Willow Oak Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Willow Oak Observatory clear sky chart from 2005-11-06 to 2026-01-02.

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	15% 6% 2 6% 4 4 5% 4 6% 8% 41%	127279
	Night	15% 7% 2 6% 4 4 4 4 5% 7% 43%	124083
Transparency	Day	40% 6% 12% 23% 16% 3	130873
	Night	39% 6% 11% 24% 17% 3	128559
Seeing	Day	19% 14% 28% 23% 12% 4	16391
	Night	18% 13% 31% 27% 11% 1	25912

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% 5% 2 4 4 3 4 4 6% 7% 48%	3605
	11th to 20th	Day	13% 9% 3 8% 6% 4 3 3 4 5% 42%	4060
	21st to end	Day	15% 9% 3 6% 4 4 4 4 5% 6% 41%	4466
February	1st to 10th	Day	14% 8% 3 7% 5% 4 5% 5% 6% 5% 38%	1800
	11th to 20th	Day	17% 8% 2 5% 5% 5% 4 3 5% 6% 39%	1813
	21st to end	Day	17% 10% 3 7% 4 4 3 2 4 5% 41%	1650
March	1st to 10th	Day	18% 6% 1 5% 4 4 4 3 5% 6% 44%	4796
	11th to 20th	Day	11% 6% 2 6% 4 4 4 4 5% 7% 47%	4980
	21st to end	Day	16% 6% 2 6% 5% 5% 5% 4 6% 7% 40%	5594
April	1st to 10th	Day	14% 8% 2 6% 5% 4 4 4 6% 8% 38%	2400
	11th to 20th	Day	13% 6% 2 6% 5% 4 4 3 5% 6% 45%	2374
	21st to end	Day	17% 6% 2 6% 4 4 4 3 5% 7% 42%	2400
May	1st to 10th	Day	23% 7% 2 6% 4 4 5% 4 7% 9% 30%	5764
	11th to 20th	Day	24% 6% 2 7% 5% 4 5% 4 6% 9% 30%	6262
	21st to end	Day	16% 5% 2 6% 4 4 5% 5% 7% 8% 38%	6934
June	1st to 10th	Day	12% 5% 2 5% 5% 5% 5% 4 7% 10% 41%	2800
	11th to 20th	Day	13% 6% 2 5% 5% 5% 5% 4 5% 9% 40%	2800
	21st to end	Day	10% 4 1 5% 4 4 5% 4 7% 10% 45%	2800
July	1st to 10th	Day	7% 5% 2 6% 5% 5% 6% 5% 7% 9% 43%	6360
	11th to 20th	Day	10% 7% 2 7% 5% 6% 6% 5% 8% 11% 32%	6297
	21st to end	Day	9% 6% 2 7% 4 5% 5% 5% 7% 10% 40%	6424
August	1st to 10th	Day	18% 5% 2 6% 4 5% 6% 5% 7% 8% 34%	4234
	11th to 20th	Day	12% 4 3 6% 5% 6% 6% 5% 7% 8% 40%	3988
	21st to end	Day	12% 5% 2 5% 4 3 4 4 5% 8% 48%	4312
September	1st to 10th	Day	14% 6% 2 5% 3 4 4 3 5% 7% 47%	2400
	11th to 20th	Day	12% 5% 2 5% 4 4 4 3 6% 8% 47%	2400
	21st to end	Day	21% 8% 2 6% 4 3 5% 4 5% 6% 37%	2111
October	1st to 10th	Day	16% 5% 1 5% 4 3 4 3 5% 7% 47%	2000
	11th to 20th	Day	16% 5% 2 5% 4 4 4 3 4 5% 48%	2000
	21st to end	Day	13% 6% 3 6% 4 3 3 3 5% 5% 48%	2200
November	1st to 10th	Day	12% 5% 2 4 3 3 3 3 5% 8% 51%	1913
	11th to 20th	Day	13% 6% 2 5% 3 4 4 3 6% 5% 48%	1864
	21st to end	Day	15% 7% 2 6% 3 5% 4 4 5% 6% 45%	1680
December	1st to 10th	Day	13% 6% 2 6% 5% 4 4 3 6% 8% 43%	2680
	11th to 20th	Day	18% 9% 3 6% 4 3 4 3 4 6% 41%	3070
	21st to end	Day	17% 7% 3 7% 4 4 4 4 5% 8% 36%	4048
January	1st to 10th	Night	12% 7% 2 6% 4 4 4 3 5% 6% 47%	6346
	11th to 20th	Night	14% 6% 2 6% 3 3 3 3 4 6% 49%	5900
	21st to end	Night	16% 8% 2 5% 4 3 4 3 4 5% 46%	6490
February	1st to 10th	Night	15% 8% 3 6% 5% 3 4 3 6% 7% 40%	2600
	11th to 20th	Night	18% 8% 2 7% 3 3 3 4 5% 6% 41%	2471
	21st to end	Night	15% 8% 3 7% 5% 4 3 3 4 6% 41%	1980
March	1st to 10th	Night	16% 7% 2 6% 3 3 3 3 4 5% 47%	5054
	11th to 20th	Night	10% 5% 2 5% 3 4 4 3 5% 7% 50%	4980
	21st to end	Night	12% 8% 2 5% 4 3 4 3 5% 6% 45%	5337
April	1st to 10th	Night	16% 8% 2 5% 6% 5% 4 3 6% 7% 38%	2000
	11th to 20th	Night	14% 6% 2 6% 4 4 4 4 5% 7% 44%	1975
	21st to end	Night	16% 6% 2 5% 4 3 4 4 6% 7% 43%	2000
May	1st to 10th	Night	24% 9% 2 7% 4 4 5% 4 6% 7% 30%	4172
	11th to 20th	Night	23% 8% 3 7% 4 3 4 3 6% 8% 31%	3629
	21st to end	Night	19% 6% 2 6% 4 4 4 4 7% 8% 36%	3920
June	1st to 10th	Night	12% 5% 2 6% 4 4 4 5% 5% 9% 45%	1600
	11th to 20th	Night	12% 5% 2 6% 4 5% 6% 5% 6% 8% 40%	1600
	21st to end	Night	10% 6% 1 5% 5% 5% 4 4 7% 10% 44%	1600
July	1st to 10th	Night	8% 5% 3 7% 6% 5% 6% 5% 7% 9% 40%	3598
	11th to 20th	Night	9% 7% 4 9% 6% 5% 7% 5% 8% 9% 31%	3643
	21st to end	Night	9% 6% 2 6% 4 5% 5% 4 6% 9% 44%	4427
August	1st to 10th	Night	19% 6% 2 6% 4 4 5% 4 6% 10% 34%	2915
	11th to 20th	Night	11% 7% 3 7% 5% 5% 5% 4 6% 8% 38%	3025
	21st to end	Night	11% 5% 2 5% 4 4 4 4 6% 9% 47%	3586
September	1st to 10th	Night	13% 6% 2 6% 3 3 4 4 5% 7% 47%	2000
	11th to 20th	Night	11% 6% 2 6% 4 3 3 3 4 8% 52%	2000
	21st to end	Night	20% 7% 2 6% 5% 4 4 3 5% 6% 38%	2285
October	1st to 10th	Night	19% 6% 2 5% 3 4 4 3 4 5% 46%	2400
	11th to 20th	Night	15% 6% 2 4 3 3 3 2 5% 5% 52%	2400
	21st to end	Night	14% 6% 2 4 3 3 4 3 5% 7% 49%	2640
November	1st to 10th	Night	11% 5% 2 5% 3 3 3 3 5% 6% 53%	2603
	11th to 20th	Night	14% 7% 3 5% 4 3 4 3 4 6% 48%	2752
	21st to end	Night	13% 5% 2 5% 5% 4 4 4 5% 7% 46%	2940
December	1st to 10th	Night	11% 6% 2 5% 4 4 4 4 5% 7% 49%	4720
	11th to 20th	Night	21% 6% 2 5% 4 4 4 4 5% 6% 39%	5345
	21st to end	Night	17% 8% 2 6% 4 4 5% 4 6% 8% 36%	7150

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	35% 4 8% 28% 21% 5%	3606
	11th to 20th	Day	45% 3 8% 21% 17% 6%	4060
	21st to end	Day	43% 4 11% 25% 14% 3	4466
February	1st to 10th	Day	45% 5% 12% 26% 11% 2	1800
	11th to 20th	Day	46% 3 10% 25% 13% 3	1822
	21st to end	Day	46% 4 11% 22% 14% 4	1650
March	1st to 10th	Day	40% 5% 9% 24% 16% 7%	4796
	11th to 20th	Day	35% 5% 10% 26% 18% 6%	4980
	21st to end	Day	43% 5% 10% 19% 19% 5%	5594
April	1st to 10th	Day	43% 6% 11% 19% 16% 5%	2400
	11th to 20th	Day	40% 5% 10% 22% 18% 5%	2374
	21st to end	Day	43% 5% 8% 22% 19% 3	2400
May	1st to 10th	Day	48% 8% 14% 19% 9% 2	5798
	11th to 20th	Day	50% 7% 11% 22% 9% 1	6266
	21st to end	Day	41% 8% 12% 22% 15% 2	6949
June	1st to 10th	Day	37% 8% 14% 24% 15% 2	2800
	11th to 20th	Day	41% 7% 12% 23% 14% 2	2800
	21st to end	Day	33% 7% 15% 25% 18% 1	2800
July	1st to 10th	Day	35% 11% 15% 20% 16% 3	6290
	11th to 20th	Day	41% 11% 17% 22% 8% 1	6222
	21st to end	Day	37% 8% 11% 22% 21% 1	6240
August	1st to 10th	Day	45% 9% 12% 25% 9%	4221
	11th to 20th	Day	38% 8% 12% 27% 14% 1	4027
	21st to end	Day	34% 6% 12% 28% 18% 2	4312
September	1st to 10th	Day	38% 6% 12% 21% 21% 4	2400
	11th to 20th	Day	35% 5% 9% 27% 21% 3	2400
	21st to end	Day	48% 5% 9% 18% 18% 2	2111
October	1st to 10th	Day	37% 5% 7% 19% 27% 5%	3260
	11th to 20th	Day	28% 4 10% 27% 22% 10%	3240
	21st to end	Day	24% 3 11% 29% 29% 4	3538
November	1st to 10th	Day	32% 4 13% 26% 21% 4	1913
	11th to 20th	Day	36% 4 10% 23% 22% 3	1864
	21st to end	Day	41% 4 11% 29% 14% 2	1680
December	1st to 10th	Day	38% 5% 16% 27% 10% 3	2680
	11th to 20th	Day	46% 4 11% 27% 10% 2	3066
	21st to end	Day	44% 6% 15% 19% 14% 1	4048
January	1st to 10th	Night	36% 4 9% 25% 22% 4	6346
	11th to 20th	Night	36% 4 9% 25% 19% 6%	5900
	21st to end	Night	41% 3 9% 27% 18% 2	6490
February	1st to 10th	Night	42% 5% 14% 24% 13% 2	2600
	11th to 20th	Night	44% 4 9% 21% 17% 4	2473
	21st to end	Night	46% 4 10% 22% 15% 5%	1980
March	1st to 10th	Night	40% 4 8% 27% 12% 9%	5054
	11th to 20th	Night	34% 4 11% 30% 15% 6%	4980
	21st to end	Night	39% 4 9% 20% 22% 4	5337
April	1st to 10th	Night	45% 5% 10% 19% 18% 4	2000
	11th to 20th	Night	39% 6% 11% 20% 19% 5%	1975
	21st to end	Night	39% 6% 10% 23% 17% 4	2000
May	1st to 10th	Night	53% 7% 10% 16% 11% 3	4201
	11th to 20th	Night	50% 7% 11% 22% 10% 1	3630
	21st to end	Night	44% 8% 13% 23% 10% 2	3927
June	1st to 10th	Night	36% 7% 11% 26% 18% 3	1600
	11th to 20th	Night	39% 8% 13% 24% 14% 2	1600
	21st to end	Night	35% 7% 13% 25% 18% 2	1600
July	1st to 10th	Night	39% 10% 16% 18% 14% 3	3560
	11th to 20th	Night	45% 12% 15% 18% 9% 1	3598
	21st to end	Night	35% 7% 13% 23% 20% 1	4297
August	1st to 10th	Night	44% 8% 17% 21% 9% 1	2904
	11th to 20th	Night	41% 7% 10% 26% 14% 1	3052
	21st to end	Night	32% 7% 12% 30% 17% 2	3586
September	1st to 10th	Night	36% 6% 12% 21% 21% 5%	2000
	11th to 20th	Night	34% 4 9% 27% 23% 3	2000
	21st to end	Night	46% 7% 9% 18% 18% 3	2285
October	1st to 10th	Night	41% 6% 10% 19% 21% 3	3920
	11th to 20th	Night	25% 3 11% 25% 29% 7%	3899
	21st to end	Night	25% 4 10% 32% 24% 5%	4252
November	1st to 10th	Night	32% 5% 10% 26% 23% 5%	2603
	11th to 20th	Night	39% 4 8% 24% 20% 5%	2752
	21st to end	Night	37% 4 11% 26% 19% 2	2940
December	1st to 10th	Night	34% 5% 15% 30% 14% 2	4720
	11th to 20th	Night	45% 4 12% 19% 17% 3	5348
	21st to end	Night	44% 7% 13% 19% 17% 1	7150

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	28% 29% 29% 11% 3	202
	11th to 20th	Day	29% 28% 32% 10% 1	300
	21st to end	Day	21% 32% 31% 12% 2 1	330
February	1st to 10th	Day	25% 28% 35% 10% 2	300
	11th to 20th	Day	34% 25% 34% 6% 1	360
	21st to end	Day	30% 22% 31% 15% 2	414
March	1st to 10th	Day	21% 27% 30% 18% 3	500
	11th to 20th	Day	26% 21% 33% 15% 4 1	500
	21st to end	Day	23% 21% 35% 16% 5% 1	550
April	1st to 10th	Day	23% 21% 33% 16% 5% 2	500
	11th to 20th	Day	19% 21% 32% 15% 11% 2	492
	21st to end	Day	23% 16% 34% 17% 8% 2	500
May	1st to 10th	Day	25% 16% 30% 18% 8% 3	503
	11th to 20th	Day	23% 15% 27% 22% 10% 3	527
	21st to end	Day	17% 8% 25% 25% 21% 4	573
June	1st to 10th	Day	14% 12% 29% 25% 15% 5%	530
	11th to 20th	Day	18% 8% 28% 26% 16% 3	530
	21st to end	Day	11% 7% 26% 35% 16% 5%	530
July	1st to 10th	Day	11% 4 22% 32% 20% 11%	525
	11th to 20th	Day	6% 3 22% 33% 26% 9%	524
	21st to end	Day	12% 3 21% 35% 23% 5%	576
August	1st to 10th	Day	13% 5% 20% 30% 23% 9%	526
	11th to 20th	Day	13% 4 20% 34% 21% 8%	519
	21st to end	Day	10% 5% 23% 28% 22% 12%	583
September	1st to 10th	Day	15% 8% 21% 24% 22% 10%	530
	11th to 20th	Day	12% 4 22% 30% 23% 9%	530
	21st to end	Day	25% 12% 22% 24% 13% 3	530
October	1st to 10th	Day	20% 9% 23% 24% 17% 7%	530
	11th to 20th	Day	18% 17% 25% 25% 12% 2	530
	21st to end	Day	16% 15% 36% 23% 8% 2	583
November	1st to 10th	Day	17% 16% 32% 24% 10% 2	458
	11th to 20th	Day	19% 18% 35% 20% 8%	404
	21st to end	Day	25% 19% 35% 16% 5%	220
December	1st to 10th	Day	23% 26% 35% 12% 3 1	220
	11th to 20th	Day	27% 24% 29% 17% 3	220
	21st to end	Day	25% 21% 32% 18% 4	242
January	1st to 10th	Night	20% 21% 40% 16% 2	805
	11th to 20th	Night	23% 20% 33% 21% 3	800
	21st to end	Night	19% 20% 42% 18% 1	880
February	1st to 10th	Night	19% 20% 37% 22% 2	800
	11th to 20th	Night	26% 24% 36% 13% 1	783
	21st to end	Night	23% 20% 39% 15% 3	660
March	1st to 10th	Night	20% 19% 39% 19% 3	800
	11th to 20th	Night	21% 19% 34% 20% 5%	800
	21st to end	Night	18% 18% 42% 17% 4	880
April	1st to 10th	Night	21% 20% 36% 19% 4	800
	11th to 20th	Night	17% 17% 35% 24% 7% 1	791
	21st to end	Night	20% 13% 34% 26% 7% 1	800
May	1st to 10th	Night	20% 10% 33% 27% 10%	663
	11th to 20th	Night	22% 8% 28% 32% 10% 1	422
	21st to end	Night	16% 6% 23% 37% 18% 1	433
June	1st to 10th	Night	12% 3 30% 35% 21% 1	400
	11th to 20th	Night	13% 5% 25% 39% 18% 1	400
	21st to end	Night	11% 3 24% 46% 17% 1	400
July	1st to 10th	Night	9% 3 18% 40% 26% 4	390
	11th to 20th	Night	8% 2 18% 41% 30% 2	408
	21st to end	Night	9% 2 18% 42% 26% 3	657
August	1st to 10th	Night	12% 3 13% 43% 27% 2	597
	11th to 20th	Night	11% 4 17% 39% 26% 3	679
	21st to end	Night	11% 3 18% 38% 26% 4	880
September	1st to 10th	Night	15% 3 21% 33% 25% 3	800
	11th to 20th	Night	12% 4 17% 36% 28% 4	800
	21st to end	Night	22% 6% 21% 33% 17% 2	800
October	1st to 10th	Night	20% 8% 21% 29% 19% 4	800
	11th to 20th	Night	17% 11% 29% 30% 13% 1	800
	21st to end	Night	18% 12% 32% 27% 11%	880
November	1st to 10th	Night	14% 11% 33% 31% 9% 1	820
	11th to 20th	Night	19% 17% 33% 25% 5% 1	840
	21st to end	Night	16% 18% 36% 23% 6%	840
December	1st to 10th	Night	22% 14% 38% 19% 6%	840
	11th to 20th	Night	19% 15% 41% 21% 4	840
	21st to end	Night	21% 16% 37% 24% 2	924

Page generated at 2025-12-30 01:14:10. Version ac07

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