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 2024-01-14.

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	14% 6% 3 5% 5% 3 5% 5% 6% 8% 41%	74697
	Night	14% 6% 2 5% 5% 3 4 5% 5% 7% 43%	75060
Transparency	Day	41% 6% 12% 23% 16% 3	74407
	Night	40% 6% 12% 23% 17% 3	74869
Seeing	Day	18% 14% 28% 23% 13% 4	13811
	Night	17% 12% 31% 27% 11% 1	23384

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	13% 7% 3 6% 5% 3 5% 5% 5% 6% 42%	1517
	11th to 20th	Day	15% 9% 3 6% 5% 3 4 5% 4 7% 40%	1647
	21st to end	Day	14% 8% 4 6% 5% 4 5% 5% 4 5% 39%	1782
February	1st to 10th	Day	14% 7% 3 7% 5% 3 5% 6% 6% 6% 37%	1620
	11th to 20th	Day	17% 7% 3 5% 5% 3 5% 4 5% 6% 39%	1627
	21st to end	Day	19% 9% 3 6% 5% 3 4 3 4 5% 39%	1480
March	1st to 10th	Day	12% 7% 2 5% 6% 3 4 3 4 7% 48%	1921
	11th to 20th	Day	16% 6% 3 6% 4 3 4 5% 5% 7% 40%	1980
	21st to end	Day	18% 6% 4 6% 6% 4 4 4 5% 6% 38%	2224
April	1st to 10th	Day	13% 7% 2 5% 6% 3 4 5% 6% 8% 40%	2160
	11th to 20th	Day	14% 5% 3 5% 5% 3 4 4 4 6% 46%	2134
	21st to end	Day	18% 5% 3 5% 5% 3 5% 4 5% 7% 41%	2160
May	1st to 10th	Day	16% 6% 3 5% 6% 3 5% 4 6% 8% 38%	2297
	11th to 20th	Day	20% 6% 2 5% 6% 3 5% 5% 5% 9% 35%	2450
	21st to end	Day	13% 6% 2 6% 5% 3 5% 5% 6% 8% 40%	2725
June	1st to 10th	Day	13% 4 3 4 5% 3 5% 5% 7% 11% 41%	2520
	11th to 20th	Day	13% 6% 2 5% 6% 3 6% 5% 6% 9% 40%	2520
	21st to end	Day	11% 3 1 4 5% 3 4 5% 7% 10% 45%	2520
July	1st to 10th	Day	10% 5% 3 5% 5% 4 6% 5% 7% 9% 43%	2520
	11th to 20th	Day	7% 3 2 4 5% 5% 6% 7% 7% 11% 44%	2499
	21st to end	Day	11% 5% 3 6% 6% 4 6% 7% 7% 9% 36%	2547
August	1st to 10th	Day	11% 5% 2 6% 6% 5% 6% 6% 8% 9% 37%	2327
	11th to 20th	Day	12% 5% 3 5% 6% 4 5% 6% 6% 7% 41%	2207
	21st to end	Day	11% 5% 2 6% 5% 3 5% 6% 6% 9% 42%	2376
September	1st to 10th	Day	15% 5% 3 5% 4 3 4 4 5% 7% 44%	2160
	11th to 20th	Day	12% 5% 2 4 4 3 4 4 6% 9% 47%	2160
	21st to end	Day	19% 7% 2 5% 5% 2 4 5% 5% 6% 40%	1901
October	1st to 10th	Day	17% 5% 2 5% 4 2 4 5% 5% 7% 45%	1800
	11th to 20th	Day	18% 5% 2 5% 4 3 3 5% 4 5% 47%	1800
	21st to end	Day	14% 6% 3 5% 5% 3 3 4 5% 5% 47%	1980
November	1st to 10th	Day	13% 5% 2 4 3 2 3 4 5% 8% 50%	1727
	11th to 20th	Day	14% 5% 3 4 3 3 4 4 6% 6% 48%	1687
	21st to end	Day	15% 6% 3 5% 4 3 4 5% 4 6% 45%	1520
December	1st to 10th	Day	17% 6% 3 4 5% 3 5% 5% 6% 8% 38%	1520
	11th to 20th	Day	16% 6% 2 6% 6% 3 3 4 5% 7% 41%	1910
	21st to end	Day	17% 7% 3 6% 5% 3 5% 4 5% 9% 35%	2772
January	1st to 10th	Night	14% 6% 3 5% 5% 3 4 4 5% 7% 43%	2645
	11th to 20th	Night	18% 6% 3 5% 4 3 4 5% 5% 7% 39%	2386
	21st to end	Night	15% 6% 2 5% 5% 3 5% 5% 5% 6% 44%	2574
February	1st to 10th	Night	16% 8% 3 5% 5% 2 5% 4 6% 8% 39%	2340
	11th to 20th	Night	17% 8% 3 7% 4 2 4 4 5% 6% 41%	2218
	21st to end	Night	16% 8% 4 5% 6% 3 4 3 4 6% 40%	1776
March	1st to 10th	Night	12% 5% 3 4 4 3 4 5% 5% 7% 47%	2036
	11th to 20th	Night	16% 7% 2 4 5% 3 5% 4 4 6% 43%	1980
	21st to end	Night	15% 7% 3 5% 6% 2 4 4 4 7% 42%	2131
April	1st to 10th	Night	15% 7% 3 5% 6% 4 5% 4 6% 8% 39%	1800
	11th to 20th	Night	14% 5% 2 6% 5% 2 4 4 5% 7% 46%	1775
	21st to end	Night	17% 6% 2 5% 4 2 3 5% 5% 8% 42%	1800
May	1st to 10th	Night	16% 8% 3 5% 4 3 5% 4 5% 6% 40%	1677
	11th to 20th	Night	18% 6% 3 5% 5% 2 5% 4 6% 9% 37%	1441
	21st to end	Night	13% 5% 2 5% 6% 2 5% 5% 7% 9% 41%	1550
June	1st to 10th	Night	12% 4 1 5% 5% 3 4 6% 6% 9% 46%	1440
	11th to 20th	Night	12% 5% 2 5% 5% 4 6% 6% 7% 8% 40%	1440
	21st to end	Night	9% 5% 2 4 5% 3 5% 6% 7% 11% 44%	1440
July	1st to 10th	Night	9% 5% 3 7% 6% 4 7% 6% 7% 9% 38%	1440
	11th to 20th	Night	7% 3 2 5% 5% 3 5% 6% 7% 10% 47%	1458
	21st to end	Night	8% 4 2 4 4 4 6% 6% 6% 12% 44%	1771
August	1st to 10th	Night	10% 5% 3 5% 5% 3 6% 7% 7% 9% 41%	1611
	11th to 20th	Night	11% 6% 2 6% 6% 4 5% 5% 6% 8% 40%	1673
	21st to end	Night	11% 5% 2 5% 6% 3 4 5% 7% 9% 43%	1980
September	1st to 10th	Night	14% 6% 2 5% 4 2 4 5% 5% 7% 46%	1800
	11th to 20th	Night	10% 5% 3 5% 4 2 4 3 3 8% 53%	1800
	21st to end	Night	17% 6% 2 5% 6% 3 4 4 5% 7% 41%	2055
October	1st to 10th	Night	20% 6% 2 4 4 3 4 4 4 5% 44%	2160
	11th to 20th	Night	15% 5% 2 4 3 2 3 3 5% 6% 51%	2160
	21st to end	Night	15% 6% 2 4 3 2 4 4 5% 7% 47%	2376
November	1st to 10th	Night	12% 5% 2 5% 4 2 3 4 5% 6% 53%	2349
	11th to 20th	Night	14% 6% 4 5% 4 2 4 3 3 6% 49%	2489
	21st to end	Night	13% 4 2 4 6% 3 5% 5% 5% 7% 46%	2660
December	1st to 10th	Night	17% 7% 3 6% 5% 3 4 5% 5% 6% 39%	2660
	11th to 20th	Night	17% 5% 2 5% 5% 3 5% 5% 5% 7% 42%	3285
	21st to end	Night	16% 7% 2 5% 5% 3 6% 6% 7% 9% 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	44% 4 11% 23% 15% 3	1518
	11th to 20th	Day	44% 4 13% 23% 12% 3	1647
	21st to end	Day	47% 3 11% 22% 13% 4	1782
February	1st to 10th	Day	46% 5% 12% 25% 10% 2	1620
	11th to 20th	Day	45% 4 10% 26% 13% 3	1636
	21st to end	Day	48% 3 11% 21% 13% 4	1480
March	1st to 10th	Day	38% 4 12% 25% 18% 2	1921
	11th to 20th	Day	43% 6% 11% 18% 17% 5%	1980
	21st to end	Day	49% 4 9% 19% 17% 3	2224
April	1st to 10th	Day	42% 6% 11% 20% 17% 5%	2160
	11th to 20th	Day	39% 5% 10% 22% 18% 5%	2134
	21st to end	Day	44% 5% 9% 21% 18% 3	2160
May	1st to 10th	Day	44% 7% 11% 21% 15% 2	2297
	11th to 20th	Day	47% 7% 12% 19% 13% 3	2454
	21st to end	Day	41% 7% 15% 23% 13% 2	2725
June	1st to 10th	Day	37% 8% 15% 24% 15% 2	2520
	11th to 20th	Day	42% 8% 12% 22% 14% 2	2520
	21st to end	Day	34% 7% 15% 26% 17% 1	2520
July	1st to 10th	Day	37% 10% 13% 21% 16% 3	2450
	11th to 20th	Day	32% 8% 14% 27% 15% 3	2424
	21st to end	Day	40% 10% 12% 20% 15% 2	2405
August	1st to 10th	Day	40% 10% 13% 25% 12% 1	2314
	11th to 20th	Day	40% 8% 13% 22% 16% 2	2207
	21st to end	Day	38% 7% 13% 24% 14% 3	2376
September	1st to 10th	Day	39% 6% 12% 20% 18% 4	2160
	11th to 20th	Day	35% 5% 9% 26% 21% 4	2160
	21st to end	Day	44% 5% 9% 19% 20% 3	1901
October	1st to 10th	Day	40% 6% 10% 25% 18% 3	1800
	11th to 20th	Day	39% 5% 10% 22% 21% 2	1800
	21st to end	Day	39% 4 11% 25% 20% 2	1980
November	1st to 10th	Day	34% 4 12% 26% 20% 4	1727
	11th to 20th	Day	36% 5% 11% 23% 22% 3	1687
	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	41% 4 10% 25% 16% 4	2645
	11th to 20th	Night	43% 6% 13% 24% 13% 1	2386
	21st to end	Night	41% 4 10% 26% 17% 3	2574
February	1st to 10th	Night	43% 5% 14% 23% 13% 2	2340
	11th to 20th	Night	44% 5% 10% 21% 17% 4	2220
	21st to end	Night	47% 3 9% 22% 14% 5%	1776
March	1st to 10th	Night	36% 5% 11% 26% 17% 3	2036
	11th to 20th	Night	42% 5% 11% 22% 17% 4	1980
	21st to end	Night	42% 5% 10% 21% 16% 5%	2131
April	1st to 10th	Night	43% 5% 10% 19% 18% 4	1800
	11th to 20th	Night	38% 6% 11% 20% 19% 6%	1775
	21st to end	Night	40% 6% 10% 23% 16% 5%	1800
May	1st to 10th	Night	44% 6% 10% 23% 13% 4	1677
	11th to 20th	Night	45% 7% 11% 18% 17% 2	1442
	21st to end	Night	38% 9% 17% 22% 12% 2	1551
June	1st to 10th	Night	35% 7% 11% 27% 17% 3	1440
	11th to 20th	Night	39% 8% 14% 23% 14% 2	1440
	21st to end	Night	34% 7% 14% 26% 17% 2	1440
July	1st to 10th	Night	41% 11% 14% 18% 12% 3	1400
	11th to 20th	Night	31% 10% 15% 25% 16% 3	1410
	21st to end	Night	31% 8% 18% 23% 17% 2	1672
August	1st to 10th	Night	37% 9% 15% 23% 14% 1	1600
	11th to 20th	Night	41% 9% 12% 21% 16% 2	1673
	21st to end	Night	35% 8% 14% 24% 16% 4	1980
September	1st to 10th	Night	37% 6% 12% 21% 19% 5%	1800
	11th to 20th	Night	33% 4 10% 26% 24% 3	1800
	21st to end	Night	42% 6% 9% 20% 19% 3	2055
October	1st to 10th	Night	42% 6% 10% 22% 17% 2	2160
	11th to 20th	Night	35% 4 10% 28% 20% 3	2160
	21st to end	Night	37% 4 10% 26% 20% 3	2376
November	1st to 10th	Night	33% 5% 9% 27% 22% 5%	2349
	11th to 20th	Night	39% 4 8% 24% 20% 5%	2489
	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	24% 31% 33% 12% 1	249
	21st to end	Day	22% 33% 30% 13% 2 1	264
February	1st to 10th	Day	25% 29% 34% 10% 3	240
	11th to 20th	Day	29% 27% 38% 6%	294
	21st to end	Day	32% 23% 30% 14% 1	346
March	1st to 10th	Day	18% 29% 30% 20% 3	420
	11th to 20th	Day	26% 20% 33% 15% 4 2	420
	21st to end	Day	22% 22% 33% 16% 5% 1	462
April	1st to 10th	Day	20% 20% 34% 18% 5% 2	420
	11th to 20th	Day	17% 19% 33% 16% 13% 3	412
	21st to end	Day	24% 16% 35% 16% 7% 2	420
May	1st to 10th	Day	24% 16% 30% 18% 9% 3	423
	11th to 20th	Day	23% 14% 28% 23% 10% 3	447
	21st to end	Day	16% 6% 24% 26% 23% 5%	485
June	1st to 10th	Day	14% 11% 28% 26% 16% 5%	450
	11th to 20th	Day	18% 9% 28% 24% 17% 4	450
	21st to end	Day	11% 7% 26% 35% 16% 5%	450
July	1st to 10th	Day	11% 4 22% 29% 21% 13%	445
	11th to 20th	Day	5% 3 21% 34% 27% 10%	444
	21st to end	Day	13% 3 22% 33% 24% 6%	488
August	1st to 10th	Day	11% 4 19% 30% 26% 9%	446
	11th to 20th	Day	12% 4 19% 33% 23% 10%	439
	21st to end	Day	9% 4 22% 28% 23% 13%	495
September	1st to 10th	Day	16% 7% 20% 21% 24% 12%	450
	11th to 20th	Day	11% 4 22% 30% 22% 11%	450
	21st to end	Day	22% 12% 22% 25% 15% 4	450
October	1st to 10th	Day	22% 8% 23% 23% 18% 7%	450
	11th to 20th	Day	18% 15% 25% 27% 13% 2	450
	21st to end	Day	17% 14% 37% 23% 7% 2	495
November	1st to 10th	Day	16% 14% 31% 26% 11% 2	392
	11th to 20th	Day	18% 17% 33% 21% 10% 1	347
	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% 23% 3	735
	21st to end	Night	19% 21% 41% 18% 1	792
February	1st to 10th	Night	19% 20% 37% 22% 2	720
	11th to 20th	Night	24% 24% 37% 14% 1	703
	21st to end	Night	23% 21% 39% 14% 2	592
March	1st to 10th	Night	19% 19% 40% 19% 3	720
	11th to 20th	Night	22% 18% 34% 20% 5%	720
	21st to end	Night	18% 17% 43% 17% 5%	792
April	1st to 10th	Night	20% 20% 37% 20% 4	720
	11th to 20th	Night	16% 16% 35% 25% 8% 1	711
	21st to end	Night	21% 12% 34% 25% 7% 1	720
May	1st to 10th	Night	18% 10% 33% 27% 11% 1	597
	11th to 20th	Night	22% 7% 28% 32% 9% 1	380
	21st to end	Night	16% 5% 22% 38% 18% 1	389
June	1st to 10th	Night	12% 2 30% 33% 22% 1	360
	11th to 20th	Night	12% 4 26% 39% 19% 1	360
	21st to end	Night	10% 3 23% 48% 16% 1	360
July	1st to 10th	Night	9% 3 17% 40% 27% 4	350
	11th to 20th	Night	7% 1 18% 40% 32% 2	366
	21st to end	Night	9% 2 18% 42% 26% 4	591
August	1st to 10th	Night	11% 3 13% 43% 28% 2	537
	11th to 20th	Night	11% 3 17% 38% 27% 3	609
	21st to end	Night	12% 3 17% 38% 27% 4	792
September	1st to 10th	Night	16% 3 20% 32% 26% 4	720
	11th to 20th	Night	12% 3 18% 35% 28% 4	720
	21st to end	Night	20% 5% 22% 33% 18% 2	720
October	1st to 10th	Night	21% 7% 21% 30% 18% 4	720
	11th to 20th	Night	16% 11% 29% 30% 13% 1	720
	21st to end	Night	19% 11% 32% 26% 11%	792
November	1st to 10th	Night	14% 11% 34% 31% 10% 2	740
	11th to 20th	Night	19% 16% 32% 26% 6% 1	760
	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

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