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 2024-11-12.

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	15% 6% 2 6% 4 4 5% 4 6% 8% 40%	101325
	Night	15% 7% 2 6% 4 4 4 4 5% 7% 42%	97805
Transparency	Day	40% 6% 12% 24% 16% 3	104963
	Night	40% 6% 11% 24% 16% 3	102307
Seeing	Day	18% 14% 28% 23% 13% 4	14980
	Night	17% 12% 31% 27% 11% 1	24421

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	16% 5% 2 5% 4 3 4 4 5% 5% 45%	2517
	11th to 20th	Day	13% 11% 3 9% 7% 5% 4 3 4 5% 37%	2840
	21st to end	Day	18% 10% 3 7% 5% 5% 4 4 4 5% 35%	3124
February	1st to 10th	Day	14% 8% 3 7% 5% 4 5% 5% 6% 6% 38%	1710
	11th to 20th	Day	17% 8% 2 5% 5% 5% 4 4 5% 6% 40%	1720
	21st to end	Day	18% 10% 3 7% 5% 4 3 2 4 5% 39%	1570
March	1st to 10th	Day	22% 7% 2 6% 4 4 4 3 5% 7% 36%	3338
	11th to 20th	Day	10% 6% 2 5% 4 4 5% 4 6% 7% 47%	3480
	21st to end	Day	20% 7% 2 6% 4 4 3 3 5% 6% 41%	3902
April	1st to 10th	Day	13% 8% 2 6% 5% 4 4 4 6% 8% 39%	2280
	11th to 20th	Day	13% 6% 2 6% 5% 4 4 3 5% 6% 46%	2254
	21st to end	Day	18% 6% 2 6% 4 4 4 3 5% 7% 41%	2280
May	1st to 10th	Day	26% 7% 3 6% 4 4 5% 3 5% 8% 30%	4012
	11th to 20th	Day	24% 6% 2 7% 6% 5% 5% 4 5% 8% 28%	4356
	21st to end	Day	10% 5% 2 6% 4 4 5% 4 7% 9% 45%	4823
June	1st to 10th	Day	12% 5% 2 5% 5% 5% 5% 4 7% 11% 41%	2660
	11th to 20th	Day	12% 6% 2 5% 5% 5% 5% 4 6% 9% 40%	2660
	21st to end	Day	10% 4 1 5% 5% 4 5% 4 7% 10% 45%	2660
July	1st to 10th	Day	6% 4 2 6% 5% 6% 6% 5% 7% 10% 42%	4440
	11th to 20th	Day	9% 6% 2 5% 5% 5% 5% 5% 7% 11% 40%	4391
	21st to end	Day	11% 7% 2 8% 5% 5% 6% 5% 7% 8% 35%	4466
August	1st to 10th	Day	18% 5% 2 6% 4 5% 6% 5% 7% 8% 33%	4104
	11th to 20th	Day	11% 4 2 6% 5% 6% 6% 5% 6% 8% 41%	3864
	21st to end	Day	12% 5% 2 5% 4 3 4 4 5% 8% 47%	4180
September	1st to 10th	Day	15% 5% 2 5% 4 4 4 3 5% 7% 46%	2280
	11th to 20th	Day	12% 6% 2 5% 4 4 4 3 6% 8% 48%	2280
	21st to end	Day	21% 7% 2 6% 4 3 5% 4 5% 6% 38%	2006
October	1st to 10th	Day	17% 6% 2 5% 4 3 4 3 5% 6% 45%	1900
	11th to 20th	Day	16% 5% 2 5% 4 4 4 3 4 5% 49%	1900
	21st to end	Day	13% 6% 3 6% 4 3 3 3 5% 5% 48%	2090
November	1st to 10th	Day	12% 6% 2 4 3 3 3 4 5% 7% 50%	1820
	11th to 20th	Day	14% 6% 2 5% 3 4 4 3 6% 5% 48%	1696
	21st to end	Day	15% 7% 2 6% 3 5% 4 4 4 6% 45%	1520
December	1st to 10th	Day	17% 7% 2 5% 5% 4 5% 4 6% 8% 37%	1520
	11th to 20th	Day	16% 7% 2 7% 5% 3 4 3 6% 7% 41%	1910
	21st to end	Day	17% 7% 3 7% 4 4 5% 4 5% 9% 35%	2772
January	1st to 10th	Night	14% 8% 2 7% 4 4 4 3 6% 7% 40%	4425
	11th to 20th	Night	19% 8% 2 5% 3 3 4 3 4 5% 44%	4120
	21st to end	Night	21% 10% 2 5% 4 3 4 3 4 5% 38%	4532
February	1st to 10th	Night	15% 8% 3 6% 5% 3 4 3 6% 7% 40%	2470
	11th to 20th	Night	17% 8% 2 7% 3 3 3 3 5% 6% 42%	2345
	21st to end	Night	16% 9% 3 7% 5% 4 3 3 4 6% 40%	1884
March	1st to 10th	Night	20% 8% 3 6% 4 4 4 4 4 5% 38%	3512
	11th to 20th	Night	11% 5% 2 5% 4 4 4 3 5% 7% 49%	3480
	21st to end	Night	14% 10% 2 5% 4 3 3 2 3 5% 47%	3729
April	1st to 10th	Night	15% 8% 2 5% 5% 5% 4 3 6% 7% 39%	1900
	11th to 20th	Night	14% 5% 2 6% 4 4 4 4 5% 7% 45%	1875
	21st to end	Night	17% 6% 2 5% 4 3 4 4 6% 7% 42%	1900
May	1st to 10th	Night	25% 9% 2 6% 4 4 4 3 5% 6% 32%	2910
	11th to 20th	Night	23% 8% 3 8% 5% 3 4 3 5% 7% 29%	2535
	21st to end	Night	11% 5% 2 5% 4 4 5% 4 8% 9% 42%	2732
June	1st to 10th	Night	12% 4 2 6% 4 4 4 5% 6% 9% 45%	1520
	11th to 20th	Night	11% 5% 2 6% 4 5% 6% 4 7% 9% 40%	1520
	21st to end	Night	9% 6% 1 5% 4 5% 4 4 7% 11% 44%	1520
July	1st to 10th	Night	6% 5% 3 8% 5% 6% 6% 5% 7% 9% 40%	2520
	11th to 20th	Night	8% 6% 3 6% 6% 5% 5% 5% 8% 9% 39%	2552
	21st to end	Night	12% 7% 3 6% 4 5% 5% 5% 5% 8% 40%	3087
August	1st to 10th	Night	19% 6% 2 6% 4 4 5% 4 6% 10% 34%	2825
	11th to 20th	Night	10% 7% 3 7% 5% 5% 5% 5% 6% 8% 38%	2930
	21st to end	Night	12% 5% 2 5% 4 4 4 4 6% 9% 47%	3476
September	1st to 10th	Night	13% 6% 2 6% 3 3 4 4 5% 7% 47%	1900
	11th to 20th	Night	10% 6% 2 6% 4 3 3 3 3 8% 53%	1900
	21st to end	Night	20% 7% 2 5% 4 4 4 3 5% 6% 39%	2170
October	1st to 10th	Night	19% 6% 2 5% 4 4 4 3 4 5% 44%	2280
	11th to 20th	Night	14% 5% 2 4 3 3 3 3 5% 6% 52%	2280
	21st to end	Night	15% 6% 2 4 3 3 4 3 5% 7% 49%	2508
November	1st to 10th	Night	11% 5% 2 5% 3 3 3 3 5% 6% 54%	2476
	11th to 20th	Night	14% 7% 3 5% 4 3 4 2 4 6% 48%	2503
	21st to end	Night	13% 5% 2 5% 5% 4 4 4 5% 7% 46%	2660
December	1st to 10th	Night	17% 7% 2 6% 5% 4 4 4 5% 7% 39%	2660
	11th to 20th	Night	17% 5% 2 6% 4 4 4 4 5% 7% 41%	3285
	21st to end	Night	16% 7% 2 6% 4 5% 6% 5% 7% 8% 35%	4884

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 10% 30% 13% 4	2518
	11th to 20th	Day	50% 4 9% 21% 13% 4	2840
	21st to end	Day	51% 5% 10% 22% 10% 2	3124
February	1st to 10th	Day	45% 5% 12% 26% 11% 2	1710
	11th to 20th	Day	45% 4 10% 25% 13% 3	1729
	21st to end	Day	48% 4 11% 21% 13% 4	1570
March	1st to 10th	Day	47% 5% 10% 24% 12% 2	3338
	11th to 20th	Day	35% 5% 10% 28% 19% 4	3480
	21st to end	Day	46% 4 10% 20% 17% 3	3902
April	1st to 10th	Day	42% 6% 11% 19% 17% 4	2280
	11th to 20th	Day	39% 5% 10% 23% 18% 5%	2254
	21st to end	Day	43% 5% 9% 21% 18% 3	2280
May	1st to 10th	Day	52% 7% 9% 21% 10% 1	4049
	11th to 20th	Day	52% 8% 13% 18% 8% 1	4360
	21st to end	Day	34% 8% 13% 26% 17% 2	4837
June	1st to 10th	Day	37% 8% 14% 24% 15% 2	2660
	11th to 20th	Day	40% 8% 12% 24% 14% 2	2660
	21st to end	Day	33% 7% 15% 26% 17% 2	2660
July	1st to 10th	Day	35% 10% 16% 21% 13% 5%	4370
	11th to 20th	Day	36% 8% 16% 28% 11% 2	4316
	21st to end	Day	43% 8% 11% 19% 17% 1	4324
August	1st to 10th	Day	45% 9% 12% 25% 8%	4091
	11th to 20th	Day	37% 8% 12% 28% 14% 1	3903
	21st to end	Day	34% 6% 12% 27% 18% 2	4180
September	1st to 10th	Day	38% 6% 12% 20% 21% 4	2280
	11th to 20th	Day	35% 5% 9% 26% 22% 4	2280
	21st to end	Day	46% 5% 9% 19% 19% 2	2006
October	1st to 10th	Day	38% 5% 7% 19% 27% 5%	3160
	11th to 20th	Day	28% 4 10% 26% 22% 10%	3140
	21st to end	Day	24% 3 11% 29% 29% 4	3428
November	1st to 10th	Day	33% 4 12% 26% 21% 4	1820
	11th to 20th	Day	36% 5% 11% 23% 22% 3	1696
	21st to end	Day	41% 4 11% 29% 14% 2	1520
December	1st to 10th	Day	45% 5% 14% 23% 11% 2	1520
	11th to 20th	Day	42% 5% 11% 30% 11% 2	1906
	21st to end	Day	45% 6% 15% 21% 12% 1	2772
January	1st to 10th	Night	42% 5% 10% 26% 14% 3	4425
	11th to 20th	Night	42% 5% 11% 26% 15% 2	4120
	21st to end	Night	48% 4 7% 27% 13% 2	4532
February	1st to 10th	Night	42% 5% 14% 24% 13% 2	2470
	11th to 20th	Night	44% 4 9% 21% 18% 4	2347
	21st to end	Night	46% 3 10% 22% 14% 5%	1884
March	1st to 10th	Night	47% 5% 9% 27% 11% 2	3512
	11th to 20th	Night	34% 4 9% 33% 16% 3	3480
	21st to end	Night	41% 4 8% 22% 20% 5%	3729
April	1st to 10th	Night	43% 5% 10% 19% 19% 4	1900
	11th to 20th	Night	38% 6% 11% 20% 19% 5%	1875
	21st to end	Night	40% 6% 10% 23% 16% 5%	1900
May	1st to 10th	Night	52% 6% 9% 19% 11% 2	2939
	11th to 20th	Night	53% 6% 11% 17% 10% 1	2536
	21st to end	Night	34% 9% 17% 25% 12% 2	2739
June	1st to 10th	Night	35% 7% 11% 27% 17% 3	1520
	11th to 20th	Night	38% 8% 13% 24% 14% 2	1520
	21st to end	Night	34% 7% 14% 25% 17% 2	1520
July	1st to 10th	Night	38% 10% 18% 18% 12% 4	2480
	11th to 20th	Night	37% 10% 16% 24% 12% 1	2504
	21st to end	Night	40% 7% 14% 20% 17% 1	2988
August	1st to 10th	Night	45% 8% 17% 21% 8% 1	2814
	11th to 20th	Night	41% 7% 10% 27% 14% 1	2957
	21st to end	Night	33% 7% 12% 30% 16% 2	3476
September	1st to 10th	Night	36% 6% 12% 20% 21% 5%	1900
	11th to 20th	Night	33% 4 9% 27% 24% 3	1900
	21st to end	Night	45% 6% 9% 19% 18% 3	2170
October	1st to 10th	Night	41% 6% 10% 19% 20% 3	3800
	11th to 20th	Night	24% 3 11% 25% 29% 7%	3779
	21st to end	Night	24% 4 11% 32% 24% 6%	4120
November	1st to 10th	Night	32% 4 10% 26% 23% 5%	2476
	11th to 20th	Night	39% 4 8% 24% 20% 5%	2503
	21st to end	Night	37% 4 11% 26% 18% 2	2660
December	1st to 10th	Night	45% 5% 12% 24% 13% 1	2660
	11th to 20th	Night	42% 4 11% 22% 18% 3	3288
	21st to end	Night	43% 8% 15% 22% 12% 1	4884

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	25% 28% 31% 12% 4	180
	11th to 20th	Day	27% 30% 31% 11% 1	270
	21st to end	Day	21% 32% 31% 13% 2 1	297
February	1st to 10th	Day	22% 27% 37% 11% 2	270
	11th to 20th	Day	30% 26% 37% 6% 1	327
	21st to end	Day	31% 21% 30% 15% 2	382
March	1st to 10th	Day	20% 27% 30% 19% 3	460
	11th to 20th	Day	26% 21% 33% 15% 4 2	460
	21st to end	Day	22% 21% 34% 17% 5% 1	506
April	1st to 10th	Day	21% 20% 34% 17% 5% 2	460
	11th to 20th	Day	17% 20% 33% 16% 12% 2	452
	21st to end	Day	23% 16% 34% 17% 8% 2	460
May	1st to 10th	Day	26% 14% 30% 18% 9% 3	463
	11th to 20th	Day	22% 14% 27% 22% 11% 3	487
	21st to end	Day	15% 7% 25% 26% 22% 5%	529
June	1st to 10th	Day	13% 11% 29% 25% 16% 5%	490
	11th to 20th	Day	17% 8% 28% 26% 17% 3	490
	21st to end	Day	11% 7% 28% 34% 16% 4	490
July	1st to 10th	Day	11% 5% 22% 31% 20% 12%	485
	11th to 20th	Day	6% 3 22% 34% 26% 9%	484
	21st to end	Day	12% 3 21% 35% 23% 5%	532
August	1st to 10th	Day	12% 5% 19% 30% 25% 9%	486
	11th to 20th	Day	12% 4 21% 33% 22% 9%	479
	21st to end	Day	9% 5% 22% 29% 22% 12%	539
September	1st to 10th	Day	16% 8% 20% 23% 23% 11%	490
	11th to 20th	Day	11% 4 23% 30% 22% 10%	490
	21st to end	Day	25% 12% 22% 24% 14% 3	490
October	1st to 10th	Day	21% 9% 23% 23% 17% 7%	490
	11th to 20th	Day	17% 16% 24% 27% 13% 3	490
	21st to end	Day	16% 13% 36% 24% 9% 2	539
November	1st to 10th	Day	16% 14% 32% 26% 11% 2	425
	11th to 20th	Day	18% 17% 34% 21% 9% 1	350
	21st to end	Day	21% 20% 34% 19% 6%	180
December	1st to 10th	Day	23% 26% 33% 13% 3 2	180
	11th to 20th	Day	27% 23% 28% 18% 4	180
	21st to end	Day	23% 21% 32% 19% 5%	198
January	1st to 10th	Night	19% 20% 41% 17% 3	760
	11th to 20th	Night	23% 19% 32% 22% 3	760
	21st to end	Night	19% 21% 41% 18% 1	836
February	1st to 10th	Night	19% 19% 37% 23% 2	760
	11th to 20th	Night	24% 24% 37% 14% 1	743
	21st to end	Night	24% 21% 39% 14% 3	628
March	1st to 10th	Night	20% 19% 40% 19% 3	760
	11th to 20th	Night	22% 19% 34% 20% 5%	760
	21st to end	Night	18% 17% 42% 17% 5%	836
April	1st to 10th	Night	21% 20% 36% 20% 4	760
	11th to 20th	Night	16% 16% 36% 25% 7% 1	751
	21st to end	Night	20% 13% 34% 25% 7% 1	760
May	1st to 10th	Night	20% 10% 33% 27% 10%	630
	11th to 20th	Night	22% 7% 29% 32% 10% 1	401
	21st to end	Night	15% 5% 22% 38% 18% 1	411
June	1st to 10th	Night	12% 2 31% 34% 22% 1	380
	11th to 20th	Night	12% 4 25% 40% 18% 1	380
	21st to end	Night	10% 2 24% 47% 16% 1	380
July	1st to 10th	Night	9% 3 18% 40% 26% 4	370
	11th to 20th	Night	7% 2 18% 41% 30% 2	387
	21st to end	Night	9% 2 18% 42% 26% 4	624
August	1st to 10th	Night	12% 3 13% 43% 28% 2	567
	11th to 20th	Night	11% 4 18% 39% 26% 3	644
	21st to end	Night	12% 3 17% 38% 26% 4	836
September	1st to 10th	Night	15% 3 20% 33% 25% 3	760
	11th to 20th	Night	12% 3 17% 36% 28% 4	760
	21st to end	Night	22% 6% 21% 32% 17% 2	760
October	1st to 10th	Night	20% 8% 21% 29% 18% 4	760
	11th to 20th	Night	16% 11% 29% 30% 13% 1	760
	21st to end	Night	18% 11% 32% 28% 11%	836
November	1st to 10th	Night	13% 11% 34% 31% 9% 2	780
	11th to 20th	Night	19% 16% 32% 26% 6% 1	765
	21st to end	Night	16% 17% 36% 23% 7% 1	760
December	1st to 10th	Night	23% 14% 37% 19% 7% 1	760
	11th to 20th	Night	18% 15% 43% 20% 5%	760
	21st to end	Night	20% 16% 37% 25% 2	836

Page generated at 2024-11-08 22:45:39. Version ac07

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