	Day	21% 21% 38% 14% 4 2	300
	11th to 20th	Day	24% 22% 40% 10% 4 1	300
	21st to end	Day	26% 26% 36% 10% 2	330
January	1st to 10th	Night	20% 19% 34% 23% 4	1000
	11th to 20th	Night	22% 19% 36% 20% 3	1000
	21st to end	Night	19% 19% 34% 25% 3	1100
February	1st to 10th	Night	24% 19% 35% 18% 3	1000
	11th to 20th	Night	22% 22% 34% 20% 2	919
	21st to end	Night	23% 20% 34% 21% 2	555
March	1st to 10th	Night	21% 18% 34% 25% 3	600
	11th to 20th	Night	21% 16% 36% 23% 3	575
	21st to end	Night	20% 18% 39% 17% 4	627
April	1st to 10th	Night	20% 20% 34% 22% 4	570
	11th to 20th	Night	17% 17% 39% 21% 6%	563
	21st to end	Night	20% 14% 41% 16% 8% 1	570
May	1st to 10th	Night	15% 12% 39% 28% 7%	570
	11th to 20th	Night	14% 9% 32% 32% 13%	561
	21st to end	Night	11% 8% 36% 25% 18% 2	617
June	1st to 10th	Night	7% 4 21% 42% 24% 1	570
	11th to 20th	Night	6% 2 32% 42% 17% 1	570
	21st to end	Night	4 3 23% 47% 22% 2	570
July	1st to 10th	Night	6% 2 17% 41% 28% 6%	555
	11th to 20th	Night	4 1 16% 45% 30% 4	552
	21st to end	Night	4 1 11% 47% 34% 3	624
August	1st to 10th	Night	3 1 18% 45% 29% 3	567
	11th to 20th	Night	8% 2 16% 47% 24% 3	564
	21st to end	Night	4 1 15% 43% 33% 4	627
September	1st to 10th	Night	10% 2 16% 41% 29% 1	570
	11th to 20th	Night	11% 2 18% 43% 22% 3	570
	21st to end	Night	9% 4 26% 35% 23% 3	570
October	1st to 10th	Night	9% 5% 30% 32% 23% 2	725
	11th to 20th	Night	15% 9% 35% 32% 9%	760
	21st to end	Night	20% 14% 29% 27% 10% 1	836
November	1st to 10th	Night	16% 12% 38% 26% 8%	780
	11th to 20th	Night	16% 15% 36% 26% 7%	895
	21st to end	Night	23% 15% 33% 22% 7%	1000
December	1st to 10th	Night	20% 15% 35% 25% 5%	1000
	11th to 20th	Night	24% 16% 32% 24% 5%	1000
	21st to end	Night	20% 22% 33% 21% 4	1100

Page generated at 2025-03-09 07:08:43. Version ac07

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