"Way The Held Out" Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the "Way The Held Out" Observatory clear sky chart from 2005-11-06 to 2026-06-01.

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	17% 13% 5% 10% 5% 5% 4 3 5% 6% 28%	132170
	Night	17% 14% 5% 11% 5% 4 4 3 4 6% 27%	130945
Transparency	Day	55% 6% 10% 18% 9% 1	135771
	Night	58% 5% 10% 16% 9% 1	135415
Seeing	Day	25% 17% 29% 18% 8% 3	16536
	Night	25% 15% 27% 22% 9% 1	26229

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	17% 21% 9% 28% 5% 4 3 2 3 3 7%	5063
	11th to 20th	Day	11% 23% 10% 22% 9% 8% 3 2 2 3 8%	5280
	21st to end	Day	17% 17% 8% 16% 10% 7% 4 2 3 3 12%	5780
February	1st to 10th	Day	16% 20% 6% 16% 9% 5% 3 2 3 4 17%	1890
	11th to 20th	Day	16% 17% 8% 16% 8% 4 3 3 4 4 16%	1843
	21st to end	Day	18% 18% 4 14% 4 4 3 4 4 5% 21%	1694
March	1st to 10th	Day	17% 13% 8% 12% 6% 3 3 3 4 6% 27%	5045
	11th to 20th	Day	12% 16% 6% 11% 4 4 3 3 4 5% 31%	5090
	21st to end	Day	22% 14% 7% 11% 4 3 3 3 3 4 26%	5586
April	1st to 10th	Day	18% 15% 3 8% 4 4 4 3 5% 7% 29%	2420
	11th to 20th	Day	20% 13% 3 6% 4 4 3 4 5% 6% 32%	2586
	21st to end	Day	20% 12% 3 7% 4 3 3 2 4 7% 34%	2730
May	1st to 10th	Day	22% 10% 3 8% 5% 4 4 4 5% 6% 30%	5992
	11th to 20th	Day	19% 10% 3 7% 4 4 4 4 6% 8% 30%	5991
	21st to end	Day	19% 9% 3 6% 4 5% 5% 4 6% 7% 34%	7088
June	1st to 10th	Day	16% 7% 2 7% 5% 4 4 5% 6% 8% 36%	2800
	11th to 20th	Day	14% 8% 3 7% 5% 5% 5% 4 6% 8% 35%	2800
	21st to end	Day	14% 6% 2 6% 5% 5% 5% 4 6% 9% 39%	2800
July	1st to 10th	Day	9% 6% 2 6% 5% 5% 5% 4 6% 10% 42%	6346
	11th to 20th	Day	13% 6% 3 6% 5% 5% 4 4 7% 9% 38%	5900
	21st to end	Day	12% 6% 2 7% 4 5% 5% 4 5% 7% 44%	6424
August	1st to 10th	Day	15% 7% 2 8% 4 5% 5% 5% 6% 9% 34%	4234
	11th to 20th	Day	15% 6% 3 8% 6% 5% 6% 4 8% 9% 31%	4174
	21st to end	Day	11% 6% 2 7% 5% 5% 6% 5% 6% 9% 39%	4512
September	1st to 10th	Day	16% 7% 2 7% 5% 4 4 3 6% 8% 39%	2398
	11th to 20th	Day	12% 5% 3 7% 5% 4 4 3 5% 8% 44%	2200
	21st to end	Day	20% 9% 2 7% 5% 4 4 4 4 6% 33%	2200
October	1st to 10th	Day	17% 7% 3 6% 4 5% 5% 4 6% 7% 38%	2005
	11th to 20th	Day	22% 9% 3 8% 6% 4 4 3 5% 7% 29%	2000
	21st to end	Day	26% 14% 3 8% 4 4 4 4 5% 5% 23%	2200
November	1st to 10th	Day	15% 14% 4 8% 5% 4 4 4 5% 6% 32%	1912
	11th to 20th	Day	19% 21% 4 9% 5% 3 3 3 4 6% 23%	1709
	21st to end	Day	20% 25% 6% 9% 4 4 4 3 5% 5% 14%	1680
December	1st to 10th	Day	18% 28% 15% 13% 4 3 3 2 3 3 9%	2680
	11th to 20th	Day	22% 25% 8% 11% 5% 3 3 3 4 4 11%	3070
	21st to end	Day	25% 25% 7% 14% 5% 4 3 2 3 3 9%	4048
January	1st to 10th	Night	15% 22% 10% 29% 5% 3 2 1 2 3 7%	7813
	11th to 20th	Night	12% 21% 11% 23% 8% 6% 3 2 2 3 9%	7680
	21st to end	Night	18% 19% 8% 14% 7% 5% 5% 3 4 4 15%	8401
February	1st to 10th	Night	18% 19% 6% 13% 8% 5% 4 3 4 4 18%	2730
	11th to 20th	Night	16% 17% 7% 16% 7% 5% 4 3 3 5% 17%	2663
	21st to end	Night	19% 15% 5% 16% 6% 4 4 3 3 4 21%	2105
March	1st to 10th	Night	20% 16% 5% 9% 6% 3 3 3 4 5% 26%	5028
	11th to 20th	Night	14% 14% 7% 7% 4 3 3 3 5% 6% 35%	5090
	21st to end	Night	19% 14% 6% 10% 4 4 4 3 3 4 30%	5588
April	1st to 10th	Night	19% 16% 5% 7% 5% 4 4 3 5% 5% 27%	2200
	11th to 20th	Night	19% 10% 3 7% 4 6% 4 4 5% 7% 31%	2001
	21st to end	Night	14% 11% 3 7% 5% 4 5% 4 5% 7% 34%	1890
May	1st to 10th	Night	26% 11% 3 8% 4 4 4 3 5% 6% 29%	4123
	11th to 20th	Night	17% 10% 3 8% 5% 4 4 3 4 7% 35%	4105
	21st to end	Night	21% 7% 2 5% 3 3 3 3 5% 9% 38%	4008
June	1st to 10th	Night	10% 7% 2 8% 5% 4 5% 4 7% 9% 40%	1600
	11th to 20th	Night	12% 6% 3 8% 5% 4 4 4 6% 8% 41%	1600
	21st to end	Night	13% 6% 2 6% 4 5% 4 4 7% 8% 42%	1600
July	1st to 10th	Night	12% 7% 2 6% 5% 4 4 3 6% 9% 43%	3598
	11th to 20th	Night	13% 6% 2 6% 4 4 5% 4 6% 9% 41%	4053
	21st to end	Night	9% 5% 2 6% 4 4 4 4 6% 7% 48%	4427
August	1st to 10th	Night	11% 8% 2 8% 5% 5% 4 4 5% 7% 40%	2915
	11th to 20th	Night	17% 7% 3 6% 4 5% 4 4 5% 7% 39%	2875
	21st to end	Night	10% 6% 2 6% 5% 5% 5% 5% 6% 8% 42%	3351
September	1st to 10th	Night	13% 6% 1 5% 4 3 4 4 5% 7% 46%	2000
	11th to 20th	Night	12% 5% 2 5% 4 3 4 3 6% 6% 50%	2199
	21st to end	Night	18% 8% 2 7% 5% 4 4 3 6% 6% 37%	2200
October	1st to 10th	Night	18% 7% 2 5% 4 3 4 3 4 7% 43%	2394
	11th to 20th	Night	20% 9% 3 7% 5% 5% 5% 3 5% 6% 33%	2400
	21st to end	Night	27% 11% 3 7% 4 4 4 3 4 5% 29%	2640
November	1st to 10th	Night	16% 12% 3 7% 5% 4 4 4 5% 7% 33%	2604
	11th to 20th	Night	19% 19% 4 8% 6% 4 4 4 4 5% 23%	2909
	21st to end	Night	21% 22% 6% 9% 5% 3 4 3 4 5% 18%	2940
December	1st to 10th	Night	16% 24% 14% 11% 5% 3 4 2 3 4 13%	4720
	11th to 20th	Night	19% 26% 9% 12% 5% 3 3 2 3 4 15%	5345
	21st to end	Night	22% 23% 7% 12% 6% 4 4 2 4 4 12%	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	85% 3 6% 4 1	5064
	11th to 20th	Day	85% 2 5% 5% 2	5280
	21st to end	Day	80% 2 6% 8% 4	5780
February	1st to 10th	Day	74% 3 8% 10% 5%	1890
	11th to 20th	Day	73% 5% 7% 10% 5%	1852
	21st to end	Day	66% 5% 10% 13% 4 2	1694
March	1st to 10th	Day	60% 4 11% 20% 5%	5045
	11th to 20th	Day	55% 5% 15% 18% 5% 2	5090
	21st to end	Day	62% 4 6% 18% 9% 1	5586
April	1st to 10th	Day	54% 6% 13% 15% 11% 1	2420
	11th to 20th	Day	53% 5% 11% 18% 11% 2	2586
	21st to end	Day	52% 4 10% 20% 11% 2	2730
May	1st to 10th	Day	54% 6% 8% 19% 8% 4	6027
	11th to 20th	Day	51% 7% 10% 21% 10% 1	5995
	21st to end	Day	48% 7% 11% 19% 12% 3	7103
June	1st to 10th	Day	44% 7% 12% 23% 12% 1	2800
	11th to 20th	Day	45% 7% 13% 21% 13% 1	2800
	21st to end	Day	41% 6% 13% 24% 14% 1	2800
July	1st to 10th	Day	38% 6% 12% 26% 17% 2	6276
	11th to 20th	Day	40% 7% 12% 24% 16% 1	5831
	21st to end	Day	38% 7% 11% 24% 19% 1	6240
August	1st to 10th	Day	44% 8% 15% 22% 11%	4221
	11th to 20th	Day	46% 8% 12% 26% 7% 1	4213
	21st to end	Day	39% 8% 12% 28% 13% 1	4512
September	1st to 10th	Day	42% 7% 13% 23% 13% 2	2398
	11th to 20th	Day	37% 7% 14% 26% 15% 1	2200
	21st to end	Day	50% 7% 11% 21% 11% 1	2200
October	1st to 10th	Day	36% 6% 15% 24% 18% 1	3265
	11th to 20th	Day	47% 5% 14% 19% 13% 3	3240
	21st to end	Day	48% 5% 14% 24% 9% 1	3538
November	1st to 10th	Day	52% 6% 10% 19% 11% 2	1912
	11th to 20th	Day	63% 5% 10% 14% 7% 2	1709
	21st to end	Day	72% 6% 8% 9% 4	1680
December	1st to 10th	Day	84% 4 6% 6% 1	2680
	11th to 20th	Day	78% 5% 6% 10% 1	3066
	21st to end	Day	83% 3 7% 6% 1	4048
January	1st to 10th	Night	86% 3 5% 5% 1	7814
	11th to 20th	Night	83% 3 6% 5% 2	7680
	21st to end	Night	75% 3 8% 11% 4	8401
February	1st to 10th	Night	71% 4 9% 11% 4	2730
	11th to 20th	Night	71% 5% 9% 11% 4	2665
	21st to end	Night	68% 4 9% 12% 7% 1	2105
March	1st to 10th	Night	60% 5% 11% 17% 7%	5028
	11th to 20th	Night	50% 5% 13% 21% 10% 1	5090
	21st to end	Night	58% 5% 9% 15% 13% 1	5588
April	1st to 10th	Night	59% 5% 10% 15% 10% 1	2200
	11th to 20th	Night	52% 6% 11% 17% 12% 2	2001
	21st to end	Night	49% 6% 12% 20% 10% 3	1890
May	1st to 10th	Night	57% 5% 9% 14% 11% 5%	4150
	11th to 20th	Night	48% 7% 10% 23% 12% 1	4106
	21st to end	Night	43% 6% 13% 23% 13% 2	4015
June	1st to 10th	Night	39% 8% 12% 24% 14% 3	1600
	11th to 20th	Night	39% 8% 12% 23% 16% 2	1600
	21st to end	Night	38% 9% 14% 23% 16% 1	1600
July	1st to 10th	Night	39% 5% 10% 25% 18% 2	3560
	11th to 20th	Night	38% 6% 12% 26% 15% 2	4003
	21st to end	Night	33% 8% 11% 20% 26% 1	4297
August	1st to 10th	Night	42% 7% 10% 27% 12% 1	2904
	11th to 20th	Night	44% 7% 11% 29% 8% 1	2902
	21st to end	Night	37% 7% 14% 28% 12% 2	3351
September	1st to 10th	Night	36% 7% 13% 26% 15% 3	2000
	11th to 20th	Night	33% 6% 14% 26% 20% 1	2199
	21st to end	Night	47% 6% 12% 20% 14% 1	2200
October	1st to 10th	Night	38% 5% 9% 24% 23% 1	3914
	11th to 20th	Night	47% 5% 12% 17% 15% 4	3899
	21st to end	Night	45% 4 15% 22% 14% 2	4252
November	1st to 10th	Night	50% 6% 13% 20% 9% 2	2604
	11th to 20th	Night	64% 5% 9% 13% 6% 3	2909
	21st to end	Night	69% 5% 10% 12% 3 1	2940
December	1st to 10th	Night	77% 5% 9% 7% 2	4720
	11th to 20th	Night	76% 3 8% 10% 2	5348
	21st to end	Night	77% 5% 8% 9% 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	38% 26% 27% 8% 2	294
	11th to 20th	Day	37% 31% 25% 5% 3	330
	21st to end	Day	33% 34% 25% 7% 2	363
February	1st to 10th	Day	33% 25% 35% 7%	330
	11th to 20th	Day	38% 28% 28% 6% 1	330
	21st to end	Day	39% 28% 28% 5%	274
March	1st to 10th	Day	33% 23% 31% 10% 3	540
	11th to 20th	Day	33% 23% 29% 11% 2 1	540
	21st to end	Day	36% 22% 27% 13% 2	594
April	1st to 10th	Day	30% 26% 27% 13% 4 1	540
	11th to 20th	Day	34% 21% 24% 13% 7% 2	532
	21st to end	Day	30% 18% 32% 15% 4 1	540
May	1st to 10th	Day	29% 20% 30% 15% 4 2	543
	11th to 20th	Day	24% 16% 31% 20% 8% 2	567
	21st to end	Day	20% 12% 30% 24% 10% 4	617
June	1st to 10th	Day	20% 15% 32% 23% 8% 2	530
	11th to 20th	Day	19% 10% 30% 26% 13% 2	530
	21st to end	Day	15% 12% 36% 24% 12% 2	530
July	1st to 10th	Day	11% 8% 27% 29% 19% 6%	525
	11th to 20th	Day	15% 9% 28% 28% 15% 5%	524
	21st to end	Day	11% 9% 33% 28% 15% 5%	576
August	1st to 10th	Day	11% 9% 27% 29% 17% 6%	526
	11th to 20th	Day	17% 8% 24% 30% 12% 9%	519
	21st to end	Day	13% 10% 27% 28% 16% 6%	583
September	1st to 10th	Day	17% 9% 25% 25% 18% 6%	530
	11th to 20th	Day	11% 5% 25% 29% 22% 8%	530
	21st to end	Day	23% 13% 27% 21% 11% 4	530
October	1st to 10th	Day	19% 13% 28% 22% 16% 2	530
	11th to 20th	Day	27% 19% 33% 14% 5% 2	530
	21st to end	Day	37% 18% 29% 14% 2	583
November	1st to 10th	Day	32% 19% 29% 14% 6% 1	390
	11th to 20th	Day	41% 24% 23% 10% 3	234
	21st to end	Day	36% 21% 35% 7%	220
December	1st to 10th	Day	38% 27% 27% 4 3	220
	11th to 20th	Day	37% 25% 28% 9% 1	220
	21st to end	Day	38% 25% 28% 8%	242
January	1st to 10th	Night	34% 25% 29% 10% 2	840
	11th to 20th	Night	37% 23% 26% 12% 2	840
	21st to end	Night	30% 24% 30% 14% 2	924
February	1st to 10th	Night	33% 21% 29% 16% 1	840
	11th to 20th	Night	29% 24% 32% 13% 1	823
	21st to end	Night	30% 23% 32% 14% 1	692
March	1st to 10th	Night	27% 19% 33% 18% 3	840
	11th to 20th	Night	28% 20% 29% 18% 4	840
	21st to end	Night	27% 18% 32% 19% 4 1	924
April	1st to 10th	Night	29% 18% 30% 19% 4	840
	11th to 20th	Night	27% 16% 30% 20% 7% 1	757
	21st to end	Night	21% 14% 31% 29% 5%	630
May	1st to 10th	Night	26% 10% 24% 31% 9%	630
	11th to 20th	Night	20% 8% 30% 32% 9% 1	617
	21st to end	Night	16% 6% 30% 28% 17% 2	455
June	1st to 10th	Night	12% 6% 35% 34% 13%	400
	11th to 20th	Night	14% 9% 24% 41% 12%	400
	21st to end	Night	12% 8% 27% 35% 19%	400
July	1st to 10th	Night	11% 3 19% 43% 19% 4	390
	11th to 20th	Night	12% 4 21% 35% 24% 3	579
	21st to end	Night	11% 4 18% 41% 25% 2	657
August	1st to 10th	Night	11% 4 19% 39% 23% 3	597
	11th to 20th	Night	12% 5% 16% 35% 28% 4	594
	21st to end	Night	14% 6% 21% 32% 24% 3	736
September	1st to 10th	Night	16% 7% 20% 31% 22% 4	800
	11th to 20th	Night	13% 5% 18% 33% 27% 4	800
	21st to end	Night	21% 11% 23% 26% 16% 4	800
October	1st to 10th	Night	20% 9% 26% 26% 17% 2	800
	11th to 20th	Night	27% 14% 29% 22% 7% 1	800
	21st to end	Night	34% 15% 30% 18% 3	880
November	1st to 10th	Night	24% 16% 29% 23% 7%	820
	11th to 20th	Night	33% 20% 28% 14% 5% 1	840
	21st to end	Night	35% 19% 30% 12% 3	840
December	1st to 10th	Night	36% 22% 28% 11% 3	840
	11th to 20th	Night	30% 30% 26% 12% 2	840
	21st to end	Night	34% 22% 30% 12% 1	924

Page generated at 2026-05-29 01:53:29. Version ac07

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