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 2024-05-13.

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	15% 9% 3 7% 5% 4 4 4 6% 7% 36%	83916
	Night	15% 9% 3 7% 5% 4 4 3 5% 7% 37%	85604
Transparency	Day	47% 5% 11% 22% 13% 2	83639
	Night	46% 5% 10% 22% 15% 3	85400
Seeing	Day	21% 12% 24% 22% 15% 7%	14240
	Night	20% 10% 22% 29% 17% 2	24395

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	15% 18% 8% 10% 4 4 4 3 5% 6% 23%	2836
	11th to 20th	Day	18% 16% 8% 12% 7% 4 3 2 3 3 23%	2840
	21st to end	Day	22% 19% 4 8% 4 4 4 3 4 5% 25%	3124
February	1st to 10th	Day	18% 14% 4 9% 4 4 4 3 5% 6% 30%	1744
	11th to 20th	Day	23% 10% 4 9% 4 4 4 3 4 5% 30%	1848
	21st to end	Day	24% 11% 3 8% 4 4 3 4 4 5% 30%	1570
March	1st to 10th	Day	22% 14% 3 7% 4 4 4 3 5% 5% 29%	3307
	11th to 20th	Day	14% 7% 3 7% 4 4 4 3 5% 6% 44%	3480
	21st to end	Day	17% 8% 3 5% 4 4 4 4 6% 7% 37%	3815
April	1st to 10th	Day	14% 9% 3 7% 5% 4 4 4 5% 7% 37%	2090
	11th to 20th	Day	15% 7% 3 7% 4 4 3 3 5% 6% 42%	2361
	21st to end	Day	17% 8% 2 7% 4 4 5% 3 5% 7% 38%	2470
May	1st to 10th	Day	16% 8% 2 8% 6% 5% 4 5% 6% 7% 33%	2470
	11th to 20th	Day	16% 8% 3 7% 6% 6% 5% 5% 5% 7% 33%	2329
	21st to end	Day	10% 5% 2 6% 6% 5% 5% 5% 8% 9% 38%	2531
June	1st to 10th	Day	11% 6% 2 6% 5% 4 6% 4 7% 9% 39%	2340
	11th to 20th	Day	8% 5% 2 8% 5% 6% 6% 5% 7% 9% 39%	2340
	21st to end	Day	10% 7% 2 5% 5% 5% 5% 5% 8% 9% 39%	2340
July	1st to 10th	Day	11% 6% 2 7% 5% 5% 5% 5% 7% 9% 38%	2340
	11th to 20th	Day	9% 7% 2 6% 6% 5% 6% 5% 9% 10% 35%	2340
	21st to end	Day	12% 6% 3 8% 6% 6% 5% 5% 6% 9% 34%	2547
August	1st to 10th	Day	11% 6% 2 7% 5% 5% 6% 5% 8% 11% 33%	2327
	11th to 20th	Day	13% 7% 2 7% 6% 5% 6% 4 8% 8% 34%	2303
	21st to end	Day	10% 5% 1 6% 5% 6% 7% 4 6% 10% 40%	2465
September	1st to 10th	Day	13% 5% 2 6% 4 4 4 4 6% 8% 44%	1980
	11th to 20th	Day	13% 5% 2 6% 4 4 4 4 5% 8% 44%	1980
	21st to end	Day	18% 6% 2 6% 4 4 3 3 4 5% 46%	1980
October	1st to 10th	Day	13% 5% 2 5% 4 4 4 3 5% 7% 48%	1942
	11th to 20th	Day	15% 6% 2 6% 3 4 3 2 5% 6% 49%	1800
	21st to end	Day	21% 8% 3 5% 4 3 3 3 4 7% 42%	1980
November	1st to 10th	Day	12% 6% 2 5% 2 3 3 4 5% 5% 52%	1690
	11th to 20th	Day	13% 9% 3 7% 5% 3 4 4 4 5% 42%	1710
	21st to end	Day	16% 10% 3 7% 3 4 4 3 4 6% 40%	1710
December	1st to 10th	Day	25% 12% 4 7% 4 4 4 3 5% 6% 26%	1710
	11th to 20th	Day	19% 14% 4 8% 4 3 4 3 4 6% 32%	2153
	21st to end	Day	17% 16% 4 9% 6% 5% 6% 4 6% 6% 22%	3124
January	1st to 10th	Night	17% 19% 6% 11% 5% 4 4 3 4 4 23%	4109
	11th to 20th	Night	21% 14% 4 10% 5% 3 4 3 4 4 30%	4120
	21st to end	Night	23% 13% 4 9% 5% 4 3 3 3 4 28%	4532
February	1st to 10th	Night	18% 11% 5% 8% 5% 5% 4 3 5% 6% 31%	2432
	11th to 20th	Night	23% 12% 4 7% 4 3 4 3 4 5% 30%	2218
	21st to end	Night	23% 13% 4 7% 4 4 4 3 5% 4 28%	1884
March	1st to 10th	Night	22% 12% 3 7% 5% 5% 4 3 4 6% 31%	3546
	11th to 20th	Night	14% 8% 3 5% 4 4 4 4 5% 6% 44%	3480
	21st to end	Night	13% 7% 2 6% 5% 5% 5% 4 6% 7% 40%	3817
April	1st to 10th	Night	13% 9% 2 7% 6% 4 4 4 5% 7% 41%	2090
	11th to 20th	Night	15% 8% 2 6% 3 3 4 3 6% 8% 42%	1784
	21st to end	Night	15% 8% 3 5% 5% 5% 4 3 5% 7% 40%	1710
May	1st to 10th	Night	17% 8% 3 6% 4 5% 5% 3 6% 8% 36%	1710
	11th to 20th	Night	14% 5% 3 7% 5% 5% 5% 5% 7% 8% 37%	1619
	21st to end	Night	9% 5% 2 6% 5% 5% 5% 4 7% 10% 41%	1744
June	1st to 10th	Night	9% 5% 2 4 4 5% 6% 5% 8% 10% 42%	1620
	11th to 20th	Night	6% 5% 2 6% 4 5% 5% 5% 8% 10% 44%	1620
	21st to end	Night	10% 5% 1 6% 4 5% 5% 5% 7% 10% 41%	1620
July	1st to 10th	Night	11% 6% 2 7% 6% 5% 5% 4 7% 8% 40%	1620
	11th to 20th	Night	6% 5% 1 5% 5% 4 5% 5% 8% 10% 45%	1620
	21st to end	Night	9% 5% 2 7% 5% 4 6% 5% 6% 9% 41%	1771
August	1st to 10th	Night	8% 5% 2 6% 5% 5% 5% 4 8% 10% 43%	1611
	11th to 20th	Night	10% 6% 3 5% 5% 5% 5% 5% 7% 9% 41%	1598
	21st to end	Night	9% 4 2 5% 3 3 4 5% 7% 9% 48%	1869
September	1st to 10th	Night	12% 4 2 6% 4 4 4 3 7% 8% 46%	1980
	11th to 20th	Night	12% 4 2 7% 5% 3 4 3 4 6% 49%	1980
	21st to end	Night	11% 6% 2 6% 4 3 4 4 5% 7% 49%	1980
October	1st to 10th	Night	13% 4 2 4 4 3 3 3 6% 6% 52%	2016
	11th to 20th	Night	17% 7% 2 6% 3 3 3 2 4 6% 47%	2160
	21st to end	Night	19% 7% 2 5% 4 3 3 3 4 6% 43%	2376
November	1st to 10th	Night	14% 6% 2 5% 4 3 4 2 3 6% 51%	2384
	11th to 20th	Night	15% 8% 2 7% 3 4 4 3 5% 6% 43%	2470
	21st to end	Night	15% 10% 2 5% 4 4 3 4 5% 6% 42%	2470
December	1st to 10th	Night	21% 15% 4 8% 4 3 3 3 5% 5% 28%	2470
	11th to 20th	Night	18% 13% 5% 7% 5% 4 4 3 5% 5% 31%	3042
	21st to end	Night	18% 16% 4 9% 6% 5% 5% 3 6% 7% 22%	4532

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% 4 8% 17% 9% 1	2837
	11th to 20th	Day	68% 2 7% 16% 6% 1	2840
	21st to end	Day	62% 3 6% 22% 6% 1	3124
February	1st to 10th	Day	57% 3 8% 20% 11% 1	1744
	11th to 20th	Day	57% 3 7% 19% 12% 2	1858
	21st to end	Day	57% 4 8% 16% 12% 3	1570
March	1st to 10th	Day	57% 4 8% 17% 12% 2	3307
	11th to 20th	Day	42% 3 9% 24% 19% 2	3480
	21st to end	Day	45% 4 10% 27% 12% 2	3815
April	1st to 10th	Day	45% 5% 10% 23% 15% 3	2090
	11th to 20th	Day	42% 4 9% 22% 19% 5%	2361
	21st to end	Day	47% 5% 10% 24% 11% 3	2470
May	1st to 10th	Day	49% 6% 12% 21% 10% 2	2470
	11th to 20th	Day	49% 7% 12% 22% 8% 1	2333
	21st to end	Day	39% 8% 16% 24% 12% 1	2531
June	1st to 10th	Day	40% 7% 12% 24% 15% 1	2340
	11th to 20th	Day	40% 9% 13% 19% 17% 2	2340
	21st to end	Day	38% 9% 14% 21% 16% 3	2340
July	1st to 10th	Day	41% 9% 16% 19% 15% 1	2275
	11th to 20th	Day	39% 8% 16% 21% 13% 3	2271
	21st to end	Day	44% 8% 13% 24% 10% 1	2405
August	1st to 10th	Day	41% 10% 18% 20% 10%	2314
	11th to 20th	Day	45% 8% 14% 23% 10% 1	2303
	21st to end	Day	38% 8% 14% 22% 15% 3	2465
September	1st to 10th	Day	38% 7% 11% 23% 16% 5%	1980
	11th to 20th	Day	38% 5% 10% 27% 17% 2	1980
	21st to end	Day	42% 4 9% 23% 21% 2	1980
October	1st to 10th	Day	35% 6% 12% 24% 20% 4	1942
	11th to 20th	Day	38% 3 11% 25% 21% 3	1800
	21st to end	Day	44% 3 8% 27% 15% 3	1980
November	1st to 10th	Day	33% 4 9% 24% 26% 3	1690
	11th to 20th	Day	44% 3 7% 23% 19% 4	1710
	21st to end	Day	46% 3 9% 22% 18% 2	1710
December	1st to 10th	Day	59% 4 10% 19% 7% 1	1710
	11th to 20th	Day	54% 3 9% 17% 15% 1	2150
	21st to end	Day	60% 5% 10% 20% 5%	3124
January	1st to 10th	Night	64% 4 8% 15% 8% 1	4109
	11th to 20th	Night	57% 4 6% 19% 12% 2	4120
	21st to end	Night	59% 3 8% 20% 9% 1	4532
February	1st to 10th	Night	55% 3 10% 20% 11% 1	2432
	11th to 20th	Night	57% 3 8% 18% 12% 2	2219
	21st to end	Night	58% 4 9% 15% 11% 3	1884
March	1st to 10th	Night	55% 4 7% 18% 14% 2	3546
	11th to 20th	Night	40% 4 9% 30% 15% 3	3480
	21st to end	Night	41% 5% 11% 24% 17% 1	3817
April	1st to 10th	Night	43% 5% 10% 22% 17% 3	2090
	11th to 20th	Night	40% 5% 10% 22% 17% 7%	1784
	21st to end	Night	44% 6% 11% 24% 13% 2	1710
May	1st to 10th	Night	46% 6% 13% 21% 11% 3	1710
	11th to 20th	Night	43% 8% 14% 23% 11% 1	1620
	21st to end	Night	37% 8% 16% 23% 15% 1	1745
June	1st to 10th	Night	33% 9% 17% 22% 17% 3	1620
	11th to 20th	Night	32% 9% 18% 20% 19% 3	1620
	21st to end	Night	35% 8% 15% 23% 14% 5%	1620
July	1st to 10th	Night	40% 7% 14% 20% 16% 3	1575
	11th to 20th	Night	31% 9% 17% 23% 16% 4	1566
	21st to end	Night	36% 8% 16% 26% 12% 2	1672
August	1st to 10th	Night	34% 8% 16% 26% 14% 2	1600
	11th to 20th	Night	37% 8% 17% 24% 13% 1	1598
	21st to end	Night	30% 8% 13% 27% 19% 4	1869
September	1st to 10th	Night	34% 7% 13% 23% 16% 7%	1980
	11th to 20th	Night	36% 7% 10% 22% 22% 3	1980
	21st to end	Night	34% 6% 10% 25% 23% 2	1980
October	1st to 10th	Night	33% 5% 11% 25% 21% 6%	2016
	11th to 20th	Night	39% 4 8% 24% 21% 4	2160
	21st to end	Night	43% 4 7% 23% 19% 4	2376
November	1st to 10th	Night	36% 4 7% 22% 24% 6%	2384
	11th to 20th	Night	42% 4 8% 20% 20% 6%	2470
	21st to end	Night	43% 4 8% 27% 16% 2	2470
December	1st to 10th	Night	58% 4 8% 19% 9% 2	2470
	11th to 20th	Night	55% 3 7% 22% 11% 1	3044
	21st to end	Night	60% 5% 9% 17% 9%	4532

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	28% 23% 31% 14% 3 1	270
	11th to 20th	Day	39% 23% 25% 12% 1	270
	21st to end	Day	30% 26% 28% 13% 2 1	297
February	1st to 10th	Day	28% 20% 34% 13% 5%	270
	11th to 20th	Day	44% 22% 26% 6% 1 1	270
	21st to end	Day	38% 20% 31% 11%	225
March	1st to 10th	Day	32% 18% 30% 14% 6%	379
	11th to 20th	Day	27% 22% 29% 17% 4 1	460
	21st to end	Day	29% 19% 29% 17% 6%	506
April	1st to 10th	Day	29% 18% 25% 18% 10% 1	460
	11th to 20th	Day	21% 21% 28% 17% 11% 3	452
	21st to end	Day	25% 14% 32% 20% 8% 2	460
May	1st to 10th	Day	22% 17% 29% 18% 10% 4	463
	11th to 20th	Day	17% 11% 28% 26% 13% 5%	456
	21st to end	Day	14% 5% 25% 24% 22% 10%	485
June	1st to 10th	Day	14% 6% 22% 27% 22% 9%	450
	11th to 20th	Day	12% 8% 19% 30% 22% 10%	450
	21st to end	Day	13% 4 26% 26% 19% 12%	450
July	1st to 10th	Day	16% 3 17% 23% 19% 22%	445
	11th to 20th	Day	8% 5% 19% 29% 24% 16%	444
	21st to end	Day	14% 5% 17% 30% 23% 13%	488
August	1st to 10th	Day	15% 4 17% 25% 27% 13%	446
	11th to 20th	Day	14% 6% 13% 28% 27% 12%	439
	21st to end	Day	13% 3 13% 27% 30% 14%	495
September	1st to 10th	Day	15% 3 14% 23% 32% 13%	450
	11th to 20th	Day	14% 3 13% 27% 27% 15%	450
	21st to end	Day	20% 9% 19% 27% 18% 8%	450
October	1st to 10th	Day	15% 8% 22% 25% 22% 8%	450
	11th to 20th	Day	18% 11% 26% 27% 15% 4	450
	21st to end	Day	26% 14% 22% 25% 12% 2	495
November	1st to 10th	Day	19% 13% 29% 25% 11% 3	288
	11th to 20th	Day	25% 15% 28% 19% 12% 1	270
	21st to end	Day	24% 17% 28% 21% 9% 1	270
December	1st to 10th	Day	32% 19% 26% 17% 5% 1	270
	11th to 20th	Day	28% 18% 29% 19% 4 3	270
	21st to end	Day	30% 18% 34% 12% 5% 1	297
January	1st to 10th	Night	28% 18% 28% 21% 5%	760
	11th to 20th	Night	32% 16% 27% 21% 4	760
	21st to end	Night	25% 17% 31% 24% 2	836
February	1st to 10th	Night	26% 14% 30% 28% 2	760
	11th to 20th	Night	33% 17% 29% 19% 2	743
	21st to end	Night	33% 17% 30% 18% 2	628
March	1st to 10th	Night	28% 15% 26% 27% 5%	760
	11th to 20th	Night	26% 15% 29% 22% 7%	760
	21st to end	Night	20% 15% 30% 27% 8%	836
April	1st to 10th	Night	20% 16% 27% 28% 8%	760
	11th to 20th	Night	21% 12% 24% 29% 13% 1	589
	21st to end	Night	18% 10% 33% 31% 8% 1	570
May	1st to 10th	Night	20% 8% 26% 31% 12% 2	570
	11th to 20th	Night	16% 7% 24% 33% 19% 2	540
	21st to end	Night	10% 4 16% 34% 30% 6%	584
June	1st to 10th	Night	11% 3 17% 36% 29% 4	540
	11th to 20th	Night	8% 3 15% 38% 31% 6%	540
	21st to end	Night	10% 2 19% 36% 29% 4	540
July	1st to 10th	Night	14% 2 10% 31% 36% 7%	525
	11th to 20th	Night	8% 2 11% 33% 41% 6%	522
	21st to end	Night	10% 4 10% 33% 35% 8%	591
August	1st to 10th	Night	9% 2 9% 33% 41% 5%	537
	11th to 20th	Night	12% 2 9% 31% 38% 8%	534
	21st to end	Night	11% 1 7% 32% 41% 8%	602
September	1st to 10th	Night	13% 3 11% 29% 39% 6%	720
	11th to 20th	Night	14% 2 13% 27% 39% 5%	720
	21st to end	Night	14% 5% 19% 29% 31% 3	720
October	1st to 10th	Night	15% 4 17% 30% 29% 4	720
	11th to 20th	Night	20% 8% 21% 32% 17% 2	720
	21st to end	Night	23% 9% 23% 30% 13% 1	792
November	1st to 10th	Night	16% 8% 22% 33% 18% 2	740
	11th to 20th	Night	19% 13% 25% 30% 11% 1	760
	21st to end	Night	21% 11% 21% 35% 11% 1	760
December	1st to 10th	Night	30% 15% 24% 26% 5%	760
	11th to 20th	Night	25% 12% 28% 28% 6% 1	760
	21st to end	Night	31% 14% 22% 29% 5%	836

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