Changstars Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Changstars Observatory clear sky chart from 2005-11-06 to 2026-02-16.

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	13% 6% 2 7% 5% 5% 5% 4 6% 8% 39%	130963
	Night	14% 7% 2 6% 4 3 4 3 5% 7% 45%	130130
Transparency	Day	40% 7% 12% 19% 15% 7%	134612
	Night	37% 6% 12% 19% 16% 9%	134532
Seeing	Day	12% 7% 20% 28% 22% 10%	16438
	Night	14% 6% 20% 32% 25% 3	26413

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	19% 10% 3 7% 3 3 3 2 3 4 42%	5289
	11th to 20th	Day	13% 6% 2 5% 4 3 3 3 3 6% 51%	5310
	21st to end	Day	27% 9% 2 9% 4 2 2 3 4 6% 32%	5812
February	1st to 10th	Day	19% 9% 3 6% 4 5% 5% 3 5% 6% 35%	2000
	11th to 20th	Day	23% 9% 3 6% 4 4 4 3 4 5% 35%	2029
	21st to end	Day	22% 8% 2 7% 3 4 4 3 4 6% 38%	1815
March	1st to 10th	Day	15% 7% 3 7% 5% 4 4 4 5% 8% 37%	4936
	11th to 20th	Day	13% 5% 2 8% 4 4 4 3 5% 5% 45%	4980
	21st to end	Day	19% 7% 2 6% 5% 4 4 3 5% 6% 40%	5462
April	1st to 10th	Day	15% 8% 3 8% 4 4 5% 3 5% 6% 38%	2200
	11th to 20th	Day	16% 8% 3 6% 4 4 4 4 5% 7% 38%	2326
	21st to end	Day	11% 7% 3 7% 5% 6% 5% 4 6% 7% 40%	2400
May	1st to 10th	Day	17% 8% 3 8% 6% 6% 5% 4 6% 7% 30%	5451
	11th to 20th	Day	11% 5% 2 7% 4 4 5% 4 6% 6% 45%	5862
	21st to end	Day	10% 7% 2 7% 6% 5% 6% 5% 8% 10% 35%	6434
June	1st to 10th	Day	7% 4 2 7% 6% 5% 6% 6% 7% 10% 41%	2600
	11th to 20th	Day	6% 4 2 6% 6% 6% 7% 7% 8% 11% 37%	2600
	21st to end	Day	7% 7% 2 7% 8% 8% 9% 7% 9% 10% 27%	2600
July	1st to 10th	Day	8% 6% 2 8% 7% 7% 8% 6% 9% 11% 28%	5900
	11th to 20th	Day	5% 6% 3 9% 7% 7% 7% 6% 8% 12% 29%	5900
	21st to end	Day	11% 7% 3 7% 6% 6% 6% 4 7% 10% 34%	6422
August	1st to 10th	Day	2 2 1 4 4 5% 5% 5% 8% 10% 55%	4234
	11th to 20th	Day	3 2 1 4 4 5% 7% 6% 9% 13% 44%	3981
	21st to end	Day	8% 5% 2 8% 6% 7% 7% 5% 8% 10% 34%	4222
September	1st to 10th	Day	7% 7% 3 10% 6% 6% 6% 6% 7% 8% 34%	2200
	11th to 20th	Day	13% 5% 2 8% 5% 5% 7% 6% 7% 8% 35%	2200
	21st to end	Day	8% 6% 2 6% 4 5% 5% 4 6% 9% 45%	2200
October	1st to 10th	Day	7% 5% 1 5% 4 4 3 4 6% 7% 55%	2079
	11th to 20th	Day	10% 5% 2 5% 4 4 4 3 5% 7% 51%	2000
	21st to end	Day	12% 5% 2 5% 4 3 4 2 5% 6% 53%	2200
November	1st to 10th	Day	11% 6% 2 6% 4 4 5% 4 5% 7% 46%	2058
	11th to 20th	Day	12% 6% 2 6% 4 4 3 3 6% 6% 46%	2100
	21st to end	Day	16% 7% 3 7% 5% 4 5% 3 5% 5% 41%	2063
December	1st to 10th	Day	26% 8% 3 6% 4 3 3 2 5% 7% 33%	3030
	11th to 20th	Day	13% 6% 3 6% 4 5% 4 3 6% 7% 43%	3481
	21st to end	Day	15% 6% 2 6% 4 4 4 3 5% 6% 45%	4587
January	1st to 10th	Night	18% 7% 2 6% 4 3 4 3 4 6% 44%	7634
	11th to 20th	Night	14% 7% 2 6% 4 3 3 3 4 5% 49%	7680
	21st to end	Night	28% 9% 2 7% 3 3 3 2 4 5% 32%	8402
February	1st to 10th	Night	19% 8% 3 6% 3 3 3 3 4 6% 40%	2636
	11th to 20th	Night	21% 8% 3 6% 3 3 3 2 3 5% 43%	2365
	21st to end	Night	22% 9% 2 6% 4 4 3 2 3 6% 40%	1815
March	1st to 10th	Night	21% 11% 3 6% 3 3 3 3 5% 6% 36%	4919
	11th to 20th	Night	18% 8% 3 6% 4 3 3 2 4 5% 45%	4980
	21st to end	Night	18% 10% 3 6% 4 3 3 3 3 5% 42%	5467
April	1st to 10th	Night	16% 7% 2 6% 5% 4 5% 3 5% 8% 40%	2200
	11th to 20th	Night	20% 5% 2 4 3 3 4 4 5% 7% 44%	2020
	21st to end	Night	13% 7% 2 6% 5% 4 4 4 6% 7% 43%	2000
May	1st to 10th	Night	24% 10% 3 7% 4 4 4 3 5% 7% 28%	4416
	11th to 20th	Night	14% 7% 2 6% 4 4 4 4 6% 8% 42%	4053
	21st to end	Night	15% 8% 3 7% 5% 5% 5% 4 8% 11% 29%	4443
June	1st to 10th	Night	5% 2 2 3 3 3 4 4 6% 12% 55%	1800
	11th to 20th	Night	5% 3 1 4 3 3 3 4 7% 11% 56%	1800
	21st to end	Night	5% 4 1 5% 4 4 6% 5% 8% 11% 46%	1800
July	1st to 10th	Night	5% 4 2 5% 4 5% 5% 5% 8% 13% 43%	4078
	11th to 20th	Night	3 3 2 6% 5% 5% 5% 5% 7% 11% 49%	4077
	21st to end	Night	6% 5% 3 7% 4 4 5% 4 6% 10% 46%	4449
August	1st to 10th	Night	1 1 1 2 1 2 2 3 4 10% 73%	2926
	11th to 20th	Night	2 2 1 3 2 2 3 4 7% 14% 60%	3069
	21st to end	Night	5% 3 1 6% 4 4 5% 5% 8% 12% 48%	3670
September	1st to 10th	Night	5% 4 2 7% 6% 5% 5% 4 7% 9% 47%	2200
	11th to 20th	Night	10% 6% 2 6% 5% 4 4 4 5% 7% 47%	2200
	21st to end	Night	8% 5% 2 4 4 3 3 4 5% 8% 54%	2200
October	1st to 10th	Night	5% 4 2 4 3 2 3 3 5% 7% 62%	2300
	11th to 20th	Night	9% 3 1 3 2 3 3 3 5% 7% 61%	2400
	21st to end	Night	11% 4 2 4 3 3 3 2 3 5% 61%	2640
November	1st to 10th	Night	10% 5% 1 4 4 3 4 3 5% 7% 54%	2460
	11th to 20th	Night	12% 5% 2 4 3 3 3 3 5% 5% 54%	2520
	21st to end	Night	16% 7% 2 6% 4 4 3 3 5% 6% 46%	2552
December	1st to 10th	Night	24% 11% 3 7% 4 4 3 3 4 5% 32%	4380
	11th to 20th	Night	14% 8% 3 7% 4 4 4 3 4 5% 45%	4946
	21st to end	Night	16% 7% 2 6% 4 3 3 3 4 5% 47%	6633

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	47% 4 6% 21% 16% 7%	5290
	11th to 20th	Day	35% 4 6% 22% 20% 13%	5310
	21st to end	Day	55% 3 5% 13% 15% 10%	5812
February	1st to 10th	Day	50% 5% 6% 13% 15% 11%	2000
	11th to 20th	Day	52% 5% 8% 12% 16% 8%	2039
	21st to end	Day	48% 6% 8% 15% 14% 9%	1815
March	1st to 10th	Day	43% 7% 10% 16% 12% 12%	4936
	11th to 20th	Day	39% 7% 7% 9% 20% 17%	4980
	21st to end	Day	44% 6% 8% 18% 19% 4	5464
April	1st to 10th	Day	46% 6% 8% 17% 14% 9%	2200
	11th to 20th	Day	45% 5% 10% 15% 15% 9%	2326
	21st to end	Day	41% 7% 10% 17% 18% 6%	2400
May	1st to 10th	Day	50% 8% 11% 18% 10% 3	5486
	11th to 20th	Day	37% 6% 12% 22% 20% 3	5866
	21st to end	Day	38% 9% 14% 29% 8% 1	6447
June	1st to 10th	Day	34% 10% 18% 24% 15% 1	2600
	11th to 20th	Day	36% 10% 14% 25% 14% 2	2600
	21st to end	Day	43% 11% 19% 19% 7%	2600
July	1st to 10th	Day	43% 11% 28% 16% 2	5836
	11th to 20th	Day	42% 11% 26% 18% 3	5831
	21st to end	Day	42% 10% 18% 24% 6%	6240
August	1st to 10th	Day	21% 8% 16% 41% 13%	4220
	11th to 20th	Day	25% 11% 31% 29% 4 1	4020
	21st to end	Day	41% 11% 17% 19% 12% 1	4221
September	1st to 10th	Day	43% 12% 13% 15% 13% 4	2200
	11th to 20th	Day	43% 10% 13% 19% 13% 3	2200
	21st to end	Day	35% 8% 12% 19% 16% 10%	2200
October	1st to 10th	Day	21% 4 6% 16% 17% 37%	3378
	11th to 20th	Day	23% 4 7% 17% 36% 13%	3240
	21st to end	Day	26% 4 6% 17% 34% 13%	3538
November	1st to 10th	Day	36% 6% 10% 19% 20% 9%	2058
	11th to 20th	Day	38% 6% 10% 14% 21% 13%	2100
	21st to end	Day	44% 5% 9% 16% 16% 10%	2063
December	1st to 10th	Day	51% 4 6% 16% 18% 4	3030
	11th to 20th	Day	38% 6% 9% 16% 20% 9%	3479
	21st to end	Day	39% 5% 7% 17% 20% 12%	4587
January	1st to 10th	Night	41% 5% 10% 17% 17% 9%	7635
	11th to 20th	Night	39% 4 4 17% 22% 14%	7680
	21st to end	Night	55% 3 5% 13% 16% 9%	8402
February	1st to 10th	Night	47% 5% 8% 10% 18% 13%	2636
	11th to 20th	Night	45% 5% 8% 14% 17% 12%	2365
	21st to end	Night	48% 5% 6% 13% 18% 10%	1815
March	1st to 10th	Night	49% 6% 8% 13% 14% 10%	4919
	11th to 20th	Night	42% 6% 6% 13% 16% 16%	4980
	21st to end	Night	45% 5% 8% 14% 20% 8%	5467
April	1st to 10th	Night	42% 7% 11% 17% 14% 9%	2200
	11th to 20th	Night	39% 5% 11% 17% 15% 13%	2020
	21st to end	Night	39% 7% 11% 20% 16% 7%	2000
May	1st to 10th	Night	54% 7% 10% 17% 9% 3	4446
	11th to 20th	Night	38% 7% 12% 23% 15% 5%	4054
	21st to end	Night	45% 10% 18% 21% 6%	4452
June	1st to 10th	Night	22% 7% 22% 31% 18%	1800
	11th to 20th	Night	23% 7% 19% 31% 17% 3	1800
	21st to end	Night	29% 12% 24% 25% 11%	1800
July	1st to 10th	Night	31% 7% 37% 23% 2	4035
	11th to 20th	Night	29% 8% 32% 27% 4	4026
	21st to end	Night	33% 10% 23% 28% 6%	4320
August	1st to 10th	Night	9% 5% 21% 50% 15%	2916
	11th to 20th	Night	15% 8% 36% 36% 4 1	3096
	21st to end	Night	27% 14% 22% 27% 9% 1	3670
September	1st to 10th	Night	31% 11% 21% 19% 13% 4	2200
	11th to 20th	Night	35% 8% 14% 24% 16% 3	2200
	21st to end	Night	28% 7% 14% 23% 18% 9%	2200
October	1st to 10th	Night	17% 4 8% 18% 18% 35%	3755
	11th to 20th	Night	15% 4 7% 23% 33% 17%	3899
	21st to end	Night	20% 4 6% 17% 36% 18%	4252
November	1st to 10th	Night	30% 6% 11% 21% 22% 10%	2460
	11th to 20th	Night	31% 5% 11% 20% 20% 12%	2520
	21st to end	Night	40% 5% 9% 19% 17% 9%	2552
December	1st to 10th	Night	54% 4 6% 15% 15% 5%	4380
	11th to 20th	Night	40% 6% 10% 12% 18% 14%	4947
	21st to end	Night	40% 5% 7% 14% 27% 9%	6633

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	22% 15% 34% 19% 8% 1	330
	11th to 20th	Day	22% 13% 34% 24% 7% 1	330
	21st to end	Day	23% 16% 31% 23% 7%	363
February	1st to 10th	Day	18% 18% 36% 21% 8% 1	330
	11th to 20th	Day	26% 18% 30% 20% 4 1	315
	21st to end	Day	26% 18% 39% 14% 3	249
March	1st to 10th	Day	21% 23% 35% 16% 5%	300
	11th to 20th	Day	16% 20% 42% 17% 4 1	300
	21st to end	Day	22% 17% 34% 19% 8% 1	330
April	1st to 10th	Day	18% 21% 33% 21% 6%	300
	11th to 20th	Day	20% 18% 27% 23% 11% 1	444
	21st to end	Day	17% 11% 32% 25% 13% 1	500
May	1st to 10th	Day	16% 11% 34% 27% 13%	523
	11th to 20th	Day	12% 6% 33% 31% 15% 2	725
	21st to end	Day	10% 5% 26% 39% 17% 3	789
June	1st to 10th	Day	6% 3 16% 38% 28% 10%	730
	11th to 20th	Day	7% 2 14% 39% 28% 10%	730
	21st to end	Day	10% 2 9% 31% 31% 17%	730
July	1st to 10th	Day	8% 3 7% 29% 33% 20%	721
	11th to 20th	Day	5% 1 3 30% 36% 25%	718
	21st to end	Day	6% 2 4 29% 36% 23%	794
August	1st to 10th	Day	3 3 26% 42% 25%	726
	11th to 20th	Day	4 4 19% 42% 31%	603
	21st to end	Day	11% 4 5% 17% 32% 31%	532
September	1st to 10th	Day	10% 2 7% 28% 36% 16%	330
	11th to 20th	Day	9% 2 7% 29% 33% 19%	330
	21st to end	Day	8% 2 6% 32% 40% 12%	330
October	1st to 10th	Day	6% 2 10% 44% 31% 8%	330
	11th to 20th	Day	10% 5% 15% 37% 24% 9%	330
	21st to end	Day	16% 9% 26% 26% 18% 4	363
November	1st to 10th	Day	13% 5% 25% 33% 18% 5%	330
	11th to 20th	Day	15% 5% 26% 35% 15% 4	330
	21st to end	Day	16% 13% 37% 20% 9% 4	330
December	1st to 10th	Day	24% 6% 33% 22% 11% 5%	330
	11th to 20th	Day	17% 14% 33% 25% 9% 2	330
	21st to end	Day	16% 11% 36% 23% 13% 2	363
January	1st to 10th	Night	20% 9% 31% 30% 9%	1050
	11th to 20th	Night	20% 10% 28% 33% 8% 1	1050
	21st to end	Night	22% 10% 28% 33% 6%	1155
February	1st to 10th	Night	24% 9% 30% 31% 6%	956
	11th to 20th	Night	26% 10% 26% 30% 8%	771
	21st to end	Night	25% 12% 34% 25% 4	495
March	1st to 10th	Night	24% 8% 30% 31% 7%	600
	11th to 20th	Night	19% 10% 31% 32% 8%	600
	21st to end	Night	17% 8% 31% 33% 10% 1	660
April	1st to 10th	Night	16% 9% 33% 35% 7%	600
	11th to 20th	Night	22% 9% 26% 33% 11%	593
	21st to end	Night	16% 9% 27% 35% 13%	600
May	1st to 10th	Night	16% 8% 27% 40% 9%	600
	11th to 20th	Night	12% 5% 25% 41% 16% 1	591
	21st to end	Night	8% 4 22% 41% 24% 2	650
June	1st to 10th	Night	5% 2 8% 45% 39% 2	600
	11th to 20th	Night	6% 2 8% 45% 37% 3	600
	21st to end	Night	6% 1 9% 29% 46% 9%	600
July	1st to 10th	Night	6% 1 5% 29% 50% 10%	585
	11th to 20th	Night	3 1 2 23% 60% 11%	582
	21st to end	Night	4 1 2 24% 58% 9%	657
August	1st to 10th	Night	1 2 23% 64% 10%	597
	11th to 20th	Night	4 2 18% 64% 11%	594
	21st to end	Night	5% 1 5% 19% 55% 16%	660
September	1st to 10th	Night	5% 1 5% 23% 54% 12%	600
	11th to 20th	Night	12% 1 5% 24% 51% 8%	600
	21st to end	Night	11% 6% 29% 46% 7%	600
October	1st to 10th	Night	6% 1 8% 35% 43% 7%	700
	11th to 20th	Night	9% 2 17% 40% 29% 3	800
	21st to end	Night	11% 6% 20% 36% 24% 3	880
November	1st to 10th	Night	11% 3 18% 40% 26% 2	820
	11th to 20th	Night	13% 5% 22% 40% 21%	840
	21st to end	Night	18% 7% 28% 32% 15% 1	872
December	1st to 10th	Night	20% 7% 26% 30% 15% 1	1050
	11th to 20th	Night	20% 9% 25% 34% 11%	1050
	21st to end	Night	21% 8% 30% 31% 10% 1	1155

Page generated at 2026-02-13 00:42:12. Version ac07

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