Oak Beach Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Oak Beach Observatory clear sky chart from 2005-11-06 to 2025-10-04.

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% 10% 4 9% 5% 5% 5% 4 5% 7% 35%	125975
	Night	14% 11% 4 10% 6% 5% 4 3 5% 6% 31%	123527
Transparency	Day	30% 39% 8% 14% 7% 1	129527
	Night	34% 39% 8% 13% 7% 1	128046
Seeing	Day	16% 7% 24% 30% 18% 5%	16640
	Night	19% 8% 28% 29% 14% 2	26066

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	11% 14% 9% 23% 7% 4 5% 3 3 4 17%	3617
	11th to 20th	Day	10% 16% 7% 18% 13% 9% 5% 3 4 4 11%	3620
	21st to end	Day	17% 15% 6% 14% 5% 4 5% 4 6% 6% 18%	4244
February	1st to 10th	Day	13% 15% 5% 12% 6% 6% 5% 4 4 7% 23%	1800
	11th to 20th	Day	11% 12% 5% 12% 9% 7% 5% 4 5% 5% 25%	1839
	21st to end	Day	13% 12% 4 8% 7% 5% 4 4 7% 7% 28%	1650
March	1st to 10th	Day	13% 10% 3 8% 7% 5% 5% 4 5% 7% 34%	4605
	11th to 20th	Day	11% 12% 5% 9% 5% 4 5% 4 6% 9% 30%	4980
	21st to end	Day	17% 12% 4 10% 6% 5% 4 3 5% 6% 29%	5465
April	1st to 10th	Day	16% 12% 3 8% 5% 4 5% 4 6% 7% 31%	2327
	11th to 20th	Day	16% 8% 3 6% 4 4 4 3 5% 7% 39%	2571
	21st to end	Day	14% 9% 3 8% 5% 5% 5% 3 5% 7% 37%	2600
May	1st to 10th	Day	15% 8% 3 7% 5% 4 4 4 5% 7% 37%	5862
	11th to 20th	Day	13% 7% 3 8% 5% 5% 5% 4 6% 8% 38%	5861
	21st to end	Day	15% 7% 2 5% 3 3 4 3 5% 7% 45%	6875
June	1st to 10th	Day	10% 7% 2 5% 5% 5% 4 4 7% 8% 44%	3000
	11th to 20th	Day	10% 5% 3 5% 5% 4 4 4 7% 9% 45%	3000
	21st to end	Day	11% 6% 2 6% 4 4 5% 5% 6% 8% 44%	3000
July	1st to 10th	Day	10% 5% 2 6% 4 4 5% 4 6% 10% 43%	6573
	11th to 20th	Day	9% 5% 2 5% 4 3 4 3 5% 8% 50%	6360
	21st to end	Day	7% 5% 2 5% 4 4 5% 4 6% 9% 49%	6680
August	1st to 10th	Day	9% 5% 2 6% 4 4 5% 5% 7% 10% 42%	4234
	11th to 20th	Day	11% 6% 2 6% 3 5% 5% 4 6% 8% 44%	4174
	21st to end	Day	9% 6% 2 6% 4 4 5% 4 6% 9% 46%	4675
September	1st to 10th	Day	10% 7% 2 6% 4 4 5% 4 6% 8% 44%	2290
	11th to 20th	Day	13% 6% 2 4 3 4 4 3 5% 9% 48%	2200
	21st to end	Day	15% 7% 3 7% 5% 4 6% 5% 5% 8% 37%	2200
October	1st to 10th	Day	15% 9% 2 7% 5% 4 4 4 4 6% 40%	2047
	11th to 20th	Day	21% 13% 3 8% 5% 5% 4 4 4 7% 28%	1900
	21st to end	Day	22% 11% 4 8% 6% 5% 5% 4 6% 7% 22%	1919
November	1st to 10th	Day	13% 14% 4 8% 6% 5% 4 3 6% 7% 29%	1762
	11th to 20th	Day	14% 16% 5% 10% 6% 5% 5% 4 5% 8% 21%	1743
	21st to end	Day	17% 19% 6% 13% 6% 5% 4 3 3 6% 17%	1600
December	1st to 10th	Day	14% 20% 10% 14% 7% 6% 4 4 4 4 12%	2573
	11th to 20th	Day	14% 19% 6% 15% 8% 6% 6% 4 4 4 14%	2625
	21st to end	Day	25% 21% 7% 12% 7% 4 4 3 3 4 10%	3504
January	1st to 10th	Night	13% 13% 7% 23% 7% 5% 4 3 4 4 17%	6348
	11th to 20th	Night	11% 15% 7% 16% 11% 8% 5% 3 5% 5% 13%	6360
	21st to end	Night	12% 15% 7% 11% 7% 5% 5% 4 5% 7% 21%	6720
February	1st to 10th	Night	13% 16% 5% 12% 6% 6% 5% 4 6% 6% 22%	2600
	11th to 20th	Night	14% 14% 5% 11% 6% 4 5% 3 5% 7% 27%	2443
	21st to end	Night	15% 14% 5% 11% 6% 5% 5% 4 5% 6% 26%	1980
March	1st to 10th	Night	13% 11% 3 8% 5% 4 4 3 5% 6% 37%	5247
	11th to 20th	Night	9% 10% 4 9% 5% 4 4 4 6% 9% 35%	4980
	21st to end	Night	17% 10% 4 10% 6% 5% 4 3 4 4 32%	5467
April	1st to 10th	Night	17% 13% 4 7% 4 4 4 3 4 7% 33%	2090
	11th to 20th	Night	18% 8% 2 6% 5% 4 4 3 5% 8% 37%	1777
	21st to end	Night	17% 10% 3 7% 5% 6% 3 3 5% 6% 35%	1800
May	1st to 10th	Night	15% 9% 4 7% 5% 4 4 3 5% 7% 36%	4033
	11th to 20th	Night	13% 8% 3 8% 5% 5% 5% 3 5% 7% 38%	4032
	21st to end	Night	18% 6% 2 5% 4 4 4 3 5% 6% 43%	3980
June	1st to 10th	Night	13% 7% 3 6% 5% 4 4 4 6% 8% 41%	1400
	11th to 20th	Night	10% 6% 3 5% 4 5% 5% 4 7% 8% 43%	1400
	21st to end	Night	8% 7% 3 6% 5% 4 4 4 6% 8% 43%	1400
July	1st to 10th	Night	9% 6% 3 7% 4 5% 5% 3 6% 8% 45%	3387
	11th to 20th	Night	9% 4 2 4 4 4 4 4 6% 8% 51%	3617
	21st to end	Night	7% 5% 2 6% 4 4 4 4 6% 8% 49%	4178
August	1st to 10th	Night	9% 5% 2 5% 4 4 5% 4 6% 8% 47%	2915
	11th to 20th	Night	11% 6% 1 5% 4 4 3 4 6% 10% 46%	2875
	21st to end	Night	7% 5% 2 7% 5% 5% 5% 3 7% 8% 47%	3223
September	1st to 10th	Night	11% 5% 2 7% 4 5% 4 3 5% 8% 47%	2085
	11th to 20th	Night	11% 5% 2 5% 3 3 4 3 6% 7% 52%	2200
	21st to end	Night	14% 8% 3 7% 5% 4 4 4 5% 6% 41%	2200
October	1st to 10th	Night	16% 7% 2 7% 5% 3 5% 3 5% 7% 41%	2200
	11th to 20th	Night	20% 12% 3 9% 5% 4 4 2 4 6% 31%	2280
	21st to end	Night	23% 12% 4 8% 5% 4 5% 3 5% 6% 25%	2677
November	1st to 10th	Night	15% 12% 3 9% 5% 5% 4 3 4 6% 34%	2536
	11th to 20th	Night	15% 16% 6% 12% 7% 5% 4 4 5% 5% 21%	2655
	21st to end	Night	20% 18% 6% 10% 6% 4 4 4 4 5% 19%	2800
December	1st to 10th	Night	14% 18% 8% 16% 6% 5% 5% 3 5% 5% 16%	4603
	11th to 20th	Night	12% 18% 6% 14% 8% 6% 5% 4 4 5% 17%	5582
	21st to end	Night	23% 20% 7% 11% 6% 5% 5% 3 4 4 12%	7457

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	51% 31% 7% 7% 3 1	3618
	11th to 20th	Day	58% 29% 3 7% 4	3620
	21st to end	Day	46% 31% 10% 9% 4	4244
February	1st to 10th	Day	24% 63% 4 7% 2	1800
	11th to 20th	Day	22% 62% 5% 7% 4	1848
	21st to end	Day	17% 64% 7% 8% 4	1650
March	1st to 10th	Day	34% 31% 11% 15% 7% 2	4605
	11th to 20th	Day	35% 31% 13% 15% 6% 1	4980
	21st to end	Day	39% 32% 10% 12% 7% 1	5465
April	1st to 10th	Day	20% 63% 4 8% 4	2327
	11th to 20th	Day	18% 63% 4 8% 6% 1	2571
	21st to end	Day	17% 62% 6% 9% 6% 1	2600
May	1st to 10th	Day	33% 30% 10% 12% 13% 3	5897
	11th to 20th	Day	29% 34% 9% 17% 10% 1	5865
	21st to end	Day	30% 29% 7% 15% 14% 5%	6890
June	1st to 10th	Day	14% 63% 5% 11% 7%	3000
	11th to 20th	Day	17% 63% 6% 9% 5% 1	3000
	21st to end	Day	14% 62% 5% 11% 7% 1	3000
July	1st to 10th	Day	30% 27% 12% 20% 11% 1	6504
	11th to 20th	Day	23% 27% 9% 29% 11%	6285
	21st to end	Day	23% 26% 12% 24% 14% 1	6489
August	1st to 10th	Day	22% 38% 9% 20% 10% 1	4221
	11th to 20th	Day	27% 38% 12% 18% 5%	4213
	21st to end	Day	23% 38% 11% 20% 9%	4674
September	1st to 10th	Day	15% 60% 9% 11% 4	2290
	11th to 20th	Day	14% 58% 5% 15% 8% 1	2200
	21st to end	Day	19% 59% 7% 10% 5%	2200
October	1st to 10th	Day	19% 37% 8% 26% 8% 2	3385
	11th to 20th	Day	30% 37% 8% 15% 10%	3140
	21st to end	Day	32% 39% 8% 17% 5%	3143
November	1st to 10th	Day	27% 56% 5% 8% 3	1762
	11th to 20th	Day	27% 58% 6% 6% 3	1743
	21st to end	Day	32% 58% 5% 4 1	1600
December	1st to 10th	Day	51% 37% 7% 4 1	2573
	11th to 20th	Day	53% 33% 6% 6% 1	2621
	21st to end	Day	61% 28% 5% 5% 2	3504
January	1st to 10th	Night	53% 28% 9% 8% 2	6348
	11th to 20th	Night	55% 29% 6% 6% 3	6360
	21st to end	Night	45% 32% 7% 11% 5%	6720
February	1st to 10th	Night	25% 62% 5% 5% 3	2600
	11th to 20th	Night	21% 62% 5% 9% 3	2445
	21st to end	Night	21% 62% 5% 6% 5% 1	1980
March	1st to 10th	Night	33% 31% 12% 17% 8% 1	5247
	11th to 20th	Night	32% 31% 16% 16% 5%	4980
	21st to end	Night	38% 31% 8% 13% 9% 1	5467
April	1st to 10th	Night	23% 62% 3 6% 5% 1	2090
	11th to 20th	Night	19% 62% 4 8% 7% 1	1777
	21st to end	Night	20% 63% 4 7% 6% 1	1800
May	1st to 10th	Night	35% 30% 7% 12% 11% 5%	4060
	11th to 20th	Night	33% 31% 7% 18% 9%	4033
	21st to end	Night	32% 30% 5% 18% 11% 4	3987
June	1st to 10th	Night	15% 62% 4 12% 8% 1	1400
	11th to 20th	Night	16% 63% 5% 8% 6% 2	1400
	21st to end	Night	15% 60% 5% 11% 9% 1	1400
July	1st to 10th	Night	31% 27% 12% 15% 12% 3	3350
	11th to 20th	Night	23% 27% 9% 26% 14%	3572
	21st to end	Night	24% 27% 12% 21% 15% 1	4056
August	1st to 10th	Night	23% 36% 9% 20% 11% 1	2904
	11th to 20th	Night	24% 39% 10% 20% 6% 2	2902
	21st to end	Night	21% 39% 10% 21% 8%	3223
September	1st to 10th	Night	15% 59% 5% 14% 6%	2085
	11th to 20th	Night	12% 58% 4 16% 9% 1	2200
	21st to end	Night	19% 58% 6% 12% 5% 1	2200
October	1st to 10th	Night	16% 38% 7% 24% 14%	3642
	11th to 20th	Night	29% 37% 6% 18% 11%	3779
	21st to end	Night	30% 37% 8% 19% 6%	4403
November	1st to 10th	Night	28% 57% 5% 7% 4	2536
	11th to 20th	Night	29% 57% 5% 6% 3	2655
	21st to end	Night	31% 58% 6% 3 1	2800
December	1st to 10th	Night	46% 37% 10% 5% 2	4603
	11th to 20th	Night	48% 33% 8% 7% 4	5585
	21st to end	Night	57% 29% 6% 5% 2	7457

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	15% 14% 39% 23% 8% 1	300
	11th to 20th	Day	19% 11% 37% 24% 8%	300
	21st to end	Day	22% 6% 33% 29% 8% 1	330
February	1st to 10th	Day	20% 10% 42% 23% 5% 1	300
	11th to 20th	Day	16% 12% 41% 25% 5%	300
	21st to end	Day	17% 16% 39% 24% 4	249
March	1st to 10th	Day	16% 13% 35% 28% 7% 1	347
	11th to 20th	Day	16% 12% 30% 32% 9% 1	500
	21st to end	Day	20% 11% 29% 31% 8% 1	550
April	1st to 10th	Day	19% 10% 30% 31% 9% 1	500
	11th to 20th	Day	21% 11% 23% 28% 13% 4	492
	21st to end	Day	18% 7% 30% 31% 13% 2	500
May	1st to 10th	Day	16% 8% 28% 31% 14% 3	503
	11th to 20th	Day	14% 5% 22% 31% 22% 6%	527
	21st to end	Day	13% 3 18% 33% 25% 7%	648
June	1st to 10th	Day	11% 3 20% 39% 22% 4	730
	11th to 20th	Day	11% 2 19% 40% 22% 5%	730
	21st to end	Day	12% 3 18% 39% 22% 5%	730
July	1st to 10th	Day	9% 2 12% 36% 32% 9%	721
	11th to 20th	Day	11% 3 14% 38% 27% 8%	718
	21st to end	Day	9% 2 13% 40% 28% 8%	689
August	1st to 10th	Day	10% 2 13% 31% 32% 12%	526
	11th to 20th	Day	11% 3 9% 31% 33% 12%	519
	21st to end	Day	13% 4 15% 29% 31% 9%	583
September	1st to 10th	Day	11% 5% 21% 29% 22% 13%	530
	11th to 20th	Day	11% 4 15% 23% 28% 19%	530
	21st to end	Day	17% 6% 20% 22% 25% 10%	530
October	1st to 10th	Day	19% 7% 24% 24% 20% 6%	502
	11th to 20th	Day	25% 11% 28% 26% 8% 2	490
	21st to end	Day	28% 12% 27% 25% 7% 1	368
November	1st to 10th	Day	19% 9% 34% 25% 11% 1	300
	11th to 20th	Day	20% 15% 34% 23% 6% 2	271
	21st to end	Day	24% 16% 36% 22% 3	200
December	1st to 10th	Day	23% 16% 36% 22% 4	200
	11th to 20th	Day	25% 21% 33% 19% 3 1	200
	21st to end	Day	29% 15% 34% 18% 4	227
January	1st to 10th	Night	21% 12% 37% 24% 6% 1	1000
	11th to 20th	Night	22% 13% 35% 24% 6% 1	1000
	21st to end	Night	18% 7% 39% 27% 8%	980
February	1st to 10th	Night	19% 9% 39% 27% 5% 1	800
	11th to 20th	Night	21% 10% 36% 27% 6%	783
	21st to end	Night	20% 10% 38% 27% 5%	660
March	1st to 10th	Night	16% 8% 36% 29% 10%	800
	11th to 20th	Night	16% 9% 37% 29% 8% 1	800
	21st to end	Night	21% 9% 29% 29% 11% 1	880
April	1st to 10th	Night	21% 9% 32% 29% 9%	690
	11th to 20th	Night	21% 6% 29% 29% 12% 3	593
	21st to end	Night	21% 6% 24% 35% 13% 1	600
May	1st to 10th	Night	15% 4 22% 42% 15% 3	600
	11th to 20th	Night	17% 4 20% 34% 22% 2	591
	21st to end	Night	15% 3 17% 35% 28% 2	540
June	1st to 10th	Night	14% 3 20% 40% 23% 2	400
	11th to 20th	Night	11% 3 19% 41% 25% 2	400
	21st to end	Night	11% 3 18% 41% 24% 4	400
July	1st to 10th	Night	7% 2 12% 45% 30% 3	485
	11th to 20th	Night	9% 2 15% 38% 28% 7%	582
	21st to end	Night	7% 3 13% 45% 28% 4	657
August	1st to 10th	Night	10% 1 12% 38% 34% 6%	597
	11th to 20th	Night	11% 2 14% 31% 35% 7%	594
	21st to end	Night	12% 3 17% 33% 30% 5%	660
September	1st to 10th	Night	12% 5% 20% 32% 23% 8%	715
	11th to 20th	Night	13% 2 16% 29% 30% 10%	800
	21st to end	Night	16% 7% 21% 27% 21% 8%	800
October	1st to 10th	Night	18% 6% 28% 27% 18% 3	773
	11th to 20th	Night	28% 12% 25% 25% 9%	760
	21st to end	Night	30% 11% 27% 27% 5% 1	836
November	1st to 10th	Night	23% 12% 28% 25% 11% 1	780
	11th to 20th	Night	24% 13% 35% 21% 8%	800
	21st to end	Night	30% 13% 32% 20% 4	800
December	1st to 10th	Night	27% 15% 35% 17% 6%	810
	11th to 20th	Night	23% 15% 37% 20% 4 1	1000
	21st to end	Night	28% 12% 36% 20% 3	1100

Page generated at 2025-10-01 01:39:04. Version ac07

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