Albireo Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Albireo 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	12% 7% 3 7% 5% 5% 5% 4 6% 7% 38%	131651
	Night	11% 8% 3 7% 5% 5% 5% 4 6% 7% 40%	130826
Transparency	Day	42% 7% 11% 23% 14% 3	135255
	Night	41% 7% 12% 23% 14% 2	135292
Seeing	Day	23% 14% 26% 21% 13% 4	16759
	Night	20% 12% 25% 29% 12% 1	26564

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	11% 12% 3 9% 4 4 5% 4 4 6% 38%	5235
	11th to 20th	Day	9% 10% 4 10% 6% 5% 5% 3 4 5% 38%	5280
	21st to end	Day	11% 7% 3 8% 6% 5% 5% 5% 6% 6% 38%	5780
February	1st to 10th	Day	11% 14% 3 7% 4 4 4 4 4 6% 37%	1890
	11th to 20th	Day	14% 9% 5% 8% 5% 5% 5% 3 5% 6% 35%	1843
	21st to end	Day	12% 10% 4 8% 5% 4 4 4 4 6% 40%	1764
March	1st to 10th	Day	14% 10% 3 7% 4 4 4 3 4 5% 41%	5045
	11th to 20th	Day	11% 7% 2 6% 4 3 3 3 5% 7% 47%	5090
	21st to end	Day	13% 9% 3 8% 5% 5% 5% 3 5% 6% 37%	5586
April	1st to 10th	Day	11% 9% 4 8% 6% 5% 5% 3 5% 6% 37%	2466
	11th to 20th	Day	11% 8% 2 6% 5% 5% 5% 4 5% 7% 43%	2494
	21st to end	Day	16% 8% 2 8% 5% 4 4 4 6% 6% 39%	2633
May	1st to 10th	Day	16% 7% 3 8% 6% 5% 6% 5% 7% 8% 30%	5875
	11th to 20th	Day	20% 9% 3 7% 5% 5% 5% 4 5% 8% 29%	5861
	21st to end	Day	15% 6% 3 7% 6% 6% 6% 4 6% 8% 33%	6844
June	1st to 10th	Day	11% 5% 2 6% 4 5% 5% 5% 8% 9% 40%	2800
	11th to 20th	Day	10% 5% 2 6% 5% 6% 6% 5% 8% 9% 37%	2800
	21st to end	Day	8% 5% 2 6% 5% 6% 6% 4 8% 10% 41%	2800
July	1st to 10th	Day	4 4 2 6% 6% 5% 5% 5% 7% 10% 44%	6183
	11th to 20th	Day	7% 6% 3 8% 6% 7% 7% 5% 8% 11% 33%	5900
	21st to end	Day	9% 6% 3 7% 6% 6% 6% 5% 7% 9% 37%	6424
August	1st to 10th	Day	13% 7% 3 6% 6% 6% 6% 6% 7% 9% 33%	4234
	11th to 20th	Day	7% 5% 2 7% 5% 6% 6% 5% 8% 9% 39%	4174
	21st to end	Day	7% 4 2 5% 4 5% 5% 4 7% 10% 47%	4315
September	1st to 10th	Day	12% 5% 2 6% 5% 4 5% 4 5% 6% 46%	2400
	11th to 20th	Day	11% 6% 2 6% 4 3 5% 4 5% 7% 46%	2285
	21st to end	Day	20% 7% 3 6% 4 5% 5% 3 5% 5% 36%	2150
October	1st to 10th	Day	14% 7% 2 5% 3 4 4 3 4 6% 47%	2000
	11th to 20th	Day	15% 8% 2 6% 4 3 4 3 4 6% 44%	2000
	21st to end	Day	13% 8% 3 6% 4 4 3 3 5% 6% 45%	2200
November	1st to 10th	Day	9% 6% 2 5% 4 3 4 4 5% 5% 53%	2057
	11th to 20th	Day	11% 9% 3 7% 4 4 4 3 5% 7% 41%	1765
	21st to end	Day	12% 9% 3 7% 6% 5% 4 3 5% 7% 39%	1680
December	1st to 10th	Day	12% 8% 3 6% 4 4 5% 3 5% 6% 44%	2680
	11th to 20th	Day	18% 10% 5% 8% 4 4 4 3 4 6% 34%	3070
	21st to end	Day	16% 9% 3 9% 6% 5% 5% 5% 6% 7% 30%	4048
January	1st to 10th	Night	9% 10% 4 11% 5% 5% 6% 5% 6% 5% 33%	7653
	11th to 20th	Night	8% 9% 3 10% 5% 5% 6% 3 5% 5% 40%	7680
	21st to end	Night	11% 9% 3 7% 4 4 4 4 5% 5% 43%	8401
February	1st to 10th	Night	11% 9% 4 7% 6% 4 5% 4 6% 6% 38%	2730
	11th to 20th	Night	12% 11% 3 9% 6% 6% 6% 3 5% 5% 34%	2663
	21st to end	Night	13% 9% 4 8% 5% 5% 5% 5% 6% 5% 35%	2037
March	1st to 10th	Night	15% 8% 2 7% 4 3 4 3 4 5% 44%	5028
	11th to 20th	Night	8% 9% 3 6% 4 3 3 2 5% 7% 48%	5090
	21st to end	Night	10% 8% 3 8% 4 5% 4 3 5% 6% 45%	5588
April	1st to 10th	Night	13% 9% 3 8% 5% 4 4 4 5% 6% 39%	2153
	11th to 20th	Night	12% 6% 2 5% 5% 4 5% 5% 5% 8% 43%	2075
	21st to end	Night	14% 6% 2 6% 4 4 5% 4 5% 6% 43%	2005
May	1st to 10th	Night	20% 8% 3 8% 5% 4 5% 4 6% 9% 29%	4042
	11th to 20th	Night	22% 9% 3 6% 4 5% 5% 4 5% 7% 31%	4032
	21st to end	Night	15% 7% 3 7% 4 5% 6% 4 6% 8% 34%	3992
June	1st to 10th	Night	10% 5% 2 5% 4 5% 4 4 6% 9% 46%	1600
	11th to 20th	Night	10% 6% 2 6% 4 4 6% 4 7% 10% 43%	1600
	21st to end	Night	7% 5% 2 5% 4 6% 6% 4 7% 10% 45%	1600
July	1st to 10th	Night	6% 5% 2 7% 5% 5% 5% 5% 7% 9% 43%	3770
	11th to 20th	Night	6% 7% 3 10% 6% 7% 7% 6% 8% 9% 32%	4057
	21st to end	Night	7% 6% 2 7% 5% 6% 6% 5% 9% 10% 37%	4427
August	1st to 10th	Night	12% 5% 3 7% 5% 5% 6% 5% 7% 9% 36%	2915
	11th to 20th	Night	9% 5% 2 6% 4 4 7% 4 7% 8% 44%	2875
	21st to end	Night	8% 4 2 6% 4 4 5% 4 7% 8% 49%	3548
September	1st to 10th	Night	12% 5% 1 5% 4 5% 4 4 4 7% 49%	2000
	11th to 20th	Night	10% 5% 2 5% 4 3 4 4 6% 7% 50%	2110
	21st to end	Night	18% 6% 2 6% 3 4 5% 4 5% 7% 40%	2250
October	1st to 10th	Night	15% 5% 2 5% 3 3 4 3 5% 5% 48%	2400
	11th to 20th	Night	12% 7% 2 5% 3 3 4 2 5% 7% 50%	2400
	21st to end	Night	15% 6% 2 6% 4 4 4 3 4 5% 47%	2640
November	1st to 10th	Night	9% 8% 3 5% 4 3 4 4 5% 6% 50%	2461
	11th to 20th	Night	12% 10% 3 7% 4 3 4 4 5% 6% 41%	2849
	21st to end	Night	12% 8% 3 8% 6% 5% 5% 4 4 5% 39%	2940
December	1st to 10th	Night	9% 8% 3 8% 4 4 6% 3 5% 8% 42%	4720
	11th to 20th	Night	15% 10% 3 8% 6% 5% 5% 4 5% 6% 34%	5345
	21st to end	Night	13% 12% 4 9% 5% 5% 6% 5% 6% 7% 28%	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	46% 8% 7% 22% 14% 3	5236
	11th to 20th	Day	45% 7% 8% 17% 17% 6%	5280
	21st to end	Day	44% 7% 10% 25% 11% 3	5780
February	1st to 10th	Day	48% 5% 11% 22% 13% 1	1890
	11th to 20th	Day	50% 5% 10% 20% 13% 2	1852
	21st to end	Day	46% 5% 12% 21% 13% 3	1764
March	1st to 10th	Day	45% 5% 8% 23% 14% 5%	5045
	11th to 20th	Day	36% 5% 10% 27% 17% 5%	5090
	21st to end	Day	47% 6% 9% 19% 16% 3	5586
April	1st to 10th	Day	46% 6% 12% 18% 14% 4	2466
	11th to 20th	Day	40% 6% 11% 21% 18% 5%	2494
	21st to end	Day	45% 5% 10% 25% 12% 2	2633
May	1st to 10th	Day	50% 7% 11% 21% 9% 2	5910
	11th to 20th	Day	52% 8% 9% 23% 7% 1	5865
	21st to end	Day	45% 9% 12% 20% 12% 2	6859
June	1st to 10th	Day	38% 9% 13% 23% 16% 1	2800
	11th to 20th	Day	40% 11% 14% 22% 12% 1	2800
	21st to end	Day	36% 9% 15% 25% 14% 2	2800
July	1st to 10th	Day	34% 10% 13% 26% 14% 3	6116
	11th to 20th	Day	41% 11% 17% 23% 8% 1	5831
	21st to end	Day	41% 7% 11% 24% 16% 1	6240
August	1st to 10th	Day	45% 10% 15% 23% 8%	4221
	11th to 20th	Day	37% 9% 12% 28% 13%	4213
	21st to end	Day	32% 8% 15% 28% 15% 1	4315
September	1st to 10th	Day	38% 7% 12% 19% 21% 3	2400
	11th to 20th	Day	37% 5% 10% 27% 18% 3	2285
	21st to end	Day	48% 7% 9% 20% 15% 1	2150
October	1st to 10th	Day	30% 4 9% 25% 26% 5%	3260
	11th to 20th	Day	35% 5% 8% 24% 19% 10%	3240
	21st to end	Day	28% 4 9% 31% 24% 4	3538
November	1st to 10th	Day	33% 5% 10% 25% 22% 4	2057
	11th to 20th	Day	42% 5% 12% 18% 20% 4	1765
	21st to end	Day	45% 5% 12% 23% 14% 1	1680
December	1st to 10th	Day	40% 7% 12% 26% 11% 4	2680
	11th to 20th	Day	50% 5% 13% 20% 10% 2	3066
	21st to end	Day	49% 8% 15% 16% 12%	4048
January	1st to 10th	Night	47% 9% 9% 20% 12% 1	7654
	11th to 20th	Night	42% 8% 9% 19% 17% 5%	7680
	21st to end	Night	42% 6% 9% 27% 14% 2	8401
February	1st to 10th	Night	45% 6% 13% 23% 12% 1	2730
	11th to 20th	Night	51% 5% 11% 19% 12% 2	2665
	21st to end	Night	48% 6% 11% 20% 11% 4	2037
March	1st to 10th	Night	41% 6% 8% 26% 13% 6%	5028
	11th to 20th	Night	37% 5% 10% 30% 15% 4	5090
	21st to end	Night	40% 6% 12% 22% 17% 3	5588
April	1st to 10th	Night	46% 6% 11% 20% 14% 3	2153
	11th to 20th	Night	38% 8% 13% 22% 15% 4	2075
	21st to end	Night	41% 7% 11% 25% 13% 3	2005
May	1st to 10th	Night	51% 8% 14% 16% 9% 2	4069
	11th to 20th	Night	52% 8% 10% 23% 8%	4033
	21st to end	Night	45% 10% 15% 21% 6% 3	3999
June	1st to 10th	Night	34% 11% 11% 27% 16% 1	1600
	11th to 20th	Night	37% 9% 15% 26% 12% 1	1600
	21st to end	Night	33% 9% 15% 25% 16% 1	1600
July	1st to 10th	Night	35% 11% 19% 21% 11% 3	3729
	11th to 20th	Night	44% 11% 17% 19% 8% 1	4006
	21st to end	Night	37% 10% 15% 24% 14%	4297
August	1st to 10th	Night	41% 10% 17% 24% 7%	2904
	11th to 20th	Night	35% 9% 14% 25% 17% 1	2902
	21st to end	Night	32% 7% 15% 32% 13% 1	3548
September	1st to 10th	Night	34% 7% 15% 21% 19% 4	2000
	11th to 20th	Night	32% 5% 12% 30% 19% 2	2110
	21st to end	Night	44% 7% 12% 21% 16% 1	2250
October	1st to 10th	Night	34% 5% 10% 26% 23% 2	3920
	11th to 20th	Night	30% 5% 10% 23% 25% 7%	3899
	21st to end	Night	27% 4 10% 29% 24% 4	4252
November	1st to 10th	Night	35% 6% 12% 23% 20% 4	2461
	11th to 20th	Night	43% 5% 9% 24% 16% 4	2849
	21st to end	Night	46% 5% 12% 20% 14% 2	2940
December	1st to 10th	Night	38% 8% 14% 27% 11% 1	4720
	11th to 20th	Night	50% 6% 13% 19% 12% 1	5348
	21st to end	Night	50% 10% 14% 16% 10%	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	39% 23% 21% 13% 3 1	325
	11th to 20th	Day	39% 19% 29% 11% 1	330
	21st to end	Day	37% 21% 29% 10% 3	363
February	1st to 10th	Day	36% 22% 26% 13% 2	330
	11th to 20th	Day	49% 17% 28% 5%	330
	21st to end	Day	35% 24% 26% 15% 1	389
March	1st to 10th	Day	30% 22% 23% 21% 3 1	540
	11th to 20th	Day	35% 21% 24% 14% 5% 1	540
	21st to end	Day	30% 20% 31% 14% 4 1	594
April	1st to 10th	Day	29% 22% 26% 16% 6% 1	540
	11th to 20th	Day	24% 20% 29% 16% 9% 2	532
	21st to end	Day	28% 17% 28% 18% 8% 1	540
May	1st to 10th	Day	25% 19% 27% 18% 8% 3	507
	11th to 20th	Day	25% 16% 28% 18% 11% 3	527
	21st to end	Day	19% 12% 26% 21% 17% 6%	573
June	1st to 10th	Day	15% 13% 30% 20% 17% 5%	530
	11th to 20th	Day	17% 10% 26% 22% 19% 6%	530
	21st to end	Day	11% 11% 27% 28% 17% 5%	530
July	1st to 10th	Day	8% 6% 28% 25% 23% 10%	525
	11th to 20th	Day	9% 6% 24% 28% 24% 9%	524
	21st to end	Day	14% 7% 23% 28% 20% 8%	576
August	1st to 10th	Day	13% 7% 20% 28% 24% 8%	526
	11th to 20th	Day	12% 6% 23% 26% 23% 11%	519
	21st to end	Day	10% 6% 22% 28% 24% 10%	583
September	1st to 10th	Day	15% 8% 20% 24% 24% 9%	530
	11th to 20th	Day	14% 4 22% 26% 25% 9%	530
	21st to end	Day	27% 9% 24% 23% 14% 3	530
October	1st to 10th	Day	19% 7% 22% 26% 20% 6%	530
	11th to 20th	Day	23% 15% 23% 23% 13% 3	530
	21st to end	Day	22% 17% 27% 22% 8% 3	583
November	1st to 10th	Day	18% 16% 27% 25% 11% 2	535
	11th to 20th	Day	26% 17% 30% 20% 8%	286
	21st to end	Day	34% 15% 28% 18% 4 1	220
December	1st to 10th	Day	31% 20% 30% 13% 4 1	220
	11th to 20th	Day	31% 19% 24% 20% 6%	220
	21st to end	Day	32% 16% 28% 17% 7%	242
January	1st to 10th	Night	25% 23% 31% 18% 2	840
	11th to 20th	Night	26% 18% 31% 23% 2	840
	21st to end	Night	25% 20% 36% 18% 1	924
February	1st to 10th	Night	23% 18% 31% 25% 2	840
	11th to 20th	Night	31% 24% 29% 15% 1	823
	21st to end	Night	29% 18% 35% 17% 1	692
March	1st to 10th	Night	24% 20% 31% 22% 3	840
	11th to 20th	Night	28% 20% 26% 22% 5% 1	840
	21st to end	Night	24% 17% 34% 19% 5% 1	924
April	1st to 10th	Night	25% 18% 29% 23% 5%	840
	11th to 20th	Night	19% 16% 30% 27% 8%	831
	21st to end	Night	23% 12% 26% 32% 7%	745
May	1st to 10th	Night	22% 9% 24% 31% 13% 1	603
	11th to 20th	Night	21% 7% 24% 35% 11% 1	591
	21st to end	Night	12% 4 24% 36% 22% 2	467
June	1st to 10th	Night	11% 6% 23% 40% 20% 2	400
	11th to 20th	Night	14% 5% 19% 44% 18% 2	400
	21st to end	Night	7% 4 24% 46% 18% 2	400
July	1st to 10th	Night	10% 2 14% 43% 27% 4	455
	11th to 20th	Night	6% 2 11% 42% 34% 4	582
	21st to end	Night	9% 3 15% 39% 32% 2	657
August	1st to 10th	Night	9% 2 13% 45% 29% 2	597
	11th to 20th	Night	10% 3 10% 44% 29% 4	594
	21st to end	Night	10% 3 16% 39% 30% 2	855
September	1st to 10th	Night	14% 3 17% 37% 24% 5%	800
	11th to 20th	Night	13% 4 15% 32% 33% 4	800
	21st to end	Night	21% 7% 19% 32% 20% 1	800
October	1st to 10th	Night	18% 7% 20% 30% 22% 3	800
	11th to 20th	Night	19% 12% 25% 32% 11% 1	800
	21st to end	Night	21% 11% 27% 31% 10% 1	880
November	1st to 10th	Night	17% 10% 26% 35% 11% 1	820
	11th to 20th	Night	24% 14% 29% 25% 8% 1	840
	21st to end	Night	24% 17% 27% 27% 5%	840
December	1st to 10th	Night	26% 16% 30% 24% 4	840
	11th to 20th	Night	22% 17% 31% 27% 3	840
	21st to end	Night	23% 16% 30% 29% 2	924

Page generated at 2026-04-29 02:21:42. Version ac07

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