Starry Night Astronomy & Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Starry Night Astronomy & Observatory clear sky chart from 2005-11-06 to 2026-02-01.

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	14% 9% 3 8% 6% 5% 5% 4 6% 7% 32%	127465
	Night	14% 9% 3 9% 6% 5% 5% 4 6% 7% 32%	124535
Transparency	Day	48% 8% 13% 19% 11% 1	130959
	Night	47% 9% 13% 19% 11% 1	129128
Seeing	Day	24% 18% 29% 18% 8% 2	16277
	Night	20% 15% 29% 26% 9% 1	25339

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	9% 9% 4 17% 12% 6% 5% 4 5% 6% 24%	3677
	11th to 20th	Day	10% 13% 5% 12% 9% 6% 5% 3 5% 6% 26%	3650
	21st to end	Day	10% 12% 6% 10% 6% 5% 5% 4 6% 6% 30%	4048
February	1st to 10th	Day	11% 11% 4 10% 7% 7% 6% 4 7% 7% 27%	1765
	11th to 20th	Day	9% 10% 5% 12% 7% 6% 5% 5% 6% 7% 28%	1948
	21st to end	Day	13% 12% 4 9% 7% 6% 5% 4 4 6% 30%	1650
March	1st to 10th	Day	15% 14% 5% 12% 6% 5% 5% 3 4 4 27%	4476
	11th to 20th	Day	12% 10% 4 8% 6% 5% 6% 4 6% 6% 33%	4646
	21st to end	Day	17% 12% 3 9% 5% 4 5% 4 4 6% 30%	5970
April	1st to 10th	Day	18% 11% 3 9% 5% 4 4 3 4 6% 33%	2400
	11th to 20th	Day	15% 10% 3 7% 6% 5% 4 4 6% 7% 34%	2374
	21st to end	Day	20% 10% 4 8% 5% 4 4 4 5% 6% 31%	2580
May	1st to 10th	Day	29% 12% 4 8% 5% 5% 4 3 5% 5% 21%	5904
	11th to 20th	Day	16% 11% 3 8% 6% 5% 5% 4 6% 7% 27%	6318
	21st to end	Day	18% 7% 3 7% 5% 5% 5% 4 7% 8% 32%	6949
June	1st to 10th	Day	18% 8% 2 8% 5% 5% 5% 4 7% 8% 29%	2800
	11th to 20th	Day	17% 6% 2 6% 5% 5% 6% 4 6% 9% 34%	2800
	21st to end	Day	17% 8% 3 7% 5% 6% 6% 4 7% 8% 29%	2800
July	1st to 10th	Day	11% 6% 2 6% 6% 5% 5% 5% 7% 10% 35%	6360
	11th to 20th	Day	8% 6% 3 7% 5% 6% 6% 5% 9% 11% 35%	6360
	21st to end	Day	12% 7% 2 6% 4 5% 5% 4 6% 8% 40%	6926
August	1st to 10th	Day	12% 6% 3 7% 6% 6% 5% 5% 7% 9% 34%	4485
	11th to 20th	Day	13% 9% 3 8% 5% 6% 5% 4 6% 9% 33%	4046
	21st to end	Day	9% 5% 2 6% 4 5% 5% 5% 7% 10% 41%	4312
September	1st to 10th	Day	11% 5% 2 5% 4 4 5% 4 6% 9% 45%	2400
	11th to 20th	Day	11% 6% 2 6% 4 4 4 3 5% 7% 49%	2259
	21st to end	Day	16% 6% 2 5% 4 4 4 4 6% 7% 43%	2200
October	1st to 10th	Day	19% 7% 2 7% 6% 4 5% 3 5% 6% 38%	2160
	11th to 20th	Day	14% 8% 4 8% 6% 4 4 3 5% 7% 38%	1930
	21st to end	Day	21% 11% 3 7% 5% 5% 4 4 5% 6% 30%	1980
November	1st to 10th	Day	11% 9% 3 7% 5% 5% 5% 4 7% 7% 37%	1851
	11th to 20th	Day	14% 10% 3 8% 5% 5% 5% 4 6% 7% 34%	1890
	21st to end	Day	16% 11% 4 10% 4 5% 5% 4 6% 6% 29%	1757
December	1st to 10th	Day	11% 14% 5% 15% 8% 6% 6% 4 6% 6% 19%	2680
	11th to 20th	Day	16% 10% 4 9% 6% 5% 6% 4 6% 6% 28%	3066
	21st to end	Day	16% 12% 4 10% 6% 6% 5% 4 5% 5% 28%	4048
January	1st to 10th	Night	10% 8% 4 15% 11% 6% 5% 5% 5% 6% 25%	6489
	11th to 20th	Night	8% 11% 5% 15% 9% 7% 6% 4 6% 7% 22%	6455
	21st to end	Night	11% 9% 4 11% 7% 6% 6% 4 6% 7% 29%	7150
February	1st to 10th	Night	10% 12% 4 10% 7% 5% 5% 5% 7% 8% 27%	2629
	11th to 20th	Night	9% 9% 4 10% 7% 7% 6% 5% 7% 7% 29%	2338
	21st to end	Night	11% 10% 3 10% 6% 5% 5% 4 7% 7% 32%	1980
March	1st to 10th	Night	18% 11% 2 8% 6% 5% 5% 4 6% 6% 28%	5364
	11th to 20th	Night	11% 9% 4 10% 6% 6% 6% 4 5% 7% 33%	5297
	21st to end	Night	17% 13% 3 9% 5% 4 4 4 5% 5% 31%	4962
April	1st to 10th	Night	16% 9% 3 8% 6% 6% 5% 4 6% 6% 33%	2000
	11th to 20th	Night	16% 9% 3 8% 5% 5% 5% 4 6% 6% 34%	1975
	21st to end	Night	18% 9% 3 6% 4 4 4 3 6% 6% 34%	1820
May	1st to 10th	Night	32% 11% 3 7% 5% 4 4 3 5% 6% 21%	4054
	11th to 20th	Night	14% 12% 3 9% 6% 5% 5% 4 5% 6% 30%	3585
	21st to end	Night	16% 7% 3 8% 6% 5% 6% 4 6% 7% 31%	3926
June	1st to 10th	Night	17% 7% 2 6% 4 5% 6% 4 6% 8% 35%	1600
	11th to 20th	Night	18% 6% 2 5% 4 4 5% 5% 5% 7% 40%	1600
	21st to end	Night	16% 5% 2 5% 5% 5% 6% 4 6% 8% 37%	1600
July	1st to 10th	Night	9% 6% 2 7% 5% 6% 4 5% 7% 10% 38%	3600
	11th to 20th	Night	11% 5% 2 9% 5% 5% 5% 4 6% 8% 39%	3600
	21st to end	Night	9% 6% 2 6% 4 4 5% 4 6% 6% 48%	3926
August	1st to 10th	Night	16% 8% 2 8% 5% 5% 4 4 5% 6% 37%	2619
	11th to 20th	Night	14% 8% 2 6% 5% 4 4 4 5% 8% 40%	2997
	21st to end	Night	9% 4 2 5% 4 3 4 4 7% 10% 48%	3586
September	1st to 10th	Night	11% 6% 1 5% 4 4 4 4 6% 7% 49%	2000
	11th to 20th	Night	11% 5% 2 6% 4 4 3 3 5% 6% 52%	2140
	21st to end	Night	13% 6% 2 5% 3 4 5% 4 6% 7% 46%	2200
October	1st to 10th	Night	18% 7% 3 7% 4 3 4 3 6% 7% 38%	2237
	11th to 20th	Night	15% 8% 3 6% 4 5% 5% 4 6% 8% 37%	2470
	21st to end	Night	20% 9% 3 6% 4 5% 5% 4 6% 8% 30%	2860
November	1st to 10th	Night	12% 9% 3 9% 6% 5% 5% 3 6% 6% 37%	2667
	11th to 20th	Night	14% 7% 3 7% 6% 5% 6% 5% 6% 7% 35%	2730
	21st to end	Night	12% 10% 4 8% 6% 4 5% 4 7% 7% 33%	2861
December	1st to 10th	Night	11% 12% 4 15% 8% 7% 6% 4 5% 7% 20%	4720
	11th to 20th	Night	18% 10% 4 10% 6% 5% 4 3 5% 6% 29%	5348
	21st to end	Night	15% 14% 5% 11% 6% 6% 5% 4 5% 5% 24%	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	55% 9% 13% 18% 5%	3678
	11th to 20th	Day	56% 8% 10% 14% 9% 3	3680
	21st to end	Day	51% 8% 9% 17% 13% 2	4048
February	1st to 10th	Day	53% 9% 13% 15% 8% 2	1765
	11th to 20th	Day	52% 10% 13% 16% 8% 2	1957
	21st to end	Day	53% 8% 11% 15% 11% 1	1650
March	1st to 10th	Day	58% 7% 8% 12% 9% 6%	4479
	11th to 20th	Day	46% 10% 15% 18% 8% 2	4646
	21st to end	Day	52% 7% 7% 13% 17% 4	5970
April	1st to 10th	Day	52% 6% 10% 19% 11% 3	2400
	11th to 20th	Day	47% 9% 11% 17% 14% 1	2374
	21st to end	Day	53% 8% 9% 17% 12% 3	2580
May	1st to 10th	Day	64% 8% 8% 14% 5% 1	5915
	11th to 20th	Day	51% 12% 14% 14% 8% 1	6322
	21st to end	Day	46% 11% 14% 22% 8% 1	6949
June	1st to 10th	Day	50% 10% 15% 16% 9%	2800
	11th to 20th	Day	45% 9% 13% 19% 13% 2	2800
	21st to end	Day	51% 10% 11% 17% 10% 1	2800
July	1st to 10th	Day	41% 8% 16% 23% 12%	6290
	11th to 20th	Day	39% 9% 17% 23% 12% 1	6285
	21st to end	Day	38% 8% 14% 29% 11%	6728
August	1st to 10th	Day	44% 10% 16% 20% 11%	4471
	11th to 20th	Day	45% 9% 16% 24% 5%	4085
	21st to end	Day	34% 8% 18% 27% 13% 1	4312
September	1st to 10th	Day	34% 8% 15% 28% 14% 1	2400
	11th to 20th	Day	35% 7% 11% 29% 18% 1	2259
	21st to end	Day	37% 9% 14% 25% 12% 2	2200
October	1st to 10th	Day	42% 7% 12% 21% 16% 1	3524
	11th to 20th	Day	41% 5% 14% 25% 14% 1	3120
	21st to end	Day	40% 7% 10% 29% 13% 1	3180
November	1st to 10th	Day	42% 9% 15% 21% 13%	1851
	11th to 20th	Day	46% 9% 13% 16% 14% 2	1890
	21st to end	Day	54% 9% 15% 16% 5% 1	1757
December	1st to 10th	Day	61% 8% 14% 13% 4	2680
	11th to 20th	Day	53% 7% 11% 17% 11% 1	3066
	21st to end	Day	57% 7% 7% 13% 13% 3	4048
January	1st to 10th	Night	53% 10% 14% 15% 8%	6489
	11th to 20th	Night	56% 12% 12% 12% 8% 1	6500
	21st to end	Night	51% 9% 9% 14% 14% 3	7150
February	1st to 10th	Night	51% 11% 13% 16% 8% 1	2629
	11th to 20th	Night	48% 11% 13% 17% 9% 1	2340
	21st to end	Night	48% 10% 14% 18% 8% 2	1980
March	1st to 10th	Night	52% 7% 11% 14% 13% 4	5374
	11th to 20th	Night	48% 10% 17% 15% 9% 2	5298
	21st to end	Night	52% 7% 7% 15% 18% 2	4962
April	1st to 10th	Night	49% 8% 11% 18% 12% 2	2000
	11th to 20th	Night	48% 8% 11% 20% 13% 1	1975
	21st to end	Night	48% 8% 10% 18% 12% 3	1820
May	1st to 10th	Night	61% 10% 10% 13% 7% 1	4060
	11th to 20th	Night	50% 13% 11% 17% 8% 1	3586
	21st to end	Night	48% 12% 14% 16% 10% 1	3927
June	1st to 10th	Night	44% 13% 14% 17% 12% 1	1600
	11th to 20th	Night	43% 11% 15% 18% 12% 2	1600
	21st to end	Night	44% 11% 13% 23% 9% 1	1600
July	1st to 10th	Night	37% 8% 20% 23% 11%	3560
	11th to 20th	Night	40% 9% 16% 22% 12%	3552
	21st to end	Night	33% 7% 15% 34% 10% 1	3809
August	1st to 10th	Night	47% 9% 13% 21% 9% 1	2609
	11th to 20th	Night	41% 7% 19% 22% 10%	3024
	21st to end	Night	27% 8% 21% 32% 12% 1	3586
September	1st to 10th	Night	33% 9% 16% 28% 14% 1	2000
	11th to 20th	Night	32% 7% 13% 29% 17% 1	2140
	21st to end	Night	36% 9% 14% 24% 15% 2	2200
October	1st to 10th	Night	45% 7% 13% 22% 11% 1	3653
	11th to 20th	Night	35% 6% 17% 25% 15% 3	4019
	21st to end	Night	42% 6% 12% 29% 10% 1	4610
November	1st to 10th	Night	45% 8% 13% 21% 10% 2	2667
	11th to 20th	Night	44% 10% 14% 19% 11% 2	2730
	21st to end	Night	47% 11% 16% 18% 7%	2861
December	1st to 10th	Night	58% 8% 16% 15% 3	4720
	11th to 20th	Night	55% 7% 10% 13% 14%	5348
	21st to end	Night	57% 8% 10% 14% 9% 3	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	24% 25% 30% 16% 4	220
	11th to 20th	Day	32% 23% 28% 13% 2 1	220
	21st to end	Day	22% 28% 31% 16% 4	242
February	1st to 10th	Day	33% 30% 28% 8% 1	290
	11th to 20th	Day	29% 26% 30% 13% 2	496
	21st to end	Day	28% 24% 30% 15% 3	414
March	1st to 10th	Day	30% 20% 33% 14% 3	500
	11th to 20th	Day	32% 26% 26% 11% 4 1	500
	21st to end	Day	33% 24% 24% 11% 5% 3	550
April	1st to 10th	Day	37% 19% 26% 12% 5% 1	500
	11th to 20th	Day	35% 23% 23% 11% 8% 1	492
	21st to end	Day	31% 24% 27% 11% 6% 1	500
May	1st to 10th	Day	34% 21% 24% 13% 6% 2	503
	11th to 20th	Day	19% 21% 33% 20% 5% 2	527
	21st to end	Day	21% 16% 32% 22% 8% 1	573
June	1st to 10th	Day	23% 17% 37% 15% 8% 1	530
	11th to 20th	Day	20% 17% 34% 19% 9% 1	530
	21st to end	Day	20% 19% 32% 19% 7% 2	530
July	1st to 10th	Day	16% 13% 33% 25% 10% 3	525
	11th to 20th	Day	13% 10% 36% 27% 12% 3	524
	21st to end	Day	14% 10% 39% 24% 11% 2	576
August	1st to 10th	Day	12% 12% 30% 26% 17% 4	526
	11th to 20th	Day	15% 12% 26% 25% 14% 8%	519
	21st to end	Day	11% 16% 27% 26% 14% 5%	583
September	1st to 10th	Day	13% 14% 27% 24% 16% 6%	530
	11th to 20th	Day	12% 14% 26% 18% 20% 11%	530
	21st to end	Day	19% 12% 29% 22% 12% 6%	530
October	1st to 10th	Day	24% 14% 23% 20% 14% 5%	530
	11th to 20th	Day	21% 20% 24% 21% 11% 2	491
	21st to end	Day	32% 17% 26% 19% 6% 1	462
November	1st to 10th	Day	25% 19% 23% 22% 10% 1	425
	11th to 20th	Day	27% 21% 28% 16% 7% 1	430
	21st to end	Day	33% 21% 26% 16% 4	297
December	1st to 10th	Day	29% 18% 34% 15% 4	220
	11th to 20th	Day	33% 25% 30% 10% 2	220
	21st to end	Day	31% 19% 28% 19% 2	242
January	1st to 10th	Night	20% 21% 36% 20% 3	840
	11th to 20th	Night	23% 20% 32% 22% 3	840
	21st to end	Night	16% 19% 37% 23% 4	924
February	1st to 10th	Night	18% 22% 38% 20% 2	801
	11th to 20th	Night	21% 21% 36% 20% 2	783
	21st to end	Night	21% 20% 34% 22% 3	660
March	1st to 10th	Night	23% 18% 34% 24% 3	800
	11th to 20th	Night	23% 20% 30% 22% 5%	800
	21st to end	Night	26% 16% 30% 21% 7%	880
April	1st to 10th	Night	24% 22% 28% 22% 5%	800
	11th to 20th	Night	22% 24% 27% 20% 7%	791
	21st to end	Night	27% 18% 28% 21% 5%	620
May	1st to 10th	Night	29% 14% 29% 22% 7%	600
	11th to 20th	Night	15% 15% 31% 31% 6% 1	404
	21st to end	Night	17% 10% 27% 35% 11%	433
June	1st to 10th	Night	18% 10% 31% 32% 10%	400
	11th to 20th	Night	18% 11% 25% 35% 11%	400
	21st to end	Night	17% 7% 27% 36% 12% 1	400
July	1st to 10th	Night	15% 6% 19% 46% 13% 2	390
	11th to 20th	Night	15% 3 24% 38% 19% 1	388
	21st to end	Night	13% 5% 24% 42% 16%	438
August	1st to 10th	Night	14% 5% 17% 42% 22%	418
	11th to 20th	Night	15% 7% 22% 35% 18% 3	665
	21st to end	Night	12% 7% 22% 36% 22% 1	880
September	1st to 10th	Night	14% 8% 26% 32% 19% 2	800
	11th to 20th	Night	14% 9% 20% 30% 23% 5%	800
	21st to end	Night	16% 9% 24% 30% 19% 2	800
October	1st to 10th	Night	23% 12% 19% 29% 16% 2	800
	11th to 20th	Night	20% 14% 24% 30% 11% 1	800
	21st to end	Night	25% 16% 27% 25% 6%	880
November	1st to 10th	Night	21% 17% 28% 26% 8%	820
	11th to 20th	Night	19% 18% 29% 25% 8% 1	840
	21st to end	Night	21% 21% 28% 25% 5%	840
December	1st to 10th	Night	24% 18% 35% 19% 4	840
	11th to 20th	Night	25% 20% 30% 22% 4	840
	21st to end	Night	26% 18% 34% 19% 3	924

Page generated at 2026-01-29 01:21:43. Version ac07

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