Cygnus Hill Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Cygnus Hill Observatory clear sky chart from 2005-11-06 to 2026-05-02.

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	16% 11% 4 9% 5% 5% 4 4 5% 7% 32%	132048
	Night	16% 12% 4 9% 5% 5% 4 3 5% 6% 31%	130427
Transparency	Day	51% 6% 11% 20% 10% 1	135552
	Night	52% 6% 11% 19% 10% 1	135010
Seeing	Day	23% 17% 31% 18% 8% 2	16282
	Night	22% 15% 31% 23% 8% 1	25569

By Month

Percentage Cloud Cover NE (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% 15% 6% 13% 8% 6% 5% 3 5% 5% 21%	4661
	11th to 20th	Day	11% 21% 8% 14% 7% 5% 5% 3 3 4 19%	4650
	21st to end	Day	11% 16% 7% 15% 8% 7% 5% 4 4 4 19%	5540
February	1st to 10th	Day	15% 15% 4 12% 6% 5% 5% 4 5% 5% 24%	1890
	11th to 20th	Day	12% 15% 5% 11% 7% 5% 5% 3 4 5% 27%	1843
	21st to end	Day	19% 14% 5% 10% 7% 5% 5% 3 4 4 23%	1709
March	1st to 10th	Day	18% 14% 4 10% 6% 4 4 3 4 6% 27%	4688
	11th to 20th	Day	11% 13% 4 8% 6% 6% 5% 4 5% 7% 31%	5090
	21st to end	Day	17% 12% 4 8% 4 4 3 3 3 4 37%	5817
April	1st to 10th	Day	21% 13% 3 8% 4 4 3 3 4 5% 32%	2520
	11th to 20th	Day	22% 10% 2 7% 4 3 4 3 4 5% 37%	2494
	21st to end	Day	21% 11% 3 7% 5% 4 4 3 5% 5% 32%	2640
May	1st to 10th	Day	24% 10% 3 8% 5% 4 4 3 5% 6% 28%	6360
	11th to 20th	Day	16% 9% 3 8% 5% 5% 4 3 5% 8% 34%	6318
	21st to end	Day	18% 9% 3 6% 4 4 4 4 6% 8% 34%	6949
June	1st to 10th	Day	19% 9% 3 8% 4 5% 5% 4 6% 6% 31%	2800
	11th to 20th	Day	14% 6% 3 6% 4 5% 5% 4 6% 9% 37%	2800
	21st to end	Day	17% 8% 3 7% 6% 5% 6% 4 6% 8% 33%	2800
July	1st to 10th	Day	12% 7% 3 7% 6% 5% 5% 5% 7% 10% 34%	6360
	11th to 20th	Day	10% 5% 2 6% 5% 5% 5% 4 7% 8% 42%	6360
	21st to end	Day	9% 5% 2 6% 4 4 6% 4 7% 9% 44%	6926
August	1st to 10th	Day	16% 8% 3 7% 4 5% 4 4 5% 8% 36%	4372
	11th to 20th	Day	16% 7% 3 6% 4 4 4 4 7% 9% 36%	4154
	21st to end	Day	14% 6% 2 6% 4 4 4 4 7% 10% 39%	4312
September	1st to 10th	Day	13% 6% 2 6% 4 5% 5% 4 6% 8% 42%	2400
	11th to 20th	Day	11% 5% 2 5% 4 4 4 3 5% 7% 49%	2400
	21st to end	Day	18% 6% 2 7% 5% 4 4 4 4 7% 39%	2119
October	1st to 10th	Day	20% 9% 2 7% 4 3 4 3 5% 6% 38%	2000
	11th to 20th	Day	19% 9% 3 7% 6% 5% 5% 3 6% 7% 30%	2000
	21st to end	Day	24% 9% 3 9% 5% 5% 4 3 5% 6% 26%	2086
November	1st to 10th	Day	15% 10% 3 8% 4 5% 5% 4 6% 6% 33%	1836
	11th to 20th	Day	16% 14% 4 9% 5% 5% 4 3 4 6% 29%	1680
	21st to end	Day	19% 16% 5% 10% 6% 5% 5% 4 5% 4 22%	1680
December	1st to 10th	Day	21% 17% 4 13% 6% 5% 4 4 5% 5% 16%	2680
	11th to 20th	Day	18% 18% 5% 11% 7% 4 3 3 4 4 24%	3066
	21st to end	Day	23% 17% 5% 13% 5% 3 3 2 3 4 22%	4048
January	1st to 10th	Night	14% 15% 8% 14% 7% 5% 4 3 4 4 21%	8234
	11th to 20th	Night	11% 18% 7% 16% 8% 5% 5% 3 4 5% 17%	8235
	21st to end	Night	13% 15% 6% 12% 6% 6% 5% 3 5% 5% 23%	8640
February	1st to 10th	Night	13% 16% 5% 10% 7% 5% 4 4 7% 6% 24%	2730
	11th to 20th	Night	12% 15% 5% 11% 5% 5% 4 3 5% 6% 29%	2663
	21st to end	Night	14% 15% 4 8% 6% 4 5% 4 5% 6% 28%	2096
March	1st to 10th	Night	20% 14% 4 10% 5% 5% 3 2 4 4 30%	5355
	11th to 20th	Night	13% 12% 3 7% 4 5% 5% 4 7% 8% 30%	5089
	21st to end	Night	18% 10% 4 8% 4 4 4 3 4 5% 35%	5355
April	1st to 10th	Night	21% 10% 3 9% 4 4 4 3 4 6% 33%	2100
	11th to 20th	Night	20% 10% 3 7% 4 5% 4 3 5% 4 36%	2075
	21st to end	Night	19% 10% 3 8% 4 4 4 3 5% 6% 34%	1999
May	1st to 10th	Night	24% 10% 3 7% 4 5% 4 4 5% 6% 26%	3602
	11th to 20th	Night	15% 8% 2 6% 3 3 4 3 5% 7% 42%	3575
	21st to end	Night	18% 8% 3 7% 5% 4 4 4 6% 6% 35%	3926
June	1st to 10th	Night	15% 8% 2 7% 6% 5% 5% 5% 6% 8% 34%	1600
	11th to 20th	Night	13% 6% 2 6% 4 5% 4 3 7% 8% 41%	1600
	21st to end	Night	17% 7% 2 7% 5% 5% 5% 4 6% 8% 35%	1600
July	1st to 10th	Night	12% 5% 3 7% 5% 5% 5% 4 7% 9% 36%	3600
	11th to 20th	Night	11% 5% 2 6% 4 4 4 3 5% 8% 46%	3600
	21st to end	Night	8% 6% 2 6% 4 4 4 4 5% 7% 49%	3926
August	1st to 10th	Night	11% 7% 2 7% 4 5% 4 3 6% 7% 43%	2773
	11th to 20th	Night	12% 7% 3 7% 4 4 4 3 6% 8% 43%	2875
	21st to end	Night	10% 5% 2 6% 5% 4 5% 4 7% 9% 42%	3572
September	1st to 10th	Night	11% 6% 2 6% 5% 4 4 3 5% 7% 47%	2000
	11th to 20th	Night	12% 4 2 4 3 4 4 4 5% 7% 52%	2000
	21st to end	Night	17% 7% 2 6% 4 4 4 3 5% 6% 43%	2279
October	1st to 10th	Night	17% 7% 3 6% 4 4 4 3 5% 7% 40%	2400
	11th to 20th	Night	20% 9% 2 8% 5% 4 5% 3 5% 6% 32%	2400
	21st to end	Night	22% 10% 3 7% 5% 4 4 3 5% 5% 31%	2752
November	1st to 10th	Night	17% 11% 3 7% 5% 4 4 4 5% 6% 35%	2678
	11th to 20th	Night	17% 11% 4 8% 5% 4 5% 3 5% 6% 32%	2940
	21st to end	Night	20% 14% 4 9% 5% 5% 5% 3 5% 6% 24%	2940
December	1st to 10th	Night	19% 16% 5% 12% 7% 5% 5% 3 5% 5% 17%	4720
	11th to 20th	Night	22% 15% 4 9% 5% 4 4 3 4 4 25%	5348
	21st to end	Night	26% 17% 4 11% 5% 4 4 2 4 4 20%	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	62% 7% 12% 14% 5% 1	4662
	11th to 20th	Day	69% 6% 7% 9% 8% 1	4680
	21st to end	Day	68% 5% 7% 10% 9% 1	5540
February	1st to 10th	Day	60% 6% 10% 16% 6% 1	1890
	11th to 20th	Day	58% 6% 10% 16% 9% 1	1852
	21st to end	Day	64% 5% 9% 12% 9% 1	1709
March	1st to 10th	Day	58% 5% 9% 13% 12% 3	4691
	11th to 20th	Day	51% 7% 16% 15% 10% 2	5090
	21st to end	Day	50% 5% 6% 19% 18% 3	5817
April	1st to 10th	Day	55% 4 8% 17% 12% 3	2520
	11th to 20th	Day	50% 6% 7% 20% 15% 2	2494
	21st to end	Day	54% 5% 8% 15% 14% 3	2640
May	1st to 10th	Day	56% 7% 9% 17% 10% 2	6371
	11th to 20th	Day	48% 8% 12% 19% 13% 1	6322
	21st to end	Day	47% 7% 12% 21% 11% 1	6949
June	1st to 10th	Day	50% 8% 10% 18% 12% 1	2800
	11th to 20th	Day	41% 8% 11% 25% 13% 2	2800
	21st to end	Day	48% 8% 12% 20% 11% 1	2800
July	1st to 10th	Day	43% 6% 14% 26% 11% 1	6290
	11th to 20th	Day	36% 7% 14% 27% 15%	6285
	21st to end	Day	34% 7% 13% 36% 10%	6728
August	1st to 10th	Day	45% 6% 14% 24% 10% 1	4358
	11th to 20th	Day	42% 8% 14% 27% 9% 1	4193
	21st to end	Day	39% 6% 14% 30% 10%	4312
September	1st to 10th	Day	38% 8% 15% 28% 12% 1	2400
	11th to 20th	Day	34% 8% 12% 31% 15%	2400
	21st to end	Day	44% 7% 13% 25% 11%	2119
October	1st to 10th	Day	43% 5% 11% 32% 9%	3260
	11th to 20th	Day	44% 6% 14% 24% 11% 1	3240
	21st to end	Day	50% 6% 15% 23% 7%	3350
November	1st to 10th	Day	48% 8% 15% 20% 8% 1	1836
	11th to 20th	Day	55% 7% 11% 16% 10% 1	1680
	21st to end	Day	63% 9% 11% 10% 7%	1680
December	1st to 10th	Day	69% 6% 13% 9% 3	2680
	11th to 20th	Day	65% 5% 10% 14% 6%	3066
	21st to end	Day	67% 5% 7% 15% 6%	4048
January	1st to 10th	Night	65% 6% 11% 12% 5%	8234
	11th to 20th	Night	68% 7% 8% 10% 7%	8280
	21st to end	Night	62% 5% 7% 13% 13% 1	8640
February	1st to 10th	Night	58% 7% 11% 16% 6% 1	2730
	11th to 20th	Night	56% 6% 11% 20% 7% 1	2665
	21st to end	Night	56% 6% 11% 17% 9% 1	2096
March	1st to 10th	Night	58% 4 7% 16% 10% 4	5365
	11th to 20th	Night	48% 8% 18% 16% 8% 2	5090
	21st to end	Night	49% 5% 8% 22% 13% 2	5355
April	1st to 10th	Night	52% 5% 7% 20% 13% 2	2100
	11th to 20th	Night	50% 6% 7% 20% 16% 1	2075
	21st to end	Night	51% 5% 8% 16% 16% 3	1999
May	1st to 10th	Night	56% 7% 10% 16% 8% 3	3608
	11th to 20th	Night	41% 7% 15% 22% 14% 2	3576
	21st to end	Night	46% 7% 14% 17% 14% 1	3927
June	1st to 10th	Night	46% 9% 13% 20% 12% 1	1600
	11th to 20th	Night	39% 7% 13% 22% 18% 2	1600
	21st to end	Night	45% 10% 13% 19% 12% 1	1600
July	1st to 10th	Night	42% 6% 16% 19% 15% 1	3560
	11th to 20th	Night	35% 6% 13% 32% 12%	3552
	21st to end	Night	33% 7% 12% 32% 14% 1	3809
August	1st to 10th	Night	39% 7% 15% 25% 13% 1	2763
	11th to 20th	Night	39% 6% 14% 31% 10% 1	2902
	21st to end	Night	36% 8% 16% 28% 11% 1	3572
September	1st to 10th	Night	35% 9% 13% 29% 14% 1	2000
	11th to 20th	Night	30% 8% 14% 33% 13% 1	2000
	21st to end	Night	41% 7% 11% 29% 12% 1	2279
October	1st to 10th	Night	40% 5% 11% 28% 15%	3920
	11th to 20th	Night	43% 5% 16% 26% 10%	3899
	21st to end	Night	49% 6% 12% 24% 8%	4438
November	1st to 10th	Night	48% 6% 14% 21% 10% 1	2678
	11th to 20th	Night	52% 6% 11% 19% 10% 1	2940
	21st to end	Night	60% 7% 12% 16% 5%	2940
December	1st to 10th	Night	67% 6% 12% 12% 3	4720
	11th to 20th	Night	62% 6% 8% 14% 9% 1	5348
	21st to end	Night	68% 6% 6% 11% 9%	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	23% 21% 40% 13% 3	220
	11th to 20th	Day	34% 18% 30% 15% 2	220
	21st to end	Day	25% 24% 32% 15% 3	319
February	1st to 10th	Day	31% 25% 33% 10% 2	330
	11th to 20th	Day	28% 28% 38% 6%	330
	21st to end	Day	33% 24% 32% 10% 1	425
March	1st to 10th	Day	27% 24% 33% 13% 3	540
	11th to 20th	Day	32% 21% 30% 13% 3	540
	21st to end	Day	33% 23% 27% 12% 5% 1	594
April	1st to 10th	Day	32% 26% 28% 10% 5%	540
	11th to 20th	Day	34% 21% 23% 14% 6% 2	532
	21st to end	Day	29% 24% 29% 12% 5% 2	540
May	1st to 10th	Day	26% 17% 32% 17% 6% 2	507
	11th to 20th	Day	19% 16% 37% 19% 8% 2	527
	21st to end	Day	21% 15% 32% 21% 8% 3	573
June	1st to 10th	Day	22% 14% 37% 21% 5% 2	530
	11th to 20th	Day	15% 17% 36% 20% 10% 2	530
	21st to end	Day	19% 17% 31% 22% 9% 2	530
July	1st to 10th	Day	14% 10% 31% 29% 14% 3	525
	11th to 20th	Day	15% 13% 32% 23% 15% 3	524
	21st to end	Day	13% 10% 35% 27% 10% 4	576
August	1st to 10th	Day	13% 9% 32% 30% 13% 3	526
	11th to 20th	Day	15% 9% 28% 27% 15% 6%	519
	21st to end	Day	15% 11% 32% 25% 11% 6%	583
September	1st to 10th	Day	14% 15% 30% 22% 11% 7%	530
	11th to 20th	Day	11% 9% 25% 25% 19% 11%	530
	21st to end	Day	19% 11% 30% 23% 13% 4	530
October	1st to 10th	Day	25% 16% 26% 18% 12% 3	530
	11th to 20th	Day	27% 20% 32% 15% 5% 1	530
	21st to end	Day	28% 21% 29% 19% 4	520
November	1st to 10th	Day	29% 19% 27% 18% 7%	410
	11th to 20th	Day	28% 20% 34% 11% 6% 1	220
	21st to end	Day	34% 21% 30% 13% 2	220
December	1st to 10th	Day	34% 19% 33% 11% 3	220
	11th to 20th	Day	35% 24% 30% 8% 2	220
	21st to end	Day	35% 18% 31% 12% 4	242
January	1st to 10th	Night	27% 19% 33% 17% 3	840
	11th to 20th	Night	30% 18% 30% 18% 4	840
	21st to end	Night	20% 17% 37% 21% 4	924
February	1st to 10th	Night	22% 22% 33% 20% 3	840
	11th to 20th	Night	23% 22% 36% 16% 3	823
	21st to end	Night	25% 20% 36% 17% 2	692
March	1st to 10th	Night	26% 17% 35% 19% 3	840
	11th to 20th	Night	27% 20% 28% 20% 4 1	840
	21st to end	Night	24% 17% 34% 18% 6%	924
April	1st to 10th	Night	28% 17% 33% 20% 3	840
	11th to 20th	Night	26% 20% 29% 19% 6% 1	831
	21st to end	Night	25% 15% 30% 22% 8% 1	739
May	1st to 10th	Night	23% 13% 27% 26% 9% 2	402
	11th to 20th	Night	15% 11% 38% 28% 7%	394
	21st to end	Night	14% 9% 38% 28% 10%	433
June	1st to 10th	Night	16% 6% 36% 35% 7% 1	400
	11th to 20th	Night	15% 11% 34% 27% 13% 1	400
	21st to end	Night	18% 8% 30% 33% 11% 1	400
July	1st to 10th	Night	11% 6% 33% 36% 13% 1	390
	11th to 20th	Night	12% 4 32% 37% 14% 1	388
	21st to end	Night	11% 4 32% 42% 11%	438
August	1st to 10th	Night	12% 5% 28% 36% 19%	507
	11th to 20th	Night	12% 5% 25% 34% 23% 2	594
	21st to end	Night	12% 7% 31% 33% 15% 2	866
September	1st to 10th	Night	13% 7% 34% 28% 17% 2	800
	11th to 20th	Night	13% 8% 23% 28% 24% 5%	800
	21st to end	Night	20% 10% 24% 27% 17% 2	800
October	1st to 10th	Night	21% 13% 28% 23% 14% 2	800
	11th to 20th	Night	26% 14% 32% 21% 7%	800
	21st to end	Night	28% 15% 29% 23% 5%	880
November	1st to 10th	Night	27% 15% 30% 21% 7%	820
	11th to 20th	Night	25% 17% 33% 16% 8% 1	840
	21st to end	Night	29% 19% 28% 18% 5%	840
December	1st to 10th	Night	28% 20% 31% 17% 4	840
	11th to 20th	Night	27% 20% 34% 16% 2 1	840
	21st to end	Night	33% 17% 30% 17% 2	924

Page generated at 2026-04-29 02:21:47. Version ac07

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