Star City Observing Site
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Star City Observing Site clear sky chart from 2005-11-06 to 2025-09-04.

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% 5% 5% 5% 4 6% 7% 35%	126040
	Night	14% 9% 3 8% 5% 4 4 4 5% 7% 37%	122699
Transparency	Day	46% 5% 10% 23% 14% 2	129738
	Night	45% 5% 10% 22% 15% 3	127070
Seeing	Day	21% 12% 24% 22% 15% 6%	16032
	Night	20% 10% 22% 29% 17% 2	26058

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	14% 20% 8% 13% 5% 4 3 3 4 5% 21%	4056
	11th to 20th	Day	15% 17% 9% 11% 6% 4 3 2 3 3 28%	4060
	21st to end	Day	18% 14% 4 8% 4 3 4 3 5% 5% 33%	4466
February	1st to 10th	Day	18% 14% 4 9% 4 4 4 3 5% 6% 30%	1836
	11th to 20th	Day	24% 11% 4 9% 4 4 4 3 4 5% 30%	1948
	21st to end	Day	23% 10% 3 8% 4 4 3 4 4 5% 31%	1650
March	1st to 10th	Day	18% 14% 3 8% 5% 4 4 3 4 5% 32%	4751
	11th to 20th	Day	13% 7% 2 7% 4 4 4 4 5% 7% 43%	4980
	21st to end	Day	16% 8% 3 6% 5% 5% 5% 3 6% 7% 37%	5465
April	1st to 10th	Day	15% 9% 3 7% 5% 4 4 4 5% 7% 37%	2200
	11th to 20th	Day	15% 7% 3 7% 4 5% 4 3 5% 7% 42%	2487
	21st to end	Day	17% 8% 2 7% 5% 4 5% 3 5% 7% 37%	2600
May	1st to 10th	Day	15% 9% 3 9% 6% 6% 5% 5% 7% 8% 26%	5862
	11th to 20th	Day	15% 9% 3 9% 6% 6% 6% 5% 6% 8% 28%	5861
	21st to end	Day	13% 7% 3 7% 5% 6% 6% 5% 9% 10% 30%	6433
June	1st to 10th	Day	11% 6% 2 6% 6% 4 6% 4 7% 9% 39%	2600
	11th to 20th	Day	8% 5% 2 7% 5% 6% 7% 5% 7% 9% 39%	2600
	21st to end	Day	9% 6% 2 5% 5% 5% 6% 5% 8% 10% 40%	2600
July	1st to 10th	Day	8% 5% 2 7% 5% 5% 5% 5% 7% 9% 42%	5900
	11th to 20th	Day	10% 6% 2 7% 5% 5% 6% 5% 9% 11% 33%	5900
	21st to end	Day	14% 8% 4 9% 6% 6% 6% 5% 6% 8% 27%	6424
August	1st to 10th	Day	10% 6% 2 7% 5% 4 6% 5% 8% 10% 36%	4234
	11th to 20th	Day	10% 6% 2 7% 5% 5% 5% 4 8% 10% 39%	4174
	21st to end	Day	6% 3 1 4 4 4 5% 4 6% 9% 55%	4461
September	1st to 10th	Day	13% 5% 1 5% 3 4 4 4 6% 8% 45%	2123
	11th to 20th	Day	13% 5% 2 6% 4 4 4 4 5% 8% 44%	2090
	21st to end	Day	19% 6% 2 6% 4 4 3 2 4 5% 44%	2090
October	1st to 10th	Day	13% 5% 2 4 4 4 4 3 5% 7% 49%	2049
	11th to 20th	Day	15% 6% 2 6% 3 4 3 2 5% 6% 50%	1900
	21st to end	Day	20% 7% 3 5% 3 3 3 3 4 7% 43%	2090
November	1st to 10th	Day	13% 6% 2 5% 2 3 3 4 5% 5% 51%	1781
	11th to 20th	Day	14% 9% 3 7% 5% 4 4 4 4 5% 41%	1800
	21st to end	Day	16% 12% 3 7% 3 4 4 3 4 6% 39%	1800
December	1st to 10th	Day	18% 11% 4 9% 5% 4 4 3 4 5% 34%	2930
	11th to 20th	Day	22% 15% 4 8% 5% 4 4 2 4 6% 26%	3373
	21st to end	Day	15% 14% 5% 10% 6% 6% 6% 4 5% 6% 22%	4466
January	1st to 10th	Night	16% 17% 7% 17% 5% 4 3 3 4 5% 20%	5889
	11th to 20th	Night	20% 13% 5% 11% 5% 4 4 3 3 4 29%	5900
	21st to end	Night	19% 10% 4 8% 5% 4 4 3 4 5% 36%	6490
February	1st to 10th	Night	19% 11% 5% 8% 5% 4 4 3 5% 6% 31%	2560
	11th to 20th	Night	24% 12% 4 8% 4 3 3 3 4 5% 29%	2338
	21st to end	Night	22% 12% 4 7% 5% 4 4 3 5% 4 29%	1980
March	1st to 10th	Night	19% 11% 3 6% 4 4 4 3 4 6% 35%	5102
	11th to 20th	Night	12% 7% 2 5% 3 3 3 4 5% 8% 48%	4980
	21st to end	Night	13% 8% 3 7% 6% 5% 5% 4 6% 7% 36%	5467
April	1st to 10th	Night	14% 9% 3 7% 6% 4 4 4 5% 7% 39%	2200
	11th to 20th	Night	14% 8% 2 6% 3 4 4 4 6% 8% 42%	1879
	21st to end	Night	15% 8% 3 6% 5% 5% 4 3 5% 8% 39%	1800
May	1st to 10th	Night	16% 9% 3 8% 6% 6% 5% 3 6% 9% 29%	4033
	11th to 20th	Night	14% 7% 3 8% 6% 5% 6% 4 6% 8% 31%	4032
	21st to end	Night	14% 8% 3 8% 5% 5% 5% 4 6% 8% 35%	4421
June	1st to 10th	Night	9% 5% 2 5% 4 4 6% 5% 8% 10% 43%	1800
	11th to 20th	Night	6% 5% 2 6% 4 5% 6% 5% 8% 10% 43%	1800
	21st to end	Night	9% 5% 1 6% 4 5% 6% 5% 7% 10% 42%	1800
July	1st to 10th	Night	8% 5% 2 7% 6% 5% 6% 5% 8% 10% 38%	4058
	11th to 20th	Night	8% 6% 2 7% 5% 5% 6% 5% 8% 10% 38%	4057
	21st to end	Night	11% 8% 3 10% 6% 5% 6% 5% 6% 9% 30%	4427
August	1st to 10th	Night	9% 6% 2 7% 5% 5% 5% 4 8% 10% 38%	2915
	11th to 20th	Night	10% 6% 3 6% 5% 4 5% 4 6% 9% 41%	2875
	21st to end	Night	5% 3 1 4 3 3 4 4 6% 8% 58%	3400
September	1st to 10th	Night	11% 4 2 5% 4 4 5% 3 7% 8% 47%	2124
	11th to 20th	Night	12% 5% 2 7% 5% 3 4 3 4 6% 49%	2090
	21st to end	Night	12% 6% 2 6% 4 3 4 4 5% 7% 47%	2090
October	1st to 10th	Night	12% 5% 2 5% 4 3 3 3 5% 6% 53%	2129
	11th to 20th	Night	16% 6% 2 6% 3 3 3 2 4 5% 49%	2280
	21st to end	Night	18% 7% 2 5% 4 3 3 3 5% 6% 44%	2508
November	1st to 10th	Night	14% 6% 2 5% 4 4 4 2 3 6% 50%	2513
	11th to 20th	Night	15% 9% 2 7% 3 4 4 3 5% 6% 42%	2600
	21st to end	Night	15% 10% 3 5% 4 4 3 4 5% 6% 41%	2600
December	1st to 10th	Night	17% 14% 5% 9% 5% 4 3 3 4 4 33%	4250
	11th to 20th	Night	20% 14% 5% 8% 5% 4 3 3 4 5% 29%	4822
	21st to end	Night	16% 16% 4 9% 6% 5% 5% 3 6% 7% 23%	6490

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	66% 4 6% 18% 7%	4057
	11th to 20th	Day	63% 2 5% 15% 13% 1	4060
	21st to end	Day	54% 3 7% 26% 9% 1	4466
February	1st to 10th	Day	57% 3 8% 20% 11% 1	1836
	11th to 20th	Day	58% 3 7% 18% 12% 2	1958
	21st to end	Day	56% 4 7% 17% 13% 3	1650
March	1st to 10th	Day	55% 4 5% 16% 16% 5%	4751
	11th to 20th	Day	40% 3 8% 21% 25% 3	4980
	21st to end	Day	46% 4 9% 26% 14% 1	5465
April	1st to 10th	Day	45% 5% 10% 22% 14% 3	2200
	11th to 20th	Day	43% 4 9% 22% 18% 4	2487
	21st to end	Day	47% 5% 10% 24% 11% 3	2600
May	1st to 10th	Day	53% 6% 11% 19% 10% 1	5897
	11th to 20th	Day	52% 7% 11% 21% 9% 1	5865
	21st to end	Day	44% 8% 14% 20% 12% 2	6447
June	1st to 10th	Day	39% 7% 13% 23% 16% 1	2600
	11th to 20th	Day	40% 9% 13% 20% 17% 2	2600
	21st to end	Day	36% 9% 15% 22% 16% 3	2600
July	1st to 10th	Day	37% 8% 15% 23% 16% 1	5835
	11th to 20th	Day	41% 9% 16% 26% 7% 1	5831
	21st to end	Day	52% 7% 10% 20% 9% 1	6240
August	1st to 10th	Day	40% 8% 15% 27% 10%	4221
	11th to 20th	Day	38% 7% 12% 30% 12% 1	4213
	21st to end	Day	26% 6% 14% 34% 19% 2	4461
September	1st to 10th	Day	36% 6% 11% 23% 18% 5%	2123
	11th to 20th	Day	38% 5% 10% 27% 17% 2	2090
	21st to end	Day	43% 4 10% 22% 20% 2	2090
October	1st to 10th	Day	29% 4 8% 22% 34% 3	3400
	11th to 20th	Day	26% 2 10% 28% 29% 3	3140
	21st to end	Day	30% 2 9% 32% 20% 7%	3428
November	1st to 10th	Day	34% 4 9% 24% 25% 3	1781
	11th to 20th	Day	45% 3 6% 23% 19% 4	1800
	21st to end	Day	47% 3 9% 22% 17% 2	1800
December	1st to 10th	Day	53% 4 6% 21% 15% 1	2930
	11th to 20th	Day	61% 3 8% 17% 11% 1	3370
	21st to end	Day	61% 5% 9% 20% 5%	4466
January	1st to 10th	Night	67% 3 8% 14% 7% 1	5889
	11th to 20th	Night	58% 3 4 17% 14% 3	5900
	21st to end	Night	51% 3 7% 24% 14% 2	6490
February	1st to 10th	Night	55% 3 10% 19% 11% 1	2560
	11th to 20th	Night	58% 3 8% 17% 11% 2	2339
	21st to end	Night	57% 4 8% 16% 12% 3	1980
March	1st to 10th	Night	51% 3 5% 21% 16% 4	5102
	11th to 20th	Night	35% 3 7% 27% 23% 3	4980
	21st to end	Night	47% 5% 9% 23% 15% 1	5467
April	1st to 10th	Night	44% 5% 10% 21% 17% 3	2200
	11th to 20th	Night	40% 5% 10% 23% 16% 7%	1879
	21st to end	Night	44% 6% 11% 24% 14% 2	1800
May	1st to 10th	Night	50% 7% 12% 20% 9% 1	4060
	11th to 20th	Night	50% 7% 14% 21% 8% 1	4033
	21st to end	Night	44% 8% 14% 20% 13% 2	4429
June	1st to 10th	Night	33% 8% 16% 22% 18% 3	1800
	11th to 20th	Night	33% 9% 17% 20% 18% 2	1800
	21st to end	Night	34% 9% 15% 22% 16% 4	1800
July	1st to 10th	Night	37% 10% 14% 24% 13% 2	4015
	11th to 20th	Night	38% 10% 18% 24% 9% 2	4006
	21st to end	Night	48% 7% 13% 20% 11% 1	4297
August	1st to 10th	Night	38% 9% 14% 26% 12% 1	2904
	11th to 20th	Night	39% 6% 12% 29% 14% 1	2902
	21st to end	Night	23% 7% 13% 33% 23% 3	3400
September	1st to 10th	Night	34% 7% 12% 22% 16% 8%	2124
	11th to 20th	Night	37% 6% 10% 22% 22% 3	2090
	21st to end	Night	36% 6% 10% 24% 22% 2	2090
October	1st to 10th	Night	28% 4 7% 23% 33% 4	3558
	11th to 20th	Night	29% 3 6% 26% 27% 9%	3779
	21st to end	Night	29% 3 7% 26% 27% 8%	4120
November	1st to 10th	Night	37% 4 8% 22% 24% 6%	2513
	11th to 20th	Night	43% 4 8% 20% 20% 5%	2600
	21st to end	Night	44% 4 8% 26% 16% 2	2600
December	1st to 10th	Night	55% 4 8% 20% 12% 1	4250
	11th to 20th	Night	58% 3 8% 20% 9% 1	4824
	21st to end	Night	59% 5% 10% 18% 8%	6490

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	30% 24% 29% 14% 3 1	300
	11th to 20th	Day	39% 23% 24% 12% 1	300
	21st to end	Day	29% 27% 28% 13% 2 1	330
February	1st to 10th	Day	27% 21% 34% 12% 5%	300
	11th to 20th	Day	46% 22% 24% 6% 1 1	300
	21st to end	Day	35% 21% 32% 12% 1	249
March	1st to 10th	Day	32% 18% 29% 15% 6%	415
	11th to 20th	Day	27% 21% 30% 17% 4 1	500
	21st to end	Day	29% 19% 29% 17% 5%	550
April	1st to 10th	Day	30% 18% 25% 17% 9% 1	500
	11th to 20th	Day	21% 22% 28% 17% 10% 2	492
	21st to end	Day	25% 14% 32% 20% 8% 2	500
May	1st to 10th	Day	22% 18% 29% 18% 10% 4	503
	11th to 20th	Day	17% 12% 27% 25% 13% 5%	527
	21st to end	Day	16% 7% 26% 22% 20% 9%	573
June	1st to 10th	Day	13% 7% 24% 27% 21% 8%	530
	11th to 20th	Day	11% 7% 19% 30% 22% 11%	530
	21st to end	Day	12% 4 24% 26% 20% 13%	530
July	1st to 10th	Day	14% 4 17% 25% 21% 20%	525
	11th to 20th	Day	8% 5% 19% 29% 24% 15%	524
	21st to end	Day	15% 6% 15% 28% 24% 12%	576
August	1st to 10th	Day	16% 5% 17% 24% 27% 12%	526
	11th to 20th	Day	13% 6% 14% 29% 27% 12%	519
	21st to end	Day	11% 3 14% 26% 31% 15%	583
September	1st to 10th	Day	14% 3 15% 23% 32% 13%	502
	11th to 20th	Day	15% 3 13% 28% 26% 14%	490
	21st to end	Day	22% 9% 19% 27% 17% 7%	490
October	1st to 10th	Day	14% 8% 22% 26% 22% 8%	490
	11th to 20th	Day	17% 11% 25% 26% 16% 5%	490
	21st to end	Day	25% 13% 22% 26% 13% 2	539
November	1st to 10th	Day	19% 13% 30% 25% 11% 3	319
	11th to 20th	Day	26% 15% 29% 19% 11% 1	300
	21st to end	Day	27% 17% 27% 20% 8% 1	300
December	1st to 10th	Day	32% 19% 28% 15% 5% 1	300
	11th to 20th	Day	30% 18% 28% 19% 4 2	300
	21st to end	Day	29% 18% 32% 14% 6% 1	330
January	1st to 10th	Night	28% 18% 28% 21% 5%	800
	11th to 20th	Night	32% 17% 27% 21% 4	800
	21st to end	Night	25% 17% 33% 24% 2	880
February	1st to 10th	Night	26% 15% 30% 27% 2	800
	11th to 20th	Night	34% 17% 30% 18% 2	783
	21st to end	Night	32% 17% 30% 20% 2	660
March	1st to 10th	Night	28% 15% 25% 27% 5%	800
	11th to 20th	Night	26% 15% 29% 23% 7%	800
	21st to end	Night	20% 15% 31% 26% 7%	880
April	1st to 10th	Night	21% 16% 27% 28% 8%	800
	11th to 20th	Night	20% 12% 25% 29% 13% 1	620
	21st to end	Night	17% 11% 32% 31% 9% 1	600
May	1st to 10th	Night	20% 8% 27% 32% 11% 2	600
	11th to 20th	Night	16% 8% 23% 34% 18% 2	591
	21st to end	Night	11% 5% 17% 33% 28% 6%	650
June	1st to 10th	Night	12% 3 17% 37% 27% 3	600
	11th to 20th	Night	8% 3 15% 40% 29% 5%	600
	21st to end	Night	10% 2 18% 35% 31% 5%	600
July	1st to 10th	Night	13% 3 10% 32% 36% 6%	585
	11th to 20th	Night	8% 3 10% 33% 42% 5%	582
	21st to end	Night	11% 4 9% 33% 35% 8%	657
August	1st to 10th	Night	11% 3 9% 34% 39% 5%	597
	11th to 20th	Night	12% 2 8% 32% 38% 8%	594
	21st to end	Night	10% 2 7% 32% 41% 8%	670
September	1st to 10th	Night	12% 3 10% 31% 38% 6%	773
	11th to 20th	Night	14% 2 14% 27% 38% 4	760
	21st to end	Night	15% 5% 19% 29% 29% 3	760
October	1st to 10th	Night	14% 4 17% 31% 30% 4	760
	11th to 20th	Night	19% 8% 20% 32% 18% 2	760
	21st to end	Night	22% 9% 23% 32% 14% 1	836
November	1st to 10th	Night	16% 9% 23% 33% 17% 2	780
	11th to 20th	Night	20% 14% 25% 30% 11% 1	800
	21st to end	Night	22% 12% 21% 34% 11% 1	800
December	1st to 10th	Night	30% 15% 25% 26% 5%	800
	11th to 20th	Night	26% 12% 28% 28% 6% 1	800
	21st to end	Night	30% 14% 22% 30% 4	880

Page generated at 2025-09-01 01:51:04. Version ac07

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